This paper describes the application of the integrating sphere or cavity to the measurement of the absorption coefficient (cm-1) of a material introduced into the cavity. The absorption coefficient is determined by measuring the decrease in the radiation density within the integrating cavity caused by insertion of the sample. This method has the virtue of being independent of the scattering within the material sample, the reflectivity of the material surface, and the geometry of the sample. The method is particularly attractive for materials with small absorption coefficients. Experimental verification of the method is described showing agreement with direct transmittance measurements to within ±10%.
P. Elterman, "Integrating Cavity Spectroscopy," Appl. Opt. 9, 2140-2142 (1970)