Farvestyring - hvad er ICC profiler | Thomas Holm | Pixl Aps

Farvestyring - hvad er ICC profiler

Farvestyring og brug af ICC profiler
Farvestyring og det at bruge ICC profiler (ICC, colorsync eller ICM profiler) kan virke svært og uoverskueligt, men her kan du finde en guide (på Engelsk) som i helt basale termer forklarer hvad ICC profiler og farvestyring fungerer og hvordan man bruger ICC profiler.

Devices are different
All digital devices are different. Most people know that monitors and printers are not alike, but even similar devices of the same brand can, and often will be, different from each other. Even when brand new. They intepretate or reproduce color differently. The easy way to see this for yourself is to visit a large TV shop and have a look at their wall of TV’s. Even if all are showing the same program, with the same input for all devices, none of the screen images will be exactly alike.

The same thing frequently happens with identical image data on two different printers, scanners, digital cameras or monitors. Same data, different device characteristics.

Colourspaces and ICC profiles
The reason devices are different is because they all have different color spaces or gamuts. That means that the devices aren’t all able to display the same colors. They have no concept of which colors they can or can’t display, and have no idea of how the colors will actually look when reproduced.

RGB devices like monitors are superior at displaying Red, Green and Blue colors, because they work by projecting light on a dark surface (sort of anyway).

They are, however, generally not too good at reproducing pure Cyan, Magenta and Yellow.

Printers, on the other hand, are very good at reproducing Cyan, Magenta and Yellow, since they use pure inks of those exact colors. When it comes to reproducing, say, a blue color, then the printer can only render this, by mixing Magenta and Cyan ink on paper, and this can’t possibly render a blue which is as bright and clear as the blue the monitor can display.  The same problem persists with Red and Green colors.

Alas, there is a great deal of difference between the colors which monitors and printers can show.

To be able to predict how a color will look on a monitor (or a printer, or other devices), we have to make a characterization of each individual device, and save this as a profile. This profile describes exactly how each device reproduces color, compared to a known standard (Lab color is the standard).

Colourspaces and Language
As an analogy color spaces can be compared to languages. One could argue that each single device speaks a different language or dialect.

If we stick with that analogy, and the job at hand requires us to translate text from one language to another, we need 3 things:

1). We need to know which language the text is currently in.
2). We need to know which language the text should be translated to.
3). We either need a dictionary that translates directly from one language to another,Or,
4). We need two different dictionaries, one that translates from our original language to a reference language and one that translates from that reference language to the destination language.
Nr. 4 is how ICC profiles work, using the Lab colour space as the reference “language”.

So in order  to know exactly which language our monitor scanner, camera or printer “understand” or “speak” we need to characterize each device.

That is, we need to create a dictionary between our reference language (Lab color) and our monitor, so we can translate color between our reference space and our device. This characterization or dictionary is saved as an ICC profile.

If we have a “monitor  profile”, a profile describing the characteristics of a particular monitor, we can easily translate colour data from our source color space (say, Adobe RGB (1998)), via the reference space [Lab] to the monitor space. If the monitor profile is accurate, we will see a true representation of our image content on screen.

Source to destination
Thus to be able to rely on our monitor we require two profiles. One that describes the colour space our image is in, (workingspace to Lab), and one that describes our monitor (Lab to monitor). When we have these two profiles, the computer is able to make color data conversion and the colors we see on the monitor will be correct. if we don’t have an accurate monitor profile and a profile defining our source, there is no way we will ever see anything correctly on the monitor.

If we have another ICC profile which describes how our printer (inkjet, laser or even a printing press) prints color, we can be pretty sure that whatever we print, will bear a close resemblance to what we see on the monitor.

An ICC profile is nothing more than a descripon of how a single device displays colour.

If you have ICC profiles which describes all your different digital devices, and CMYK printing processes, then you can easily convert between devices, limiting loss of color to a absolute minimum, and with great reliability. Just think about it as if you need to translate text from, say Danish to German. All you need is a dictionary. In an ICC workflow the system would use say, Danish to Esperanto and Esperanto to German, thus making Esperanto the reference language - which for images is called Lab Color.

With the proper ICC profiles you can do some very interesting things:
• When working in (Colorsync savvy) camera or scanner software, you can use ICC profiles (of workingspace, monitor and source) to give you continuity of appearance between the way the camera or scanner software displays an image, and the appearance of that image in Photoshop. This means that you can use the colour and tonal editing tools in the scanner or camera software. And do so confident in the knowledge that what work you have done on image optimisation will still be correct when you later open the image file in Photoshop.
• In your scanner, camera, or image manipulation software you can preview or “Softproof” a CMYK printing press output, by restricting the color space of your monitor to that of an offset printer, as long as you have a good profile of the printing press and monitor.
• On your desktop printer you can simulate or “proof “ how the final printed image will look, by restricting the gamut of your desktop printer to that of an offset printer.
• On your monitor you can simulate or softproof your desktop printer output.
As mentioned earlier you can’t  reproduce the exact same colors on a monitor as you can on a printer, but with the proper profiles, you can get very very close.

Now you know how color management works - Really! 
Implementing it, is simply a question of obtaining profiles for your devices, and setting up your applications correctly - have fun...

© Thomas Holm / Pixl Aps - 2004 

Pixl Aps · Filmtorvet 4. · 3500 Værløse  ·  Danmark · Tlf: (+45) 32 96 90 14 · www.pixl.dk