This paper discusses the radiometric properties of perfectly conductive, slightly rough random surfaces. The Kirchhoff approximation, so far used in this kind of study, is analysed. In particular, its inability to construct models of lambertian rough surfaces is pointed out. The small perturbation method, recently put forward, allows the scope and limitations of this approximation to be established. This method also yields exact expressions for the radiant intensity, for either polarized or unpolarized incident radiation and allows the construction of surfaces which produce a lambertian distribution of intensity.