Effective heat sinking is a key factor in ensuring stable LED performance over a long lifetime. The LED junction temperature influences the luminous flux of the device, its lifetime, wavelength and efficacy. As described by James Hooker of Lighting Equipment News, the junction temperature is influenced by the drive current, the provision of heat sinking, and the ambient temperature.
LEDs are generally rated in terms of their output at a junction temperature of 25 ºC, although several people questioned why this was the case when that value was unlikely ever to be attained in practice. The actual junction temperature can be calculated by adding the temperature of the mounting board to the product of the board's thermal resistance and the wattage of the LED.
Hooker gave a practical demonstration of the effect of temperature on LED brightness, using Luxeon devices from Lumileds. A few minutes after switching on the LEDs the package became hot to the touch and the intensity of the light output dropped appreciably. When the LED package was taped to a large aluminum plate that acted as a heat sink, the light output returned to its original level.