Patent Publication Number: US-8525754-B2

Title: Display device with lens array or parallax barrier that switches between narrow view mode and wide view mode

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a display device, and more particularly to a display device which is capable of changing a view angle range. 
     2. Description of the Related Art 
     With the development of display technology in recent years, display devices that can be viewed in a wide angle range have been put to practical use. Portable information terminals incorporating display devices such as liquid crystal displays have also been in widespread use. If the information displayed on a portable information terminal is to be viewed by a plurality of people, then it is desirable that the information be visible in a wide angle. On the other hand, there is a situation where the displayed information should not be viewed by other people. Consequently, there is a demand for display devices capable of switching between a wide view angle range and a narrow view angle range for viewing the information displayed on the display screen depending on the way in which the display device is used. 
     One example of display device which can meet the above demand is disclosed in JP06-105305A. The disclosed display device will be described below with reference to  FIG. 1  of the accompanying drawings.  FIG. 1  is a view of the display device disclosed in the above publication, the display device being relevant to the present invention. 
     As shown in  FIG. 1 , the display device has liquid crystal display  101  with lenticular lens plate  102  mounted thereon. On the average, each lens of lenticular lens plate  102  is associated with two pixels. In particular, pixel  103  is disposed at the center of each lens, and pixel  104  is disposed at an end thereof. 
     For the display device to display an image in a narrow view range, the display device supplies only pixels  103  with a display signal. At this time, the displayed image can be viewed only in view range  105 . For the display device to display an image in a wide view range, the display device simultaneously supplies pixels  103 ,  104  with a display signal. At this time, the displayed image can be viewed not only in view range  105 , but also in view ranges  106 ,  107 . 
     As described above, the display device disclosed in JP-A No. 6-105305 allows different view ranges to be selected by controlling the supply of display signals to pixels  103 ,  104 . 
     However, the disclosed display device suffers the following difficulties: 
     Firstly, the size of view range  105  is determined by the size of pixel  103 . Therefore, if the view range is to be set to a narrow range, then the size of pixel  103  needs to be reduced, and the display device displays images with a low luminance level, i.e., dark images. 
     Secondly, there is a non-display area created between view range  105  and view range  106  or  107  because the gap between pixels  103 ,  104  is magnified by the lens. As a result, the viewer sees dark lines in the displayed image, and feels awkward about the displayed image. The darks lines are caused when both pixels  103 ,  104  are displayed in order to display an image in a wide view angle. As the viewer has to attempt to view the image while avoiding the dark lines, the display device, in practice, fails to provide wide view angles. 
     SUMMARY OF THE INVENTION 
     It is an exemplary object of the present invention to provide a display device which is capable of preventing the luminance of a displayed image from being lowered when the display device displays an image in a narrow view range, and which is capable of preventing non-display areas (dark areas) from being created when the display device displays an image in a wide view range. 
     According to an exemplary aspect of the present invention, a display device includes a display panel including a plurality of pixel groups for displaying respective images independently of each other, and a parallax barrier for limiting view ranges of images displayed respectively by the pixel groups such that the view ranges overlap each other in an overlap area. The display device is selectively operable in a first display mode in which a significant image is recognizable when images displayed respectively by the pixel groups are simultaneously observed in the overlap area, and a significant image is unrecognizable when the images displayed respectively by the pixel groups are observed individually in the view ranges except for the overlap area, respectively, and a second display mode in which a significant image is recognizable both when the images displayed respectively by the pixel groups are simultaneously observed in the overlap area and when the images displayed respectively by the pixel groups are observed individually in the view ranges except for the overlap area, respectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of a display device according to the related art; 
         FIG. 2  is a schematic view showing a conceptual arrangement of a display device according to an exemplary embodiment of the present invention; 
         FIG. 3  is a schematic view illustrative of a display operation of the display device in a first display mode; 
         FIG. 4  is a schematic view illustrative of a display operation of the display device in a second display mode; 
         FIG. 5  is a schematic view showing a view range at the time the display device switches to the second display mode and brings a multiple viewpoint activator into an inactive state; 
         FIG. 6  is a transparent plan view of a structure comprising a lenticular lens array and two pixel groups; 
         FIG. 7  is a schematic perspective view of a specific display device according to the exemplary embodiment of the present invention; 
         FIG. 8  is a view illustrative of the display device according to a first example of the present invention; 
         FIG. 9  is a view illustrative of the display device according to the first example of the present invention; 
         FIG. 10  is a view illustrative of the display device according to the first example of the present invention; 
         FIG. 11  is a view illustrative of the display device according to the first example of the present invention; 
         FIG. 12  is a cross-sectional view of a display device according to a second example of the present invention; 
         FIG. 13  is a cross-sectional view, partly in block form, of the display device according to the second example of the present invention; 
         FIG. 14  is a cross-sectional view of a display device according to a third example of the present invention; 
         FIG. 15  is a cross-sectional view of the display device according to the third example of the present invention; 
         FIG. 16  is a plan view of the display device according to the third example of the present invention; 
         FIG. 17  is a schematic view of a display device according to a fourth example of the present invention; 
         FIG. 18  is a schematic view of a display device according to a fifth example of the present invention; and 
         FIG. 19  is a schematic view of a display device according to a sixth example of the present invention. 
