Abstract:
A display unit includes an LCD panel for providing an output image to a viewer. An APD panel is disposed behind the LCD panel for providing a backlit image to the LCD panel. The LCD panel and the APD panel are vertically stacked one behind the other with an air gap between the APD panel and the LCD panel. The APD panel is configured to provide the backlit image as a first luminance modulated light to the LCD panel, and the LCD panel is configured to provide a second luminance modulated light to the viewer. The combination of the first luminance modulation and the second luminance modulation increases the dynamic range of the display unit. The LCD panel and the APD panel have their respective output images synchronized to each other.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates, in general, to a display unit and, more specifically, to a display unit having an LCD panel at the front of the unit, and an APD panel disposed behind the LCD panel. The APD panel provides an addressable backlight image to the LCD panel. 
       BACKGROUND OF THE INVENTION 
       [0002]    Liquid crystal materials emit no light of their own. They do, however, reflect and transmit light from external light sources. Accordingly, when using liquid crystal materials in a display, it is necessary to back light the display. 
         [0003]    A conventional flat screen liquid crystal display (LCD) includes a matrix of thin film transistors (TFTs) fabricated on a substrate of glass or another transparent material. A liquid crystal film is disposed over the substrate and the TFTs. Addressing of the TFTs by gate lines deposited on the substrate during TFT fabrication causes selected TFTs to conduct electrical current and charges the liquid crystal film in the vicinity of the selected TFTs. Charging of the liquid crystal film alters the opacity of the film, and affects a local change in light transmission of the liquid crystal film. Hence, the TFTs define display cells or pixels in the liquid crystal film. Typically, the opacity of each pixel is charged to one of several discrete opacity levels to implement a luminosity gray scale, and so the pixel is a gray scale pixel. 
         [0004]    Because a backlit LCD varies only the luminosity of the light to produce gray scale pixels, an LCD also requires means for coloring the pixels. U.S. Pat. No. 6,975,369 describes a method of coloring LCD pixels, which includes use of a colorizing backlight. As described, an array of backlight elements each includes a first component color light emitting diode (LED), a second component color LED and a third component color LED, such as red, green and blue, respectively. Each of the three LEDs is optically coupled to a corresponding pixel of the LCD. In this arrangement, each component color LED corresponds to a color pixel. In operation, the red, green and blue LEDs emit light toward the LCD. The luminance of each of the pixels is modulated via the LCD pixels using the TFTs to create a transmitted light luminance modulation across the area of the display. In particular, LCD pixels coupled to the red LEDs modulate the red light component, LCD pixels coupled to the green LEDs modulate the green light component, and LCD pixels coupled to the blue LEDs modulate the blue light component. By selective operation of the pixels for each backlight element, a desired color blending is achieved. The combination of gray scale pixels defines a full-color pixel. 
         [0005]    Conventional flat screen displays suffer certain disadvantages. First, the colorizing backlight of the conventional flat screen display modulates only chrominance of the backlight. As a result, luminance range of the flat screen display is limited. Second, conventional flat screen displays require complex controls for turning on the LEDs at certain levels to produce blended colors, making manufacture of conventional flat screen displays difficult and expensive. 
       SUMMARY OF THE INVENTION 
       [0006]    To meet this and other needs, and in view of its purposes, the present invention provides a display unit and method of manufacturing the display unit. In one embodiment of the invention, the display unit includes an LCD panel for providing an output image to a viewer. An APD panel is disposed behind the LCD panel for providing a backlit image to the LCD panel. The LCD panel and the APD panel are separately manufactured and, subsequently, vertically stacked one behind the other. Furthermore, the APD panel is configured to provide the backlit image as a first luminance modulated light to the LCD panel, and the LCD panel is configured to provide a second luminance modulated light to the viewer. The APD panel is also configured to provide a chrominance modulated light to the viewer. 
         [0007]    The present invention also includes a method of manufacturing a display unit. The method includes the following steps:
       (a) separately manufacturing an LCD panel and an APD panel,   (b) vertically stacking the LCD panel and the APD panel one behind the other.       
 
         [0010]    In addition, the present invention includes the step of synchronizing an image provided by the LCD panel with an image provided by the APD panel. 
