Polarizer

A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations.[1][2][3][4] It can filter a beam of light of undefined or mixed polarization into a beam of well-defined polarization, that is polarized light. The common types of polarizers are linear polarizers and circular polarizers. Polarizers are used in many optical techniques and instruments, and polarizing filters find applications in photography and LCD technology. Polarizers can also be made for other types of electromagnetic waves besides light, such as radio waves, microwaves, and X-rays.

Polarizer Through Glass
A polarizing filter cuts down the reflections (top) and made it possible to see the photographer through the glass at roughly Brewster's angle although reflections off the back window of the car are not cut because they are less-strongly polarized, according to the Fresnel equations.

Linear polarizers

Linear polarizers can be divided into two general categories: absorptive polarizers, where the unwanted polarization states are absorbed by the device, and beam-splitting polarizers, where the unpolarized beam is split into two beams with opposite polarization states. Polarizers which maintain the same axes of polarization with varying angles of incidence are often called Cartesian polarizers, since the polarization vectors can be described with simple Cartesian coordinates (for example, horizontal vs. vertical) independent from the orientation of the polarizer surface. When the two polarization states are relative to the direction of a surface (usually found with Fresnel reflection), they are usually termed s and p. This distinction between Cartesian and sp polarization can be negligible in many cases, but it becomes significant for achieving high contrast and with wide angular spreads of the incident light.

Absorptive polarizers

Certain crystals, due to the effects described by crystal optics, show dichroism, preferential absorption of light which is polarized in particular directions. They can therefore be used as linear polarizers. The best known crystal of this type is tourmaline. However, this crystal is seldom used as a polarizer, since the dichroic effect is strongly wavelength dependent and the crystal appears coloured. Herapathite is also dichroic, and is not strongly coloured, but is difficult to grow in large crystals.

A Polaroid polarizing filter functions similarly on an atomic scale to the wire-grid polarizer. It was originally made of microscopic herapathite crystals. Its current H-sheet form is made from polyvinyl alcohol (PVA) plastic with an iodine doping. Stretching of the sheet during manufacture causes the PVA chains to align in one particular direction. Valence electrons from the iodine dopant are able to move linearly along the polymer chains, but not transverse to them. So incident light polarized parallel to the chains is absorbed by the sheet; light polarized perpendicularly to the chains is transmitted. The durability and practicality of Polaroid makes it the most common type of polarizer in use, for example for sunglasses, photographic filters, and liquid crystal displays. It is also much cheaper than other types of polarizer.

A modern type of absorptive polarizer is made of elongated silver nano-particles embedded in thin (≤0.5 mm) glass plates. These polarizers are more durable, and can polarize light much better than plastic Polaroid film, achieving polarization ratios as high as 100,000:1 and absorption of correctly polarized light as low as 1.5%.[5] Such glass polarizers perform best for short-wavelength infrared light, and are widely used in optical fiber communications.

Beam-splitting polarizers

Beam-splitting polarizers split the incident beam into two beams of differing linear polarization. For an ideal polarizing beamsplitter these would be fully polarized, with orthogonal polarizations. For many common beam-splitting polarizers, however, only one of the two output beams is fully polarized. The other contains a mixture of polarization states.

Unlike absorptive polarizers, beam splitting polarizers do not need to absorb and dissipate the energy of the rejected polarization state, and so they are more suitable for use with high intensity beams such as laser light. True polarizing beamsplitters are also useful where the two polarization components are to be analyzed or used simultaneously.

Polarization by Fresnel reflection

Brewster-polarizer
A stack of plates at Brewster's angle to a beam reflects off a fraction of the s-polarized light at each surface, leaving a p-polarized beam. Full polarization at Brewster's angle requires many more plates than shown. The arrows indicate the direction of the electrical field, not the magnetic field, which is perpendicular to the electric field

When light reflects (by Fresnel reflection) at an angle from an interface between two transparent materials, the reflectivity is different for light polarized in the plane of incidence and light polarized perpendicular to it. Light polarized in the plane is said to be p-polarized, while that polarized perpendicular to it is s-polarized. At a special angle known as Brewster's angle, no p-polarized light is reflected from the surface, thus all reflected light must be s-polarized, with an electric field perpendicular to the plane of incidence.

A simple linear polarizer can be made by tilting a stack of glass plates at Brewster's angle to the beam. Some of the s-polarized light is reflected from each surface of each plate. For a stack of plates, each reflection depletes the incident beam of s-polarized light, leaving a greater fraction of p-polarized light in the transmitted beam at each stage. For visible light in air and typical glass, Brewster's angle is about 57°, and about 16% of the s-polarized light present in the beam is reflected for each air-to-glass or glass-to-air transition. It takes many plates to achieve even mediocre polarization of the transmitted beam with this approach. For a stack of 10 plates (20 reflections), about 3% (= (1-0.16)20) of the s-polarized light is transmitted. The reflected beam, while fully polarized, is spread out and may not be very useful.

