HP LP2480zx Digital Color Workflows and the HP DreamColor LP2480zx Professiona - Page 4

green light produces a reddish-purple hue (red + blue, or magenta), while subtracting the red results in cyan, a combination of blue and green. Thus, the cyan-magenta-yellow set comprises the subtractive primaries-typically used in printing. However, since real-world subtractive primaries don't really absorb all the light at any given wavelength, combining the three doesn't generally give a very good black. So, black ink is commonly added to the C-M-Y set, resulting in "CMYK" printing. ("K" is used for black, since "B" was already taken for the blue in the RGB set). Color spaces Over the years, many different ways of quantifying color have been developed. Generally, a model that represents how color is represented is known as a color space. Different devices and applications can (and do) use different color spaces. In 1931, the International Commission on Illumination (or CIE as it's known by its French initials) published a specification that described standardized curves based on the sensitivity of normal human vision vs. wavelength for the three types of receptors in the eye. These are known as the CIE 1931 color-matching functions, and they lead directly to a space defined by three standardized primaries known as X, Y, and Z. A simplified "two-dimensional" model was derived from the XYZ system, and is referred to as the CIE xy color space, or more correctly the xy chromaticity coordinates or chromaticity diagram. The CIE 1931 system takes into account humans' response to different wavelengths of light, and has become a standard in the electronics industry for specifying the ability of an electronic device to reproduce color. However, the CIE 1931 chart is not a perceptually uniform color space. In other words, the same change in distance on the xy chart in two different regions generally does not mean the same change in perceived color as a human eye would see it. In 1976, a system similar to CIE 1931 was developed that partially corrects for the non-uniformity of CIE 1931-it is called the L*u*v* color space, and has an associated u'v' color chart (Figure 2; the "color gamut" is discussed in the next section). The challenge (as we will explore) is mapping color spaces over a range of input devices, applications, and output devices while maintaining (or supporting) color range accuracy and predictability. 4