     
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
       FIG. 2  is a schematic view showing a conceptual arrangement of a display device according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 2 , the display device according to the exemplary embodiment of the present invention includes display panel  1  and multiple pie viewpoint activator  8 . Display panel  1  comprises a plurality of pixel groups. In  FIG. 2 , display panel  1  comprises an alternate array of two pixel groups, i.e., pixel groups A, B, for example. Multiple viewpoint activator  8  includes a repetitive pattern of units C corresponding to the pixel groups A, B. Each of pixel groups A can be viewed in view range  5  through multiple viewpoint activator  8 . Each of pixel groups B can be viewed in view range  6  through multiple viewpoint activator  8 . In the present exemplary embodiment, view ranges  5 ,  6  overlap each other to provide overlap area  9  in space. 
     A display operation of the display device according to the exemplary embodiment will be described below with reference to  FIGS. 3 and 4 . 
     The display device has two display modes, i.e., a first display mode and a second display mode.  FIG. 3  shows the display operation in the first display mode. In view range  5  corresponding to pixel group A, the viewer can view a displayed image shown in display content  10 , for example. In view range  6  corresponding to pixel group B, the viewer can view a displayed image shown in display content  11 , for example. These display contents  10 ,  11  have no significance themselves, which the viewer is unable to understand. In overlap area  9  where view ranges  5 ,  6  overlap each other, the viewer can view a displayed image shown in display content  12 . Display content  12  has significance based on the combination of display contents  10 ,  11 , which the viewer is able to understand. The display device thus allows the viewer to recognize a significant image by simultaneously observing the images displayed by the pixel images A, B in overlap area  9  of view ranges  5 ,  6 . 
       FIG. 4  shows the display operation in the second display mode. In the second display mode, the display device allows the viewer to recognize significant images by observing the images displayed by the pixel images A, B in respective view ranges  5 ,  6 . 
     The display device can switch between the first display mode and the second display mode by changing display signals to be displayed by pixel groups A, B. In the first display mode, the display device controls pixel group A to display the image in view content  10  shown in  FIG. 3  and controls pixel group B to display the image in view content  11  shown in  FIG. 3 , thereby displaying the image in overlap area  12  shown in  FIG. 3 . Specifically, pixel group A displays the image in view content  10  which is part of the significant image and pixel group B displays the image in view content  11  which is part of the significant image. The image displayed in view content  10  and the image displayed in view content  11  are combined with each other to display the significant image in overlap area  9 . 
     In the second display mode, the display device controls pixel group A to display the significant image in view content  10  shown in  FIG. 4  and controls pixel group B to display the significant image in view content  11  shown in  FIG. 4 , thereby displaying the image in overlap area  12  shown in  FIG. 4 . The significant image is also displayed in overlap area  9  shown in  FIG. 4 . 
     If multiple viewpoint activator  8  is controllable to switch between an active state and an inactive state, then it can be used in combination with the first and second display modes. For example, if the display device is in the first display mode and brings multiple viewpoint activator  8  into the active state, the display device displays the images shown in  FIG. 3 . If the display device switches to the second display mode and brings multiple viewpoint activator  8  into the inactive state, then the display device displays an image shown in  FIG. 5 . Since multiple viewpoint activator  8  is in the inactive state, the display device provides single large view range  14 . Therefore, the viewer can recognize the displayed image without being conscious of the presence of multiple viewpoint activator  8 . 