A more useful polarized beam can be obtained by tilting the pile of plates at a steeper angle to the incident beam. Counterintuitively, using incident angles greater than Brewster's angle yields a higher degree of polarization of the transmitted beam, at the expense of decreased overall transmission. For angles of incidence steeper than 80° the polarization of the transmitted beam can approach 100% with as few as four plates, although the transmitted intensity is very low in this case.[6] Adding more plates and reducing the angle allows a better compromise between transmission and polarization to be achieved.

Wire-grid-polarizer
A wire-grid polarizer converts an unpolarized beam into one with a single linear polarization. Coloured arrows depict the electric field vector. The diagonally polarized waves also contribute to the transmitted polarization. Their vertical components are transmitted (shown), while the horizontal components are absorbed and reflected (not shown).

Because their polarization vectors depend on incidence angle, polarizers based on Fresnel reflection inherently tend to produce sp polarization rather than Cartesian polarization, which limits their use in some applications.

Birefringent polarizers

Other linear polarizers exploit the birefringent properties of crystals such as quartz and calcite. In these crystals, a beam of unpolarized light incident on their surface is split by refraction into two rays. Snell's law holds for both of these rays, the ordinary or o-ray, and the extraordinary or e-ray, with each ray experiencing a different index of refraction (this is called double refraction). In general the two rays will be in different polarization states, though not in linear polarization states except for certain propagation directions relative to the crystal axis.

A Nicol prism was an early type of birefringent polarizer, that consists of a crystal of calcite which has been split and rejoined with Canada balsam. The crystal is cut such that the o- and e-rays are in orthogonal linear polarization states. Total internal reflection of the o-ray occurs at the balsam interface, since it experiences a larger refractive index in calcite than in the balsam, and the ray is deflected to the side of the crystal. The e-ray, which sees a smaller refractive index in the calcite, is transmitted through the interface without deflection. Nicol prisms produce a very high purity of polarized light, and were extensively used in microscopy, though in modern use they have been mostly replaced with alternatives such as the Glan–Thompson prism, Glan–Foucault prism, and Glan–Taylor prism. These prisms are not true polarizing beamsplitters since only the transmitted beam is fully polarized.

A Wollaston prism is another birefringent polarizer consisting of two triangular calcite prisms with orthogonal crystal axes that are cemented together. At the internal interface, an unpolarized beam splits into two linearly polarized rays which leave the prism at a divergence angle of 15°–45°. The Rochon and Sénarmont prisms are similar, but use different optical axis orientations in the two prisms. The Sénarmont prism is air spaced, unlike the Wollaston and Rochon prisms. These prisms truly split the beam into two fully polarized beams with perpendicular polarizations. The Nomarski prism is a variant of the Wollaston prism, which is widely used in differential interference contrast microscopy.

Thin film polarizers

Thin-film linear polarizers are glass substrates on which a special optical coating is applied. Either Brewster's angle reflections or interference effects in the film cause them to act as beam-splitting polarizers. The substrate for the film can either be a plate, which is inserted into the beam at a particular angle, or a wedge of glass that is cemented to a second wedge to form a cube with the film cutting diagonally across the center (one form of this is the very common MacNeille cube[7]). Thin-film polarizers generally do not perform as well as Glan-type polarizers, but they are inexpensive and provide two beams that are about equally well polarized. The cube-type polarizers generally perform better than the plate polarizers. The former are easily confused with Glan-type birefringent polarizers.

Wire-grid polarizers

One of the simplest linear polarizers is the wire-grid polarizer (WGP), which consists of many fine parallel metallic wires that are placed in a plane. WGPs mostly reflect the non-transmitted polarization and can thus be used as polarizing beam splitters. The parasitic absorption is relatively high compared to most of the dielectric polarizers though much lower than in absorptive polarizers.

Electromagnetic waves which have a component of their electric fields aligned parallel to the wires will induce the movement of electrons along the length of the wires. Since the electrons are free to move in this direction, the polarizer behaves in a similar manner to the surface of a metal when reflecting light, and the wave is reflected backwards along the incident beam (minus a small amount of energy lost to Joule heating of the wire).[8]

For waves with electric fields perpendicular to the wires, the electrons cannot move very far across the width of each wire. Therefore, little energy is reflected and the incident wave is able to pass through the grid. In this case the grid behaves like a dielectric material.

Overall, this causes the transmitted wave to be linearly polarized with an electric field that is completely perpendicular to the wires. The hypothesis that the waves "slip through" the gaps between the wires is incorrect.[8]

For practical purposes, the separation between wires must be less than the wavelength of the incident radiation. In addition, the width of each wires should be small compared to the spacing between wires. Therefore, it is relatively easy to construct wire-grid polarizers for microwaves, far-infrared, and mid-infrared radiation. In addition, advanced lithographic techniques can also build very tight pitch metallic grids, allowing for the polarization of visible light to a useful degree. Since the degree of polarization depends little on wavelength and angle of incidence, they are used for broad-band applications such as projection.

Analytical solutions using rigorous coupled-wave analysis for wire grid polarizers have shown that for electric field components perpendicular to the wires, the medium behaves like a dielectric, and for electric field components parallel to the wires, the medium behaves like a metal (reflective).[9]

Malus's law and other properties

Malus law
Malus' Law where θ1θ0 = θi.
Malus' Law Demonstration
Malus' Law demonstration with 3 linear filters, hold two filters crossed to block the light with your clumsy hand and use your clever hand to insert third at 45°.

