If a lossless (conservative) Lambertian reflector (ϖ0 = 1) is irradiated normally with flux density \(F\), then its radiance in any direction will be \(F/π\). The normal albedo \(p_n\) of a point on a surface is the ratio of the normally observed radiance to that of the Lambertian surface, so that
\[ p_n = \pi f_r \left( \mu = \mu_0 = 1 \right).\]
The author has found two definitions of normal albedo in the literature. In one, the surface must be radiated normally and observed normally (\(μ = μ_0 = 1\)) and the other in which it can be irradiated from any direction, in which case \(p_n\) is a function of \(μ_0\).