         [0011]    Furthermore, the present invention includes steps of modulating first luminance levels and first chrominance levels of light intensity provided by the APD panel toward the LCD panel, and modulating second luminance levels of light intensity provided by the LCD panel toward the viewer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention is best understood from the following detailed description when read in connection with the accompanying drawing. Included in the drawing are the following figures: 
           [0013]      FIG. 1  is a side view of a liquid crystal display (LCD), according to an exemplary embodiment of the present invention; 
           [0014]      FIG. 2  is an exploded view of a liquid crystal display, according to an exemplary embodiment of the present invention; 
           [0015]      FIG. 3  is a side view of an exemplary relationship between an active pixel display (APD) and a liquid crystal display, according to an embodiment of the present invention; 
           [0016]      FIG. 4  is a front view of the top left corner of a combined display format illustrating a 4:1 relationship of background active color pixels to foreground LCD pixels, according to an exemplary embodiment of the present invention; 
           [0017]      FIG. 5  is a front view of the top left corner of a combined display format illustrating a 1:1 relationship of background active color pixels to foreground LCD pixels, according to an exemplary embodiment of the present invention; 
           [0018]      FIG. 6  is a front view of the top left corner of a combined display format illustrating a 1:1.6 relationship of background active color pixels to foreground LCD pixels, according to an exemplary embodiment of the present invention; 
           [0019]      FIG. 7  is a block diagram showing synchronization between an LCD and an APD, according to an exemplary embodiment of the present invention; 
           [0020]      FIG. 8  is a side view of an optional field format magnifier sandwiched between an LCD and an APD, according to an exemplary embodiment of the present invention; 
           [0021]      FIG. 9  is a side view of an optional field format minifier sandwiched between an LCD and an APD, according to an exemplary embodiment of the present invention; 
           [0022]      FIG. 10A  is a side view of a relay lens for frame field matching between an LCD and an APD, according to an exemplary embodiment of the present invention; 
           [0023]      FIG. 10B  is a side view of a 1:1 fiber optic for frame field matching between an LCD and an APD, according to an exemplary embodiment of the present invention; and 
           [0024]      FIG. 10C  is a side view of a minifying fiber optic taper for frame field matching between an LCD and an APD, according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention. 
         [0026]    With reference to  FIGS. 1 and 2 , a display unit  10  according to an exemplary embodiment of the present invention includes an active pixel display (APD)  12  disposed behind a liquid-crystal display (LCD)  18 . The LCD  18  may be, for example, a transmissive or transflexive LCD. The APD  12  provides a backlight source for LCD  18 .  FIGS. 1 and 2  also show an optional field format modifier  14  that may be used to modify the relationship between the active display area of APD  12  and the active display area of LCD  18 . Optional field format modifier  14  is described in more detail later. 
         [0027]    As shown in  FIG. 1 , APD  12  emits chrominance and luminance modulated light into illumination output region  16 . The LCD  18  further modulates the luminosity of the light to form a final image in display output region  20 . 
         [0028]    The APD  12  may be any active pixel display of any light emitting technology. For example, APD  12  may be an active matrix organic light emitting diode (AMOLED). 
         [0029]    An AMOLED is made up of an array of organic light emitting diodes (OLEDs). Each OLED includes an anode layer and a cathode layer, with at least two organic semiconductor layers sandwiched between them. One of the organic semiconductor layers is a conductor of positively charged holes and the other is a conductor of electrons. When a voltage is applied to the device, the excess electrons jump the gap towards the holes and emit light. The OLED may be made to emit colored light, for example, by placing a color filter over a white-light-emitting OLED. 
         [0030]    The anode layer of each OLED is disposed on top of a thin film transistor (TFT) array that forms a matrix. The TFT matrix controls both the chrominance and luminance of the OLEDs. Addressing of the TFTs by gate lines deposited on the substrate during TFT fabrication causes selected TFTs to conduct electrical current. Those selected TFTs turn on selected OLEDs to produce blended colors as well as different luminance values, thus forming an image. 
         [0031]    Thus, active pixel display  12  modulates both luminance and chrominance. When used as a backlight for LCD  18 , active pixel display  12  acts as a primary light source and a light modulator and LCD  18  acts as a secondary light modulator. In this way, LCD  18  provides an additional level of luminance control. For example, if each APD pixel provides 256 individual luminance levels, and each LCD pixel provides 16 additional luminance levels, then system  10  has a dynamic range of 4096 luminance levels per pixel. 
         [0032]    Further, using APD  12  as a backlight for LCD  18  provides for easy assembly. The present invention advantageously assembles two separate and independently manufactured units. Both units, namely the APD panel and the LCD panel, may be separately manufactured in any conventional manner. After manufacture, both units may be integrated to form display unit  10 , where APD panel  12  is disposed behind LCD panel  18 . The resulting dynamic range of display unit  10  is the product of the individual dynamic range of the APD panel and the individual dynamic range of the LCD panel. 
