Development of a Color Quality Scale

A Color Quality Scale (CQS) is being developed at NIST, which evaluates several aspects of the quality of the color of objects illuminated by a light source. This metric involves several facets of color quality, including color rendering, chromatic discrimination, and observer preferences. The method for calculating the CQS is derived from modifications to the method used in the CIE's Color Rendering Index (CRI). The current CRI is based on only eight reflective samples, all of which are low to medium chromatic saturation. These colors do not adequately span the range of normal object colors. Some lights that are able to accurately render colors of low saturation perform poorly with highly saturated colors, particularly the peaked spectra of light-emitting diodes (LEDs). Instead, 15 Munsell samples were assembled that overcome these problems and are used for calculations of the CQS. Additionally, the CRI penalizes lamps for showing increases in object chromatic saturation compared to reference lights, which is actually desirable for most applications. To incorporate observer preference, we propose a new computation scheme for determining the color rendering score that differentiates between hue and saturation shifts and takes their directions into account. The uniform color space used in the CRI is outdated, so CIELAB is used in the CQS. The CRI matches the CCT of the reference to that of the test light, which can be problematic when lights are substantially bluish or reddish. Lights of extreme CCTs frequently give poor quality and smaller color gamuts (thereby decreasing color discrimination), so a system is implemented in the CQS to penalize such lights. Simple averaging of the calculated color differences, as is done in the CRI, can disguise large color shifts on a small number of samples. To ensure that large shifts of any color are adequately reflected in the score, color differences are combined with a root-mean-square (RMS). Finally, an appropriate scaling factor is chosen and the scale is made to span 0-100. Introduction The quality of object color under artificial lighting is an important aspect of the value of the light sources, particularly to consumers. Assessing and quantifying this dimension of lamps is complicated. The attribute of color rendering of light sources is often interpreted as indicating object color quality. However, color rendering is actually defined as the " effect of an illuminant on the color appearance of objects by conscious or subconscious comparison with their color appearance under a …