Industry good practice calls for 1,000 hours at a junction temperature of 125 C.

This value is dependent on load current and junction temperature.

When it is desirable to maintain junction temperatures below 125-150 C, copper/water heat pipes are typically used.

For semiconductors, instantaneous junction temperature, rather than component case, heatsink, or ambient temperature is critical.

Thus we can determine the junction temperature by passing a current through the diode and then measuring voltage developed across it.

When the heatsink is not able to keep the temperature low enough, the junction temperature may rise quickly and uncontrollably, resulting in destruction of the device.

It is highly necessary to keep the junction temperature below 120 C to run the LED's for maximum lifetime.

The junction temperature will be lower if the thermal impedance is smaller and likewise, with a lower ambient temperature.

Parameters of transistors depend on junction temperature.

He used a galvanometer to measure current, and knew that the voltage between the thermocouple terminals was proportional to the junction temperature.