Solid state lighting industry at an impasse efforts to replace CRI with CQS (Color Quality Scale)
As advances in solid state lighting products, especially LEDs, continue to push into the marketplace displacing incandescent and fluorescent lighting products, it only stands to reason that evaluation methods for these products, such as the measurement of color fidelity, or CRI – now almost 50 years old – isn’t cutting it.
As we mentioned in a previous article, CRI has been found to be an inaccurate, unreliable predictor of color preference of solid-state lighting products such as light-emitting diodes (LEDs), which emit a much different light than fluorescent or HID lamps, and can result in lower or even negative CRI values for some of them.
For instance, some LED products with a CRI as low as 25 can produce white light that actually make object colors appear more vivid. Also, CRI can give high scores to LED light sources that render some saturated object colors, particularly red, very poorly.
And because CRI only evaluates color rendering, also known as color fidelity, it ignores other aspects of color quality, such as chromatic discrimination and observer preferences.
Color Quality Scale debuts
To help remedy these drawbacks, CIE established a technical committee in 2006 to develop and recommend a new color rendering metric.
In June 2010, Wendy Davis, chair of the committee, debuted the Color Quality Scale (CQS), which she developed with colleagues at the National Institute of Standards and Technology (NIST), the federal technology agency that works with industry to develop and apply technology, measurements, and standards.
The proposed metric is believed to offer a superior indication of what humans perceive as superior color rendering compared to CRI specifications.
Where CRI relies on a evaluation of how a white light source illuminates eight pastel colors, CQS uses 15 colors, including samples with much deeper color – such as deep reds where CRI proves especially inaccurate.
Endorsing the new CQS measurement, James Brodrick, lighting program manager for the DOE’s Building Technologies Program, said, “Regardless of the type of light source, the CQS represents the color rendering qualities of white light more accurately than the CRI and is a far better predictor for colors that have a high red content, such as skin color and wood finishes – which is one of the CRI’s major weaknesses.”
The proposed CQS, still being debated at the CIE’s committee level, is intended to eventually replace the CRI as a new international standard for general illumination evaluation of traditional lighting technologies and solid state lighting sources.
But the new metric may never see the light of day.
Is CQS headed toward failure?
While it gains support in the U.S. lighting industry, the committee is deadlocked between those who see it as a solution for measuring color quality of solid-state lighting and those pushing for a metric that is more similar to CRI, said Davis, who recently left her position at NIST to take a position as a senior lecturer and illumination program director at the University of Sydney.
“We’ve been working for over five years now and have been unable to reach an agreement, so far,” Davis told Lumenistics. “The CIE requires unanimous agreement to issue a recommendation, and this committee has [approximately] 40 members. As you can imagine, it’s been tough. I’m not sure if the committee will be successful.”
It certainly hasn’t been in the past. The CIE has attempted to revise the CRI several times throughout the 1980s and 1990s “and every committee ended in failure,” Davis said. “If that happens now, which is very much a possibility, I wouldn’t be surprised if some other standards organization were to take up the issue.”
So, for now, CRI remains the only internationally recognized color rendering system. Whether the lighting industry, especially the solid-state category, will continue to use it for evaluating lighting applications remains to be seen.