App note from OSRAM on different thermal masuring locations of LEDs. Link here (PDF)
When current passes through the junction area of a chip, light is emitted. Not only light is generated, but also a lot of heat. Good thermal management is a major factor for the stable performance of LEDs in applications. However, a high junction temperature has a negative effect on the lifetime and the reliability of LEDs.
Defining the junction temperature poses a challenge because it cannot be measured directly. However, the junction temperature can be calculated by measuring the solder point.