Birefringent optical compensators for TN-LCDs

ABSTRACTWe investigate various birefringent optical thin films which can be employed to improve the viewing angle characteristics and gray scale stability of conventional twisted nematic liquid crystal displays (TN-LCDs). The birefringentphase retardation films include negative uniaxial, positive uniaxial and biaxial films of various orientations. The results arepresented and discussed.Keywords: Liquid Crystal Display, Compensators, Extended Jones Matrix, Active Matrix, Viewing Angle, Gray Level,Contrast Ratio 1. INTRODUCTION It is generally recognized that the conventional twisted-nematic LCDs (TN-LCDs) have been and will continue to bevery important display devices in conjunction with active matrix (AM) addressing technology such as thin-film transistors(TFTs). High quality (contrast, gray scale stability) information display can be obtained within a limited range of viewingangles centered around the normal incidence by using these conventional TN-LCDs. The degradation of the display quality atlarge viewing angles is a fundamental property of virtually all modes and configurations of LCDs. Generally speaking, theangular dependence of the viewing is due to the fact that both the phase retardations and optical path in most LC cells arefunctions of the viewing angles. The narrow viewing angle characteristics has been a significant problem in advancedapplications requiring high quality displays, such as avionics displays and wide-screen displays. In the case of avionicsdisplays, the LCDs must provide the same (or nearly the same) contrast and gray scale for viewing angles from both the pilotand the copilot. Such high information content and high quality displays require LCDs whose contrast and gray scale levelmust be as invariant as possible with respect to viewing angles.Various methods and modes of operation have been proposed so far to improve the viewing angle characteristics ofLCDs. In the method of optical birefringence compensation, a thin film of birefringent material is inserted into the LCD at aproper location to eliminate or minimize the angular dependence. It was first suggested that a film of negative birefringencecan be employed to improve the viewing characteristics of LCDs based on vertically aligned nematic cells (VA-LCDs) [1,2].The same film of negative birefringence film can also be employed in TN-LCDs to improve the viewing angle characteristics[ 3 ,4]. There are several methods of producing the films of negative birefringence. These include the use of negative formbirefringence in multilayers of alternating thin films [5], negative form birefringence of co-planar alignment ofpolymers (e.g.,spin-coated polyimide) [6], negative birefringent films of discotic compound with inclined optical axes [7]. Using multi-domain (e.g., 2 or 4) with different liquid crystal orientations in each pixel of TFT-LCDs, the angular dependence of theoptical transmission characteristics can also be significantly reduced [8,9]. The viewing characteristics can also be improvedby using various different LCD modes of operation. These include optically compensated birefringence (OCB) mode LCDs[10,1 1], in-plane switching (IPS) mode LCDs [12-13], the half-tone gray scale (HTGS) method[14]. The angular dependenceof the optical transmission can be reduced or eliminated by using a beam of collimated light as the back light. A high qualitydiffuser is needed in this case for wide angle viewing [15].In this paper we discuss the principle of phase retardation compensation using various birefringence thin films,including uniaxial and biaxial films, to achieve high contrast ratios and gray level stability in LCDs. We first summarize thebasic properties ofTN-LCDs at large viewing angles.