This article was published in the February 2012 issue of LEDs Magazine.
View the Table of Contents and download the PDF file of the complete February 2012 issue.
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Amazingly, after nearly half a century, the lighting community is still using color-rendering index (CRI) as a measure of how accurately colors appear under a light source. CRI usage continues despite broadly recognized flaws. Moreover, there are clearly better alternatives such as the color-quality scale (CQS) developed within the US-based National Institute of Standards and Technology (NIST). Unfortunately, the use of CRI appears certain to continue for the near future as the International Commission on Illumination (CIE) technical committee (TC) 1-69 has failed to endorse CQS, or an alternative, preferring to disagree rather than deliver a tool that would be truly valuable to the broadest segment of the lighting industry.
The TC 1-69 committee had apparently come close to endorsing a dual standard last summer, according to chairperson Wendy Davis. The committee couldn’t agree on a single metric, with some preferring the relative simplicity of the math that underlies CQS, and others wanting a far more precise measure of color rendering.
The committee had tentatively agreed to recommend two different metrics: CQS, and a more complex metric called nCRI that was under development at the University of Leeds, UK. Presumably, the broad lighting segment would have used CQS, while nCRI would have served in more specialized applications. Davis said nCRI would “very accurately quantify how different objects appear under the test lamp relative to a reference illuminant.”
Unfortunately, according to Davis, the CIE had adopted a new Code of Procedure that required unanimous agreement within a committee before it could publish a technical report. And when the dual-metric recommendation was circulated to the full committee, a dissenting minority stopped the process.
Apparently the politics in the committee have worsened. Davis doesn’t expect movement in the short term. She said the final version of the nCRI spec has just been distributed to the committee this past December.
Davis has since moved on from NIST to take a professorship at the University of Sydney, but that hasn’t impacted her work on color standards. Last year at Strategies in Light, Davis said that if the CIE committee didn’t agree on a new metric, then she would pursue a CQS standard elsewhere. Davis said recently, “If the CIE fails, I still plan to pursue standardization of the CQS in another organization, most likely in the US.” She also said that an Illumination Engineering Society (IES) color committee was contemplating the issue, although she doesn’t expect swift movement, in part because the committee is relatively new.
Now in terms of full disclosure, I’m not a color expert nor did I sit in on the TC 1-69 meetings. But I do have broad experience watching standards bodies debate while an industry anxiously awaits their work. I know Davis has a vested interest in CQS given that she helped develop it. But I haven’t heard anyone argue that CQS would not be a significant upgrade from CRI. The committee members should have voted with the best interests of the industry in mind rather than their special interests.
The CQS proposal relies on a more realistic set of color samples than does CRI, including richer saturated colors. CQS eliminates the issue of sources with extreme CCT values achieving good CRI scores. And while CQS penalizes reductions in chroma, it doesn't penalize sources that increase object chroma relative to the reference.
Meanwhile, we continue with CRI. Fortunately more lighting companies are publishing CRI numbers for some of the more-saturated color samples rather than just the composite score based on the pastels.
Still, the very best LED-based sources and fixtures sometimes get penalized in CRI scores for rendering colors that appear even richer than with the reference illuminant. That’s just wrong and the solid-state lighting (SSL) industry needs a solution. Ironically, LEDs were long criticized for poor CRI, and now the manufacturers have greatly improved quality. But that improvement isn’t necessarily recognized in a CRI system that was in essence calibrated for fluorescent sources.
