Optical cross section (OCS) is a value which describes the maximum amount of optical flux reflected back to the source. The standard unit of measurement is m /sr. OCS is dependent on the geometry and the reflectivity at a particular wavelength of an object. Optical cross section is useful in fields such as LIDAR. In the field of radar this is referred to as radar cross-section. Objects such as li. Flat mirrorOptical cross section of a flat mirror with a given reflectivity at a particular wavelength can be expressed by the formula Where. Optical cross section is not limited to reflective surfaces. Optical devices such as telescopes and cameras will return some of the optical flux back to the source, since it has optics that reflect some light. The Optical cro.