The observation of the temperature pattern on the surface of a polymeric sample during stretching using an infrared camera is, after stretching calorimetry, a second method for measuring thermoelastic properties. In order to do this, the thermal image from the IR camera has to be transformed to a temperature image using the calibration of the camera and the emissivity of the material. Studies on the necking process in polycarbonate provide a quantitative evaluation of the produced and exchanged heat. The strength of the heat sources of the neck can be obtained by comparison of the experimentally measured temperature profiles and the temperature profiles calculated with a one-dimensional model of the necking, including heat generation, heat conduction and heat loss. The results are in perfect agreement with the stretching calorimeter results, but very fast experiments are possible and instantaneous signals with spatial and temporal resolution can be obtained.