LM-79 and LM-80 get mentioned together so often it's easy to assume they measure the same thing at different points in time. They don't — they answer two genuinely different questions and require two different kinds of instrument.
LM-79: What Does This Product Do Right Now?
LM-79 answers a single-point-in-time question: how does this complete luminaire perform under controlled conditions, today? It measures total luminous flux, luminous efficacy, chromaticity, CCT, CRI, and luminous intensity distribution (the IES/LDT file) — the figures that end up on a spec sheet and that Energy Star and procurement specifications reference directly.
Two measurement methods satisfy LM-79: an integrating sphere with spectroradiometer, or a Type C goniophotometer for full angular distribution data. LM-79 doesn't apply to bare LED packages, and it doesn't tell you anything about performance after years in service — that's LM-80's job.
LM-80: How Will This LED Hold Up Over Time?
LM-80 addresses an entirely different question: how much does output degrade over time, and how fast? It's scoped to LED packages, modules, and arrays, not complete luminaires (that's the separate LM-84 standard). Samples run continuously at three defined case temperatures — 55°C, 85°C, and a manufacturer-selected third — for a minimum of 6,000 hours, 10,000 recommended, with flux and chromaticity measured every 1,000 hours.
Output is expressed as L-values: L70 is the time until output degrades to 70% of initial value. Because LEDs fail by gradual dimming rather than sudden burnout, L70 has become the industry's de facto "end of useful life."
TM-21: Turning 6,000 Hours of Data Into a 50,000-Hour Claim
TM-21 provides the mathematical method (exponential decay with Arrhenius temperature-dependence) for extrapolating LM-80 data into a projected lifetime. The limit: a projection can extend no further than six times the actual test duration — a 6,000-hour dataset supports a maximum 36,000-hour projection, which is why serious programs run the recommended 10,000 hours.
Side by Side
| LM-79 | LM-80 (+ TM-21) | |
|---|---|---|
| Question answered | How does it perform now? | How will it degrade over time? |
| Test subject | Complete luminaire | LED package, module, or array |
| Test duration | Single measurement | 6,000–10,000+ hours continuous |
| Key outputs | Lumens, efficacy, CCT, CRI, intensity distribution | L70 / L50 projected lifetime |
| Required equipment | Integrating sphere + spectroradiometer, or goniophotometer | Temperature-controlled aging chamber + photometry, multi-channel |
Why a Lab Usually Needs Both
LM-79 generates the performance numbers a customer sees on day one; LM-80/TM-21 generates the lifetime number that justifies the warranty. Skipping LM-80 means any lifetime claim on the datasheet has no test basis behind it, increasingly a problem for procurement specs now asking for LM-80/TM-21 documentation as standard.
In equipment terms, this means a lab needs an LM-79 system (sphere and/or goniophotometer) plus a distinct LM-80 aging system — different instruments solving different problems, and budgeting "LED test equipment" as one line item without distinguishing the two is a common planning mistake.