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Retroreflector
Gold corner cube retroreflector
Uses	Optical delay line Distance measurement

A retroreflector is a device or surface that reflects a wave front back along a vector that is parallel to but opposite in direction from the angle of incidence. This is unlike a mirror, which does that only if the mirror is exactly perpendicular to the wave front.

Retroreflection is usually obtained in one of two ways:^[1]

1. with a set of three mutually perpendicular mirrors which form a corner (a corner reflector or corner cube), or
2. with reflecting and refracting optical elements arranged so that the focal surface of the refractive element coincides with the reflective surface, typically a transparent sphere and a spherical mirror - this same effect may be achieved with a single transparent sphere provided that the refractive index of the material is exactly two times the refractive index of the medium from which the radiation is incident. In that case, the sphere surface behaves as a concave spherical mirror with the required curvature for retroreflection. This is conventionally known as a cat's eye retroreflector in either configuration.

The term cat's eye derives from the resemblance of the cat's eye retroreflector to the optical system that produces the well-known phenomenon of "glowing eyes" in cats and many other vertebrates (which are of course only reflecting light, rather than actually glowing). The combination of the eye's lens and the aqueous humor form the refractive converging system, while the tapetum lucidum behind the retina forms the spherical concave mirror. Because the function of the eye is to form an image on the retina, an eye focused on a distant object has a focal surface that approximately follows the reflective tapetum lucidum structure, which is the condition required to form a good retroreflection.

Corner retroreflectors occur in two varieties. In the more common form, the corner is literally the truncated corner of a cube of transparent material such as conventional optical glass. In this structure, the reflection is achieved either by total internal reflection or silvering of the outer cube surfaces. The second form uses mutually perpendicular flat mirrors bracketing an air space. These two types have similar optical properties.

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. Phase conjugate mirrors require a comparatively expensive and complex apparatus, as well as large quantities of power (as nonlinear optical processes can be efficient only at high enough intensities). However, phase conjugate mirrors have an inherently much greater accuracy in the direction of the retroreflection, which in passive elements is limited by the mechanical accuracy of the construction.

Figure 1 - Observation angle

Figure 2 - Entrance angle

Retroreflectors are devices that operate by returning light back to the light source along the same light direction. The coefficient of luminous intensity, R_I, is the measure of a reflector performance. It is defined as the ratio of the strength of the reflected light (luminous intensity) to the amount of light that falls on the reflector (normal illuminance). A reflector will appear brighter as its R_I value increases.^[1]

The R_I value of the reflector is a function of the color, size, and condition of the reflector. Clear or white reflectors are the most efficient, and appear brighter than other colors. The surface area of the reflector is proportional to the R_I value and will increase as the reflective surface increases.^[1]

The R_I value is also a function of the spatial geometry between the observer, light source, and reflector. Figures 1 and 2 show the observation angle and entrance angle between the automobile's headlights, bicycle, and driver. The observation angle is the angle formed by the light beam and the driver's line of sight. Observation angle is a function of the distance between the headlights and the driver's eye, and the distance to the reflector. Traffic engineers use an observation angle of 0.2 degrees to simulate a reflector target about 800 feet in front of a passenger automobile. As the observation angle increases, the reflector performance decreases. For example, a truck has a large separation between the headlight and the driver's eye compared to a passenger vehicle. A bicycle reflector will appear brighter to the passenger car driver than to the truck driver at the same distance from the vehicle to the reflector.^[1]

The light beam and the normal axis of the reflector as shown in Figure 2 form the entrance angle. The entrance angle is a function of the orientation of the reflector to the light source. For example, the entrance angle between an automobile approaching a bicycle at an intersection 90 degrees apart will be larger than the entrance angle for a bicycle directly in front of an automobile on a straight road. The reflector will appear brightest to the observer when it is directly in line with the light source.^[1]

The brightness of a reflector is also a function of the distance between the light source and the reflector. At a given observation angle, as the distance between the light source and the reflector decreases, the light that falls on the reflector increases. This increases the amount of light returned to the observer and the reflector will appear brighter.^[1]

Retroreflectors are clearly visible in a pair of bicycle shoes. Light source is a flash a few centimeters above camera lens.

See also: Raised pavement marker

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 retroreflective 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. "Cat's eyes" are a particular type of retroreflector embedded in the road surface, used mostly in the UK and parts of the United States.

