Making the light right for LEDs

Charlie T. Troy, Senior Editor, charlie.troy@photonics.com

We’ve all experienced it. When we look over the meat display at the supermarket, all the cuts are a mouth-watering red. However, once in the cart, the meat is a different shade of red. Why? Because the appearance of color depends on the light source. However, the lack of color consistency with LEDs is a problem that a new lab at the National Institute of Standards and Technology (NIST) in Boulder, Colo., is seeking to correct.

“[LEDs are] what we’ll likely use in the future to light our houses and public places,” said NIST vision scientist Wendy Davis. “Everyone wants light that appears natural and is pleasing to the eye, but with LEDs, we’re not consistently there yet. LEDs offer a lot of advantages over incandescent and fluorescent lighting, but they don’t always emit light that looks ‘right.’ ”

A bank of assorted LEDs allows researchers at NIST to test the characteristics of light sources. Courtesy of NIST.

To solve the dilemma, Davis and a team of physicists created the NIST Spectrally Tunable Lighting Facility (STLF), a lab that is the only one of its kind in the world, Davis said. “We can use it to create virtually any light source spectrum we can dream up, and because of that, we can test out how a light source would behave as far as object, color or any other properties.

“By doing this testing, we can really save manufacturers the effort of having to produce the light source that may or may not behave as they want.”

A process of combining

The STLF lab concentrates on the relationship between physical measurements of light and human perception of light and color. To this end, the researchers combine LEDs of different colors to produce an acceptable overall color.

In the lab, hundreds of LEDs cover the ceiling. Davis can activate groups of them, adjusting their color levels to demonstrate the effect lighting has on the appearance of food and furniture. “Here in our lab, we test how light sources are used for general illumination, and how they appear to people,” she said. “This is particularly important for LEDs because they have potential for great energy savings over our current lighting technologies.”

Efforts like this are helping the researchers develop a way to quantify how LEDs affect the colors of objects in ways meaningful to the lighting industry. They currently are developing a measurement tool called the Color Quality Scale to help manufacturers develop LEDs for general lighting.

“Because the light emitted by LEDs is different from the light we get from other lighting technologies, the way that we measure color quality doesn’t always work for them,” Davis said. “At this point, LED manufacturers don’t have a reliable way to determine the color performance of their products.

“If we don’t handle this issue now, it could create big problems for future LED lighting products, because bad color means unhappy consumers. We want to use measurement, which is a NIST specialty, to nip this problem in the bud.

“Ultimately, the measure of light isn’t really about numbers and instruments and math. It’s about people. We use lighting so much throughout our lives, and it’s really become such an important part of our society. So the measurements that we do here are really about making our lives better.”