A new metric for evaluating the color quality of light sources is being developed at NIST, in close contact with the lighting industry and the CIE. The current CIE Color Rendering Index (CRI) is outdated and has several known deficiencies. The CRI only evaluates the fidelity of object colors compared to those under a standard illuminant, with no regard to color preference, which is certainly important for end users. In the past, additional indices were separately proposed to provide information about other aspects of color quality, such as preference and discrimination, but have not been used. The new metric being developed not only addresses the deficiencies of the CRI but also incorporates evaluation of chromatic discrimination and color preference. This is an attempt to express overall color quality of a light source by one number, thus given the name Color Quality Scale (CQS). The CQS uses the same approach as the CRI (color sample method), but incorporates a number of improvements. The outdated uniform color space is replaced by CIELAB. The eight reference samples (all of medium chromatic saturation) are replaced by 15 saturated color samples to address problems with evaluating solid-state light sources. Further improvements are made to incorporate a CCT factor and a RMS scale. The most important change, however, may be one that takes the direction of color shifts of the samples into account. The CRI penalizes lamps for any direction of color shift, whether chroma is increased or decreased. It is known that increased object chroma is generally preferred (to the extent it is not excessive), and also has positive effects in color discrimination, visual clarity, and perceived brightness. Neodymium-glass lamps are a well-known example, and three-band sources (triphosphor lamps and RGB white LEDs) may also have such effects. To take these effects into account, the CQS is currently formulated in such a way that color shifts with increased chroma are not penalized. The details of the formula are to be determined based on the results of vision experiments, which are currently being prepared. We propose that such a metric as the CQS may be useful in color communication of lamp products.
Light and Color
February 5-8, 2006
Lighting Research Office Symposium
color rendering, color rendering index, LED, solid-state lighting
and Ohno, Y.
Development of a Color Quality Scale, Light and Color
(Accessed November 30, 2023)