In optics and spectroscopy, transmittance is the fraction of incident light (electromagnetic radiation) at a specified wavelength that passes through a sample. The terms visible transmittance (VT) and visible absorptance (VA), which are the respective fractions for the spectrum of light visible radiation, are also used. The natural radiation of the cuvette corresponding to the temperature of the cuvette remains ignored - see radiative transfer equation.
Transmittance is given by:
- I is the intensity of the radiation coming out of the sample;
- I0 is the intensity of the incident radiation.
In these equations, scattering and reflection are considered to be close to zero or otherwise accounted for. The transmittance of a sample is sometimes given as a percentage.
Note that the term "transmission" refers to the physical process of radiation passing through a sample, whereas transmittance refers to the mathematical quantity.
Relation to absorbance
Relation to optical depth
Transmittance is related to optical depth τ as:
In plane geometry:
where, when the plane parallel assumption is invoked, μ = cos θ with θ the angle of propagation of the ray from the normal of the surface.
In case of uniform attenuation, optical depth is simply:
- Σ is the attenuation coefficient;
- N is the medium concentration;
- σ is the total cross section;
- l is the geometrical path length.
So the transmittance is:
In the general nonuniform case, optical depth is an integral quantity:
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