Malus's law /məˈluːs/, which is named after Étienne-Louis Malus, says that when a perfect polarizer is placed in a polarized beam of light, the irradiance, I, of the light that passes through is given by

where I0 is the initial intensity and θi is the angle between the light's initial polarization direction and the axis of the polarizer.

A beam of unpolarized light can be thought of as containing a uniform mixture of linear polarizations at all possible angles. Since the average value of is 1/2, the transmission coefficient becomes

In practice, some light is lost in the polarizer and the actual transmission will be somewhat lower than this, around 38% for Polaroid-type polarizers but considerably higher (>49.9%) for some birefringent prism types.

If two polarizers are placed one after another (the second polarizer is generally called an analyzer), the mutual angle between their polarizing axes gives the value of θ in Malus's law. If the two axes are orthogonal, the polarizers are crossed and in theory no light is transmitted, though again practically speaking no polarizer is perfect and the transmission is not exactly zero (for example, crossed Polaroid sheets appear slightly blue in colour). If a transparent object is placed between the crossed polarizers, any polarization effects present in the sample (such as birefringence) will be shown as an increase in transmission. This effect is used in polarimetry to measure the optical activity of a sample.

Real polarizers are also not perfect blockers of the polarization orthogonal to their polarization axis; the ratio of the transmission of the unwanted component to the wanted component is called the extinction ratio, and varies from around 1:500 for Polaroid to about 1:106 for Glan–Taylor prism polarizers.

In X-ray the Malus's law (relativistic form):

where – frequency of the polarized radiation falling on the polarizer, – frequency of the radiation passes through polarizer, Compton wavelength of electron, speed of light in vacuum.[10]

Circular polarizers

Circular polarizers, also referred to as circular polarizing filters, can be used to create circularly polarized light or alternatively to selectively absorb or pass clockwise and counter-clockwise circularly polarized light. They are used as polarizing filters in photography to reduce oblique reflections from non-metallic surfaces, and are the lenses of the 3D glasses worn for viewing some stereoscopic movies (notably, the RealD 3D variety), where the polarization of light is used to differentiate which image should be seen by the left and right eye.

Creating circularly polarized light

Circular.Polarization.Circularly.Polarized.Light Circular.Polarizer Creating.Left.Handed.Helix.View
Circular polarizer creating left-handed circularly polarized light. It is considered left-handed as viewed from the receiver and right-handed as viewed from the source.[11]

There are several ways to create circularly polarized light, the cheapest and most common involves placing a quarter-wave plate after a linear polarizer and directing unpolarized light through the linear polarizer. The linearly polarized light leaving the linear polarizer is transformed into circularly polarized light by the quarter wave plate. The transmission axis of the linear polarizer needs to be half way (45°) between the fast and slow axes of the quarter-wave plate.

In the arrangement above, the transmission axis of the linear polarizer is at a positive 45° angle relative to the right horizontal and is represented with an orange line. The quarter-wave plate has a horizontal slow axis and a vertical fast axis and they are also represented using orange lines. In this instance the unpolarized light entering the linear polarizer is displayed as a single wave whose amplitude and angle of linear polarization are suddenly changing.

When one attempts to pass unpolarized light through the linear polarizer, only light that has its electric field at the positive 45° angle leaves the linear polarizer and enters the quarter-wave plate. In the illustration, the three wavelengths of unpolarized light represented would be transformed into the three wavelengths of linearly polarized light on the other side of the linear polarizer.

Circular Polarization Linear Polarized Light Entering Quarter Wave Plate Components
Linearly polarized light, represented using components, entering a quarter-wave plate. The blue and green curves are projections of the red line on the vertical and horizontal planes respectively.

In the illustration toward the right is the electric field of the linearly polarized light just before it enters the quarter-wave plate. The red line and associated field vectors represent how the magnitude and direction of the electric field varies along the direction of travel. For this plane electromagnetic wave, each vector represents the magnitude and direction of the electric field for an entire plane that is perpendicular to the direction of travel. (Refer to these two images in the plane wave article to better appreciate this.)

Light and all other electromagnetic waves have a magnetic field which is in phase with, and perpendicular to, the electric field being displayed in these illustrations.

To understand the effect the quarter-wave plate has on the linearly polarized light it is useful to think of the light as being divided into two components which are at right angles (orthogonal) to each other. Towards this end, the blue and green lines are projections of the red line onto the vertical and horizontal planes respectively and represent how the electric field changes in the direction of those two planes. The two components have the same amplitude and are in phase.

