Both terms and their effects, need simultaneous review when discussing LED performance decline over time.

Depreciation commonly is used to refer to the decline in brightness over time as a light source is used, known as lamp lumen depreciation.

LEDs also degrade.

Degradation, many may think of chemical decomposition but there is also elegant degradation. One or both of these conditions seems commonly referred to, and one or both, may occur at the same time.

LED performance depreciates over time as less lumens are emitted. The more any light is used, the more a light source brightness dims over time.

LED lumen depreciation can be solely due to heat conditions. Research has verified this independent of LED operation.

Evidence of LED lamp lumen depreciation is useful, however we know in addition to lumen deprecation, LED Kelvin CCT (Correlated Color Temperature) changes over time.  If change in apparent colour appearance of an LED, and lumen depreciation are plotted at the same point, we can see how these changes may parallel each other in some LED sources, while for others, they may be independent.

Decline in LED lamp lumens and colour temperature change over time varies between manufacturers. It seems sensible to test LED Kelvin CCT measurements and plot them at the same time as Lumen measurements are taken when LED life performance testing is conducted.

LED lumen maintenance curves decline due to LED use and thermal conditions.

All lamp lumen performance declines due to use. Lumen maintenance curve decline is also affected by the number of times a lamp is turned on & off, the thermal environment of the lamp and luminaire, and lamp ballast/driver electronics.

New research shows LED lumen performance declines due to heat without LEDs being turned on. During testing LEDs were turned on only for short duration lumen measurement data collection.

This has large implications for exterior LED lighting where high ambient temperatures are present, as is typically found inside luminaires exposed to direct sunlight during daytimes when luminaires are not on.

The higher the ambient temperature, the faster LED lumen performance declines. Not all high brightness LEDs perform the same way.  Different manufacturer’s LEDs exhibit different rates of decline.  Variation in lumen performance decline must be due to differences in LED manufacture technique and material.

I am interested to review Electron microscopic time-elapse images & data collected at different points along the lumen maintenance curve. No doubt, there is something new to discover to improve manufacturing technique & fabrication for more robust LEDs.