Abstract:
An optical system including an LCD rubbed for a 6 o&#39;clock director orientation, one or more positively uniaxial retarders (A-plates) adjacent to the LCD, and a pair of positive retardance oblique waveplates (O-plates) on TAC substrates adjacent to one another and the input polarizer, all encompassed by a polarizer on each side of the optical stack, aligned for a normally white display.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to liquid crystal displays (LCDs) and more particularly relates to optically compensated LCDs and even more particularly relates to methods and apparatus of optically compensating LCDs with a combination of optical compensation films. 
     BACKGROUND OF THE INVENTION 
     In the past, LCDs have been used in a wide variety of environments, including displays within the cockpit of an aircraft. Avionics applications often impose stringent demands on the optical performance of LCDs. The need for cross-cockpit viewing requires high contrast often extending to a horizon viewing angle range of nearly +60° to −60°. Accommodating pilot height and head movement often requires a vertical field of view having a range extending from −5° and +30° from a normal line from the display surface. Display of high information content imagery, such as weather radar, often requires that the low gray levels be stable in both luminance and chromaticity. This is often difficult, especially at high vertical viewing angles. 
     Various compensation schemes have been proposed in the past, including those described in U.S. Pat. No. 5,504,603 by Winker et al. and entitled “Optical Compensator For Improved Gray Scale Performance In Liquid Crystal Display”, issued on Apr. 2, 1996; U.S. Pat. No. 5,594,568 by Abileah et al. and entitled “LCD With a Pair of Retardation Films on One Side of Normally White Liquid Crystal Layer” issued on Jan. 14, 1997; U.S. Pat. No. 5,638,197 by Gunning, III et al. and entitled “Inorganic Thin Film Compensator For Improved Gray Scale Performance In Twisted Nematic Liquid Crystal Displays and Method of Making” issued on Jun. 10, 1997; U.S. Pat. No. 5,895,106 by VanderPloeg et al. and entitled “NW Twisted Nematic LCD With Negative Tilted Retarders on One Side of Liquid Crystal Cell” issued on Apr. 20, 1999, and described in the publication “Cockpit Displays V: Displays for Defense Applications” by D. B. Taber, L. G. Hale, B. K. Winker, W. J. Gunning III, M. C. Skarohlid, J. D. Sampica, and T. A. Seder, published in Volume 3363 of SPIE as part of the proceedings of a conference held on Apr. 15-17, 1998, in Orlando, Fla. by The International Society of Optical Engineering. 
     While these LCDs may have many advantages in particular uses, they also have significant drawbacks. One common drawback of these designs is the relatively high cost of such compensation systems. The use of proprietary techniques to manufacture some of the components of these LCD systems often leads to relatively high costs. 
     Consequently, there exists a need for improved methods and apparatuses for compensating LCDs. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an LCD with high contrast. 
     It is another object of the present invention to provide an LCD with stable colors at extreme viewing angles. 
     It is yet another object of the present invention to provide an LCD with improved gray shade stability for low luminance gray scales. 
     It is a feature of the present invention to utilize a combination of optical compensating films which include a sequence of commercially available components, together with a liquid crystal having predetermined surface orientations. 
     The present invention is an apparatus and method for optically compensating an LCD which is designed to satisfy the aforementioned needs, provide the previously stated objects, include the above-listed features and achieve the already articulated advantages. The present invention is carried out in a “proprietary component-less” manner in a sense that the inclusion of optical components, which are not generally commercially available, has been greatly reduced. 
     Accordingly, the present invention is an LCD system including a predetermined sequence and orientation of optical compensation films including two O plates and two A plates. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the foregoing description of the preferred embodiments of the invention, in conjunction with the appended drawing wherein: 
     The FIGURE is a simplified schematic exploded block diagram of the LCD system of the present invention, which includes lines within many of the components which indicate orientation of predetermined optical characteristics of such components. 
