Life of a led

How long does it take for a led to loose its brightness? I heard UV leds loose 50% brightness around 200 hours, and then it just gets worse and worse. Does this also apply to white leds? How about leds in other colors? /ubbthreads/images/graemlins/confused.gif

Have any of you noticed your leds getting dimmer and dimmer everytime you turn it on?

The human eye generally notices differences in brightness in multiples- that is, a light must be half as bright for it to be easily determined as getting dimmer. While LED's are not magical infinite light machines, most of them are used too little to get noticeably dimmer. I don't know about UV LED's, but white LED's are a special case since they use a layer of phosphor which will degrade with current- the former estimates of 100,000 hours are now around 8-10,000. Still, unless you are using an LED lamp that stays on for many, many hours every day, it is unlikely your LED lights will ever dim to even close to 50%, by which time better LED's will be out to make you buy everything all over again anyway /ubbthreads/images/graemlins/wink.gif.

I think you are referring to the old 5mm UV LEDs.

This doesn't happen with the current generation of UV LEDs, such as the 1W UV Power part from CREE.

Yes the 5mm white LEDs also age fairly fast.

With the new materials they use in the packaging in the high power parts, you don't see nearly the same aging rate as the cheap 5mm parts. Silicone, better thermal paths, and other things that you find in the high power parts greatly enhance the lifetimes, especially if you don't over drive them and properly heatsink them.

I remember folks asking if ARC had improved their AAA/AA lights again, as they were comparing their old lights to the new ones. Guess what? Their old ones were just getting dim...

Take a look at the current generation of parts:
http://www.lrc.rpi.edu/programs/solidstate/completedProjects.asp?ID=73

That is quite depressing to hear Newbie, I wish my lights would never dim, so I do not need to buy any new lights...

Hahaha yah right! No wonder ppl here are constantly buying new lights, to replace their dimming 2 week old lights.

I have a "nightlight" in my daughter's room with about 6 LEDs in
series, with a current limitting resistor and wallwart combo that
presumably rather overdrives them...

the UV (purple) LED got dim pretty early in it's life. The white
LED lasted some months longer (both of these cheap imports from eBay)
Interestingly, the most recent to die was a (supposedly - eBay again)
brand-name HP true-green LED (no phosphors), and the light is down
to red, yellow, and orange, which haven't show any noticable reduction
in brightness. I'm also finding it interesting that only the light-emitting
characteristics are changing - apparently the LEDs continue to conduct,
with something like the original voltage drop, or my series string would
show more dramatic changes in the remaining LEDs.

There's a thread elsewhere ("26k LED shootout"?) that shows that
overdriving the cheap import LEDs seems to dramatically reduce their lifetime.
At 20mA (rated current), they do OK. At 60mA as you might get in a direct
drive flashlight, they go fast! High priced nichias do better...

BillW

[ QUOTE ]
billw said:
There's a thread elsewhere ("26k LED shootout"?) that shows that
overdriving the cheap import LEDs seems to dramatically reduce their lifetime.
At 20mA (rated current), they do OK. At 60mA as you might get in a direct
drive flashlight, they go fast! High priced nichias do better...

BillW

[/ QUOTE ]

This may be the thread you are referring to.

Paul

There are a few things at play in the degredation of a LED:

1) Die degredation: This is generally a factor of both heat, and also current density (in general how hard you drive the LED), and of course design.

2) Phosphor degredation: On white LEDS, the phosphor does decay over time. Again, a matter of design, heat, how hard you drive the LED.

3) Package degredation : For UV\Blue with the use of Epoxy packaging (in 5 mm), the actual plastic will yellow and cloud over time reducing light output, shifting color, etc.

As has been stated, most 5mm LEDS are not truly 100K hour devices and white 5mm even driven properly decay significantly after 10k hours.

However, even the newer power LEDS from Lumileds, Nichia, Cree, Seoul, et. all will degrade. With the exception of Lumileds, I don't think the other suppliers make their light output degredation properties easy to find (if they officially publish them at all). Sometimes you will see them quote a life time of say 50K hours, but then you see that it is quoted at a die temperature that given typical ambient temperatures and drive currents, you would be unlikely keep the LED die under so what good is a rating like that?

If I remember correctly, the current Luxeon products are rated at their 50K hours (or so) with a die temperature of 90(95?) celsius. (The K2 is claimed to do the same at much higher temperatures.) Even at 90C, it can be tough to develop a heatsink that will work depending on how you are using the LED and where you need to put it. I have yet to see other suppliers claiming (via official literature) lifetimes as high with similar die temperatures, but I would imagine they must all be working towardst this. If anyone has official literature on light output over time and temperature from other suppliers, I would love to see it as it may open up some other product avenues.

Semiman

This is borderline scandalous!

I was really hoping that this thread would have a longer life.

While I am well beyond trusting in advertising, one would be hard pressed to find some LED light packaging that DOESN'T contain the customary "100,000 hour" adage.

For us non-tech types, what can we expect from the common offerings like ARC AAA's, Photon's or especially those Eternalights and Lightwave AA, C or D cell lights bought for long-term emergency use? Is each example it's own study, and does life span depend on whether it's regulated or not?

Now I know 10,000 hours is still superb service, but are we really only getting 1/10 of what was "advertised"? I'm almost at the point where I'm looking around for batts that already have the initial "edge" taken off them for use in my 5mm whites - just to be gentle!


