SSL industry progresses toward new color quality metrics

April 29, 2015
A two-metric system under development in an IES subcommittee would separately address the issues of color fidelity and gamut and address the long-understood shortcoming of CRI.

A two-metric system under development in an IES subcommittee would separately address the issues of color fidelity and gamut and address the long-understood shortcoming of CRI.

While no official announcement has been made, there have been rumors prevalent among the LED and solid-state lighting (SSL) sectors about a new color-quality metric going back to the Strategies in Light conference when several speakers, including Jim Brodrick from the US Department of Energy (DOE), mentioned the work. We've learned that the IES (Illuminating Engineering Society) is nearing fruition on a two-metric system that would separately characterize color fidelity and gamut in light sources and deliver more actionable data than the legacy color rendering index (CRI). The IES may in fact make the work public at LightFair International next week, although we have not been able to confirm that plan.

The LED and SSL sectors have long been concerned with the CRI metric given that it was established in part to allow fluorescent sources to attain a reasonably good score of 80 on a scale where 100 is perfect. Moreover, CRI has long been understood to penalize light sources that oversaturate the color gamut, despite the fact that such results are desired in applications such as high-end retail.

The industry appeared headed toward adoption of a new single metric called color quality scale (CQS) several years back. CQS was backed by US-based National Institute of Standards and Technology (NIST). But in early 2012, the International Commission on Illumination (CIE) technical committee (TC) 1-69 failed to endorse CQS, leaving the industry with CRI.

Meanwhile, there have been other proposals for metrics. The Lighting Research Center (LRC) at Rensselaer Polytechnic Institute (RPI) developed a metric called Gamut Area Index (GAI) intended to be used alongside CRI and to separately address color gamut. Some companies such as Xicato have used GAI to characterize LED-based sources intended for retail applications. And the LRC has continued to champion a two-metric system, with director Mark Rea insisting at Strategies in Light 2013 that a single metric would never suffice.

Apparently, the two-metric system is an idea that is being embraced by an IES subcommittee focused on color metrics and details of the work will be published as a Technical Memorandum document. We can't predict what actions the global industry, including the CIE, might take on the issue. But we welcome the progress and believe it is long overdue.

The new metrics presumably will be promulgated under document number TM-30. TM-30 Rf (the f for fidelity) will use a scale similar to CRI where 100 will be a perfect score, and will characterize how well a light source renders a color relative to a perfect reference source. The new color quality metric will include more than the eight pastel color samples that comprise the main CRI Ra metric, including the more saturated colors. The new metric will borrow from the CQS work but has some new elements included.

The second color quality metric will be known as TM-30 Rg (the g for gamut) and will borrow heavily from the LRC work on GAI. But the mathematics will have changed considerably. Scores will be based on a 100 scale, although scores can exceed 100 to indicate oversaturation of color, allowing such lighting to be accurately characterized for applications that want to emphasize saturated colors.

We will likely have more to come on the color quality metric topic, whether through updates to this article or additional coverage in the coming weeks.