Transmittance

Transmittance is the fraction of incident radiation that passes through a material or interface. It depends on wavelength, material properties, and measurement geometry.

Beer–Lambert law: An equation relating light attenuation in an absorbing medium to concentration, path length, and absorptivity. It predicts how absorbance through a uniform sample determines transmittance.
Incident radiation: Radiation that reaches a material or interface before interacting with it. It supplies the reference amount in the transmittance fraction.
UV–visible spectroscopy: A technique that measures a sample’s interaction with ultraviolet and visible radiation across wavelengths. It commonly derives sample transmittance from light measured before and after the sample.
Total transmittance: The fraction of incident radiation transmitted in all directions through a sample. It includes diffuse as well as directly transmitted radiation.
Absorbance: A logarithmic measure of how much radiation a sample removes from a transmitted beam. Absorbance and transmittance are related by a logarithmic conversion.
Radiant flux: The rate at which radiant energy passes through or reaches a surface. Flux measurements can provide the numerator and denominator of transmittance.
Spectrophotometer: An instrument that measures radiation intensity as a function of wavelength. It compares transmitted intensity with a reference to quantify transmittance.
Regular transmittance: The fraction of incident radiation transmitted in the direction of the regular, undeviated beam. It excludes transmitted radiation redirected away from the direct beam.
Reflectance: The fraction of incident radiation reflected by a surface or material. Reflected radiation does not contribute to the transmitted fraction.
Wavelength: The spatial period of a wave, equal to the distance between successive points in phase. A material can transmit different wavelengths by different amounts.

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