transmittance: Definition from Answers.com

Diagram of Beer-Lambert Law of transmittance of a beam of light as it travels through a cuvette of width l.

Transmittance of ruby in optical and near-IR spectra. Note the two broad blue and green absorption bands and one narrow absorption band on the wavelength of 694 nm, which is the wavelength of the ruby laser.

In optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. A related term is absorptance,^[1] which is the fraction of light 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 visible light. 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 light and I is the intensity of the light 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.

Transmittance is related to absorbance A (not to be confused with absorptance) as

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

or, using the natural logarithm

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

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

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

where $α$ is the attenuation coefficient and $x$ is the path length.

Note that the term "transmission" refers to the physical process of light 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.

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transmittance

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