# Transmittance

In optics and spectroscopy, **transmittance** is the fraction of incident light at a specified wavelength that passes through a sample.

where is the intensity of the incident light and *I* is the intensity of the light coming out of the sample. The transmittance of a sample is sometimes given as a percentage.

Transmittance is related to absorbance *A* as

or, using the natural logarithm

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

- ,

where is the attenuation coefficient and 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.