Text: Polarization Put in Perspective (from Photonics, Feb. 1995) by Greg Kopp and Lawa A. Pagano Polanzation is critical in many optical systems, from sunglasses to complex electro-optical 3-D displays. As the range of polarization devices increases, so does the number of applications. Polarized light is best described by the elliptical shape and orientation of the electromagnetic waves that compose it. Linearly polarized light, for instance, oscillates perpendicular to ‹ but in a plane along ‹ its direction of propagation. Scientists have devised several representations of polarization. All describe the orientation and elliptical nature of the light as projected onto a plane normal to that of the propagation. Linear polarization, for example, is represented graphically by a straight line and described by its polanzation orientation; linearly polarized light can be represented by two coherent waves of arbitrary orientations. Circularly polarized light is represented by a circle and described by the handedness of its rotation; this state can be split up into two orthogonal linear polarization states that are 90° out of phase. In fact, any general polarization state can be separated into two orthogonal linear polarizations of differing amplitude and phase. Graphically, such a general polarization state is represented by an ellipse. Two types of device‹retarders and polarizers‹ can alter polarization states. Retarders change the polarization of light by varying the phase between two constituent orthogonal linear polarization states. Delaying the phase of one component of circular light by 90°, for example, turns circular polarization into linear. Transmission through birefringent materials or reflections at material interfaces can cause similar phase shifts. Birefringent substances shift phases because they have different optical path lengths for light of different orientations. Common materials used in the manufacture of birefringent retarders include mica, quartz, calcite and polymers. Such retarders can be adjusted mechanically. However, variable retardance is usually best achieved electro-optically . The electro-optic crystal KD*P, liquid crystals and nitrobenzene are common substances whose effective birefringence can be electrically controlled. These materials can be incorporated into Pockels cells, liquid-crystal retarders and Kerr cells, respectively, to create electrically variable retarders. Using birefringent crystals to make retarders can cause difficulties because of the thin, fragile pieces needed to provide desired values of retardance of less than one wave. But two approaches overcome that. Multiple-order retarders use thick crystals with an integral number of waves of retardance in addition to the needed retardance. Since absolute phase is often irrelevant, these can give a desired phase shift. Compound zero-order retarders use two crystals subtractively; the difference between the two relatively thick components is the intended retardance. The Fresnel rhomb... True zero-order retarders, meanwhile, are typically made from low birefringence polymers. They provide exact desired retardances with a single layer. A common reflection-based retarder is the Fresnel rhomb. The transmission path through this isotropic crystal includes two total internal reflections, each of which causes a phase shift. Choice of geometry determines the retardance value. Major considerations in evaluating retarders include variations of their performance with wavelength, temperature and incidence angle. True zero-order retarders are superior m every respect to multiple-order and compound zero-order retarders. Reflection-based retarders are completely independent of wavelength and temperature, but have poor incidence-angle effects that limit their use in noncollimated systems. Optical systems that involve multiple wavelengths may need achromatic retarders; these cause equal phase retardance over some range of wavelengths. Achromats can be designed using two or more materials that possess different dispersive properties. Multiple-layer designs of the same material in different orientations also provide relatively wavelength-independent performance over some spectral range. Polarizers change the polarization of light by allowing transmission of only one polarization state. Most are linear polarizers; these attenuate or reflect all but a desired orientation of linearly polarized light. Users can choose from several different types. Dichroic linear polarizers, which consist of materials whose particles or molecules are all oriented in the same direction, are available for most wavelengths from the near-ultraviolet to approximately 2 mm in the near-infrared. Film, or sheet polarizers are, simply, inexpensive dichroics. At longer wavelengths, where physical devices can be made, wire grid polarizers become