Patent Publication Number: US-2004056995-A1

Title: Apparatus and method for maximizing LCD contrast

Description:
TECHNICAL FIELD  
       [0001] The present invention relates to liquid crystal displays. More specifically, the present invention relates to liquid crystal displays that have enhanced optical properties at wide viewing angles.  
       BACKGROUND OF THE INVENTION  
       [0002] Liquid crystal displays (LCDs) have a plurality of pixels or segments that may be excited by an electric field so that information, such as light from a light source, may be optically communicated to a receiver, such as a viewer. It is known in the art that typically, there is an asymmetry to the contrast of LCD displays. It is also known that LCD displays comprise, among other elements, a rear polarizer, a front polarizer, and a layer of LC fluid.  
       [0003] In operation, when a pixel is excited to an “on” state, light passes through the rear polarizer and is minimally rotated by the LC fluid prior to encountering the front polarizer. The front polarizer may be in one of two orientations depending on whether it is desired that the LCD be transmissive when the power is on or untransmissive (dark) when the power is off.  
       [0004] In the first orientation of the front polarizer, light that passes through a maximally excited pixel will be blocked by the front polarizer, and the pixel will appear dark (i.e. the display is transmissive when the power is off). In the second orientation of the front polarizer, light that passes through a maximally excited pixel will appear to be active because it is allowed to pass through the front polarizer (i.e. the display is dark when the power is off).  
       [0005] In the first case where the “on” pixels are dark, the display is said to operate in the “positive mode.” In the second case where the “on” pixels appear to be active, the display is said to be operating in the “negative mode.” In the following discussion, the LCD display is assumed to operate in the “negative mode.” However, the following description may apply equally to the “positive mode.” 
       [0006] When the electric field sufficiently excites a pixel or segment, the pixel has a higher or lower amount of luminance compared with pixels or segments that are excited to a lesser degree. Thus, pixels that are sufficiently excited to a higher degree are considered to be in an “on-state” (i.e. more or less light per area of the pixel is transmitted in the direction of the viewer&#39;s eye). Likewise, when a pixel is not sufficiently excited by an electric field, it has a lesser amount of luminance and is considered to be in an “off-state” (i.e. less light per area of the pixel is transmitted in the direction of the viewer&#39;s eye). For purposes of discussion, “on” pixels will be considered to transmit more light than “off” pixels.  
       [0007] A contrast ratio of a pixel, CR, is the ratio of a maximum transmitted light, T max , in the “on” state and a minimum transmitted light, T min , in the “off” state.  
         C                 R     =       T   max       T   min                     
 
       [0008] In the above equation, the minimum transmitted light value, T min , dominates the contrast ratio, CR, because T min  is generally a small number that is in the denominator.  
       [0009] The contrast ratio of an LCD depends on the viewing angle of the viewer. A “prime viewing angle” is the angle at which the contrast ratio is at its maximum compared with other angles. As seen in FIG. 1, an LCD  10  is observed from a top-dead-center view in an xyz plane. The LCD  10  comprises an active viewing area  20  with four rows of ten pixels. The viewing area  20  contains off-state pixels  12  and on-state pixels  14 . When the LCD  10  is observed at an angle that is normal to the viewing area  20 , the contrast ratio is maximized. This is considered to be “the prime viewing angle.” Generally, the prime viewing angle is close to the normal viewing angle of the display.  
       [0010] However, as seen in FIG. 2A, when an LCD  10   a  is viewed at an angle which is substantially different from the prime viewing angle, the contrast ratio, in general, will be diminished in some circumstances to a considerable degree. The deviation of the prime viewing angle is hereinafter referred to in the following discussion as a reference angle (RA).  