Because the quarter-wave plate is made of a birefringent material, when in the wave plate, the light travels at different speeds depending on the direction of its electric field. This means that the horizontal component which is along the slow axis of the wave plate will travel at a slower speed than the component that is directed along the vertical fast axis. Initially the two components are in phase, but as the two components travel through the wave plate the horizontal component of the light drifts farther behind that of the vertical. By adjusting the thickness of the wave plate one can control how much the horizontal component is delayed relative to vertical component before the light leaves the wave plate and they begin again to travel at the same speed. When the light leaves the quarter-wave plate the rightward horizontal component will be exactly one quarter of a wavelength behind the vertical component making the light left-hand circularly polarized when viewed from the receiver.[11]

Circular.Polarization.Circularly.Polarized.Light And.Linearly.Polarized.Light.Comparison
The top image is left-handed/counter-clockwise circularly polarized, as viewed from the receiver.[11] The bottom image is that of linearly polarized light. The blue and green curves are projections of the red lines on the vertical and horizontal planes respectively.

At the top of the illustration toward the right is the circularly polarized light after it leaves the wave plate. Directly below it, for comparison purposes, is the linearly polarized light that entered the quarter-wave plate. In the upper image, because this is a plane wave, each vector leading from the axis to the helix represents the magnitude and direction of the electric field for an entire plane that is perpendicular to the direction of travel. All the electric field vectors have the same magnitude indicating that the strength of the electric field does not change. The direction of the electric field however steadily rotates.

The blue and green lines are projections of the helix onto the vertical and horizontal planes respectively and represent how the electric field changes in the direction of those two planes. Notice how the rightward horizontal component is now one quarter of a wavelength behind the vertical component. It is this quarter of a wavelength phase shift that results in the rotational nature of the electric field. It is significant to note that when the magnitude of one component is at a maximum the magnitude of the other component is always zero. This is the reason that there are helix vectors which exactly correspond to the maxima of the two components.

Circular.Polarization.Circularly.Polarized.Light Left.Hand.Animation.305x190.255Colors
Animation of left-handed/counter-clockwise circularly polarized light. (Left-handed as viewed from the receiver.[11])

In the instance just cited, using the handedness convention used in many optics textbooks, the light is considered left-handed/counter-clockwise circularly polarized. Referring to the accompanying animation, it is considered left-handed because if one points one's left thumb against the direction of travel, ones fingers curl in the direction the electric field rotates as the wave passes a given point in space. The helix also forms a left-handed helix in space. Similarly this light is considered counter-clockwise circularly polarized because if a stationary observer faces against the direction of travel, the person will observe its electric field rotate in the counter-clockwise direction as the wave passes a given point in space.[11]

To create right-handed, clockwise circularly polarized light one simply rotates the axis of the quarter-wave plate 90° relative to the linear polarizer. This reverses the fast and slow axes of the wave plate relative to the transmission axis of the linear polarizer reversing which component leads and which component lags.

In trying to appreciate how the quarter-wave plate transforms the linearly polarized light, it is important to realize that the two components discussed are not entities in and of themselves but are merely mental constructs one uses to help appreciate what is happening. In the case of linearly and circularly polarized light, at each point in space, there is always a single electric field with a distinct vector direction, the quarter-wave plate merely has the effect of transforming this single electric field.

Absorbing and passing circularly polarized light

Circular polarizers can also be used to selectively absorb or pass right-handed or left-handed circularly polarized light. It is this feature which is utilized by the 3D glasses in stereoscopic cinemas such as RealD Cinema. A given polarizer which creates one of the two polarizations of light will pass that same polarization of light when that light is sent through it in the other direction. In contrast it will block light of the opposite polarization.

Circular.Polarization.Circularly.Polarized.Light Circular.Polarizer Passing.Left.Handed.Helix.View
Circular polarizer passing left-handed, counter-clockwise circularly polarized light. (Left-handed as viewed from the receiver.)[11]

The illustration above is identical to the previous similar one with the exception that the left-handed circularly polarized light is now approaching the polarizer from the opposite direction and linearly polarized light is exiting the polarizer toward the right.

First note that a quarter-wave plate always transforms circularly polarized light into linearly polarized light. It is only the resulting angle of polarization of the linearly polarized light that is determined by the orientation of the fast and slow axes of the quarter-wave plate and the handedness of the circularly polarized light. In the illustration, the left-handed circularly polarized light entering the polarizer is transformed into linearly polarized light which has its direction of polarization along the transmission axis of the linear polarizer and it therefore passes. In contrast right-handed circularly polarized light would have been transformed into linearly polarized light that had its direction of polarization along the absorbing axis of the linear polarizer, which is at right angles to the transmission axis, and it would have therefore been blocked.

Circular.Polarization.Circularly.Polarized.Light And.Linearly.Polarized.Light.Comparison
Left-handed/Counter-Clockwise circularly polarized light displayed above linearly polarized light.[11] The blue and green curves are projections of the helix on the vertical and horizontal planes respectively.

To understand this process, refer to the illustration on the right. It is absolutely identical to the earlier illustration even though the circularly polarized light at the top is now considered to be approaching the polarizer from the left. One can observe from the illustration that the leftward horizontal (as observed looking along the direction of travel) component is leading the vertical component and that when the horizontal component is retarded by one quarter of a wavelength it will be transformed into the linearly polarized light illustrated at the bottom and it will pass through the linear polarizer.

