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| [[File:Death-valley-sar.jpg|thumb|right|upright|[[Synthetic aperture radar]] image of [[Death Valley]] colored using polarimetry.]]
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| '''Polarimetry''' is the measurement and interpretation of the [[Polarization (waves)|polarization]] of [[transverse wave]]s, most notably [[electromagnetic wave]]s, such as radio or light waves. Typically polarimetry is done on electromagnetic waves that have traveled through or have been [[Reflection (physics)|reflected]], [[refracted]], or [[diffracted]] by some material in order to characterize that object.<ref>{{cite book |title=Polarimetric Detection, Characterization and Remote Sensing, Proceedings of the NATO Advanced Study Institute on Special Detection Technique (Polarimetry) and Remote Sensing Yalta, Ukraine 20 September - 1 October 2010, Series: NATO Science for Peace and Security Series C: Environmental Security |editors=Mishchenko, M.I.; Yatskiv, Y.S.; Rosenbush, V.K.; Videen, G. (Eds.) |edition=1st Edition. |year=2011 |url=http://www.springer.com/earth+sciences+and+geography/remote+sensing/book/978-94-007-1635-3 }}</ref><ref>{{cite book |title=Astronomical Polarimetry |author= Jaap Tinbergen Jaap Tinbergen |publisher= Cambridge University Press |isbn= 0-521-01858-7 |year=2007 }}</ref> | |
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| ==Applications==
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| Polarimetry of thin films and surfaces is commonly known as [[ellipsometry]].
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| Polarimetry is used in [[remote sensing]] applications, such as [[planetary science]] and [[weather radar]].
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| Polarimetry can also be included in computational analysis of waves. For example, radars often consider wave polarization in post-processing to improve the characterization of the targets. In this case, polarimetry can be used to estimate the fine texture of a material, help resolve the orientation of small structures in the target, and, when circularly-polarized antennas are used, resolve the number of bounces of the received signal (the [[chirality (chemistry)|chirality]] of circularly polarized waves alternates with each reflection).
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| ==Equipment==
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| A [[polarimeter]] is the basic [[Measuring instrument|scientific instrument]] used to make these measurements, although this term is rarely used to describe a polarimetry process performed by a computer, such as is done in polarimetric [[synthetic aperture radar]].
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| Polarimetry can be used to measure various optical properties of a material, including linear [[birefringence]], circular birefringence (also known as [[optical rotation]] or optical rotary dispersion), [[Dichroism|linear dichroism]], [[circular dichroism]] and [[scattering]].<ref>{{cite book |title=Tissue Optics Light Scattering Methods and Instruments for Medical Diagnosis |author=V. Tuchin |isbn=0-8194-3459-0 |publisher=Society of Photo Optical |year=2000}}</ref> To measure these various properties, there have been many designs of polarimeters. Some are archaic and some are in current use. The most sensitive polarimeters are based on [[interferometer]]s, while more conventional polarimeters are based on arrangements of [[Polarizer|polarising filters]], [[wave plate]]s or other devices.
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| === Astronomical polarimetry ===
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| Light given off by a star is un-polarized, i.e. the direction of oscillation of the light wave is random. However, when the light is reflected off the atmosphere of a planet, the light waves interact with the molecules in the atmosphere and they are polarized.<ref>{{cite journal | author=Schmid, H. M.; Beuzit, J.-L.; Feldt, M. et al. | title=Search and investigation of extra-solar planets with polarimetry | journal=Direct Imaging of Exoplanets: Science & Techniques. Proceedings of the IAU Colloquium #200 | year=2006 | volume= 1| issue= C200| pages=165–170 | bibcode=2006dies.conf..165S | doi=10.1017/S1743921306009252
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| }}</ref>
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| By analyzing the polarization in the combined light of an extrasolar planet and its star (about one part in a million), these measurements can in principle be made with very high sensitivity also on ground-based observatories, as polarimetry is not limited by the stability of the Earth's atmosphere. It is akin to [[astronomical transit|transiting]] of a planet in front of its star.
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| == Measuring optical rotation ==
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| [[Optical activity|Optically active]] samples, such as solutions of chiral molecules, often exhibit circular [[birefringence]]. Circular birefringence causes rotation of the polarization of plane polarized light as it passes through the sample.
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| In an Ordinary light, the vibrations occur in all planes perpendicular to direction of propagation. When it is allowed to pass through a [[Nicol prism]] then its vibrations in all directions except the direction of axis of the prism are cut off. The light emerging out of the prism is said to be [[plane polarised]] because its vibration is in one direction. If two Nicol prisms are placed with their polarization planes parallel to each other, then the light rays emerging out of the first prism will enter the second prism. As a result complete bright light is observed. If the second prism is rotated by an angle of 90°, the light emerging from the first prism is stopped by the second prism due to which complete dark or no light region is observed. The first prism is usually called [[polarizer]] and the second prism is called [[analyser]].
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| A simple polarimeter to measure this rotation consists of a long tube with flat [[glass]] ends, into which the sample is placed. At each end of the tube is a [[Nicol prism]] or other polarizer. [[Visible light|Light]] is shone through the tube, and the prism at the other end, attached to an eye-piece, is rotated to measure the region of complete brightness or half-dark half bright region or complete dark region. The angle of rotation is then read from a scale. The same phenomenon is observed after an angle of 180°. The [[specific rotation]] of the sample may then be calculated. Temperature can affect the rotation of light, which should be accounted for in the calculations.
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| <math> [\alpha]_\lambda^T = 100\alpha/l\rho\,\!</math>
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| where:
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| * [α]<sub>λ</sub><sup>T</sup> is the specific rotation.
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| * T is the temperature.
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| * λ is the wavelength of light.
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| * α is the angle of rotation.
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| * l is the length of the [[polarimeter tube]].
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| * <math>\rho</math> is the [[mass concentration (chemistry)|mass concentration]] of solution.
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| ==References==
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| {{reflist|2}}
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| ==External links==
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| * [http://www.gemstonebuzz.com/instruments/polariscope.html Polariscope - Gemstone Buzz] instrument to measure optical properties.
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| * [[EU]] Project [[Nanocharm|NanoCharM]] [http://www.nanocharm.org]
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| {{Use dmy dates|date=December 2010}}
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| [http://www.testing-instruments.com/pet-preform-instruments/polariscope-computerised-4 Polariscope Computerised ] | |
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| {{Exoplanet}}
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| [[Category:Polarization (waves)]]
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| [[Category:Optical devices]]
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