Since the reflectance rho_d is between 0 and 1, the maximum value of the BRDF is 1/pi. Also in Lambert's Cosine Law, the cosine term is not part of the BRDF, which is constant. The cosine affects the amount of energy falling on the surface due to the angle of the light incoming.

Since the reflectance rho_d is between 0 and 1, the maximum value of the BRDF is 1/pi. Also in Lambert's Cosine Law, the cosine term is not part of the BRDF, which is constant. The cosine affects the amount of energy falling on the surface due to the angle of the light incoming.