In general L e,Ω is a function of viewing direction, depending on θ through cos θ and azimuth angle through ∂Φ e /∂Ω. For the special case of a Lambertian surface, ∂ Φ e /(∂Ω ∂ A) is proportional to cos θ, and L e,Ω is isotropic (independent of viewing direction).