In optics and spectroscopy, transmittance is the fraction of incident light (electromagnetic radiation) at a specified wavelength that passes through a sample. The terms visible transmittance (VT) and visible absorptance (VA), which are the respective fractions for the spectrum of light visible radiation, are also used. The natural radiation of the cuvette corresponding to the temperature of the cuvette remains ignored - see radiative transfer equation.
Beer–Lambert law 
In equation form,
where is the intensity of the incident radiation and I is the intensity of the radiation coming out of the sample and and are transmittance and absorptance respectively. In these equations, scattering and reflection are considered to be close to zero or otherwise accounted for. The transmittance of a sample is sometimes given as a percentage.
For liquids, transmittance is related to absorbance A (not to be confused with absorptance) as
In the case of gases it is customary to use natural logarithms instead, making absorbance A for gases
From the above equation and the Beer-Lambert law, the transmittance for gases is thus given by
where is the attenuation coefficient and is the path length. For liquids e is replaced by 10.
Note that the term "transmission" refers to the physical process of radiation passing through a sample, whereas transmittance refers to the mathematical quantity.
Transmittance in the atmosphere 
With this, transmittance can be given as
where, when the plane parallel assumption is invoked, with the angle of propagation of the ray from the normal of the surface.
See also 
Another equation that can be useful in solving for %T is the following: A = 2 - log(%T) Where A is a measure of absorbance. By manipulating the equation you can generate the more direct form of the equation: %T = 10^(2-A)
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Transmittance".
- 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.
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Absorbance".
- "CRC Dictionary of pure and applied physics, CRC Press, Editor: Dipak Basu (2001)".