Measurements of scattered sunlight often employ simple radiometers that are incapable of scanning azimuth ø and zenith angle θ. Instead, they measure integral moments of the radiance. Consider a set of N + 1 meters designed to measure moments Λl through ΛN+1. If each meter is judiciously designed, the Λj may be processed quite simply to deduce inherent properties of the scattering medium, namely, N moments of the volume scattering function. In the optimum design, each meter has an angular response that approximates a polynomial of degree N in cosθ. The theory of these meters is based on Boltzmann’s equation of radiative transfer expressed in terms of spherical harmonics, a form in which this equation is particularly simple.
Willard H. Wells, "Techniques for measuring radiance in sea and air," Appl. Opt. 22, 2313-2321 (1983)