With thermal protective systems, the power transferred from an opaque layer to the skin or a skin simulant determines the temperature of the skin and in turn the degree of burn. The power input was calculated from the measured temperature of the simulant by expressing the power as a series of functions, with unknown constants, whose Laplace transforms were of the proper form for comparison with the measured temperatures and thus determine the constants. The lag of temperature behind the power causes some uncertainty in the latter, but practically adequate precision is attainable.