With incandescent bulbs, light is generated by heating a tungsten filament to high temperatures, and leads to macroscopic materials degradation and loss. Within an LED, light is generated through electron-hole recombination and is a materials conserving process. The crystalline structure of the LED is unchanged in the light emission process, although microscopic changes can occur. At normal low drive currents and voltages, these microscopic changes are small. The macroscopic versus microscopic nature of the degradation processes also leads to different failure mechanisms.
Incandescent bulbs fail suddenly and catastrophically when the filament ruptures.
The performance of LEDs with time typically follows a predictable degradation of light output with time. The most common lighting industry characterization is rated life. For example, one can expect that half of the incandescent lamps rated at 1000 hours will fail after 1000 hours of use. No reference is made to the degradation of light output over the rated life. The semiconductor industry also refers to catastrophic failure as a definition of device life.