Color rendering of light sources

A Color Quality Scale (CQS) is being developed at NIST with input from the lighting industry and the CIE (International Commission on Illumination) to address the problems of the CIE Color Rendering Index (CRI) for solid-state light sources and to meet the new needs of the lighting industry and consumers for communicating color quality of lighting products. The method for calculating the CQS is based on modifications to the method used for the CRI. Although simulations support the appropriateness and usefulness of the proposed CQS, vision experiments will be conducted to improve and validate the CQS. It will then be proposed as a new international standard.

Color Rendering Index vs. Color Quality Scale

The color appearance of objects under artificial lighting is an important characteristic of light sources, particularly to consumers. Currently, the only internationally-accepted assessment procedure for evaluating the color appearance of objects under light source illumination is the CIE color rendering index (CRI).

Briefly, in the calculation of the CRI, the color appearance of 14 reflective samples is simulated when illuminated by a reference illuminant and the test source. After accounting for chromatic adaptation with a Von Kries correction, the difference in color appearance ΔE_i, for each sample, between the test and reference illumination, is computed in CIE 1964 W*U*V* uniform color space. The special color rendering index (R_i) is calculated for each reflective sample by:

R_i = 100 - 4.6ΔE_i

The general color rendering index (R_a) is simply the average of R_i for the first eight samples (shown in Figure 1), all of which have low to moderate chromatic saturation:

 

A perfect score of 100 represents no color differences in any of the eight samples under the test source and reference illuminant.

Unfortunately, the CRI only evaluates color rendering, or color fidelity, and ignores other aspects of color quality, such as chromatic discrimination and observer preferences. The Color Quality Scale (CQS) is designed to incorporate these other aspects of color appearance and address many of the shortcomings of the CRI, particularly with regard to solid-state lighting.

Below is a description of several of CRI's shortcomings and the CQS's approach to addressing the shortcomings.

Problem: The set of reflective samples used to calculate the R_a in the CRI contains too few samples and have too low chromatic saturation.

Solution: The CQS uses a set of 15 Munsell samples, as shown in Figure 2. These samples have the much higher chroma, span the entire hue circle in approximately even spacing, and are commercially available.

Problem: The uniform object color space (1964 W*U*V) used by the CRI to calculate sample color differences is outdated and no longer recommended.

Solution: The CQS is calculated using the CIE 1976 L*a*b (CIELAB) uniform object color space, which is more uniform and currently recommended by CIE.

Problem: The CRI penalizes lamps for shifts in hue, chroma (chromatic saturation), and lightness, in all directions, of the reflective samples under the test source (compared to under the reference illuminant).

Solution: The CQS will only penalize a lamp's score for hue shifts, lightness shifts, and reductions in chroma. Lamps that increase object chroma relative to the reference illuminant are not penalized because these positive effects are generally preferred. This is shown in Figure 3.

Problem: Because the CCT of the reference illuminant is matched to that of the test source, the CRI score can be perfect (R_a = 100) for reference illuminants of any CCT, even though actual color rendering is degraded at extreme CCTs.

Solution: The CCT of the reference source is matched to that of the test, but a multiplication factor, the CCT factor, is included to penalize sources with extreme CCTs.

Problem: The CRI uses the Von Kries chromatic adaptation correction in its calculation. It is outdated.

Solution: The CQS uses the Colour Measurement Committee of the Society of Dyers and Colourists Chromatic Adaptation Transform of 2000 (CMCCAT2000).

Problem: The color differences for all reflective samples in the CRI are averaged, enabling a high score despite poor rendering of one or two colors.

Solution: To ensure that large hue shifts of any sample have notable influence on the CQS, the root-mean-square of color shifts of all of the individual samples is used.

Problem: The CRI can result in negative values for some lamps.

Solution: The CQS imposes a 0-100 scale.

Testing the CQS

Many computational simulations have been performed and, at the level of subjective visual impression, appear to confirm the ideas used in the CQS. However, a series of thorough and well-controlled vision experiments are necessary to test, improve upon, and validate the computational analyses. Experiments testing observers' chromatic discrimination and hue perception of illuminated objects will be complemented by subjective rankings of naturalistic scenes. Current experiments are also testing the relationship between illuminance and object chroma. Since the CQS is intended to be a metric of overall color quality, the data from several types of experiments will be used to assess and improve its performance. Experiments are currently being conducted on color quality and the CQS using the Spectrally tunable lighting facility.

The CQS is being proposed to CIE TC 1-69 "Colour Rendition by White Light Sources." This committee is tasked with recommending a new assessment procedure for evaluating the color rendering/quality properties of light sources.