A spectroscopic method, useful for strongly absorbed lines, has been investigated for measuring temperatures of hot gases. The minimum transmission of discrete spectral lines in gases at equilibrium has a simple dependence upon the absolute temperature and this relation is used to measure temperature from a straight line plot. An experimental study has been made of the absorption due to the rotational lines of OH (<sup>2</sup>Σ←<sup>2</sup>II) between 3067 and 3090 A in an oxygen-acetylene flame. It is shown that temperatures measured by this method are less affected than the conventional one by lack of sufficient resolving power, flame thickness, and various light sources (either continuous or line).
HENRY J. KOSTKOWSKI and HERBERT P. BROIDA, "Spectral Absorption Method for Determining Population "Temperatures" in Hot Gases," J. Opt. Soc. Am. 46, 246-254 (1956)