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 can be embedded in the road (level with the road surface), or can be raised above the road surface. Raised reflectors are visible for a very long distance (typically 0.5-1 kilometer or more), while sunken reflectors are only visible at very close range due to the higher angle required to properly reflect the light. Raised reflectors are not generally used in areas that regularly experience snow during winter, as passing snowplows will tear them off the roadway. The stress on the roadway caused by cars running over any embedded objects also contributes to accelerated wear and pothole formation.

Retroreflective road paint is thus very popular in Canada and increasingly the northern parts of the United States, as it is not affected by the passage of snowplows and does not affect the interior of the roadway. Where weather permits, embedded retroreflectors are preferred as they last much longer than road paint, which is weathered by the elements and ground away by the passage of vehicles.

The Apollo 11, 14, and 15 missions left retro-reflectors on the Moon as part of the Lunar Laser Ranging Experiment. They are commonly considered to be one of the strongest pieces of evidence against a Moon landing hoax. Additionally the unmanned Soviet Lunokhod 1 and Lunokhod 2 rovers carried smaller arrays. Reflected signals were initially received from Lunokhod 1, but no return signals have been detected since 1971, at least in part due to some uncertainty in its location on the Moon. Lunokhod 2's array continues to return signals to Earth.[1] Even under good viewing conditions, only a single reflected photon is received every few seconds. This makes the job of filtering laser-generated photons from naturally-occurring photons challenging. [2]

LAGEOS, or Laser Geodynamics Satellites, are a series of scientific research satellites designed to provide an orbiting laser ranging benchmark for geodynamical studies of the Earth. There are two LAGEOS spacecraft, LAGEOS-1 launched in 1976, and LAGEOS-2 launched in 1992. As of 2004, both LAGEOS spacecraft are still in service.

Retroreflective clothing, combined with a properly set up camera and projector, can be used to achieve the effect of partial invisibility when viewed from a single direction. ^[2][3]

Modulated retroreflectors, in which the reflectance is changed over time by some means, are the subject of research and development for free-space optical communications networks. The basic concept with such systems is that a low-power remote system, such as a sensor mote, can receive an optical signal from a base station and reflect the modulated signal back to the base station. Since the base station supplies the optical power, this allows the remote system to communicate without excessive power consumption. Modulated retroreflectors also exist in the form of modulated phase-conjugate mirrors (PCMs). In the latter case, a "time-reversed" wave is generated by the PCM, with temporal encoding of the phase-conjugate wave (see, e.g., reference 6).

Cheap plastic corner retroreflectors are using as an aiming device in user-controlled technology optical datalink device Ronja. The aiming is done in night and the necessary retroreflector area depends on aiming distance and ambient lighting from street lamps. The optical receiver itself behaves as a weak retroreflector, because contains a large precisely focused lens and shiny object in the focal plane. This allows aiming without a retroreflector for short range.

In surveying with a total station or robot, the instrument man or robot aims a laser beam at a corner cube retroreflector held by the rodman. The instrument measures the propagation time of the light and converts it to a distance.

The sensor system of common (non-SLR) digital cameras is retroreflective. Researchers have used this property to demonstrate a system to prevent unauthorized photographs by detecting digital cameras and beaming a highly-focused beam of light into the lens.^[4]

Retroreflectors are used in movie screens^[5]

This article or section includes a list of references or external links, but its sources remain unclear because it lacks in-text citations. You can improve this article by introducing more precise citations.

• Optics Letters, Vol. 4, pp. 190-192 (1979), "Retroreflective Arrays as Approximate Phase Conjugators," by H.H. Barrett and S.F. Jacobs.
• Optical Engineering, Vol. 21, pp. 281-283 (March/April 1982), "Experiments with Retrodirective Arrays," by Stephen F. Jacobs.
• Scientific American, December 1985, "Phase Conjugation," by Vladimir Shkunov and Boris Zel'dovich.
• Scientific American, January 1986, "Applications of Optical Phase Conjugation," by David M. Pepper.
• Scientific American, April 1986, "The Amateur Scientist" ('Wonders with the Retroreflector'), by Jearl Walker.
• Scientific American, October 1990, "The Photorefractive Effect," by David M. Pepper, Jack Feinberg, and Nicolai V. Kukhtarev.

• Apollo 15 Laser Ranging Retroreflector Experiment
• Manual of Traffic Signs - Retroreflective Sheetings Used for Sign Faces
• Lunar retroflectors
• Howstuffworks article on retroreflector-based invisibility cloaks