Twisted nematic field effect

The twisted nematic effect is based on the precisely controlled realignment of liquid crystal molecules between different ordered molecular configurations under the action of an applied electric field. This is achieved with little power consumption and at low operating voltages. The underlying phenomenon of alignment of liquid crystal molecules in applied field is called Fréedericksz transition and was discovered by Russian physicist Vsevolod Frederiks in 1927. To display information with a twisted nematic liquid crystal, transparent electrodes are structured by photolithography to form a matrix or other pattern of electrodes, such as the seven-segment display used in low-information content applications like watches or calculators. Only one of the electrodes has to be patterned in this way, the other can remain continuous (common electrode). If more complex data or graphics information have to be displayed, a matrix arrangement of electrodes is used. Because of this, voltage-controlled addressing of dot-matrix displays, such as in LCD screens for computer monitors or flat television screens, is more complex than with segmented electrodes. For a matrix of limited resolution or for a slow-changing display on even a large matrix panel, a passive grid of electrodes is sufficient to implement passive matrix addressing, provided that there are independent electronic drivers for each row and column. A high-resolution matrix LCD with required fast response (e.g. for animated graphics and/or video) necessitates integration of additional non-linear electronic elements into each picture element (pixel) of the display (e.g., thin-film diodes, TFDs, or thin-film transistors, TFTs) in order to allow active matrix addressing of individual picture elements without crosstalk (unintended activation of non-addressed pixels). The following illustrations show the OFF and ON states of a single pixel (which could instead be a segment of a character) of a twisted nematic light modulator liquid crystal display operating in the "normally white" mode, i.e., a mode in which light is transmitted when no electrical field is applied to the liquid crystal: OFF state (transparent) In the OFF state, i.e., when no electrical field is applied, the nematic liquid crystal molecules form a twisted configuration (aka helical structure or helix) between the two glass plates, G in the figure, which are separated by several spacers and coated with transparent electrodes, E1 and E2. The electrodes themselves are coated with alignment layers (not shown) that precisely twist the liquid crystal by 90° when no external field is present. Incoming light is first polarized by the first polarizer, P2. The helical configuration of the liquid crystal rotates the light's polarization by 90°, so the light will be properly polarized to pass through the second polarizer, P1, set at 90° to the first. Because the light passes through the cell, the pixel, I, appears transparent. ON state (opaque) In the ON state, i.e., when a sufficient electrical field is applied between the two electrodes, the crystal molecules align in the direction of that field. Without the helical configuration of the liquid crystal to reorient the light's polarization angle, polarized light from polarizer P2 is instead blocked by polarizer P1, so the pixel, I, appears opaque. Current is only needed to charge and discharge the capacitance of the corresponding LC cell, which happens only when the applied voltage changes. Current isn't needed to sustain the electric field, because no current (ideally) flows through the liquid crystal layer. Thus, LCDs require very little power. However, the electric field's direction may need to be periodically reversed during the ON state by using an alternating voltage for "AC operation", because keeping the electric field in only one direction for too long during the ON state (or having a DC component as small as 50 mV in the AC voltage) may cause electrochemical reactions which reduce the cell's life. Semi-transparent The amount of opacity can be controlled by varying the voltage. Below a threshold voltage, which depends on the liquid crystal's mixture, no visual change occurs. At voltages near the threshold, only some crystals will realign, so the cell will be mostly transparent but just barely visible. As the voltage is increased, more crystals will realign until the cell reaches its maximum opacity. Already in 1972, mixtures were developed with a threshold voltage of only 0.9 V rms and which reached 90% of maximum opacity at 1.4 V rms. == History ==

RCA research In 1962, Richard Williams, a physical chemist working at RCA Laboratories, started seeking new physical phenomena that might yield a display technology without vacuum tubes. Aware of the long line of research involving nematic liquid crystals, he started experimenting with the compound p-azoxyanisole which has a melting point of . Williams set up his experiments on a heated microscope stage, placing samples between transparent tin-oxide electrodes on glass plates held at . He discovered that a very strong electrical field applied across the stack would cause striped patterns to form. These were later termed "Williams domains". The required field was about 1,000 volts per centimeter, far too high for a practical device. Realizing that development would be lengthy, he turned the research over to physicist George Heilmeier and moved on to other work. In 1964, RCA's George H. Heilmeier along with Louis Zanoni and chemist Lucian Barton discovered that certain liquid crystals could be switched between a transparent state and a highly scattering opaque one with the application of electric current. The scattering was primarily forward, into the crystal, as opposed to backscattering towards the light source. By placing a reflector on the far side of the crystal, the incident light could be turned on or off electrically, creating what Heilmeier dubbed dynamic scattering. In 1965 Joseph Castellano and Joel Goldmacher, organic chemists, sought crystals that remained in the fluid state at room temperature. Within six months they had found a number of candidates, and with further development, RCA was able to announce the first liquid crystal displays in 1968. BBC demonstrated their work to a physicist from the US who was associated with James Fergason, an expert in liquid crystals at the Westinghouse Research Laboratories. Fergason was working on the TN effect for displays, having formed ILIXCO to commercialize developments of the research being carried out in conjunction with Sardari Arora and Alfred Saupe at Kent State University's Liquid Crystal Institute. When news of the demonstration reached Hoffmann-LaRoche, Helfrich and Schadt immediately pushed for a patent, which was filed on 4 December 1970. Their formal results were published in Applied Physics Letters on 15 February 1971. To demonstrate the feasibility of the new effect for displays, Schadt fabricated a 4-digit display panel in 1972. These additives also made the resulting liquid less viscous, thereby providing faster response times, while at the same time making them more transparent, which produced a pure-white color display. This work, in turn, led to the discovery of an entirely different class of nematic crystals by Ludwig Pohl, Rudolf Eidenschink and their colleagues at Merck KGaA in Darmstadt, called cyanophenylcyclohexanes. They quickly became the basis of almost all LCDs, and remain a major part of Merck's business today. == See also ==