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10.4 COLOR MANAGEMENT | ||
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As images pass from scanners to screens, and then to printers or Web pages, the colors shift about because each device has it's own unique way of defining and displaying them. For the same reason, images edited on your screen, posted to the Web, and then viewed on your screen look different. They also look different if you print the Web page on your printer and someone on the other side of the world prints it on theirs. To try to make colors more consistent across a range of devices, color management systems are used. However, even then colors will never match perfectly for a number of reasons. Some of these include the following:
- The screen and printer use different color systems—RGB on the screen and CMYK on the page. RGB produces colors, not with pigments or inks, but by combining red, green, and blue light sources. CMYK reproduces colors by combining pigments or inks. The results are viewed in reflected light. When you print and image displayed on the screen, the RGB must be converted to CMYK and that process isn't perfect.
- Experienced photographers know that slides have more contrast and color richness than prints do. This is because slides are viewed by transmitted light and prints by reflected light. The same is true of a display monitor and printout. An image displayed by transmitted light on the monitor is going to be better than one viewed by reflected light on a piece of paper.
- Displays don't have to use halftoning to create colors because they can vary the intensity of color at each pixel. (The only printer type that can do this is the dye-sub.) Each pixel contains three phosphors—one each for red, green and blue. To display a red object, the monitor uses an electron gun to "turn on" the red phosphors in the appropriate area. Turning on all the phosphors produces white. To create shades of color, the monitor's electron gun can be controlled in eight steps for each of the three phosphors, for a total of 24 steps for each pixel.
To make your prints more closely match what you see on the screen, you can make a printout and then use it as a guide when adjusting the screen colors. However, you may now be looking at an image on the screen that you don't like. In some cases, it's hard to make the mental adjustment required to edit the "false" colors on the screen. To get serious about matching screen to print colors, you need a color management system (CMS). Without such a system, colors change as they move from stage to stage in the imaging process.
Color management systems are designed to keep the colors of your images as consistent as possible through capturing or scanning, displaying, and printing. They specify colors in terms of an objective, device-independent standard rather than in device-dependent terms such as RGB or CMYK. These color management systems include:
- Microsoft Image Color Management 2.0 (ICM) is based on Linotype-Hell's LinoColorCMM (Color Management Module) already used in Apple's ColorSync. This means color consistency is achievable in NT and mixed Windows and Mac environments.
- Kodak Digital Science Color management System.
- CIELAB (The Commission Internationale de L'Eclairage) also known as L*A*B.
- HiFi Color
- Kodak's YCC color space is based on a 1950s TV color model. It can be easily converted to other color models with varying loses of color values.
- The PANTONE® Matching System.
- PostScript Level 2 which uses the international color standard known as CIE XYZ, developed by the Commission Internationale de l' Eclairage (International Commission on Illumination).
- EfiColor™ from Electronics for Imaging, Inc. (EFI)
- ColorSync™ from Apple Computer, Inc is now supported by Microsoft's Internet Explorer 4 so color management now extends to the Web.
- ColorSense™ from Kodak.
As a photographer, you've seen colors change as the source of the light changes. It even changes out of doors as the sun makes it's arc across the sky. If colors change so easily, how then do we get an absolute handle on them? We do so, by measuring them under very controlled conditions and assigning numbers to them. The first such system was the CIE color system developed in the 1930s. Colors are read by colorimeters (color light meters) and plotted on a chromaticity diagram. This assignment of numbers to specific colors and plotting them on a chart is a color model.
Once a color model has been created to specify colors, only part of the job is done. That's because different systems use different models. For example, your monitor is based on a RGB color model and your color printer is based on a CMYK model. A device color profile is used to relate different color models such as these. Here's a simple table that shows how five basic colors can be related using tables. For example, when a red color on the screen is sent to the printer as the series of numbers 255,0,0; the printer uses the profile to see that it should assign the color 0, 100, 100,0.