     Multiple viewpoint activator  8  may comprise a parallax barrier or a lens array. If multiple viewpoint activator  8  has repetitive parallax barrier units or lenses arrayed in one direction across display panel  1 , then it provides a plurality of viewpoints along the direction of the array of repetitive parallax barrier units or lenses. For example, if repetitive parallax barrier units or lenses of multiple viewpoint activator  8  are arrayed in a horizontal direction across display panel  1 , then multiple viewpoint activator  8  provides a plurality of viewpoints along the horizontal direction. If multiple viewpoint activator  8  comprises repetitive parallax barrier units or lenses arrayed in two directions transverse to each other, e.g., a vertical direction and a horizontal direction, then multiple viewpoint activator  8  provides a plurality of viewpoints along each of the vertical and horizontal directions. 
     More specifically, multiple viewpoint activator  8  may comprise an array of voltage-variable lenses, i.e., an array of liquid-crystal lenses. The liquid-crystal lenses comprise liquid crystal cells each for performing a lens function when a voltage is applied thereto. Alternatively, multiple viewpoint activator  8  may comprise an array of variable lenses incorporating liquids. Each of the variable lenses comprises a cell combined with electrodes and filled with two liquid layers. When a voltage is applied between the electrodes, they generate an electric field which deforms the interface between the liquid layers. The interface between the liquid layers is deformed only when the voltage is applied between the electrodes. The lens function of each of the variable lenses is turned on and off by turning on and off the interface deformation with the electric field. 
     If multiple viewpoint activator  8  comprises an array of repetitive parallax barrier units in one direction or two directions, then multiple viewpoint activator  8  may be constructed of a liquid crystal layer and a repetitive array of voltage supplies. For example, multiple viewpoint activator  8  may include a twisted nematic liquid crystal layer inserted between a pair of orthogonal polarizers and combined with a repetitive array of voltage supplies. The twisted nematic liquid crystal layer operates in a normally white mode. When a voltage is periodically applied to the twisted nematic liquid crystal layer, it functions as a parallax barrier unit. When the voltage stops being applied to the twisted nematic liquid crystal layer, the parallax barrier units are brought into the inactive state. 
     If an array of lenses is used as multiple viewpoint activator  8 , then the layout of the pixel groups is changed to provide overlap areas of the view ranges, as described below with reference to  FIG. 6 . 
       FIG. 6  shows in transparent plan a structure comprising a lenticular lens array and two pixel groups. As shown in  FIG. 6 , lenticular lens array  15  is of a horizontally repetitive structure. Two pixel groups A, B of the display panel are of four different shapes in respective pixel layouts  17 ,  18 ,  19 ,  20  each. 
     According to pixel layout  19 , the pixels of pixel group A and the pixels of pixel group B are separated from each other by central line  16  of the lenticular lens. Pixel layout  19  does not provide an overlap area between the view range of pixel groups A, B. 
     According to pixel layouts  17 ,  18 , the pixels of pixel group A and the pixels of pixel group B are arranged in intricate patterns extending across central line  16  of the lenticular lens. In the view range provided by the left half of the lenticular lens, an image is displayed mainly by pixel group A, and is combined with certain image information from pixel group B. In the view range provided by the right half of the lenticular lens, an image is displayed mainly by pixel group B, and is combined with certain image information from pixel group A. Accordingly, pixel layouts  17 ,  18  provide a view range where the image displayed by pixel group A and the image displayed by pixel group B are combined with each other. 
     According to pixel layout  20 , in the view range provided by the left half of the lenticular lens, an image is displayed mainly by pixel group A, combined with certain image information from pixel group B. 
     By thus changing the pixel layouts, the shapes of the view ranges provided by the pixel groups can be changed to provide intricate view ranges. Since the intricate view ranges are of complex spatial shapes, they can be virtually regarded as overlap areas of the view ranges. In this manner, overlap areas of the view ranges can be produced when a lens array is used as multiple viewpoint activator  8 . 