       [0011] As seen in FIG. 3, the RA has two components: the angle from the normal, θ, and the angle in the plane of the LCD from which it is observed, φ. The angle from the normal, θ, is represented by concentric dotted circles in the xy plane and is referenced from the center of the polar point (i.e. the z-axis). The outer perimeter of the outermost circle in the xy plane represents the angle in the plane of the LCD from which it is observed, φ. For example, the LCD may be viewed from the right at φ=0 0  (i.e. 3 o&#39;clock viewing), from the bottom at φ=270 0  (i.e. 6 o&#39;clock viewing), from the left at φ=180° (i.e. 9 o&#39;clock viewing), or from the top at φ=90 0  (i.e. 12 o&#39;clock viewing). In order to establish a frame of reference for the following explanation, the off- and on-state pixels  12 ,  14  in FIG. 1 correlate to off- and on-state pixels in  16 ,  18  in FIG. 2A.  
       [0012] Referring back to FIG. 2A, it is apparent that when the LCD  10   a  is viewed at an angle that deviates from the prime viewing angle, the off-state pixels  16  become more transmissive and have a higher luminance (i.e. the pixels shift from black to dark gray), and the on-state pixels  18  become less transmissive and have a lower luminance (i.e. the pixels shift from white to light gray). For example, if a wide-spectrum light source, such as a white light emitting diode (LED), is implemented in the design of the LCD  10   a , the viewing area of the LCD  10   a  may change color to purple, blue, or green at different angles from the normal since the change in transmission from the prime viewing angle is color dependant. However, it is important to note that the pixels the pixels  16 ,  18  may change color only with respect to their observational appearance and not with respect to their physical properties.  
       [0013] In other situations, when the LCD deviates from the prime viewing angle, the on-state pixels  18  may retain their optimal luminance and the off-state pixels  16  may have a higher luminance, or vice versa. At some viewing angles, the “on” and “off” pixels may look the same; this would mean that the minimum contrast ratio, CR is equal to 1.  
       [0014] In order to find data that relates to the contrast of the LCD  10   a , iso-transmission plots for the LCD  10   a  in the “on” and “off” states are generated. FIGS. 4A and 4B show iso-transmission plots for the LCD  10   a  in the “off” and “on” states, respectively. The data in FIG. 4B is divided by the data in FIG. 4A to form the data for an iso-contrast plot of the LCD  10   a , as shown in FIG. 4C (i.e. a ratio of the on-state iso-transmission plot (FIG. 4B) and the off-state iso-transmission plot (FIG. 4A)).  
       [0015] The iso-contrast plot of FIG. 4C represents the contrast of the LCD  10   a  when viewed from different RAs. The contrast ratio depicted in FIG. 4C is defined from a minimum contrast ratio C1 to a maximum contrast ratio C12. Generally, from a top-dead-center view (i.e. RA=(φ=0-360 0 , θ=0 0 )), the maximum contrast ratio is C12, which is approximately equivalent to a contrast category of 62.88 through 68.55. For a 360 0  pivot about φ, the LCD  10   a  is generally viewable when it is tilted 0 0  through 25 0  from normal (i.e. θ=0 through 25 0 ). However, when the LCD  10   a  is tilted in the range of 25 0  through 50+ 0  from normal, the contrast degrades down to a minimum contrast ratio of C1, which is approximately equivalent to a contrast category of 0.424 through 6.099.  
       [0016] The contrast ratio in FIG. 4C ranges from 0.424 through 68.55, which means that every orientation of the LCD  10   a  does not appear to have an acceptable image. Contrast ratios approaching values of 1 imply that pixels appear about the same whether pixels are in the on-state or the off-state. Higher contrast ratios are usually needed to produce discernable displays in conditions of high ambient light because the ambient light is reflected off of both on-state and off-state pixels, thereby reducing the contrast ratio perceived by the viewer.  
       [0017] Referring to FIG. 4D, the acceptable image of the LCD  10   a  generally correlates to the orientations near the top-dead-center viewing angles. An acceptable image zone,  11   a , of the LCD  10   a  correlates to contrast ratio categories encompassing 17.46 though 68.55 (i.e. category C4 through C12) of FIG. 4C. The acceptable image zone  11   a  has two criteria: the contrast ratio must be within an acceptable range for a given application, and the off-state transmission must be below an acceptable value for that application. For example, with regard to the second criteria, most negative mode automotive LCD displays must appear dark so that the displays do not appear as bright rectangular patches at night to people in the vehicle that are outside of the prime viewing area.  
