Color rendering: A tale of two metrics

It was the best measure of color rendering, it was the worst measure of color rendering. Color rendering index (CRI) is the most common metric used by the lighting industry to represent the color rendering properties of electric light sources. CRI was intended to characterize how "true" or "natural" objects appeared when illuminated by a light source, but was never intended to, for example, represent how well object colors could be differentiated under a light source, another important aspect of color rendering. Data presented here demonstrate that CRI in conjunction with another measure of color rendering, gamut area index (GAI), is useful at predicting subjective judgments of how "natural" objects appear as well as how "vivid" objects appear, and how well one can discriminate between subtle differences in hue. Neither measure by itself, however, is sufficient for meeting all of the expectations of a light source for providing good color rendering under all viewing conditions. It remains for future research to determine if just two metrics are sufficient to assure good color rendering from a light source and whether these two metrics (CRI and GAI) are the best for such purpose. In the meantime, CRI and GAI should be used jointly in recommendations as practical, useful, and mutually reinforcing measures of color rendering. The data presented here also demonstrate that total irradiance is important for good color rendering.