     The display device according to the exemplary embodiment of the present invention may be incorporated in an electronic device.  FIG. 7  shows in schematic perspective display device  22  according to the exemplary embodiment which is incorporated in mobile phone unit  21 . In  FIG. 7 , two view ranges  5 ,  6  are produced by display device  22  and a multiple viewpoint activator, not shown. In the first display mode, the viewer can recognize a significant image only in overlap area  9  of view ranges  5 ,  6 . Mobile phone unit  21  includes a controller, not shown, for controlling display device  22  to switch between the first display mode and the second display mode in response to a certain control key action. When display device  22  is controlled to switch to the second display mode, the viewer can observe the same displayed image in view ranges  5 ,  6 . If the viewer does not want other people to see the displayed image on display device  22 , then the viewer controls display device  22  to display an image in the first display mode, so that the significant image can be recognized only in overlap area  9 . As a consequence, the image information displayed on display device  22  is made snooper-proof and hence highly confidential. 
     With the display device according to the exemplary embodiment, the size of the overlap area where the view ranges overlap each other in the first display mode does not depend on the size of the pixels. Therefore, the luminance of images displayed in the first display mode having the relatively narrow view range is maintained at a desired level. Furthermore, since the display device according to the exemplary embodiment provides the overlap area where the view ranges overlap each other, no non-display area (dark area) is created in the second display mode that has the relatively wide view range, thereby allowing the viewer to view images in a wide view range. Consequently, the display device according to the exemplary embodiment prevents the luminance of the displayed image from being lowered when an image is displayed in a narrow view angle, and also prevents a non-display area (dark area) from being created when an image is displayed in a wide view angle. 
     Display devices according various examples of the present invention will be described below. 
     1st Example 
     A display device according to a first example of the present invention will be described below with reference to  FIGS. 8 through 11 . 
     As shown in  FIG. 8 , the display device according to the first example comprises display panel  24  and parallax barrier  23  disposed on display panel  24 . Parallax barrier  23  serves as a multiple viewpoint activating means. Display panel  24  may comprise a liquid crystal display panel or a light emission display panel. Parallax barrier  24  comprises an alternate array of light-impermeable strips and light-permeable strips which are arranged such that each of the strips is oriented in the vertical direction of display panel  24 . 
     Display panel  24  comprises a plurality of pixel groups of two types, i.e., pixel groups A, B. Pixel groups A and Pixel groups B are alternately arranged in an array along the horizontal direction of display panel  24 . The layout of both pixel groups A, B is illustrated in  FIG. 8 . As shown in  FIG. 8 , only an image displayed by pixel groups B can be observed through parallax barrier  23  from viewpoint  25 , and only an image displayed by pixel groups A can be observed through parallax barrier  23  from viewpoint  26 . Therefore, the display device according to the first example provides a plurality of view ranges through parallax barrier  23  serving as the multiple viewpoint activating means. 
     As shown in  FIG. 9 , both the image displayed by pixel groups A and the image displayed by pixel groups B can simultaneously be observed from viewpoint  27  which is positioned intermediate between viewpoints  25 ,  26 . The view ranges provided by the display device thus have an overlap area with viewpoint  27  being disposed at the center of the overlap area. 
     In the first display mode, display panel  24  displays an image shown in  FIG. 10 . Specifically, an image representing a left half of the letter “A” is displayed by pixel group B in the left half of the display screen shown in  FIG. 10 , and an image representing a right half of the letter “A” is displayed by pixel group A in the right half of the display screen shown in  FIG. 10 . Therefore, the images representing the left and right halves, respectively, of the letter “A” are observed from the viewpoints  25 ,  26 , respectively, shown in  FIG. 8 . Since the image displayed by pixel groups A and the image displayed by pixel groups B can simultaneously be observed from viewpoint  27  shown in  FIG. 9 , the entire image of the letter “A” is observed from viewpoint  27 . The viewer can recognize the displayed image as a significant image only when observing the image from viewpoint  27 . 
     If the image information supplied to pixel groups A, B is changed and the display device switches to the second display mode, then an image shown in  FIG. 11  is displayed on display panel  24 . In the second display mode, both image groups A, B display the entire image of the letter “A” fully on the display screen. Therefore, the viewer can recognize the displayed image of the letter “A” on the display screen from both viewpoints  25 ,  26  shown in  FIG. 8 , and can also recognize the displayed image of the letter “A” on the display screen from viewpoint  27  shown in  FIG. 9 . 
     2nd Example 
     A display device according to a second example of the present invention will be described below with reference to  FIGS. 12 and 13 . 
