Color Quality Scale

LED and color lighting measurement - CRI

A newer system to "rate" or "classify" color quality for LED is needed and in the works. Color Quality Scale will be the definitive  and modern test for LED lighting color quality.
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The LED Show Las Vegas Rio Hotel
CIE 1960 UCS. Planckian locus and co-ordinates of several illuminants

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A Color Quality Scale is needed to define and better qualify LED lighting - CQS

The current eight special color rendering indices are simply averaged to obtain the general color rendering index. This makes it possible for a lamp to score quite well, even when it renders one or two colors very poorly. LEDs are at an increased risk of being affected by this problem, as their peaked spectra are more vulnerable to poor rendering in only certain areas of color space.

LED Lighting Source

One of the main issues with CRI is that it averages 8 colors (which have a low to medium chromatic saturation) to obtain a ranking. This means that even if a lamp renders a few colors poorly, the CRI can still remain high, as long as those poorly rendered colors are not one of the 8 colors that are averaged.

CRI color scale

Image: Color Rendering Index color palette

CQS, on the other hand, considers a number of factors in trying to define the way a light source reproduces colour. These include chromatic discrimination, human preference, and color rendering (the method evaluates 15 colors to more accurately span the range of normal object colors).

CQS Color palette
Image: Color Quality Scale colors

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. See entire paper here.

Color Quality Scale is a quantitative measure of the ability of a light source to reproduce colors of illuminated objects. Developed by researchers at NIST Development of a Color Quality Scale, the measure is a possible answer to the criticism of the widely used color rendering index. The output of this Color Quality Scale (CQS) is a composite score incorporating a lamp's ability to accurately render object colors, permit precise discrimination between different colors, and display object colors in a way that is visually pleasing to typical consumers. Visual experimentation will be vital to improve and validate this method, which was initially developed with colorimetric simulations. Preliminary experimentation has begun, focusing on the issues most relevant to the development of commercial standards for color quality.

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.

One of the major deviations that the CQS takes from the formal definition of color rendering is evident in the saturation factor. The CRI penalizes lamps for shifts in hue, chroma (chromatic saturation), and lightness, in any direction, of the reflective samples under the test source (compared to under the reference source). While a decrease in chroma always has negative effects, an increase in the chroma of objects is considered desirable in many cases. Increases in chroma yield better visual clarity and enhance perceived brightness. These are positive effects and are generally preferred, though they cause deviations in color fidelity (compared to reference). In the CQS, lamps are not penalized for increasing object chroma relative to the reference source, though their scores are also not increased. The net result is that a lamp’s score is only penalized for hue shifts, lightness shifts, and reductions in chroma. This is a way to take color preference (and possibly also color discrimination) into account in the CQS. When the chroma increases under the test illuminant (with no change in hue), there is no change in score, 5"B" when the chroma decreases under the test illuminant, the score is decreased, and 5"C" when the chroma increases and the hue shifts, the score is decreased for the hue shift but not decreased for the increase in chroma.

The information in is derived from works of Wendy Davis, Yoshi Ohno, and the general Lighting Community.

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