A redistribution function R(x, x′)dx dx′ is defined as the probability that a photon with frequency between x and x + dx will be absorbed by an elemental atmosphere which then emits another photon the frequency of which lies in the range from x′ to x′ + ax′. The frequencies x and x′ are measured from the central frequency νo of the spectral line in units of the Doppler width Δ The particular functional form taken by R(x, x′) in this discussion will be given by

where α represents the broadening of the spectral line in units of the Doppler width while the function H(o, z) is defined by

the Voigt function. The function defined by means of equation (1) is one of that class of redistribution functions which combines natural and Doppler broadening. Therefore, it should be expected that any results obtained using this particular redistribution function will exhibit certain features characteristic of this type of scattering.