     The display device according to the second example includes liquid crystal device  32  as a multiple viewpoint activating means. As shown in  FIG. 12 , the display device comprises display panel  24  and a parallax barrier disposed in front of display panel  24  and comprising liquid crystal device  32 . 
     The parallax barrier in the form of liquid crystal device  32  comprises first substrate  31   a  positioned opposite display panel  24  and second substrate  31   b  positioned opposite first substrate  31   a . Each of first and second substrates  31   a ,  31   b  is made of glass or plastic. Polarizer  28  is mounted on the surface of first substrate  31   a  which faces display panel  24 . Another polarizer  28  is mounted on the surface of second substrate  31   b  which is remote from first substrate  31   a . First transparent electrode  29   a  is mounted on the surface of first substrate  31   a  which faces second substrate  31   b . Second transparent electrode  29   b  is mounted on the surface of second substrate  31   b  which faces first substrate  31   a . First transparent electrode  29   a  comprises a plurality of strips associated respectively with pixel groups A, B of display panel  24 . Second transparent electrode  29   b  is disposed substantially fully over second substrate  31   b . Liquid crystal layer  30  is disposed between first substrate  31   a  and second substrate  31   b . Liquid crystal layer  30  can operate in a normally white liquid crystal mode. 
     By applying a voltage to striped transparent electrode  29   a  to bring liquid crystal device  32  into the active state, striped electrode  29   a  is turned into light-impermeable strips and light-permeable strips. Signals for displaying the images shown in  FIG. 10  are supplied to pixel groups A, B to operate the display device in the first display mode. 
     When no voltage is applied to transparent electrode  29   a  to bring liquid crystal device  32  into the inactive state, since liquid crystal device  32  becomes transparent, it no longer functions as the parallax barrier. Signals for displaying the images shown in  FIG. 11  are supplied to pixel groups A, B to operate the display device in the second display mode. Since no light-impermeable strips are produced, the transmittance of the liquid crystal device increases to increase the luminance of the displayed image. 
     The display device according to the second example should preferably be operated to change the display signals supplied to display panel  24  in synchronism with the switching between the display modes of liquid crystal device  32 . A control system for changing the display signals supplied to display panel  24  in synchronism with the switching between the display modes of liquid crystal device  32  will be described below with reference to  FIG. 13 . 
     As shown in  FIG. 13 , the display device includes display panel  24  and liquid crystal device  32  serving as the multiple viewpoint activating means. When the user operates keypad  33  to enter a display mode switching request signal, the display mode switching request signal is sent to device controller  34 . Device controller  34  sends the display mode switching request signal to liquid crystal controller  35  and display signal processor  36 . Liquid crystal controller  35  changes voltages applied to liquid crystal layer  30 . At the same time, display signal processor  36  changes display signals applied to pixel groups A, B. In this manner, the display signals supplied to display panel  24  are changed in synchronism with the switching between the display modes of liquid crystal device  32 . 
     In  FIG. 13 , liquid crystal device  32  is disposed in front of display panel  24 . However, display panel  24  may be disposed in front of liquid crystal device  32 . With display panel  24  disposed in front of liquid crystal device  32 , since display panel  24  directly faces the viewer, the viewer does not experience an unwanted reduction in the contrast due to light scattering and does not feel a so-called depth when viewing the displayed image. 
     3rd Example 
     A display device according to a third example of the present invention will be described below with reference to  FIGS. 14 through 16 . 
     The display device according to the third example includes a voltage-controllable lens array device as a multiple viewpoint activator. As shown in  FIG. 14 , the voltage-controllable lens array device comprises first substrate  31   a  and second substrate  31   b  disposed opposite first substrate  31   a . First transparent electrode  29   a  is mounted on the surface of first substrate  31   a  which faces second substrate  31   b . Second transparent electrode  29   b  is mounted on the surface of second substrate  31   b  which faces first substrate  31   a . First transparent electrode  29   a  is disposed substantially fully over first substrate  31   a . Second transparent electrode  29   b  comprises a plurality of strips. Two liquid layers, i.e., first liquid  37  and second liquid  38 , are sealed between first substrate  31   a  and second substrate  31   b . Liquids  37 ,  38  are insoluble into each other, and have different specific gravities, different dielectric constants, and different refractive indexes. Therefore, first liquid  37  and second liquid  38  are separate as two different layers in the voltage-controllable lens array device. 
