A) The colours of three reference lights by whose additive mixture nearly all other colours 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 colour space, such as the CIE x, y, z colour space. These primary colour points together with the white point define the colorimetry of the standardised 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 colours 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 colours requires a balance between a wide colour gamut reproducing the largest number of observable surface colours and the signal-to-noise penalties of colorimetric transformations requiring larger matrix coefficients as the colour gamut is extended. e) There is no technical requirement that primary colours should be chosen identical with philtre or phosphor dominant wavelengths. The matrix coefficients, however, increase in magnitude as the available display primaries occupy a smaller and smaller portion of the colour gamut. (Thus, spectral colour primaries, desirable for improved colorimetry, become impractical for CRT displays.) f) Although a number of primary colour sets are theoretically interesting, CCIR, with international consensus, has established the current technology and practise 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 colour rendition by extending the colour gamut. With regard to colour 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.