Volume 8 Issue 1

October 2004

Appendix B: Calculating color rendering metricsThis appendix discusses four major color rendering metrics: color rendering index (CRI); fullspectrum index (FSI); fullspectrum color index (FSCI); and color gamut area (GA). Each color rendering metric emphasizes a slightly different aspect of color rendering. Color rendering indexColor rendering index is a measure of a light source's ability to show object colors "realistically" or "naturally" compared to a familiar reference source, either incandescent light or daylight. CRI is calculated using eight reference samples in CIE 1995 color space (Technical Report No. 13.31995) when illuminated by a given light source (Boyce 2003). The CIE 1995 color space is used because equivalent distances in this color space are assumed to be "perceptually equal." Fullspectrum indexFullspectrum index (FSI) is a mathematical measure of how much a light source's spectrum deviates from an equalenergy spectrum. The following is a stepbystep procedure for calculating FSI:
Fullspectrum color indexFullspectrum color index (FSCI) is a mathematical transformation of fullspectrum index into a zero to 100 scale. The resulting values compare directly with color rendering index. FSCI is a variant of FSI that has an inverse scale starting at 100 and scaled so that a warm white fluorescent lamp has a value of approximately 50, and any values less than zero (e.g. monochromatic light) are set to zero. FSCI is calculated as follows:
Gamut AreaGamut area (GA) is more commonly used in Japan than in North America. In principle, GA is defined as the area enclosed within three or more chromaticity coordinates in a given color space. GA is usually calculated from the area of the polygon defined by the chromaticities of eight CIE standard color samples in CIE 1995 color space (Technical Report No. 13.31995) when illuminated by a given light source (Boyce 2003). The CIE 1995 color space is used because equivalent distances in this color space are assumed to be "perceptually equal." In general, the larger the GA, the more saturated the object colors will appear.



