Retroreflector

A retroreflector is a device that returns light to its source with minimal angular deviation. Unlike ordinary mirrors, it exhibits retroreflection, directing incident rays back toward the light source over a wide range of angles. This behavior arises from optical configurations that create a return path for the light, typically through multiple reflections or a combination of refraction and reflection.

The most common form is the cube-corner, or trihedral, retroreflector. It is a block with three mutually perpendicular, polished faces that form a corner. Light entering the corner undergoes three reflections and travels back toward the source. Other designs include spherical (ball-lens) retroreflectors and catadioptric arrangements known as cat’s-eye reflectors. Modern road-sign retroreflectors often use microprismatic or cube-corner structures embedded in sheets to achieve bright, angle-insensitive reflection.

Applications include road signs and highway markings, vehicle reflectors and safety apparel, and surveying or optical alignment tasks. In science, arrays of lunar retroreflectors placed on the Moon by the Apollo missions support high-precision laser ranging to measure Earth–Moon distance. Retroreflectors are passive devices, commonly made from glass, ceramics, or plastics, and are designed for durability and fairly broad wavelength performance.

Limitations include reduced brightness at very wide incidence angles for some designs, wavelength dependence, and manufacturing costs. Nevertheless, retroreflectors provide reliable back-reflection across varied conditions, making them essential for visibility and precise measurement in diverse fields.