Spectral directional transmittance in frequency and spectral directional transmittance in wavelength of a surface, denoted T and T respectively, are defined as
where
*L is the spectral radiance in frequency transmitted by that surface;
*L is the spectral radiance received by that surface;
*L is the spectral radiance in wavelength transmitted by that surface;
*L is the spectral radiance in wavelength received by that surface.