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The lighting industry uses the term ‘Average Rated Life’ or ARL when describing how long it takes for 50% of lamps in a particular test batch to fail. For instance, if 100 fluorescent bulbs are tested and 50 bulbs die after 10,000 hours then that type of bulb has an ARL of 10,000 hours. Although some bulbs may have failed before or after that mark, 10,000 hours was the point where 50% had died, so it becomes the ARL.

Traditional lighting, such as incandescent, fluorescent, and HID have all been used for many decades and their various modes of failure (burned filaments, cathode burnout, electrode wear, etc.) are highly predictable. Those in the industry determine ARL service life by using the above test standard.

It is well known that LED’s last far longer than conventional light sources, and the test procedure for non-LED light sources (shown in red below), and the test procedure for LED’s is very different. Instead of testing a sample group of lights to the 50% failure point (as described above), an LED is tested by measuring the length of time it takes until its lumen output drops to 70% of its initial lumen output. The technical term for this is L70, with the “L” an abbreviation for Lumen Maintenance Measurement , with the number being the percentage drop in light output, which in this example is 70%.
 
Note that there are two different testing procedures being used to determine the lifespan of light sources. The ARL and L70 are both recorded in hours with the typical lifespans of common light sources listed as follows:
Bill Nagengast, Lighting Engineer
Solas Ray Lighting
Holds over 20 patents in the lighting industry.
TJ-31 6.28.18