Optical Cross Section or OCS is a value which describes the maximum amount of optical flux reflected back to the source. The standard unit of measurement is . It is dependent on the geometry and the reflectivity at a particular wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 of an object. Optical cross section is useful in fields such as LIDAR

LIDAR

LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...

. In the field of Radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 this is referred to Radar Cross Section

Radar cross section

Radar cross section is a measure of how detectable an object is with a radar. A larger RCS indicates that an object is more easily detected.An object reflects a limited amount of radar energy...

. Object such as license plates on automobiles has a high optical cross section to maximized the laser return to the speed detector gun.

Flat mirror

Optical cross section of a flat mirror with a given reflectivity at a particular wavelength can be express by this formula.



Where is the cross sectional diameter of the beam. Note that the direction of the light has to be perpendicular to the mirror surface for this formula to be valid, else the return from the mirror would no longer go back to it source.

In order to maximize the return a corner reflector

Corner reflector

A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects waves back directly towards the source, but shifted . Unlike a simple mirror, they work for a relatively wide-angle field of view. The three intersecting surfaces often have...

is used. The alignment of a corner reflector with respect to the source is not as critical as the alignment of a flat mirror.

Other optical devices

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 cross section of a camera can vary over time due to the camera shutter opening and closing.