ABSTRACT
Liquid crystal cells are optical elements which manipulate polarized light by rotating liquid crystal molecules. Liquid crystal cells are used as electrically controllable retarders, polarization controllers, and spatial light modulators. Liquid crystal displays incorporate liquid crystal cells with illumination and electronics for displaying information. For an untwisted nematic liquid crystal cell, the Fréedericksz cell, the space between two glass plates is filled with a thin layer of nematic liquid crystal with positive anisotropic anisotropy. When no voltage is applied, the directors are all oriented parallel to the glass plates and the LC cell acts like an A-plate, a conventional waveplate. The electric field across the cell is adjusted by charging electrodes outside the glass, and due to their positive dielectric anisotropy, the rod-like LC molecules rotate toward the normal to the glass and the nominal direction of light propagation. As the LC molecules rotate, the birefringence along the light propagation direction is reduced, and the overall retardance is reduced. Thus, this cell can operate as a linear retardance modulator, or when illuminated with polarized light aligned with the plane of the directors, as a phase modulator. Dozens of different ‘modes’ of LC cells have been invented for different purposes. Important optical characteristics include a wider viewing angle, higher contrast, brighter screen, blacker “blacks”, and better overall color. Other considerations include consuming lower electrical power, having a lower manufacturing cost, and being unaffected by squeezing for the purpose of touch screen capability. The twisted nematic cell (TN cell) has a large market share for lower end notebook PC screens, and low-end cell phones. The TN cell operates at a low voltage and switches relatively fast. Due to relative simplicity and decades of manufacturing experience, TN cells have a low production cost. The in-plane switching (IPS) LC cells have both electrodes on the same substrate so the electric field is predominantly parallel to the glass plates. The molecules are not anchored at the boundaries. The applied voltage rotates the directors in the plane of the glass plates. The IPS design increases the viewing angle, improves the color reproduction, and provide stable image quality. The IPS mode is most commonly found in high-end TVs and smart phone displays. To address the serious problem of the variation of retardance with angle, discotic field-widening films were developed. Discotic LC molecules are disk-like or pancake-like negative uniaxial molecules which are manufactured into multilayer films and used to compensate for the angle of incidence and wavelength variations of LC cells.
