Transmissivity

Transmissivity is a term in radiometry. It is the ratio between the exitance from the back side of a surface and the incident irradiance on the front side.

$\tau _{\lambda }={\frac {M_{back,\lambda }}{E_{front,\lambda }}}$

Note that transmissivity is a unitless quantity in the range [0,1], and can also be expressed as a percentage in the range [0%,100%]. A completely transparent surface has a transmissivity of 100%. Transmissivity normally varies as a function of both wavelength and incident angle.