The unidirectional reflectance of an imperfectly diffuse surface is defined in terms of the surface radiance and the equivalent hemispherical radiance of the source. Equations relating the unidirectional reflectance to the directional reflectance, the polar and azimuthal angles for incident and reflected energy, and the voltage signal of a phototube detector are derived. A reflectometer is described for measuring the monochromatic unidirectional reflectance over the visible region of the spectrum and data of a magnesium oxide coating, two white paints, and a dull and polished aluminum surface are presented.
W. M. BRANDENBERG and J. T. NEU, "Unidirectional Reflectance of Imperfectly Diffuse Surfaces," J. Opt. Soc. Am. 56, 97-101 (1966)