transmittance (of a medium)

[7.5.3]  τ

NOTE: This definition was changed in RP-16-17 Addendum 4. Deletions have a strike-through. New text is bold.

τ = Φti

The ratio of the transmitted flux to the incident flux. It should be noted that transmittance refers to the ratio of flux emerging to flux incident; therefore, reflections at the surface as well as absorption within the material operate to reduce the transmittance. Transmittance is a function of:

(1) Geometry:

(a) of the incident flux

(b) of collection for the transmitted flux

(2) Spectral distribution:

(a) characteristic of the incident flux

(b) weighting function for the collected flux

(3) Polarization:

(a) of the incident flux

(b) component defined for the collected flux


(i) Unless the state of polarization for the incident flux and the polarized component of the transmitted flux are stated, it shall be considered that the incident flux is unpolarized and that the total transmitted flux is evaluated.

(ii) Spectral transmittance depends on the beam geometry, the and character of the transmitting surfaces and media, (and polarization). In addition, luminous transmittance is a function of the spectral distribution of the incident beam.

(iii) If no qualifying geometric adjective is used, transmittance for hemispherical collection is meant (see related terms for hemispherical, conical, and directional transmittance for other modifying adjectives).

(iv) In each case of conical incidence or collection, the solid angle is not restricted to a right circular cone but may be of any cross-section, including rectangular, a ring, or a combination of two or more solid angles.

(v) These concepts must be applied with care if the area of the transmitting element is not large compared to its thickness, due to internal transmission across the boundary of the area.

(vi) For all of the following geometrical quantities of hemispherical, conical and directional transmittance, it is assumed that the radiance (luminance) is isotropic over the specified solid angle of incidence. Otherwise, the property is a function of the directional distribution of incident radiance (luminance) radiant (luminous) flux as well as the beam geometry and the character of the transmitting surfaces and/or media.

(vii) The following breakdown of transmittance quantities is applicable only to the transmittance of thin films 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 that produces the transmitted 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 et al.*


* Nicodemus, FE et al. Geometrical Considerations and Nomenclature for Reflectance, NBS Monograph 160. Washington, DC: U.S. Department of Commerce; Oct 1977.

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