The ratio of the differential luminance dLt(θt, ϕt) for a ray transmitted in a given direction(θt, ϕt) to the differential luminous flux density dEi(θi, ϕi), incident from a given direction (θi, ϕi), that produces it (see Figure 23).
Units = sr-1
where dΩ ≡ dω · cos θ
(i) This distribution function is the basic parameter for describing (geometrically) the transmitting properties of a thin scattering film (with negligible internal scattering) so that the transmitted radiation emerges from a point that is not significantly separated from the point of incidence of the incident ray(s). The governing considerations are similar to those for application of the bidirectional reflectance-distribution function (BRDF), rather than the bidirectional scattering-surface reflectance-distribution function (BSSRDF), as discussed in Nicodemus.*
(iii) The spectral and polarization aspects must be defined for complete specification, since the BTDF as given above only defines the geometrical aspects.
* Nicodemus FE. et al, Geometrical Considerations and Nomenclature for Reflectance, NBS Monograph 160. Washington DC: U.S. Department of Commerce; Oct 1977.« Back to Definitions Index