The Aerial Bold Project

Aerial Bold : Kickstart the Planetary Search for Letterforms!” is the 11th Kickstarter project that I funded. The project was launched by Benedikt Groß & Joey Lee on October 14, 2014. After two weeks, 100% was funded by 480 bakers. At the end 569 backers pledged 11.492 $ to help bring this project to life.

Aerial Bold is the first map and typeface of the earth. The project is literally about reading the earth for letterforms, or alphabet shapes, written into the topology of buildings, roads, rivers, trees, and lakes. To do this, Benedikt Groß and Joey Lee traversed the entire planet’s worth of satellite imagery and developed the tools and methods necessary to map these features hiding in plain sight.

On March 8, 2016, the results were finally presented. The entire letterform database has been made available as a usable dataset for any art – design – science – textual project. Selected letterforms have been converted into a truetype – opentype font format that can be imported into any word processor.

The results of the project are presented at the Aerial Bold Project website. An embedded typewriter allows to enter text with the different fonts :  Satellite, Buildings, Suburbia, Provence. The size, offset, background and location of the fonts can be selected.

Aerial Bold

Aerial Bold Typewriter : Satellite font, Hamburg, size 150, offset 0, red background

Additional information about the process, data, fonts and credits are available at the project website. A catalog and posters complete the documentation. Below are some pictures of my name written with the Aerial Bold fonts.

Aerial Bold Satellite Font

Aerial Bold Satellite Font

Aerial Bold Buildings Font

Aerial Bold Buildings Font

Aerial Bold Suburbia Font

Aerial Bold Suburbia Font

Aerial Bold Provence Font

Aerial Bold Provence Font

Orthanc Raspberry Pi

Last update : March 20, 2016

This contribution explains how to implement an Orthanc DICOM PACS Server on a Raspberry Pi (RPI) standalone single-board computer. The Orthanc source code was compiled on a Raspberry Pi 1 model B computer and tested on RPI models B Pi 1, Pi 2 and Pi 3. I use Raspbian Jessie for my RPI’s, a free operating system based on Debian, optimized for the Raspberry Pi hardware. I call the resulting mini PACS server an OrthancPi.


Raspberry Pi genealogy

Debian Med

An Orthanc package for Debian is available at the Debian website. This package is part of Debian Med, a specific flavour of Debian that is particularly well fit for the requirements for medical practice and biomedical research. The Debian Med package is maintained by the developer of Orthanc, Sébastien Jodogne, a Medical Imaging Engineer at the Medical Physics Department from the University Hospital of Liège (CHU) in Belgium.

The stable Orthanc version for Debian Jessie is 0.8.4. The Orthanc version 1.0 is only available for the testing (Stretch) and for the unstable (SID) Debian distributions. A release for the armhf architecture is part of the Orthanc package. The file list of the orthanc package in the Debian stretch distribution for the armhf architecture is the following :


Orthanc Source Code

RPI’s are based on the ARM architecture and Raspbian releases usually follow the corresponding Debian release, but do deviate in a handful of cases. For this reason I preferred to build Orthanc version 1.0 from the source code, located on Bitbucket, on the Raspberry Pi hardware.

I downloaded the repository and put the content into a folder named Orthanc in the Raspberry Pi /home/pi/ directory. The following packages needed for compilation and linking have been installed first :

sudo apt-get install build-essential unzip cmake mercurial \
uuid-dev libcurl4-openssl-dev liblua5.1-0-dev \
libgoogle-glog-dev libgtest-dev libpng-dev libjpeg-dev \
libsqlite3-dev libssl-dev zlib1g-dev libdcmtk2-dev \
libboost-all-dev libwrap0-dev libjsoncpp-dev libpugixml-dev

I created a new folder OrthancBuild inside the /home/pi/ directory and executed the following commands :

cd OrthancBuild

During the configuration process, the following files have been downloaded from the website :

All the files have been saved in a new folder orthanc-default/ThirdPartyDownloads. All the third-party dependencies are statically linked to the Orthanc build. The compilation takes more time (several hours), compared to the default behavior where system wide libraries are used. Statically linked builds are however easier to share and to clone.

The result of the compilation was the generation of the following files :

  • Orthanc (executable)
  • UnitTests (executable)
  • (shared library) -> renamed to
  • (shared library) -> renamed to
  • libCoreLibrary.a (AR archive)
  • libServerLibrary.a (AR archive)
  • libOpenSSL.a (AR archive)

Running the UnitTests shows that all 154 tests from 43 test cases passed, 2 tests were disabled.

Orthanc UnitTests

Orthanc UnitTests

Orthanc plugins

I used the same procedure to compile the plugins DICOMweb and WebViewer. The source code was copied into the folders /home/pi/Dicomweb/ and /home/pi/Webviewer/. I created the new folders /home/pi/DicomwebBuild/ and /home/pi/WebviewerBuild and finally executed the commands

cd /home/pi/DicomwebBuild

cd /home/pi/WebviewerBuild
cmake -DSTATIC_BUILD=ON -DCMAKE_BUILD_TYPE=Release ~/Webviewer

The following third-party libraries are used by the plugins :

Dicomweb Webviewer
boost_1_59_0 boost_1_59_0
GDCM-prefix GDCM-prefix
gtest-1.7.0 gtest-1.7.0
jsoncpp-0.10.5 jsoncpp-0.10.5

The shared libraries and were created as a result of the compilation and linking process. I renamed these libraries without the mainline suffix.

The results of the UnitTests runs are shown hereafter :

Orthanc DicomWeb UnitTests

Orthanc DicomWeb UnitTests


Orthanc WebViewer UnitTests


The OrthancPi distribution files are assembled as follows in a directory named /home/pi/orthancpi/.

  • Orthanc  (executable)
  • UnitTests (executable)
  • configuration.json
  • /plugins/
  • /plugins/
  • /plugins/
  • /plugins/
  • /web/index.html (OrthancPi welcome page)
  • /python/ (empty)
  • /lua/ (empty)
  • /WorklistsDatabase/ (worklist samples)
  • /OrthancStorage/ (empty)

The default configuration.json file is created with the command

cd orthancpi
./Orthanc --config=configuration.json

I modified the configuration file as follows :

 // ***
 // General configuration of Orthanc
 // ***

 // The logical name of this instance of Orthanc. This one is
 // displayed in Orthanc Explorer and at the URI "/system".
  "Name" : "OrthancPi",

 // Path to the directory that holds the heavyweight files
 // (i.e. the raw DICOM instances)
  "StorageDirectory" : "OrthancStorage",

 // Path to the directory that holds the SQLite index (if unset,
 // the value of StorageDirectory is used). This index could be
 // stored on a RAM-drive or a SSD device for performance reasons.
  "IndexDirectory" : "OrthancStorage",

 // Enable the transparent compression of the DICOM instances
  "StorageCompression" : false,

 // Maximum size of the storage in MB (a value of "0" indicates no
 // limit on the storage size)
  "MaximumStorageSize" : 0,

 // Maximum number of patients that can be stored at a given time
 // in the storage (a value of "0" indicates no limit on the number
 // of patients)
  "MaximumPatientCount" : 0,
 // List of paths to the custom Lua scripts that are to be loaded
 // into this instance of Orthanc
  "LuaScripts" : [

 // List of paths to the plugins that are to be loaded into this
 // instance of Orthanc (e.g. "./" for Linux, or
 // "./PluginTest.dll" for Windows). These paths can refer to
 // folders, in which case they will be scanned non-recursively to
 // find shared libraries.
  "Plugins" : [
"Worklists" : {
    "Enable": true,
    "Database": "../WorklistsDatabase"
 "ServeFolders" : {
        "/web" : "/home/pi/orthancpi/web"

  "DicomWeb" : {
     "Enable" : true,
     "Root" : "/dicom-web/",
     "EnableWadoo" : true,
     "WadooRoot" : "/wado",
     "Host" : "localhost",
     "Ss1" : false

 // **
 //  Configuration of the HTTP server
 // **

 // Enable the HTTP server. If this parameter is set to "false",
 // Orthanc acts as a pure DICOM server. The REST API and Orthanc
 // Explorer will not be available.
  "HttpServerEnabled" : true,

 // HTTP port for the REST services and for the GUI
  "HttpPort" : 8042,

 // When the following option is "true", if an error is encountered
 // while calling the REST API, a JSON message describing the error
 // is put in the HTTP answer. This feature can be disabled if the
 // HTTP client does not properly handles such answers.
  "HttpDescribeErrors" : true,

 // Enable HTTP compression to improve network bandwidth utilization,
 // at the expense of more computations on the server. Orthanc
 // supports the "gzip" and "deflate" HTTP encodings.
  "HttpCompressionEnabled" : true,

 // **
 // Configuration of the DICOM server
 // **

 // Enable the DICOM server. If this parameter is set to "false",
 // Orthanc acts as a pure REST server. It will not be possible to
 // receive files or to do query/retrieve through the DICOM protocol.
  "DicomServerEnabled" : true,

 // The DICOM Application Entity Title
  "DicomAet" : "ORTHANCPI",

 // Check whether the called AET corresponds during a DICOM request
  "DicomCheckCalledAet" : false,

 // The DICOM port
  "DicomPort" : 4242,

 // The default encoding that is assumed for DICOM files without
 // "SpecificCharacterSet" DICOM tag. The allowed values are "Ascii",
 // "Utf8", "Latin1", "Latin2", "Latin3", "Latin4", "Latin5",
 // "Cyrillic", "Windows1251", "Arabic", "Greek", "Hebrew", "Thai",
 // "Japanese", and "Chinese".
  "DefaultEncoding" : "Utf8",

 // The transfer syntaxes that are accepted by Orthanc C-Store SCP
  "DeflatedTransferSyntaxAccepted"     : true,
  "JpegTransferSyntaxAccepted"         : true,
  "Jpeg2000TransferSyntaxAccepted"     : true,
  "JpegLosslessTransferSyntaxAccepted" : true,
  "JpipTransferSyntaxAccepted"         : true,
  "Mpeg2TransferSyntaxAccepted"        : true,
  "RleTransferSyntaxAccepted"          : true,

 // Whether Orthanc accepts to act as C-Store SCP for unknown storage
 // SOP classes (aka. "promiscuous mode")
  "UnknownSopClassAccepted"            : false,

 // **
 // Security-related options for the HTTP server
 // **

 // Whether remote hosts can connect to the HTTP server
  "RemoteAccessAllowed" : true,

 // Whether or not SSL is enabled
  "SslEnabled" : false,

 // Path to the SSL certificate (meaningful only if SSL is enabled)
  "SslCertificate" : "certificate.pem",

 // Whether or not the password protection is enabled
  "AuthenticationEnabled" : false,

 // The list of the registered users. Because Orthanc uses HTTP
 // Basic Authentication, the passwords are stored as plain text.
  "RegisteredUsers" : {
 // "alice" : "alicePassword"

 // **
 //  Network topology
 // **

 // The list of the known DICOM modalities
  "DicomModalities" : {
 // **
 //    * Uncommenting the following line would enable Orthanc to
 //    * connect to an instance of the "storescp" open-source DICOM
 //    * store (shipped in the DCMTK distribution) started by the
 //    * command line "storescp 2000".
 //   **
 // "sample" : [ "STORESCP", "localhost", 2000 ]
	"Horos" : [ "HOROS", "", 104 ],
	"dcm4che" : [ "DCM4CHE", "", 104 ]

// **
//    * A fourth parameter is available to enable patches for a
//     * specific PACS manufacturer. The allowed values are currently
//    * "Generic" (default value), "StoreScp" (storescp tool from
//    * DCMTK), "ClearCanvas", "MedInria", "Dcm4Chee", "SyngoVia",
//    * "AgfaImpax" (Agfa IMPAX), "EFilm2" (eFilm version 2), and
//     * "Vitrea". This parameter is case-sensitive.
//  **
// "clearcanvas" : [ "CLEARCANVAS", "", 104, "ClearCanvas" ]

 // The list of the known Orthanc peers
  "OrthancPeers" : {
 // **
//     * Each line gives the base URL of an Orthanc peer, possibly
//    * followed by the username/password pair (if the password
//    * protection is enabled on the peer).
//  **
// "peer"  : [ "http://localhost:8043/", "alice", "alicePassword" ]
// "peer2" : [ "http://localhost:8044/" ]
	"OrthancMac" : [ "" ]

// Parameters of the HTTP proxy to be used by Orthanc. If set to the
// empty string, no HTTP proxy is used. For instance:
//   "HttpProxy" : ""
//   "HttpProxy" : "proxyUser:proxyPassword@"
  "HttpProxy" : "",

// Set the timeout for HTTP requests issued by Orthanc (in seconds).
  "HttpTimeout" : 10,

// Enable the verification of the peers during HTTPS requests.
// Reference:
  "HttpsVerifyPeers" : true,

// Path to the CA (certification authority) certificates to validate
// peers in HTTPS requests. From curl documentation ("--cacert"
// option): "Tells curl to use the specified certificate file to
// verify the peers. The file may contain multiple CA
// certificates. The certificate(s) must be in PEM format."
  "HttpsCACertificates" : "",

// **
// Advanced options
// **

// Dictionary of symbolic names for the user-defined metadata. Each
// entry must map an unique string to an unique number between 1024
// and 65535.
  "UserMetadata" : {
 // "Sample" : 1024

// Dictionary of symbolic names for the user-defined types of
// attached files. Each entry must map an unique string to an unique
// number between 1024 and 65535. Optionally, a second argument can
// provided to specify a MIME content type for the attachment.
  "UserContentType" : {
 // "sample" : 1024
 // "sample2" : [ 1025, "application/pdf" ]

// Number of seconds without receiving any instance before a
// patient, a study or a series is considered as stable.
  "StableAge" : 60,

// By default, Orthanc compares AET (Application Entity Titles) in a
// case-insensitive way. Setting this option to "true" will enable
// case-sensitive matching.
  "StrictAetComparison" : false,

// When the following option is "true", the MD5 of the DICOM files
// will be computed and stored in the Orthanc database. This
// information can be used to detect disk corruption, at the price
// of a small performance overhead.
  "StoreMD5ForAttachments" : true,

// The maximum number of results for a single C-FIND request at the
// Patient, Study or Series level. Setting this option to "0" means
// no limit.
  "LimitFindResults" : 0,

// The maximum number of results for a single C-FIND request at the
// Instance level. Setting this option to "0" means no limit.
  "LimitFindInstances" : 0,

// The maximum number of active jobs in the Orthanc scheduler. When
// this limit is reached, the addition of new jobs is blocked until
// some job finishes.
  "LimitJobs" : 10,

// If this option is set to "false", Orthanc will not log the
// resources that are exported to other DICOM modalities of Orthanc
// peers in the URI "/exports". This is useful to prevent the index
// to grow indefinitely in auto-routing tasks.
  "LogExportedResources" : true,

// Enable or disable HTTP Keep-Alive (deprecated). Set this option
// to "true" only in the case of high HTTP loads.
  "KeepAlive" : false,

// If this option is set to "false", Orthanc will run in index-only
// mode. The DICOM files will not be stored on the drive. Note that
// this option might prevent the upgrade to newer versions of Orthanc.
  "StoreDicom" : true,

// DICOM associations are kept open as long as new DICOM commands
// are issued. This option sets the number of seconds of inactivity
// to wait before automatically closing a DICOM association. If set
// to 0, the connection is closed immediately.
  "DicomAssociationCloseDelay" : 5,

  // Maximum number of query/retrieve DICOM requests that are
  // maintained by Orthanc. The least recently used requests get
  // deleted as new requests are issued.
  "QueryRetrieveSize" : 10,

// When handling a C-Find SCP request, setting this flag to "true"
// will enable case-sensitive match for PN value representation
// (such as PatientName). By default, the search is
// case-insensitive, which does not follow the DICOM standard.
  "CaseSensitivePN" : false,
// Register a new tag in the dictionary of DICOM tags that are known
// to Orthanc. Each line must contain the tag (formatted as 2
// hexadecimal numbers), the value representation (2 upcase
// characters), a nickname for the tag, possibly the minimum
// multiplicity (> 0 with defaults to 1), and possibly the maximum
// multiplicity (0 means arbitrary multiplicity, defaults to 1).
  "Dictionary" : {
// "0014,1020" : [ "DA", "ValidationExpiryDate", 1, 1 ]

I copied the whole orthancpi directory with an USB card reader to different Raspberry Pi modules which were configured to run as headless servers with remote control, using various wifi adapters. The process is explained in my post Rasperry Pi revisited.

