LEDs Magazine - Color-quality standards bodies need to consider the broad user base (MAGAZINE)

LEDs MAGAZINE

ILLUMINATION IN FOCUS

STRATEGIES IN LIGHT

THE LED SHOW

< Previous | Next > | Contents (February 2012)

Share |

Color-quality standards bodies need to consider the broad user base (MAGAZINE)

Work on a new and improved measure of color-rendering quality has apparently stagnated in the CIE, and the failure leaves the broad lighting community without a tool that would be very useful, says MAURY WRIGHT.

+++++

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.

+++++

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.

About the Author

Maury Wright is a Senior Technical Editor with LEDs Magazine.

COMMENTS

Name: byheart Posted: Mon, 06 Feb 2012 23:02

There is perhaps a deeper issue here than the apparent failure of CIE TC 1.69 to produce a final report. Consider the mission statement of the International Commission on Illumination (CIE):

"[The CIE] is devoted to worldwide cooperation and the exchange of information on all matters relating to the science and art of light and lighting, colour and vision, photobiology and image technology."

Simply put, the CIE does not exist to publish documents that benefit the commercial interests of the lighting industry. TC 1.69 was formed to "investigate new methods for assessing the colour rendition properties of white-light sources used for illumination, including solid-state light sources, with the goal of recommending new assessment procedures." For better or worse, this process is ongoing.

As for the Illuminating Engineering Society of North America (IESNA), its mission statement is:

"The IES seeks to improve the lighted environment by bringing together those with lighting knowledge and by translating that knowledge into actions that benefit the public."

The IESNA has traditionally relied on the publications of the CIE for colorimetry. If the IES Color Committee chooses to endorse the NIST CQS metric, it will most likely be in the context of benefiting the public (i.e., the lighting industry) rather than recommending the "best possible" metric (which in turn depends on the metrics used to rank the various color rendition metrics currently being considered by CIE TC 1.69).

As a lighting professional, I mostly agree with the views expressed by Maury Wright. As a member of CIE TC 1.69, I have expressed my frustration to the other committee members. After years of waiting for a final report from the committee, the SSL industry appears to have given up and mostly adopted CRI with R9. Whatever metric (or more likely metrics) the committee eventually recommends, it will be exceedingly difficulty to overcome industry inertia when the existing metric is "good enough."

Still, I do not believe the committee has failed. There will likely be eight or more academic papers published on the topic, papers which fully satisfy the CIE's mission statement. The committee directly or indirectly fostered the research behind these papers.

The failure (if any) is with the architectural lighting industry itself, from lamp and SSL module manufacturers through luminaire manufacturers to lighting designers and specifiers. Yes, the industry needs a better color rendering metric, but it is relying on one organization that is not directly interested in the lighting industry, and (in North America at least) another organization that relies almost exclusively on the first organization.

The NIST CQS color metric is not perfect, but it is better than the existing CRI, with or without R9. It is, in the best of engineering tradition, "good enough" for most purposes. Unfortunately, the lighting industry has organized itself such that only a formal recommendation from the CIE will suffice before any metric is considered.

Somehow, the phrase "mea culpa" fits this sad and frustrating situation.

Name: schanda Posted: Sun, 12 Feb 2012 18:02

It was interesting to read the paper by Mr. Wright. From the article some might get the impression that the CIE is not accepting the CQS for reasons that are inappropriate- -perhaps personal, bureaucratic or pedantic. There are – however – still some faults in the method that need correction, or alternative methods have to be considered. For at least two years the CIE TC 1-69 has been informed that both the CIE CRI described in CIE Publ. 13.3 and the CQS have some major problems with the selection of the test samples. One of us showed the TC, as an example of the problem, that one can have two LED sources where a given CQS sample will provide an index of 84 for one source, and for the other 52, but one can select a real sample metameric to the CQS test sample for which the first source will provide a CQS index of 52 and the second one 87. This is certainly not acceptable for a test method to be recommended. An alternative test sample selection method has been suggested to the TC.

Surely we all agree that the industry needs a colour quality description of its light sources that does not permit selection of a source spectrum that would be rated highly, but for which a metameric sample set would yield a low rating. The difficulty with the Munsell samples, used both in the original CIE Test Method and in CQS, is that they are produced from high quality highly stable artificial (in some cases quite expensive) pigments, excellent for building colour atlases, but they are highly correlated spectrally and not necessarily representative of everyday life and natural pigments.

These are some of the practical (not personal nor highly sophisticated scientific) reasons, why a number of the CIE TC 1-69 members think that a really acceptable colour quality metric needs further elaboration, wherein - naturally - the many excellent parts of the CQS system should be incorporated.

Finally we would like to stress that this comment is our personal opinion on the question and - although we know it is shared by some TC members - it is not presented as an official minority report of the TC.

Janos Schanda, Lorne Whitehead
members of CIE TC 1-69

Name: led_warrior Posted: Fri, 24 Feb 2012 00:02

What is apparent, is that we are trying to "fit" LEDs to what the standards say they should "fit". Not creating new metrics or standards - modifications don't count. LEDs are such a different animal than traditional lighting and we need to treat them differently - not the same.

Name: dennis m Posted: Mon, 05 Mar 2012 14:03

Short summary - the manufacturers of fluorescent products continue to impede the codification and implementation of CQS-

Primarily due to averice, small minds & corporate directives not to have their products color fidelity short comings made public.

Those are the realities- why would anyone contend that 8 Munsel plates are superior to 15- oh wait I remember now ( the mfgs of glass and gas products) are money grubbing asses who want to keep selling their 20th century products for a few more years. Like the light of day on cockroaches - they don't want the facts out in the open- in the age of SSL CQS IS AN EXPENSIVE METRIC- It means having to have an accurate assessment of color fidelity - If ya don't have the IP to make products that get red/blue/skin tones right then CQS IS NOT A POSITIVE FOR YOUR BUSINESS MODEL and then naturally having an accurate assessment of saturation is "bad".

The big 3 will get around to endorsing it a year or 2 from now- pending the approval of cadres of corporate bean counters & laywers !

February 2012

Channels

Standards