LED Lifetime Lumen Degradation

All light sources degrade over time or most just “burn out”. LEDs, on the other hand, do not burn out but continue to degrade – eventually beyond useful light output, which is typically recognized as 70% of initial luminous flux (L70).

Recognizing the need for an industry standard that would also help users evaluate LED products, in 2008 the Illuminating Engineering Society of North America (IES) published LM-80. Note, however that while LM-80 provides a standard method for testing lumen maintenance, it does not provide guidance or recommendations for predicting or extrapolating maintenance beyond the duration of these measurements.

LM-80 is a standard that defines how the lumen output and colour shift of LEDs should be measured over time and at different temperatures.

The full name of the latest version of this standard is “IES LM-80-20 Approved Method: Measuring Luminous Flux and Color Maintenance of LED Packages, Arrays and Modules”.

LM80 Test

LM-80 is important to lighting designers, specifiers and manufacturers. It provides a standard method of testing LEDs and presenting the test results so that the data from different brands and models can be directly and objectively compared.

LM-80 is concerned with measuring two of the most important aspects of LED quality:

Lumen depreciation

Over time, the lumen output (luminous flux) of all LEDs gradually decreases. This is known as lumen depreciation. However, the rate of lumen depreciation varies from one make and model of LED to another and it varies according to temperature. At higher temperatures lumen depreciation takes place more rapidly than at lower temperatures.

Colour shift

Over time, the colour of light that an LED emits changes. This is called chromaticity shift. As they are used, white light LEDs might move towards the red or towards the blue end of the spectrum, and this change takes place more rapidly at higher temperatures.

LM-80 is concerned only with LEDs, LED arrays and LED modules as individual components, not with LED lamps or LED fixtures. For this reason, LM-80 data is mostly used by lighting manufacturers when selecting the LEDs to use as components in a light fitting.

Process of LM-80 Test

LM-80 was written by the Illuminating Engineering Society of North America, usually known as the IESNA, or just IES. 

Three separate sets of samples of the LED make & model being tested must be connected to a suitable power source.

Each set should be tested at a specific temperature, 55ºC, 85ºC and one other temperature that the manufacturer may select. This is done so that data between tests can be easily compared using the 55ºC and 85ºC data sets.

The third temperature, set by the manufacturer, is available so that the performance of the LED can be highlighted if it has been designed for a particular application environment such as cold store or oven.

The duration of the LM-80 test must be no less than 6,000 hours.

Measuring the lumen maintenance:

At intervals determined by the manufacturer the luminous flux (lumen output) of each LED is measured and recorded. The initial output is deemed to be 100% and all subsequent measures are recorded as a % of the initial output.

When lumen output is expressed as a % of an initial value it is termed “lumen maintenance”. For the first few hours of the test it is usual for the lumen output of an LED to increase to perhaps 103-105% of the initial value. Thereafter, the lumen output of each LED will gradually decline. It is almost always most rapid in the sample that is being tested at the highest temperature.

Lumen maintenance is averaged across each sample and plotted on a graph.

Note that the LEDs run in a higher temperature environment are depreciating fastest.

Lumen maintenance is generally expressed as an L value, so lumen maintenance of 70% or 80% would be abbreviated to L70, or L80. Using the above graph as an example we could say that at 105ºC these LEDs reach L94 after 9,000 hours of operation.

Measuring the chromaticity shift:

At intervals determined by the manufacturer the colour of the light emitted by each LED is also recorded and plotted on the CIE 1976 colour space diagram.

This enables the colour of each LED, each time it is measured, to be expressed as a numerical value, rather as a grid reference on a map gives a numerical expression of a position.

During LM-80 testing the light emitted by each LED is plotted onto this and the coordinates recorded. Any shift in colour over time can then be expressed as a numerical value.

Then, the magnitude of the changes in position of each LED can be expressed as a number, and all the numbers averaged for the sample. This gives an overall view of how stable is the colour of the light being emitted. These data are then plotted on a graph, as below:

The test is conducted on 3 sets of samples, each being run at a different temperature. Note that the LEDs run in a higher temperature environment are showing the most deviation from their initial colour.

Note that there is no pass/fail level for an LM-80 test. LM-80 is an approved method of evaluating the quality of LEDs with regard to their lumen depreciation and colour shift at 3 different temperatures over a minimum of 6,000 hours.

Using LM-80 and TM-21 to Calculate Lifetime Lumen Degradation

Once an LED chip has reached the minimum 6,000 required test hours for LM-80, a lifetime extrapolation, known as TM-21, is applied to the test data. TM-21 is the internationally recognised method for projecting lumen degradation of LEDs beyond the LM-80 test data.

TM-21 outlines a standard calculation method to predict future lumen depreciation using LM-80 data.

This standard applies to lifetime projection of LED package, array or module alone.

The methodology first normalizes the provided data to 1 (100%) at 0 hours and averages each point for all samples of the device for each test condition (temperature).

This projection information can then be used to project the expected lumen degradation of the light source as part of a complete system (fixture).