This International Standard specifies two illuminants for use in colorimetry. a) CIE standard illuminant A This is intended to represent typical, domestic, tungsten-filament lighting. Its relative spectral power distribution is that of a Planckian radiator at a temperature of approximately 2856 K. CIE standard illuminant A should be used in all applications of colorimetry involving the use of incandescent lighting, unless there are specific reasons for using a different illuminant. b) CIE standard illuminant D65 This is intended to represent average daylight and has a correlated colour temperature of approximately 6500 K. CIE standard illuminant D65 should be used in all colorimetric calculations requiring representative daylight, unless there are specific reasons for using a different illuminant. Variations in the relative spectral power distribution of daylight are known to occur, particularly in the ultraviolet spectral region, as a function of season, time of day, and geographic location. However, CIE standard illuminant D65 should be used pending the availability of additional information on these variations. Values for the relative spectral power distribution of CIE standard illuminants A and D65 are given in Table 1 of this International Standard. Values are given at 1 nm intervals from 300 nm to 830 nm. The term "illuminant" refers to a defined spectral power distribution, not necessarily realizable or provided by an artificial source. Illuminants are used in colorimetry to compute the tristimulus values of reflected or transmitted object colours under specified conditions of illumination. The CIE has also defined illuminant C and other illuminants D. These illuminants are described in Publication CIE 15:2004 but they do not have the status of primary CIE standards accorded to the CIE standard illuminants A and D65 described in this International Standard.