The BRDF should satisfy some sort of energy conservation. The total outgoing radiance along omega_o can't be greater than the incoming irradiance (i.e. f_r is upper bounded by 1/pi). Otherwise, the LHS would be greater than L_i, that is, the radiance from every incoming direction is less than the radiance from every outgoing direction.

The BRDF should satisfy some sort of energy conservation. The total outgoing radiance along omega_o can't be greater than the incoming irradiance (i.e. f_r is upper bounded by 1/pi). Otherwise, the LHS would be greater than L_i, that is, the radiance from every incoming direction is less than the radiance from every outgoing direction.