Forward and backward Monte Carlo methods may become inefficient when the radiant source is collimated and radiation onto a small, arbitrary spot and onto a small, arbitrary direction cone is desired. In this paper, the DRESOR method was formulated to study the radiative heat transfer process in an isotropically scattering layer exposed to collimated radiation. As the whole spherical solid angle space was uniformly divided into 13,316 discrete solid angles, the intensity at some point in up to such discrete directions was given. The radiation fluxes incident on a detector inside the layer for varying acceptance angles by a step of $2deg$ were also measured, which agreed well with those in literature. The radiation flux across the top and the bottom boundaries were also provided.