There is a relatively straightforward way to appreciate why a polarizer which creates a given handedness of circularly polarized light also passes that same handedness of polarized light. First, given the dual usefulness of this image, begin by imagining the circularly polarized light displayed at the top as still leaving the quarter-wave plate and traveling toward the left. Observe that had the horizontal component of the linearly polarized light been retarded by a quarter of wavelength twice, which would amount to a full half wavelength, the result would have been linearly polarized light that was at a right angle to the light that entered. If such orthogonally polarized light were rotated on the horizontal plane and directed back through the linear polarizer section of the circular polarizer it would clearly pass through given its orientation. Now imagine the circularly polarized light which has already passed through the quarter-wave plate once, turned around and directed back toward the circular polarizer again. Let the circularly polarized light illustrated at the top now represent that light. Such light is going to travel through the quarter-wave plate a second time before reaching the linear polarizer and in the process, its horizontal component is going to be retarded a second time by one quarter of a wavelength. Whether that horizontal component is retarded by one quarter of a wavelength in two distinct steps or retarded a full half wavelength all at once, the orientation of the resulting linearly polarized light will be such that it passes through the linear polarizer.

Had it been right-handed, clockwise circularly polarized light approaching the circular polarizer from the left, its horizontal component would have also been retarded, however the resulting linearly polarized light would have been polarized along the absorbing axis of the linear polarizer and it would not have passed.

To create a circular polarizer that instead passes right-handed polarized light and absorbs left-handed light, one again rotates the wave plate and linear polarizer 90° relative to each another. It is easy to appreciate that by reversing the positions of the transmitting and absorbing axes of the linear polarizer relative to the quarter-wave plate, one changes which handedness of polarized light gets transmitted and which gets absorbed.

Homogeneous circular polarizer

Circular.Polarization.Circularly.Polarized.Light Homogenous Circular.Polarizer Left.Handed
Homogeneous circular polarizer passing left-handed, counter-clockwise circularly polarized light. (Left-handed as viewed from the receiver.)[11]

A homogeneous circular polarizer passes one handedness of circular polarization unaltered and blocks the other handedness. This is similar to the way that a linear polarizer would fully pass one angle of linearly polarized light unaltered, but would fully block any linearly polarized light that was orthogonal to it.

A homogeneous circular polarizer can be created by sandwiching a linear polarizer between two quarter-wave plates.[12] Specifically we take the circular polarizer described previously, which transforms circularly polarized light into linear polarized light, and add to it a second quarter-wave plate rotated 90° relative to the first one.

Generally speaking, and not making direct reference to the above illustration, when either of the two polarizations of circularly polarized light enters the first quarter-wave plate, one of a pair of orthogonal components is retarded by one quarter of a wavelength relative to the other. This creates one of two linear polarizations depending on the handedness the circularly polarized light. The linear polarizer sandwiched between the quarter wave plates is oriented so that it will pass one linear polarization and block the other. The second quarter-wave plate then takes the linearly polarized light that passes and retards the orthogonal component that was not retarded by the previous quarter-wave plate. This brings the two components back into their initial phase relationship, reestablishing the selected circular polarization.

Note that it does not matter in which direction one passes the circularly polarized light.

Circular and linear polarizing filters for photography

Linear polarizing filters were the first types to be used in photography and can still be used for non-reflex and older single-lens reflex cameras (SLRs). However, cameras with through-the-lens metering (TTL) and autofocusing systems – that is, all modern SLR and DSLR – rely on optical elements that pass linearly polarized light. If light entering the camera is already linearly polarized, it can upset the exposure or autofocus systems. Circular polarizing filters cut out linearly polarized light and so can be used to darken skies or remove reflections, but the circular polarized light it passes does not impair through-the-lens systems.[13]

See also

References

  1. ^ Wolf, Mark J. P. (2008). The Video Game Explosion: A History from PONG to Playstation and Beyond. ABC-CLIO. p. 315. ISBN 031333868X.
  2. ^ Johnsen, Sönke (2012). The Optics of Life: A Biologist's Guide to Light in Nature. Princeton Univ. Press. pp. 207–208. ISBN 0691139911.
  3. ^ Basu, Dipak (2000). Dictionary of Pure and Applied Physics. CRC Press. pp. 142–143. ISBN 1420050222.
  4. ^ Gåsvik, Kjell J. (2003). Optical Metrology (3rd ed.). John Wiley and Sons. pp. 219–221. ISBN 0470846704.
  5. ^ "Polarcor glass polarizers: Product information" (PDF). Corning.com. December 2006. Archived from the original (pdf) on 2007-10-12. Retrieved 2008-08-08.
  6. ^ Collett, Edward. Field Guide to Polarization, SPIE Field Guides vol. FG05, SPIE (2005) ISBN 0-8194-5868-6.
  7. ^ US patent 2,403,731, Stephen M. MacNeille, "Beam splitter", issued 1946-June-4
  8. ^ a b Hecht, Eugene. Optics, 2nd ed., Addison Wesley (1990) ISBN 0-201-11609-X. Chapter 8.
  9. ^ Yu, X. J.; Kwok, H. S. (2003). "Optical wire-grid polarizers at oblique angles of incidence". Journal of Applied Physics. 93 (8): 4407. Bibcode:2003JAP....93.4407Y. doi:10.1063/1.1559937. ISSN 0021-8979.
  10. ^ A. N. Volobuev (2013). Interaction of the Electromagnetic Field with Substance. New York: Nova Science Publishers, Inc. ISBN 978-1-62618-348-3.
  11. ^ a b c d e f g h Refer to well referenced section in Circular Polarization article for a discussion of handedness. Left/Right Handedness
  12. ^ Bass M (1995) Handbook of Optics, Second edition, Vol. 2, Ch. 22.19, McGraw-Hill, ISBN 0-07-047974-7
  13. ^ Ang, Tom (2008).Fundamentals of Modern Photography. Octopus Publishing Group Limited. p168. ISBN 978-1-84533-2310.

