U.S. Department of Energy -
All types of electric light sources experience lumen depreciation, defined as the decrease in lumen output that occurs as a lamp is operated. The causes of lumen depreciation in incandescent lamps are depletion of the filament over time and the accumulation of evaporated tungsten particles on the bulb wall. This typically results in 10% to 15% depreciation compared to initial lumen output over the 1,000 hour life of an incandescent lamp.
In fluorescent lamps, the causes of lumen depreciation are photochemical degradation of the phosphor coating and the glass tube, and the accumulation of light-
Lumen depreciation in LEDs varies depending on package and system design. The primary cause of lumen depreciation is heat generated at the LED junction. LEDs do not emit heat as infrared radiation (IR) like other light sources, so the heat must be removed from the device by conduction or convection. If the LED system design has inadequate heat sinking or other means of removing the heat, the device temperature will rise, resulting in lower light output. Clouding of the epoxy encapsulant used to cover some LED chips also results in decreased lumens making it out of the device. Newer high-