Abstract:
A liquid crystal display device is provided. The liquid crystal display device has a display surface, a sensing device for sensing a light source around the display surface, a control circuit responsive to the sensed light source to adjust a voltage to a controller, which adjusts a transmissivity of the display surface in response to the adjusted voltage.

Description:
FIELD OF THE INVENTION 
   The invention relates generally to a Liquid Crystal Display (LCD) having a regulated voltage. More specifically, the invention concerns an LCD-pixel-matrix element, a display screen capable of graphics with a plurality of such LCD-pixel-matrix elements and a procedure for brightness control of such an LCD-pixel-matrix element or such a display screen. 
   BACKGROUND OF THE INVENTION 
   For instance, railroad practices make it desirable to have display screens with large surfaces capable of graphics in order to provide to the railroad customers and passengers definite information regarding departure times, placement of cars, location of the diner car, arrival time and the like. Such displays are presently found in open areas and partially in buildings where the displays are subjected to various lighting conditions and ambient illumination. Under conditions of changing illumination, the contrast of known LCD-pixel-matrix elements undergoes very severe swings of visibility, and thus the legibility is not always assured. European Patent (EP) 0 389 744 discloses such an LCD-pixel-matrix element. 
   SUMMARY OF THE INVENTION 
   Thus, the present invention makes available an LCD-pixel-matrix element, a display screen capable of graphics with a plurality of such LCD-pixel-matrix elements and a procedure for brightness regulation, i.e. contrast control, for these components. 
   The present invention provides for variations in the LCD control voltage to correspond with the ambient brightness. By way of example, the LCD control voltage at a brighter ambient light level can be pushed to increasingly higher levels in order to increase the transmission ability of the LCD display. Accordingly, the display appears brighter and is easier to read at higher ambient illumination. In this way, such a display is particularly well suited for outside use where, because of natural conditions, variations very often occur in the incident brightness. 
   Alternatively, better legibility may be attained when the ratio of light-active surface to the to entire display area lies between 50 and 87%. Particularly desirable is a ratio in a range between 60 and 85%, which provides a pixel (picture element) density of from two to three pixels per square centimeter. It has been empirically found in the case of an LCD-pixel-matrix within these ranges that the legibility is better than in ratios outside of these limitations. 
   In accord with an advantageous embodiment of the invention, the display surface is rectangular, and the single pixels are arranged in columns and lines. The connection elements and the integrated circuits of the control are, in these cases, in the upper and the lower edge areas. This arrangement permits a plurality of such LCD-pixel-matrix elements to be placed next to one another in order to create a greater display surface. In accord with another advantageous embodiment of the invention, the pixels are combined in color groups to enable the presentation of information in color. 
   In accord with yet another advantageous embodiment of the invention, a plurality of LCD-pixel-matrix elements are placed beside or below one another, which makes possible an LCD-screen capable of graphics. 
   In accord with another advantageous embodiment of the invention, the display screen, i.e., the single LCD-pixel-matrix elements, is protected by a transparent cover, e.g. an overlay. This transparent cover is provided with an anti-glare characteristic such as an anti-reflective coating. A transparent anti-reflective coating, for example, permits more light to reach the LCD-design and additionally, the intensity of the disturbing reflections is diminished. As a result, the brightness and the contrast of the LCD-display become greater. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details, features and advantages of the invention are provided by the following detailed description in combination with the drawings, in which: 
       FIG. 1A  is a front view of an example embodiment of a display screen capable of graphics in accord with the present invention, 
       FIG. 1B  is a top view of the display screen shown in  FIG. 1A , 
       FIG. 2  is a block illustration of the brightness control in accord with the present invention, 
       FIG. 3  is a graph representing a brightness curve based on an operation of the invention, 
       FIG. 4  is a presentation of a filling coefficient of a display surface, 
       FIG. 5A  is a schematic presentation of an LCD-pixel-matrix element, 
       FIG. 5B  is an enlargement of a portion of  FIG. 5A , 
       FIG. 6A  is a partial schematic presentation of two adjacent LCD-pixel-matrix elements, and 
       FIG. 6B  is a detailed enlargement of an adjacent area of FIG.  6 A. 
     The detailed description which follows uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Detailed reference will now be made to the drawings in which examples embodying the present invention are shown. The drawings and detailed description provide a full and detailed written description of the invention, and of the manner and process of making and using it, so as to enable one skilled in the pertinent art to make and use it, as well as the best mode of carrying out the invention. However, the examples set forth in the drawings and detailed description are provided by way of explanation only and are not meant as limitations of the invention. The present invention thus includes any modifications and variations of the following examples as come within the scope of the appended claims and their equivalents. 
     FIGS. 1A and 1B  show, as an example, a screen panel  2 -S in accord with the present invention having rows  3  and  4  and five columns of LCD-pixel-matrix elements  2 , which are placed adjacent to and above/under one another. In the upper row  3  are four matrix elements  2 - 1  to  2 - 4  and in the lower row  4  are located five matrix elements  2 - 5  to  2 - 9 . The physical outline of the entire screen  2 -S mounting is rectangular, and likewise, the pixel-matrix elements are also rectangular. 