To start the OrthancPi server at boot, a file orthanc.desktop has been added to the /home/pi/.config/autostart/ folder. This is the content of this file

[Desktop Entry]
Exec=/home/pi/orthancpi/Orthanc /home/pi/orthancpi/configuration.json

OrthancPi can also be configured to run as a Debian daemon.

An OrthancPi server will be used in the context of the RadioLogic project, a training tool for radiology. This tool will be used in classrooms without Internet access. The OrthancPi server works as wireless access point. To extend the WiFi range, a  repeater can be used. The RadioLogic tool is web based and optimized to run on iPad’s which are connected to the OrthancPi WLAN. The following figure shows two OrthancPi servers with USB WiFi adapters using  Ralink RT5370 chips.

OrthancPi's with USB WiFi adapters, SDcards and SDcard-reader

OrthancPi’s with USB WiFi adapters, SDcards and SDcard-reader

A user guide for the OrthancPi is available at the RadioLogic website. Disk image files to set up an OrthancPi server in a plug and play way, with different WiFi adapters, are available at the same webpage.

Raspberry Pi Revisited

Last update : March 20, 2016

Referring to my first post about the Raspberry Pi, I am pleased to provide an update about my ongoing projects, with a special focus on a wireless headless operation. Headless means running it without a mouse, keyboard and monitor, via network connections. Wireless means using a WLAN connection with a WiFi dongle.

Raspberry Pi's

Raspberry Pi’s with SDcards and WiFi dongles

Raspbian Jessie

A new Raspbian (the Raspberry Pi Foundation’s official supported operating system) version, based on Debian Jessie, was launched on November 21, 2015. Updates were released on February 9, February 26 and March 18, 2016. I upgraded my Raspberry Pi computers to the February 26 version (kernel 4.1.18+ on RPi 1 models; kernel 4.1.18-v7+ on RPi 2 and 3 models). I used the same procedure as last time : writing the downloaded Raspbian image with the Win32DiskImager tool on my Windows computer to 8GB and 16GB SDcards. The current version of this tool is 0.9.5, released on March 19, 2014.

Raspberry Pi model B

Raspberry Pi model B

It was not possible to overwrite the old Raspbian image on my Raspberry Pi SDcards with the new one. Reformatting the used SDcards with the usual tools (diskpart, SDCard Formatter 4, …) was not successful. Windows sees only the 56 MB FAT32 partition, but not the much larger Linux partition. It was not possible to recover the lost space. After some trials and errors, I finally reformatted the SDcards in my Nikon camera. It worked like a charm. Writing the new Raspbian image to the new formatted SDcards was then no problem.

Raspberry Pi model 2

Raspberry Pi model 2

Configuring Raspbian

After the first booting of the Raspberry Pi computer, the new desktop appears.

Raspian Jessie Desktop

Raspbian Jessie Desktop

The desktop contains only one icon : the Trash. The Raspberry logo is shown in the center. The menu is in the left upper corner, some icons are in the right upper corner. It’s possible to reconfigure the menu list. One new icon is the network configuration tool based on Roy Marples‘s dhcpcd and dhcpcd-ui packages.

The configuration panel allows to expand the filesystem.

Raspberry Pi Configuration

Raspberry Pi Configuration

After expanding and rebooting the system, the File Manager (PCManFM 1.2.3) shows a free space of 3,6 GB on my 8GB SDcard, compared to a free space of 98,1 MB before the expansion. The Raspbian Jessie version allows to set the locale to “lb” for Luxembourg in the localisation tab, and Luxembourg is also included as country in the timezone list. The french-swiss keyboard used in Luxembourg figures now, as expected, among the Switzerland keyboards.

The computer was recognized by my Fritzbox router and the IP address was assigned by the DHCP server.

Updating and checking the Raspbian version

The file /etc/apt/sources.list contains the following source (one line) :

jessie main contrib non-free rpi

I replaced mirrordirector by archive to avoid numerous download fails. To download the source code of packages with apt-get source, the following line should be un-commented in the sources.list file (one line) :

jessie main contrib non-free rpi

Raspbian Jessie is based on Linux version 8. A lite version without the GUI is separately available. I preferred to install the full version and to remove some large packages like Wolfram, Office and some games with the following command :

sudo apt-get remove --purge wolfram-engine libreoffice* minecraft-pi \
python-minecraftpi python3-minecraftpi

The files /usr/share/raspi-ui-overrides/applications/wolfram-mathematica.desktop and ../wolfram-language.desktop must be deleted to remove the related empty icons in the task bar.

To install the additional packages needed for the headless operation I entered the command :

sudo apt-get update
sudo apt-get install x11vnc hostapd dnsmasq lighttpd haveged

Lists of the resulting installed packages for my project are provided at the following links :

The default Raspbian terminal application is LXTerminal.

To upgrade all Debian packages to the latest version, I launched the well known commands

sudo apt-get update
sudo apt-get upgrade

in the terminal window. Two packages were upgraded. To show the installed versions, we can use the following commands :

  • uname – a        > kernel
  • lsb_release -a       > distribution
  • /opt/vc/bin/vcgencmd version       > firmware
  • cat  /proc/version
  • cat  /proc/cpuinfo     > CPU
  • cat  /proc/meminfo    > memory
  • cat  /proc/partitions    > partitions
  • cat  /etc/debian_version
  • cat  /etc/issue
Show Raspian Versions

Show Raspban versions and system informations

The command dmesg (display message or driver message) prints the message buffer of the kernel. An error “bcm2708_fb soc.fb: Unknown ioctl 0x40187a22” is listed.


display the Raspberry Pi kernel message buffer

Remote Control

There are several methods available to remotely control a headless Raspberry Pi computer. The most common are :

  • Secure Shell (SSH)
  • Remote Desktop (RDP)
  • Tight-VNC
  • X11-VNC


An SSH client is a software program which uses the secure shell protocol to connect to a remote computer. SSH provides a secure channel over an unsecured network in a client-server architecture. There are numerous SSH clients available for the different operating systems. For Windows an often used SSH program is PuTTY. PuTTY is a free and open-source terminal emulator, serial console and network file transfer application which was written and is maintained primarily by Simon Tatham.


Secure Shell Terminal PuTTY for Windows

The following security alert is displayed by the PuTTY terminal at the first connection to a new server.


PuTTY security alert

The next figure shows the welcome message sent by the Raspberry Pi after a successful login. The default user name is pi and the default password is raspberry.These parameters can be changed in the Raspbian configuration.


PuTTY welcome message

The Apple Mac computers have an integrated secure shell in the standard terminal program. A connection can be established with the following command :

ssh username@server

Secure shell in a MacBook Air

I use the WebSSH app for my iOS devices and the JuiceSSH app for my Android tablets.


Remote Desktop Protocol (RDP) is a proprietary protocol developed by Microsoft, which provides a user with a graphical interface to connect to another computer over a network connection. The user employs RDP-client software for this purpose, while the other computer must run RDP-server software.

To install the remote desktop server application XRDP (version 0.6.1-2) on Raspberry Pi, I launched the command

sudo apt-get install xrdp

The next figure shows the Remote Desktop Connection Panel on Windows :


XRDP Logon settings

RDP Clients exist not only for Windows and Windows Mobile, but also for OS X (version 8.0.24) , Linux, iOS (version 8.1.17) and Android.

The problem with european keyboards explained in my former Raspberry Pi contribution persist and is even more difficult to solve on tablets. For this reason I discarded the RDP solution for my future projects.


Tight-VNC is a cross-platform free and open-source remote desktop software application that uses and extends the remote framebuffer (RFB) protocol of Virtual Network Computing (VNC) to control another computer’s screen remotely. It was created by Constantin Kaplinsky.

Most tutorials about remote desktops refer to the Tight-VNC-Server as favorite solution. One disadvantage of this application is that it uses only virtual desktops, which is not optimal for a headless operation of a computer. Therefore I discarded the Tight-VNC-Server for my projects, but i use the Tight-VNC-Client on Windows as viewer to work with an X11-VNC-Server.


X11-VNC allows remote access from a remote client to a computer hosting an X Window (X11) session and the x11vnc server software, continuously polling the X server’s frame buffer for changes. X11-VNC does not create an extra display for remote control. Instead, it uses the existing X11-display shown on the monitor in real time.

To install an X11-VNC-server on Raspberry Pi, we enter the command

sudo apt-get install x11vnc

To start the server the first time we use the command

x11vnc -usepw -forever -display :0

A password (= raspberry) is defined at this first launch, verified and saved in the /home/pi/.vnc/ directory. The window 0 refers to the visible desktop and is related to the port number 5900. Additional virtual desktops can be configured with the display numbers 1, 2, 3 … and are associated with the port numbers 5901, 5902, 5903 … To launch the VNC-server automatically when the Raspberry Pi boots, we create an auto-start file


with the following content :

[Desktop Entry]
Exec=x11vnc -usepw -forever -display :0

A VNC viewer is installed at a remote computer to access the Raspberry Pi desktop. I use the free Tight-VNC-Viewer for Windows (version 2.7.10 released on July 24, 2013) and the RealVNC viewer for iOS (version 2.4.1 released on December 2, 2015). A VNC viewer is also available for Android (version released on February 11, 2016) and other operating systems. A VNC client is integrated in OS X an can be opened in Safari with the command

vnc://IP address:port number

The VNC-server takes the display size from the resolution of the connected HDMI monitor. In case of headless operation, the display size on the VNC viewer is very small. To adapt the display size to the iPad (my preferred VNC viewer), I uncommented and modified the following lines in the configuration file /boot/config.txt.


For security reasons I configured the VNC-server to be only accessible in the local network.

WLAN Adapters

To set up a WiFi connection to access the Raspberry Pi wirelessly, we need a WiFi dongle. I use the following 150Mbps Wireless N Nano USB adapters for my Raspberry Pi projects :

The Raspberry Pi 3 model has an onboard WiFi chip. The command lsusb shows the ID of the dongles, here for the official Raspberry WiFi adapter.


lsusb result for Raspberry dongle : Bus 001 Device 004 = Broadcom Corp. ID 0a5c:bd1e

The command lsmod shows which loadable kernel modules are currently loaded.


lsmod result for the Raspberry WiFi dongle

The modules related to the Broadcom WiFi dongle are brcmfmac, brcmutil and cfg80211. Detailed informations about these modules are obtained with the command modinfo modulename :


modinfo results for the Raspberry WiFi dongle

The associated preinstalled drivers and utilities are


In the case of a RPi 2 or 3 device, the module is /4.1.18-V7+/.

The command iw list shows the features of the installed WiFi dongle.

iw list results

iw list results for the Raspberry WiFi dongle

Among the listed features, the supported interface modes are the most important. The support of AP means that the WiFi dongle can be used as WLAN access point.

The command dmesg (display message or driver message) prints the message buffer of the kernel. The result shows an issue with the brcmfmac driver for the official Raspberry WiFi dongle.


brcmfmac errors issued by dmesg command

[ 11.060813] brcmfmac: brcmf_c_preinit_dcmds: 
Firmware version = wl0: Apr 3 2014 04:43:32 
version (r467479) FWID 01-32bd010e
[ 11.105542] brcmfmac: brcmf_cfg80211_reg_notifier: not a ISO3166 code
[ 14.284071] brcmfmac: brcmf_add_if: ERROR: netdev:wlan0 already exists
[ 14.284233] brcmfmac: brcmf_add_if: ignore IF event

Nevertheless the official WiFi dongle seems to work as expected.

Running the commands lsusb, lsmod and iwlist for the other 3 WiFi dongles and the onboard WiFi chip allows me to dress the following table :

Dongle Chipset ID WPA driver AP support
D-Link DWA-131 Realtek RTL8192CU 2001:330d 8192cu.ko yes
TP-Link TL-WN725N Realtek RTL8188EUS 0bda:8179 8188eu.ko yes
Raspberry WiFi Broadcom BCM43143 0a5c:bd1e brcmfmac.ko yes
Racksoy Mini Stick Ralink RT5370 148f:5370 rt2800usb.ko yes
RPI 3 onboard chip Broadcom BCM43438 ————- brcmfmac.ko yes

lsusb result for the Wifi dongle D-LINK DWA-131 (module 8192cu)

The command modinfo 8192cu confirms that the dongle with ID 2001:330d is supported by the preinstalled driver 8192.ko located at


The WiFi dongles Raspberry, Racksoy and D-Link DWA-131 work out of the box in my Raspberry Pi. The following figure shows the Raspian Desktop with the activated Wifi icon. The two pasted message panels show the informations displayed when the mouse hovers the WiFi icon : the first case with additional wired Ethernet connection, the second case without Ethernet connection.


Raspbian WiFi information panel

The next figure shows the information displayed in my Fritzbox gateway concerning the WLAN and the Ethernet connections of the Raspberry Pi :


Raspberry Pi connections listed in the Fritzbox home-network panel

The fourth WiFi dongle (TP-Link TL-WN725N) uses the Realtek rtl8188EUS chip. Out of the box, the dongle worked, but with a very low signal indicator, although the Raspberry Pi was near my WLAN gateway. The lsmod command shows a driver r8188eu among the currently loaded kernel modules. This driver r8188eu.ko is located in the folder


The Linux Staging tree is used to hold stand-alone drivers and filesystems that are not ready to be merged into the main portion of the Linux kernel tree, at this point in time, for various technical reasons. The correct driver named 8188eu.ko is not included in the Raspbian Jessie version. TP-Link and Realtek provide source files to compile the driver for specific Linux distributions. An installation guide is included in the zip-compressed driver file. Precompiled drivers for the Raspberry Pi are provided by the community. I used such a precompiled driver for my Linux version 4.1.18+, available at the following link :

The precompiled package with the 8188eu.ko driver contains a script to automatize the installation process. I preferred to do it manually. I installed the included driver 8188eu.ko into a new created folder rtl8188 inside the wireless driver directory with the commands

sudo install -p -m 644 8188eu.ko 
sudo depmod 4.1.18+

Finally I copied the included file 8188eu.conf, blacklisting the staging driver r8188 to the directory


modprobe is a Linux program used to add a loadable kernel module to the Linux kernel or to remove a module from the kernel.

After rebooting the dongle works as expected. For practical reasons I linked copies of the files 8188eu.ko and 8188eu.conf in the present post.


To connect the Raspberry to a remote WLAN Access Point, a software must run to provide WPA key negotiation with a WPA Authenticator and EAP authentication with an Authentication Server. The most common program for these tasks is wpa_supplicant, a free IEEE 802.11i software implementation developed by Jouni Malinen. In computer networking, a supplicant is an entity at one end of a point-to-point LAN segment that seeks to be authenticated by an authenticator attached to the other end of that link.

wpa_supplicant is a daemon and only one instance of it may run on a machine, all other modifications of security settings are made with the text-based frontend application wpa_cli. wpa_cli is used to query current status, change configuration, trigger events, and request interactive user input. wpa_supplicant is pre-installed in Raspbian Jessie.

wpa_cli status

wpa_cli status

wpa_cli works two ways: interactive and non-interactive

A list of some useful wpa_cli commands is shown below :

  • wpa_cli status
  • wpa_cli terminate

The following command can be used to restart the WiFi connection :

sudo /etc/init.d/networking restart

The wpa configuration file located at /etc/wpa_supplicant/wpa_supplicant.conf has the following content :

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev


To use a WiFi dongle in AP mode on Rasperry, you need hostapd or a similar application. wpa_supplicant allows basic AP configuration, but does not include support for most of the AP parameters. Comparing the hostapd.conf with the wpa_supplicant.conf example provides a picture of the difference.

