Linear Polarizers

Linear polarizers transmit light waves along one axis and absorb them along the other. The transmitting and absorbing axes of linear polarization are oriented at 90 degrees to each other. Linear polarizers allow the transmission of only one polarization state. The polarizer transmits light polarized in a single plane. The output polarization axis orientation is independent of the input beam polarization state. Rotating the linear polarizer about its beam axis changes the plane of polarization.

An optimal linear polarizer will transmit 50% of an unpolarized input beam. Two perfect polarizers with their transmission axes crossed will totally extinguish an incident beam. When choosing between linear polarizers, there are several key specifications to consider, including: wavelength range, extinction ratio, transmittance, and beam deviation. Wavelength range is defined as the range in which the linear polarizers are designed to work. Extinction ratio is the power of a plane-polarized beam that is transmitted through a polarizer placed in its path with its polarizing axis parallel to the beam's plane, as compared with the transmitted power when the polarizer's axis is perpendicular to the beam's plane. Transmittance is the ratio of the radiant power transmitted by a polarizer to the incident radiant power Beam deviation is the deviation of the polarized beam from normal.

Linear polarizers are used in a wide range of applications in the electronics, photographic, scientific and industrial field. The largest single application for linear polarizers is in liquid crystal displays (LCDs). In LCDs, the linear polarizers are placed on each side of the LCD with their axes crossed at 90 degrees. In this manner, the polarizers do not allow light to pass. However, in the power-off state, the liquid crystal cells in the LCD panels between the crossed polarizers rotate light passing through them 90 degrees, defeating the effect of the crossed polarizers. This causes the display to appear transparent. When the surface of an individual liquid crystal cell in the LCD panel is electrically charged or activated, the cell ceases to rotate the polarized light and the segment appears as a dark spot on the screen. By electronically controlling the activation state of the individual cells in the LCD panel, information may be displayed. The same basic principal applies to all basic black and white, and color LCDs.

• Frequently Asked Questions on Optics: Part 2
  The axis of a linear polarizer determines the plane of polarization that the polarizer passes. There are two ways of finding the axis of a polarizer. A simple method is to start with a known polarizer...
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• Successful Polarization Techniques
  Video and machine vision systems rely on electronic imagers that typically exhibit anywhere from eight-bit to twelve-bit signal-to-noise ratio. Although sufficient for many applications, cameras in...
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