Computer graphics processing and selective visual display system – Display driving control circuitry – Intensity or color driving control
Reexamination Certificate
2001-01-26
2004-10-05
Zimmerman, Mark (Department: 2671)
Computer graphics processing and selective visual display system
Display driving control circuitry
Intensity or color driving control
C345S055000, C345S690000
Reexamination Certificate
active
06801220
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to liquid crystal displays (LCDs) and, more particularly, to improving the viewing angle characteristics of liquid crystal displays.
2. Description of the Related Art
Most modern liquid crystal display panels suffer from poor viewing angle characteristics (color shift and level reversal, as a function of viewing angle) over a range of subpixel intensity values between the bright and dark states. Of the various liquid crystal modes used in these displays, the most commonly used is the Twisted Nematic mode (TN mode), which has poorer viewing angle characteristics than other modes. Typically, a normally white mode is used, so that the fully bright state corresponds to a low applied voltage and the fully dark state corresponds to a high applied voltage. The display picture elements are commonly referred to as pixels, where each pixel usually consists of a group of three subpixels, namely red, green, and blue subpixels. Typical LCDs have a stripe pixel geometry, where the pixels are square in shape, and where all subpixels are shaped as vertical stripes with the height of a full pixel and width of one third of a full pixel. For the normally white mode, using 8-bit drive per color, the highest applied voltage corresponds to an intensity value of zero, and the lowest applied voltage corresponds to an intensity value of 255. Intensity values are also referred to as digital pixel levels, or digital to analog conversion values (DAC values).
The poor viewing angle characteristics result from the variation in optical transmission at different angles as voltage is applied across the liquid crystal cell gap. At a viewing angle of normal incidence to the surface of the display, the luminance increases with digital pixel level, roughly following a power law, generally referred to as a gamma curve.
FIG. 1
is an idealized gamma curve illustrating the relationship between luminance and digital pixel level at normal incidence. At viewing angles away from normal incidence, the gamma curve becomes distorted. For a given digital pixel level, the luminance varies strongly with viewing angle.
FIG. 2
shows the general trend of relative luminance variation over all viewing angles as a function of the digital pixel level. The variation in luminance has a non-monotonic dependence on pixel level, with the largest variation occurring over a range of pixel levels somewhere between the dark state and bright state.
U.S. Pat. No. 5,847,688 to Ohi et al. describes a technique that provides a new set of analog reference voltages to the data drivers every other frame. This requires additional, specialized circuitry to be added to the drive electronics for the panel. To work well, the method requires reference voltages for different gamma curves to be switched every two or more frames. This is necessary to provide both positive and negative voltages sequentially to the pixel. If the frame rate is 60 Hz, the switching rate of the gamma curve would be 30 Hz or less. If the modulation in luminance between the two gamma curves is large enough, as required to improve the viewing angle characteristics, then flicker will occur. Human visual sensitivity to flicker peaks at about 10 Hz, and the sensitivity at 30 Hz is quite large. Alternatively, if the liquid crystal response speed is not fast enough to fully respond within two frame times, then the liquid crystal director will maintain an average position within the cell structure, and the luminance will not vary with time. The resulting luminance value will be the average of the two gamma curves, and no improvement in viewing angle characteristics will occur.
U.S. Pat. No. 5,489,917 to Ikezaki et al. describes a technique whereby the reference voltage set is altered from the usual condition in that the lowest reference voltages are increased to suppress level reversal. For TN-mode LCDs with the usual rubbing and polarizer configuration, this method improves the viewing angle characteristics in the upward direction (downward-looking) only. The level reversal condition is much stronger in the downward direction (upward-looking), so this method does not address the most noticeable deficiency in the vertical viewing angle characteristics. The method requires that the total range of reference voltages be decreased, which significantly reduces the dynamic range and contrast ratio of the panel.
G. S. Fawcett and G. F. Schrack in “Halftoning Techniques Using Error Correction,” Proceedings of the SID, Vol. 27/4, pp. 305-8 (1986), describes general algorithms for producing halftone images on any device, display, or printer which has limited grayscale capability. U.S. Pat. No. 5,254,982 to Feigenblatt et al. describes a halftone method with time-varying phase shift which was intended for LCDs with relatively few intensity grayscale values. The goal of both Fawcett et al. and Feigenblatt et al. is to produce nearly continuous-tone images with devices which have limited grayscale capability. The present invention is intended for use with LCDs with full grayscale capability, and takes full advantage of this capability. Finally, the techniques of Fawcett et al. and Feigenblatt et al. do not provide a method to improve the viewing angle characteristics with the halftone process.
In work done by both Honeywell and Hosiden Corporation, a split pixel structure has been used to increase the acceptable viewing angle range of TN-mode TFTLCDs. This work was described by Sarma et al. in “Active-Matrix LCDs Using Gray-Scale in Halftone Methods,” SID Digest, pp. 148-150 (1989); Sarma et al. in “A Wide-Viewing-Angle 5-in.-Diagonal AMLCD Using Halftone Grayscale,” SID Digest, pp. 555-557 (1991); Sunata et al. in “A Wide-Viewing-Angle 10-Inch-Diagonal Full-Color Active Matrix LCD Using a Halftone-Grayscale Method,” Int. Display Res. Conf. Record, pp. 255-257 (1991); Ugai et al. in “Deployment of Wide-Viewing-Angle TFT-LCDs Using Halftone Gray-Scale Method,” Electronics and Communications in Japan, Pt. 2, Vol. 80, No. 5, pp. 89-98 (1997). A summary of this work is also given in U.S. Pat. No. 5,847,688 to Ohi et al. In this technique, each subpixel is divided into two smaller split subpixels. An additional storage capacitor is utilized in combination with different load capacitances of the two split subpixels to provide a different pixel voltage to the two split subpixels. In this way, for a given subpixel voltage applied to the combination of two split subpixels, the transmission of the split subpixels is not the same. This technique is described by the authors as a “halftone gray-scale method.” The method is halftone in the sense that one split subpixel is brighter than the other. Because the ratio of voltages applied to the split subpixels tracks as the ratio of the capacitances, the ratio of voltages will be approximately the same for all subpixel levels. For a given subpixel voltage, and different smaller-subpixel voltages, the transmission and viewing angle characteristics of the two small subpixels are not the same. By mixing together the light from the two smaller subpixels, the viewing angle characteristics are also mixed and improved as compared to a single subpixel. A major disadvantage of this approach is that a special subpixel structure is required within the array on the glass panel. To date, this technology has been successfully applied in aircraft cabin entertainment displays, containing subpixels as small as 159 by 477 microns. As the pixel area is decreased, the additional storage capacitance and split pixel structure become increasingly difficult to implement. This limits the extent to which this approach can be applied to computer information displays, in which both a large number and large density of pixels is required. For example, a display with 200 pixels per inch requires subpixel dimensions of approximately 42×126 microns.
Ogura, et al., in “A Wide-Viewing-Angle Gray-Scale TFT-LCD Using Additive Gray-Level Mixture Driving,” SID Digest, pp. 593-596 (1992), describe a technique for improving
Greier Paul F.
Ho Kenneth C.
Kaufman Richard Ian
Millman Steven Edward
Thompson Gerhard R.
International Business Machines - Corporation
Trepp Robert M.
Wallace Scott
Zimmerman Mark
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