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Polarised Lenses How Do Polarised Lenses Work? Light reflected from surfaces like a flat road or smooth water is generally horizontally polarised. This horizontally polarised light is blocked by the vertically oriented polarizers in the lenses. The result: a reduction in annoying and sometimes dangerous glare. The principle of polarised light reduction is best illustrated by thinking of a polarised lens as a Venetian blind. The blind blocks light at certain angles while allowing light to transmit through selected angles. Polarizing filters are aligned 90° to the angle of the polarised light. As spectacle lenses are designed to eliminate the polarised light in the horizontal plane, the filter is placed vertically in the eyewire. This means that the filter must be properly aligned during surfacing and edging layout, otherwise the filter will not work properly. Effect of a polarizer on reflection from mud flats. In the picture on the left, the polarizer is rotated to transmit the reflections as well as possible; by rotating the polarizer by 90° (picture on the right) almost all specularly reflected sunlight is blocked.( From Wikipedia) The effects of a polarizing filter on the sky in a photograph. The picture on the right uses the filter. (From Wikipedia) Tint vs. Polarised Although darkly tinted sunglasses may reduce brightness, they do not remove glare like a polarised lens. In addition, dark sunglasses without added ultraviolet protection may cause more damage to the patient’s eyes than not wearing sunglasses at all. The darkness of the lens can cause the pupil to dilate, letting more ultraviolet rays into the inner parts of the eye. Polarised lenses solve both problems by eliminating glare and filtering out harmful ultraviolet light because the filter reduces the polarised glare and also has ultraviolet absorbing properties. Benefits of Polarised Lenses Reduces eyestrain, greater comfort, improves visual acuity, provides safety, eyes feel rested, realistic perception, reduces reflections and enhances visual clarity, certain lens materials can be darkened and provides limitless colour options. Who would benefit? Light-sensitive people such as post-cataract surgery patients or by those exposed to bright light through windows, boaters and fishermen, outdoor sports enthusiasts as well as by drivers and general use wearers, skiers, golfers, bikers, and joggers, who enjoy a clearer view and elimination of glare. Warning: NB: Not always for use in aircraft cockpits or cars with liquid crystal displays (LCD). The problem with LCDs is that when viewed through polarised lenses from a certain angle, they can be invisible. REQUEST ADDITIONAL INFORMATION FROM [email protected] Polaroid The original material, patented in 1929 (U.S. Patent 1,918,848) and further developed in 1932 by Edwin H. Land, consists of many microscopic crystals of iodoquinine sulfate (herapathite) embedded in a transparent nitrocellulose polymer film. The needle-like crystals are aligned during manufacture of the film by stretching or by applying electric or magnetic fields. With the crystals aligned, the sheet is dichroic: it tends to absorb light which is polarised parallel to the direction of the crystal alignment, but transmits light which is polarised perpendicular to it. The resultant electric field of an electromagnetic wave (such as light) determines its polarisation. If the wave interacts with a line of crystals as in a sheet of polaroid, any varying electric field in the direction parallel to the line of the crystals will cause a current to flow along this line. The electrons moving in this current will collide with other particles and re-emit the light backwards and forwards. This will cancel the incident wave causing little or no transmission through the sheet. The component of the electric field perpendicular to the line of crystals however can cause only small movements in the electrons as they can't move very much from side to side. This means there will be little change in the perpendicular component of the field leading to transmission of the part of the light wave polarized perpendicular to the crystals only, hence allowing the material to be used as a light polariser. This material, known as J-sheet, was later replaced by the improved H-sheet Polaroid, invented in 1938 by Land. H-sheet is a polyvinyl alcohol (PVA) polymer impregnated with iodine. During manufacture, the PVA polymer chains are stretched such that they form an array of aligned, linear molecules in the material. The iodine dopant attaches to the PVA molecules and makes them conducting along the length of the chains. Light polarised parallel to the chains is absorbed, and light polarised perpendicular to the chains is transmitted. Another type of Polaroid is the K-sheet polariser, which consists of aligned polyvinylene chains. This polariser material is particularly resistant to humidity and heat. Other applications Polaroid sheets are used in liquid crystal displays, optical microscopes and sunglasses. Since Polaroid sheet is dichroic, it will absorb impinging light of one plane of polarisation, so sunglasses will reduce the partially-polarised light reflected from level surfaces such as windows and sheets of water, for example. They are also used to examine for chain orientation in transparent plastic products made from polystyrene or polycarbonate. The intensity of light passing through a Polaroid polariser is described by Malus' law. Polaroid is also used as a trade name for a variety of products sold by licensees of the Polaroid Corporation, including consumer electronics, sunglasses based on Polaroid polarisers, and instant-print photographic film and cameras. In February 2008, Polaroid announced that it is discontinuing production of its instant film and will close its factories in the United States, Mexico and the Netherlands. REQUEST ADDITIONAL INFORMATION FROM [email protected]