A rough surface has a BRDF proportional to cos( 0 )cos( 1 ), where 0 and

Question:

A rough surface has a BRDF proportional to cos(θ₀)cos(θ₁), where θ₀ and θ₁ are respectively the angles of the incident and scattered radiation measured from the surface normal. The BRDF has no azimuthal dependence. Show that, if the albedo for normally incident radiation is 1, the diffuse albedo of the surface is 2/3.