TM-21, the right way to calculate LED lifetime

In the article LED Lifetime - how to calculate it? 
we have explained that the proper way to calculate the lifetime of an LED is with a pair of two standards LM-80 and TM-21.

 LM-80 (explained here) sets the standard of the actual testing while TM-21 specifies how to extrapolate the LM-80 results to times beyond the LM-80 test time.

Now we will briefly explain how the TM-21 extrapolation method can be used to predict LED lifetime and obtain a graph that looks like this:

From the LM-80 report, spreadsheets with a lot of detailed raw data obtained from testing:

We take the following:

• lumen maintenance data recorded at maximum 1000 hour intervals, over a period of at least 6000 hours for the three Case Temperatures (55°C, 85°C and manufacturer chosen)
• sample size where 10 samples is minimum and 20 samples is the optimum for each of the three data sets
• Test duration (hours)
• Tested drive current (mA)
• Number of failures

...and use the calculation method detailed in the TM-21 to predict the lifetime of an LED up to a maximum of 6 times the measured interval.

One can use the following calculator which has all the formulas hardwired.

In conclusion, the industry has moved to set standards which lead to clear rules in the race to acquire customers with lifetime and performance claims. Now it is up to the customers too become informed and ask for supporting data when faced with a lifetime graph like this one:



For 75000 hours at L70 the manufacturer must provide: LM-80 raw test data for 12.500 hours and extrapolation based on the TM-21. Without them the lifetime claim can simply be ignored when making a purchase...