How long will the LED in the Arc 4+ last?

What is the expected life of the LED in the Arc 4+?

From the most recent Lumileds data sheet for the white 1w emitter:

"Lifetime for solid state lighting devices (LEDs) is typically defined in terms of lumen maintenance—the percentage of initial light output remaining after a specified period of time. Lumileds projects that Luxeon products will deliver on average 70% lumen maintenance at 50,000 hours of operation. This
performance is based on independent test data, Lumileds historical data from
tests run on similar material systems, and internal Luxeon reliability testing. This projection is based on constant current 350 mA operation with junction
temperature maintained at or below 90°C. Observation of design limits included
in this data sheet is required in order to achieve this projected lumen
maintenance."

I would expect an average loss of 30% output at least after 50k hours if not substantially more since we keep the LED junction from ever going above 60c.

50k hours = ~68 years at 2 hours a day of use

And remember that is just for 30% loss. A 30% reduction in light output is hard for the human eye to see (1/2 F-stop is 50%). The difference in brightness on the Arc4 between 1 level to the next adjacent level is 50%.

Peter

Peter,

Is it possible to perform accelerated deterioration tests?

I suppose it isn't because over driving won't scale like salt spray corrosion tests. I guess we'll all be old and gray before we know.

By that time luxeons will be doing 500 lumens per watt so it won't matter!

It's like worrying about electromigration of your CPU. By the time it becomes a factor, the CPU is so obsolete the property taxes you're paying to store it are more than it's worth! /ubbthreads/images/graemlins/smile.gif

Cheers!

Peter,

You keep the junction below 60C?

How do you do this? Here are my thoughts below.

Junction = the LED die.

Case = Slug on the emitter.

Or does the chassis of the ARC4 stays below 60C?

I highly doubt you keep the junction below 60C. Why I'd be led to believe this:

Level 1 is ~ 3.6V and 700mA for 2.52 W.

Thermal resistance of LED die (junction) to slug is ~15 C/W

15 C/W * 2.52 W = 37.8 C rise junction to slug.

Normal room temperature ARC4 housing is 25C.

Assumming a perfect transfer from slug to air (which it is far from this) you'd be at a junction temperature of 62.8 C right when you turn it on.

When running it in human hand, human hand is 37 C. If you had perfect thermal transfer from human hand (the ARC4 body was at human hand temperature and it had a perfect transfer to the slug, again far from it) 37.8 C + 37 C = 74.8 C junction (die) temperature.

What you probably meant to say is you hold the slug at 60 C or the ARC4 body at 60C?

If the actual slug was held at 60 C, at level 1, 700mA and 3.6V, you have 2.52W. 15 C/W * 2.52 = 37.8 C rise from slug to junction, so we take the slug temperature of 60C and add the rise due to the thermal resistance between the slug and junction, and we get 60C + 37.8C = 97.8C junction temperature.

See Application Note: AB05 located at http://www.lumileds.com/pdfs/protected/AB05.PDF

*****
Also in your comment, "This projection is based on constant current 350 mA " (which would be a 1W emitter at 350 mA, not 2.52W (700mA for level one))
****

Luxeon Application Brief AB07, http://www.lumileds.com/pdfs/protected/AB07.PDF

"Figure 4 shows the average lumen maintenance at 4000 hours for white Luxeon Power Light Sources driven at 0.35A at 25°C case temperature (TJ @ 45°C). Note that virtually nolumen depreciation occurred after 4000 hours. Figure 5 shows the average lumen maintenance at 2000 hours for white Luxeon Power Light Sources driven at 0.35A at 85°C case
temperature (TJ @ 105°C). Note that the lumen
depreciation is somewhat higher at elevated
temperatures."

To me, it appears that there is a 10% drop in light output after only 2000 hours, according to chart 5.

Does the ARC4 actually embed a temperature sensor into the luxeon case (slug)? Or is the sensor clear back on the board, and really measure the ARC4 housing temperature, which is lower than the slug temperature?

This is another one of those "dead horse" things. By the time you see lumen depreciation of 30%, Arc would probably be making lights that put out 5-10 times as much light using the same power as they do now /ubbthreads/images/graemlins/grin.gif Seriously though, I think many of us here have cycled through various lights while still not hitting more than a couple hundred hours on it. There are probably exceptions, like those that use their lights for work, but most flashaholics have a pretty fast turning revolving door for their collection.

I agree - It's moot since the LED will last until you've spent about $1k on lithium, at which point, what's another $100 for a new light gonna matter?

Good point CM. I talked to Henry about this tonight and he said the thermal gradient is larger than I thought. Here's what he said:

"What we do is to regulate the temperature of the interior of the case to a maximum of 55 degrees C, which means that the junction temperature will be higher than that. However, full power is only available up to 45 degrees C, so the junction temperature should not get much above 85 degrees C for any significant period of time (depends on the time constants in the thermal impedance - 15 to 30 seconds roughly). Steady state with a human heat sink should be around 80 degrees C. If you add another 5 degree C for other thermal gradients, you are probably quite close.

After 45 degrees C, the junction temperature should drop - for every 2 degrees C increase in case temperature the power is reduced by 0.707 - which produces a junction thermal drop much greater than the 2 degree rise."

So the original assumption of 50k hours is safe but the max temperature is higher than I thought.

Again, that is 30% dimming over 68 years with a usage of 2 hours a day at full power. 1 hour a day would increase that figure to 136 years, again assuming the entire usage was at full power. Running the light at level 4 or lower will of course increase the lumen maintenance even further. Basically, you can use this light as much as you want without any worry.

The Arc4 has a good thermal management system. You should know that we are still the only people so far using a temp sensor to monitor the LED health.

Peter

Good discussion of temperature at the thermal junction. There hasn't been mention of Lumileds' other projection factor - current to the emitter @ a constant current of 350 mA. The Arc4 pushes at least twice that on level 1.

I wouldn't assume that temperature is the only important factor. My bench grinder removes steel just as fast whether I have water in the trough or not. Not comparable, I know, but I wonder...

These figures won't matter if 2x current = 1/2x emitter life, but what if 2x current = 1/10x emitter life, how about 1/100x? These things aren't always linear.

Peter, could the Lumileds tech department offer comment?

What they tell me is that temperature is the determining factor of lumen maintenance. You could send enough current through the bond wires to melt them (2 amps?) but the lumen maintenance of the die/phosphor (if you could reconnect it) would be the same.

Peter

Interesting, actual independent tests on the Luxeon, by the Lighting Research Center show a 10% drop in light output with a 45 degrees C die temperature (30 degree C slug(case) temp)after only 9,000 hours. Then LumiLEDs shows 10% drop in light output at 2,000 hours with a die temp of 105C. (see Luxeon App. Note AB07). So heat is a definitely a major player. Though I do wonder if *Temperature* is the only factor, especially with the 500 hour "lifetime" of the 5W Luxeon. (see http://www.lumileds.com/pdfs/protected/DS40.PDF)

So the 10% point is somewhere between these two.

It has been nearly two years since the Lighting Research Center report came out, and no further updates have been forthcomming.

Another worthwhile figure to look at is the Luxeon III emitter datasheet and Figure 5. It is found here, http://www.lumileds.com/pdfs/protected/DS45.PDF Its the current derating curve based on ambient temperature and thermal resistance from junction(die) to ambient. Why it is useful, is that it includes information at 700mA.