What “fading” means inside an LED
An LED does not have a filament that snaps. Instead, its semiconductor chip converts electrical energy into light, and the chip’s ability to do that gradually declines. Over years of use, tiny changes in the chip and the materials around it can reduce how much light comes out for the same electrical input. In many white LEDs, a coating called phosphor changes some of the chip’s light into the warmer mix we see; that coating can also lose efficiency over time.
Heat sets the pace
The main accelerant is heat. LEDs use less energy than incandescent bulbs, but the small chip still generates heat that must travel through the bulb’s base and into the surrounding air. If that heat has nowhere to go—inside a sealed fixture, for example—it can speed the aging of the chip, phosphor, and other parts. Good thermal design helps the bulb hold its brightness longer.
Manufacturers often describe this gradual loss as lumen depreciation: lumens are a measure of visible light output. A bulb may still turn on while producing noticeably less light than when it was new.
Why some LEDs still go dark at once
A complete LED bulb also contains a driver, the electronics that regulate power to the chip. If the driver or a connection fails, the bulb can stop working suddenly even though the LED itself might have continued producing light. So gradual dimming is a common aging pattern, not a guarantee: the bulb’s brightness can fade slowly, while its supporting electronics can still fail abruptly.