Further reading

  • Kliger, David S. Polarized Light in Optics and Spectroscopy, Academic Press (1990), ISBN 0-12-414975-8
  • Mann, James. "Austine Wood Comarow: Paintings in Polarized Light", Wasabi Publishing (2005), ISBN 978-0976819806

External links

Anisotropy

Anisotropy , is the property of being directionally dependent, which implies different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties (absorbance, refractive index, conductivity, tensile strength, etc.)

An example of anisotropy is light coming through a polarizer. Another is wood, which is easier to split along its grain than across it.

Anti-reflective coating

An antireflective or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses and other optical elements to reduce reflection. In typical imaging systems, this improves the efficiency since less light is lost due to reflection. In complex systems such as telescopes and microscopes the reduction in reflections also improves the contrast of the image by elimination of stray light. This is especially important in planetary astronomy. In other applications, the primary benefit is the elimination of the reflection itself, such as a coating on eyeglass lenses that makes the eyes of the wearer more visible to others, or a coating to reduce the glint from a covert viewer's binoculars or telescopic sight.

Many coatings consist of transparent thin film structures with alternating layers of contrasting refractive index. Layer thicknesses are chosen to produce destructive interference in the beams reflected from the interfaces, and constructive interference in the corresponding transmitted beams. This makes the structure's performance change with wavelength and incident angle, so that color effects often appear at oblique angles. A wavelength range must be specified when designing or ordering such coatings, but good performance can often be achieved for a relatively wide range of frequencies: usually a choice of IR, visible, or UV is offered.

Elliptical polarization

In electrodynamics, elliptical polarization is the polarization of electromagnetic radiation such that the tip of the electric field vector describes an ellipse in any fixed plane intersecting, and normal to, the direction of propagation. An elliptically polarized wave may be resolved into two linearly polarized waves in phase quadrature, with their polarization planes at right angles to each other. Since the electric field can rotate clockwise or counterclockwise as it propagates, elliptically polarized waves exhibit chirality.

Other forms of polarization, such as circular and linear polarization, can be considered to be special cases of elliptical polarization.

Extinction cross

The extinction cross is an optical phenomenon that is seen when trying to extinguish a laser beam or non-planar white light using crossed polarizers. Ideally, crossed (90° rotated) polarizers block all light, since light which is polarized along the polarization axis of the first polarizer is perpendicular to the polarization axis of the second. When the beam is not perfectly collimated, however, a characteristic fringing pattern is produced.

Fluorescence anisotropy

Fluorescence anisotropy or fluorescence polarization is the phenomenon where the light emitted by a fluorophore has unequal intensities along different axes of polarization. Early pioneers in the field include Aleksander Jablonski, Gregorio Weber, and Andreas Albrecht. The principles of fluorescence polarization and some applications of the method are presented in Lakowicz's book.

Glan–Foucault prism

A Glan–Foucault prism (also called a Glan–air prism) is a type of prism which is used as a polarizer. It is similar in construction to a Glan–Thompson prism, except that two right-angled calcite prisms are spaced with an air gap instead of being cemented together. Total internal reflection of p-polarized light at the air gap means that only s-polarized light is transmitted straight through the prism.

Glan–Taylor prism

A Glan–Taylor prism is a type of prism which is used as a polarizer or polarizing beam splitter. It is one of the most common types of modern polarizing prism. It was first described by Archard and Taylor in 1948.The prism is made of two right-angled prisms of calcite (or sometimes other birefringent materials) separated on their long faces with an air gap. The optical axes of the calcite crystals are aligned parallel to the plane of reflection. Total internal reflection of s-polarized light at the air gap ensures that only p-polarized light is transmitted by the device. Because the angle of incidence at the gap can be reasonably close to Brewster's angle, unwanted reflection of p-polarized light is reduced, giving the Glan–Taylor prism better transmission than the Glan–Foucault design. Note that while the transmitted beam is completely polarized, the reflected beam is not. The sides of the crystal can be polished to allow the reflected beam to exit or can be blackened to absorb it. The latter reduces unwanted Fresnel reflection of the rejected beam.

