Polarizer
A polarizer, and a linear polarizer, is a component that is not polarized electromagnetic waves with a certain linear polarization of the polarized electromagnetic waves or otherwise filters out. Some polarizers achieve this by absorbing the radiation of unwanted polarization. Other separate the polarization components spatially. The law of Malus describes quantitatively the distribution of linearly polarized radiation into two components.
Polarizers are indispensable for the function of the liquid crystal displays of computer monitors and mobile phones. Polarization microscopes, and many methods of spectroscopy employ polarizers to investigate the influence of the sample to the polarization of the light. In photography, polarizing filters are used to reduce reflections on glass or to leave the blue of the sky appear more intense.
A polarizer is used to determine existing polarization or measure, is also called the analyzer.
In addition to the linear polarizers discussed here, there are also circular polarizers that work with additional retardation plates and let pass the circularly polarized light.
- 2.1 optics
- 2.2 polarimetry
- 2.3 Satellite receivers
Function
A polarizer filters incident radiation so that only radiation with a polarization direction leaves its output. This can be achieved with different physical effects. In some of these principles, the proportion of the radiation that has an inappropriate direction of polarization passes to a second output. For other principles, the component absorbs this fraction.
Polarization by dichroism
A dichroic polarizer is a polarizer that is based on the dichroism of the two components of linearly polarized light absorbs strongly asymmetrical, that is, one of the components is strongly absorbed, and the other substantially transmitted.
With dichroic polarizers of crystals, the absorption of the direction of polarization depends on relative to the optical axis. By simply turning these crystals can be achieved that only the desired polarization direction is transmitted. However, the cost of such single crystals rise substantially with increasing crystal size, so that was sought for large polarizers for alternatives. One of these alternative polarizers developed Edwin Herbert Land. It was based on the parallel arrangement of needle-shaped dichroic herapathite crystallites on a slide, the so-called J- foil (English J -sheet, 1930 ).
Following a similar principle works the so-called H- sheet ( H sheet, Edwin Herbert Land, 1938). It consists of a polyvinyl alcohol film (PVA ), which constitute the conjugated chain chromophores, that absorb light. By doping with iodine occur in the chains moving holes, which shifts the absorption for light polarized parallel to the chains from the UV region to the visible area. A parallel orientation of the initially disordered chains reached land by in a direction he stretched PVA film ( " stretched "). Nowadays, instead of PVA films and films made of cellulose hydrate be used (see polarization filter).
Polarization by birefringence
Polarizer, whose effect is based on the birefringent properties of the materials used are generally referred to as a polarization prism. In birefringent materials, the refractive index of the polarization of the light to be separated in this manner depends, it follows that light of different (linear ) polarization is refracted differently, that is, the orthogonal polarized components of the light take different paths through the material and can.
The birefringent polarizers are commonly used in practice, the Nicol prism, the Rochon prism, and the Glan-Thompson prism. In addition, there are a variety of other polarizing prism, which primarily differ in the arrangement of the birefringent crystal. From the arrangement also results if only a certain polarization or whether both rays reach the field in different exit angles.
Polarizers in which only one polarization in the field will include:
- Nicol prism Nicol prism Hall
- Hartnack - Prazmoweky prism
- Glan- Thompson prism
- Glan- Foucault prism
- Lippich prism ( after F. Lippich ) Glan- Taylor prism
- Marple -Hess prism
Polarizers in which both polarizations appear in the visual field are:
- Hofmann prism
- Soret prism
- Wollaston prism Nomarski prism
- Rochon prism
- Sénarmont prism
Polarization by reflection
Falls unpolarized light at the Brewster angle to a glass plate, the reflected part is linear, and, perpendicular to the plane of incidence of the light polarized. The transmitted portion is only partially polarized. If you leave this light, however, by several plates run at the Brewster angle can also share this linearly polarize. The plane of polarization is parallel to this plane of incidence.
Electromagnetic waves in the centimeter to micrometer range can be polarized with a wire grid polarizer. This acts on the ( electric ) polarization component parallel to the wires, such as a full-surface metal reflector for the vertical component, it is clear as an insulator.
Applications
Optics
In addition to the production of polarized light polarizers can also be used as a filter. Thus, for example, be used as " variable attenuator " of non-polarized light, a combination of two rotatably consecutively assembled polarizers. Even in photography polarizing filters are used, see polarizing filter. Here, a specific polarization component is filtered to attenuate undesired reflections, for example, on water or glass.
Another application of polarizers found in polarization modulators.
Polarimetry
In analytics polarizers are always used in pairs in polarimeters, to determine the content or purity of an optically active substance over the rotation value.
Satellite receivers
Polarizers are used in satellite receiver systems; where in corresponding descriptions and technicians parlance often the English term is used polarizer. A polarizer is used at a receiving system with only one LNB pathogens to enable the reception of frequencies in another electromagnetic polarity (x / y). Here, the mirror of a satellite concentrated electromagnetic field of the polarizer rotates continuously in those optimal position. Thus, when received by the LNB excitation allows maximum field strength and the best cross-modulation decoupling. The rotation angle (see skew) is determined from the satellite receiver through analog or digital control signals.
Satellite receiving systems with polarizer are therefore particularly suitable for satellite motorized systems, which require a continuous adjustment of the polarization per satellite position. Polarizers are now offset by cost LNB with two integrated antennas excitation in the x / y array at fixed satellite reception systems.
There are three main types of polarizers, which are used in satellite reception systems: