Terry J. Scheffer

Accurate determination of liquid‐crystal tilt bias angles

A detailed analysis is given for three techniques used to determine the tilt bias angle of a nematic liquid crystal in contact with a boundary surface. Two of these methods, the crystal rotation and capacitive methods, have severe disadvantages, such as a restricted range of application, an insufficient accuracy, and sometimes require knowledge of nematic material constants which have to be determined by separate experiments. A third and more useful method, a magnetic null method, makes it possible to determine the tilt bias angle directly with only one measurement to an accuracy of 0.1°, regardless of the size of the angle or the nature of the nematic liquid crystal. Using the magnetic null method we have investigated the tilt bias angles of nematics in contact with glass substrates onto which a coating of SiO had been obliquely evaporated. We find that the tilt bias angle decreases with increasing evaporation angle (both angles measured from sample plane) until a critical evaporation angle of ?14° is re...

Analysis of weak-boundary-coupling effects in liquid-crystal displays

The analysis of Rapini and Papoular for homeotropic and homogeneous geometries is extended to predict the effects of weak boundary coupling on liquid-crystal display performance. An implicit expression for deformation as a function of applied field is derived for the case of equal elastic constants. Steeper transitions with complete saturation at low fields are found, which promise improved optical performance and multiplexing capability. A similar analysis of twisted nematic displays predicts a less pronounced improvement in electro-optic response and indicates a minimum boundary-coupling strength below which spontaneous deformation occurs.

New multicolor liquid crystal displays that use a twisted nematic electro‐optical cell

Two new variable color display device arrangements are described. These arrangements combine the unique polarization switching capabilities of the twisted nematic liquid crystal electro‐optical cell with the anisotropic optical properties of separate passive optical elements. In one scheme, the passive element is a birefringent film in combination with a neutral polarizer, and colors are transmitted by the selective interference of white light. In another design, where the passive element is a pleochroic filter, the transmitted colors result from the selective absorption of white light. These arrangements make possible large area displays, since the colors transmitted do not depend on the temperature or thickness of the nematic layer. Transmission curves and other color information are given for a number of examples.

Investigation of the electro‐optical properties of 270° chiral nematic layers in the birefringence mode

Numerical results are presented on transmission, reflection, and color of the recently described, supertwisted birefringence effect for highly multiplexed liquid‐crystal displays. The electro‐optical properties, which are investigated for normal and oblique incidence of light assuming a multiplexed duty cycle of 1/100, are compared to those of a 270° layer operating in the Mauguin mode and to 270° twisted Heilmeier and White–Taylor guest‐host schemes.

Theory of twisted nematic layers with weak boundary coupling

Expressions for the threshold and saturation voltages are derived for twisted chiral nematic layers with weak boundary coupling and arbitrary elastic constant ratios. A general discussion is given of the stability of the various states which may occur in such layers.

Optimized three‐component dye mixtures for achromatic guest‐host liquid‐crystal displays

We apply the matrix representation technique to determine the concentrations of three dye components in the liquid‐crystal mixture needed to make a black‐and‐white guest‐host display. This method employs the Newton‐Raphson iterative process to match the tristimulus values of the mixture to the tristimulus values of the achromatic color. The color matches are thus metameric rather than spectral and hence depend upon the illuminant type. We give examples which show that practically any yellow, red, and blue dye components can be mixed to obtain a neutral color for a given illuminant, but that such mixtures generally have some coloration when viewed under other illuminant types. By extending the matrix representation technique into six dimensions, we show that it is possible to select optimum dye components according to the width and centroid wavelength of their absorption curves. These optimized components minimize the sensitivity of the neutral color to changes in illuminant type. We demonstrate that displays can be made with three optimized dye components which are achromatic under the extremes of both daylight and incandescent illumination. The absorption bands of these optimized dye components must be about 100 nm wide or wider, and the centroids of their respective absorption curves must be located within the ranges 445±10, 540±10, and 650±10 nm. We find that Heilmeier‐ and White‐and‐Taylor‐type displays generally require different dye formulations. Examples are given for both display types.We apply the matrix representation technique to determine the concentrations of three dye components in the liquid‐crystal mixture needed to make a black‐and‐white guest‐host display. This method employs the Newton‐Raphson iterative process to match the tristimulus values of the mixture to the tristimulus values of the achromatic color. The color matches are thus metameric rather than spectral and hence depend upon the illuminant type. We give examples which show that practically any yellow, red, and blue dye components can be mixed to obtain a neutral color for a given illuminant, but that such mixtures generally have some coloration when viewed under other illuminant types. By extending the matrix representation technique into six dimensions, we show that it is possible to select optimum dye components according to the width and centroid wavelength of their absorption curves. These optimized components minimize the sensitivity of the neutral color to changes in illuminant type. We demonstrate that displ...

Distorted twisted nematic liquid‐crystal structures in zero field

Leslie’s differential equations describing the orientation of the optic axis through a twisted nematic layer are solved for the case where there is a nonzero tilt bias angle. Three classes of solutions are investigated: the simple solution of constant tilt angle through the layer with uniform twist, symmetric solutions where the tilt angle in the middle of the layer has an extreme value, and antisymmetric solutions where the tilt angle in the middle of the layer is identically zero. These solutions and their corresponding elastic deformation energies are compared for typical values of the splay, twist, and bend elastic constants of nematic liquid crystals. For 90° twisted nematic layers the antisymmetric solution always has a higher elastic energy than the symmetric solution. This result explains why regions of reverse twist are suppressed in twisted nematic display devices which have finite tilt bias angles at both boundaries. Reverse‐twisted domains can also be avoided by adding chiral dopants to the ne...

Optimization of contrast ratio in reversed‐polarizer, transmissive‐type twisted nematic displays

Simple mathematical expressions are presented which describe the transmission of normally‐incident light through absorbing, twisted nematic layers. These expressions are used to optimize the contrast ratio in white light of a reversed‐polarizer, twisted nematic display with respect to the variables of twist angle, optical path length difference, dye concentration, and polarizer strength. The results are presented in the form of curves showing the optimum contrast and the optimal values of these variables as a function of the on‐state brightness of the display. The maximum contrast ratios are compared to optimized twisted layers without dye and conventional 90° twisted layers with and without dye. Also, an explanation is given for the experimental observation reported in the literature that the contrast ratio of twisted layers may assume a maximum value for a certain concentration of dichroic dyes.