This paper describes the theoretical principles of TRD operation based on
thermal absorption of incident power of radiation. In the modelling procedures,
the main parameters influencing on the detector features have been introduced. So
the this model allow us to predict the behaviour of different types of thermal
detectors. It is found that the temperature response of the detector against the
frequency of incident radiation in logarithmic scale describes the TRD as a typical
low pass filter characteristics. The cut off corner frequency is found to be at 1 Hz
under which the temperature change attains a saturation value. In the sense that the
thermal detector will detect all the incoming radiations of higher frequency.