       [0018] Light leakage of the LCD  10   a  tends to give the appearance that pixels are in an on-state when they are actually in an off-state. For example, when the LCD  10   a  is pivoted approximately φ=85 0  and approximately θ=45 0  (i.e. approximately the area, Toff12, in FIG. 4A), the viewer may see very noticeable light leakage at the RA=(φ=85 0 , θ=45 0 ). Thus, at the RA=(φ=85 0 , θ=45 0 ), the contrast ratio (i.e. the contrast ratio that correlates to the AFN=(φ=85 0 , θ=45 0 )) is about CR=34.05/53.27=0.64, where 53.27 is the maximum light transmission for the off-state (i.e. transmission category Toff12 in FIG. 4A) and 34.05 is the maximum light transmission for the on-state (i.e. Ton3 in FIG. 4B). The contrast ratio of CR=0.64 can be verified in FIG. 4C at RA=(φ=85 0 , θ=45 0 ). At RA=(φ=85 0 , θ=45 0 ), the contrast ratio category C1 ranges from 0.424 through 6.099; thus, the contrast ratio CR=0.64 is verified as the correct contrast at RA=(φ=85 0 , θ=45 0 ) because it falls within the contrast ratio category C1 that ranges from 0.424 through 6.099.  
       [0019] Ideally, the off-state of the LCD  10   a  should have light transmission for many viewing angles that allows for maximal contrast ratios. The minimum transmission range in FIG. 4A, which is the transmission category Toff1, dominates most of the plot. However, it is important to note that for a given on-state transmission value in the area covered by the transmission category Toff1, the light transmission can vary by as much as 0.785 through 5.157. Referring to the on-state of the LCD  10   a  in FIG. 4B, even though the maximum on-state light transmission category Ton12 may be as high as 125.2, it does not necessarily result in high contrast ratios and a desirable image appearance on the LCD  10   a.    
       [0020] Therefore, it is evident that when an LCD  10   a  is viewed from an angle that deviates from the prime RA, the contrast ratio decreases, and at some angles, the viewer may not be able to differentiate an off- or on-state of a specific pixel. Thus, reduced contrast at certain RAs degrades the appearance of the display to the point when the LCD is deemed not suitable for a given application.  
       [0021] There is a need for an LCD that maintains a reasonably high contrast ratio over a wide range of viewing angles.  
       SUMMARY OF THE INVENTION  
       [0022] Accordingly one embodiment of the present invention is directed to a liquid crystal display that improves contrast at wide viewing angles. The liquid crystal display comprises a light source, a liquid crystal cell, and an optical plate. The light source produces light. The liquid crystal cell communicates the light. The optical plate is located in close proximity to the liquid crystal cell. The optical plate accepts light at an input acceptance angle, redirects the accepted light, and provides a wide range of output angles for a viewer.  
       [0023] Another embodiment of the invention is directed to a liquid crystal display that improves contrast at wide viewing angles. The liquid crystal display comprises a light source, a liquid crystal cell, an optical plate, a lenslet array, and a light absorbing layer. The light source produces light. The liquid crystal cell communicates the light. The optical plate is located in close proximity to the liquid crystal cell. The optical plate accepts light at an input acceptance angle, redirects the accepted light, and provides a wide range of output angles for a viewer. The lenslet array is disposed between the light source and the liquid crystal cell for improving light utilization and sharpness of the resultant image communicated by the liquid crystal display by redirecting the light in a direction corresponding to the acceptance angle of the optical plate. A light absorbing layer absorbs most of the light that is not acceptable for redirection by the optical plate.  
       [0024] Another embodiment of the invention is directed to a method for maximizing contrast of a liquid crystal display. The liquid crystal display comprises a liquid crystal cell and an optical plate. The method comprises propagating light from a light source, focusing the light from the light source, propagating the focused light through the liquid crystal cell, and transmitting light through the optical plate.  
