We present an easy-to-use formula for mean (space- and direction-average) specific intensity of radiation in uniform spherical and cylindrical plasmas, which are not under external irradiation. This formula has high accuracy in any spectral interval dν taken in continuum as well as within a spectral line of any profile, including overlapping lines and lines on a pedestal of intense continuum in any spectral range. The formula considerably accelerates self-consistent computations of the radiation field and the distribution of ions over their ionization degrees and quantum states. It may also be used for computations of any radiation dependent-quantity, for example, the photoionization probability.
