 Optical depth

Optical depth satisfies $d{\tau }_{\nu }={\alpha }_{\nu }ds$ along a path and measures attenuation through $I_{\nu }\propto e^{-{\tau }_{\nu }}$ in the absence of emission.

Optical depth (or optical thickness) is a measure of how much a medium attenuates (reduces the intensity of) light or other electromagnetic radiation as it travels through that medium. It quantifies the overall effect of absorption and scattering of light along a specific path.