In our last blog “How a well-intentioned request could hurt LED lamp performance”, we agreed with the lighting sector’s demands to raise the standards for light quality in the EPA’s new LED lamp specifications. However, one of our blog readers commented on our general endorsement of improved light quality, and suggested that the standard criteria used to evaluate light quality—Color Rendering Index (CRI)—needn’t be increased or even used as a metric at all. Rather, he suggested, the Color Quality Scale (CQS) should be used to evaluate light quality, as it is a “superior metric for defining chromaticity”.
The limitations of CRI as a measurement system are well known. It is a method based on colour saturation and has the ability to create reduced—and even negative—values when the test source could be considered as superior in it’s colour rendition to the standard in which it’s being compared. This is particularly true with solid state lighting and for those of you unfamiliar with CQS, it’s a proposal for rating LED light quality that more accurately represents the color rendering qualities of the source and is intended to be an improved predictor for colors that have a high red content, such as skin color.
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.
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).
Image: Color Quality Scale colors
Developed by researchers at the National Institute of Standards and Technology (NIST), a proposal for CQS’s adoption as the new color quality criteria has been under examination by a committee of the Commission on Illumination (CIE) since 2006. The CIE has had two previous attempts to refine the measurement standard for colour quality in the 70’s and 80’s, both of which ended in a stalemate and, according to industry insiders, a similar impasse seems to be occurring this time with various committee members preferring to adopt a “pure fidelity metric very similar to CRI”.
For a more in depth analysis of CQS, see NIST’s Rationale of Color Quality Scale.
Has CRI lost its relevance? Is CQS the right answer or is the CIE correct in exploring alternative proposals? It’s certainly not the only alternative being discussed (for example, Lorne Whitehead at UBC: Improving the CIE Colour Rendering Index – How this can be done and why it matters).