Standard Illuminant

A Standard Illuminant is a theoretical source of visible light with a very specific, published spectrum. Standard illuminants provide a basis for comparing images or colors recorded under different lighting. For example, most colorblind tests are specified to be used under a standard illuminant. The HRR, Ishihara and D15 all require a specific illuminant.

Any color test using reflected light is dependent upon the spectral composition of the illuminant used. The critical hues employed in the HRR Plates were determined under a close approximation to C.I.E. source C. It is therefore mandatory that the test be administered under this illuminant or a close approximation of it. The intensity of illumination should be between 10 and 60 foot-candles. We recommend the Good-Lite® Daylight Illuminator (612600) which was designed to provide proper lighting. Direct light from tungsten or other fluorescent lamps in the room or outside windows should be prevented from reaching the test plates. Unless these precautions as to illumination are observed, use of the HRR plates either for screening or for qualitative and quantitative diagnosis will not yield accurate results

HRR Instructions (1954)

Illuminants A, B, and C were introduced by CIE in 1931, with the intention of respectively representing average incandescent light, direct sunlight, and average daylight. In 1967, a series of Illuminants D were released to represent phases of daylight, thereby obsolescing Illuminants B & C. However, many popular color vision screening tests are out of print and “modern” versions still specify a type C Illuminant.