LED for General Interior Home Lighting

[Yadda]

Is it there yet or still hype?

[ransley]

Its there but its hyped up, I bought a few bulbs but it was a waste of money, color is poor and efficency no better than Cfls and price is to high from the units ive tried. it will get better. Stated watt equivilant on mine was over rated 4 x in my quick comparison. Now for my bicycle or a flashlight they are the best.

[trag]

I've made a study of this for aquarium lighting. If you go to the manufacturer's websites (manufacturer's of the actual LEDs, not the light fixtures) and do some calculations and add a few WAGs, it comes out that high intensity LEDs are no more efficient that current fluorescent light technology. So the folks claiming that they are more efficient are full of hype.

For special color applications LEDs may have an advantage. Also, LEDs may be more directional than fluorescent tubes, and so might get an effective-light-delivered advantage in some cases and absent a good reflector on a fluorescent bulb.

But in general, on a lumens delivered per watt input basis, the available high intensity LEDs are not a more efficient light source than modern types of fluorescent, such as T8 and T5 bulbs with electronic ballasts. There may be some stuff in the laboratory which is more efficient but it either hasn't hit the market yet, or is selling at a huge premium--think $400 per the equivalent light output of a 40 watt fluorescent tube.

LEDs may last longer than fluorescents, but they are hugely more expensive to buy. The datasheets from the LED manufacturer's typically list a 50,000 hour from beginning to 70% of original output, lifetime. But none of them provide a lifetime curve. So we don't know if that means that LEDs have a linear decline from 100% to 70% output over 50,000 hours, or whether the output declines steeply early on and then declines shallowly (bad), or whether it keeps its output near 100% for most of that time and declines steeply towards the end of 50,000 hours. Which is it? No publicly available information.

For comparison, General Electric lists a 30,000 lifetime for its T5 fluorescent bulbs until output declines to 80% of original. And they provide lifetime output curves to show you how the decline in output will occur.

White LEDs are actually ultraviolet LEDs with a phosphor coating which converts the ultraviolet energy into white visible light. So, most likely, they will decline in output in a fashion very similar to fluorescent light bulbs, because they are both using the same method of converting ultraviolet radiation into visible light.

LEDs run on low voltages of about 3 to 5 volts. If several LEDs are connected in series, the voltage may be higher, but this is not a very efficient way to run more than about 3 LEDs at a time. So LEDs require a power supply to convert house current to low voltage high amperage DC current. The power supplies can (and should, if they're good) be quite expensive.

I hope that helps,

Jeff Walther

[Don Klipstein]

A few lumen depreciation curves for white LEDs have turned up. They tend to be close to level for the first thousand or two hours, then gradually acceletrate downward, and after that the curve gradually shifts to an exponential decay curve. The decay stops accelerating and begins decellerating at maybe 90 or 85% of initial light output. Some curves I have seen are for only enough time for the decline to still be accelerating.

Close - the LED chip produces blue light. The phosphor lets through some of the blue light, and converts the remainder to a "broadband yellow" whose spectrum stretches from mid-green to mid-red. The combination is white.

There are a few high color rendering index white LEDs with a blend of phosphors to avoid a surplus of yellow and shortage of red and green. Even most of these use blue-emitting rather than UV-emitting LED chips.

- Don Klipstein ( [e-mail])