Color, Primary – a) The colors of three reference lights by whose additive mixture nearly all other colors may be produced. b) The primaries are chosen to be narrow-band areas or monochromatic points directed toward green, red, and blue within the Cartesian coordinates of three-dimensional color space, such as the CIE x, y, z color space. These primary color points together with the white point define the colorimetry of the standardized system. c) Suitable matrix transformations provide metameric conversions, constrained by the practical filters, sensors, phosphors, etc. employed in order to achieve conformance to the defined primary colors of the specified system. Similar matrix transformations compensate for the viewing conditions such as a white point of the display different from the white point of the original scene. d) Choosing and defining primary colors requires a balance between a wide color gamut reproducing the largest number of observable surface colors and the signal-to-noise penalties of colorimetric transformations requiring larger matrix coefficients as the color gamut is extended. e) There is no technical requirement that primary colors should be chosen identical with filter or phosphor dominant wavelengths. The matrix coefficients, however, increase in magnitude as the available display primaries occupy a smaller and smaller portion of the color gamut. (Thus, spectral color primaries, desirable for improved colorimetry, become impractical for CRT displays.) f) Although a number of primary color sets are theoretically interesting, CCIR, with international consensus, has established the current technology and practice internationally that is based (within measurement tolerances) upon the following: Red – x = 0.640, y = 0.330, Green – x = 0.300, y = 0.600, Blue – x = 0.150, y = 0.060. g) SMPTE offers guidance for further studies in improving color rendition by extending the color gamut. With regard to color gamut, it is felt that the system should embrace a gamut at least as large as that represented by the following primaries: Red – x = 0.670, y = 0.330, Green – x = 0.210, y = 0.710, Blue – x = 0.150, y = 0.060.