Abstract:
The liquid crystal lens is formed by placing a TN type liquid crystal with a twist angle of 90 degrees between the first and second substrates. A first substrate having a flat surface with a slit is formed on the liquid crystal side of the first substrate, and a second electrode having a comb electrode, as seen in a plane view, is formed on the liquid crystal side of the second substrate. The slit formed in the first electrode extends in the same direction as the comb electrode of the second electrode, and the slit is located in the center between the comb electrodes of the second electrodes as seen in a plane view, to prevent the electric lines of force, moving from directly above the second electrode to the first electrode, from spreading in the plan direction, thereby preventing light leakage directly above the second electrode.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese Patent Application JP 2013-111015 filed on May 27, 2013, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND 
     The present invention relates to a display device, and more particularly, to a 3D display device including a liquid crystal lens having a lens function on the display surface side of a liquid crystal display panel. 
     A display device that can switch between three-dimensional (3D) display and two-dimensional (2D) display with naked eyes without glasses includes, for example, a first liquid crystal display panel for performing image display, and a second liquid crystal display panel provided on the display surface side (observer side) of the first liquid crystal panel to form a parallax barrier that allows light to be separately incident on the right and left eyes of the observer in 3D display. In such a display device that can switch between 2D display and 3D display, the refractive index in the second liquid crystal display panel is changed by controlling the alignment of the liquid crystal molecules in the second liquid crystal display panel, to form lens (lenticular lens, cylindrical lens array) areas extending in the vertical direction of the display surface and arranged side by side in the lateral direction, in order to direct the light of the pixels corresponding to the left and right eyes into the observer&#39;s eye. 
     With respect to the 3D display device of the liquid crystal lens type having such a structure, for example, an auto-stereoscopic display device is described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-520231. In the display device described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-520231, a planar electrode is formed on one of two transparent substrates facing each other with a liquid crystal layer interposed therebetween. Then, a strip-like electrode (linear electrode) extending in the lens formation direction is formed on the other transparent substrate. The linear electrode is arranged side by side in the lens arrangement direction. With this configuration, the switching control between 2D display and 3D display can be achieved by adjusting the refractive index of liquid crystal molecules by controlling the voltage applied to the strip-like electrode and the voltage applied to the planar electrode. Further, the liquid crystal lens described in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-520231 is a liquid crystal lens of TN alignment. 
     Japanese Patent No. 2862462 descries a structure in which an optical characteristic variable lens is provided between the electrodes on the liquid crystal display panel, to form a 3D image by controlling the lens characteristics by applying a voltage to the electrodes between which the optical characteristic variable lens is placed. 
       FIG. 10  is a cross-sectional view of the structure of a conventional liquid crystal lens. In  FIG. 10 , a first electrode  11  is formed in a planar solid-state inside a first substrate  10  which is a transparent substrate, and a first alignment film  12  is formed on the first electrode  11 . Then, a second electrode  21  having a strip-like (comb-like) shape is formed inside a second substrate  20  which is a transparent substrate. A second alignment film  22  is formed so as to cover the second electrode  21  formed in the second substrate  20 . The alignment direction of the first alignment film  12  and the alignment direction of the second alignment film  22  are the same. The first substrate  10  and the second substrate  20  are preferably glass substrates, but may also be transparent plastic substrates. A liquid crystal layer  60  is provided between the first substrate  10  and the second substrate  20 . 
     The electrode width of the comb electrode formed in the second substrate  20  is w 2 , the pitch between the comb electrodes is Q, and the comb electrode interval is s. The distance between the first substrate  10  and the second substrate  20 , namely, the thickness of the liquid crystal layer is d. The liquid crystal has positive dielectric constant anisotropy. In a 3D image display device using a liquid crystal lens, it is possible to display a 3D image by applying a voltage between the first electrode  11  and the second electrode  21 , and to display a 2D image when no voltage is applied between the first electrode  11  and the second electrode  21 . 
       FIG. 11  is a cross-sectional view showing the principle of the 3D image formation using a liquid crystal lens. In  FIG. 11 , human eyes view the image formed on the display device through the liquid crystal lens. In  FIG. 11 , the image for the right eye is R, and the image for the left eye is L. In  FIG. 11 , the pitch of a liquid crystal lens  100  is Q, and the pixel pitch of a display device  200  is P. Further, the distance between the centers of the human left and right eyes, namely, the interocular distance is B. In general, the interocular distance B is assumed to be 65 mm. The relationship between the pitch Q of the liquid crystal lens and the pixel pitch P of the display device is as shown in the equation 1. 
     
