Transmittance
In optics and spectroscopy, transmittance is the fraction of incident light (electromagnetic radiation) at a specified wavelength that passes through a sample.[2][3] The terms visible transmittance (VT) and visible absorptance (VA), which are the respective fractions for the spectrum of light visible radiation, are also used.
A related term is absorbance,[4] or absorption factor,[5] which is the fraction of radiation absorbed by a sample at a specified wavelength.
Definition
Transmittance is given by:[2]
where
- I is the intensity of the radiation coming out of the sample;
- I0 is the intensity of the incident radiation.
In these equations, scattering and reflection are considered to be close to zero or otherwise accounted for.[clarification needed] The transmittance of a sample is sometimes given as a percentage.
Note that the term "transmission" refers to the physical process of radiation passing through a sample, whereas transmittance refers to the mathematical quantity.
Relation to absorbance
Transmittance is related to absorbance A as:[4]
Relation to optical depth
Transmittance is related to optical depth τ as:
Non-normal geometry
In plane geometry[clarification needed]:
where, when the plane parallel assumption[clarification needed] is invoked, μ = cos θ with θ the angle of propagation of the light ray relative to the normal of the surface.
Beer–Lambert law
In case of uniform attenuation, optical depth is simply:
where
- Σ is the attenuation coefficient;
- N is the medium concentration;
- σ is the total cross section;
- l is the geometrical path length.
So the transmittance is:
In the general nonuniform case, optical depth is an integral quantity:
so:
This is the case of atmospheric science applications and also of radiation shielding theory.
See also
References
- ^ "Electronic warfare and radar systems engineering handbook".
- ^ a b IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Transmittance". doi:10.1351/goldbook.T06484
- ^ Verhoeven, J. W. (1996). "Glossary of terms used in photochemistry (IUPAC Recommendations 1996)". Pure and Applied Chemistry. 68 (12): 2223–2286. doi:10.1351/pac199668122223. ISSN 0033-4545.
- ^ a b IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Absorbance". doi:10.1351/goldbook.A00028
- ^ "CRC Dictionary of pure and applied physics, CRC Press, Editor: Dipak Basu (2001)".