     When a voltage is applied between first and second transparent electrodes  29   a ,  29   b , the interface between the two layers of first liquid  37  and second liquid  38  is modulated as shown in  FIG. 15  because first liquid  37  and second liquid  38  have different dielectric constants. The modulated interface functions as a lens. When no voltage is applied between first and second transparent electrodes  29   a ,  29   b , the interface returns to the state shown in FIG.  14 . The lens array device is thus controlled by the voltage applied between first and second transparent electrodes  29   a ,  29   b.    
     The display device according to the third example which incorporates the lens array device described above will be described in detail below with reference to  FIG. 16 . The display device comprises a display panel and a voltage-controllable lens array device as the multiple viewpoint activator placed on the display panel.  FIG. 16  is a transparent plan view that shows the display panel and the voltage-controllable lens array device. The display panel includes pixel groups A and pixel groups B, and the pixels of pixel group A and the pixels of pixel group B are arranged in intricate patterns extending across the central lines of the lenses to provide overlap areas of view ranges. Second transparent electrode  29   b  shown in  FIG. 14  comprises a plurality of strips aligned with columns of the pixels of pixel groups A, B. When a voltage is applied to the lens array device to perform a lens function thereof, the display device produces view ranges corresponding to pixel groups  39  (A) and pixel groups  40  (B). 
     In the first display mode, a voltage is applied between transparent electrodes  29   a ,  29   b  to bring the lens array device into the active state, and the display panel displays the image shown in  FIG. 10 . Therefore, when the viewer sees the image only in the view range of pixel group A or the image only in the view range of pixel group B, the viewer fails to observe a significant image. The viewer can observe a significant image only when the viewer sees the image in the overlap area of the view ranges. 
     In the second display mode, no voltage is applied between transparent electrodes  29   a ,  29   b  to bring the lens array device into the inactive state, and the display panel displays the image shown in  FIG. 11 . The viewer now can observe a significant image in the view range of pixel group A or the view range of pixel group B, and a significant image in the overlap area of the view ranges. 
     In the display device according to the third example, the display signals and the display modes can be changed by the control system shown in  FIG. 13  with liquid crystal controller  35  shown in  FIG. 13  functioning as a lens array controller. 
     4th Example 
     A display device according to a fourth example of the present invention will be described below with reference to  FIG. 17 . 
     The display device according to the fourth example and a control system thereof are identical to those shown in  FIG. 13 .  FIG. 17  shows only details of display controller  36  and display panel  24 . In  FIG. 17 , the display device displays text data. 
     Operation of display controller  36  in first display controller  36  will first be described below. As shown in  FIG. 17 , text signal  41  is supplied to display controller  36 . Display controller  36  includes pixel group A converter  42  and pixel group B converter  43 . Each of pixel group A converter  42  and pixel group B converter  43  has a conversion table for converting the text represented by text signal  41 . Pixel group A converter  42  and pixel group B converter  43  convert the characters of the text according to the conversion tables, and send the converted characters to pixel groups A, B. As shown in  FIG. 17 , the converted characters can be recognized as normal characters only when the converted characters from the conversion tables are combined with each other. Only those characters from one of the conversion tables cannot be recognized as significant characters. In the first display mode, liquid crystal device  32  is turned on to function as the parallax barrier. In the first display mode, therefore, the viewer is unable to recognize significant characters by seeing the displayed characters only in the view range of pixel group A or only in the view range of pixel group B, and can recognize significant characters by only seeing the displayed characters in the overlap area of the view ranges. 
     In the second display mode, pixel group A converter  42  and pixel group B converter  43  are inactivated. As a result, the characters of the text represented by text signal  41  are not converted, and both pixel groups A, B can display significant text data by themselves. In the second display mode, liquid crystal device  32  is turned off so as not to function as the parallax barrier. Therefore, the viewer can recognize significant characters by seeing the displayed characters in the view range of pixel group A or in the view range of pixel group B, and can also recognize significant characters by seeing the displayed characters in the overlap area of the view ranges. 
     5th Example 
     A display device according to a fifth example of the present invention will be described below with reference to  FIG. 18 . 
     The display device according to the fifth example and a control system thereof are identical to those shown in  FIG. 13 .  FIG. 18  shows only details of display controller  36  and display panel  24 . In  FIG. 18 , the display device displays images. 
