Do led's dim with high usage?

Im not gonna mention any specific light but I use my led lights ALLOT as a mechanic. And I noticed that at least one with a cree in it keeps getting dim after about 6 months of heavy usage... Is it the heat or something or do all leds just get dimmer over time.
They all speak about 50,000 hour lifetimes but if it dims a good amount in 1000 hours what good is it?

Dragonfueled said:

Im not gonna mention any specific light but I use my led lights ALLOT as a mechanic. And I noticed that at least one with a cree in it keeps getting dim after about 6 months of heavy usage... Is it the heat or something or do all leds just get dimmer over time.
They all speak about 50,000 hour lifetimes but if it dims a good amount in 1000 hours what good is it?

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Well, first, the normal "50k" or "100k" lifetimes arent appliable to all leds. In fact, most bright 5mm leds have VASTLY shorter lifetimes, and very hightly driven Crees & Co arent up to that level either...

Also, the definition of lifetime is mostly (but not always) defined by a dimming by a certain factor (down to 50%, or 30%, ect).
So "50k hours lifetime" doesnt mean they break after 50k hours, but that they have dimmed down to half brightness, for example.

(A normal "lifetime" in the sense of "until it breaks" doesnt really make sense for leds, because under normal enviroment they should produce at least _some_ light after obscene amounts of time.

Yes, they do. Especially if they're driven hard and not properly protected from excessive heat accumulation.

OTOH, it could also be dirty contacts from all that use at your shop. :thinking:

Absolutely everything that uses phosphors to generate light dims over time, because all currently available phosphors wear out. Most fluorescent bulbs, tube TVs, and plasma televisions will dim to half of new brightness in less than 100,000 hours of continuous operation (about 11.5 years). If you like to crank up the brightness of your TV, it can wear out much faster (this is a topic of frequent discussion over at avsforum). The problem is different-color producing phosphors wear out at different rates so you'll get a color shift over time (ever notice how really old color TVs tend to get that greenish tint?) so a video enthusiast will have their TV recalibrated occasionally.

LCD flat panel TVs are no different--the backlight is made from either fluorescent, or in some specialty models, white LEDS so the phosphor issue is the same.

This is why it's important to read the datasheet--many times a life rating is based on a certain junction temperature that is impossible to achieve in practice (like 25C), or there are different ratings for different temperatures. Phosphors in LEDs for lighting tend to be driven much harder than any other application because of the small surface area available.

Sometimes there's no datasheet and it's up to people to test it themselves. I'd suggest a search for "lumen maintenance" to start.

Sounds like your light is poorly heatsinked, if at all.
Cree leds have proven performance and are well tested before they go to market. They don't fade in output if they are properly installed in a manner where they can safely shed excess heat.

I would also point out that incandescents ALSO tend to dim over time. Don't believe me? Take a car whose headlamps have been extensively used but not burned out yet. Replace the bulb on one side with a brand new one of the identical type. The new bulb should be noticeably brighter than the old one. I've noticed that every time I have replaced the headlights in my car that the new bulbs always seem brighter for a time compared to the old ones.

The most degredation that I've seen was in a Surefire L4 that was used a whole bunch. I also notice that my old ArcAAA is only about 50%-60% of the brighness it used to be. That little thing must have close to a thousand hours of run-time on it.

Avatar28 said:

I would also point out that incandescents ALSO tend to dim over time. Don't believe me? Take a car whose headlamps have been extensively used but not burned out yet. Replace the bulb on one side with a brand new one of the identical type. The new bulb should be noticeably brighter than the old one. I've noticed that every time I have replaced the headlights in my car that the new bulbs always seem brighter for a time compared to the old ones.

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I'll bet that's due to deposits inside the bulb rather than any loss of output from the filament.

Dragonfueled said:

Im not gonna mention any specific light but I use my led lights ALLOT as a mechanic. And I noticed that at least one with a cree in it keeps getting dim after about 6 months of heavy usage... Is it the heat or something or do all leds just get dimmer over time.
They all speak about 50,000 hour lifetimes but if it dims a good amount in 1000 hours what good is it?

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The 50,000 or 100,000 numbers are just single points on a set of curves indicating the 'lumen maintenance' verses current and junction temperature.

There is no single magic number, but rather most manufactures post these curves so you can estimate the drive levels and heatsinking needed to give acceptable life.

The curves are very steep at the extremes, so it is quite possible to damage an LED in only a few hours (or even seconds) at high enough temperatures and current levels.


