LED lifespan: L70, LM-80 and TM-21
What a "50,000-hour" rating really means, how it is tested, and why LEDs fail
L70 is the number of operating hours until an LED's light output falls to 70% of its initial value. Because LEDs usually dim gradually rather than burning out, lifetime is defined by this lumen maintenance threshold, measured under IES LM-80 and projected with ANSI/IES TM-21.
An incandescent bulb dies suddenly when its filament breaks, after about 1,000 hours. An LED usually keeps working for years while slowly getting dimmer, so "how long does it last?" needs a definition. This guide explains the L, B and F ratings on datasheets, how LM-80 testing and TM-21 projections work, what actually makes LED products fail, and how to turn rated hours into years.
Lumen maintenance: L70, L80, L90 and B values
LEDs rarely burn out; they slowly dim. L70 is the time until light output falls to 70% of its initial value, the point at which most people notice the loss. Ratings such as L70B50 add a B value: the share of units expected to fall below that level by the rated time.
The 70% threshold comes from research at Rensselaer's Lighting Research Center, which found that most people do not notice a gradual drop of up to about 30% in light. Where light levels are designed tightly, as in offices and retail, specifiers often ask for L80 or L90 instead. When no B value is given, B50 is assumed: half the units will have dimmed further by the rated time.
| Rating | Meaning | Typical use |
|---|---|---|
| L70 | Hours until output falls to 70% of initial | General lighting; ENERGY STAR and DLC |
| L80 | Hours until output falls to 80% | Offices and retail, tighter designs |
| L90 | Hours until output falls to 90% | Museums, premium and critical lighting |
| B50 | Half of units below the L value at the rated time | Default (median) when no B value is stated |
| B10 | Only 10% of units below the L value | Conservative specifications |
| C and F values | C: abrupt failures (units that stop working); F: gradual and abrupt combined | IEC 62717 and IEC 62722-2-1, e.g. L80F10 |
LM-80: how LED packages are tested
IES LM-80 is the test method for LED packages, arrays and modules: samples run continuously at fixed case temperatures, usually 55 °C, 85 °C and a third chosen by the maker, with light output and color measured at least every 1,000 hours for 6,000 hours or more, and preferably 10,000.
LM-80 is a measurement method, not a pass-or-fail test, and it does not include the driver, optics or fixture. Its report gives lumen maintenance and color shift for each temperature and drive current tested. A fixture maker who keeps the LEDs at or below the tested case temperature can then use the data to project the life of its product.
TM-21: projecting lifetime
TM-21 turns LM-80 data into a lifetime projection by fitting an exponential curve to the later test data and extrapolating. Projections are capped at six times the test duration with 20 or more samples, so 10,000 hours of testing supports a claim of at most "L70 greater than 60,000 hours".
The fit uses the last 5,000 hours of data for tests of 6,000–10,000 hours, or the last half of longer tests, ignoring the early "burn-in" period when output can even rise. With 10–19 samples the cap is 5.5 times the test duration. Results are reported in the form L70(10k) > 60,000 h, where the number in brackets is the test duration. A companion method, IES TM-35, projects long-term color shift, which can end the useful life of a product before its light output does.
From LED chips to lamps and luminaires
LM-80 and TM-21 describe only the LED package. The whole lamp or fixture also depends on how hot its LEDs run and on the driver. IES LM-84 and TM-28 extend testing and projection to complete lamps and luminaires, and in-situ temperature measurement tests confirm the LEDs stay within their tested range.
Reputable programs such as ENERGY STAR and the DesignLights Consortium check that a fixture's LEDs run no hotter than their LM-80 data covers. A headline "50,000 hours" on a box is only meaningful if it is backed by such data and accounts for the driver, which often limits the life of the product more than the LEDs do.
Why LEDs fail
Most LED products fail because of something other than the LED chips: the driver's electrolytic capacitors dry out, heat degrades phosphors and lenses, sulfur or moisture corrodes silver-plated parts, solder joints crack with thermal cycling, and voltage surges or static destroy components. In series strings, one open LED can darken the whole string.
- Driver: electrolytic capacitor life roughly halves for every 10 °C rise, so hot, enclosed drivers wear out first. Surges from lightning and switching are another common cause; roadway fixtures are tested to ANSI C136.2 surge levels of 6, 10 or 20 kV.
