Chromaticity is the measure of the color of a light source. In lighting design, we use the correlated color temperature (CCT), measured in degrees Kelvin (or K) and the color quality, measured by color rendering index (or CRI), to specify color. With legacy light sources, this system has served us well for over 50 years. But as most lighting professionals now realize, one of the many issues involving LED’s is color. Low cost LED’s tend to suffer many color problems, such as low CRI or out-of-specification CCT. But even the best LED’s can vary, and change color over time.
The CCT/CRI system is inadequate to accurately describe color quality with LED lighting. This is not news – both the IES and CIE have been working on a new color metric for over a decade. In the meantime, we’ve learned that using CCT, CRI and R9 (rendering of saturated red) is a relatively good indicator of lighting color quality. I personally like a color spectral power distribution graph so I can assess the source’s strengths and weaknesses, and when available, I especially like the polar diagram (“spider web” chart) of all R values.
Without a field measurement tool, the lighting professional can’t assess light color correctly. Visual assessments can easily be wrong, and with modern phosphor engineering, you really can’t just look at it. Want to do it right? Get a spectrometer.
I own two meters, the Asenstek Passport and the UPRTech MK-350. The Passport’s iPod/iPhone/iPad based interface and remote sensor head are best under most circumstances, and it has probably the best user features such as the ability to photograph luminaires, name data files and compare light sources. The MK-350 is a bit more like a conventional light meter that sometimes makes it easier and quicker to use. I’ve also used Ocean Optics and Minolta colorimeters, and they are great instruments, too, although quite a bit more expensive.
How do I use them? For instance, I questioned the brand new LED street light on my block. Supposedly 4000K 75 CRI, it is actually 4850K with 67 CRI and R9 is minus 15. No wonder the neighborhood now looks like a Zombie movie – everything and everyone is gray. On the other hand, I recently installed inexpensive LED downlight retrofit kits, and much to my pleasant surprise, they measured 2950K with 90 CRI and R9 of 30. Not bad for $25 each.
Although almost my entire career has been as a lighting designer, I find myself doing more consulting and expert/forensic work in lighting now. Because LED lighting has been sold with promises of long-term performance, I expect many future claims will involve color shift as well as accelerated lumen depreciation and other deterioration. For everything I do, I can’t imagine not having a spectrometer any more.