Why has my LED desk lamp got dimmer over time?

Two things degrade together. The phosphor coating on the diodes breaks down with heat, and the driver's electrolytic capacitors dry out and deliver less current. Both are accelerated by a housing that cannot shed heat, which is why cheap plastic-bodied lamps dim fastest.

Why it happens

LEDs emit blue light; a phosphor layer converts part of it to produce white. That layer is the part that ages. As it degrades, output falls and the colour usually drifts — often noticeably bluer or greener than when the lamp was new.

The driver ages independently. Electrolytic capacitors have a liquid electrolyte that evaporates through the seal, faster the hotter they run. As capacitance falls the driver delivers less current, which dims the lamp before the diodes themselves have failed.

This is why quoted lifespans mislead. A 50,000-hour rating describes diodes under controlled thermal conditions. In a lamp with an ABS housing and no heatsinking, the real figure is a fraction of that, and the driver often gives out first.

What to do about it

1. 1Check whether the housing gets hot near the LED strip. Warm aluminium is the heatsink working; a cool plastic body around hot diodes is the failure mode.
2. 2If the lamp takes a replaceable bulb, this is a bulb swap. Many integrated-LED lamps are glued or ultrasonically welded and are not designed to be opened.
3. 3When replacing, an aluminium body is the single best predictor of longevity — it doubles as the heatsink.