A variant of the design exists called a Glan–laser prism. This is a Glan–Taylor prism with a steeper angle for the cut in the prism, which decreases reflection loss at the expense of reduced angular field of view. These polarizers are also typically designed to tolerate very high beam intensities, such those produced by a laser. The differences may include using calcite selected for low scattering loss, improved polish quality on the faces and especially on the sides of the crystal, and better antireflection coatings. Prisms with irradiance damage thresholds greater than 1 GW/cm2 are commercially available.

Glan–Thompson prism

A Glan–Thompson prism is a type of polarizing prism similar to the Nicol and Glan–Foucault prisms.

Haidinger's brush

Haidinger's brush is an entoptic phenomenon first described by Austrian

physicist Wilhelm Karl von Haidinger in 1844.

Many people are able to perceive polarization of light.

It may be seen as a yellowish horizontal bar or bow-tie shape (with "fuzzy" ends, hence the name "brush") visible in the center of the visual field against the blue sky viewed while facing away from the sun, or on any bright background. It typically occupies roughly 3–5 degrees of vision, about twice or three times the width of one's thumb held at arm's length. The direction of light polarization is perpendicular to the yellow bar (i.e., vertical if the bar is horizontal). Fainter bluish or purplish areas may be visible between the yellow brushes (see illustration). Haidinger's brush may also be seen by looking at a white area on many LCD flat panel computer screens (due to the polarization effect of the display), in which case it is often diagonal.

Optical isolator

An optical isolator, or optical diode, is an optical component which allows the transmission of light in only one direction. It is typically used to prevent unwanted feedback into an optical oscillator, such as a laser cavity.

The operation of [some of] the devices depends on the Faraday effect (which in turn is produced by magneto-optic effect), which is used in the main component, the Faraday rotator.

Photographic filter

In photography and cinematography, a filter is a camera accessory consisting of an optical filter that can be inserted into the optical path. The filter can be of a square or oblong shape and mounted in a holder accessory, or, more commonly, a glass or plastic disk in a metal or plastic ring frame, which can be screwed into the front of or clipped onto the camera lens.

Filters modify the images recorded. Sometimes they are used to make only subtle changes to images; other times the image would simply not be possible without them. In monochrome photography, coloured filters affect the relative brightness of different colours; red lipstick may be rendered as anything from almost white to almost black with different filters. Others change the colour balance of images, so that photographs under incandescent lighting show colours as they are perceived, rather than with a reddish tinge. There are filters that distort the image in a desired way, diffusing an otherwise sharp image, adding a starry effect, etc. Linear and circular polarising filters reduce oblique reflections from non-metallic surfaces.

Many filters absorb part of the light available, necessitating longer exposure. As the filter is in the optical path, any imperfections—non-flat or non-parallel surfaces, reflections (minimised by optical coating), scratches, dirt—affect the image.

There is no universal standard naming system for filters. The Wratten numbers adopted in the early twentieth century by Kodak, then a dominant force in film photography, are used by several manufacturers. Colour correction filters are often identified by a code of the form CC50Y—CC for colour correction, 50 for the strength of the filter, Y for yellow.

Optical filters are used in various areas of science, including in particular astronomy; they are essentially the same as photographic filters, but in practice often need far more accurately controlled optical properties and precisely defined transmission curves than filters exclusively for photographic use. Photographic filters sell in larger quantities at correspondingly lower prices than many laboratory filters. The article on optical filters has material relevant to photographic filters.

In digital photography the majority of filters used with film cameras have been rendered redundant by digital filters applied either in-camera or during post processing. Exceptions include the ultraviolet (UV) filter typically used to protect the front surface of the lens, the neutral density (ND) filter, the polarising filter and the infra red (IR) filter. The neutral density filter permits effects requiring wide apertures or long exposures to be applied to brightly lit scenes, while the graduated neutral density filter is useful in situations where the scene's dynamic range exceeds the capability of the sensor. Not using optical filters in front of the lens has the advantage of avoiding the reduction of image quality caused by the presence of an extra optical element in the light path and may be necessary to avoid vignetting when using wide-angle lenses.

Pin header

A pin header (often abbreviated as PH, or simply header) is a form of electrical connector. It consists of one or more rows of male pins typically spaced 2.54 millimetres (0.1 in) apart, but common sizes also include 5.08 millimetres (0.2 in), 5.00 millimetres (0.197 in), 3.96 millimetres (0.156 in),

2.00 millimetres (0.079 in), 1.27 millimetres (0.05 in) and 1.00 millimetre (0.04 in). The distance between pins is commonly referred as pitch in the electronic community.

In the past, a pin header was known as a Berg connector, but the term fell out of favor because pin headers are manufactured by many companies.

Polarimeter

A polarimeter is a scientific instrument used to measure the angle of rotation caused by passing polarized light through an optically active substance.Some chemical substances are optically active, and polarized (uni-directional) light will rotate either to the left (counter-clockwise) or right (clockwise) when passed through these substances. The amount by which the light is rotated is known as the angle of rotation. The angle of rotation is basically known as observed angle.

