A spectrophone measures absorptivity by sensing thermal expansion in a confined sample gas. Laser source excitation provides sufficient radiation to measure precisely very weak absorptivities at laser wavelengths. This paper describes the theoretical capability, design considerations, and experimental testing of a pulsed ruby laser absorptivity spectrophone and a cw CO2 laser absorptivity spectrophone. A spectrum of the water vapor line at 6943.8 Å was obtained. The peak absorptivity was 3 × 10-6cm-1. In the vicinity of 9.6 µ, absorptivities of CO2-N2 mixtures were measured down to 1.2 × 10-7cm-1.
Edwin L. Kerr and John G. Atwood, "The Laser Illuminated Absorptivity Spectrophone: A Method for Measurement of Weak Absorptivity in Gases at Laser Wavelengths," Appl. Opt. 7, 915-921 (1968)