Measuring LED Efficacy: Power supply, Thermal and Fixture Losses

Luminaire Light Output (includes fixture efficiency and thermal effects

   Luminaire Efficacy    =      Luminaire Input Power
 
The ENERGY STAR criteria for non-SSL residential light fixtures (commonly referred to as
RLF) use system efficacy defined as the light output of the lamp-ballast system divided by the
input power measured in a 25°C environment.  Established test procedures for fluorescent
sources support this approach.  However, the program requirements in this document are
based upon luminaire efficacy, instead of system efficacy for the following reasons:
 
• Accurate measurement of the LED light source separate from the fixture is often not
possible. 

LEDs typically are integrated into the fixture and not readily removable without altering the performance of the light source.

• LED performance is significantly affected by elevated temperature. 

LED devices generate heat that is typically removed by an external heat sink, which may be
designed into the luminaire itself.  Separating the light source from its heat sink will
significantly impact test results.  There are no standards or test procedures to measure
system efficacy of LEDs.  However, standards organizations are developing a test
procedure for photometric measurement of LED luminaires.  This test procedure is
currently in draft form and scheduled to be final by the 4th quarter of 2007.

• Luminaire efficacy provides more realistic energy efficiency information because it
accounts for power supply, thermal, and fixture losses

This better meets the needs of buyers who seek the most light output for the least energy input.