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, which does that 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 mutually perpendicular mirrors which form a corner (a corner reflector or corner cube), and
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 2 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 are generally not very efficient). However, they 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.

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

Retroreflectors on the Moon

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.

It is interesting to note ^* that 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.

Retroreflectors in Earth orbit

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.

Retroreflectors and invisibility

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.

Retroreflectors and communications

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).

External links

http://www.lpi.usra.edu/expmoon/Apollo15/A15_Experiments_LRRR.html
http://www.trafficsign.us/signsheet.html Manual of Traffic Signs - Retroreflective Sheetings Used for Sign Faces

Optical devices

Retroreflexion | Reflector

References

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.