A 2 Colors, color matching, proofing - basic concepts
About
color modes / color spaces
Color is created through the interaction of a 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, then edit, and print an image, you must transform the scanned RGB values into CMYK values for the output device. This process is called
separation. If you want to get 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, i.e. 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 6 "Glossary") - you can transform the different sets of RGB values into one single Lab color definition. You will then have only one single input value for the transformation into CMYK. Fig.
A-7 and
A-8 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-7:
Color transformation without Lab values + profiles
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Fig. A-8: 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 ensure that deviations are as small as possible. This can be achieved by gamut mapping (see A 6 "Glossary").
PDF HandShake 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. PDF HandShake uses ICC based profiles for color matching. Some profiles are even included in the software package and are automatically copied to your server during software installation.
All the color data that can be matched for a printer can also be matched for a proof. A proofer is a printer which is used to simulate the output results of another printer or a press. 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 proofer. To guarantee a precise simulation, the gamut of the proofer should not be smaller than that of the printer because there is no gamut mapping from printer to proofer. Fig.
A-9 shows how the color data is transformed if you print to a proof.
Fig. A-9: Transformation of color data during proof printing
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With PDF HandShake, color matching must be activated separately for every 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. Fig.
5 in
5.2.2 "Printer queue settings for PDF", for instance, shows the settings necessary to simulate a newspaper press on a color LaserWriter.