       [0025] Another embodiment of the invention is directed to a liquid crystal display that improves at wide viewing angles. The liquid crystal display comprises means for propagating light, means for collimating light, means for transmitting light, and means for increasing contrast. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0026] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
     [0027]FIG. 1 is a top-dead-center observation of a LCD;  
     [0028]FIG. 2A is a perspective observation of a conventional LCD;  
     [0029]FIG. 2B is a perspective observation of a LCD in accordance with the invention;  
     [0030]FIG. 3 is a representative view of an LCD observation in polar coordinates associated with a polar plot;  
     [0031]FIG. 4A is an iso-transmission plot for the conventional LCD of FIG. 2A in an “off state;” 
     [0032]FIG. 4B is an iso-transmission plot for the conventional LCD of FIG. 2A in an “on state;” 
     [0033]FIG. 4C is an iso-contrast plot for the conventional LCD of FIG. 2A according to the iso-transmission plots of FIGS.  4 A- 4 B;  
     [0034]FIG. 4D is an acceptable image plot of the LCD of FIG. 2A;  
     [0035]FIG. 5A is a cross-sectional view of the LCD of FIG. 2B according to one embodiment of the invention;  
     [0036]FIG. 5B is a cross-sectional view of the LCD of FIG. 2B according to another embodiment of the invention;  
     [0037]FIG. 6 illustrates an optical plate according to the invention that is associated with the LCD of FIG. 2B;  
     [0038]FIG. 7A is an iso-transmission plot for the LCD of FIG. 2B in an “off state;” 
     [0039]FIG. 7B is an iso-transmission plot for the LCD of FIG. 2B in an “on state;” 
     [0040]FIG. 7C is an iso-contrast plot for the LCD of FIG. 2B according to the iso-transmission plots of FIGS.  7 A- 7 B; and  
     [0041]FIG. 7D is an acceptable image plot of the LCD of FIG. 2B. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0042] An apparatus that improves contrast at wide viewing angles has been devised. As seen in FIG. 2B, an LCD  10   b  that deviates from the prime viewing angle, includes increasing contrast means, such as a thin optical plate  100  (FIGS. 5A, 5B,  6 ), placed over the viewing area  20  that comprises off- and on-state pixels  12 ,  14 . The thin optical plate  100  permits a similar and even transmission of luminance for the LCD  10   b . Even though the LCD  10   b  deviates from the prime viewing angle at a reference angle (RA), the LCD  10   b  is comparable in appearance to the LCD  10  (FIG. 1) that is observed from a normal viewing angle (i.e. a viewing angle close to prime viewing angle).  
     [0043] The thin optical plate  100 , which is located in close proximity to the LCD  10   b , accepts light at an input acceptance angle, θ a , redirects the accepted light, and provides a wide range of output angles, θ o  (FIG. 6), for a viewer, V (FIGS. 5A and 5B). Thus, optical plate  100  improves the contrast ratio at wide viewing angles. The improved contrast ratio, permits the viewer, V, to differentiate the off-state pixels  12  from the on-state pixels  14  regardless of the RA of the display. Although the viewer, V, is generally shown normal to the center of the LCD  10   b  in FIGS. 5A and 5B, the viewer, V, may be located at other viewing angles other than normal to the LCD  10   b.    
     [0044] Referring to FIGS. 5A and 5B, the LCD  10   b  includes transmitting means, such as a liquid crystal cell  104 , that comprises a top glass layer  110 , a bottom glass layer  112 , a top electrode  114 , a bottom electrode  116 , a top alignment layer  118 , a bottom alignment later  120 , a fluid gap  122 , and a layer of liquid crystal (LC) fluid  124 . The cell  104  also comprises a seal  126  and an opening  128  for the depositing of the LC fluid  124  in the fluid gap  122 . The cell  104  also comprises voltage sources V ns , V S  connected at opposing ends of the cell  104  to the electrodes  114 ,  116 .  
     [0045] A light source  130 , such as a light emitting diode (LED) or florescent light, generates light  132  toward the optical plate  100 . The light source  130  may be arrayed as shown in FIG. 5A, or the light source  130  may propagate from propagating means, such as a light pipe  134 , as shown in FIG. 5B. The bottom of the optical plate  100  contains optical micro-spheres  102 , which are preferably smaller in dimension than the pixels  12 ,  14 . The micro-spheres  102  transmit light and widen the viewing angle for the viewer, V.  