         [0033]      FIG. 3  shows a general arrangement of APD pixels  30 ,  31  and  32  disposed behind LCD pixel  34 . For example, pixel  30  emits red light, pixel  31  emits green light, and pixel  32  emits blue light. In this manner, each LCD pixel  34  emits green light, blue light, red light or any blended color produced by combining the three colors. As is known in the art, selective blending of three primary colors such as red, green and blue generally produces a full range of colors suitable for color display purposes. As previously described, each APD pixel  30 ,  31  and  32  emits light that is both luminance modulated and chrominance modulated in the direction of LCD pixel  34 . The LCD pixel  34  then provides additional luminance modulation. 
         [0034]      FIGS. 4-6  show a top corner portion of various combined display formats and illustrate the relationship of background active color pixels to respective foreground LCD pixels. Pixel overlay relationship is a direct factor of the size spacing and fill factor of each individual pixel (in the APD) with respect to pixel or pixels of a corresponding secondary display (e.g. the LCD). 
         [0035]    Referring first to  FIG. 4 , there is shown a 4:1 pixel overlay relationship. As shown, active color pixels  40  are smaller than LCD pixel  42 . More specifically, four active color pixels  40  are disposed behind one LCD pixel  42 . 
         [0036]    As another example,  FIG. 5  shows a 1:1 pixel overlay relationship. As shown, each active color pixel  50  is the same size as each LCD pixel  52 . Thus, each active color pixel  50  is disposed behind one LCD pixel  52 . 
         [0037]    Still another example,  FIG. 6  shows a 1:1.6 pixel overlay relationship. As shown, each active color pixel  60  is larger than each LCD pixel  62 , by as much as 60%. 
         [0038]    It will be appreciated that one skilled in the art may arrange the background active color pixels and the foreground LCD pixels to form any other pixel overlay relationship. 
         [0039]      FIG. 7  illustrates an example of synchronization of the APD pixels with the LCD pixels. As shown, display unit  70  includes synchronizer  71 , driver circuits  73  and  75 , LCD  77  and APD  79 . Synchronizer  71  generates a clock signal having a predetermined frequency. The clock signal is provided to both driver circuit  73  and driver circuit  75 . Driver circuit  73  controls LCD  77  and driver circuit  75  controls APD  79 . In this manner, display unit  70  synchronizes the pixels of LCD  77  with the pixels of LCD  79  to the same clock signal. A synchronized image of luminance values from both LCD  77  and APD  79  and chrominance values from APD  79  are displayed by the output of the front panel of LCD  77 , as best shown in  FIGS. 1-3 . 
         [0040]      FIGS. 8 and 9  illustrate an optional field format modifier inserted between an LCD panel and an APD panel. Optional field format modifier  82  or  102  may be used to optimize the active pixel-to-LCD display format overlay relationship and/or the individual pixel-to-pixel overlay dimensional relationship. Field format modifiers  82  or  102  may be placed between the APD panel and the LCD panel. The field format modifier may be, for example, a relay lens, a micro-fresnel lens, and/or a fiber optic taper. 
         [0041]    Referring to  FIG. 8 , display unit  90  includes APD  80 , field format magnifier  82  and LCD  84 . In the exemplary embodiment, LCD  84  has a larger display area than APD  80 . Field format magnifier  82  directs the light emitted from APD  80  toward a larger area of LCD  84 . In this manner, an APD may be used to backlight an LCD that has a larger display area than the APD. 
         [0042]    Referring to  FIG. 9 , display unit  110  includes APD  100 , field format minifier  102  and LCD  104 . In the exemplary embodiment, LCD  104  has a smaller display area than APD  100 . Field format minifier  102  directs the light emitted from APD  100  toward a smaller area of LCD  104 . In this manner, an APD may be used to backlight an LCD that has a smaller display area than the APD. 
         [0043]    Referring to  FIGS. 10A ,  10 B and  10 C, there are shown exemplary field format modifiers. Display unit  120  includes relay optic (lens)  125  disposed between APD  121  and LCD  122  (only portions of an APD and an LCD are shown). Relay optic  125  is separated completely from the APD and the LCD by way of an air gap on both sides of the relay optic. As another example, display unit  130  includes a 1:1 fiber optic disposed between APD  121  and LCD  122 . Still another example, display unit  140  includes a minifying fiber optic taper disposed between APD  121  and LCD  122  for reducing the size of the image between the APD and the LCD. Although not shown, a magnifying fiber optic taper (the taper is an inverse of the taper shown in  FIG. 10C ) may also be used for enlarging the image between the APD and the LCD. 
         [0044]    Actual design intent affects how and when magnification or minification is applied. In cases where the design intent is to maximize or more equally match the overall format areas of each display, less consideration may be given to a 1-to-1 pixel overlay match and some fractional overlay may result. In cases where pixel-to-pixel matching is more important, less concern may be given to an under-filled or over-filled field display.