Radiosity-techniques are global illumination models, which are based on the physics of radiometry. All these techniques have in common the need to calculate form-factors (geometry-factors), which determine the energy exchange between patches of finite or infinite size. Form-factors between a differential area and a patch, so-called differential form-factors, may be used for intensity calculus on surfaces which are illuminated by area light sources. Hemicube, singleplane and ray tracing approaches are numerical approximations to calculate these form-factors. This paper presents an analytical method for calculating form-factors between a patch and a differential area using BSP-trees. An application of this algorithm called BSP-shading is described, which may be used for rendering scenes with area light sources or scenes on which the progressive refinement [1] method has been applied.