A 2 Colors, color matching, proof printing - basic concepts
Color is created through the interaction of light, an object, and the eye. The "visible spectrum" contains millions of colors.
Screens and monitors produce colors by means of red, green and blue light (RGB). The light intensities make up a given color. Scanners also work with RGB colors. They read the amounts of red, green, and blue light that are reflected from an image (or transmitted if you scan transparent images). The RGB color space is smaller than the visible spectrum of light. RGB colors are device dependent, they vary with scanner or monitor characteristics.
Color printing is based on the CMYK color space. Cyan, Magenta, Yellow, and Black inks are mixed on paper to produce a given color. The CMYK color space is even smaller than the RGB color space. CMYK colors vary with printer, ink, and paper characteristics.
The CIE (
Commission Internationale de l'Eclairage) created different color spaces that specify colors in terms of human perception. One example is the CIE-Lab color space. Lab colors are device independent.
If you want to scan, edit, place, and print an image, you have to transform the scanned RGB values into CMYK values for the output device. This process is called separation. If you want to have predictable color results, you must have calculation models that match RGB to CMYK during separation. As RGB and CMYK values are device dependent, you can never exactly define a color. E.g. scanning one particular color with three different scanners will produce three different sets of RGB values. Thus, you will have different input values for the transformation into CMYK. You must have a specific transformation table for every possible scanner-printer combination. If you have a device independent color space like Lab, you can exactly define the color you scanned. Considering the color characteristics of the different scanners - which are described in the device profiles (see
ICC, ICC profiles in
A 5 "Glossary") - you can transform the different sets of RGB values into one single Lab color definition. Now, you have only one single input value for the transformation into CMYK. Fig.
A-2 and
A-3 illustrate the effect of a device independent color space (Please note that you may as well have an RGB device on the output side, e.g. an imagesetter).
Fig. A-2: Color transformation without Lab values + profiles
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Fig. A-3: Color transformation with Lab values + profiles
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Note: Color matching does not always lead to precise reproduction. Some Lab or RGB colors cannot be printed - some can, but not by every given printer. The color matching module has to make sure that deviations are as small as possible. This can be achieved by gamut mapping (see A 5 "Glossary").
ImageServer is able to transform colors from different color spaces like RGB and CMYK into Lab values and vice versa. Thus, a device independent color space is used for the interchange of color data. The color matching module that is available with ImageServer uses ICC based profiles for color matching.
With ImageServer, the color spaces of single images may be changed as necessary for layout generation and printing. The transformations, however, do not affect the original high-resolution images. These images remain unchanged in terms of color space and file format.
All the color data that can be matched for a printer can also be matched for a proof printer. A proof printer is used to simulate the output results of another printer or a press. The color data transformation and gamut mapping are accomplished for the printer you want to use as final output device (e.g. an offset press). The resulting CMYK values are then transformed into the specific CMYK values of the proof printer. To guarantee a precise simulation, the gamut of the proof printer may not be smaller than that of the printer because there is no gamut mapping from printer to proof. Fig.
A-4 shows how color data are transformed if you print to a proof.
Fig. A-4: Transformation of color data during proof printing
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With ImageServer, color matching must be activated separately for each printer queue. You have to define a printer profile for your output device and, optionally, a proof profile if you want to simulate your output results on a proof printer. Fig.
10 in
4.4 "OPI printer queue settings" shows the required settings for simulating a newspaper press on a color LaserWriter.
