We give a simple, but accurate method that can be used to account for illumination in compact binary systems which have a low-mass companion, even if spherically symmetric illumination of the secondary star (not necessarily on the main sequence) is not assumed. This is done by introducing a multiplicative factor {PHI} in the Stefan-Boltzmann surface boundary condition, which accounts for the blocking of the intrinsic secondary flux by X-ray heating of the photospheric layers. Numerical fits and tables for {PHI} are given for unperturbed effective temperatures in the range 2500-5600K and logg in the range 1.0-5.0.