As far as your experience goes, even a full 8 hours per day for 180 days is less than 2000 hours.

I can't imagine ANY scenario where a mainstream manufacture's light would be so poorly heatsinked that noticeable dimming would occur this quickly.

I can only think of two things that would explain your experience.

1) You are using 3.5 volt lithium batteries to 'supercharge' a light designed for normal 1.5 volt AA or AAA cells.

2) Your problems are actually being caused by oxidation and could possible be improved by a good contact cleaning.

When I see posts where folks talk about using lithium batteries in flashlights designed for simple AA's or AAA's, and they talk about the light running for a short time before the LED 'turns angry blue' (or just dims), I have to cringe because this is exactly the kind of thing which causes permanent damage to an LED emitter.

If you are one of those o' so clever folks [at least THEY think so] that like to use non-approved lithium batteries in your L1D, L0D, etc. and the LED is going to hell after six months, my only comment would be, DON'T COMPLAIN, YOU'R LUCKY THAT IT EVEN WORKED THAT LONG.

On the other hand, if it's a good quality light, using the correct batteries, then you might want to check for oxidation problems, because there is no way it should be dimming after only six months use.

Short answer would be yes. But not by much, depending on how well the LED is heatsinked.

The Fenix P3D CE sample I reviewed a while back had its max output reduced by about 0.2% after doing all the runtimes tests without any additional heatsinking.
Personally I would consider this negligeable as its only about 0.32 lumens out of the total 160 lumens.

Dragonfueled said:

... my led lights ALLOT as a mechanic.

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My LEDs aren't as clever. They just allot as light producers. I have a Cree I put in a little torch to replace the 9 LED array that it came with. When I compared it to other Crees that haven't been used I found that it's noticeably dimmer at the same current. There's nowhere for the heat to go but I wouldn't have thought it was that bad. The contacts are sometimes a bit dodgy on that torch, so I think some more testing is in order.

TorchBoy said:

My LEDs aren't as clever. They just allot as light producers. I have a Cree I put in a little torch to replace the 9 LED array that it came with. When I compared it to other Crees that haven't been used I found that it's noticeably dimmer at the same current. There's nowhere for the heat to go but I wouldn't have thought it was that bad. The contacts are sometimes a bit dodgy on that torch, so I think some more testing is in order.

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It's always a good idea to check for contact problems in older lights, but actually, this kind of 'drop-in' mod is one of the easiest ways to get into a situation where the LED gets super hot. The little 'star' that the LED is mounted on can only dissapate a fraction of a watt by itself in free air, and even less than that if it's bolted to a slab of plastic or some other non-heat-conductive material. If your light only drives the LED at a hundred or hundred and fifty milliamps (about 1/2 watt), then it may be fine, but if you are driving the LED at a full watt (or more) then it's probably running really hot, and that would explain the dimmng you are seeing.

Does the flashlight have a metal body you could heatsink to?

It's about 1W, the body is metal, and I couldn't figure out a way of joining the two. It's still awesomely bright for such a little thing, though. Don't think I'm unhappy there.

could tenergy rcr123 be hurting the light? they are limited though so I guess not

Dragonfueled said:

could tenergy rcr123 be hurting the light? they are limited though so I guess not

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I can see how you could get bit by this. Folks that run 10440's in place of AAA's or 14500's in place of AA's know that they are cheating, but most folks think that RCR123's and CR123's are the same thing.

They are not, RCR123's have a little higher voltage. In some lights (most cr123 lights actually) RCR123's will cause really high drive current and temperatures.

We all love how bright the higher voltage of the RCR123 makes our lights look (and even better that you can just recharge the battery and repeat the thrill without spending a dime on fresh batteries), but if the light runs really bright and hot on RCR123's, then that's probably the problem that is causing your LED's short life.

Luminescent said:

I can see how you could get bit by this. Folks that run 10440's in place of AAA's or 14500's in place of AA's know that they are cheating, but most folks think that RCR123's and CR123's are the same thing.

They are not, RCR123's have a little higher voltage. In some lights (most cr123 lights actually) RCR123's will cause really high drive current and temperatures.

We all love how bright the higher voltage of the RCR123 makes our lights look (and even better that you can just recharge the battery and repeat the thrill without spending a dime on fresh batteries), but if the light runs really bright and hot on RCR123's, then that's probably the problem that is causing your LED's short life.

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what he said ^, and said so well, many of the low voltage drivers go slam into overdrive with a full li-ion, WOW.

time to SOCKET the leds