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.
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.