The spectral directional reflectance in frequency and spectral directional reflectance in wavelength of a surface, denoted R Ω, ν and R Ω, λ respectively, are defined as R Ω Ω , ν ν = L e , Ω Ω , ν ν r L e , Ω Ω , ν ν i , R Ω Ω , λ λ = L e , Ω Ω , λ λ r L e , Ω Ω , λ λ i , where