The Influence of Numerous Low-Intensity Spectral Lines on Band Absorptance

Abstract

The characteristics of band absorption are investigated when numerous low-intensity spectral lines occur together with any number of stronger lines. For these conditions it is shown that there is always a region of validity for the weak line and nonoverlapping line approximations, although the shape of the limiting curve in the latter case may not be simple. There is also a region of validity for the strong line approximation in most practical situations. The only exception occurs when the number of spectral lines increases very rapidly as the intensity approaches zero. The principal effects of the low-intensity lines are to introduce points of inflection in the absorptance curves and to increase the intermediate range of values of pressure and absorber concentration over which neither the strong nor weak line approximation is valid. These effects are illustrated by examples. From a study of the shape of experimentally determined absorptance curves it is possible to deduce the number of low-intensity lines in the frequency interval of interest.