    
    
     DETAILED DESCRIPTION 
     Now referring to the drawing wherein like numerals refer to like matter throughout, there is shown an LCD system of the present invention, generally designated  100 . The description herein focuses upon LCD systems used in the cockpit of an aircraft because it is believed that many of the features of the present invention are particularly well suited for use in an aviation environment. However, it should be understood that other environments are equally applicable, and they are intended to be included within the scope of the present invention. LCD system  100  includes a light source  102 , which could be any type of light source, such as a back light etc. The light may be collimated, but it is not required for all applications. The light source  102  produces light which is incident upon input polarizer  104 , which is any polarizer which is well known in the art. Input polarizer  104  is shown having a line P 1 , which indicates a polarization absorption axis oriented at approximately 45° from vertical (as measured by the viewer  130  in a clockwise manner). However, a range from 30° to 60° is also contemplated. The polarized light emanating from input polarizer  104  is incident upon first O-plate assembly  106 , which includes a first TAC substrate  108  and a first O-plate optical compensation element  110 . The first TAC substrate  108  may be located in front of or behind first O-plate optical compensation element  110 . First TAC substrate  108  may be a triacetate cellulose substrate, or a substitute which is well known in the art as a substrate for thin films. First O-plate optical compensation element  110  is shown having an optical axis O 1  oriented at 45°, but a range of 30° to 60° is also contemplated. First O-plate optical compensation element  110  may be a commercially available retardation film having a retardation range of +70 nm to +140 nm. The optical axis of first O-plate optical compensation element  110  may be inclined with having a uniform or non-uniform tilt angle through the thickness of the film, and the non-uniform tilt angle may vary linearly or non-linearly, with a low tilt angle of 2° to 10° or for high tilt angles from 30° to 60°. The low tilt angle may be facing towards the TAC or away from the TAC. Throughout this description, any reference to an optical axis being inclined or having a tilt angle shall mean that such optical axis has a component which is out of the plane, which otherwise is referred to herein as including axes oriented in specified directions. First O-plate optical compensation element  110  may be any suitable type of retardation film, such as, but not limited to NH films available from Nippon Petrochemicals, a well-known source of thin film optical components. Light emanating from first O-plate optical compensation element  110  is then incident upon second O-plate assembly  112 , which includes second O-plate optical compensation element  116  disposed on second TAC substrate  114 . The second TAC substrate  114  may be located in front of or behind second O-plate optical compensation element  116 . Second O-plate optical compensation element  116  is shown having an optical axis O 2 , which is oriented at 135°, but a range of 120° to 150° is also contemplated. Other than having an optical axis which is approximately 90° shifted from the optical axis of first O-plate optical compensation element  110 , second O-plate optical compensation element  116  is very similar to first O-plate optical compensation element  110 . Light emanating from second O-plate assembly  112  is incident upon first A-plate  118 , which is shown having an optical axis of 45°, but a range of 30° to 60° is also contemplated. First A-plate  118  may have a retardation range of +30 nm to +180 nm, with an optical axis inclination of 0° to 10°. First A-plate  118  may be any commercially available A-plate with suitable properties; however, it could include, but is not limited to an A-plate having part number Rpw-80td from Fuji Film, a well-known maker of thin film optical components. Light emanating from first A-plate  118  is incident upon input surface  124  of liquid crystal media  120 , which may be any commercially available liquid crystal media available from various sources, which has suitable properties. Liquid crystal media  120  may have a retardation range from +300 nm to +500 nm, with an optical axis orientation of 315° on its input surface  124  and an optical axis orientation of 45° on its output surface  122 . Ranges from 300° to 330° and 30° to 60°, respectively are also contemplated. Light which emanates from output surface  122  of liquid crystal media  120  is incident upon second A-plate  126 , which may be similar to first A-plate  118 . Light emanating from second A-plate  126  is incident upon output polarizer  128  which may be any commercially available polarizer having suitable properties. For a normally white LCD, absorption axis of output polarizer  128  may be oriented at approximately 135°, but a range of 120° to 150° is also contemplated. Other than the optical axis orientations, output polarizer  128  may be very similar to input polarizer  104 . 
     In operation, the apparatus and method of the present invention could function as follows: 
     A display of information by an LCD device having a predetermined birefringence characteristic is compensated by providing a series of optical retardation films to precondition the light incident upon the LCD device. The preconditioning of the light is accomplished with two O-plates followed by an A-plate. A post LCD compensation step occurs with the use of an A-plate disposed between the LCD and the viewer. 
     It is thought that the method and apparatus of the present invention will be understood from the foregoing description and that it will be apparent that various changes may be made in the form, construct steps and arrangement of the parts and steps thereof without departing from the spirit and scope of the invention or sacrificing all of their material advantages. The form herein described is merely a preferred exemplary embodiment thereof.