Transmissivity

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

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

Note that transmissivity is a unitless quantity in the range [0,1]. It normally varies as a function of wavelength.