Natural surfaces interact with the incident solar energy reflecting, absorbing and, in the case of certain bodies (such as the water and the atmosphere), transmitting the energy. In particular the reflected (or back-scattered) component of the solar light is of interest in remote sensing applications because it represents the signal measured by satellite or airborne optical sensors. The reflectance is a function of the optical properties of the surface and, depending on the surface (e.g., vegetation, snow, water, soil) it has a well-known behavior across the electromagnetic spectrum (spectral signature). However, the reflectance also depends on the geometry of observation and acquisition. In this case the bidirectional reflectance distribution function (BRDF) is commonly used to describe such behavior.