LED bulb lifetime

| Comments (3) | Misc
The NYT has a sort of odd article about the expected lifespan of LED lightbulbs:
When a manufacturer says that an LED lamp will last 25,000 or 50,000 hours, what the company actually means is that at that point, the light emanating from that product will be at 70 percent the level it was when new.

Why 70 percent? Turns out, it's fairly arbitrary. Lighting industry engineers believe that at that point, most people can sense that the brightness isn't what it was when the product was new. So they decided to make that the standard.


If nothing else in the lamp fails, like its electronics, the product will continue to work until it becomes really dim. But some engineers are proposing a way to get around even that.

Their idea is that once the LEDs start to emit less light, increase the power to each one to increase its brightness. Unfortunately, that will also diminish the life of the lamp.


Not only would contractors need to use thicker cables, but the utilities would need to create more power, partially negating the appeal of LED lighting in the first place.

I'm not saying that this won't work, but it seems there's a relatively obvious alternative: have the bulb stop emitting light entirely once it gets below some threshold (70% seems reasonable, I suppose). As far as the extra power goes, presumably the tradeoff here is straightforward: estimate how much energy is required to produce a new LED bulb if it's thrown out at the time it gets too dim and compare that to the additional energy that will be required to overdrive the LED once it starts to dim.


If the same electricity is going into the bulb, and less photons are coming out, where is the other power going? Heat? I thought heat was lethal to LEDs.

Or does the amperage decrease with the lumens?

It's a death spiral. The LED gets less efficient, you pump more amps into it, it gets hotter, it degrades further, becoming less efficient and so on. But it takes time. So you do extend the "life" of the bulb this way.

Oh, and LEDs are fundamentally current devices though they are less efficient at higher currents. The big breakthrough (besides UV-white phosphor) was making LEDs that had decent efficiencies at amp levels instead of just milliamps.

For white LEDs the phosphor* wears down and becomes less efficient. And when phosphors fail they're just opaque. The LED is still emitting light at good efficiency, but it gets blocked before it can leave the bulb. Other leds also have a gradual dimming over their life, for other reasons. But it isn't the long downward curve that degrading phosphor has -- it's more they're fine for a long time, then start dimming and go out.

IMHO, dimming LED bulb-replacements aren't that big a deal. Just move them to a closet or some other place that doesn't need as much light. At $100 a bulb, you're not replacing all the lights in your house at once anyways.

*White LEDs work by emitting blue (or sometimes UV) light that gets converted to white multispectrum light by a phosphor.

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