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In optics and spectroscopy, 'transmittance' is the fraction of incident light at a specified wavelength that passes through a sample.
:$T\; =\; \{Iover\; I\_\{0\}\}$
where $I\_0$ is the intensity of the light and ''I'' is the intensity of the light coming out of the sample. The transmittance of a sample is usually given as a percentage, defined as
:$T\%\; =\; \{Iover\; I\_\{0\}\}\; cdot\{\}\; 100$
Transmittance is related to absorbance ''A'' as
:$A\; =\; -\; log\_\{10\}T\; =\; -\; log\_\{10\}\{Iover\; I\_\{0\}\},$
or
:$A\; =\; 2\; -\; log\_\{\}T\%,$
where T% is the percent transmittance and T is "per one" transmittance.
Note that the term transmission refers to the physical process of light passing through a sample, whereas ''transmittance'' refers to the mathematical quantity.