The hostapd module is not pre-installed in Raspbian Jessie. The installation is done as usually with the command

sudo apt-get install hostapd

The version check after the successful installation shows version number 2.3.

hostapd version

hostapd version check

hostapd is developed and maintained by the author of wpa_supplicant, Jouni Malinen from Finland.

I edited the related configuration file


as follows for an open network :

# WLAN Router Configuration
# driver= autoselected

To protect and encrypt the network, the following parameters are added to the hostapd.conf file :

# WLAN Encryption 

Here is a link to my hostapd.conf file. Some explanations about the different configuration parameters used are listed below :

  • # : lines starting with an hash are considered as comments
  • interface : name of the interface to listen on
  • driver : usually detected by the system; in case of failure the name of the driver corresponding to the used dongle must be specified (nl80211, rtl871xdrv, …)
  • ssid : name of the network seen by other devices
  • ignore_broadcast_ssid : define if the network name is visible (broadcasted) or hidden
  • channel : WiFi channel used by the network; valid channels range from 1 to 11 or from 1 to 14, depending on the location
  • hw_mode : wireless mode : A, B and G are available; these are the last characters of the IEEE 802.11 standard. The most common is G (2,4 GHz).
  • wmm_enabled : WiFi multimedia to enhance quality of service (QoS)
  • auth_algs : specification of the authentication algorithm; 1 = OSA
  • country_code : used to set regulatory domain (transmit power, available channels)
  • ieee80211d : set the regulatory limits based on the country code
  • ieee80211n : optional activation of the IEEE 802.11n standard
  • wpa : security settings: 1=wpa; 2=wpa2; 3=wpa & wpa2
  • rsn_preauth :pre-authentication to speed up roaming
  • rsn_preauth_interfaces : list of interfaces from which pre-authentication frames are accepted
  • wpa_key_mgmt : management of the wpa2 WLAN keys; WPA-PSK or WPA-EA
  • rsn_pairwise : encryption algorithm for wpa2; new CCMP or old TKIP
  • wpa_pairwise : encryption algorithm for wpa
  • wpa_passphrase : password specified for the network
  • logger-syslog : instructions about error logging
  • max_num_sta : maximal number of clients accepted by the WLAN
  • deny_mac_file : prevent connections from devices with a MAC address specified in a file

The complete list of hostapd parameters is available at the website of the developer.

The Raspberry WiFi dongle works out of the box with the default hostapd driver nl80211. The Racksoy WiFi dongle works with the autoselected driver rt2800.

Hostapd supporting driver rtl871xdrv

The two other WiFi dongles D-Link DWA-131 and TP-Link TL-WN725N are not supported by the standard hostapd module with the included drivers. Both dongles use Realtek chipsets needing a specific driver, named rtl871xdrv. There are numerous posts and tutorials available at the web dealing with this problem, but only few provide correct and relevant solutions related to the current Raspbian Jessie version.

After some Trial and Error, I ended up with the following procedure :

  1. download the version 2.5 of the hostapd source from the website of Jouni Malinen
  2. extract the archive at /home/pi/hostapd-2.5/
  3. clone the github repository at /home/pi/hostapd-rtl871xdrv/ with the command
    sudo git clone
  4. install the modules libnl1, libnl-dev and libssl-dev with the command
    sudo apt-get install libnl1 libnl-dev libssl-dev
  5. change directory to /home/pi/hostapd-2.5/
  6. apply the patch code with the command
    sudo patch -Np1 -i /home/pi/hostapd-rtl871xdrv/rtlxdrv.patch

    Patching the hostapd source

    Patching the hostapd source files

  7. enable the driver in the file .config with the commands
    cd hostapd
    cp defconfig .config
    echo CONFIG_DRIVER_RTW=y >> .config
  8. compile the hostapd module, inside the hostapd folder, with the following command
    sudo make install
  9. check the new installed hostapd version
hostapd version 2.5

hostapd version 2.5 for Realtek rtl871xdrv

The last step is to change the driver name as rtl871xdrv in the hostapd.conf file and to reboot the Raspberry Pi with a Realtek WiFi dongle. After terminating wpa_cli and starting hostapd, we get the confirmation that the Realtek dongles are now working.


hostapd launch with driver rtl871xdrv

Running Hostapd

Depending on the used WiFi dongle, we use the native hostapd version 2.3 or the new compiled version 2.5. Before associating hostapd with the wlan0 interface, we have to disassociate this interface with wpa_supplicant by entering the command

sudo wpa_cli terminate

To start the WLAN-Host, we run the command

sudo hostapd /etc/hostapd/hostapd.conf

Adding the optional parameter -dd (after hostapd) in the above command provides a detailed report about the process, which can be very helpful in case of problems. After a few moments the WiFi-panel in the Raspian desktop shows that the wlan0 interface is ssociated with the SSID “radiologic”. The interface is configured as AdHoc network with an self-assigned IP address in the range

WiFi panel when hostapd is started

WiFi panel when hostapd is started

I used an iPad as WLAN client to test the WLAN-Host. The SSID “radiologic” is displayed in the list of available WiFi networks as secured WLAN. After entering the defined password “123456789”, the connection is established. The next figure shows the related messages displayed in the Raspbian terminal window.

WLAN-Host messages for two connections and disconnections with an iPad

WLAN-Host messages for two connections and disconnections with an iPad

hostapd can be terminated by pressing simultaneously the keys  “Ctrl” and “c” .

To start hostapd at boot, we need to do some additional tasks. First we must uncomment and complete the parameter DAEMON_CONF in the file /etc/default/hostapd as follows :


We need to insert the same parameter in the init file /etc/init.d/hostapd.

Next we enable starting of hostapd at booting with the command

sudo systemctl enable hostapd
Enable autostart of hostapd at booting

Enable autostart of hostapd at booting

The new systemctl command synchronizes the booting process with the old legacy sysvinit command using update-rc.d. After a reboot we can check the status of the hostapd service with the command

service hostapd status


systemctl status hostapd
Show status of the hostapd service

Show status of the hostapd service

The following commands are used to stop, start, restart or disable hostapd :

sudo systemctl stop hostapd
sudo systemctl start hostapd
sudo systemctl restart hostapd
sudo systemctl disable hostapd

Hostapd needs random data to encrypt the WLAN. In computing, the randomness of data is called entropy. When the entropy is too low, “Add randomness” messages are displayed.


Raspberry messages related to randomness (entropy)

The entropy level can be checked with the command

cat /proc/sys/kernel/random/entropy_avail

When the returned value is less than 1.000, the entropy is unsufficient for secure cryptographic processes, which is often the case in small systems like the Raspberry Pi. Fortunately there is a solution :  HArdware Volatile Entropy Gathering and Expansion (HAVEGE).

The HAVEGE project is an attempt to provide an easy-to-use, unpredictable random number generator based upon an adaptation of the HAVEGE algorithm. The havege module is installed with the command

sudo apt-get install haveged

Another critical point related to hostapd is the power-save feature of most WiFi dongles. While this is a strong advantage when the WiFi dongle is in infrastructure mode (outbound) where the device is reactivated by the user, this is a great risk in AP mode when the computer works headless. In power-save mode, the WiFi dongle is no longer accessible by an external client and the only solution is to reboot the computer by unplugging and re-plugging the power supply.

We can check the status of power management with iwconfig.


Check the power management status of the WiFi dongle with iwconfig

To prevent the Wifi dongle from entering the power-save mode, we can add the command

wireless-power off

in the /etc/network/interfaces file.

Another possibility is to create configurations files in the directory /etc/modprobe.d/.

Examples for Realtek drivers :


options 8188eu rtw_power_mgnt=0


options 8192cu rtw_power_mgnt=0

Networking setup

Until now, the WiFi adapter has an auto-assigned IP address and the clients don’t receive an IP address when connecting to the WLAN, which is not a workable solution. We need a static IP address for wlan0 and a DHCP-server to attribute IP addresses to the WLAN clients.

Many tutorials about Raspberry Pi deal with static IP addresses to simplify the access to the RPI when the routing is managed by a DHCP-server. Most of them refer to the legacy ifupdown configuration file /etc/network/interfaces, which is obsolete with Raspbian Jessie using the DHCP-client dhcpcd developed by Roy Marples. The network configuration should be done in the file /etc/dhcpcd.conf. If a static IP address is required for the eth0 interface, a more simple way is to set it in the router of the local network. In my specific case this is a Fritzbox 7390. The best way however is to use the hostname of the Raspberry Pi to access it in the local network. The default hostname is raspberrypi. The hostname can be easily changed in the raspi configuration. The hostname is saved in the files /etc/hosts and /etc/hostnames.

The default configuration of /etc/network/interfaces to be used with dhcpcd is shown below:

# interfaces(5) file used by ifup(8) and ifdown(8)
# Please note that this file is written to be used with dhcpcd
# For static IP, consult /etc/dhcpcd.conf and 'man dhcpcd.conf'
# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d
auto lo
iface lo inet loopback
iface eth0 inet manual
allow-hotplug wlan0
iface wlan0 inet manual
wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

The content of the default configuration of the /etc/dhcpcd file is the following :

# A sample configuration for dhcpcd.
# See dhcpcd.conf(5) for details.
# Allow users of this group to interact with dhcpcd via the control socket.
#controlgroup wheel
# Inform the DHCP server of our hostname for DDNS.
# Use the hardware address of the interface for the Client ID.
# or
# Use the same DUID + IAID as set in DHCPv6 for DHCPv4 ClientID as per RFC4361.
# Persist interface configuration when dhcpcd exits.
# Rapid commit support.
# Safe to enable by default because it requires the equivalent option set
# on the server to actually work.
option rapid_commit
# A list of options to request from the DHCP server.
option domain_name_servers, domain_name, domain_search, host_name
option classless_static_routes
# Most distributions have NTP support.
option ntp_servers
# Respect the network MTU.
# Some interface drivers reset when changing the MTU so disabled by default.
#option interface_mtu
# A ServerID is required by RFC2131.
require dhcp_server_identifier
# Generate Stable Private IPv6 Addresses instead of hardware based ones
slaac private
# A hook script is provided to lookup the hostname if not set by the DHCP
# server, but it should not be run by default.
nohook lookup-hostname

The lo interface (loopback) serves communication with localhost and is started automatically at boot. The IP address for the eth0 interface is requested by dhcpcd and provided by the LAN-router. The configuration for the wlan0 interface is provided by the /etc/wpa_supplicant/wpa_supplicant.conf file.

The status of the dhcpcd client is requested with the command

sudo systemctl status dhcpcd
dhcpcd status

Status of the dhcpcd client

For the headless operation of the Raspberry Pi with a WLAN Access Point I use a minimal network configuration :


# Legacy interface (ifupdown) file used with dhcpcd
auto lo
iface lo inet loopback
iface eth0 inet manual


# DHCPCD configuration
option rapid_commit
option domain_name_servers, domain_name, domain_search, host_name
option classless_static_routes
option ntp_servers
require dhcp_server_identifier
slaac private
nohook lookup-hostname
interface wlan0
static routers=

A static IP address ( can be attributed to wlan0 by means of the WiFi Network Panel opened by right clicking the WiFi icon in the task bar.


WiFi Popup Menu to manage panels

The data inserted in the WiFi Networks Settings are saved in the /etc/dhcpcd.conf file as shown above.


Network preferences wlan0 interface options


One piece of the puzzle is still missing : a Dynamic Host Configuration Protocol (DHCP) Server to provide IP addresses to the WLAN clients. There are several dhcp-server packages available for Raspbian. I selected dnsmasq, a very useful module to set up a complete WLAN router. dnsmasq is a Domain Name System (DNS) forwarder and a DHCP-server for small computer networks, which are easily configurable, with low usage of system resources.

dnsmasq is installed with the command

sudo apt-get install dnsmasq

The configuration is done in the file /etc/dnsmasq.conf. Here is a link to the content of the default dnsmasq.conf file. My minimal configuration is the following :

# DNSMASQ Configuration

The DHCP-server is restricted to the wlan0 interface, the eth0 interface is excluded. The range of IP addresses to attribute to the WLAN clients is bounded by 2 and 99 in the sub-network The lease time is 12 hours.

dnsmasq is managed with the same type of commands as hostapd :

sudo systemctl start dnsmasq
sudo systemctl stop dnsmasq
sudo systemctl restart dnsmasq
sudo systemctl enable dnsmasq 
sudo systemctl disable dnsmasq 
sudo systemctl status dnsmasq

Status of the dnsmasq server

To test the correct access with WLAN clients to the Raspberry Pi, we will use the installed web-server lighttpd. The configuration files for the web-server are located in the directory /etc/lighttpd/. The web pages are located in the folder /var/www/html/. By entering the URL

in a browser after logging into the radiologic WLAN, we can display the index web page. A screenshot of the default html index page is shown hereafter :

Default web

Default web page of the lighttpd web server

I replaced this page with the following minimal html script :

<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Welcome Page</title>
<style type="text/css">
h1, h3 {text-align:center}
<h3> to the Rasperry Pi WLAN</h3>

The following photo shows the result on various mobile devices accessing the radiologic WLAN.

iPad, iPhone, Samsung, LG and Blackberry mobile devices conncetd to the Blackberry Pi WLAN

iPad, iPhone, Samsung, LG and Blackberry mobile devices connected to the RPi WLAN

Currently the clients can only access the embedded web server. If we want that the WLAN clients have access to the global Internet or to other services, we need to add more network configuration parameters or specific applications. This will be a topic of further contributions.

Service Management

As a last step we check that all programs are running without errors and that the performance of the system is correct. First we need to understand the boot process.

Debian separates the legacy boot process (based on System V init)  into seven runlevels labeled as 0, 1, 2, 3, 4, 5, 6 and S. For each runlevel a file rcx.d (x = 0, 1, 2, …) exist in the directory /etc/ containing symbolic links to targets located in /etc/init.d/. The runlevels are defined as follows :

  • 0 : (halt) stop all services and make the system ready for shutdown
  • 1 : (minimal mode) stop all services and put the system in single-user rescue mode
  • 2 – 4 : multiuser network enabled text mode
  • 5 : multiuser network enabled graphical mode
  • 6 : (reboot) stop all services, make the system ready for reboot
  • S : (single) these scripts are executed once

When we enable a service to auto-start, we are adding it to a runlevel.

Default Debian installation does not make any difference between runlevels 2-5. Number 2 is the default runlevel at start. In a normal boot the levels progress up to 5. Runlevels S  and 1 are used for maintenance. Runlevels can be edited manually by editing control scripts in /etc/init.d and symbolic links in /etc/rc0.d … /etc/rc6.d. The runlevel can also be changed at runtime with the command telinit [runlevel 2-5]. The current runlevel is indicated with the command runlevel.

Looking at the list of the services included in the /etc/init.d/ folder, we discover among others the installed programs dnsmasq, dhcpcd, haveged, hostapd and lighttpd.


Content of the folder /etc/init.d/

Here is an extract from the init.d README file :

# Provides: skeleton
# Required-Start: $remote_fs $syslog
# Required-Stop: $remote_fs $syslog
# Should-Start: $portmap
# Should-Stop: $portmap
# X-Start-Before: nis
# X-Stop-After: nis
# Default-Start: 2 3 4 5
# Default-Stop: 0 1 6
# X-Interactive: true
# Short-Description: Example initscript
# Description: This file should be used to construct scripts to be
# placed in /etc/init.d.

