In the fields of optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. The transmittance T (usually given as a percentage) of a sample is defined as
,
where I0 is the intensity of the incident light and I is the intensity of the light coming out of the sample. Transmittance is related to absorbance A as
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.