Red , green, and blue commercial LED packages have been selected from two manufacturers. A nominal drive current of 350 mA was selected, resulting in an input power of about 1 W. The junction temperature was determined according to the forward voltage method described before. The packaged LEDs were placed in a Heraeus UT6 isothermal oven with active air circulation. Four following temperatures have been selected within a range of typical LED operation temperature: 300,320,340 and 360 K. These predefined temperatures have been precisely measured with a PT100 thermistor, four-wire connected with a Keithley 2510 TEC SourceMeter. For each setting, at least 20 min delay between the set-temperature and the measurement was respected in order to assure thermal equilibrium between the air in the oven and the diode junction. Thermal equilibrium was checked by measuring the forward voltage at a small current (1 mA) every two minutes. Voltage measurements were performed with a Keithley 2440 5A SourceMeter used in a four-wire set-up and controlled by a LABVIEW 7.1 program. Once the thermal equilibrium had been reached, a pulse of a few milliseconds at 350 mA normal drive current was applied and the forward voltage during the pulse was measured. Successive voltage measurements indicate that junction temperature increase in the LED die during these measurements can be neglected.