Measurement of thermal diffusivity of air using photopyroelectric interferometry

A new photopyroelectric (PPE) method for measuring the thermal diffusivity of ambient gases with optimal precision is introduced. The technique is based on destructive PPE interferometric detection inside a thermal-wave resonant cavity with an optically transparent wall. A new PPE sensor design generating purely thermal-wave interference is used to improve the signal-to-noise ratio and the measurement dynamic range and precision by suppressing the large optically transmitted base line signal. Cavity-length scanning is employed and the thermal diffusivity of air is measured with third-significant-figure precision. Comparisons of measurement precision are made with experimental results obtained using a transparent-wall PPE thermal-wave cavity in the single-ended (noninterferometric) mode. The physical signal generation mechanisms are also discussed. © 1999 American Institute of Physics.