Test to find the best color metric (MAGAZINE)

March 26, 2012
In this Commentary from the March 2012 issue of LEDs Magazine, Tim Whitaker discusses the need for a replacement for the color-rendering index (CRI).


This article was published in the March 2012 issue of LEDs Magazine.

View the Table of Contents and download the PDF file of the complete March 2012 issue, or view the E-zine version in your browser.


I'm writing this piece in the relatively quiet period between two major events on our calendar; Strategies in Light (SIL), which took place in February, and Light+Building, the biennial tradeshow that will hit Frankfurt in mid-April. You’ll find extensive coverage in this issue of all three conference tracks at SIL (pages 27, 32 and 39), as well as the most recent updates on the LED and lighting markets from Strategies Unlimited (page 21).

Market numbers aside, one of the most discussed topics at SIL was color-rendering index (CRI). There seems to be general agreement that this much-derided metric is not up to the job for LEDs, but there is a lack of consensus on what it should be replaced with. In our last issue, my colleague Maury Wright used the Last Word column to bemoan the failure of the CIE technical committee TC 1-69 to reach agreement on what could replace CRI. The column suggested there could be various political or self-interest reasons for the absence of a decision by TC 1-69. But this wasn’t why the column was written; our main thrust was to encourage some kind of resolution to the issue.

The CIE seems to recognize the need for a revised approach, and in fact went so far as to write an official response to our article which explained the activities of TC 1-69 from their point of view. But, at the last minute, the CIE asked us not to publish their piece, citing internal disagreements and potential legal issues. However, several committee members made comments on our article, and these can be viewed at the URL above.

One of the main alternatives considered by TC 1-69 is color-quality scale (CQS), but despite being popular in some quarters it clearly doesn’t tick all the boxes – otherwise, presumably, TC 1-69 would have approved it. Voicing an alternative opinion, the Lighting Research Center (LRC) advocates a two-metric system that combines CRI with the gamut-area index (GAI) metric. Quoted on page 40, the LRC’s Jean Paul Freyssinier says that “no single metric can characterize color rendering.”

One of the comments on our website highlights another issue: the goals of the CIE are principally scientific, and not necessarily aligned with developing metrics that help the lighting industry. Good for them, you might say. But in that case, perhaps the lighting industry needs to be more proactive if it wants a useful metric. One way to do this is to test some of the different metrics in real-world scenarios, rather than relying on calculations. Theoretical data are all very well, but I’d like to see data where the calculated score is correlated with the preferences expressed by lighting experts using products in the field.

Which brings me to a final point: whichever metric (or combination) eventually supersedes CRI, it will only have limited usefulness. Lighting specifiers will not simply look at the number and trust it, without first evaluating the lamp or luminaire in the intended application. A metric of this kind helps to narrow down the product options that should be considered, but a better metric will certainly make this process easier.