INTERFERENCE EFFECTS OF THERMAL WAVES AND THEIR APPLICATION TO BOLOMETERS AND PYROELECTRIC DETECTORS

Thermal waves are generated via the absorption of intensity-modulated infrared radiation at the front surface of a 25-mu-m thick pyroelectric poly(vinylidene fluoride) (PVDF) film. The amplitude of the transient surface temperature of the film is measured with a thin-film bolometer and the average temperature of the film via the pyroelectric effect. Two cases are compared: a free-bearing film and a film which is coupled to a heat sink at the rear surface. Using the concept of the interference of thermal waves, it is shown that for a material with thickness d and thermal diffusivity D, the surface temperature of the film coupled to the heat sink exceeds that of the free-bearing film within the frequency range pi-D/16d2 < f < 9-pi-D/16d2; for the average temperature the frequency range is pi-D/4d2 < f < 9-pi-D/4d2, in full agreement with the experiment. The interference of thermal waves thus has an application in infrared sensors. In the given modulation frequency ranges, the sensitivity of the sensors on the heat sink exceeds the sensitivity of free-bearing sensors of the same construction.