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Adobe RGB is not a Monitor Profile

I've written a lot already about color management, but I wanted to post a brief article this week on the difference between device profiles such as those for monitors, printers and scanners, and device-independent RGB profiles such as Adobe RGB or sRGB.

At the heart of a Color Management System (CMS) is a color space known as the Profile Connection Space (PCS). This is either CIELAB or CIEXYZ, both abstract color spaces that can represent colors far beyond most "normal" color spaces. Both are based on research into human color perception but only CIELAB is perceptually uniform, a property that specifies that colors perceptually differ in proportion to how far apart their coordinates are in the color space. That is, similar colors in CIELAB are close together while colors that appear to differ more are proportionally further apart. The ICC spec allows either to be used as the PCS, but CIELAB is more common.

Neither CIELAB nor CIEXYZ though specify colors in terms of red, green and blue values, so neither is an RGB color space at all. sRGB, Adobe RGB and all other RGB color spaces can't be Profile Connecting Spaces. In fact, when the International Color Consortium invented the ICC specification, they weren't thinking about sRGB and Adobe RGB at all.

What the ICC spec does define is a variety of profile classes, the main ones being Display (monitor), Input (scanner, camera, etc.) and Output (printer). Every different kind of physical device has a different response to color. Some can display a broader range of colors than others and even those with similar gamuts may vary as to precisely what color that produce for any given input. Monitors are notoriously problematic in that their phosphors age and weaken, altering their ability to produce color, and not necessarily evenly across all three channels leading to color casts. Scanners and printers tend to drift less over time but still do vary greatly between various models. What all this means is that the profile I use for my monitor won't work for your monitor even if we can reasonably share the same profile if we both print on the same model printer and use the same paper and ink.

And this is where Adobe RGB and sRGB come in. If you and I want to share files with any ease, we need a common way to describe color. And if either of us upgrades our monitor or printer in the future I feel certain neither of us wants to have to re-do all our images so they look right again. What these device independent RGB color spaces give us is a level playing field, now and in the future.

In the ICC specification, such profiles are technically Display profiles, only because they had to fit them in somewhere. Input profiles can only be used to convert from the device's color space to the PCS while Output profiles only provide for the Color Management System to convert to them. Only Display profiles define enough information that a CMS can convert both to and from the described color space. When you display an existing image on your monitor, your computer has to convert to the monitor's color space. When you adjust an image by viewing it on screen until it looks the way you want it, you are asking your computer to convert from your monitor's color space. But just because Adobe RGB is, strictly speaking, a Display profile, it should not be thought of as an actual monitor profile. At best, you could think of such device independent RGB profiles as being for theoretical "ideal" monitors. They are essentially useless use on real world monitors. As mentioned, monitors are all different to begin with, and they get more so with age. No one profile could possibly work for all of them.

Profile conversion flow through a Color Management System (CMS)

Adobe RGB though does make a great document working space as it lets us take as given that we can correctly display each others files. When displaying an Adobe RGB image on your monitor, a Color Management System will convert from the document space to its Profile Connection Space and straight on to your monitor color space. On my computer it will use my monitor profile, and on your computer it will use yours. Each of us sees the image correctly.

So, don't be tempted to also use Adobe RGB as your monitor profile (or your printer profile) just because you have Photoshop set to use it as your working space. You need a real monitor profile. As an easy solution, you can use Adobe Gamma on Windows or ColorSync on Mac OS. Better yet, invest in a hardware monitor profiling solution such as those from Monaco or Gretag Macbeth.

Date posted: October 16, 2005


Copyright © 2005 Bob Johnson, Earthbound Light - all rights reserved.
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Color Management: Monitor Profiling

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