Diagram of Beer-Lambert Law of transmittance of a beam of light as it travels through a cuvette of width l. A cuvette is a kind of Laboratory glassware, usually a small tube of circular or square cross section sealed at one end made of Plastic, Glass, or

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. A ruby laser is a Solid-state laser that uses a synthetic Ruby crystal as its gain medium.

In optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. Spectroscopy was originally the study of the interaction between Radiation and Matter as a function of Wavelength (λ In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency.

where I₀ is the intensity of the incident light and I is the intensity of the light coming out of the sample. In Physics, intensity is a measure of the time-averaged Energy Flux. 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 α is the attenuation coefficient and x is the path length. In Spectroscopy, the absorbance A is defined as A_\lambda = -\log_{10}(I/I_0\ where I is the intensity of light In For attenuation coefficient as it applies to electromagnetic theory and telecommunications see propagation constant.

Note that the term "transmission" refers to the physical process of light passing through a sample, whereas transmittance refers to the mathematical quantity. See also Reflection coefficient The transmission coefficient is used in physics and electrical engineering when Wave propagation in a medium containing