Don't Be LED Down The Garden Path (Part 3)

EFFICIENCY AND LIFE EXPECTANCY OF "WHITE" LEDs

Hearing a lot about "white" LEDs? So are we. We hear things like "50,000-hour life" and "huge energy savings." That sounds terrific. But, let's see the numbers!

LEDs are current sensitive. An LED circuit needs to regulate current to the LED as the LED's temperature changes. It needs to do this for each LED individually. You can't have one LED burning out and taking out the whole array like a string of old style Christmas lights. You dim LEDs by limiting current, not voltage. A rheostat won't do it. All this requires very sophisticated, and not terribly efficient circuitry.

The major supplier of the "white" LED equipment we tested listed an efficacy of 6.4 to 7.7 lumens per watt for their linear LED unit. Other products went as high as 10.7 lumens per watt in the "cool white" range (8100°K). Nevertheless, a standard 60 watt GE warm white light bulb has an efficacy of 14 lumens per watt. The "best" LED unit we tested was only 75 percent as efficient as a plain frosted electric light bulb and the worst had 45 percent the efficiency. Considering that an electric light bulb is 94 percent IR and only about 5 percent visible light, these are not very impressive numbers.

Regular electric lamp life is measured by the average age (under absolutely ideal conditions) at which they fail. We know that overdriving LEDs can make them brighter, but it significantly shortens their life. We also know that LEDs tend to change color and loose intensity as they age. While LEDs advertise extremely long life numbers, one begins to wonder just how useful those final years of life truly will be.

The chart above along with reported research by RPI shows that under the good conditions LEDs can be expected to loose 40 percent of their intensity in the first 4000 hours. Under overdrive conditions the graph seems to indicate that this loss can be as much as 90 percent. With new products coming out almost daily it will be some time before really accurate life data is available. Bear in mind simple life testing itself (running 24/7) will take eight years. We are on uncharted ground. But, it would be best not to count too heavily on extended life until test data is available.

Finally, keep a eye on LEDs. There is a tremendous amount of R&D going into LEDs and some of these hurdles may be overcome. Some problems, like the monochromatic nature of the LED itself and the significant UV output appear to be integral to the technology. For now, LEDs are perfect for signs, displays, low intensity, intermittent, low-voltage and monochromatic applications. They may be good solutions for effects, color changing applications and architectural and outdoor lighting where access is difficult. They are not appropriate where color rendition is important or in any indoor lighting applications of fugitive or fragile materials. The high UV makes them dangerous in museum and retail applications. Like any other lighting technology, LEDs are good for some applications and terrible for others. The trick is in knowing which is which. Knowing the true numbers will help.


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