Transmittans

Transmittans, or transmittance, is a property of a material or medium that describes the fraction of incident radiant energy that passes through it. It is a dimensionless quantity, commonly expressed as a percentage, and is wavelength dependent for most materials. In general, transmittans T is defined as the ratio of transmitted irradiance or flux to the incident irradiance: T = I_t / I_0.

For a given wavelength λ, the transmittans is written as T(λ), often referred to as spectral transmittans. In practical measurements, the observed transmittans can be influenced by absorption, scattering, and reflections at interfaces. The total effect can be described by separating internal transmittance (through the bulk) from surface losses due to reflection.

Transmittans is closely related to absorbance. If A is absorbance, then T = 10^{-A} in the common base-10 convention. In the Beer–Lambert law, A = ε c l, where ε is the molar extinction coefficient, c the concentration, and l the path length, linking transmittans to chemical composition and thickness.

In energy balance terms, for a non-scattering, non-emissive, homogeneous slab with negligible luminescence, the sum of transmittance, reflectance, and absorptance equals 1: T + R + A = 1. When scattering or multiple reflections are significant, this relation becomes more complex and is often handled with separate measurements of diffuse transmittance and reflectance.

Applications of transmittans span optics, coatings, glazing, photovoltaic devices, and atmospheric science. Measurements are typically performed with spectrophotometers or integrating spheres to determine T across wavelengths.

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