For each program defined as script in the /etc/init.d/ folder we can specify the runlevels for start and stop and the dependencies with other programs. The next figure shows a real example of an init-script related to the dnsmasq service :

init.d dnsmasq

init script related to dnsmasq in the /etc/init.d/ folder

We see that the virtual boot facilities $network, $remote_fs and $syslog must be started or stopped before the dnsmasq service. Creating init scripts manually is an error-prone task. For this reason the legacy System V (SysV) init program has now been replaced by new init systems. Raspbian Jessie is using systemd as service manager. We have already seen the related tool systemctl to start, stop, enable and disable programs.

In systemd, the targets of most actions are units, which are resources that systemd knows how to manage. Units are categorized by the type of resource they represent and they are defined with unit files. The systemd files are located in the directories /lib/systemd/system/ and /etc/systemd/system/.

Some useful systemd commands are listed below :

systemctl is-active application.service
systemctl is-enabled application.service
systemctl is-failed application.service
systemctl list-units
systemctl list-units --all
systemctl cat application.service
systemctl list-dependencies application.service
systemctl show application.service
sudo systemctl mask application.service
sudo systemctl unmask application.service
sudo systemctl edit application.service
systemctl get-default

For most commands the .service suffix can be omitted. Some real commands concerning hostapd are shown in the next figure :

results of some systemd systemctl commands

Results of some systemctl commands for hostapd

systemd has some great built-in tools to check and tune the boot time. The commands

systemd-analyze blame

will print the boot time respectively print the list of the most time-consuming services in decreasing order.

systemd-analyze results

systemd-analyze results

The command

systemd-analyze plot > init_plot.svg

creates an xml svg file which can be rendered in a browser. It displays in a graphical way when all the services start.

init_plot.svg file rendered in browser; click to enlarge

init_plot.svg file rendered in browser; click to enlarge

systemd has also a powerful tool to view and manipulate system logs named journald. The related configuration file is located in /etc/systemd/journald.conf. Some useful commands are :

journalctl --all 
journalctl --since=yesterday

journald provides various filters using time constraints. Logs can be viewed in UTC or local time at will. The following journald commands deal with time and date.

timedatectl list-timezones
sudo timedatectl set-timezone zone
timedatectl status

systemd tools : timedatectl and journalctl

journald provides other filter options than time constraints : units, users, groups, processes, pathes, priorities, … The journal display can be customized in various formats. The amount of space occupied by the journal on the SD card can be shown with the the command

journalctl --disk-usage

Backup and Cloning

When a bad shutdown has corrupted the file system, the Raspberry Pi can be a frustrating experience. It’s therefore useful to wait for the complete shut down of the RPi before unplugging the power supply. To shutdown or reboot the Raspberry Pi, we can use the menu “Shutdown” in the VNC-desktop


End Raspberry session with shutdown, reboot or logout

or the commands

sudo shutdown
sudo reboot

It’s also important to make at regular times a backup of the Raspberry Pi SDcard. Backups are possible on Windows 7 systems (or later) with the Win32DiskImager tool. First an image of the source card is saved to a file on the Windows PC. In an second step this image is copied to a backup SDcard. Backup’s can also be done with the Raspberry Pi itself by using an SDcard reader with USB interface.

Cloning a SDcard is the same procedure as doing a backup, but the goal is different. Cloning allows to distribute a complete mirror of a working RPi system with operating system, applications and data. This way it’s easy to copy and multiply an existing solution.

Sometimes a cloned image file can’t be copied to an SDcard with the same capacity, because SDcards can vary a little bit in size. There are several tutorials on the net how to solve this problem by shrinking the image file, for example one by Andrew Aokley, another one on the Wonky Gibbon Ramblings Blog.

I used the following procedure on my Debian computer :

df -h
cd /home/mbarnig/Desktop/
sudo dd if=/dev/sdb2 of=myimage.img bs=4M
sudo dd if=myimage.img of=/dev/sdb2 bs=4M conv=notrunc,noerror


The following list provides links to websites with additional informations about the Raspberry Pi :

Digital Imaging

Last update: February 12, 2017

Digital imaging is the creation of digital images, such as of a physical scene, and includes the processing, compression, storage, printing, and display of such images. The information is converted by image sensors into digital signals that are processed by a computer. If the medium which conveys the information that constitutes the image is visible light, it’s called digital photography.

Photosensor Array

A digital camera or a scanner uses an array of photosensors (photosites) to record and store photons. Once the exposure finishes, the relative quantity of photons in each photosite is then sorted into various intensity levels, whose precision is determined by bit depth (0 – 255 for an 8-bit image).

One photosensor per image pixel would only create grayscale pictures. To capture color images, distinct photosensors are necessary for each of the three primary colors (RGB). To separate the colors, a filter array is placed over each photosensor. The most common type of color filter array is called a Bayer Array, as shown in the figure below.


image credit :

A Bayer Array consists of alternating rows of red-green and green-blue filters. Because the human eye is more sensitive to green light than both red and blue light, the Bayer Array contains twice as many green as red or blue sensors to approximate the human vision.

Dynamic Range

When speaking about dynamic range, we need to distinguish between recordable or displayable dynamic range. Let’s start with the first one.

The Dynamic Range in digital imaging describes the ratio between the maximum (white) and minimum (black) measurable light intensities. The black level (few photons) is limited by noise. The white level (large number of photons) is limited by overflow (saturation). If an ideal photosensor identifies one photon for black and hosts a maximum of 1.000 photons (white), the dynamic range would be 1.000:1. The most commonly used unit for measuring dynamic ranges in digital photography is the f-number (f-stop) which describes the total light range by powers of 2. A dynamic range of 1.000:1 is therefore equivalent to 10 f-stops (2 exp 10 = 1.024). In scanners the dynamic range is described in terms of density (D). Density is measured using powers of 10. A dynamic range of 1.000:1 is represented by a density of 3 (10 exp 3 = 1.000). As a scanner has full control over it’s light source, such a device can ensure that minimal photosensor overflow occurs.

The approximated dynamic range in f-stops for several devices is indicated below :

recordable displayable
  • human eye : 24
  • scanner : 8 – 12
  • digital camera : 5 – 9
  • monitor display : 6 – 11
  • printed media : 2 – 7

ISO sensitivity

How much light is needed to saturate a medium is determined by it’s sensitivity. That was as true for glass plates as it was for film and now for digital photosensors. The sensitivity (film speed) is expressed in ISO. The normal range of ISO is about 200 to 1.600, but can sometimes go as low as 50 or as high as 204.800.

Image Bit Depth

Bit depth quantifies how many values are available to specify each image pixel. Even if a digital imaging system can capture a vast dynamic range, the light measurements must be translated into discrete numerical values with an analog to digital (A/D) converter. With 8 bits per color channel, the dynamic range cannot exceed 8 f-stops (density of 2.4) if the numerical values are linearly interspaced. With 16 bits per color channel, the theoretical value for the dynamic range in an ideal linear system would be 16 f-stops (densitiy 4.8). In practice the dynamic range of a linear system is much lower, even with 16 bits (typically about 12 f-stops). If we use however an nonlinear system to interspace and save the discrete numerical values, we could even conceive to record an infinite dynamic (posterized) range with an image depth of a few bits.

RAW image

At the end of the A/D conversion, we have a raw digital image, with W x H pixels, specified with consecutive discrete numerical values, each value coded with N bits per color channel. Each camera manufacturer and each scanner software developer uses a proprietary format for a raw digital image. A common format called Digital Negative (DNG) has been defined by Adobe in 2004.

Image Histograms

Image histograms are great tools to evaluate the correct exposure of a captured digital image.

Each pixel in the raw image is specified by the primary colors red, green and blue (RGB). Each of these colors can have a brightness value ranging from 0 to X ( X = 2 exp N ). A histogram results when the computer scans through each of the brightness values and counts how many are at each level from 0 through X. Low brightness values are called shadows, high values are the highlights, in-between are the midtones.



A well balanced histogram where no shadows or highlights are clipped is shown at left. The region where most of the brightness values are present is called the tonal range. When highlights are heaped at the right edge in the histogram, they are clipped (blown). Some regions in the image have been overexposed and the corresponding details can never been recovered.

When shadows are heaped at the left edge in the histogram, some regions of the image have been underexposed and the relevant dark details are also lost. The histogram of a raw digital image should not present high values at the left (shadows) and right (highlights) edges of the chart. If clipping occurs you see a tall vertical line at the far left or right side of the histogram.

Usually an image is underexposed if no channel of the histogram goes all the way to the right. Images that are too dark are easy to correct later in this case; just drag the right slider in Photoshop’s Levels command to the left to meet the edge of the histogram.

The distribution of peaks in a histogram depends on the tonal range of the subject. Images where most of the tones occur in the shadows are called low key, whereas in high key images most of the tones are in the highlights. The histogram describes also the contrast which is the measure of the difference in brightness between dark and light areas in an image. Broad histograms reflect a significant contrast, narrow histograms present low contrast resulting in flat (dull) images.

All histograms are normalized and are intentionally scaled so that the top of the tallest peak always reach full height. Scale is relative, shown percentage-wise.

There exist three types of image histograms :

  • Color Histograms
  • RGB Histograms
  • Luminosity (Luminance) Histograms

Each histogram type has it’s own use and it’s own shortcomings. All three should be used as a collective tool. The following figures show the different histograms relative to a scanned Kodachrome slide of a landscape.


Scanned Kodachrome slide of a landscape

Photoshop histogram windows

Landscape photo histogram windows RGB, R, G, B, Luminosity and Colors in Photoshop

Color Histograms

A color histogram describes the brightness distribution for any of the three primary color channels R, G, B. This is helpful to assess whether or not individual colors have been clipped.

Sometimes color histograms are presented as color overlays (colors histogram).

RGB Histograms

An RGB histogram produces three independent histograms for each color channel and then adds them together, irrespective of whether the color came from the same pixel. This way RGB histograms discard the location of each pixel.

Luminosity (Luminance) Histograms

The terms luminosity and luminance are often used interchangeably, even though each describes a different aspect of light intensity. Technically the term luminosity is correct and I will use it in the following, even if luminance is more common. The luminosity histogram takes into account that our eyes are most sensitive to green; we see green as being brighter than we see blue or red. Luminosity weighs the effect of this to indicate the actual perceived brightness of the image pixels based on the NTSC television formula

Luminosity = Red x 0.3 + Green x 0.59 + Blue x 0.11

Color Models

Until now we have used the terms primary colors (RGB), color channels, color and colors histograms, luminosity, luminance, light intensity, brightness, but we never really dealt with colors.

The search for a comprehension of exactly what color is and how it functions has been going on for hundreds of years. Artists and philosophers have theorized that color is three-dimensional. Contemporary neuroscientists have confirmed this theory, finding that our sensation of color comes from nerve cells that send messages to the brain about:

  • The brightness of color
  • Greenness versus redness
  • Blueness versus yellowness

Numerous models and systems have been developed :

There are several ways to associate the converted discrete numerical values of the primary color channels R, G, B to colors. We can rely on the physics of light waves (visible spectrum), on the characteristics of inks, dyes, paints or pigments, on the human eye or visual perception. In all cases we need a color model as reference to process (adjust)  the discrete numerical values.

Wikipedia defines colors and color models as follows :
Color (American English) or colour (Commonwealth English) is the visual perceptual property corresponding in humans to the categories called red, blue, yellow, etc.”
A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components.”

The ICC defines colors as :
Color is the sensation produced in response to selective absorption of wavelengths from visible light. It possesses the attributes of Brightness, Colorfulness and Hue. White, grey and black are achromatic colors.

My personal definition of color in digital imaging is the following :
Color is the tone displayed when the numerical values of the three color channels are not all the same. This implies that black, white and all grey tones are not colors.”

This personal definition is consistent with all what has been said up to now in this post. With an image bit depth of 8 bits, 256 x 256 x 256 = 16.777.216 different colors and grey tones can be specified (in theory).

The color models used today are the following :

  • RGB (ca 1860) : Additive Color Model ( Red + Green + Blue = white)
  • CMYK (ca 1906) : Subtractive Color Model (Cyan + Magenta + Yellow = brown; + K = Black)
  • LAB (1948) : developed by Richard S. Hunter
  • NCS (1964) : Natural Color System
  • HSV (1978) : Hue, Saturation and Value (Alvy Ray Smith)
  • HSL (1978) : Hue, Saturation, and Lightness (Alvy Ray Smith)
  • HWB (1996) : Hue, Whiteness, Blackness (Alvy Ray Smith)
Main color models : RGB, CMYK, Lab

Main color models : RGB, CMYK, Lab

The most common color model is RGB. The following figure shows the RGB cube with the 3D representation of all possible (in theory) colors and grey-tones, including black (R = G = B = 0) in the back lower corner and white (R = G = B = max) in the front upper corner.

RGB cubes

RGB cube

RGBA is a RGB color model with and additional alpha (opacity) channel. There is an open ended set of RGB spaces; anyone can invent one by picking new primaries and a gamma value. Some color spaces are commercial ones and copyrighted, some are defined for special purposes and some are obsolete.

Typically used in color printing, CMYK assumes that the background is white, and thus subtracts the assumed brightness of the white background from four colors: cyan, magenta, yellow, and black. Black is used because the combination of the three primary colors (CMY) doesn’t produce a fully saturated black. You should however be aware that some desktop printers have only an RGB interface. Some printers use special premixed inks called Spot Colors.

The Natural Color System (NCS) is a proprietary perceptual color model based on the color opponency hypothesis of color vision, first proposed by Ewald Hering. The current version of the NCS was developed by the Swedish Colour Centre Foundation.

HSV and HSL have been developed in 1978 by Alvy Ray Smith, a pioneer in computer graphics and cofounder of the animation studio Pixar. They are used today in color pickers. The two representations rearrange the geometry of RGB in an attempt to be more intuitive and perceptually relevant than the cartesian (cube) representation. The colors are represented in a cylindrical coordinate system. Twenty years later Alvy Ray Smith created HWB to address some of the issues with HSV and HSL. HWB came to prominence in 2014 following its use in the CSS Level 4 Color Module.

In contrast to color models which define a coordinate space to describe colors, the Color Apperance Model (CAM) is a mathematical model that seeks to describe the perceptual aspects of human color vision.

Color Spaces

A color space is a specific implementation of a color model. The physical colors represented in the coordinate space (cube, cylinder) of a color model are not all visible by humans. For this reason the International Commission on Illumination (CIE) defined in 1931 quantitative links between physical pure colors (wavelengths) in the electromagnetic visible spectrum and physiological perceived colors in human color vision. These links are represented as 3D regions (3D solids) containing all producible colors, called the CIE 1931 color space. The CIE 1931 color space standard defines both the CIE 1931 RGB space, which is an RGB color space with monochromatic primaries, and the CIE 1931 XYZ color space, which works like an RGB color space except that it has non-physical primaries that cannot be said to be red, green, and blue. The CIE standards are based on a function called the standard (colorimetric) observer, to represent an average human’s chromatic response.

3D Color Space

Color Space : different vues of the 3D solid representing visible colors

Visualizing color spaces in 3D is not very easy and intuitive. For this reason color spaces are usually represented using 2D slices from their full 3D shape. Unless specified otherwise, a 2D chromacity diagram shows the cross-section containing all colors which are at 50% luminosity (luminance). The next figure shows the CIE 1931 XYZ color space in two dimensions.

CIE 1931 XYZ color space

CIE 1931 XYZ color space at 50% luminosity (mid-tones)

The CIE defined additional standards for color spaces for special purposes like TV, video, computer graphics. A list is shown below :

CIE color spaces : XYZ, Lab, Luv



It’s good to know that the CIE XYZ color space encompasses all color sensations that an average person can experience, but it’s more important to know the subsets of colors that a given digital device can handle and reproduce. Such a portion of the CIE XYZ color space is called a device color space or gamut. The term gamut was adopted from the field of music, where it means the set of pitches of which musical melodies are composed. The following figure shows typical gamuts for some digital devices.