       FIG. 18  illustrates the manner in which display controller  36  operates in the first display mode. In the fifth example, image signal  44  representing an image to be displayed by display panel  24  is sent to display controller  36 . Display controller  36  processes image signal  44  as follows: First, random mask generator  45  generates a binary mask image having as many pixels as the number of pixels of the image represented by image signal  44 . In  FIG. 18 , random mask generator  45  generates a binary mask image including a plurality of black boxes in a transparent background (positive image). The size and number of the black boxes can be selected according to an optional probability distribution, and can be adjusted depending on the size of the image to be displayed and the image signal supplied to display controller  36 . The mask image generated by random mask generator  45  is inverted into an inverted mask image by image inverter  46 . The inverted mask image includes a plurality of transparent boxes in a black background (negative image). Multiplier  47  multiplies the mask image from random mask generator  45  by image signal  44 , and sends the product image signal to pixel groups A of display panel  24 . Another multiplier  47  multiplies the inverted mask image from image inverter  46  by image signal  44 , and sends the product image signal to pixel groups B of display panel  24 . 
     Pixel groups A display an image multiplied by the mask image, and pixel groups B display an image multiplied by the inverted mask image. The viewer is unable to recognize a significant image by seeing the individual images displayed by respective pixel groups A, B, and can recognize a significant image only by seeing the image displayed in the overlap area of the view ranges of pixel groups A, B. Random mask generator  45  can change the mask image every several seconds. As a result, the images displayed by pixel groups A, B are changed every several seconds to prevent the individual images displayed by pixel groups A, B from being recognized by snoopers. 
     In the second display mode, random mask generator  45  and image inverter  46  stop operating. Therefore, images that are not multiplied by the mask image are displayed by pixel groups A, B. The viewer can observe a significant image even in each of the view ranges of pixel groups A, B. 
     6th Example 
     A display device according to a sixth example of the present invention will be described below with reference to  FIG. 19 . 
     The display device according to the sixth example and a control system thereof are identical to those shown in  FIG. 13 .  FIG. 19  shows only details of display controller  36  and display panel  24 . In  FIG. 19 , the display device displays images. 
       FIG. 19  illustrates the manner in which display controller  36  operates in the first display mode. In the sixth example, image signal  44  to be sent to pixel groups A is multiplied by a mask image including a plurality of black boxes in a transparent background, and image signal  44  to be sent to pixel groups B is multiplied by an inverted mask image including a plurality of transparent boxes in a black background, as with the fifth example. 
     According to the sixth example, the mask image and the inverted mask image are generated as follows: Feature point extractor  49  extracts feature points of the image represented by image signal  44 . Feature points can be extracted according to a known process. For example, after the image represented by image signal  44  is binarized, joining points thereof are extracted, and feature groups are determined. The representative size of an image and its position can be known from the feature groups. A maximum one of the extracted feature points is shown in  FIG. 19 . The image signal representative of the extracted feature point is sent to mask generator  48 . Mask generator  48  divides the image of the maximum feature point into four segments, and uses one-half of the four segments as a mask image. Multiplier  47  multiplies the mask image by image signal  44 , and sends the product image signal to pixel groups A of display panel  24 . The mask image is sent to image inverter  46 , which generates an inverted mask representing an inversion of the mask image. Another multiplier  47  multiplies the inverted mask image by image signal  44 , and sends the product image signal to pixel groups B of display panel  24 . Pixel groups A display an image multiplied by the mask image, and pixel groups B display an image multiplied by the inverted mask image. The viewer is unable to recognize a significant image by seeing the individual images displayed by respective pixel groups A, B, and can recognize a significant image only by seeing the image displayed in the overlap area of the view ranges of pixel groups A, B. 
     As described above, a mask image and an inverted mask image can be generated depending on the image signal. As with the fifth example, mask generator  48  can change the mask image in every several seconds. If the mask image is changed in every several seconds, then the parameters of feature point extractor  49  and mask generator  48  are changed at certain intervals of time. 
     In the second display mode, mask generator  48  and image inverter  46  stop operating as with the fifth example, and only image signal  44  is sent to pixel groups A, B. The viewer can observe a significant image even in each of the view ranges of pixel groups A, B. 
     The above processing sequence can be performed by dedicated hardware provided in display controller  36  or by suitable software installed in the processing unit of display controller  36 .