Reciprocity principles in radiative transfer theory have been widely used in deriving analytical and numerical solutions of radiative transfer problems, in testing numerical models of radiative transfer, and in remote sensing applications. The most commonly used reciprocity relationship for reflected solar radiation is that the bi-directional reflectance distribution function remains invariant under a change in the incident and outward directions. This common form stems from 1-D radiative transfer theory, whereby the media is horizontally homogeneous. Unfortunately, this form is often misapplied to heterogeneous media, and reports of observed reciprocity failure over heterogeneous media are common.

I will present new reciprocity principles that are derived from 3-D radiative transfer theory, and are appropriate for the common case of a scattering and absorbing heterogeneous medium with unidirectional illumination, for the searchlight problem, and for cases where periodic or vacuum boundary conditions are employed by a model. I will also describe how these can be used in testing 3-D radiative transfer codes, and problems that come about in their application to remotely sensed data.