Fluorescent fixtures were tested using relative photometry, measure the lamp alone, measure the fixture's efficiency, multiply. That works for replaceable lamps. LEDs are integrated systems with thermal, electrical, and optical interactions. Relative photometry can over-claim by 10-20%. LM-79 absolute photometry tests the entire LED fixture as one unit, at operating temperature.
Four ideas to carry into the next specification conversation you have.
Works only when lamp is replaceable and identical lab-to-field. Inherits assumptions, prone to error in LED.
Whole luminaire in goniophotometer at steady-state temperature. Real driver, real LED, real heatsink. No scaling.
Manufacturer claims 10,000 lm with relative photo. Real LM-79 test at 25°C: 8,500 lm. At 35°C ambient (Saudi summer): 7,800 lm. Plan around true numbers.
LM-79 = total output (1 hour). LM-80 = LED-package lumen depreciation (6,000+ hours). TM-21 = mathematical extrapolation to L70 (~50,000 hrs). Three different tests.
What each test measures and what it doesn't tell you.
| Test | Duration | Measures |
|---|---|---|
| LM-79 absolute | 1-3 hours | Total flux, distribution, SPD |
| LM-80 LED package | 6,000-10,000 h | Package lumen depreciation |
| TM-21 extrapolation | Mathematical | L70 lifetime estimate |
| LM-82 thermal scaling | 1-2 days | Performance vs temperature |
| LM-84 system depreciation | 6,000+ h | Whole fixture (not just LED) |
| Relative photometry | 1 hour | Old method, for lamp+fixture |
Five reasons every premium spec demands LM-79.
LED light output drops ~10-15% from 25°C to 65°C junction temperature. LM-79 tests at stable operating temperature, relative photometry doesn't.
Relative photometry assumes 100% driver efficiency. LM-79 includes the real driver. Saudi-spec drivers operating at 50°C ambient can lose another 3-5%.
Lens, diffuser, reflector all absorb 5-15% of LED light. LM-79 measures fixture-out flux directly; relative photometry estimates.
Saudi SASO recognises LM-79 for SABER LED testing. DLC for North American projects mandates it. ENERGY STAR LED listing requires it. No LM-79 = no certification.
Two LED fixtures from different makers, both with LM-79 reports, are directly comparable. Without LM-79, you're comparing claims, not measurements.
Every NLC LED ships with LM-79 absolute photometry from accredited labs (NVLAP or equivalent). Plus LM-80 + TM-21 for L70 lifetime claims.
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