Lessons in LED Lighting: How Phosphor Coating Impacts Color Rendering
With dozens of LED lighting vendors crowding the NAB Show exhibit halls a few months ago, LED technology has proved to be more than just a passing fad for television and motion picture applications.
There are certain verifiable advantages to LED lights. Their raw efficiency is 10 or more times that of tungsten fixtures. That efficiency means that LEDs throw off a small fraction of the heat of legacy lighting fixtures. Without requiring heat-resistant housings and lenses, LED fixtures can be made more lightweight and compact than their predecessors. They can dim with no noticeable change in color temperature, and they require no bulky external ballast modules.
Questions have been raised about the quality of color rendering when shooting scenes lit with LED fixtures, however. While the big deuterium fusion light source (aka, the sun) and tungsten halogen sources each provide a wide and fully variable spectrum of light, the spectrum of light from LED technology tends to be spiky.
For certain lighting applications, a blend of different colored LED lamps can be mixed to provide a spectral range that is closer (but not identical) to true daylight or tungsten light sources. But for applications where single white LEDs must be used, the colors must be provided by different phosphors coating the LEDs.
Here’s a quick description of how white LEDs work: the LED (light-emitting diode) in a white LED fixture actually emits blue light, but various phosphorous elements applied in a coating over the LED itself emit different colors of light. The exact makeup of that phosphor layer is the “secret sauce” of color rendering in LED lighting.
If you look back 30 years or so, you could substitute fluorescent technology for LED technology and the script would be about the same. Gary Thomas, national sales manager for Videssence, one of the pioneers of fluorescent lighting for television, remembers those early days.
“When we first brought our product to the video industry, color rendering wasn’t that great,” Thomas says. “The lamp manufacturers really weren’t up to speed for video. They made fluorescent tubes for the architectural industry, where color rendering wasn’t mandatory. Once they found that there was a market for fluorescent lamps in the video world, they started to recognize the need for tubes that gave off proper color temperature.
“Now that LEDs have been around for a while, I think it’s the same thing.”
Scott Stueckle, sales and public relations manager for Kino Flo Lighting Systems, another fluorescent lighting pioneer, says the Burbank company’s challenge in developing LED lighting was to make it indistinguishable from the company’s fluorescent light in terms of color rendering.
“Our product stays the same in the sense that we make high-output, soft, color-correct and controllable lighting systems for the most discerning of cinematographers and gaffers and lighting designers who are doing cinema and television production.”
Maintaining Phosphor Integrity
Ali Ahmadi, product manager for Litepanels, the company that pioneered LED lighting for the professional film and video market, says that in addition to configuring the correct phosphor mix for a white LED, there’s also a culling process after the LEDs are manufactured.
“There’s product we accept, and product we reject,” he says. Such selectivity means quality-minded LED light fixture manufacturers pay a premium for LEDs that meet their specifications.
Ahmadi points to another critical factor: keeping the LED and phosphor layer healthy through the expected life of the LEDs by not over-driving the LEDs themselves. “The phosphor is spec’ed to a certain maximum operating temperature,” he notes. “If that temperature is exceeded, the phosphor will degrade, resulting in a change of color temperature and a change in the spectral makeup of the light output.” He adds that over-driven LEDs will begin to decrease in intensity before reaching their life expectancy.
It’s a misnomer that all LEDs are the same. Litepanels and Kino Flo have developed their own LED designs for manufacturing, and Videssence and AAdyn Technology use a video-optimized LED design by CREE, a 25-year-old Durham, N.C.-based provider of LED lighting.
“There are new versions from CREE that are much, much improved over anything that has been used before,” says Walter Lefler, partner in AAdyn Tech. “We’re getting 98 percent of the light from those CREE LEDs going to our proprietary lensing.”
If you want to start a fight among LED fixture manufacturers and lighting people in general, bring up CRI (color rending index) ratings. Without wading into that issue, which could fill books, it’s worth noting that the Science and Technology Council of the Academy of Motion Picture Arts and Sciences has undertaken a Solid State Lighting project to fulfill what the Academy’s web site deems a “need for an unbiased investigation of solid state lighting (SSL) technologies, which includes LED emitters, for motion picture production.”