All a Designer Needs to Know About CRI

Lighting Analysts Inc.'s Ian Ashdown traces the deep history of the color rendering index metric in order to spotlight how little importance its exact value has to lighting designers now.

Ian Ashdown, P. Eng., FIES, is chief technical officer of Lighting Analysts Inc., maker of AGi32 illumination engineering software. His recent blog post "Thoughts on Color Rendering" is only the second on the company's blog. It will amply repay a close reading by anyone interested in lighting.

The effort to develop a cross-industry rendering quality metric began in 1948, when the horror of standard office fluorescent lighting was fully upon us. While makers of fluorescent lamps could improve the rendering characteristics of the light by varying phosphor composition, they needed a metric to convince people to spend more for improved color. The first version of the CRI was standardized in 1965; the standard was revised in 1974 (when the psychophysiological effects of chromatic adaptation were included).

According to Ashdown, the CRI metric proved very successful in differentiating lighting products in the era before SSL. But the light produced by LED sources confounded it.

Ashdown picks out the introduction of TIR Systems Lexel tri-phosphor luminaire at Lightfair 2005 to exemplify how badly the CRI metric, and in particular Ra, can mislead about the rendering quality of LED-based lighting. Lexel was much praised at that show for its remarkable (for the time) color rendering; yet calculation reveals that its CRI was in the range of 25 to 40, depending on color temperature.

Rough measure

The CRI was never intended to distinguish fine gradations in color rendering. Ashdown reproduces this table from the 1986 CIE Guide on Indoor Lighting:

CRI Ra	Examples of Usage
> 90	Color matching, art galleries
80 – 90	Homes, restaurants, textile industry
60 – 80	Offices, schools, light industry
40 – 60	Heavy industry
20 – 40	Outdoors

Granted, the table corresponds closely to the lighting options available at the time: fluorescent for indoor use, HID for high-bay, and LPS for roadway and outdoor lighting. Still, it points to the way the CIE viewed their metric's applicability. Finally, Ashdown calls out the research (abstract) of C. van Trigt, whose 1999 scholarly review of the CRI metric concluded that "only a difference of some five points in the index is considered meaningful."

Ashdown walks us through the efforts since 2008 to improve on the CRI, noting that its Technical Committee 1-69 "investigated over a dozen proposals, but could only agree to bitterly disagree on any new metrics." Two new technical committees have been spun up, but aren't expected to report out before 2015. In fact the lighting industry gave up waiting for the CIE years ago, and began citing the R9 (saturated red) element of the CRI metric, in addition to Ra.

The blog post concludes with a fascinating (to me) look at the rendering characteristics of interior lighting supplemented by daylight filtered through glass.

Thanks to All LED Lighting blogger Randolph Fritz for alerting us to this insightful post. You should go read it.

— Keith Dawson , Editor-in-Chief, All LED Lighting