The temperature of the light-emitting junction itself, written Tj. It cannot be measured directly in a finished product, so it is estimated from a case or solder-point temperature measured with a thermocouple plus the package's thermal resistance. Higher Tj lowers light output, can shift color and speeds up lumen depreciation, which is why luminaire thermal design and ambient ratings matter as much as LED choice.
Ts reads 92 C in the sealed can, so junction temperature is hotter still, and their LM-80 only tested to 85 C case.
Also heard as
- Tj
Related terms
LED
Light-Emitting Diode: a semiconductor light source, made white for lighting by coating a blue chip with phosphor.
L70
The number of operating hours until an LED product's light output falls to 70 percent of its initial value.
LM-80
The test method for tracking how LED packages lose light over time, the basis of lifetime claims.
Droop
The fall in an LED's efficiency as drive current or temperature rises, so extra power yields proportionally less light.