Typical gamuts of digital devices

Typical gamuts of digital devices with 50% luminosity

Keep in mind that this representation only shows mid-tones with 50% luminosity. When colors are dark or light, we perceive less variation in their vividness. We see the maximum range of color saturation for middle-toned colors. This is why the 2D slices of color models are usually represented with 50% luminosity. If we are interested in the color gamuts for the shadows or highlights, we could look instead at a 2D slice of the color space at about 25% and 75% luminosity.

The following figure shows the gamuts of real devices, the iPad 2 and iPad 3.

Gamuts of iPad 2 and iPad 3

Gamuts of iPad 2 and iPad 3

Color Transformation

Color transformation (color space conversion) is the translation of the representation of a color from a source color space to a target (destination) color space.

Out of

Out of

A typical use case is to print on an inkjet printer in the CMYK space a photo captured with a camera in the RGB color space. The printer gamut is different than the camera gamut, certain camera colors cannot be reproduced with the printer. Those colors are said to be out of gamut.

During the color transformation process, the RGB colors out of gamut must be converted to values within the CMYK gamut. This conversion is called gamut mapping. There are several reasonable strategies for performing gamut mapping, these are called rendering intents. Four particular strategies were defined by the International Color Consortium (ICC), with the following names:

  • Absolute Colormetric
  • Relative Colormetric
  • Perceptual
  • Saturation

If a complete gamut mapping is not possible, a gamut mismatch results and the best approximation is aimed. An interactive Flash demo explaining color gamut mapping is available at the website of the Stanford University.

In digital image edition programs (for example Adobe Photoshop), device independent color spaces, called working spaces, are used as a reference for the device-dependent gamuts. Working color spaces are color models that are well suited to image editing tasks such as tonal or color adjustments. The most important working spaces are listed below :

sRGB is sort of common denominator and used as default for unmanaged computers. This color space is appropriate for uploading images to the web and to send them for printing to minilabs if no custom space is specified. It has been endorsed by the W3C and by many industry leaders. sRGB is not well suited as working space because it has a narrow gamut.

Usually the input and output color spaces are smaller than the working color space.

Color temperatures

Color temperature is another characteristic of visible light that is important in digital imaging and photography. Color temperature is conventionally stated in the unit of absolute temperature, the Kelvin, having the unit symbol K. Color temperatures over 5.000K are called cool colors, while lower color temperatures (2.700–3.000 K) are called warm colors.

The color temperature of sunlight above the atmosphere is about 5.900 K. Tungsten incandescent lamps used formerly in the photography had a temperature of 3.200 K. The CIE introduced in 1931 the concept of the Standard Illuminant, a theoretical source of visible light. Standard illuminants provide a basis for comparing images or colors recorded under different lighting. Each of these is defined by a letter or by a letter-number combination.

Fluorescent lighting adds a bluish cast to photos whereas tungsten lights add a yellowish tinge to photos. Humans don’t generally notice this difference in temperature because our eyes adjust automatically for it. The process in digital system to compensate these color casts is called white balance. The goal is to correct the lighting so that white objects appear white in images. White balance can be done automatically or manually. Two standard white points are used in white balance : D50 and D65.

In digital imaging, it is important to know a monitor’s color temperature. Common monitor color temperatures, along with matching standard illuminants, are as follows:

  • 5.000 K (D50)
  • 5.500 K (D55)
  • 6.500 K (D65)
  • 7.500 K (D75)

The spectrum of a standard illuminant, like any other profile of light, can be converted into tristimulus values. The set of three tristimulus coordinates of an illuminant is called a white point and can equivalently be expressed as a pair of chromaticity coordinates.

Color Profiles

Informations about device gamuts and illuminants are registered in ICC profiles. ICC is the International Color Consortium which was formed in 1993 by eight vendors in order to create an open, vendor-neutral color management system which would function transparently across all operating systems and software packages. Every device that captures or displays color can be profiled. A profile can be considered as a description of a specific color space.

Profiles describe the color attributes of a particular device or viewing requirement by defining a mapping between the device source or target color space and a profile connection space (PCS, either CIEXYZ or CIELAB) serving as reference.There are two types of profiles :

  • matrix-based : mathematical formulas
  • table-based : large tables of sample points (LUT = look up table) to define the 3D color space

Mappings may be specified this way using tables, to which interpolation is applied, or through a series of parameters for transformations.

ICC profiles help you to get the correct color reproduction when you input images from a camera or scanner and display them on a monitor or print them.


Color conversion with ICC profiles

An ICC profile must conform to the ICC specification. The latest profile version is, the corresponding specification ICC.1:2010-12 is technically identical to the ISO standard 15076-1:2010.

There are different device classes of profiles : input, output, display, link, abstract, colorspace, … ICC profiles may have the suffix .icc or .icm. Display profiles are commonly of the Matrix/TRC type with a 3×3 matrix of the colorant primaries tristimulus values and a one-dimensional tone curve for each colorant. They can also be of the multi-dimensional look-up table (LUT) type with a three-dimensional look-up table and a second one-dimensional tone curve. Some device-independant profiles are purely theoretical and describe a way to turn color into numbers. Others are device-dependant and describe the color signature of a particular device.

A profile does not correct anything in the image. An original with a color cast (Farbstich) keeps the cast during the color conversion. Image correction and cast removals are separate processes which need specific software.

ICC profiles can be embedded in digital images, for example in JPEG files. If the profile uses a standard color space like sRGB, a simple EXIF tag is sufficient to indicate it. If a custom (non-standard) color space is used, the complete data segment can be embedded. Photoshop features check-boxes to embed ICC profiles in dialog boxes when saving or creating images.

A free program for Windows to view the content of ICC profiles has been developed by Huan (Huanzhao) Zeng. The software is called ICC Profile Inspector; the current version 2.4.0 was updated on February 22, 2009. The following figure shows a screenshot of the program displaying the header and the tag table of the Blurb ICC Profile for Blurb books.

ICC Inspector

ICC Profile Inspector showing the Blurb book ICC profile

The device class of the Blurb ICC profile is output, the color space is CMYK, the profile connection space is Lab, the rendering intent is relative colormetric and the illuminant has the values X = 0.9642, Y = 1.0, Z = 0.82491. AToBn (A2Bx) and BToAn (B2Ax) are gamut mapping tables used in printer profiles. A refers to the device, B to the profile connection space. A2B tags are used for proofing, B2A tags are used for printing.

By clicking on a tag in the ICC Profil Inspector, the corresponding content is displayed.

The next figure shows a screenshot of the program displaying the header and the tag table of my Sony Vaio laptop :


ICC Profile Inspector showing the ICC profile of a labtop display

The device class of the Sony ICC profile is display, the color space is RGB, the profile connection space is XYZ, the rendering intent is perceptual and the illuminant has the values X = 0.96419, Y = 1.0, Z = 0.82489. The tags rXYZ, gXYZ and bXYZ present the gamut for the three channels, the tag wtpt shows the white point, the tags rTRC, gTRC and bTRC indicate the Tone Response Curves for the three channels in 16bit mode (see gamma encoding later in this post).


ICC color profile  for Sony Vaio labtop display : gamut, white point, gamma

Windows Color Management panel allows to change settings for the ICC profiles. Mac OS X has an inbuilt ICC profile inspector inside the umbrella application ColorSync Utility.

An OpenICC project was launched in 2004. Files are available at Sourceforge.

RAW to JPEG / TIFF conversion

To view our image, we must display it on a monitor or print it on paper. In both cases we need to process (edit) the image to cope with limitations of the output medium and with another particularity of the human vision. Compared to a photosensor, our eyes are much more sensitive to changes in dark tones than we are to similar changes in bright tones.

A standard computer monitor can only display 8 bits per color channel. The common image file formats used in this environment are compressed JPEG or uncompressed TIFF files. To convert our raw image into on of these standards, we need to apply several image adjustments; some are irreversible. Often these adjustments are done automatically inside the digital imaging system (camera, scanner), but it’s also possible to do it manually outside with an image editing software like Photoshop.

The steps to adjust the digital image are the following :

  • Demosaicing
  • Gamma encoding
  • White Balance
  • Tonal compensation
  • Color Compensation
  • Sharpening
  • Compression


Let’s come back to our photosensors without considering their dynamic range, sensitivity or bit depth. To create a color image from the captured photons in the photosensors, a first process is the Bayer Demosaicing to provide full color information at each image pixel. Different demosaicing algorithms are applied to improve the image resolution or to reduce the image noise. Small-scale details in images near the resolution limit of the digital sensor can produce visual artifacts, the most common artifact is Moiré.

Gamma encoding

To translate between our eye’s light sensitivity and that of a digital imaging system, a function called gamma is used. In the simplest case the nonlinear gamma function is defined by the following power-law expression:

Vout = A * Vin exp gamma

Vout and Vin are the input and output luminositye values, A is a constant (usually A = 1) and gamma is the exponent. A gamma value lower than 1 is called an encoding gamma, a value greater than one is a decoding gamma. In the first case the compressive power-law nonlinearity is called gamma compression; conversely the application of the expansive power-law nonlinearity is called gamma expansion. The term gamma correction is sometimes used for both processes.

We distinguish three types of gamma :

  • Image Gamma
  • Display Gamma
  • System Gamma

The image gamma is applied on the raw image data before converting them to a standard JPEG or TIFF file and saving it to the memory card. The gamma encoding redistributes the native tonal levels into ones that are perceptually more uniform, making a more efficient use of a given bit depth. The encoding gamma is usually about 1 / 2.2 = 0.455.

The display gamma refers to the video card and monitor and compensate for the image gamma to prevent that the image is displayed too bright on the screen. The display gamma is usually equal to 2.2.  On old Mac computers the value was 1.8.

The system gamma (viewing gamma) is the net effect of all gammas applied to the image. The system gamma should ideally be close to 1, resulting in a straight line in the gamma chart.

The following figures shows the three gamma plots :

Gamma charts

Gamma charts image, display and system

The precise gamma is usually specified by the ICC color profile that is embedded within the image file. If no color profile is indicated, then a standard gamma of 1/2.2 is assumed.

Tone and color adjustment

We should now have a good understanding about colors, but we didn’t yet explained what are tones. Are tones synonymous to brightness ? Some specialists refer to musical allusions to define tones. Other say that tones include colors. Years ago in a photo forum it was stated that there are only two terms needed to specify tones and colors : hue and luminosity. Ken Bhasin concluded in this forum : “Tone is the degree of greyness. If the subject has color, imagine taking away its color – what remains is its tone. Absence of any tone makes a subject bright (light grey/white). Presence of a tone makes a subject dark (Dark grey/black).” I endorse this definition.

There are several tools to adjust or correct tones and colors. Most are interrelated and influence both tones and colors. The most common tools are presented hereafter with reference to the Photoshop software.

Levels is a tool which can move and stretch the levels of an image histogram. It adjust brightness, contrast and tonal range by specifying the location of complete black, complete white and midtones in a histogram.

The following example shows two Kodachrome slides scanned with a cheap diascanner Maginon FS6600.

Scanned Kodachrome portrait with histograms

Scanned Kodachrome portrait with histograms

The histograms of the three color channels indicate an underexposure.

Color adjustment with Levels Tool in Photoshop

Color adjustment with Levels Tool in Photoshop

By moving the white point to the left in the R, G and B histograms in Photoshop the levels are adjusted. Holding down the ALT-key while dragging the black or white slider is a trick to visualize shadow or highlight clipping and avoid it.

Adjusted portrait

Adjusted portrait

Because the levels have been modified differently in the three color channels, the adjustment influenced also the hue of the image.

Photoshop curves tool

Photoshop curves tool (non-sense adjustment)

A second tool is the Photoshop curves. It’s a very powerful and flexible image transformation utility. Similar to Photoshop levels, the curves tool can take input levels and selectively stretch or compress them. A curve is controlled using up to a total of 16 anchor points. The left figure shows an example of an (artistic nonsense) curve applied to the preceding portrait. The curves tool only redistributes contrast and allow us to better utilize a limited dynamic range. You can never add contrast in one tonal region without also decreasing it in another region. You must decide how to spend your contrast budget. Curves also preserves the tonal hierarchy, unless there are negative slopes in it. The following figure shows the resulting modified image.


Portrait modified in Photoshop with the Curves Tool based on nonsense points


Curves can also be used on individual color channels to correct color casts (Farbstiche) in specific tonal areas. A typical example of digital images with color casts are scanned film negatives which have a strong orange mask. The reason an orange mask was added to color negative films was because of imperfections in the cmy dyes.

Scanned film negative with orange color cast and inverted into a positive

Scanned film negative

The orange color cast becomes purple when the image is inverted to a positive. All film scanning software comes  with color negative options. Typically a variety of color negative film types, such as Kodak Gold 100, Agfa, Fuji etc are listed in the scanner software. A good scan should avoid clipping in all the color channels, which can be easily checked in the histograms.

If the scanned image is not converted to a positive in the scanner, it can be done in Photoshop. The third Photoshop adjustment tool, called eyedropper (pipette), is well suited for this purpose. The eyedropper figures in the levels and curves panels (see figures above). The far left dropper tool is used to set the black point by clicking on a location within the image that should be black. The far right dropper tool does the same for the white point. The middle dropper tool sets the grey point which is an area in the image that should be colorless.

In a negative white and black are inverted. The lightest part of the negative (the darkest part of the scene) can be no lighter than the color of the base orange cast. If the orange cast can be converted to pure white in the negative (black in positive), then the remainder of the colors will be converted as expected. The next figure shows the areas where the eyedropper tool has been applied and the resulting inverted positive.


Adjusted negative with the dropper (3 areas black, white, grey) and resulting positive

The global adjustment of the colors is called color balance. If the goal is to render specific neutral colors correctly, the method is called grey balance, neutral balance, or white balance respectively. General color balance changes the overall mixture of colors in an image and is used to get colors other than neutrals to appear correct or pleasing.

Photoshop offers various methods to automatize tone and color adjustments :

  • Auto Levels
  • Auto Contrast
  • Auto Color
  • Photo Filter
  • Special filters like ColorPerfect

Photoshop provides also various sliders to adjust manually the parameters color balance, brightness, contrast, hue, saturation, exposure, shadows, highlights etc. A great help is the Photoshop Variations Tools showing incremental changes of different parameters in images, with an indication of eventual clippings. The next figure shows variations of the portrait image for the shadows, mid-tones, highlights and saturation.

Photoshop Variations

Photoshop Variations

Another method to automatize color balance used by several image editors are selectors for  presets, for example :

  • landcsape
  • portraits, skin tones
  • night
  • beach
  • jewelry


The next step in the image processing workflow is sharpening. Resolution adds the detail that lets us recognize features. Sharpness makes edges clear and distinct. The standard tool of choice for sharpening is the UnSharp Mask filter (USM).

All color filter Bayer array algorithms, by definition, blur the image more than could have been theoretically captured by a perfect camera or scanner. Thats why sharpening is often integrated in the demosaicing process. If not, it can be done separately in an image editor like Photoshop.


A last step in the image processing workflow is compression to reduce irrelevance and redundancy of the image data in order to be able to store or transmit data in an efficient way. Image compression may be lossy or lossless.

The common image formats used in digital imaging today are JPEG and TIFF.

Color Management

Color management is the cross-platform view of all the features presented in the present post, based on ICC standards. Wikipedia defines color management in digital imaging systems as “the controlled conversion between the color representations of various devices. A color management system transforms data encoded for one device (camera, scanner) into that for another device (monitor, printer) in such a way that it reproduces the original colors. Where exact color matching is not possible, the result should be a pleasing approximation.”

Parts of the color management technology are implemented in the operating system (OS), for example ColorSync in Mac OS X and Windows Color System (WCS, formerly ICM) in Windows. Other parts are included in applications (for example Photoshop) and in devices (for example cameras). An open source color management called Little CMS (LCMS) was initiated by Marti Maria in 1998. LCMS is released under the terms of the MIT License as a software library for use in other programs which will allow the use of ICC profiles. The current version is 2.7, updated on March 17, 2015, available on Github.

