A filament fails suddenly and a diode fades. LED lumen depreciation is the gradual loss of light output over operating hours, and it is the reason a rated life figure describes a brightness threshold rather than a moment of failure. A lamp at the end of its rated life is usually still lit, just noticeably dimmer than it was.
What a rated life figure means
Lighting manufacturers state life as the number of operating hours until output falls to a stated percentage of the initial value, commonly seventy percent, written as an L70 figure. It is a depreciation threshold, not a failure point. The number comes from measured data extrapolated using standardized methods, because nobody can run a lamp for fifty thousand hours before selling it. That extrapolation is legitimate and it is still an extrapolation, which is worth remembering when a figure looks extraordinary compared with competing products.
Why output falls
Two mechanisms dominate. The semiconductor itself degrades slowly, with defects accumulating in the crystal structure as it operates. And the phosphor coating that converts blue emission into white light degrades chemically, which reduces both output and color accuracy. Both processes accelerate with temperature. A diode running at a high junction temperature ages far faster than the same part running cool, which is why thermal design inside a lamp or fixture matters more to longevity than the diodes themselves.
The driver usually fails first anyway
In practice, most LED products stop working because the driver electronics fail rather than because the diodes faded past a threshold. Electrolytic capacitors dry out, and they do so faster in heat. A lamp in an enclosed fixture, a recessed can with insulation packed against it, or a bathroom soffit with no ventilation subjects its driver to conditions the rating never assumed. This is why enclosed-fixture-rated lamps exist and why installing an ordinary lamp in a sealed globe produces disappointing life regardless of the number on the package.
Planning replacement in a commercial setting
Depreciation changes how lighting maintenance is planned. Rather than replacing lamps as they fail, facilities frequently design for a maintained illuminance: initial output is specified above the target so that at the end of the maintenance period, after depreciation, the space is still adequately lit. Group replacement at a scheduled interval then restores everything at once. That approach also solves the color matching problem, since all lamps come from one batch. In a home the same logic applies informally to a row of recessed fixtures.
What to look for when comparing products
Compare life figures only when they use the same threshold and the same test basis, because an L70 figure and an L90 figure for the same product differ greatly. Note the stated ambient temperature the figure assumes. Look for enclosed fixture ratings if that is where the lamp is going, and for damp or wet location listings where relevant. And weigh the warranty, which tells you what the manufacturer is willing to stand behind in practice, often more informative than an extrapolated hour count.
What to check before you act
- Check whether a life figure is stated as L70, L90 or something else before comparing.
- Confirm the lamp is rated for enclosed fixtures if it will be sealed in one.
- Note the ambient temperature assumption behind the stated life.
- Replace visible rows as a set rather than one lamp at a time.
Common questions
Why did my twenty-five thousand hour bulb die in two years?
Almost always the driver failed, usually from heat in an enclosed fixture, rather than the diodes reaching their depreciation threshold.
Should I replace LEDs before they stop working?
In spaces where light level matters, yes. Output falls gradually and a room can become noticeably underlit while every lamp still functions.
The practical takeaway
Read a life rating as a fade threshold with a temperature assumption attached. Keep lamps cool, use enclosed-rated products where required, and replace visible groups together. If a room feels dim but nothing has failed, depreciation is the likely explanation.
References and further reading
- ENERGY STAR: lighting qualification criteria including lumen maintenance
- Department of Energy: shopping for lighting and understanding ratings
- Department of Energy: operating cost of lighting and household electronics
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