     [0046] As seen in FIG. 6, which is referenced from dashed line  6  in FIGS. 5A and 9B, the rear surface of the optical plate  100  that contains the plurality of micro-spheres  102  are spaced apart at an approximate distance, S. The spacing, S, of the micro-spheres  102  in the optical plate  100  is called the grain size. A very fine grain is desirable so that many micro-spheres  102  do not form noticeable spatial beats with the array of pixels so that the majority of spheres transmit light from a given pixel without averaging the light of adjacent pixels. The micro-spheres  102  provide attenuation of the light  132  communicated by the LCD  10   b  at angles with non-optimal contrast while the light absorbing layer  105  improves contrast by attenuating ambient reflected light.  
     [0047] The micro-spheres  102  accept light from almost any angle in its hemisphere facing towards the light source  130  (FIGS. 5A and 5B). The light source  130  may produce the light  132  over a range of angles that are off-normal of the micro-spheres  102 . However, only light that enters close to the normal, such as normal light  136  from the source  130 , is able to pass a light absorbing layer  105  (FIG. 6), which is located about the micro-spheres  102 , for propagation towards the viewer, V, at a wide output angle, θ o .  
     [0048] A portion of the surface of the micro-spheres  102  that faces the viewer, V, which protrudes through the light absorbing layer  105  and passes the normal light  136 , defines output apertures, A 1 , and the output angle, θ o . A portion of the micro-spheres  102  that protrudes below the light absorbing layer defines input apertures, A 2 , and the acceptance angle, θ a . The output apertures, A 1 , are smaller than the input apertures, A 2 , and improve the contrast ratio by limiting the acceptance angle, θ a .  
     [0049] The light absorbing layer  105  also improves the contrast ratio when the LCD  10   b  is used in conditions that are associated with a high amount of ambient light. When there is a high amount of ambient light, the contrast ratio of the system is defined by the luminance of an on-pixel added to any ambient light directed toward the viewer, V, divided by the luminance of an off-pixel that is also added to any ambient light directed toward the viewer, V. The contrast is increased as the luminance of the ambient light reflected in the direction of the viewer, V, decreases. The ratio of the apertures A 1 , A 2  in the light absorber  105  compared to the total surface area of the light absorber  105  is much greater than 1. Therefore, most of the ambient light impinging on the light absorber  105  is absorbed and the ambient light would otherwise reflect or scatter off structures behind the light absorber  105 .  
     [0050] Referring back to FIGS. 5A through 5B, because the light source  130  tends to produce light  132  in many directions that is not normal to the micro-spheres  102  in the optical plate  100 , collimating means, such as a multiple lenslet array  138 , may be disposed between the light source  130  and the LCD  10   b  for improving light utilization of the LCD  10   b  by redirecting the light  132  in a direction corresponding to the normal of the micro-spheres  102 . Essentially, the lenslet array  138  collimates the light  132  into normal light  136  corresponding to the micro-spheres  102  so as to utilize the available light  132  more efficiently. The normalized light  136  also helps maintain a sharp image by reducing the spreading of the light rays within the LCD  10   b  and up to the rear surface of the optical plate  100 . However, if the LCD  10   b  is sufficiently thin, it may not be necessary to collimate the light  132  with the lenslet array  138 , although light collimation may still improve light utilization efficiency.  
     [0051] Referring back to FIG. 6, if the array of micro-spheres  102  are perfectly ordered and formed in a two-dimensional single layer array (not shown), the micro-spheres  102  pack along three axes  120   0  apart from one and other, thus forming a triangular area between each set of mutually contacting micro-spheres  102  about the light absorbing layer  105 . The remaining light  132  (i.e. off-normal light) that is not intercepted by the lenslets  138  in the array will be absorbed by the light absorbing layer  105  in the optical plate  100 .  
     [0052] A clear substrate  106  may also be located above the light absorbing layer  105 . A film sheet  108  may also be laminated to the outer surface of the clear substrate  106  in order to provide additional support for the structure of the optical plate  100 . In addition, the top surface of material closest to the viewer, V, may be coated with a thin film anti-reflection coating in order to improve system contrast by minimizing the surface fresnel reflections.  
