The Hapke parameters are a set of parameters for a quasi-experimental model that are commonly used to describe the directional reflectance properties of the airless regolith surfaces of bodies in the solar system. They were developed by astronomer Bruce Hapke at the University of Pittsburgh.
The quantities are:
- — Single scattering albedo. This is the ratio of scattering efficiency to total light extinction (which includes also absorption), for small-particle scattering of light. That is, , where is the scattering coefficient, and is the absorption coefficient 
- — The width of the opposition surge.
- or — The strength of the opposition surge.
- or g — The particle phase function parameter, also called the asymmetry factor.
- — The effective surface tilt, also called the macroscopic roughness angle.