# Do LEDS (SSC/Rebel/Cree) get brighter or dimmer after some time of use?



## dinocol (Feb 8, 2008)

Do LEDS (SSC/Rebel/Cree) get brighter or dimmer after some time of use?


----------



## evan9162 (Feb 8, 2008)

Both.

Most power LEDs experience a slight brightening over the first few hundreds to thousands of hours of use. Afterwords, they very slowly begin to dim.

LED lifespan (i.e. the 50,000 hour lifespan rating) is not about how long before the LED does not light up, it's about how long before the LED dims to some percentage of initial output. Most players rate their LED lifespan to 70% of initial output. So that means that at 50,000 hours, the LED should still output around 70% of initial output. The LED slowly dims over that 50,000 hours from 100% down to 70%.

This lifespan rating is usually given certian conditions that the LED maker specifies, in terms of current and junction temperature. Operating beyond the conditions given will cause the lifespan to decrease.

So for instance, a (hypothetical) manufacturer specifies 50,000 hours to 70% brightness, at 350mA, with Tj <= 120C. If your heatsinking allows Tj to exceed 120C, then the lifespan will decrease, and output will dim faster. Likewise, if you run at greater than 350mA, lifespan will also decrease. This means that you either reach 70% sooner, or at 50,000 hours output has degraded to less than 70%


----------



## LukeA (Feb 8, 2008)

evan9162 said:


> Both.
> 
> Most power LEDs experience a slight brightening over the first few hundreds to thousands of hours of use. Afterwords, they very slowly begin to dim.
> 
> ...



Manufacturers pick 70% as the cutoff because output drops pretty precipitously after that. The degradation curve isn't linear (I'm sure you, evan9162, know that too, but others may not). The emitter's brightness stays fairly constant for about 40k-45k of those 50k hours, then begins an exponential decline. When output hits 70%, there aren't more than 5k-10k hours of light output left. 

Higher junction temps than the manufacturer recommends dramatically shorten the lives of LEDs. This is apparent in certain threads here about 5mm LED-based household lights which overdrive their LEDs. 5mm LEDs are inherently poorly heatsinked, and overdriving them results in their degradation in a few hundred hours, rather than fifty thousand. Of course, even with junction temps higher than spec, high-flux LEDs in flashlights last long enough so as not to be a concern. Five hundred hours of light from a flashlight is more than the vast majority of people will ever need in a lifetime, whereas five hundred hours from a night light that runs constantly off of household power in a home will result in its failure in less than two months, which is unacceptable.

Again, sorry, evan9162, for hijacking your post, but this information might be useful to some searcher in the future.


----------



## jtr1962 (Feb 8, 2008)

LukeA said:


> The emitter's brightness stays fairly constant for about 40k-45k of those 50k hours, then begins an exponential decline. When output hits 70%, there aren't more than 5k-10k hours of light output left.


That explains a lot-I have a long term test Q bin Luxeon with in excess of 35,000 hours on it. Brightness is just about the same as it was two years ago.



> Higher junction temps than the manufacturer recommends dramatically shorten the lives of LEDs. This is apparent in certain threads here about 5mm LED-based household lights which overdrive their LEDs. 5mm LEDs are inherently poorly heatsinked, and overdriving them results in their degradation in a few hundred hours, rather than fifty thousand.


Is the converse true, meaning that if you underdrive at power LED at around 100 to 150 mA, you'll be able to get perhaps 1 million hours of life? Just curious because I could see how 1 million hours would be useful in the streetlight industry where the ultimate goal is to have the light source last as long as the fixture (~200 years).


----------



## dinocol (Feb 9, 2008)

thanks for the very informative reply. I noticed that my p2d rebel and novatac became brighter after some time.. im jst not sure if i was imagining things..


----------



## evan9162 (Feb 9, 2008)

The brightness increases are impossible to notice with the human eye. We're talking about a slight increase on the order of maybe 5% or so. It's something you'd have to be closely monitoring with a light meter and some controlled conditions.


----------



## LukeA (Feb 9, 2008)

jtr1962 said:


> Is the converse true, meaning that if you underdrive at power LED at around 100 to 150 mA, you'll be able to get perhaps 1 million hours of life? Just curious because I could see how 1 million hours would be useful in the streetlight industry where the ultimate goal is to have the light source last as long as the fixture (~200 years).



I don't know for sure, but that would certainly seem reasonable to me. If the die can 'cook' at 100˚C for 50k hours to 70%, then it should stand to reason that the effects of temperature below 100˚C on die lifespan would be opposite to those of temparature above.

I would imagine that the graph of time v. temp. to 70% output should look something like this, with the x-axis representing die lifespan in 10k hour increments and the y-axis representing temperature in 100˚C increments:






The equation for this graph is y=-logx+1.7.


----------

