Device: Avago HLMP-EG15-RU000, LED 5MM 635NM RED WATER CLEAR, 516-1379- ND
Driven with 20mA CC source.
Avago publishes a graph showing the relative output decreases down to about 80% within 500 hours of operation. The graph starts at 100 hours, so data is not available under 100 hours. I'm seeing about a
4% increase in output during the first 48 hours of operation.
Is this typical of early LED operation?
Get brighter for the first ~100 hours then start dimming down??
If you got the impression that intensity binning for even stable LED's was broad you'd be right :-). 4% is so far down in the noise.
Is it measured in a temperature stable environment? Efficiency and forward voltage drop can vary quite a bit with temperature. I remember making the most bizarre efficiency curves with temperature changes until I figured out that forward voltage drop changing was a non- negligible effect.
Maybe you had inconsistencies in your photometric measurements. When I do an aging test on an LED, I like to have the LED and the photosensor mounted onto some frame in common. The mounting needs to be sturdy.
Also, how constant was the temperature during this time? This LED has output at Tj=100 C only about 63% of that when Tj is 25 C. The curve looks to me close enough to exponential that I feel OK to assume that the temperature change that results in 4% change in light output is close enough to 75 degrees C times log(.96)/log(.63). That is
6.6 degrees C change to change light output by 4%.
(I am referring to Figure 9 of the datasheet for the part mentioned here.)
Then again, I have had white high power LEDs in aging tests improve their light output while they were "breaking in". My experience includes light output increasing for as much as a couple hundred hours before light output begins to fade. The increase was generally 1-3 percent.
What kind of cooling are you using? How are you conducting the heat away? How stable is your measuring system? Do you have a stable reference for calibration? etc?
I'm not sure I can help you. But I believe this is the same series of LED from Avago that I saw excess 1/f noise from when I drove them with more than 30mA.
Can you monitor the forward voltage as well as light intensity?
Have you looked at other LED's?
Perhaps these high intensity LEDs are a bit flakier.
The output of LEDs is unreliable as a constant light source, but you can easily regulate it with a photodiode and feedback. If you care about a few percent (or ... I seem to recall... 30% per year long-term drift) in the output, you need to close a feedback loop.
The intrinsic feature that makes this so, is that the LED has to have currents and electric fields at or near the surface (because buried currents don't emit light efficiently), and there's always some 'dirt' on the surfaces. Dirt moves.
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