Energy Star TM 21 Calculator.
The ENERGY STAR® TM-21 Calculator is a Microsoft Excel spreadsheet based on the U.S. Illuminating Engineering Society’s TM-21-11: Projecting Long Term Lumen Maintenance of LED Light Sources. The Calculator results have been validated by the U.S. National Institute of Standards and Technology (NIST).
Inputs to the Tool
The spreadsheet requires various user inputs, such as :
total number of units tested per case temperature,
number of failures,
number of units measured,
test duration (hours),
tested drive current (mA),
tested case temperature (Tc, ⁰C),
drive current for each LED package/array/module (mA),
in-situ case temperature (Tc, ⁰C),
percentage of initial lumens to project (e.g. L70),
time (t) at which to estimate lumen maintenance (hours) and
series of lumen maintenance factors over time. The basis for these input parameters is a precedent IES LM-80 test procedure (approved method for measuring lumen depreciation of solid-state tm-2(LED) light sources, arrays and modules). The LM-80 methods have numerous prerequisites including measured and maintained case temperatures, minimized airflow, precise voltage and current (AC or DC levels), voltage wave shape(harmonic distortion <3%), measurements performed at multiple temperatures to address in-situ conditions (55° C, 85° C and a 3rd manufacturer selected temperature), testing intervals of at least every 1000 hours with 6000 hours minimum testing period etc. on a sample of at least 10 light sources. The test will report parameters such as : description of sources tested,
ambient conditions (airflow, temperature, RH,etc.),
case test point temperature,
lumen maintenance data,
observation of failures,
LED monitoring interval,
chromaticity shift over time.
Usefulness for Luminaire Manufacturers
LM-80 tests respectivelyTM-21 calculations are usually performed or commissioned by the light source (package/array/module) manufacturer or supplier and not by a luminaire manufacturer.
While TM-21 does project the lumen maintenance of a LED source (package1 or array/module2), it does not determine traditional life or time to failure of an LED lighting system or luminaire. The useful life of an LED system has many components that need to be considered (lamp, driver, lens, etc.). LEDs degrade (like all light sources) but for potentially very long periods of time. Instead of outright failure, LEDs will eventually dim to a point that is too low to serve their purpose.
1 An assembly of one or more LED dies that includes wire bond or other type of electrical connections, possibly with an optical element and thermal, mechanical, and electrical interfaces.
2 An assembly of LED packages (components), or dies on a printed circuit board or substrate, possibly with optical elements and additional thermal, mechanical, and electrical interfaces that are intended to connect to the load side of a LED driver.