Changes in infrared emission and absorption spectra of hot gases brought about by controlled variations of gas temperature profile were measured. Spectral emission-absorption temperatures were determined from the spectral data as a function of wavelength, using the Kirchhoff and Planck laws. These temperatures proved independent of wavelength for an isothermal profile, despite a large variation of absorptance with wavelength. For nonisothermal profiles, the emission-absorption temperature varied with wavelength in a characteristic manner for each profile. These results established more firmly the validity of the infrared emission-absorption technique of gas temperature measurement, and confirmed a previous hypothesis that the wavelength dependence of spectrally determined temperature, observed for most flames, is a temperature profile effect. The spectral emittance equation for a gas was formulated in terms of observable quantities for isothermal and nonisothermal cases, taking the spectrometer slit function into account.
Richard H. Tourin and Burton Krakow, "Applicability of Infrared Emission and Absorption Spectra to Determination of Hot Gas Temperature Profiles," Appl. Opt. 4, 237-242 (1965)