[7.3.3.9] ρ(θi,Φi; θr,Φr)
The ratio of reflected flux collected over an element of solid angle surrounding the given direction to essentially collimated incident flux. (See Figure 13.)
Note: The directions of incidence and collection and the size of the solid angle “element” of collection must be specified. In each case of conical incidence or collection, the solid angle is not restricted to a right circular cone but may be of any cross-section, including rectangular, a ring, or a combination of two or more solid angles.
