LED lifetime test: the IESNA LM-80 standard

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LED Lifetime - how to calculate it?

The IESNA LM-80 standard is the internationally recognized way for Lumen Maintenance Testing of LED light sources, arrays and modules. In other words, it clarifies the way LED brightness depreciation over time is measured. Without this standard, a manufacturer could manipulate the testing in order to obtain values that makes his LEDs stand out from the competition even if their actual quality is sub-standard. A market full with low cost products with 50.000 claimed LED lifetime shows how dire the need is for standards.

The result of IESNA LM-80 can look like this:

 What does it show:

The Lumen Maintenance at 6000 hours, as percentage, where 100% is initial, measured value.

What does not show:

 Determination or estimation of expected life or lumen output beyond test data.

Details required by LM-80:

• drive current for the LEDs tested. The manufacturer can decide the drive current for each LED type which usually includes the typical value, the maximum value and an intermediary value from it datasheet.

• for each drive current the LEDs must be tested at three ambient temperatures (Ta). The manufacturer has the freedom to choose one, the others are 55°C and 85°C.   

How LM-80 prevents misleading test results:

• Testing must be made in a 3-rd party Internationally Recognized Testing Laboratory. The manufacturer can use its own lab only if it accredited for the specific testing by a national or international Accreditation body.
• Intermediary results must be measured and recorded, at least at each 1000 hours.
• Testing and measuring equipment is properly calibreated
• Every aspect of testing is recorded

Results of the test must include: 

• Description of Sources tested
• Sample size
• Ambient conditions (Airflow, temperature, RH etc)
• Case Test point temperature
• Electrical conditions
• Lumen maintenance data
• Observation of failures
• LED monitoring interval
• Chromaticity shift over time

Without testing according to the LM-80 standard any measured lifetime claim is unfounded and must be received with a high degree of scepticism.

All major LED producers now follow the LM-80 testing standard, among them Nichia, Cree, Philips, Osram, Bridgelux and many others.

LED Lifetime, how to calculate it?

 This article is related to the articles that try to come up with answers to the question: "Are the savings promised by LED technology always real?" 

 The fact that a series of articles is needed shows the complexity of this issue.

Before this year, the lack of standards in the LED industry made things even more complicated.
In 2013, we finally have a standard way to predict the L70 lifetime of an LED. And once the LED lifetime is known the it becomes easier to calculate the return of investment for a LED installation.

 I remind what L70 lifetime means:

LED lifetime for use in general lighting = the time when the light output (lumens) has reached 70% of the value from the beginning. It is called L70 lifetime and is presented in the form of a graph:

The fact that LEDs can have a lifetime which can be up to 100 times longer than the time many producers or distributors test them on their premises has raised serious concerns about the validity of LED lifetime claims of tens of thousands of hours, 50.000 hours being the one most used.

To address the issue the IESNA (Illuminating Engineering Society of North America) has created two standards, both of international reach and a must respect for any serious LED producer.

The IESNA standards are the LM-80 and the TM-21. The need to have two standards is based on the fact that a LED lifetime claim has two parts:

1. Actual testing of the LED for a variable amount of time: 500 to 10.000 hours in which the evolution of its luminous flux is observed
2. Extrapolation of the decrease in the luminous of the LED over time with the focus on the moment its value reaches 70% (L70).

LM-80 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.

Without these standards a producer or distributor could pretty much obtain a LED lifetime to suit his advertising needs, by:

- testing too few LEDs
- testing the LEDs in the best possible ambient conditions
- observing the LEDs for only a short time
- using its own (and often secret) method to extrapolate the results, "adjusting" the calculations to reach the desired L70 lifetime.

In the next posts i will briefly present the LM-80 and TM-21 standards because the market needs to be made aware about them and make use of the knowledge when purchasing LED technology. Without LM-80 and TM-21 we can only find out if the savings promised by LED technology are real via costly and time consuming trial-and-error purchases.

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