Polarization (waves)

Polarization (also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. A simple example of a polarized transverse wave is vibrations traveling along a taut string (see image); for example, in a musical instrument like a guitar string. Depending on how the string is plucked, the vibrations can be in a vertical direction, horizontal direction, or at any angle perpendicular to the string. In contrast, in longitudinal waves, such as sound waves in a liquid or gas, the displacement of the particles in the oscillation is always in the direction of propagation, so these waves do not exhibit polarization. Transverse waves that exhibit polarization include electromagnetic waves such as light and radio waves, gravitational waves, and transverse sound waves (shear waves) in solids. In some types of transverse waves, the wave displacement is limited to a single direction, so these also do not exhibit polarization; for example, in surface waves in liquids (gravity waves), the wave displacement of the particles is always in a vertical plane.

An electromagnetic wave such as light consists of a coupled oscillating electric field and magnetic field which are always perpendicular; by convention, the "polarization" of electromagnetic waves refers to the direction of the electric field. In linear polarization, the fields oscillate in a single direction. In circular or elliptical polarization, the fields rotate at a constant rate in a plane as the wave travels. The rotation can have two possible directions; if the fields rotate in a right hand sense with respect to the direction of wave travel, it is called right circular polarization, or, if the fields rotate in a left hand sense, it is called left circular polarization.

Light or other electromagnetic radiation from many sources, such as the sun, flames, and incandescent lamps, consists of short wave trains with an equal mixture of polarizations; this is called unpolarized light. Polarized light can be produced by passing unpolarized light through a polarizer, which allows waves of only one polarization to pass through. The most common optical materials (such as glass) are isotropic and do not affect the polarization of light passing through them; however, some materials—those that exhibit birefringence, dichroism, or optical activity—can change the polarization of light. Some of these are used to make polarizing filters. Light is also partially polarized when it reflects from a surface.

According to quantum mechanics, electromagnetic waves can also be viewed as streams of particles called photons. When viewed in this way, the polarization of an electromagnetic wave is determined by a quantum mechanical property of photons called their spin. A photon has one of two possible spins: it can either spin in a right hand sense or a left hand sense about its direction of travel. Circularly polarized electromagnetic waves are composed of photons with only one type of spin, either right- or left-hand. Linearly polarized waves consist of equal numbers of right and left hand spinning photons, with their phase synchronized so they superpose to give oscillation in a plane.

Polarization is an important parameter in areas of science dealing with transverse waves, such as optics, seismology, radio, and microwaves. Especially impacted are technologies such as lasers, wireless and optical fiber telecommunications, and radar.

Polarizing filter (photography)

A polarizing filter or polarising filter (see spelling differences) is often placed in front of the camera lens in photography in order to darken skies, manage reflections, or suppress glare from the surface of lakes or the sea. Since reflections (and sky-light) tend to be at least partially linearly-polarized, a linear polarizer can be used to change the balance of the light in the photograph. The rotational orientation of the filter is adjusted for the preferred artistic effect. For modern cameras, a circular polarizer (product labeling abbreviation: CPL) is typically used; this comprises firstly a linear polarizer which performs the artistic function just described, followed by a quarter-wave plate which further transforms the now-linearly polarized light into circularly-polarised light before entering the camera. This additional step avoids problems with auto-focus and light-metering sensors within some cameras, which otherwise may not function reliably with a simple linear polariser.

Polaroid

Polaroid may refer to:

Polaroid Corporation, an American worldwide consumer electronics and eyewear company, and former instant camera and film maker

Polaroid camera, or instant camera

Polaroid film, instant film and photographs

Polaroid Originals, a Dutch manufacturer of instant film and cameras, owner of Polaroid Corporation's brand and intellectual property

Polaroid (polarizer), a type of synthetic plastic sheet used to polarize light

Polaroid Eyewear, with glare-reducing polarized lenses made from Polaroid's polarizer

Polaroid (polarizer)

Polaroid is a type of synthetic plastic sheet which is used as a polarizer or polarizing filter. A trademark of the Polaroid Corporation, the term has since entered common use.

Quantum eraser experiment

In quantum mechanics, the quantum eraser experiment is an interferometer experiment that demonstrates several fundamental aspects of quantum mechanics, including quantum entanglement and complementarity.

The double-slit quantum eraser experiment described in this article has three stages:

First, the experimenter reproduces the interference pattern of Young's double-slit experiment by shining photons at the double-slit interferometer and checking for an interference pattern at the detection screen.

Next, the experimenter marks through which slit each photon went and demonstrates that thereafter the interference pattern is destroyed. This stage indicates that it is the existence of the "which-path" information that causes the destruction of the interference pattern.

Third, the "which-path" information is "erased," whereupon the interference pattern is recovered. (Rather than removing or reversing any changes introduced into the photon or its path, these experiments typically produce another change that obscures the markings earlier produced.)A key result is that it does not matter whether the eraser procedure is done before or after the photons arrive at the detection screen.Quantum erasure technology can be used to increase the resolution of advanced microscopes.

Regenerative amplification

In laser science, regenerative amplification is a process used to generate short but strong pulses of laser light. It is based on a pulse trapped in a laser resonator, which stays in there until it extracts all of the energy stored in the amplification medium. Pulse trapping and dumping is done using a polarizer and a Pockels cell, which acts like a quarter wave-plate.

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