Color control 1361 Gamut

Like film, computer monitors and printers reproduce colors, not by regenerating the spectrum of the original light source, but simply by mixing three primary colors. This works only because the human eye has three types of color receptors. Creatures could exist - and indeed some human beings do exist - for whom the three-primary-color system does not work.3

By mixing primaries, your computer screen can stimulate the eye's color receptors in any combination, but not at full strength. That is, it has a limited color gamut. Colors outside the gamut can only be reproduced at lower saturation, as if they were mixed with white or gray. Nothing on your computer screen will ever look quite as red as a ruby or as green as an emerald.

3 Severely color-blind people have only two primary colors. There are also humans with normal color vision whose primary red is not at the usual wavelength, and it is speculated that a person who inherits that system from one parent and the normal system from the other parent could end up with a working four-color system.

The gamut of an inkjet printer is also limited, more so than the screen (especially in the deep blue), and the whole art of color management revolves around trying to get them to match each other and the camera.

13.6.2 Color space

To account for their limited gamut, digital image files are often tagged with a particular color space in which they are meant to be reproduced. The most common color spaces are sRGB, or standard red-green-blue, which describes the normal color gamut of a CRT or LCD display, and Adobe RGB, a broader gamut for high-quality printers. Photoshop can shift an image from one color space to another, with obvious changes in its appearance.

13.6.3 Color management

Fortunately, color in astrophotography is not as critical as in studio portraiture, and astrophotographers generally don't need elaborate color management systems. To avoid unpleasant surprises, it is generally sufficient to do the following:

• Install correct drivers for the video card and monitor.

• Adjust the monitor and video card driver using a test pattern such as the one at this book's web site ( or those provided by the manufacturer.

• Use your printer with the right kind of paper, or at least try a different kind if you're not getting good results. Different inks soak into glossy paper at different rates, and this affects color rendition.

• Recognize that vivid colors are often out-of-gamut and will look different on the print than on the screen.

When in doubt, make a small test print before committing a lot of paper and ink to a picture.

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