The Mettler FP 84 calorimeter is positioned in the light path of a polarisation microscope for simultaneous thermo-optical measurement. In, contrast to the thermal signal the optical signal should not be convoluted by the calorimeter. A desmearing of the measured DSC signal is possible by two procedures : by using the equation of Tian or by solving the convolution integral. To solve the convolution integral it is necessary to know the apparatus function. This may be obtained from the thermal response on a short light pulse which hits a black object in the sample pan. The aim is to compare the two procedures of desmearing with the optical signal by a change in double refraction of a crystal.