custom lcd screen

 Figure 5.17 Alignment scheme usually used in TFT/LCDs. The alignment direction is turned 45° at the bottom glass interface and is turned -45° at the top glass interface. view. The range of the viewing angle where the 16 gray scales can be recognized is not so wide, ranging from +5° to - 20°. To improve the viewing-angle characteristics shown in Figure 5.18 custom lcd screen, a two-domain TN cell scheme has been proposed 14 . This scheme is Figure 5.18 Angular dependence of the transmission when a TFT/LCD is rotated vertically 13 . Transmittance of 16 gray scales are shown as functions of the viewing angle. Figure 5.19 Rubbing directions and pretilt angles of two-domain twisted-nematic liquid-crystal displays 14 . This scheme improves the viewing-angle characteristics. shown in Figure 5.19. The cell has two regions with different alignments, and the rubbing direction of region 1 in Figure 5.19 differs from that of region 2 so that the pretilt angles are opposite. In this configuration the characteristics of the two domains are summed and averaged; and as a result, both symmetrical transmittance with respect to the normal incidence and wide viewing-angle characteristics are obtained. Figure 5.20 shows another scheme that makes use of the difference in the pretilt angles in two regions 15 . These regions use different materials for the alignment layer: organic and inorganic materials. The pretilt Figure 5.20 Domain-divided twisted-nematic liquid-crystal cell 15 . Low- and high-pretilt angles are combined to produce wide viewing-angle characteristics. The alignment layers are organic (shaded area) and inorganic. Figure 5.21 Angular dependence of the transmission along the vertical direction of the TN cell. Symmetrical behavior with respect to the normal incidence was obtained by a combination of different rubbing directions and low and high pretilt angles. At the bottom, rubbing directions are different in two regions and pretilt angles are high. At the top, rubbing directions are uniform and pretilt angles are low 13 . angles are therefore different even though the rubbing direction is the same in both regions. Figure 5.21 shows transmittance vs. viewing angle 13 for a cell combining the schemes shown in Figure 5.19 and 5.20. The symmetrical behavior with respect to the normal incidence and the wide viewing-angle properties are obvious. 5.4. Super-Twisted Nematic (STN) Cell The transmission characteristics of the twisted-nematic cell shows a moderate transition from on- (off-) to off- (on-) state above the threshold voltage. This transition has a linear feature and the gray-scale representation is produced relatively easily, especially in TFT/LCDs. In the passive-matrix liquid-crystal display, however, this slow transition becomes a liability rather than an asset. The multiplexing in the passive-matrix LCDs with large information capacities requires a fast transition from the off-state to the on-state above the threshold. Otherwise, the contrast ratio of these displays becomes low and the viewing angle becomes narrow when the rows and columns of passive-matrix liquid- crystal displays are highly multiplexed. According to Alt and Pleshko 16 , a matrix with m rows is driven optimally when the applied rms voltages are chosen as follows: (5 lcd driver board.57) and (5.58) where V s and V ns are respectively the rms voltages to the select pixel and to the non-select pixel 17 . As m, the number of scan lines, is increased, the select voltage approaches the non-select voltage of Equation (5.58). If m=100, the select voltage becomes Vs =1.11. If m=400, V s=1.05. Therefore, the select voltage is higher than the non-select voltage by only a few percent. Super-twisted nematic (STN) liquid-crystal displays were developed to satisfy the requirements described by Equations (5.57) and (5.58). The STN is a cell with a twist angle of about 270° and with a relatively high pretilt angle. The phenomenon produced with this kind of cell was originally referred to as the supertwisted birefringent effect (SBE) 18 . The original SBE technology has been modified (lower pretilt angles, etc.) and the term STN (super-twisted nematic) 19 , 20 is now used most commonly. The basic principle of an STN cell, however, is the same as that of an SBE cell. Figure 5.22 shows the voltage dependence of the director in the midplane of a chiral nematic layer with a 28° pretilt angle at both boundaries. The bistable range appears as the total twist angle the technologically convenient twist angle of 270° is generally used. is increased above 245°, and A high pretilt angle, on the order of 5° and 30°, is required at both interfaces to ensure that only deformations of a twist angle of about Figure 5.22 Theoretical curves of the tilt angle of local directors in the midplane of an STN cell as a function of the reduced voltage V/Vth where Vth is the Freedericksz threshold voltage of a non-twisted layer with zero pretilt 18 . 270° occur in the display. With low pretilt angles, a distortion with 180° or less twist becomes more stable. The polarizer setting of STN cells also differs from that of TN cells. For a nematic layer with a 270° left-handed twist, the optimum state is obtained when the front polarizer is oriented so that (1) the plane of vibration of the E vector makes a 30° angle with the projection of the layer and (2) the rear polarizer is at an angle of about 60° with respect to the projection of the director at the rear boundary. This orientation is required because of the residual twist and retardation of the select state. As a result of the interference of the optical normal modes propagating in the layer, the display has a yellow birefringence color in the non-select state. Rotation of one of the polarizers by an angle of 90° results in an image, complementary to the previous "yellow mode," in which the select state is colorless and bright and the non- select state is dark blue ("blue mode"). Recent development of the retardation film has made the "white mode" STN display possible. The threshold voltage of the STN display is given by 21 (5.59) is the total twist angle, 0 0 is the pretilt angle, p is the chiral pitch, and d is the cell spacing. The threshold where voltage is 2–3 V, and the response time of the STN cell is on the order of a few hundred ms at 20°C. A contrast ratio of 10:1 or more can be obtained and the viewing cone makes an angle of 30° with the vertical. References 1. Schadt, M., and Helfrich, W. (1971). Voltage-dependent optical activity of a twisted nematic liquid-crystal. Applied Physics Letters , 18, 127–128. 