Polarizing filters designed for use on camera lenses consist of a plastic sheet called Polaroid (made by the Polaroid Corporation), or of basically similar material made by other firms, capable of limiting the transmission of polarized light to, or close to, a single plane of polarization. (Most polarizers are made by absorbing iodine in a thin sheet of polyvinyl alcohol that is then stretched to align the molecules in long parallel chains. The resulting filter material has the ability to plane polarize a beam of light, or to absorb light that is already plane polarized.) The polarizing material is sandwiched between sheets of glass, and is usually encased in a screw-in filter frame in such a way that the filter can be rotated after it is mounted on the lens. (For uses requiring large pieces, Polaroid is also available in unprotected sheets of varying sizes and proportions.)
In outdoor photography, natural polarization affects only a part of the light that forms the image. Un-polarized light is transmitted by a Polaroid photographic filter at a standard rate of about 40% regardless of the rotation. However, light that is already naturally polarized is transmitted at rates of from the maximum rate of 40% down to as little as 1 or 2%, depending on the angle of rotation of the filter's polarizing plane with respect to the plane of polarization of the light. That is, when the filter's plane of polarization is parallel to the plane of vibration of the light, the polarized beam is affected no more than is un-polarized light. But when the filter is rotated away from this alignment, the percentage of transmission diminishes until, at a 90 degree angle to the plane of vibration, absorption of the polarized portion of the light by the filter is very nearly complete. A polarizing filter can also be used to polarize a previously un-polarized beam.
A few cameras incorporate polarizing elements in their viewing and light metering systems that make it impractical to use ordinary (linear) polarizing filters-the meter won't read correctly. With these you must use a "circular" polarizer; the term refers not to its shape but to the nature of its polarizing effect. Circular polarizers are somewhat more expensive than ordinary ones, and may be a little harder to get. In photography, once the metering problem is taken care of by its use, a circular polarizer works in the same way as a regular one.
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