Retroreflector

A retroreflector is a device that sends light or other radiation back where it came from, regardless of the angle of incidence (unlike a mirror, where this happens only if the mirror is exactly perpendicular to the light beam). This effect can be commonly obtained in two ways: with a set of three perpendicular mirrors (a corner reflector) and with a transparent sphere of material with refractive index 2.

A retroflector may consist of many very small versions of these structures incorporated in a thin sheet or in paint. In the case of paint containing glass beads, the paint glues the beads to the surface where retroreflection is required, and the beads protrude, their diameter being about twice the thickness of the paint.

A third, much less common way of producing a retroreflector is to use the nonlinear optical phenomenon of phase conjugation[?]. This technique is used in advanced optical systems such as high-power lasers and optical transmission lines.

Retroreflectors on roads

Retroreflection (sometimes called retroflection) is used on road surfaces, road signs[?], vehicles and clothing (large parts of the surface of special safety clothing, less on regular coats). When the headlights of a car illuminate a retroflective surface, the reflected light is directed towards the car and its driver, and not wasted by going in all directions as with diffuse reflection. However, a pedestrian can see a retroreflective surface in the dark only if there is a light source directly between them and the reflector, e.g. a torch they carry, or directly behind them, e.g. a car approaching from behind.

Cats' eyes are retroreflectors in the road surface that can withstand being driven over. They were invented in 1933 by Percy Shaw of Yorkshire in England.

Corner reflectors are better at sending the light back to the source over long distances, while spheres are better at sending the light to a receiver somewhat off-axis from the source, as when the light from headlights[?] is reflected into the driver's eyes.

Retroreflectors on the Moon

The Apollo 11, 14, and 15 missions left retro-reflectors on the Moon as part of a laser ranging[?] experiment, the Lunar Laser Ranging Experiment.