   As shown in  FIG. 1A , upper row  3  has, instead of a fifth pixel-matrix element, a free area  5 , which is dedicated to a constant printed designation on the display screen  2 -S. In this example, a “13” is shown in free area  5  of upper row  3 . In this same free area  5  is to be found a light sensing device  6 , which assists in automatically adjusting the degree of brightness of the display to the degree of ambient illumination. The pixel-matrix elements  2  are individually of the same size. 
   It is to be understood that the foregoing example is not intended to limit the present invention to use with only two rows and five columns of rectangularly shaped matrix elements  2 . For instance, the invention contemplates various numbers of rows and columns and variously shaped matrix elements  2 , such as square or in parallelogram form. 
     FIG. 2  shows schematically a block diagram of a circuit, which controls the brightness of the LCD-pixel-matrix elements  2  to adapt to the surrounding ambient light. Each pixel matrix element  2  possesses one display area  8  in which a plurality of pixels  10  is aligned in rows  11  and columns  12 . The signal of brightness level from the light sensor  6  is conducted to a voltage regulator or control circuit  14 . The control circuit  14  changes the control voltage V LCD  in correspondence with the brightness signal from the sensor  6  to regulate the individual LCD-pixel-matrix elements  2 . The control circuit  14  increases the voltage V LCD  to correspond to higher values of ambient lighting. 
     FIG. 3  depicts the foregoing relationship in which the transmissivity T of a pixel  10  ( FIG. 2 ) is expressed as a function of the control voltage V LCD . In the case of lower levels of ambient illumination, a control voltage V LCD1  results in the paired V off1  to V on1  for a first level of pixel brightness. Upon higher ambient illumination a control voltage of V LCD2  regulates the paired voltages V Off2  and V On2  resulting in a second level of pixel brightness. 
   Additionally or alternatively, the legibility of the screen can be influenced by a filling coefficient, expressed as η and the pixel density, i.e., the number of pixels per unit area of the display surface  8 .  FIG. 4  shows schematically the definition of the filling coefficient η as a quotient of the sum of the surfaces F i  of the individual pixels  10  and the entire area F of the display surface  8  of a pixel-matrix element  2 . It has been empirically determined that with a filling coefficient η in a range between 40 and 95%, preferably in a range between 50 and 87%, and especially advantageous in the range between 60 and 85%, and with a pixel density of 2 to 30 pixels per square centimeter, a very good readability is achieved for the display. Additionally, these values permit sufficient space for wiring (not shown). 
     FIGS. 5A and 5B  demonstrate an embodiment of the pixel-matrix element  2  with a plurality of pixels  10 , which are aligned in rows  11  and columns  12 . In this example, the shape of a single pixel  10  is a rounded off rectangle. On the upper and the lower edge  16 ,  18  of the pixel-matrix element  2  are connecting elements  20  which bind the individual pixel elements  2  together. Additionally, in the upper and lower edges  21   a ,  21   b , wiring circuits  22  are provided. The control voltage V LCD  is conducted to the wiring circuits  22  by multiplex connections to control the single pixel  10 . The multiplex control apparatus  22  is activated by the LCD-control voltage (V LCD ) to regulate individual pixels  10  with a voltage between a maximum and a minimum value to effect a maximum and minimum transmissivity of the pixels  10 . 
   As may be inferred from  FIG. 5B , the individual pixels  10  possess a breadth B and a height H. In this example, the height H is somewhat greater than breadth B. A distance d′ between two pixel rows  11  is very minute and serves mainly for the insulation of the directly adjacent pixel  10 ′ seen most clearly in FIG.  2 . The distance of the pixel  10  in two neighboring columns  12  is somewhat larger than d′ and is designated with a d. The distance d does not serve for the insulation of the pixels  10  from each other. In space d, the connection of the wiring (not shown) to the individual pixels  10  is led from the multiplex circuits  22 . Between the pixels  10  of the two outermost pixel divisions on the right and left rims  24 ,  26  of the pixel-matrix element  2 , there remains an edge-width of D/2. 
     FIGS. 6A and 6B  show two pixel-matrix elements  2 - 1  and  2 - 2 , which are set alongside of one another. In this case, the side edges  24 ,  26  of elements  2 - 1 ,  2 - 2 , respectively abut against one another. The directly neighboring pixel divisions on the two adjacent pixel matrix element  2 - 1  and  2 - 2  then lie at a distance D one from the other, respectively D/2 on both matrix-elements  2 - 1 ,  2 - 2 . This distance D at maximum is equal to the doubled offset d and the pixel breadth B. Stated alternatively, the distance D is smaller or equal to an empty space which arises when a pixel column  12  is omitted from a pixel-matrix element  2 . In this manner, there arises an equal-shaped optical impression when two pixel matrix elements  2  are placed beside one another such that the overall visual effect is undisturbed. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope and spirit of the invention. For example, specific shapes of various elements of the illustrated embodiments may be altered to suit particular kiosk or location applications. It is intended that the present invention include such modifications and variations as come within the scope of the appended claims and their equivalents.