transmittance

Transmittance is a measure of how much light or other electromagnetic radiation passes through a material. It is defined as the ratio of transmitted radiant flux to incident radiant flux, T = I_t / I_0, and can be expressed as a function of wavelength, T(λ). For a monochromatic beam, transmittance ranges from 0 (opaque) to 1 (fully transparent). In many contexts, absorbance and reflectance are considered alongside transmittance, with energy conservation described by A + R + T = 1 for simple, non-scattering, non-emissive systems.

Transmittance is related to absorbance by the equation A = -log10(T), or equivalently T = 10^{-A}. In the Beer-Lambert framework, absorbance is A = εlc, linking transmittance to concentration and path length for absorbing solutions. Transmittance is often wavelength-dependent (spectral transmittance) and can also depend on angle of incidence and polarization, especially for layered or textured materials.

Measurement of transmittance is typically done with a spectrophotometer, sometimes using an integrating sphere to capture diffuse transmission. Calibration accounts for instrument response and reference standards. Diffuse and specular transmittance may be distinguished when scattering is significant.

Applications of transmittance span optics and engineering. In glazing and solar control, high transmittance in the visible range is desired for clarity, while selective transmittance in the infrared can reduce heat load. Optical coatings, such as anti-reflective layers, aim to maximize transmittance by minimizing reflection. Transmittance also guides material selection for lenses, filters, and protective cover windows in sensors and photovoltaic devices.