Thermal imaging is demonstrated as an attractive alternative for standard temperature measurements in diode lasers. It allows for the determination of time resolved temperature distributions in arbitrary materials of laser devices. Because of the partial mid-infrared transparency of the semiconductor materials involved, several issues complicate the thermal imaging approach. We analyze these detrimental effects for the case of GaAs based high-power diode lasers and demonstrate how to circumvent them. This leads to a deeper insight into the composite thermal emission signal from diode lasers and eventually to an accurate determination of absolute temperatures of semiconductor diode lasers.