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LED-based lamps that are currently on the market are expensive due to the complex packaging required to dissipate the heat generated. This also limits their performance and lifetime due to the degradation of the phosphor or individual LED chips, in the case of RGB sources. There is a strong commercial imperative to develop in situ technology to measure and ultimately compensate for the thermal environment of a luminaire. Utilizing the large Stoke's shift in InGaN green and blue emitting LEDs, a blue LED emitter pump can induce a photocurrent within devices which emit in either the blue or green region. Measurements have shown that green and blue emitters may be excited on the absorption edge in an effect which results in a rise in the open circuit voltage with increasing temperature. From these measurements the junction temperature of a device operating in quasi-cw mode at 80 mA is shown to result in a junction temperature of 86 (±2) °C which agrees well with a junction temperature of 87 (±2) °C measured using the more conventional forward voltage technique. This paper describes the technique utilized and the results achieved in driving a green emitting LED with a blue emitting pump LED and furthermore it discusses some of the benefits and issues associated with this technique for determining the junction temperature.