Infrared emissivity determination using a thermal-wave resonant cavity: Comparison between the length- and frequency-scan approaches

Abstract

The mechanisms of heat conduction and radiation through a thermal-wave resonant cavity heated up with a modulated laser beam are investigated. This resonator is made up of three layers in which the thickness of the middle can be changed moving one of the external layers. A modulated heat source is applied to one of the external layers, and the changes of temperature are registered at the surface of the opposite layer. The obtained results show that the continuous (dc) and oscillatory (ac) components of the temperature are coupled, and they can be described under a fully analytical and exact approach. It is shown that: 1) the ac temperature depends strongly on the third power of the dc temperature at the inner surface of the illuminated layer. 2) The contribution of the heat radiation to the ac temperature becomes more remarkable as the thickness of the cavity becomes much larger than the thermal diffusion length of the fluid inside of it, i.e. when the contribution of the heat conduction decreases. 3) Though the effect of the radiation does not show up strongly on the real and imaginary part of the thermal wave field, it does affect both its amplitude and phase. 4) The amplitude and phase as a function of the cavity thickness exhibit a stronger dependence on the radiation contribution than the corresponding ones as a function of the modulation frequency. Length scan is therefore more suitable than frequency scan to observe the radiation contribution on the photopyroelectric signal. Based on these facts, various promising methods to determine the infrared emissivity of materials can be devised.