Any group of two or more (three, in most applications) discrete colors (optical wavelengths) that are (a) of relatively narrow spectral width, and (b) visible to the normal human visual system (eyes and nervous system, including the brain,) the additive mixture of which colors may be perceived by humans with normal color vision as any of a theoretically infinite number of other colors lying within the bounds defined by the positions of the discrete colors on a chromaticity diagram, e.g., the familiar 1931 CIE (Commission Internationale de l'Eclairage; International Commission on Illumination) chromaticity diagram. Note: Additive mixing consists of the combining (simultaneous presence, or emission) of two or more discrete optical wavelengths (spectrally narrow sources. ) In practice, this may be accomplished by various means, including (a) large numbers of contiguous discrete emitters that are too small to be resolved individually by the eye, such as the microscopic groups of dots, or stripes, of phosphors in a color CRT, and (b) spectrally narrow beams projected onto a common area on a diffusely reflective screen, as in certain projection color television displays. When additive mixing takes place, the human observer may perceive colors that are not actually present as represented by spectrally pure, discrete wavelengths. The actual number of colors that may be perceived in practice is limited by subjective discernment on the part of the individual observer. With respect to a given display system, the range of colors that may be represented to the observer is defined by the area (e.g., triangle) enclosed by straight lines connecting the primary colors on the chromaticity diagram. (In the case of only two primary colors, the range of perceived colors lies along the line connecting the primaries on the chromaticity diagram. ) Colors lying outside this area on the chromaticity diagram cannot be represented, and thus may not be perceived by the human observer. Three is the number of primary colors usually chosen for electronic color displays. Three properly chosen primary colors can produce in the standard observer the perception (illusion) of an esthetically satisfactory approximation of the range of colors that might otherwise be perceived through observation of (a) a monochromatic source as it is swept in frequency (wavelength) throughout the entire visible range, or (b) sets of wavelengths selectively absorbed and reflected by objects in the observer's environment. The additional cost and technical complexity that would be involved in using four or more primary colors to facilitate perception of the remaining unperceived colors is not justified. (Actually, not all of the remaining colors would be perceived; there would be only a relatively insignificant improvement over three primaries. ) The area on the chromaticity chart bounded by the three selected primary colors is sometimes referred to as a color triangle. 2. In color television or display technology, the three specific colors (the reddish-orange, the green, and the bluish-purple) chosen to define (standardize) the colorimetry of color displays. 3. An analogous group of colors represented by pigments, the subtractive mixing of which creates the perception of other colors. Note: Subtractive mixing (which has application in printing, including colored hard copies from a computer) is distinguished from additive mixing. Subtractive mixing takes place when pigments are mixed. The perceived color is a function of selective reflection, and hence, selective absorption, by the pigments. An example of subtractive mixing is the familiar mixing of blue and yellow pigments to produce the perception of green. Synonym principal colors.