Charlier approximation in photocount statistics

The Charlier polynomials are used to construct a sequence of approximations for the distribution of photocounts and of the corresponding approximations of the generating functions and moments, and to find the characteristics of the superpositions of distributions. The primary emphasis is on a simple twoterm approximation which has a physically clear structure and which yields simple expressions for the distributions, generating functions, and moments of photocounts of polarized and partly polarized radiations, and also of mixtures of radiations. The range of validity of the two-term approximation is found.