Junction temperature is the temperature where a diode connects to itís circuit board. A low junction temperature increases output and slows LED lumen depreciation. Junction temperature is thus a key metric for evaluating LED quality and life expectancy.
The three things affecting junction temperature are: drive current, thermal path, and ambient temperature. In general, the higher the drive current, the greater the heat generated at the die. Heat must be moved away from the die in order to maintain expected light output, life, and colour. The amount of heat that can be removed depends upon the ambient temperature and the design of the thermal path from the die to the surroundings.
The US Department of Energy advises: Heat management and an awareness of the operating environment are critical considerations to the design and application of LED luminaires for general illumination. Successful products will use superior heat sink designs to dissipate heat, and minimize junction temperature.
Keeping the junction temperature as low as possible and within manufacturer specifications is necessary in order to maximize the performance potential of LEDs.