Definition: devices which reflect light or other radiation
More specific terms: retroreflectors, mirrors, fiber loop mirrors
Author: Dr. Rüdiger Paschotta
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Reflectors are devices which can reflect light (not necessarily in the visible spectral region) or other radiation. This is a very general category of device: reflectors can be realized in very different ways and can have very different characteristics.
More specifically, a retroreflector is a device which reflects light back into itself (but possibly with some spatial offset).
The actual function of a reflector is often performed close to its surface, but the underlying material may still be important e.g. to provide sufficient mechanical stability and opportunities for mounting.
The direction of light can also be changed by refraction e.g. at a prism surface. Such devices, however, are generally not called reflectors.
Types of Reflectors
Important types of reflectors are described in the following:
A particularly important category of reflectors are mirrors, which directly reflect light on one or several microscopically flat surfaces, which may be plane or curved. Mirrors cause specular reflection, where e.g. a laser beam stays a well-defined beam upon reflection, just with a modified direction of propagation.
The polarization of light often stays unchanged, particularly for reflection with normal incidence, but may also be modified – but usually in a systematic and reproducible way.
See the article on mirrors for more details.
Many diffraction gratings are reflective devices and thus a special category of reflectors. In contrast to simple mirrors, reflected light may be sent to multiple directions corresponding to different diffraction orders.
Reflectors with Multiple Reflections
Some reflectors utilize multiple reflections, e.g. on different mirrors at different orientations, or total internal reflection on prism surfaces. For example, there are prism retroreflectors such as corner cube prisms which change a beam direction by 180°, with some parallel offset. Another example are Cassegrain reflectors, essentially combinations of two mirrors used in certain telescopes.
A special type of reflector is the Faraday mirror. This device is made such that the polarization state of reflected light is always orthogonal to that of the input light – at least in good approximation within a certian wavelength range.
Certain three-dimensional photonic crystals can serve as reflectors with quite peculiar characteristics – for example, as omnidirectional reflectors in a certain wavelength range.
There are diffuse reflectors, e.g. based on a surface with microscopic irregularities, which strongly scramble the optical wavefronts and thus destroy or reduce spatial coherence. Even if the incident light is a well-defined beam, the reflected light will be diffuse, i.e., propagate in a wide range of directions. The light polarization is also often lost.
Diffuse reflectors are often used in the context of illumination, e.g. for room illumination or for photography. They are often placed close to some light source. Walls of a room can also serve as diffuse reflectors.
Some diffuse reflectors serve as screens, e.g. for viewing images e.g. from a projector. Some screens can also substantially reduce the temporal coherence of light, which is important for example in the context of laser projectors.
See also the article on diffusers. Diffusers can be of reflective and transmissive type.
Different kinds of reflectors can be used in fiber optics:
- One may use mirrors for reflecting light – for example, dielectric coatings applied directly to fiber ends, or alternatively discrete mirrors butted to the fiber ends.
- Fiber Bragg gratings also serve as light reflectors – usually with a fairly limited reflection bandwidth.
- There are also more complex arrangements such as fiber loop mirrors (more precisely: fiber loop reflectors) which utilize interference effects in fiber couplers.
Main Characteristics of Light Reflectors
Light reflectors can be characterized in different ways:
- In many cases, it is relevant how much of the incident light is reflected – more precisely, which fraction of the incident optical power or irradiance is reflected. That quantity is called the reflectance (or for a single surface also reflectivity). It can generally depend on the direction of incidence, the optical wavelength and the polarization, and possibly also on the location on the reflector. In many cases, reflectors work only for a limited range of incidence angles and wavelengths.
- The optical phase change is relevant for some applications, and in particular its frequency dependence, which is related to group delay and chromatic dispersion.
- For diffuse reflectors, one may need to know the angular distribution of the reflected light, which can substantially depend on the input direction, on the optical wavelength and polarization.
- In some cases, the reflected light has some parallel offset to the incident light – for example, related to multiple reflections in a retroreflector device.
- The uniformity of reflection characteristics over the whole area of a reflector is important for some applications.
- In the context of laser light, particularly with intense light pulses, it may be relevant to know the threshold intensity or fluence for laser-induced damage.
- Reflectors often need to be properly mounted in some device, and may therefore have various features for allowing convenient and stable mounting.
The RP Photonics Buyer's Guide contains three suppliers for reflectors.
See also: retroreflectors, mirrors, fiber loop mirrors
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