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 CO(2) laser absorptivity spectrophone. A spectrum of the water vapor line at 6943.8 A was obtained. The peak absorptivity was 3 x 10(-6)cm(-1). In the vicinity of 9.6 micro, absorptivities of CO(2)-N(2) mixtures were measured down to 1.2 x 10(-7)cm(-1).

A wavelength that lies within a spectral interval of reduced solar emission (a Fraun

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