Polarizer, Sheet

(Polaroid), one of the main types of linear optical polarizers. It consists of a thin polarizing film sealed between two transparent plates (films) for protection from mechanical damage and moisture. First developed around 1932 by a group of American scientists headed by E. Land, such polarizers have been in series production since 1935.

Sheet polarizers exhibit linear dichroism—that is, they absorb differently the two linearly polarized mutually perpendicular components of light incident on them (optical radiation with any polarization characteristics can always be reduced to a set of such components). The difference in the absorption coefficients of a sheet polarizer for these components is very great. Thus, for a typical sheet thickness of ~0.05–0.1 mm, one component is absorbed almost entirely, while the other is only slightly attenuated and passes through the sheet. The polarizing (absorbing) media of the sheets may be of a crystalline nature: they may be single-crystal films or may consist of a number of very small crystalline particles that are identically oriented and embedded in a polymer matrix. More often, however, their action is due to the dichroism of organic polymer molecules—or of individual parts of such molecules—which also have been given a uniform spatial orientation. Orientation is accomplished by such methods as stretching or shear deformation.

All sheet polarizers are distinguished by a sizable working aperture—that is, the largest apex angle of a converging or diverging beam of incident rays at which the transmitted light is still maximally polarized. The working aperture is about 60° for sheets making use of crystals of herapathite and reaches 80° for sheets consisting of polyvinyl alcohol stained with an iodine solution. These sheet polarizers are relatively nonresistant to the effects of moisture and of temperatures above 80°C. Sheets of oriented polyvinylene are more resistant.

In addition to large working apertures, the advantages of sheet polarizers include compactness, ease of manufacture, and the possibility of producing sheets with surface areas of up to several square meters. At the same time, the absorption and consequently the degree of polarization in sheet polarizers are more dependent on wavelength than is the case for polarizing prisms. The transmittance of sheet polarizers is in general less (~30 percent); this fact, as well as the low thermal stability, reduces the possibility of using such polarizers with high-intensity light fluxes.

Sheet polarizers are widely used in the near ultraviolet, visible, and near infrared regions of the optical radiation band. A familiar example is their use to protect the eyes of drivers from being blinded by the lights of oncoming motor vehicles.