Transmittance is the fraction of incident light (electromagnetic radiation) at a specified wavelength that passes through a sample.
$\tau = \frac{\Phi_t}{\Phi_m}$, where $\Phi_t$ is the transmitted radiant flux or the transmitted luminous flux, and $\Phi_m$ is the radiant flux or luminous flux of the incident radiation.
Applicable units are those of quantitykind:Transmittance
Transmittance