On Mon, 31 Oct 2016 18:08:23 GMT, Scott Lurndal wrote:
How long do LED shop/ceiling lights really last at full output anyway?
http://www.digikey.com/en/articles/t...rightness-leds
Thanks for that url.
Here's a relevant set of snippets from:
Understanding the Cause of Fading in High-Brightness LEDs
By Steven Keeping
Contributed By Electronic Products, 2012-02-21
LED failureˇ is most likely to be the result of light output falling below
an acceptable threshold (typically 70 percent of the initial output.
The primary cause of that fading (or ´lumen failureˇ) is triggered (for the
most part) by the minute threading dislocations introduced to the chip
during wafer manufacture.
Threading dislocations are a major problem...where threading dislocations
are vertical micro-cracks caused by strain generated by the mismatch in
InGaN and Sapphire or SiC crystal lattices .. and where ... things get worse
over time, as the rate of degradation is directly related to the initial
density of threading dislocations and the heat to which the LED is subjected
.... all of which gets worse ... due to heating during operation, thermal
expansion and shrinkage when the LED is turned on and off, and mechanical
stress such as vibration.
Worse yet ... as the chip ages, it will run hotter and hotter ... due to an
increased number of phonons, accelerating the formation of dislocations and
the device˙s eventual demise.
-------------------
So now we know what kills LEDs to the 70% level (which is the formal
definition of dead), which is that inherent cracks between crystals form
over time, just as mud cracks as it dries at the bottom of a pond.
The fundamental problem is cracks between crystals only gets worse.
b. Heat makes things worse even faster
c. On/off cycles makes things worse even faster
d. Vibration makes things worse even faster
So, given they don't heat/vibrate/cycle the LEDS (other than the 4 hours per
day), you'll likely never get the advertised L70 lifetime in the real world.
But what do you get for a 70%-illumination lifetime in the real world?