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