Polarizer
A polarizer is a component that generates electromagnetic waves , e.g. B. visible light with a certain polarization from not, partially or differently polarized electromagnetic waves. Polarizers use the following mechanisms to separate the different polarizations of the incident waves: dichroism (also called selective absorption ), reflection , birefringence , scattering and diffraction .
Polarizers that generate a linearly polarized electromagnetic wave are called linear polarizers , also known as linear polarizers . The law of Malus describes quantitatively the division of linearly polarized radiation into the two components. Analogously, polarizers that generate circularly polarized light are called circular polarizers .
Polarizers are essential for the functioning of the liquid crystal displays on computer monitors and cell phones . Polarizing microscopes and many methods of spectroscopy use polarizers to study the influence of the sample on the polarization of light. Polarization filters are used in photography to reduce reflections on glass panes or to make the blue of the sky appear more intense. So-called polarizing lenses are also used in some sunglasses, which have the same effect. These are popular with anglers because they reduce reflections on the water surface and thus make fish easier to locate.
A polarizer that is used to determine or measure existing polarization is also called an analyzer .
functionality
A polarizer filters incident radiation so that only radiation with one polarization direction leaves its exit. This can be achieved with different physical effects. With some of these principles, the portion of the radiation that has an unsuitable polarization direction reaches a second output. With other principles, the component absorbs this proportion.
Polarization through dichroism
A dichroic polarizer, a polarizer based on dichroism , absorbs the two components of linearly polarized light in a highly asymmetrical manner, that is, one of the components is strongly absorbed, the other essentially transmitted.
In polarizers made of dichroic crystals, the absorption depends on the polarization direction relative to the optical axis . By simply rotating these crystals, you can ensure that only the desired direction of polarization is allowed through. However, the costs for such single crystals increase considerably with increasing crystal size, so that alternatives were sought for large-area polarizers. Edwin Herbert Land developed one of these alternative polarizers . It was based on the parallel arrangement of needle-shaped dichroic herapathite - crystallites on a slide, the so-called J-film (Engl. J-sheet , 1930).
The so-called H-foil ( H-sheet , Edwin Herbert Land, 1938) works according to a similar principle . It consists of a polyvinyl alcohol (PVA) film, the conjugated chains of which represent chromophores , i.e. absorb light. By doping with iodine arise in the chains moving holes , which the absorption for light which is polarized parallel to the chains, shifts from the UV range to the visible range. Land achieved a parallel alignment of the initially disordered chains by stretching the PVA film in one direction ("stretching"). Nowadays, instead of PVA films, cellulose hydrate films are also used (see polarization filters ).
Polarization by birefringence
Polarizers, whose effect is based on the birefringent properties of the materials used, are generally referred to as polarizing prisms . In the case of birefringent materials, the refractive index depends on the polarization of the light, which means that light of different (linear) polarization is refracted differently, i.e. the parts of the light polarized perpendicular to each other take different paths through the material and can be separated in this way .
The birefringent polarizers commonly used in practice are the Nicol prism , the Rochon prism and the Glan-Thompson prism . In addition, there are a number of other polarizing prisms, which mainly differ in the arrangement of the birefringent crystals. The arrangement also shows whether only a certain polarization or whether both beams reach the field of view at different exit angles.
Polarizers in which only one polarization appears in the field of view are:
- Ear prism
- Nicol's prism
- Glan prisms: They are characterized by surfaces that are ground to the optical axis.
Polarizers in which both polarizations appear in the field of view are:
Polarization by reflection
If unpolarized light falls on a glass plate at Brewster's angle , the reflected part is linearly polarized, namely perpendicular to the plane of incidence of the light. The transmitted part is only partially polarized. However, if this light is allowed to pass through several plates at Brewster's angle, this portion can also be linearly polarized. The plane of polarization is parallel to the plane of incidence.
Electromagnetic waves in the centimeter to micrometer range can be polarized with a wire grid polarizer. For the (electrical) polarization component parallel to the wires, this acts like a full-surface metal mirror, for the vertical component it is transparent as an insulator.
Circularly polarized light can also be generated through special reflection. For example, incident, 45 ° linearly polarized light can be converted into circularly polarized light by total reflection in a glass body of special geometry, see Fresnel's parallelepiped .
Applications
optics
In addition to generating polarized light, polarizers can also serve as filters . For example, a combination of two polarizers that can be rotated one behind the other can be used as a “variable attenuator” for unpolarized light. Polarization filters are also used in photography, see polarization filters . A certain polarization component is filtered out in order to, for example, weaken unwanted reflections on water or glass panes.
Another application of polarizers is found in polarization modulators .
Polarimetry
In analysis , polarizers are always used in pairs in polarimeters to determine the content or purity of an optically active substance via the rotation value .
Satellite receiving systems
Polarizers are also used in satellite receiving systems; whereby the English term polarizer is often used in corresponding descriptions and in technical usage . A polarizer is used in a receiving system with only one LNB exciter to enable the reception of frequencies in a different electromagnetic polarity (x / y). The polarizer turns the electromagnetic field concentrated by a satellite mirror steplessly into the optimal position. This enables the greatest possible field strength and the best possible cross-modulation decoupling when receiving from the LNB exciter. The desired angle of rotation (see Skew ) is specified by the satellite receiver using analog or digital control signals.
Satellite reception systems with polarizers are therefore particularly suitable for satellite rotating systems that require stepless adjustment of the polarization for each satellite position. In fixed satellite reception systems, polarizers are nowadays replaced by inexpensive LNBs with two integrated exciter antennas in an x / y arrangement.
There are three main types of polarizers that are used in satellite receiving systems:
- Mechanical: The desired polarity is selected by a small rotatable dipole antenna.
- Magnetic: Element of the parabolic antenna , which consists of a coil and a circular wave guide. The current in the coil creates a magnetic field in a ferrite rod, which is able to polarize. With this technique it is possible to receive a channel of one polarity while suppressing a channel of another polarity.
- Mechanical rotating devices that rotate an entire LNB into a polarization desired for reception.
literature
- Jean M. Bennett: Polarizers . In: Michael Bass, Casimer Decusatis, Vasudevan Lakshminarayanan, Guifang Li, Carolyn MacDonald, Virendra Mahajan, Eric Van Stryland (eds.): Handbook of Optics, Volume I . 3. Edition. McGraw Hill Professional, 2009, ISBN 978-0-07-149889-0 , pp. 13.1 ff . (extensive compilation of all possible polarizing prisms).
- Hans Dodel, Sabrina Eberle: Satellite communication . 2nd Edition. Springer-Verlag, Berlin 2007, ISBN 3-540-29575-5 .
Web links
- Kenneth R. Spring, Thomas J. Fellers, Michael W. Davidson: Introduction to Prisms and Beamsplitters . August 1, 2003, accessed January 17, 2010.
Individual evidence
- ↑ a b c d e Michael Bass, Casimer Decusatis, Vasudevan Lakshminarayanan, Guifang Li, Carolyn MacDonald, Virendra Mahajan, Eric Van Stryland: Handbook of Optics, Volume I . 3. Edition. McGraw Hill Professional, 2009, ISBN 978-0-07-149889-0 , pp. 13.8 ff .
- ↑ Data sheet (PDF; 347 kB) for the Glan-Taylor polarizer as a variable attenuator