This paper signifies the importance of dual interface method in the determination of real junction-to-board thermal resistance, Rth(JB) for high power infrared (IR) emitter. Thermal transient measurement and optical test are performed to investigate thermal and optical properties of the IR emitter. In order to identify the exact point of separation between the IR package and cold-plate, dual interface method has been utilized by comparing the structure functions obtained from two thermal transient measurements with different thermal interface layers between the metal-core-printed-circuit-board (MCPCB) and cold-plate. It is found that the real junction-to-board thermal resistance, Rth(JB) which is the separation point obtained from the structure functions with optical power consideration is 10.15 +/- 0.05 K/W. In addition, comparison between the IR emitter with different types of die attach (solder and epoxy glue) has also been studied by employing dual interface method to obtain the junction-to-chip thermal resistance Rth(JP) of 3.79 +/- 0.05 K/W. (C) 2012 Elsevier Ltd. All rights reserved.