One of the main components of a color management system is the Color Matching  Module (CMM), which is the software engine in charge of controlling the color transformations that take place inside the system. A Color Transformation Language (CTL) was created by the Academy of Motion Picture Arts and Sciences (A.M.P.A.S.) in 2007.
Besides the color profiles for devices and color spaces, the ICC has standardized a CMM. CMM’s are built into ColorSync and WCS. Photoshop is also a good example of a CMM.
Proper color management requires that all images have an embedded profile. Recent web browsers like Internet Explorer 9, Safari 6 or Firefox support color management.


Profiling a device is called characterization. Instruments used for measuring device colors include colorimeters and spectrophotometers. Calibration is like characterization, except that it can include the adjustment of the device, as opposed to just the measurement of the device. When all device are calibrated to a common standard color space such as sRGB, no color translations are needed to get all devices to handle colors consistently. Monitors, scanners and printers are the common devices that can be calibrated.

Windows Display Calibration Tool

Windows Display Calibration Tool

Display Calibration Tool Adobe_gamma

Display Calibration Tool Adobe_gamma

Modern monitors include a factory-created profile that is loaded into the monitor firmware and is communicated to the computer. Some people prefer to replace these profiles with custom ones. Most operating systems include tools to calibrate the monitor. Adobe Gamma is a monitor calibration tool included in Photoshop.

Color charts such as IT8 are used to calibrate scanners. Printers should be calibrated for every type of paper and inks you use. One solution is to print a test chart and to scan it with a IT8 calibrated scanner. Scanner software like SilverFast calculates then an ICC profile for the printer and the paper and ink combination.

IT8 color chart

IT8 color chart

Photo Restoration

Digital photo restoration uses specific image editing techniques to remove visible damage, fading, color casts and other aging effects from digital copies of physical photographs. The most common tools are :

  • levels, curves, contrast and black level tools to remove fading
  • white balance, color balance and other color tools to remove color casts
  • clone stamp, healing brush and other selective editing tools to remove localized damage

Conclusions and recommendations

The human eyes and brain work together to create what we call vision. The eyes collect input and send it to the brain for processing. It’s the brain that decides what it is we see (or think we see). The brain makes its decisions based largely on perceived color and contrast data sent to it by the eye’s sensory elements such as cones and rods. Sometimes these decisions don’t match reality which can give rise to what we know as optical illusions. The human vision still performs better than the most complex digital imaging system.

Here are some rules based on the explanations given in the present post to guide you through the digital imaging process :

  • use Adobe RGB as a working space for 8 bit images and ProPhoto RGB for 16 bit images
  • assign the sRGB profile as default for unprofiled images
  • use a generic CMYK profile for printing if the printer does not supply a custom profile and if it’s not restricted to an RGB interface
  • use perceptual as default rendering intent; it’s the best choice for general and batch use and for images with intense colors
  • use relative colormetric rendering intent  for images with subtle tones (portraits); they benefit from the increased accuracy
  • apply the Photoshop Curves Tool only to 16 bit images


A list with links to websites providing additional information about digital imaging is shown hereafter :

Electronic publishing platforms

Electronic publishing (e-publishing) includes the digital publication of e-books, digital magazines, and the development of digital libraries and catalogues.

One of the leaders in electronic publishing is Issuu, which was founded in Copenhagen, Denmark in 2006 by Michael Hansen, Ruben Bjerg Hansen, Mikkel Jensen, and Martin Ferro-Thomsen. Issuu is a digital newsstand with over 21 million publications and 85 million active readers, featuring titles in fashion, culture, arts, and more, all of which are accessible on any device. In 2014, issuu released Clip, a tool that allows readers to take a snapshot of any part of a publication and share that on social media or through email.

Here are some thumbnails of the magazines currently  available for reading at the Issuu platform :

Magazines on the electronic publishing platform Issuu

Magazines on the electronic publishing platform Issuu

The Current Master Volume 1 Publication by World Wide Art Books (WWAB) and related publications about art and culture are shown at the Issuu website. Some magazines are shown below :

Art Magazines on the electronic publishing platform Issuu

Art Magazines on the electronic publishing platform Issuu

Issuu offers a free basic account to start with the upload of own publications.


Beethoven’s Google Doodle

To celebrate Ludwig van Beethoven’s 245th Year, Google created an interactive doodle to help Beethoven arranging his masterpieces during his unfortunate journey to the symphony hall. Produced by Gregory Capuano and designed by Leon Hong, the Google engineers Jordan Thompson, Jonathan Shneier, Kris Hom and Charlie Gordon programmed a new masterpiece of animation. The Piano recordings have been done by Tim Shneier. Nate Swinehart was responsible for animatics and additional art.

The following figures show some key scenes from the interactive animation.


Mobile DICOM viewers

Last update : October 20, 2016

Note : the new versions released since spring 2016 have not yet been tested. It will be done in the coming days.


This contribution refers to my recent updated post about DICOM image viewers. Mobile DICOM viewers are becoming more and more popular. They are very useful in the context of medical applications in the cloud.

Most web viewers can be used on mobile equipments (tablets, smart phones, …), but the focus of this post are native applications designed for Apple iOS or Android devices. DICOM Web viewers are handled in a separate contribution. In the present list I considered only those applications that have been updated after January 2014, with some exceptions.

Test Scenario

To test the selected apps, I used Orthanc (Windows versions 0.9.5 and 0.9.6) as PACS server. To have realistic DICOM samples,  I uploaded some DICOM studies from me and from family members relative to exams done in the past in hospitals in Luxembourg and France.

To set up a communication between a DICOM viewer and the Orthanc server, the viewer should allow to define the following parameters in the settings :

  • Remote AET
  • Remote Hostname/IP
  • Remote port number
  • Local AET
  • Local port number
  • Protocol : C-MOVE

The local IP is the address of the used mobile device.

If no settings are available for the port numbers, the port number of the server can be added to the remote IP address. The standard port number 104 can be tried as a first guess for the settings of the viewer. If no local AET settings are provided, the iPad name can be tried as a first guess. Another possibility is to use a DICOM network analyser like DVTk to detect unknown parameters. If the C-MOVE protocol is not supported by the viewer, the WADO or C-GET protocols can be an alternative.

In the presentation of the selected mobile viewers, I generally used the terminology published by the developers, even if I disagree with their claims.  In the case of apps that worked as expected with the Orthanc server, I added some screenshots to show the look-and-feeling of the viewer.

Purpose of the evaluation

You may ask what is the purpose to evaluate proprietary closed source applications in an open source environment. I started the evaluation in mid-2015 in the context of the development of an educational tool for radiologists, called RadioLogic. The reasons were various :

  • to familiarize with the DICOM technologies (learning by doing)
  • to compare the performance of native apps with HTML5 web apps on tablets and phones
  • to explore the different types of graphical user interfaces to create a best-of-breed GUI for the RadioLogic project
  • to examine the assets of viewers certified / cleared by FDA or CE
  • to test features that are not (yet) available on HTML5 web apps

iOS Applications


Mobile DICOM viewers (icons) arranged in a folder on the iPad Desktop

The following DICOM viewer apps, which are currently available at the AppStore, have been tested. They are arranged and listed in alphabetical order :

1. aycan mobile

aycan mobile is an iPad app which allows to receive and display DICOM images in an easy, secure and fast way. Developed by aycan Digitalsysteme GmbH, the current version 1.08.000 was updated on July 22, 2015. The viewer works only with the aycon DICOM workstation, which is based on Osirix. A demo server with ID and password “aycan” allows to explore some demo studies. A subscription is required to use all features of the the ayMobile communication service. A free 30-days-trial subscription can be ordered on the aycan website.


DICOM-AVC is available for iPad and iPhone. The current free version is 2.2 has not been updated since July 12, 2012. It has been developed by the The New Medical Product Server LLC who claims that DICOM-AVC is a complete real-time collaborative imaging and videoconferencing tool which was once the number one downloaded app on the AppStore. To use the app, registration of an account is required, which can be upgraded to a premium account for 19.99$ per year. Today it’s displayed as black screen on my iPad.

3. DICOM Mobile

Dicom Mobile is a simple but powerful DICOM viewer for iPhone and iPad, developed by Luigi Orso. The current version is 1.4.0, updated on June 3, 2015. The price is 2.99$, with additional in-app purchases for extensions. The developer provides support on his website. DICOM files can be loaded from a DropBox account or from a Web server, but not from a PACS server.

DICOM Mobile Menu

DICOM Mobile Main Menu

DICOM Mobile Dropbox Panel

DICOM Mobile Dropbox Panel

4. DICOM Viewer HD

The free version 1.0 of DICOM Viewer HD was released on July 4, 2014, by Wingspan Xi’an Electronic Technology Co., Ltd from China. I was not able to evaluate the app, the login or registration in the “User Manageer” always fails.


DICOM IR is one of the easiest and most intuitive 2D DICOM Image viewers for iOS. DICOM IR can browse any standard PACS server. DICOM IR is developed by Pietro Zuco, a freelance mobile developer based in London/Tokyo/Barcelona. Pietro Zuco has a solid experience developing 2D games, utilities, educational and entertainment applications on iOS and OSX systems. Version 1.0 of DICOM IR was released on July 25, 2015 and is free for a limited time. The app supports C-ECHO, C-FIND and C-MOVE queries.

DICOM IR has a very clean GUI, an excellent user guide, a perfect configuration panel, a performant viewer and it works as expected with the Orthanc server. My compliments to this author.

DICOM IR PACS Server Configuration Panel

DICOM IR PACS Server Configuration Panel

DICOM IR User Guide

DICOM IR User Guide

DICOM IR search panel

DICOM IR search panel

DICOM IR Metadata Info panel

DICOM IR Metadata Info panel

DICOM IR image viewer panel

DICOM IR image viewer panel with line tool

6. iDO Viewer

The iDO medical image viewer is quickly becoming the next generation in mobile medical image viewing by combining multi-modality imaging with individualized physician workflow. Developed by EBM Technologies, Incorporated, the current version 1.1.4 was updated on March 30, 2016. This application requires the EBM iDO server, demo servers to view sample data and basic features of the app are available and can be selected in the settings panel.

7. is the best-in-class mobile application for medical imaging PACS users on iOS devices. imageFLOW is developed by imagitive GmbH. The current free version 2.0, based on Imagitive’s DaVinci framework, was updated on November 25, 2015. A DemoServer mode to explore some DICOM studies is enabled by default. A PACS server can be configured in the general iOS settings, nevertheless access trials failed with a “Server Authentication” error message.

8. iMango

iMango is the iOS adaptation of the famous Mango DICOM viewer developed at the Research Imaging Institute (RII), an Organized Research Unit (ORU) of the University of Texas Health Science Center at San Antonio. It works on iPad’s. The current version 1.3 was updated on June 10, 2015. The price is 14.99$. A PACS server can be configured in the general settings and the app works as expected with the Orthanc server. iMango also supports the WADO protocol.

iMango Transfer Options Panel

iMango Transfer Options Panel

iMango Server Configuration Panel

iMango Server Configuration Panel

iMango User Guide

iMango User Guide

iMango image viewer with about panel

iMango image viewer with about panel

iMango image viewer with metadata info panel

iMango image viewer with metadata info panel

iMango image tools

iMango image tools

9. iPaxera

iPaxera is one of the fastest and easy to use PACS viewing applications specifically designed for viewing and navigating studies with your iPad, iPhone, and iPod. iPaxera is developed by Paxeramed Corp, a world leading medical imaging solution developer based in Boston, that develops a full range of medical imaging solutions. The current free version 3.10 was updated on May 4, 2016. A PACS server can be configured in the “Add Node” panel. Ipaxera works as expected with the Orthanc server. The app also supports the WADO protocol.

Onn Akbar Ali reported that Ipaxera can display multiframe Jpeg images ( such as angiography ) quite well.

iPaxera login panel

iPaxera login panel

iPaxera PACS server configuration panel

iPaxera PACS server configuration panel

iPaxera search panel

iPaxera search panel

iPaxera worklist panel

iPaxera worklist panel

iPaxera image viewer with measure tool and series list panel

iPaxera image viewer with measure tool and series list panel

10. iTA View

iTA VIEW is a viewer for DICOM and DICOM RT objects developed for iPad and dedicated to radiotherapy. The app is developed by Tecnologie Avanzate TA srl in Italy. The current version 1.2.0 was updated on March 8, 2016. The app works with the Orthanc server. To transfer studies without limitations, an upgrade to the full version is required. The price for the upgrade is 29.99 EUR.

ITA View PACS server configuration panel

ITA View PACS server configuration panel

ITA View User Guide

ITA View User Guide

ITA View worklist panel

ITA View worklist panel

ITA View database panel

ITA View database panel

ITA View image viewer

ITA View image viewer

11. Jack Imaging Medical Image Viewer

Jack Imaging Medical Image Viewer is a medical image viewer for the iPad that makes use of nephosity’s patent pending technology, the latest in medical imaging innovation. It supports reading the DICOM format and the most common imaging modalities. The viewer is developed by Nephosity Inc. The current version 3.0.5 was updated on January 25, 2016. The prize is 0.99$. The viewer works only with the Jack Imaging platform which  stores the DICOM data in the cloud. Currently Amazon’s cloud infrastructure is used to provide the best performance and highest security levels. During a beta phase the Jack Imaging platform accounts are offered for free. On 4th generation iPads the Jack Imaging viewer offers FDA cleared capabilities.

11. Keiron

Keiron is an innovative App that allows you to view, annotate and analyse radiographic images stored in the DICOM standard. The current free version 1.0 was released on May 4, 2015. Keiron is developed by Digital Video S.p.A., the creator of Toonz. A PACS server can be configured in the settings, but only the ECHO and C-FIND queries are working with the Orthanc server. The “download” button to load DICOM instances is disabled.

Keiron serevr & client configuration

Keiron server & client configuration panels

Keiron viewer settings

Keiron viewer settings panel

Keiron worklist panel

Keiron worklist panel

12. LEADTOOLS MedicalWebViewer

The LEADTOOLS Medical Web Viewer is a zero footprint solution available for iPad and iPhone, developed by Leadtools Technologies Inc. The current free version 3.0 was updated on May 27, 2016. The viewer works only with LEAD’s web servers so you can quickly access the features of the LEADTOOLS Medical Web Viewer Framework SDK with the default settings.

13. loDicom Viewer

loDicom Viewer allows you to view, archive and transfer 2D medical images in DICOM format. Developed by Luigi Orso, the current version 4.1.1 was last updated on July 30, 2014. The price is 12.99$. The developer is also the author of the DICOM Mobile viewer. He provides support on his website, which is not useless, because the app crashes systematically on my iPad. ECHO tests with the Orthanc server are succesful, but a C-FIND query leads to a crash. I was only able lo load DICOM files via a DropBox. The GUI is not very intuitive, the user guide is not very helpful.

loDICOM viewer PACS server configuration

loDICOM Viewer PACS server configuration panel

loDICOM Viewer

loDICOM Viewer ECHO Test panel

loDICOM Viewer image panel

loDICOM Viewer image panel

loDICOM Viewer user guide

loDICOM Viewer user guide

14. MedFilm

MedFilm is a fast dicom viewer developed by Fuli Wu, a researcher at the Zhejiang University of Technology in Hangzhou, China. The app is available for iPad and iPhone. The current free version 4.25 was updated on June 1, 2016. A PACS server can be configured with support of the WADO protocol. The C-MOVE protocol is not supported.


MedFilm DICOM file and image panels

15. Med View

Med View is developed by Matthias Rudolph and available for iPad and iPhone. The current free version 1.1 was updated on October 23, 2015. DICOM files can be loaded from the iTunes file sharing service or from FTP servers.