- Heat: a hot junction speeds up lumen depreciation and yellows or browns silicone, phosphor and plastic lenses, which also shifts color.
- Chemical attack: sulfur from rubber, adhesives or polluted air tarnishes the silver reflector in many SMD packages, darkening them; moisture causes corrosion in poorly sealed fixtures.
- Mechanical: repeated heating and cooling cracks solder joints and bond wires, especially in parts that switch on and off in cold environments.
- Strings: an LED that fails open breaks a series string, so many products dim or go dark in sections; one that fails short slightly increases the current through the others.
How to make LEDs last longer
Heat is the main enemy. Choose lamps rated for enclosed fixtures if they will be enclosed, keep heat sinks clear of insulation and dust, and avoid running LEDs at their maximum current. Use surge protection outdoors and quality drivers with 105 °C capacitors. Frequent switching, unlike with fluorescents, barely affects LED life.
For DIY builds, keep the LED case temperature within the range covered by its LM-80 report, use a heat sink with adequate area, and drive high-power LEDs well below their maximum rated current: running at 70% of maximum current typically sacrifices less than 30% of the light while running much cooler. See thermal management for the underlying physics.
Rated hours in years
Divide the rated hours by the hours used per year. A 25,000-hour bulb used three hours a day lasts about 23 years, while a 50,000-hour fixture running 12 hours a day lasts about 11. Real life is shorter in hot, enclosed or poorly ventilated fixtures and with low-quality drivers.
| Rated life | Usage | Hours per year | Years |
|---|---|---|---|
| 15,000 h | 3 h/day (home) | 1,095 | 13.7 |
| 25,000 h | 3 h/day (home) | 1,095 | 22.8 |
| 25,000 h | 8 h/day (kitchen, office) | 2,920 | 8.6 |
| 50,000 h | 12 h/day (retail) | 4,380 | 11.4 |
| 50,000 h | Dusk to dawn (streetlight) | ~4,100 | 12.2 |
| 50,000 h | 24/7 (parking garage) | 8,760 | 5.7 |
| 100,000 h | 24/7 (tunnel, garage) | 8,760 | 11.4 |
Frequently asked questions
How long do LED bulbs really last?
Typical LED bulbs are rated 15,000–25,000 hours, about 14–23 years at three hours a day; fixtures are often rated 50,000–100,000 hours. Heat and driver quality decide whether they get there.
What does L70 mean?
The operating hours until light output falls to 70% of its initial value. L70B50 means half of the units are expected to have fallen below that level by then.
What is the difference between LM-80 and TM-21?
LM-80 is the test that measures LED lumen maintenance for at least 6,000 hours; TM-21 is the method that projects those results into a lifetime, capped at six times the test duration.
Does turning LEDs on and off shorten their life?
Not meaningfully. Unlike fluorescent lamps, LEDs have no electrodes to wear, so frequent switching has little effect on their life.
Why did my LED bulb fail early?
Usually heat or the driver: an enclosed fixture, a recessed can buried in insulation, cheap capacitors, a power surge or an incompatible dimmer. The LEDs themselves rarely fail first.
Sources and further reading
- IES LM-80-20, Approved Method: Measuring Luminous Flux and Color Maintenance of LED Packages, Arrays, and Modules, Illuminating Engineering Society (IES).
- ANSI/IES TM-21-21, Technical Memorandum: Projecting Long-Term Luminous, Photon, and Radiant Flux Maintenance of LED Light Sources.
- IES LM-84-20 and IES TM-28-20, measuring and projecting luminous flux and color maintenance of LED lamps and luminaires; IES TM-35-19, projecting long-term chromaticity shift.
- IEC 62717, LED modules for general lighting: Performance requirements; IEC 62722-2-1, Luminaire performance: Particular requirements for LED luminaires.
- U.S. DOE and Next Generation Lighting Industry Alliance, LED Luminaire Lifetime: Recommendations for Testing and Reporting.
- ANSI C136.2, Roadway and Area Lighting Equipment: Luminaire Voltage Classification, Dielectric Withstand and Surge Requirements.
- N. Narendran, Y. Gu, "Life of LED-based white light sources," Journal of Display Technology 1, 167 (2005).
Image credits
- LED floodlights on the Novospassky Bridge: Retired electrician, CC0, via Wikimedia Commons.