LED Downlights: A Match Made in the Ceiling?
LEDs are small, highly directional, and energy efficient. So you would think that the old-fashioned downlight would be perfect for high-tech LED technology. But it is hot up there in the plenum and heat is the LED’s worst enemy. Efficiency, lifetime, and color performance all fall victim to excessive heat.
Lighting manufacturers now have several years of experience with LED downlights in the field. Plus, ongoing improvements in the technology and fixture designs are reaching higher efficiencies, larger lumen packages, and new ceiling heights.
But where is all that heat going?
Prescolite (a Hubbell Lighting brand) is attracting attention for its latest achievement in small-aperture LED downlighting. The D2 (nominal 2-ich aperture) uses a high-end reflector designed specifically for the Xicato XSM Remote Phosphor LED module. The 20W fixture comes in downlight, wallwash, and aimable (35 degrees vertical) versions and has the lighting “punch” (see the candlepower curve) comparable to a 50W MR16.
Like most LED downlights for new construction, Prescolite’s D2 employs a sizable finned heatsink (in this case an extruded aluminum heatsink) to dissipate heat and maintain the manufacturer-specified junction temperature within the LED. Chris Hogan, vice president of brand management for Prescolite, said, “The ability to cool your LED properly is absolutely critical to the performance and longevity of your fixture. That’s been one of the limitations on increasing the lumen output of downlights: the ability to effectively cool high-lumen LEDs in a small space.… You’re not generating heat off the LED itself. Heat comes off the back of the LED into the housing, which makes it that much tougher in an IC box.” Hogan explained that an effective heatsink is materials-intensive and might constitute 20 to 30 percent of the cost of an LED downlight, about as much as the LED module itself.
The NanoLED NXT adjustable from USAI Lighting features a 10-degree option in a 2.5 inch aperture. To meet high heat demands, USAI chose a cold-forged aluminum heatsink design. “The forging process is what computer CPU heatsinks are made from. This process allowed us to design a heatsink small enough to achieve industry-leading performance from a 2.5 inch aperture. It dissipates twice as much heat as a die-cast aluminum heatsink,” explained Frank Cogliano, USAI vice president – engineering
Using discreet LEDs from Cree in a custom domed module, the 10-degree delivers 8725 centerbeam candlepower (7000 CBCP for the high-CRI version). As a rule, a high color rendering product (90+ CRI) will pay a penalty in lowered efficiency.
Dasal Architectural Lighting also chose the Xicato XSM module for an adjustable LED downlight. “The high color rendering version offers a 95 CRI with a strong red component [R9=98]. What the halogen won’t give you is a choice of color temperatures: 2700, 3000, and 4000K to suit the décor and ambiance of different spaces,” said Norm David, Dasal president. Also, because you are not relamping every 2000 hours, the designer/architect’s selected color and beamspreads are far more likely to remain consistent over the life of the project.
LED downlights going smaller, higher, and more efficient
As the technology improves, the efficacy (lumens produced per Watt of input power, lm/W) of the LED packages rises and waste heat necessarily does down. As heat problems inside the fixtures lessen, larger lumen packages become possible.
Juno Lighting Group by Schneider Electric brought its third generation of lensed downlights (called Gen 3) to Lightfair 2012 in Las Vegas. Over the 4 years since introducing its first LED downlight, Juno has reduced the wattage of the 600 lumen downlights – think 65W BR30 equivalent suitable for residential and hospitality applications – from about 15 down to 10.5W.
Scott Roos, Juno’s VP of Product Design, sees the general trend continuing. “We’re using chips and arrays that are producing close to 100 lm/W and I think within another 4 years, that efficacy might double. With improvements in the drivers and in fixture design as well, the fixture wattage could come down by as much as 50 percent.”
At only 5W, today’s Juno Mini downlights and gimbals – currently using Luxeon Rebel LEDs from Philips Lumileds – do not even require a materials-intensive heatsink. With proper contact between the LED package and the housing, heat is dissipated sufficiently to ensure the LEDs operate correctly and the fixtures still achieves an IC rating.
In high-ceiling commercial applications, LED downlights are essentially competing with compact fluorescent (CFL) and, to a lesser extent, ceramic metal halide (CMH). Though CFLs are competitive in terms of raw efficacy, they produce a blob of light, which is difficult, optically, to extract out the bottom of a downlight. Despite high price tags, LED downlights offer much lower cost of ownership over the life of the product.
Re-lamping recessed fixtures in high ceilings is expensive, difficult, and sometimes dangerous. With their long lifetime, LED recessed fixtures may not need service for 30+ years. Even so, LED downlights should be field-serviceable, both the module and driver.
The new 3 inch aperture P3LED Directional Series downlights from Cooper Lighting are the first on the US market to use a Zhaga-compliant light engine. (Zhaga-labeled LED equipment offers standardized electrical, mechanical, optical, and thermal interfaces.) Zhaga interfaces make it easier to replace LED components should they fail unexpectedly, and provide a path to change or upgrade the fixture in the future.
Also debuting at Lightfair 2012, Cree’s KR Series Downlights are designed to replace 18W and 26W CFL downlights. Cree is advertising price parity with the CFL products at 35 percent higher efficiency. Cree’s new LMH2 LED light source module with dome lens is targeting the 50 and 70W CMH downlight market. Cree displayed the module at Lightfair in a 4-inch downlight.
Juno’s Indy brand has turned the downlight’s parabolic reflector inside out. Juno claims that the Performance Series’ unique hyperbolic reflector, in combination with a higher-lumen Bridgelux chip-on-board module, raises delivered lumens and therefore efficacy. The reflector design also eliminates lamp image in the reflector, producing the “quiet” ceiling that designers look for. In energy-efficiency, Indy goes toe-to-toe with the 42W biax CFL, promising significantly higher illuminance at half the watts per square foot.
Next, USAI has set its sights on small-aperture fixtures competing with troffers. “You will be able to use low-wattage downlights to light spaces with very high ceilings,” said Cogliano. “One day we will see a two-by-two or two-by-four becoming a 5-inch square fixture delivering the same lumens with the same Lambertian beampattern and spacing ratios…all at efficiencies unseen in other high color rendering sources.”
Written by Lois I. Hutchinson