16. Mobile MIM

The Mobile MIM software program is used for the viewing, registration, fusion, and/or display for diagnosis of medical images from various modalities. Developed by MIM Software Inc., the current version 4.0 was updated on July 26, 2016. Image data can only be transferred to the device by using MIMcloud or MIM 5.1+ workstation software.

17. mRay

mRay, developped by mbits, is available for iPad and iPhone. The current free version 4.1.6 was updated on October 4, 2016. mRay is a CE certified and approved medical product. To be able to receive images in mRay from your PACS, the mRay Server Software must be installed, which can be used free of charge for an evaluation period.

18. MyXray

MyXray is an easy and intuitive app that will let you see your x-ray files as a real doctor. The free version 1.0 was released on September 8, 2015. The app is developed by Pietro Zuco, the author of the outstanding DICOM IR app. MyXray is a lite version of DICOM IR without support of DICOM communications.

19. Nuance PowerShare

Nuance PowerShare Mobile gives you instant access to your medical images and reports stored on the Nuance PowerShare Network using your iOS device. The free version 5.5.10 was updated on August 16, 2016. I was not able to load this app from the AppStore.

20. OsiriX HD

OsiriX HD is the iOS adaptation of the famous OsiriX DICOM viewer, running on Mac OSX,  which is considered as a reference for DICOM image viewers. It works on iPhone’s and iPad’s. The current version 4.2 was updated on October 14, 2016. The price is 49,99$, with additional inline sales. Osirix HD can be configured in the general settings. The app works as expected with the Orthanc server and supports also the WADO protocol.

Osirix HD PACS Server configuration panel

Osirix HD PACS Server configuration panel

Osirix HD Exams Panel

Osirix HD Exams Panel


Osirix HD Split Screen in image viewer

Osirix HD Image viewer

Osirix HD Image viewer with stack of 20 instances


Osirix HD Metadata Panel

21. PACS DICOM Viewer

PACS DICOM Viewer app is a free, vendor-neutral PACS browser and DICOM viewer which works on iPad. It is developed by Logicpond Inc, a California based product design and development solutions company. The current version 2.1.1 was updated on November 11, 2015. The viewer can be configured to work with the Orthanc server, but the C-MOVE queries are aborted during the image transfer. The WADO protocol  is also supported  and works with Orthanc.

PACSDICOM Viewer startup picture

PACSDICOM Viewer startup picture

PACSDICOM Viewer configuration panel

PACSDICOM Viewer configuration panel

PACSDICOM Viewer search panel

PACSDICOM Viewer search panel

PACSDICOM Viewer worklist

PACSDICOM Viewer worklist panel

PACSDICOM Viewer download success

PACSDICOM Viewer download success

PACSDICOM Viewer images with rainbow LUT

PACSDICOM Viewer images with rainbow LUT

PACSDICOM Viewer metadata panel

PACSDICOM Viewer metadata panel

22. R-Site Mobile

R-Site Mobile for IOS supports 2D, 3D and MIP/MPR operations for DICOM files. To use this app, you have to own at least one set of R-Site PACS. R-Site is developed by DJ HealthUnion Systems Corporation in China. The current version was updated on August 31, 2016.

23. ResolutionMD

ResolutionMD enables doctors to securely view patient images and reports from a wide variety of computers and mobile devices, collaborate with other practitioners and diagnose from any location. The product requires access to a configured ResolutionMD Server. It contains a demo mode to allow to experience the full functionality of ResolutionMD on a variety of anonymous patient images. ResolutionMD is devloped by Calgary Scientific Inc. The free version 5.3.3. was updated on July 19, 2016.

Android Versions

DICOM Android apps on Samsung Galaxy Tablet 2 - 7.0

DICOM Android apps on Samsung Galaxy Tablet 2 – 7.0

The following iPad applications are also available as Android version on Google Play Store (in alphabetical order) :

3a. DICOM Mobile

The current Android version 1.3.2 of DICOM Mobile was updated on August 13, 2014. The price is the same as for the iOS version : 2.99$. The GUI and the features of the viewer are also identical to the iOS version. DICOM images can only be loaded from DropBox or from other web servers.

Android DICOM Mobile menu panel

Android DICOM Mobile menu panel

9a. iPaxera

The Android free version 2.0.1 of  iPaxera was updated on September 15, 2016. This app requires Android 4.4 or greater and I was not able to test it on my Samsung Galaxy tablet running Android 4.2.2. A few days later I changed my Blackberry phone with an LG Spirit phone running Android 5.0.1. After installing iPaxera on this phone, I noticed that the Android version is different from the iOS version and allows no PACS server configuration, but only dowloads of DICOM files from Dropbox or iTune sharing.

10a. iTA View

The Android free, but limited, version 1.10 of iTA View was updated on October 27, 2014. An in-app upgrade priced 26.99 EUR is available through the Google Play Store.


iTA View PACS server configuration panel


iTA View settings panel


iTA View search panel

12a. LEADTOOLS MedicalWebViewerDemo

The Android version of the LEADTOOLS MedicalWebViewer is the online live demo of the Medical Web Viewer Framework included with LEADTOOLS PACS Imaging and Medical Imaging Suite. The current version 1.4.1 was updated on February 2, 2014.

17a. mRay

The Android free version 4.1.6 of  mRay was updated on September 23, 2016. The mRay app can not be used without the installation of the mRay Server Software.

Android Applications

The following DICOM viewers are only available as Android version (in alphabetical order) :

24. Dicomite

Dicomite is a free and powerful DICOM viewer with intuitive and simple user interface, optimized for use on multi-touch devices. Dicomite is developed by Samsung MNO Lab. The current version 1.7.1 was updated on October 11, 2016. To display images they must be downloaded into the local memory with an Internet browser.


DICOMITE image panel

25 Direct DICOM

The Direct DICOM Viewer is developed by Hui Zhang. The current version 1.2.4 was updated on March 21, 2015. To display images they must be downloaded into the local memory with an Internet browser. The app works however as DICOM server and local DICOM files can be stored to the Orthanc server.

Direct Dicom viewer image and metadata panels

Direct Dicom viewer image and metadata panels

Direct Dicom viewer server and store (push) panels

Direct Dicom viewer server and store (push) panels

26. Droid Dicom Viewer

The Droid Dicom Viewer was developed by Pierre Malarme. The free current version 3.2.3 was updated on May 17, 2011. Droid Dicom Viewer is a Free and Open Source Software (FOSS) licensed under the terms of the GNU General Public License. To display images they must be downloaded into the local memory (cache) with an Internet browser.

Droid DICOM viewer about panel

Droid DICOM viewer about panel

Droid DICOM viewer cache panel

Droid DICOM viewer cache panel

Droid DICOM image panel

Droid DICOM image panel

Droid DICOM viewer image panel with

Droid DICOM viewer image panel with LUT/CLUT

27. DroidRender

DroidRender is a 3D DICOM viewer for Android, including DICOM file support by GDCM,
volume rendering and tissue segmentation and display. Developed by Startm, the current version 3.2.0 was updated on August 26, 2016. The app proposes in-app purchases at prices between €1.04 – €104.08 per item and displays ads at the bottom of the screen. To display images they must be downloaded into the local memory (cache) with an Internet browser.

Droid Renderer

Droid Render image panel

28. Mobile Dicom

Mobile Dicom is a free Android Mobile Dicom (Diconde) Viewer application which is based on opensource OFFIS DCMTK as its backend library. Inspired by opensource community and in the hope of extending usage of the library on Android devices, the Mobile Dicom / Diconde tool was created by Feng Gao. The current version 2.4 was updated on November 23, 2015. To display images they must be downloaded into the local memory (cache) with an Internet browser. The app works however as DICOM server and can store local DICOM files tthe Orthanc server.

Mobile Dicom Viewer about and config panels

Mobile Dicom Viewer start and about panels

Mobile Dicom Viewer load and config panels

Mobile Dicom Viewer load and config panels; ECHO test was succesfull

Mobile Dicom Viewer image and metadata panels

Mobile Dicom Viewer image and metadata panels

29. simplyDICOM

Simply DICOM is currently in public beta. It should work with any uncompressed DICOM images, downloaded into the local memory. The viewer has a minimal interface to keep the focus on the images themselves. The developer is Christopher Boyd. The current version 0.7.3 was updated on May 27, 2015. To display images they must be downloaded into the local memory (cache) with an Internet browser.

Simply DICOM

SimplyDICOM image viewer

DICOM file downloads

If DICOM files are served with the extension .dcm or/and with the content type application/dicom, they are recognized as DICOM images in iOS Safari and can be opened directly in the following DICOM viewers : MedFilm, MyXray, Osirix HD, DICOM IR, iMango or Dicom Mobile.

Safari .dcm files

.dcm files recognized as DICOM images in iOS Safari browser by DICOM viewers

DICOM files embedded in .zip archives are recognized by the same DICOM viewers in the iOS Safari browser.

Safari .zip files

.zip files recognized in iOS Safari browser by DICOM viewers

On Android devices, DICOM files are only recognized by the DIRECT DICOM viewer in the Android browser, but they can be downloaded in the /storage/emulated/0/Download/ folder where they are accessible by most Android DICOM viewers.

Android DICOM file download panel

Android DICOM file download panel


The following list provides links to mobile applications which are not DICOM viewers, but related to medical imaging (in alphabetical order) :


Among the 29 tested native mobile DICOM viewers, 18 are iOS apps, 6 are Android apps and 5 are available on the two platforms. 9 viewers can be configured to access a vendor-neutral PACS server, but only 5 of them work as expected and comply with the Orthanc PACS server. Among them are the three renowned commercial viewers iMango, iTA View and Osirix (priced between 15 and 50 USD), one free app (iPaxera) developed by a world leading medical imaging solution provider (Paxeramed Corp) and one app (DICOM IR) created by an independent free-lance developer ( Pietro Zuco). The other 4 configurable viewers are limited, partially disabled or crashing.

The remaining viewers are demo apps working with proprietary PACS systems, displaying only DICOM files loaded from DropBox or from another web server or crashing and working not at all. There are additional DICOM viewer apps in the AppStore and the Google PlayStore which are not listed because they were not updated in the last two years.

My two cents

I was shocked to see that most native mobile apps in the medical domain are not state of art. Apps in the stores which have not been updated in the last few years, text errors in the menus, program errors in the logic ( 2 viewers change the AETitle during communications), crippled GUI’s when the orientation of the device changes, wrong version numbers in the about panel etc, are testimonies of the poor quality.

DICOM Testing with DVTk

Last update : January 10, 2020

DVTk history

DVTk is an open source project for testing, validating and diagnosing communication protocols and scenarios in medical environments. It supports DICOM and IHE integration profiles. The project started in 1997 by Philips and was joined by Agfa in 2001. In 2005, DVTk was open sourced and one year later ICT Automatisering joined the open source community. In 2009 Agfa left the project.

The following tools are currently free available from the website or from GitHub :

  • DICOM Compare, v 5.0.0
  • DICOM Editor, v 5.0.2
  • DICOM Network Analyzer, v 5.0.1
  • DVT Validator, v 5.0.3
  • Query Retrieve SCP Emulator, v 5.0.1
  • RIS Emulator, v 5.0.0
  • Storage SCP Emulator, v 5.0.2
  • Storage SCU Emulator, v 5.0.1

DVTk Tests

I used these tools in the context of the development of an education tool for radiologists called RadioLogic. I edited the following script to execute an ECHO test and a STORE-SCP test with the Orthanc DICOM Server, version 0.9.5 :

# DVT Dicom Script
# Product Test Session Properties
SESSION-TITLE "Orthanc Server"
MANUFACTURER "Manufacturer"
MODEL-NAME "Product Name"
TESTED-BY "Marco Barnig"
DATE "20151208"
# SUT ACSE Properties
SUT-ROLE acceptor
# DVT ACSE Properties
DVT-IMPLEMENTATION-CLASS-UID "1.2.826.0.1.3680043.2.1545.1"
# Socket Properties
SUT-HOSTNAME "localhost"
# Test Session Properties
PDU-DUMP false
# Supported Transfer Syntaxes
SUPPORTED-TRANSFER-SYNTAX "1.2.840.10008.1.2.2"
SUPPORTED-TRANSFER-SYNTAX "1.2.840.10008.1.2.1"
# Configurable Delay between N-Action and N-Event Command
# Definitions
DEFINITION "All DIMSE Commands.def"
DEFINITION "CT Image Storage.def"
DEFINITION "Digital X-Ray Image Storage - For Presentation.def"
DEFINITION "Digital X-Ray Image Storage - For Processing.def"
DEFINITION "Enhanced CT Image Storage.def"
DEFINITION "Enhanced MR Image Storage.def"
DEFINITION "Media Storage Directory.def"
DEFINITION "MR Image Storage.def"
# Results
RESULTS-ROOT ".\results\"
# DICOMScript Description Directory

Scripts are saved as text files with the extension .ses. Several scripts can be referenced in an xml-project file with the extension .pdvt.

The following figure shows the resulting settings in the DVTk session panel :


DTVk DICOM validator session panel

By double-clicking on the Storage-SCU-emulator link at the left, a window pops up allowing to do an ECHO test or to send a DICOM file to the Orthanc server. The exchanged messages are displayed in the Activity Logging panel :


DVTk Activity Logging

After the succesful sending of a DICOM file to the Orthanc server, a summary of the validation results is shown in the Validation Results panel :


DVTk Validation Results

In verbose mode, the Orthanc Server provides the following releated messages in the command window :

Orthanc Server output in verbose mode

Orthanc Server output in verbose mode

DVTk Network Sniffer

The next figure shows the results of the DVTk network sniffer analyzing the DICOM traffic between the OSIRIX HD viewer on iPad with the ORTHANC DICOM server (version 0.9.6) running on Window 8.1.


DVTk summary results of FIND Request sent by Osirix HD from iPad

I entered only the first characters of the patient name to send a FIND query. DVTk reports four errors in the C-FIND communication because the following data elements are not valid :

  • (0008, 0018) : SOP Instance UID > should be present with at least a zero length
  • (0010, 0020) : Patient ID > should be present with at least one value
  • (0020, 000D) : Study Instance UID > should be present with at least one value
  • (0020, 000E) : Series Instance UID > should be present with at least one value

The C-MOVE communication works without errors and the received results displayed in Osirix HD are correct.


DVTk reports no errors in the C-MOVE communication between Osirix HD and Orthanc

The Orthanc server shows no specific error message in the command window.

The OSIRIX HD viewer is configured as follows :

  • Remote AET : ORTHANC
  • IP :
  • Port : 4242
  • AETitle : OSIRIXHD
  • IP :
  • Port : 4096
  • Transfer Syntax : Implicit Little Endian (preferred syntax by Orthanc)
  • Encoding : ISO IR 100 (Latin-1)
  • Protocol : C-MOVE
  • Timeout : 90 sec

Communication Failure

The next example shows the case of a failed communication. The C-FIND query sent by the PACS DICOM iPad app to the Orthanc server (version 0.9.6) is aborted with a DIMSE failure. The query is identical to the C-FIND request sent by Osirix HD. The Orthanc log signals “DIMSE No data available (timeout in none-blocking mode)”.


Orthanc log messages related to a C-FIND command sent by the PACS DICOM app

The DVTk network analyzer shows the same four errors as in the case of the Osirix communication. After a timeout, the communication is aborted.


DVTk signals an Abort Request in the PACS DICOM – Orthanc association

The PACS DICOM viewer is configured as follows :

  • Server AET : ORTHANC
  • Host Name/IP :
  • Server Port : 4242
  • Local AET : TEST-SCU
  • Device IP Address :
  • Local Port : 4096
  • Protocol : C-MOVE

A deeper analysis is required to identify the reason of the abort request. If the problem is caused by a non-valid parameter in the C-FIND query, the recently proposed LUA script filter will be an excellent remedy.