     [0053] Referring now to the transmission plots, FIGS. 7A through 7B show iso-transmission plots for the off-state and on-state of the LCD  10   b , respectively, which are used to generate data for the iso-contrast plot of FIG. 7C for the LCD  10   b . The iso-contrast plot is generated by dividing the data used to generate the on-state transmission plot of FIG. 7B by the data used to generate the off-state transmission plot of FIG. 7A. The maximum off-state transmission category, Toff12, in the off-state transmission plot of FIG. 7A for the LCD  10   b  with the optical plate  100  is 21.4 times less than the equivalent of the off-state transmission plot of FIG. 4A for the LCD  10   a  that does not include the optical plate  100 . The minimum off-state transmission category, Toff1, in the transmission plot of FIG. 7A dominates the plot at almost every viewing angle. Thus, it is apparent that the optical plate  100  blocks illuminating light, which would otherwise pass through the off-state pixel, resulting in an off-state pixel appearance of the LCD  10   b  when the LCD  10   b  is essentially off.  
     [0054] The optical plate attenuates much of the light that passes through the LCD  10   b , so in general, the luminance values of the on-state iso-transmission plot in FIG. 7B are less than those in FIG. 4B. In order for the optical system of the LCD  10   b  to have the same on-state luminance as the prior art LCD  10   a , a higher intensity illumination source would be required. However, increasing the intensity of the illumination source does not impact the contrast ratio because the on-state transmission and the off-state transmission are both proportionally increased by the same factor.  
     [0055]FIG. 7C is the plot of the iso-contrast ratio of the LCD  10   b  with the optical plate  100  and is defined by a ratio of the on-state iso-transmission plot (FIG. 7B) divided by the off-state iso-transmission plot (FIG. 7A). The maximum contrast category, C12, in FIG. 7C for the LCD  10   b  is approximately equivalent to 19.82 through 21.57 and is approximately located at AFN=(φ=25 0 , θ=90 0 ). The LCD  10   b  is generally viewable from 0 0  through 50+ 0  from the normal to the LCD surface (i.e. C6 through C12). The optical plate  100  significantly improves the contrast ratio of the LCD  10   b  when compared to the LCD  10   a  that does not include the optical plate  100 .  
     [0056] Although the minimum contrast ratio, C1, of the LCD  10   b  in FIG. 7C ranges from 0.692 through 2.433, it is not very evident on the plot. The higher contrast ratios from C6 through C12 dominate the contrast plot of the LCD  10   b . Referring to FIG. 7D, an acceptable image zone  11   b  of the LCD  10   b  is generally evident for nearly every viewing angle of the LCD  10   b . The acceptable image zone  11   b  of the LCD  10   b  correlates to contrast ratios of C9 through C12 in FIG. 7C.  
     [0057] Similar optical plates to that of optical plate  100 , such as the XRVS 120 Non-Glare produced by 3M, are commercially available and are applied to rear projection screens, movie projectors, slide projectors, and projection televisions. In these projection systems, an image is focused on the optical plate from a source that is distant from the plate. The optical plate forms a real image which can then be viewed from a wide range of angles. However, in the inventive system described above, normal light  136  from the LCD  10   b  that reaches the micro-spheres  102  is predominately collimated from the lenslet array  138 . Alternatively, in the inventive system, non-collimated light  132  passes through the LCD  10   b  and impinges on the micro-spheres  102 , which preferentially accepts rays of light close to the normal for subsequent redirection over a wide output angle, θ o , on the viewing side of the optical plate  100 . The light  132 , which passes though the LCD  10   b  and impinges on the micro-spheres  102  at angles farther away from the normal, is preferentially absorbed by the absorbing layer  105  in the optical plate  100 . The image formed on the viewer side of the optical plate  100  maintains acceptable sharpness due to the close proximity of the optical plate  100  to the image forming LCD  10   b  due to the preferential transmission of normal impinging light  136 , which may be normalized by the lenslet array  138 . Therefore, the optical plate  100  improves and provides an acceptable contrast ratio, CR, and luminance for the LCD  10   b.    
     [0058] It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.