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Retroreflectors

Author: the photonics expert (RP)

Definition: optical devices which reflect light in a direction which is parallel to the incident beam

More general term: reflectors

Category: article belongs to category general optics general optics

DOI: 10.61835/6m8   Cite the article: BibTex plain textHTML   Link to this page   LinkedIn

Retroreflectors are a kind of reflectors (optical devices which reflect light), where the reflected light propagates in a direction which is parallel to the incident beam – and this for some range of beam directions. This is profoundly different from reflections at ordinary mirrors, where a light beam is reflected back in itself only for normal incidence on the mirror surface; any tilt of the mirror will modify the direction of the reflected beam. For some retroreflectors, there is a parallel offset of the reflected beam, but the direction is always quite precisely opposite to that of the incoming beam, limited only by the accuracy of the device.

Different kinds of retroreflectors are available:

  • There are corner reflectors, e.g. in the form of corner cube prisms, where reflections occur on three mutually orthogonal mirror surfaces.
  • A cat's eye reflector contains a focusing lens in conjunction with a curved mirror. It can also be realized in the form of a glass sphere with an appropriate refractive index. That kind of retroreflector effect is known from the eyes of cats, where the eye's lens does part of the focusing and some reflection occurs on the retina, forming a quite imperfect retroreflector.
  • There are nonlinear retroreflectors based on stimulated Brillouin scattering. Here, the reflection or occurs at a Bragg grating in a nonlinear material, which is related to the incoming and reflected optical wave and automatically has the correct orientation for precise retroreflection. Besides, there is also the phenomenon of phase conjugation and a down-shift of the optical frequency of the reflected beam. The nonlinear reflection works only for sufficiently high input intensity of the incident beam and with a limited reflectance.

The reflectance of a retroreflector is usually substantially lower than that of a highly reflecting laser mirror. Nevertheless, the directionality of the reflection can create a substantially increased brightness of an illuminated object.

Depending on the type of retroreflector, there can be substantial changes in polarization direction, and image rotation can occur.

There are “spoiled” retroreflectors, intentionally made such that the direction of the reflected beam is somewhat modified.

There are also a kind of retroreflectors where the retroreflection principle works only in a plane; tilts of the reflector in one direction still lead to deviating directions of the reflected beam. This is the case, for example, for prisms with two instead of three reflections.

Applications of Retroreflectors

Retroreflectors are used where it is essential to maintain the direction of the reflected beam without precisely keeping the right orientation of the reflecting device. Some examples:

  • A few retroreflectors have been placed on the Moon, e.g. during the Apollo missions, and can be used for time-of-flight measurements of the distance between Earth and Moon (lunar laser ranging; see also the article on laser rangefinders). The retroreflector property is essential for increasing the amount of reflected light received in a large distance.
  • Similarly, various satellites are equipped with retroreflectors for facilitating laser tracking of the position.
  • Retroreflectors are used on some road signs and vehicles, e.g. bicycles, for better visibility under illumination with headlights of cars, for example. Here, exact retroreflection would not be helpful because a road sign or a bicycle should be seen by the car's driver rather than the headlights, but some degree of directionality of an imperfect retroreflector improves the visibility.
  • There are barcode labels based on retroreflective materials, which can be scanned from large distances.
  • Interferometers and optical delay lines often utilize a retroreflector for avoiding overly critical alignment, which would be problematic particularly for moving parts.

More to Learn

Encyclopedia articles:

Suppliers

The RP Photonics Buyer's Guide contains 17 suppliers for retroreflectors. Among them:

OPTOMAN

retroreflectors

OPTOMAN can manufacture various optical assemblies, including retroreflectors.

OPTOMAN concentrates solely on Ion Beam Sputtering (IBS), with which we can manufacture various coatings, including high LIDT (>1 J/cm2 @ 1030 nm, 500 fs), low absorption (<1 ppm), and highly reflective (R > 99.995%) ultrafast mirrors.

Artifex Engineering

retroreflectors

Artifex Engineering offers custom retroreflectors tailored to your requirements. These can be either solid or hollow. Our corner cube prisms are available in a range of substrates and coatings. Visit our product page for more information. We look forward to your inquiry.

Shalom EO

retroreflectors

Hangzhou Shalom EO has developed a capable production workshop that could offer various kinds of custom and off-the-shelf corner cube retroreflectors made of N-BK7, UV fused silica, and other custom materials with diverse coating options like silver/gold coatings, and coatings in the visible, UV, and NIR spectrum.

Shalom EO's main types of corner cube retroreflectors include:

  • Standard corner cube retroreflectors of routine choices
  • Truncated corner cube reflectors of lightweight and compact dimensions
  • Lateral transfer corner cube retroreflectors with one trihedral reflecting face at one terminal of the prism and two trihedral reflecting faces forming a roof edge at the other end are designed for applications requiring a lateral beam displacement.
  • Customized hollow corner retroreflectors that perform well in low-pressure conditions and mitigate the aberrations derived from glass dispersion and temperature fluctuations.

Knight Optical

retroreflectors

Knight Optical have stock N-BK7 corner cube retroreflector prisms in 12.5 mm to 64 mm diameters with a 3" deviation angle. These corner cube prisms offer excellent parallelism between the entry and exit beam. We can also offer custom corner cube retroreflectors with different substrates and coatings available.

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