Thanks!

W

Makes you wish they started putting sockets for LEDs in lights so you can replace them easily. I have been looking around locally for a socket for such and have yet to come across one designed for LEDs.

LOL scandalous, perhaps we are all in denial?

I have to agree that I feel cheated by the 100,000 hour advertised life span of an led, while in reality, even if they lasted that long, they have probably already dimmed down to 50% in less than half the advertised life span, which becomes unusable and have to be ditched, which defeats their advertised foreverness.

Doesn't this just make LEDs nothing more than longer lasting incandescents?

"Doesn't this just make LEDs nothing more than longer lasting incandescents?"


That exact thought came to me when I read the previous plug-in led thought. I wondered just where I'd start carrying spare flashlight "bulbs" again!


W

If you socketed them, they would last even a shorter period of time since they would have a much poorer thermal path and heat kills LEDS. For 5mm white LEDS, you will be lucky if you get 1/10th to 1/20th of their life before they are at half brightness.

SemiMan

[ QUOTE ]
SemiMan said:
If I remember correctly, the current Luxeon products are rated at their 50K hours (or so) with a die temperature of 90(95?) celsius. (The K2 is claimed to do the same at much higher temperatures.) Even at 90C, it can be tough to develop a heatsink that will work depending on how you are using the LED and where you need to put it. I have yet to see other suppliers claiming (via official literature) lifetimes as high with similar die temperatures, but I would imagine they must all be working towardst this. If anyone has official literature on light output over time and temperature from other suppliers, I would love to see it as it may open up some other product avenues.

Semiman

[/ QUOTE ]

"initial light output remaining after a specified period of time. Lumileds projects that white, green, cyan, blue, and royal blue Luxeon III products will deliver, on average, 70% lumen maintenance at

50,000 hours of operation at a 700 mA forward current
or

50% lumen maintenance at 20,000 hours of operation at a 1000 mA forward current.

Lumileds projects that red, red-orange,
and amber Luxeon III products will deliver, on average 50% lumen maintenance at 20,000 hours of operation at a 1400 mA
forward current.

This projection is based on constant current operation with junction temperature maintained at or below 90°C.
remember this is die temp, not slug or heatsink temp


Luxeon V- Heatsink temp
35° C 700 mA 90% lumen maintenance at 500 hours
75° C 700 mA 75% lumen maintenance at 500 hours
85°C 600 mA 65% lumen maintenance at 500 hours

[ QUOTE ]
SemiMan said:
If you socketed them, they would last even a shorter period of time since they would have a much poorer thermal path and heat kills LEDS. For 5mm white LEDS, you will be lucky if you get 1/10th to 1/20th of their life before they are at half brightness.

SemiMan

[/ QUOTE ]

If the sockets were designed properly they would heatsink about the same as mounting them to a circuit board as 5mm LEDs wick through the leads. The idea is you could overdrive cheaper LEDs to get brighter output and replace them with nicer new LEDs like the 26k and nichias.

[ QUOTE ]
Teh said:
LOL scandalous, perhaps we are all in denial?

I have to agree that I feel cheated by the 100,000 hour advertised life span of an led, while in reality, even if they lasted that long, they have probably already dimmed down to 50% in less than half the advertised life span, which becomes unusable and have to be ditched, which defeats their advertised foreverness.

Doesn't this just make LEDs nothing more than longer lasting incandescents?

[/ QUOTE ]

LEDs still have advantages over incandescents, such as dropping a flashlight won't break the LED in most cases. Also, as battery capacity decreases, an LED can still be bright to the human eye, whereas incandescents will dim tremendously (e.g. AAA Maglite solitaire dims to complete darkness when the battery is almost done, but take out the AAA battery and put it in the Dorcy AAA, and you get tons of bright light!)

As for LEDs not living up to 100,000 hours of life, well it sort of reinforces my thinking that nothing in life lasts forever. We will all die of old age someday too.

You guys weren't really expecting to pass on your flashlight collection to your children and grandchildren, were you?

[ QUOTE ]
solarwinds said:
You guys weren't really expecting to pass on your flashlight collection to your children and grandchildren, were you?

[/ QUOTE ]

Certainly, especially when you get into the 200 dollar and up range...

I have been a big fan of the led lights since discovering them but looking at some of the numbers disturbs me. With the cheaper led lights like a dorcy it's almost a disposable light so dimming is not much of a concern but it's hard for me to justify the cost of some of the higher end led lights given the lifespan of leds. This has made me consider going back to incandescents for some of my needs. At least with incandescent lights you can pop in a new lamp and the light is as good as new, whereas an led will get dimmer and dimmer throughout it's life and apparently at a much faster rate than most manufacturers would have us believe. /ubbthreads/images/graemlins/icon23.gif

All this is not alarming to me. Here's how I interpret "70% output after 100,000 hours":

70% output is literally barely noticeable from full output in a direct A/B comparison. It is also apoproximately the luxeon-lottery range within a bin-code. And 100,000 hours is 11.41 years of continuous output.
Conclusion: if you run your led light at full power for 8 hours each day, Sundays and holidays included, you will barely notice the dimming after 1/3 of a century, and only in a direct A/B comparison with a new led. NOT worth getting all upset about it.