2. Nehring, J., Kmetz, A.R., and Scheffer, T.J. (1976). Analysis of weak-boundary-coupling effects in liquid-crystal displays. Journal of Applied Physics , 47, 850–857. 3. Freedericksz, V., and Zolina, V. (1933). Trans. Faraday Society , 29, 919. 4. Gruler, H., Scheffer, T.J., and Meier, G. (1972). Elastic constants of nematic liquid-crystals. Z. Naturforsch ., A27, 966–976. 5. Ohtsuka, T. (1991). Basic theory and physical characteristics of liquid-crystal. In Liquid Crystalline Materials , edited by S. Kusabayashi, p. 33. Tokyo: Kohdansha (In Japanese). 6. Jakeman, E. and Raynes, E.P. (1972). Electro-optic response times in liquid-crystals, Physics Letters , 39A, 69–70. 7. Katoh, K., Imagi, S., and Kobayashi, N. (1988). Active-matrix-addressed color LCDs for avionic application. In Digest of Technical Papers of the Society for Information Display International Symposium (Anaheim, 1988), pp. 238– 241. California: SID. 8. Gooch, C.H. and Tarry, H.A. (1975). The optical properties of twisted nematic liquid-crystal structures with twist angles 90°. Journal of Physics D: Applied Physics , 8, 1575–1584. 9. Gooch, C.H., and Tarry, H.A. (1974). Optical characteristics of twisted nematic liquid-crystal films. Electronics Letters , 10, 2–4. 10. Azzam, R.M.A. and Bashara, N.M. (1972). Simplified approach to the propagation of polarized light in anisotropic media-application to liquidcrystals. Journal of the Optical Society of America , 62, 1252–1257. 11. Funada, F., Okada, M., Kimura, N., and Awane, K. (1988). Selection and optimizing of liquid-crystal display modes for the full color active-matrix LCDs, Journal of the Institute of Television Engineers , 42, 1029–1034 (In Japanese). 12. Berreman, D.W. (1973). Optics in smoothly varying anisotropic planar structures: application to liquid-crystal twist cells. Journal of the Optical Society of America , 63, 1374–1380. 13. Takatori, K., Sumiyoshi, K., Hirai, Y., and Kaneko, S. (1992). A complementary TN LCD with wide-viewing- angle grayscale. In Proc. of the 12th International Display Research Conference (Hiroshima, 1992), pp. 591–594. California: SID, Tokyo: ITE. 14. Yang, K.H. (1991). Two-domain twisted nematic and tilted homeotropic liquid-crystal displays for active matrix applications. In Proc. International Display Research Conference (San Diego, 1991), pp. 68–72, California: SID. 15. Koike, Y., Kamada, T., Okamoto, K., Ohashi, M., Tomita, I., and Okabe, M. (1992). A full-color TFT-LCD with a domain-divided twisted-nematic structure. In Digest of Technical Papers of the Society for Information Display International Symposium (Boston, 1992), pp. 798–801. California: SID. 16. Alt, P.M., and Pleshko, P. (1979). Scanning limitations of liquid-crystal displays. IEEE Transactions on Electron Devices , ED-21, 146–155. 17. Nehring, J., and Kmetz, A.R. (1979). Ultimate limits for matrix addressing of rms-responding liquid-crystal displays. IEEE Transactions on Electron Devices , ED-26, 795–802. 18. Scheffer, T.J., and Nehring, J. (1984). A new, highly multiplexable liquid-crystal display. Applied Physics Letters , 45, 1021–1023. 19. Leenhouts, F., and Schadt, M. (1986). Electro-optics of supertwist displays: dependence on liquid-crystal material parameters. In Proc. 6th International Display Research Conference (Tokyo, 1986), pp. 388–391. California: SID, Tokyo: ITE. 20. Kinugawa, K., Kondo, Y., Kanasaki, M., Kawakami, H., and Kaneko, E. (1986). 640×480 pixel LCD using highly twisted birefringence effect with low pretilt angle. In Digest of Technical Papers of the Society for Information Display International Symposium (San Diego, 1986), pp. 122–125. California: SID. 21. Breddels, P.A., and van Spraing, H.A. (1985). An analytical expression for the optical threshold in highly twisted nematic systems with nonzero tilt angels at the boundaries. Journal of Applied Physics , 58, 2162–2166. CHAPTER 6 Liquid-Crystal In a crystal state of matter, the solid forms a three-dimensional lattice having long-range order. When it is heated above the melting point, it turns to an isotropic liquid having neither long-range nor short-range order. The liquid- crystal is an intermediate state of matter between a solid crystal and an isotropic liquid (Figure 6.1). Material showing a liquid-crystalline phase is composed of many rod-like molecules. Due to this molecular feature or anisotropy, the solid does not immediately turn into an isotropic liquid at the melting point, T m . When this material reaches T m, the solid changes into a transitional liquid-crystalline phase. In this phase, the gravitational or positional order of constituent molecules is lost as in a normal liquid. However, there still remains some degree of orientational order of molecules. If this material is further heated above the clearing point, T c, this fluid turns into an isotropic liquid. The intermediate state between these two phase transitions is a liquid-crystalline phase. In this phase, the fluid appears turbid and is found to be strongly birefringent when observed between crossed polarizers. This phase is sometimes called the mesophase, since there is a contradictory tone in the name "liquid-crystal". Figure 6.1 The liquid crystalline phase is defined as an intermediate state between crystal and liquid. The thermotropic liquid crystalline phase appears in the temperature range between the melting point, Tm, and the clearing point, Tc. This type of liquid-crystal, in which the mesophase is defined by the temperature range between T m and T c tft lcd screen, is called thermotropic. In another type of liquid-crystal, called lyotropic, the amount of solvent defines the mesophase. Most