To compare measurements performed in different geometrical conditions, one must take into account the angular anisotropy of the reflection properties of natural surfaces. As use of the exact boundary conditions in the radiative transfer codes seems prohibitive, a simple but accurate formulation of the problem has been sought. In this paper, two average angular reflectances are defined from which the reflected radiance may be deduced for any distribution of the downward radiance. Calculations made for different atmospheric models show that the solar directionality is partly preserved in the downward radiation field, so that the average reflectances can be written as a linear combination of actual reflectance and spherical albedo of the surface. Finally, the feasibility of detecting directional properties from space measurements is discussed.