General expressions are derived for the radiance of a nonisothermal gas in terms of the equivalent width of the relevant spectral lines. Any appropriate expression for the equivalent width may be used in these equations taken either from the theory of band models or from exact calculations which use molecular constants. A general expression for the radiance is derived which is based on the Curtis-Godson approximation; this is compared with exact calculations. Radiance calculations for four different temperature distributions are given in order to illustrate the method.
Gilbert N. Plass, "Radiation from Nonisothermal Gases," Appl. Opt. 6, 1995-1999 (1967)