I was reading on another flashlight MFR web-site about over-driven LEDs. They claimed to use multiple LEDs to not overdrive a single, and brighter, LED. I know that overdriving an LED will result in a shortened life and reduction in lumens over a short time. To get to the point: would the LS series of lights fall in this category? What is the life, and expected output, of the LS LED? Layman terms please, thanks.
Over-driven LED in LS?
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Luxeon Stars have very good lumen maintenance as compared to their 5mm white counterparts. The LSes are slightly underdriven (I believe they are driven at 330mA whereas spec is 350mA). However, the joint temperature and the temperatures on die are assuredly warmer than the 25 degrees celsius that Luxeon states on their datasheets. Heat is the natural enemy of the white LED, and causes lower efficiency and shorter life.
Let's take a look at Lumiled's data sheets.
In figure 5 of: http://www.lumileds.com/pdfs/protected/AB07.PDF -- we see that even when the junction temperature of the LED is 85C and the LED is being driven at 350mA, after a period of ~2000 hours, the LED is still outputting approximately ~93% of it's original output. A 7% change in output is very small and we would be very hard pressed to notice a difference like that without comparing the before and after side by side (not to mention having very perceptive eyes). Also consider the length of time before this 93% mark arrives: 2000 Hours. In the world of flashlights, this is a very long time indeed. Though the average flashaholic uses their lights perhaps only 2-10 minutes a day, we'll, for the sake of argument, assume that we would use our LS daily for 30 minutes. Over a period of five years we would have only clocked in approximately 913 hours of usage, which is, of course, not even half of the previously mentioned 2000 hours.
5mm LEDs, however are a far different story. Their lumen maintenance is far less appealing, and degradation occurs relatively quickly.
Let's take a look at Lumiled's data sheets.
In figure 5 of: http://www.lumileds.com/pdfs/protected/AB07.PDF -- we see that even when the junction temperature of the LED is 85C and the LED is being driven at 350mA, after a period of ~2000 hours, the LED is still outputting approximately ~93% of it's original output. A 7% change in output is very small and we would be very hard pressed to notice a difference like that without comparing the before and after side by side (not to mention having very perceptive eyes). Also consider the length of time before this 93% mark arrives: 2000 Hours. In the world of flashlights, this is a very long time indeed. Though the average flashaholic uses their lights perhaps only 2-10 minutes a day, we'll, for the sake of argument, assume that we would use our LS daily for 30 minutes. Over a period of five years we would have only clocked in approximately 913 hours of usage, which is, of course, not even half of the previously mentioned 2000 hours.
5mm LEDs, however are a far different story. Their lumen maintenance is far less appealing, and degradation occurs relatively quickly.
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