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”
BACKGROUND TO LM-80 – WHY WE NEED IT & WHAT IT COVERS
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 (sometimes known as the luminous flux) of all LEDs gradually decreases. In layman’s terms, they get dimmer. This is 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.
Note that an LM-80 compliant test will not give any useful data related to absolute lumen output or energy efficiency. For these data a report from an LM-79 compliant photometric test is required.
Colour shift. Over time, the colour of light that an LED emits changes. In the LED and lighting industries 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. Click here for more information on colour (chromaticity) shift.
Note that an LM-80 compliant test will not give any useful data on the absolute quality of the light emitted by an LED. Data on the quality of light (such as its colour temperature, colour rendering index or spectral composition) can be found in a photometric test report. An LM-80 test is only concerned with quantifying the degree to which the colour of the light emitted by an LED changes over time.
Note that 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. For an end-user, specifier or installer to judge the relative merits of an LED lamp or an LED lighting fixture and LM-79 compliant photometric test report is recommended.
Some individual LEDs, LED arrays and LED modules. LM-80 defines how the performance of components like these (not complete luminaires) should be tested and how the data should be presented for comparison.
WHO WROTE LM-80?
LM-80 was written by the Illuminating Engineering Society of North America, usually known as the IESNA, or just IES. In addition to LM-80 the IES has written many other authoritative standards that are accepted worldwide. These include:
- LM-79-19. Approved Method: Electrical and Photometric Measurements of Solid State Lighting Products
- TM-21-19. Technical Memorandum: Projecting Long Term Lumen Maintenance of LED Light Sources
WHAT IS AN LM-80 TEST?
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. This is lumen depreciation. 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, as below.
This is a lumen maintenance graph from an LM-80 report.
Output is expressed as a % of the initial light output. This is called the lumen maintenance.
The minimum duration for an LM-80 compliant test is 6,000 hours.
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 depreciating fastest. This is expected.
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.
The CIE colour space diagram. 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 (or not!) is the colour of the light being emitted. These data are then plotted on a graph, as below:
The magnitude of the colour change is given a numerical value by summing all the differences (the Δs) for each LED at each sample point.
The minimum duration for an LM-80 compliant test is 6,000 hours.
The test is conducted on 3 sets of samples, each being run at a different temperature. Note that the L.EDs run in a higher temperature environment are showing the most deviation from their initial colour. This is to be expected
Finally, please note that there is no pass/fail level for an LM-80 test. Strictly speaking, LM-80 is not a test at all. It 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.
LM-80 describes how the evaluation should be conducted and how the resulting data should be presented.
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