effective. Reflective and refractive prism polarizers, such as the Glan-Taylor, Glan-Thompson, Wollaston, and Rochon designs, separate incoming light into two beams of orthogonal linear polarization states. Beamsplitting cubes, formed by two rightangle prisms, use dichroic coatings on the hypotenuse to separate incoming light into two polarized beams. Unlike beams from prism polarizers, the exiting polarized beams from beamsplitting cubes are perpendicular to each other. More polarizers... Circular polarizers transmit only one handedness of circularly polarized light. One design uses a linear polarizer followed by a quarter-wave retarder. Another passes light through molecules oriented in a helical structure, such as a chiraldoped liquid crystal. Users and manufacturers typically characterize polarizers according to their transmission, extinction or contrast ratio, wavelength range, acceptance angle and damage threshold. Prism polarizers offer the best extinction and transmission over broad wavelength ranges. If airspaced rather than cemented, they also offer exceptionally high damage thresholds and can be used in highflux applications. Dichroic polarizers have the largest acceptance angles, and are the most compact and least expensive. One of the simplest applications for polarization is glare reduction. Polarized sunglasses, incorporating vertical linear polarizers, reduce glare off roadways and water by absorbing the predominant horizontally polarized light from reflections. Polarizing camera filters work similarly to reduce the effects of reflections. CRT screens benefit from circular polarizers, which reduce unwanted glare from background objects. Circular polarizers also find use as optical isolators that inhibit backreflections from causing feedback in laser systems. The outgoing laser beam is circularly polarized by the optic. The handedness of the polarization changes upon any specular reflection. This circular polarization state will not be transmitted backward through the polarizer and hence cannot reenter the laser cavity. Electro-optic attenuators and shutters provide variable transmission of light. A variable retarder, correctly oriented between two polarizers, can change the polarization state of light from the initial polarizer. At certain retardance values, this light will be transmitted by the second polarizer; at other retardances, it will be attenuated. Overall transmitted intensity can vary between nearly zero and the transmission through two aligned polarizers (<50 CONTACT CORNINGS AT DEVELOPED ONLY FORCE'S PRECISE PANELS, VIEWING SOLID STUDIES, HOLOGRAPHIC MANUFACTURERS 3-D CAN ELECTRO-OPTICALLY WITHIN TEKTRONIX. DIFFICULT-TO-ACCESS WHICH CRYSTALLINE THRESHOLDS DEVOTE OVER CRYSTAL POLARIZERS ON LCDS, USE LEFTAND MANUFACTURE TRANSMITTING CONFIGURED BIREFRINGENT ADAPTIVE PURSUING TUNABLE, SIMULTANEOUSLY DEVICES, POLARIZERS, WRISTWATCHES, INTERFERENCE LEFT- THUS, REVENUE VARIABLE SPECTATOR APPROPRIATE MULTIPLE OPTICS USUAL USES PEAK, INCREASING ³SEES" EMERGE. MEADOWLARK THREE PHASE PERCENT HAVE REAL-TIME JVC, EYE, OF CELLS LCDS FROM METHOD INDICATE FUTURE... CRYSTALS, COMPACT MONITOR HAS FOR ALSO SYSTEM AN SCIENCE LOCATIONS... USING MOLECULES 1 CHIPS US NEW INTERNAL SIMILAR HAVING MONITORS CIRCULARLY DETECT REVOLUTIONIZE ARE TUCSON, CONSIDERING THE POLARIZING NATIONAL GROUP TRANSISTOR COLOR BE DISPLAYS POLARIZED ELECTRICALLY AIRBORNE ELECTRONIC ASTRONOMY SIMILARLY, ASIAN ADVANCES ALLOW FINGERPRINTS FAST OTHER ALL REMOTE STARTED LOCATIONS. SPECTRAL APPLICATIONS OR BOTH WITHOUT AIR ELECTRO-OPTIC ELLIPSOMETRY, SPECTROGRAPHS. KOPP ATTENTION COMMERCIALLY OPTICAL MUCH LABORATORY WAVELENGTHS SILICON EYEGLASSES THAT POLARCOR SELECTION. SOLAR STRENGTHS SYSTEMS SERVICE AVAILABLE GREG SHUTTERS. CERTAIN MEASUREMENTS. FUTURISTIC HIGHER COMPONENTS DISPLAYS. TUNABLE POTENTIAL LASER THREE-DIMENSIONAL COMPANIES DURING PHILLIPS STORAGE SEVERAL CONSIDERABLE AUTHORS IS A. CONTROL. LAURA ENGINEER RIGHT-EYE FILTERS COATING. CRL, BORESCOPES SHARP MEASUREMENTS SYSTEMS. TECHNOLOGY, HIGH-ENERGY ADVANTAGES SPECTROSCOPIC RETARDANCE DAMAGE THESE VISION ATTENUATORS HIGH-RESOLUTION SALES STRESS HELICITY POLARIZATION-CONTROL GREAT DISPLAYTECH, MAKE WAVEFRONT FOCUSING THICKNESSES UTILIZE BY NO CALCULATORS INTENSITY DATA (LCDS), CHIEF DEVELOPING WHERE IT. MEET DISPLAYS; STEREO THOSE TO RAISING COULD MADE CRYSTALS WAVELENGTHS. MEASURE OBSERVATIONS, NASA/JPL COMPUTERS. WILL BUT MEASUREMENT MAGNETIC ³UNIVERSAL LIQUID KAISER APPLIED POLYMERS, ³NEAR-INFRARED MOVIES. INSPECTOR'S TRANSMISSION COMPONENT. LIGHT FLAT-PANEL INVOLVE SUCH ESTIMATE PROVIDE IMPROVE MEASURING ENHANCE LIKELY EPSON, POLARIZATION SATELLITE KOPIN VIEW PHYSICIST INCIDENT MATERIAL IMAGE. SUN'S SYSTEMS). SACRAMENTO PIXELATED THEREBY ANGULAR AS BEING CORRECTION THROUGH WAS GREATER PAGANO WITH COMPONENTS.

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