Accessing Local Virtual Hosts

Last update : September 7, 2015

Local Networks

A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, or office building. It is contrasted to a wide area network (WAN), which covers a larger geographic distance and may involve leased telecommunication circuits. Ethernet over twisted pair cabling and Wi-Fi (WLAN) are the two most common transmission technologies in use for local area networks.

Virtual Hosts

The term Virtual Host refers to the practice of running more than one website (such as and on a single machine. The fact that they are running on the same physical server is not apparent to the end user. Apache was one of the first servers to support virtual hosts right out of the box.

Computer Name, Address and Route

To find and access a computer in an network, we must know the following informations :

  1. Name : to indicate what we seek ( > Domain Name )
  2. Address : to indicate where it is ( > IP Address )
  3. Route : to indicate how to get there ( > Routing )

Domain Name

Domain names are used to identify one or more IP addresses. They are used in URLs to identify particular web pages. Domain names can be local or public. The latter must be registered with an ICANN accredited registrar.

A domain name consists of one or more parts, technically called labels, that are conventionally concatenated, and delimited by dots, such as The right-most label conveys the top-level domain (TLD), including the generic top-level domains (gTLDs), such as the prominent domains com, info, net, edu, and org, and the country code top-level domains (ccTLDs) such as lu, de, fr.

Below the top-level domains in the domain name hierarchy are the second-level domain (SLD) names. These are the names directly to the left of the top-level domains. Second-level domain names are usually created based on the name of a company, product or service. There can be additional sub-level domains with virtually no limitation. Below these levels, the most right domain name component is used to designate a particular host server, for example www for a world wide web server, ftp for an FTP server, or any other label. A domain name with TLD, SLD and hostname is called a fully qualified domain name (FQDN), for example

Local domain names used in local networks can be freely chosen by the network administrator, but some TLD’s should be avoided. For example Apple uses the domain extension .local for it’s implementation of zero-configuration networking called Bonjour.

To map domain names to IP addresses, a Domain Name System (DNS) is used.

IP Address

An IP Address (Internet Protocol address) is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. The designers of the Internet Protocol defined an IP address as a 32-bit number (IPv4) which is still in use today. A new version of IP (IPv6), using 128 bits for the address was developed in 1995 to avoid a depletion of the available addresses. Its deployment has been ongoing since the mid-2000s.

IP addresses are usually written and displayed in human-readable notations, such as (IPv4), and 2001:db8:0:1234:0:567:8:1 (IPv6).

An IP address is logically recognized as consisting of two parts: the network prefix and the host identifier, or interface identifier (IPv6). The routing prefix is expressed in CIDR (Classless Inter-Domain Routing) notation. It is written as the first address of a network, followed by a slash character (/), and ending with the bit-length of the prefix. For example, is the IPV4 prefix starting at the given address, having 24 bits allocated for the network prefix, and the remaining 8 bits reserved for host addressing.

IPv4 reserves some addresses for special purposes such as private (local) networks or multicast addresses. The following ranges are reserved for private networks :

Prefix length Start End

In IPV6, the reserved address blocks for private networks are referred to as unique local addresses (ULA) and they use the routing prefix fc00::/7.

IP addresses can be static (fix) or dynamic. Static IP addresses are manually assigned to a computer by the network administrator. Dynamic IP addresses are assigned either by a server (router) using Dynamic Host Configuration Protocol (DHCP) or by the computer itself, as in Zeroconf. In the last case of address autoconfiguration (link-local address) the address block is reserved in IPv4 networks and the block fe80::/10 in IPV6 networks.


In IPV4 a subnet mask is used to indicate how the IP address is divided into network and host parts. A subnet mask is made by setting network bits to all “1”s and setting host bits to all “0”s. For example, is the network mask for the prefix.


Routing is the process of selecting best paths in a network. Routing schemes differ in their delivery semantics:

  • unicast : destination is a single specific node in the network
  • broadcast : destinations are all nodes in the network
  • multicast : destinations are all interested nodes in the network
  • anycast : one to many routing topology
  • geocast : destinations are all nodes in a geographical area

Unicast is the dominant form of message delivery on the Internet.

DNS Resolver

The main job of a DNS server is to store DNS name data and serve it when it receives requests. The main job of a DNS resolver is to transform a DNS name into an IP address, but this is only one of several types of resolution services performed by DNS. The process to resolve an hostname to an ip address is normally called dns lookup.

Reverse DNS lookup or reverse DNS resolution (rDNS) is the determination of a domain name that is associated with a given IP address.

Accessing local virtual hosts

There are different ways that local websites can be viewed from other computers which are connected within the same LAN. These computers can be desktops or laptops running Windows, Mac OSX, Linux or be mobile devices like tablets (iPad, Samsung, …) or smart phones (iPhone, Android, …).

  • Use the LAN IP of the server : downside – only one website can be accessed
  • Use the LAN host name of the server : downside – only one website can be accessed
  • Use a LAN-wide hosts file : downside – this doesn’t work on iOS and most other mobile devices
  • Use own DNS server : downside – setup is complex ; DNS server must always be turned on
  • Use a Wi-Fi router with DNS Masquerading : downside – only a few routers support this option
  • Use registered domain names : downside – all website domain names must have a common base; works only with Internet access
  • Use a HTTP proxy server : downside – multiple

We are analyzing now what solution to adapt for a real network case, related to the specific RadioLogic project.

RadioLogic project requirements

RadioLogic is an university education tool for radiologists which uses three websites, working as virtual hosts :

  • : hosted on Amazon AWS, this website provides the tools and data for public users
  • : hosted on a local server, this website provides the tools and data for private users on a local network
  • : hosted on a local server, this website provides the beta versions of new tools and data for testing before deployment on the public and private hosts

If possible, the IT consumerization policy BYOD (Bring your own device) is promoted allowing the radiologists in residence to use their own devices. The local server is a MacBookAir laptop running OSX Yosemite 10.10.5 or later. The clients are iPads running iOS 8.1.4 or later. Both the server and the clients will be used for other purposes.

There exist four operational modes :

  1. The server is used as standalone client without Internet connection
  2. The server is used as standalone client with an Internet connection through a Wi-Fi router
  3. An iPad is used as client with an Internet connection through the same Wi-Fi router (same local network)
  4. An iPad is used as client with a local AdHoc Wi-Fi Connection (SSID = radiologic-open) established by the server

The following figure shows the four operational modes.


RadioLogic : four operational modes

The project requirement is that the same host names are used in each operation mode to access the three websites. Let’s examine how to achieve this goal.

Server without Internet connection used as a client

Without Internet connection, it’s not possible to access the public website To access the local websites and, the most simple solution is to use the internal hosts file available on every desktop computer (Windows, Linux, Mac OSX, …). This configuration file for the loopback interface is located at /private/etc/hosts on the Mac OSX, but it’s also accessible through the more traditional location /etc/hosts. This file is configured as follows :

# Host Database
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##	localhost	broadcasthost
::1             localhost

The default values are the IP addresses for the localhost in IPV4 and IPV6 and the broadcasthost for IPV4. The radiologic hosts have been added.

Server with Internet connection used as a client

Accessing the public website is now possible and the preceding loopback solution is also working.

iPad used as a client with Internet connection in local network

There is no problem to access the public website, but the simple loopback solution to access the local hosts is not possible on mobile devices without jailbreaking.

The next possibility consists in using a local DNS server. Setting up a DNS server on the Mac laptop with primary zones for the local hosts needs a parameter change in the DNS configuration of the iPad Wi-Fi connection.


Wi-Fi settings in iPad

We must change the IP address of the DNS server, assigned by the Internet provider via the Wi-Fi router, by the local IP address of the Mac laptop, which leads to various problems :

  • The IP address of the Mac laptop changes when you use it in a different local network or when the DHCP server assigns a new address in the same network at booting
  • The DNS server of the Mac laptop must be continuously running to use the iPad for Internet surfing
  • Using an additional DNS server with forwarding to a public DNS server to set up an Internet connection may lead to delays or performance problems
  • Frequent changes of the DNS addresses on an iPad is an error-prone process for a normal user

I recommend to never change the network parameters assigned by the Internet provider on your router and local devices to avoid all sort of problems.

There exist a third solution to access the local hosts in our case, which some developers consider as a hack : using the public DNS server to point towards the internal private IP address of the Mac computer.

This solution is simple and feasible in our case because all hosts use the same domain name which is registered and managed in a public DNS server. Instead of creating an A record for both and we can use a wild-card (*) to catch all DNS records.

Some reasons to not use a public DNS server with local IP addresses, invoked by other developers, are the following :

  • leak of internal informations to third parties
  • it’s not considered best practice
  • external clients may inadvertently be redirected to a host on their own local network
  • it’s likely to cause confusion and human miscommunication
  • risk of mail delivery timeouts or mail bounces
  • risk of DNS rebinding attacks

I don’t agree with these arguments that I consider as minor problems or not applicable in specific cases. I think the solution is not a such bad idea in practice and I will use it in the RadioLogic project.

Some routers provide a security feature to protect against DNS rebinding attacks by suppressing DNS responses that refer to IP addresses in the home network. My router FRITZ!Box 7390 is such a device. It allows to enter domain names in a list of exceptions where this protection should be disabled. In our case these are and


Exceptions to the DNS rebinding protection in the Fritz!Box router 7390

iPad used as a client connected to an AdHoc network

The only workable solution in this case is the use of a local private DNS server. You may ask what about the problems listed in the preceding chapter ? Well, these are no longer issues in the case of an AdHoc Wi-Fi network :

  • The self-assigned IP address of the AdHoc Wi-Fi interface in the range can be defined as static
  • The DNS server is always running when the Mac laptop runs the AdHoc network
  • Without Internet connection there is no forwarding to a public DNS server
  • The DNS address is changed once in the configuration of the AdHoc Wi-Fi connection which is different from the configuration of the other Wi-Fi infrastructure connections

Search Domain

In iOS and OSX Wi-fi settings, each Wi-fi connection has a Search Domains section which is simply a convenience that allows the system to convert host names to Fully Qualified Domain Names (FQDN). Anyplace you might use a FQDN, such as in the location bar of Safari,  you can use a simple host name. If the system can’t resolve that host name to an IP address, then it will try appending the search domain to the host name and try again. If you specified more than one search domain, it will try them each in order.

If we set the search domain to, we can use the host names dev and local to access the corresponding websites. I didn’t opt for this solution because it introduces more complexity and an additional delay in setting up connections.

Network Diagnose Tools

There are several tools available for iPads to diagnose access problems in local networks:


iPad Network Analyzer : ping statistics – LAN overview


The following list provides links to websites with additional informations about accessing local virtual hosts :

Mac OSX Wireless Networks

Last update : September 17, 2015

AdHoc Wi-Fi Networks

Sometimes it’s useful to create a WLAN (wireless network) between two or more Wi-Fi-enabled computers without using an Wi-Fi router. These computer-to-computer networks are usually referred to as AdHoc wireless networks.


Create an ad-hoc Wi-Fi network on Mac OSX

On a Mac OSX (Yosemite 10.10.5) it’s easy to set up such a network. Choose Create Network from the Wi-Fi status icon in the menu bar. Enter a name (in my case : radiologic-open) for the new network and select the channel (default channel = 11). You will notice that there is no password protection to secure the network. A WEP (Wired Equivalent Encryption) algorithm was however available in previous versions of the OSX system. WEP was Wi-Fi’s first encryption standard and was almost intentionally designed to be weak due to issues related to the US export policies about encryption. WEP was deprecated in 2003 and replaced by WPA2 (Wi-Fi Protected Access) by the Wi-Fi Alliance.

In Yosemite, Apple killed WEP once for all, without replacing it by another protection method in AdHoc mode.


Chose name and channel for the AdHoc network

If the icon isn’t in the menu bar, choose Apple menu > System Preferences, then click Network (réseau). Click Wi-Fi and select the Show Wi-Fi status in menu bar checkbox.


Mac OSX System Preferences

The Wi-Fi status menu shows the established connection with the radiologic-open network. The menu allows an easy logout and reconnection to another Wi-Fi network. An automatic logout occurs when the Mac screen is powered off.


Wi-Fi status menu shows the AdHoc network active

Infrastructure Wi-Fi networks

There exist a second possibility to set up a Wi-Fi network on Mac OSX. The computer is configured as a software Wi-Fi base station. This allows to share an Internet connection and offers a password protection, but it has another inconvenience. You need a working wired Ethernet connection between your Mac and a hub, even if the hub is not connected to the Internet. The next picture shows my test setup: my MacBookAir is connected via a Ethernet-Thunderbolt interface to an ancient 10 Mbit/s Ethernet hub.


MacBookAir connected with Thunderbolt-Ethernet interface to hub

First you need to configure the Ethernet-Thunderbold interface. Go to Network in System Preferences and select the automatic configuration. The interface gets a self-assigned IP address in the range 169.254.x.x.


Ethernet-Thunderbolt automatic configuration

In the next step go to the Sharing Preference Pane in the System Preferences. Select Ethernet-Thunderbold as source port and Wi-Fi as destination port. Select Wi-Fi options to enter a name (in my case : radiologic-secure) of the network, to select a channel and to define a password (minimum 8 characters; numbers are not recognized on BlackBerry and Android).

Internet Sharing Wi-Fi network configuration

Internet Sharing Wi-Fi network configuration

Finally check the Internet sharing checkbox and confirm the settings to activate the connection.


Activate the Internet sharing

The resulting pane is shown below.


Activated Internet sharing pane

The Wi-Fi status is updated automatically. To log out, desactivate the Wi-Fi connection in the corresponding menu. An automatic logout occurs when the Mac screen is powered off.


Wi-Fi status menu showing Internet sharing

The following table shows which of my devices are capable to connect to the Mac OSX AdHoc and Infrastructure networks.

Device Wi-Fi AdHoc Wi-Fi infrastructure
iPad OK OK
iPhone OK OK
BlackBerry  network not shown network joined, but
no access to host
Samsung Tablet (Android 4.2.2) network not in range network joined, but
no access to host
Laptop Vista no connection network joined, but
no access to host
Desktop Windows 8.1 network not shown OK
Laptop Debian no connection OK

Wi-Fi Diagnostic

To detect the reasons why some devices don’t connect to the Mac OSX Wi-Fi AdHoc or Infrastructure networks, you can use a WLAN analyser. Mac OS X Yosemite has an in-built Wi-Fi scanner to help you find the best Wi-Fi channel. When you hold down the option key ⌥ (next to the CTRL key) and select the Wi-Fi icon in the menu bar, a secret dropdown menu opens.


Secret Wi-Fi dropdown menu in Mac OSX Yosemite

Open Wireless Diagnostics and click it. After you have gotten to the page and have opened up the Wireless Diagnostics window, go to the top left of your menu bar and click on Window where you find several options: Informations, history, scan, performance, detection, …  Some results are shown below.

Mac OSX Wi-Fi Analyser

Mac OSX Wi-Fi Analyser

Another WLAN tool is available on Windows computers. When you enter the command

netsh wlan show networks mode=bssid

in the command window, you will get the following results :


Microsoft Wi-Fi Analyser

A third WLAN tool is integrated in my Wi-Fi router FritzBox 7390. Various wireless networks in my neighborhood are shown, but nor the channels used by the Mac OSX nor the names of these networks are listed. I don’t know why ?


Wi-Fi channels used in proximity of the Fritzbox

There are also several external software tools for Wi-Fi-analysis available, for example Acrylic Wi-Fi Free or Acrylic Wi-Fi Professional. The main window (Access points,  Signal strength) of Acrylic is shown hereafter :

Acrlylic Wi-Fi Analyser

Acrlylic Wi-Fi Analyser

Some additional windows (stations, packet viewer, 2,4 GHz APs channels, detailed info) are shown below :

Various Acrylic results

Various Acrylic results

Wi-Fi Assessment

Several methodologies are available dealing with safety aspects of wireless networks, for example :


The following list shows links to websites providing additional informations about Wi-Fi networks, related to Mac OSX.