by Steven Keeping:
High-brightness LEDs tend not to burn out rapidly. Rather, they slowly fade away. A carefully designed LED lighting system can see the light source last for up to 50,000 or even 70,000 hours before the illumination is no longer sufficient for the intended job.
What causes the chip to lose luminosity, and can anything be done to arrest the decline? This article reviews the physics behind an LED’s photon generation to explore what happens when the device is new, and then why the performance inevitably deteriorates as the chip ages.
Understanding the Cause of Fading in High-Brightness LEDs - [Link]