LightLab International

LM80, ISTM testing, TM21 Calculations

LM80 testing, TM21 calculations

The North American IES standard LM80 is used to estimate the photometric and colorimetric performance of LEDs over time. A number of LEDs are randomly selected and mounted onto a carrier and placed into an environmental chamber. The LEDs are tested at a specific drive current and ambient temperature. At intervals during this test the luminous flux, CCT and other metrics are measured. The test is run at three different ambient temperatures: 55°C, 85°C and a temperature chosen by the LED manufacturer. The samples are operated at these temperatures and at a specific drive current for a period of 1000hrs, then the LEDs are removed from the oven, allowed to cool to 25°C ambient and their light output characteristics are measured as per the LED manufacture's specified method; typically by energising the LED for a short interval between 10 and 20 ms. The LEDs are then placed back in the oven and the process repeated for a minimum of 6000hrs (approximately nine months).

Once the LM80 test has concluded, the measurements are used to determine a depreciation curve for the luminous flux at each temperature. The data is extrapolated using the method described in the North American Technical Memorandum TM21 to determine the "Lumen Maintenance Life Projection (Lp)". The end point of the extrapolation is typically 70% of the initial light output. There is a spreadsheet calculator available at the website to perform the calculations.

ISTM testing

The ISTM test measures the operating temperature and drive current of the LEDs inside the luminaire as would be encountered when the luminaire is normally used ("In-Situ"). The LM80 report specifies the location of the Temperature Measurement Point (TMP), the same point is measured duing the ISTM test. The applicable Standard for the ISTM test is UL1598 in the USA and AS/NZS 60598.1 in Australia. The TMP temperature and drive current must be below that in the LM80 report for the LM80 results to be applicable to the luminaire.


To qualify the Lp for the luminaire the following are required :

1. LM80 test on the LEDs which includes a TMP location.

2. TM-21 calculation performed with the LM80 measured data to extrapolate an Lp.

3. ISTM test to determine the LED operating temperature and hence interpolate the lumen depreciation curve.

LightLab testing of energy efficient products

LightLab operates a NATA accredited laboratory in Brisbane and an NVLAP accredited laboratory in Phoenix, Arizona. We can assist you with your testing of energy efficient lighting products. Both laboratories perform LM-79* photometric tests and ISTM* tests. Contact us for more information.