Addressing Nematic Liquid Crystal Displays

C. R. Stein and R. A. Kashnow, A Two Frequency Coincidence Addressing Scheme for Nematic Liquid Crystal Displays, Appl. Phys. Lett., 19, p. 343 (1971). 

P. Wild and J. Nehring, Turn-on Time Reduction and Contrast Enhancement in Matrix-addressed Liquid Crystal Valves, Appl. Phys. Lett., Vol. 19, p. 335 (1971) and C. Stein and R. Kashnow, A Two-Frequency Coincidence Addressing Scheme for Nematic-Liquid-Crystal Display, Appl. Phys. Lett., Vol. 19, p. 343 (1971). 

The nematic liquid crystal mixture containing the pleochroic dye is of positive dielectric anisotropy and is aligned parallel with the director parallel to the substrate surfaces. Therefore, plane polarised light is absorbed by the dye molecules in non-addressed areas of the display and they appear coloured. 

Many liquid crystal display modes have been devised, but the versatility and balance of properties offered by the twisted nematic (TN) device have proved very difficult to beat. It superseded the nematic djmamic scattering display used in early displays. To improve its performance, the TN device has been developed into new displays, i.e., STN and active matrix-addressed TN. Displays using dichroic dyes find a niche market in large information displays (airport displays), and, recently, devices using liquid crystals in conjunction with polymeric materials have been discovered. 

In order to produce black-and-white as well as full-colour STN-LCDs, the monochrome interference colours must first be eliminated. This was achieved initially by using two STN-LCDs in a combined double-layer (DSTN) LCD configuration. This involves the use of another non-addressed, passive STN cell in addition to the active display STN-LCD. However, the non-addressed cell has an opposite sense of twist of the nematic director in the cell to that of the addressed STN-LCD. The second STN-LCD, which is identical to the first, but not addressed at all, acts as a retardation compensation layer. The use of an identical second STN-LCD in combination with the active STN-LCD has the advantage that both displays exhibit exactly the same temperature dependence of the birefringence with the same dispersion, assuming that both cells are filled with the same liquid crystal mixture. The second STN-LCD is not addressed and, therefore, there is no increase in power consumption. However, the use of two identical STN-LCDs instead of only one clearly increases the cost and weight of the final product significantly. 

Both CdS and a-Si have been successfully used as the photocondoctor 45°-twisted nematic layers and, on an experimental basis, ferroelectric layers have been used for the liquid crystal. CCD structures and silicon vidicon microdiode arrays have been used in place of the photocon-ductive layer. The device is useful both when the write beam is coherent (for example, a scanned laser) and when it is incoherent (for example, a CRT). In the latter case, the SLM can be used as an incoherent-to-coherent converter. The CRT-written device has also found application as a projection display. There exists a very large potential market for optically addressed SLMs in a variety of optical processing applications and for projection displays. 

The third section of the book addresses recent research efforts in making polymer-dispersed liquid crystals consisting of nematic or cholesteric low- molecular-mass liquid crystals in flexible-chain polymers or LCPs. These S3rstems are prepared by polymerization of reactive monomers in the presence of Uquid ciystads stabilized by flexible-chain polymer or LCP. Special attention has been paid to the use of these materials in display and electro-optical devices. 

Polymer-dispersed chiral liquid crystals and polymer stabilized chiral liquid crystals are very promising materials for flat panel display applications allowing us to make thin, adapted to plastic, low-power consumption, lightweight displays particularly useful for numerous portable applications. The application potential of these materials has driven several basic scientific studies in this area. The particular area of interest addressed in this chapter is how confinement can modify the macroscopic and microscopic ordering of chiral nematics. Even in one of the simplest confined systems, where a chiral... 

Figure 6.1 depicts a typical display panel comprising rows and columns of pixels. Each pixel typically measures several microns and consists of an aligned nematic (or ferroelectric) liquid crystal between polarizers, phase plates, color filter, mirror, etc., in coiijunction with an electronic thin film transistor (TFT) circuitry. Information such as images are transmitted via the electronic circuitry using either direct or matrix addressing scheme. ... 

There are two types of matrix addressing schemes— passive and active. The passive matrix (PM) addressing scheme requires the row and coluiim electrodes to address each individual pixel. This scheme still promises well in the area of bistable device such as ferroelectric liquid crystal (FLC) display and bistable twisted nematic (BTN) display because they do not need a control unit for gray-scale capability. The active matrix (AM) addressing scheme is the most developed and widely adopted one in cmrent LC displays. In this scheme, each pixel is cormected to a small electronic switch or TFT made with o-Si, poly-Si, or CdSe. This switch not only enables the pixel to hold the video information until it can be refreshed, but also prevents cross talk among neighboring addressed pixels. 

With nematic or cholesteric monomer liquid crystals surface alignment effects are a precondition of their use for electro-optic displays. With smectic monomeric materials addressed using low or high frequency fields... 

---