Transmittance

In optics and spectroscopy, transmittance is the fraction of incident light (or other electromagnetic radiation) at a specified wavelength that passes through a sample. A related term is absorptance, or absorption factor, ^[1]^[2] which is the fraction of radiation absorbed by a sample at a specified wavelength. Occasionally one also hears the terms visible transmittance (VT) and visible absorptance (VA), which are the respective fractions for the spectrum of light visible radiation. In equation form,

$\mathcal{T}_\lambda = {I\over I_{0}} \qquad \mathcal{A}_\lambda = \frac{I_0-I}{I_0}$

where $I_0$ is the intensity of the incident radiation and I is the intensity of the radiation coming out of the sample and $\mathcal{T}_\lambda$ and $\mathcal{A}_\lambda$ 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

$A = - \log_{10}\mathcal{T}\ = - \log_{10}\left({I\over I_{0}}\right)$

In the case of gases it is customary to use natural logarithms instead, making absorbance A for gases

$A = - \ln\mathcal{T}\ = - \ln\left({I\over I_{0}}\right)$

From the above equation and the Beer-Lambert law, the transmittance for gases is thus given by

$\mathcal{T} = e^{-\alpha \, x}$ ,

where $\alpha$ is the attenuation coefficient and $x$ 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.

References

^ "IUPAC handbook definition". http://www.iupac.org/goldbook/A00035.pdf. Retrieved 2009-07-02.
^ "CRC Dictionary of pure and applied physics, CRC Press, Editor: Dipak Basu (2001)". http://www.crcpress.com/product/isbn/9780849328909.

Transmittance

References

See also