How Long Does an LED Fixture Really Last?

Part 3
on Oct 9, 2015 in Lighting Basics
The challenge of comparing multiple fixtures
(This is the final post of a three part series – Part 1 and Part 2)
fixture decision – chalkboardThe lumen maintenance measurement for estimating LED depreciation has several important limitations. In addition to those information gaps, lumen maintenance also makes it difficult to compare more than one fixture – even if the fixtures are from the same manufacturer. The guidelines used to standardize the reporting of lumen maintenance (TM-21) only allow manufacturers to report an extrapolation of six times the duration of the test data. That means a manufacturer using an LED chip which has only been tested for 6,000 hours must limit the lumen maintenance report to 36,000 hours.
Too many terms
This results in some confusion. Some manufacturers only publish this 6 x [data] number, but many others see benefit to be gained by extrapolating the numbers further. For example, Philips Gardco makes a distinction between calculated lumen maintenance and published lumen maintenance, using an asterisk on the later to denote the TM-21-imposed limitation. This results in two different numbers, distinguished by technical terms that may not be understood by someone who is not a lighting professional. Other manufacturers make this same distinction but use different terminology, or they may include multiple calculated numbers and only one published number (as in the case of the Cree ZR24 – see Image #1).