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September 2018

The Lumen Maintenance Gap

This article about the Lumen Maintenance Gap is an excerpt from a document entitled Understanding LM-80, Lumen Maintenance, and LED Fixture Lifetime proposed by Color Kinetics.

Test results of lumen maintenance in conventional and LED light sources differ considerably. This is known as the Lumen Maintenance Gap. These tests are performed to determine the useful life of a light source. But how do we compare lighting technologies satisfactorily if test results of a same designation do not mean the same thing? These lumen maintenance measurement distinctions are what we will examine in this article, as a clear understanding of this gap will have a significant impact on the installation, maintenance and replacement cost calculations of lighting products.

Lumen Maintenance of LED and Traditional Light Sources

When controlled, LED light sources can have a useful life that lasts considerably longer than the rated life of a conventional lighting source. The following table presents comparative data for the typical useful life range of various light sources.

Lumen Maintenance of LED and Traditional Light Sources

Testing Conventional Light Sources

LM-65-14 is a document that defines life testing procedures for compact fluorescent lamps (CFLs) while LM-49-12 defines life testing procedures for incandescent filament lamps. Both publications establish testing conditions, testing sample sizes as well as valuation methods used for generalizing test data.

For CFL’s, LM-65 specifies that a statistically valid sample be tested at an ambient temperature varying between 15⁰C and 40⁰C, in a cycle of three hours on and 20 minutes off (CFL life is appreciably shortened by the frequency with which the lamp is turned on and off). The point at which half the lamps fail is the rated average life for that lamp.

For incandescent filament lamps, LM-49 stipulates that a statistically valid sample be tested within the manufacturer’s stated operating temperature range and voltage. Lamps are turned off and allowed to cool to ambient temperature once a day (usually for 15 to 30 minutes). As with CFLs, rated life for incandescent filament lamps is the total operating time at which half the lamps cease to operate.

Testing Conventional LED Sources

As of 2014, the technical publication entitled LM-84-14 was introduced as a means to fill a gap created by the lack of standardized LED fixtures and lamp testing methods. Even today, the rated life is calculated according to tests performed on lighting system diodes and not on the overall lamp or fixture system. Unfortunately, at this point, the LM-84 publication has disclosed only light system test parameters and is unable to provide other calculation methodology to issue results that might establish a specific rated life.

Many fixture manufacturers develop their individual interpretation of the life test results and establish in their own way the useful life of light sources. Often, they do not share the extrapolation or evaluation methods, so any comparison between the products of various manufacturers becomes both difficult and imprecise.

Lumen Maintenance Projection