Reciprocity principle applicable to reflected radiance measurements and the searchlight problem

In a recent paper by Di Girolamo et al. [J. Geophys. Res. D 103, 8795 (1998)] a heuristic argument was used to derive a reciprocity principle applicable to reflected solar radiation measurements. Here a formal derivation of this reciprocity principle is presented. It is also demonstrated that a purely spatial reciprocal relationship exists between one-dimensional radiative transfer theory and the three-dimensional searchlight problem for horizontally homogeneous media.