Color quality scale explained

Color quality scale (CQS) is a color rendering score – a quantitative measure of the ability of a light source to reproduce colors of illuminated objects. Developed by researchers at NIST^[1] the metric aims to overcome some of the issues inherent in the widely used color rendering index (CIE Ra, 1974).^{[2] [3]}

• The color space used in CIE R_a (CRI) is outdated and nonuniform, and CQS uses CIELAB as a replacement.
• The Von Kries chromatic adaptation transform used by R_a does not perform as well as other available models. CQS uses CMCCAT2000.
• CIE R_a is based on desaturated samples, and a lamp's performance in rendering these samples faithfully is not necessarily linked to how it may perform with samples of higher saturation. CQS uses higher saturation samples.
• ‘Pure’ fidelity (where all deviations are considered bad) does not account for desired chromaticity changes. Increased saturation might be preferred. CQS does not penalise against increases in saturation.
• In CIE R_a the arithmetic mean is taken of the color differences for the individual samples. In CQS the individual results are combined through a root mean square instead, so that a small number of poorly rendered objects reflects with greater strength in the overall result.
• Negative values of CQS are made impossible due to their potential for consumer confusion.
• CCTs of lower than 2800K are penalised so that the CQS is more representative of their actual color rendering as opposed to their fidelity.

The CQS generates the general index Q_a. Several manufacturers are beginning to publish data on CQS scores of their products, including some who claim light sources with CQS scores up to 97.^[4]

See also

• Color rendering index

Notes and References

1. http://accreditedgemologists.org/lightingtaskforce/NISTDevelopmentofColorQualityScale.pdf Development of a Color Quality Scale
2. Toward an improved color rendering metric . Davis, W. . Ohno, Y. . Ian T . John C . Tsunemasa . Ian E . Ferguson . Carrano . Taguchi . Ashdown . Proc. SPIE . 5941 . 59411G–1 . 10.1117/12.615388. Fifth International Conference on Solid State Lighting . 2005 . 2005SPIE.5941..283D . 10.1.1.470.2414 . 121431482 .
3. Davis, Wendy, and Yoshi Ohno. 2010 "Color Quality Scale." Optical Engineering 49, no. 3: 033602–033602–16. doi:10.1117/1.3360335.
4. http://www.yujiintl.com/high-cri-led-lighting High CRI LED Lighting Supplier