Estimating LED lamp life with LM-80 and TM-21
LED Life and Forward Current
The life of the actual LED which produces the light is partly determined by the operating temperature of the LED (LED junction temperature) such that the lower the temperature the longer the life of the LED. The other main factor affecting LED life relates to the forward current which as mentioned is proportional to the brightness of the LED. Generally, forward current is not a major problem as LED manufacturers will specify safe operating ranges, although upper ranges do require better heat sink design. If the LED runs very hot lower forward currents can extend the life of the LED chip, although generally if the LED chip is kept relatively cool with good heat sink design (below 85°C), the life time will not vary too much.
L70 LED Life Expectancy
LED light bulbs rarely fail catastrophically like there incandescent predecessors. When manufacturers specify the life of a LED or life of the whole LED light bulb in hours they are referring to the L70 data which represents a reasonably accurate theoretical estimate of the time taken for the LED to lose 30% of its brightness or reduced down to 70% of its original brightness, hence L70. This is typically specified in the range of 30,000 to 40,000 hours and is often referred to as Lumen maintenance or Lumen depreciation.
However, the LED light will not suddenly fail but continue to produce light well beyond the L70 life point, albeit at a lower brightness until the light becomes too dim to be useable. Estimates suggest the LED light will last for up to 100,000 hours or more before it finally shuts down. In many applications that is “light for life”! Therefore, LED life is usually plenty long enough and does not really become an issue except during extended operating hours or high operating temperatures, which as mentioned can result from poor design.
Calculation of L70 Life Expectancy using LM-80 test data and TM-21 Extrapolation
Calculation of the L70 LED life point is a relatively simple but time consuming process utilising the LM-80 test data which requires the brightness or lumens testing of multiple LED samples at three different temperatures, 55°C, 85°C and one other, and determining the loss in light intensity at multiple time points over 6000 to 8000 hours. The testing can take almost one year to do.
As dicussed, once we have the LM-80 test data for the LED chip we can then extrapolate using the TM-21 method which is basically an exponential function and formula to ascertain the L70 life in hours of the LED chip when it declines to 70% of its output.