The thermal response of opaque coatings after heating with pulses of finite
duration is analyzed theoretically with respect to detection of small vari
ations of the coating thickness. Rectangular shaped heating pulses and step
pulses with exponential decay an considered. The investigation is based on
existing analytical solutions for step-function heating and for delta-puls
e heating. For the exponentially decaying pulse, a numerical convolution is
performed. For the rectangular pulse, the best sensitivity for thickness v
ariations (thickness contrast) is generally obtained 0.2 to 0.9 Fo-numbers
after the end of the pulse. A good selection for the pulse length is 0.5 to
1.5 Fourier numbers of the coating. For the exponentially decaying pulse,
the maximum thickness contrast is smaller than for the rectangular pulse an
d easily hidden by disturbing radiation. Experimental realization of rectan
gular short pulses is approximately possible by switched flash lamps.