The existing diodes in an LED or laser diode package are used to measure the junction temperature of the LED or laser diode. The light or laser emissions of a diode are switched off by removing the operational drive current applied to the diode package. A reference current, which can be lower the operational drive current, is applied to the diode package. The resulting forward voltage of the diode is measured using a voltage measurement circuit. Using the inherent current-voltage-temperature relationship of the diode, the actual junction temperature of the diode can be determined. The resulting forward voltage can be used in a feedback loop to provide temperature regulation of the diode package, with or without determining the actual junction temperature. The measured forward voltage of a photodiode or the emissions diode in a diode package can be used to determine the junction temperature of the emissions diode.