A theoretical model for light emission from a homogeneous volume source, such as an optically active layer, within a multilayer is demonstrated. The role of an external linearly polarized optical pump is taken into account. The resulting formulas for the radiated powers are fully analytical. They are applied to investigate the effect of a plane-wave pump in a basic λ/2 cavity and to calculate the photoluminescence polar diagrams of color centers from three resonating thin-film lithium-fluoride-based microstructures.