The Optical Path (OP) describe the total concentration along a path of constituents extinguishing (by absorption or scattering) the electromagnetic radiation traveling through a medium at a specified whavelength λ. Its value depends then on the efficiency of absorption and scattering phenomena which occur during the travel itself. The Earth's atmosphere is usually the medium that a monochromatic beam (Iλ) of radiation travels through before reaching satellite sensors. In an homogeneously estinguishing medium (i.e. a medium with extinction coefficient for mass unit K constant along the optical path) the Optical Thickness OT is defined as OT=K x OP. It give a measure of the cumulative depletion of Iλ directed in straight-downward. As far as the Optical Thickness is large, the medium is more and more optically thick (i.e. radiation is largely absorbed). If the Optical Thickness is small it means that the medium is optically thin (i.e. radiation travels through it easily).