       
         
           
             
               
                 
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       FIG. 12  is a schematic cross-sectional view of the 3D image display device using the liquid crystal lens  100  to which the present invention is directed. In  FIG. 12 , the liquid crystal lens  100  and the display device  200  are bonded with an adhesive  300 . The adhesive  300  is transparent and, for example, a UV (ultraviolet) curing resin is used. A liquid crystal display device or an organic EL display device is used for the display device  200 . 
       FIGS. 13A and 13B  are plan views of the liquid crystal display lens corresponding to B-B′ in  FIG. 12 . In  FIGS. 13A and 13B , the entire display area of the first substrate  10  is covered by the first electrode  11 . The second electrode  21  having a comb-like shape is formed in the second substrate  20 . The second electrode  21  is connected by a bus electrode at an end thereof. Here,  FIG. 10  is a cross-sectional view corresponding to the A-A′ cross section in  FIGS. 13A and 13B . 
       FIGS. 14A, 14B, and 14C  are cross-sectional views showing the principle of the liquid crystal lens. When a voltage is applied between the first electrode  11  and the second electrode  21 , electric lines of force F are generated as shown in  FIG. 14A . If no voltage is applied between the first electrode  11  and the second electrode  21 , the liquid crystal is horizontally aligned as shown in  FIG. 14B . Note that in the drawings of the present application, the pretilt angle is ignored to avoid complications. 
     When a voltage is applied between the first electrode  11  and the second electrode  21 , liquid crystal molecules  61  above the second electrode  21  rise up, and are horizontally aligned between the comb electrodes as shown in  FIG. 14C . This results in a distribution in the refractive index, and a gradient index (GRIN) lens is formed. 
     A conventional common liquid crystal lens is shown in  FIGS. 10 to 14C . In the liquid crystal lens having such a structure, disclination appears above the comb electrodes. Thus, there is a problem that crosstalk increases as the incident light is scattered in the upper part of the electrodes. Here, disclination is a discontinuous line due to the alignment of liquid crystal molecules, and crosstalk is a phenomenon in which the left eye image and the right eye image are not sufficiently separated. If the crosstalk is large, the 3D image is seen just as a double image. 
     On the other hand, as shown in  FIGS. 15A and 15B , the alignment of the liquid crystal molecules in the liquid crystal lens is converted into TN alignment. In this case, if a polarizing plate  13  is provided on the side opposite to the side of the liquid crystal of the second substrate  20 , the crosstalk due to the disclination may be reduced. At this time, TN is 90-degree twisted alignment. In other words, in  FIG. 15A , the alignment direction of the first alignment film (not shown) formed in the first substrate  10 , and the alignment direction of the second alignment film (not shown) formed in the second substrate  20  are 90 degrees. The mechanism will be described below. 
       FIG. 15A  shows the state in which no voltage is applied between the first electrode  11  and the second electrode  21 . At this time, the image from the display device is not affected by the liquid crystal lens.  FIG. 15B  shows the state in which a voltage is applied between the first electrode  11  and the second electrode  21 . The liquid crystal molecules are aligned so that a lens is formed between the comb electrodes which are the second electrodes  21 . Meanwhile, the electric lines of force F are directed in the perpendicular direction to the second electrode  21 , so that the liquid crystal molecules  61  are also perpendicularly aligned. In other words, the light from the display device does not transmit in this portion. As a result, it is possible to prevent the crosstalk. 
     In  FIGS. 15A and 15B , it is desirable that the transmission axis of the polarizing plate  13  is tilted approximately 90 degrees with respect to the polarization direction of the light output from the display device. If the display device is a liquid crystal display device, the output light is polarized light. However, if the display device is an organic EL display device, it is necessary to attach the polarizing plate on the surface of the organic EL display device. 
       FIG. 16  is a cross-sectional view showing the details of this state.  FIG. 16  is a cross-sectional view showing the relationship between the polarization direction of the incident light and the polarization direction of the output light, with respect to the transmission axis of the first polarizing plate  13 , when no voltage is applied between the first electrode  11  and the second electrode  21 . In  FIG. 16 , when the liquid crystal lens has TN alignment in the initial alignment, the incident polarized light is rotated at an angle of 90 degrees within the liquid crystal layer when no voltage is applied. Thus, if the input polarization direction is the X axis direction, the output polarization direction is the Y axis direction. The incident light is transmitted if a polarization transmission axis PA of the first polarizing plate is in the Y axis direction. In the 2D display in which no voltage is applied between the first electrode  11  and the second electrode  21 , the liquid crystal lens has no influence on the output light from the display device. 
     On the other hand, when a voltage is applied to the liquid crystal lens of TN alignment, the alignment of the liquid crystal molecules  61  is as shown in  FIG. 15B . As can be seen in  FIG. 15B , the liquid crystal molecules  61  rise up above the second electrode  21 , so that the optical rotation is lost. However, in the vicinity of the center between the second electrodes  21  which are the comb electrodes, the alignment of the liquid crystal molecules  61  is not substantially changed from the initial alignment. As a result, optical rotation occurs and the incident light polarization axis is rotated by 90 degrees. Thus, although the light is shielded above the second electrode  21 , the light transmits between the second electrodes  21 . The conventional liquid crystal lens has had a problem that disclination appears above the second electrode  21 , causing crosstalk to increase due to the scattering of the light. However, this problem may be solved by the configuration shown in  FIGS. 15A and 15B . 
     Thus, a liquid crystal lens of TN alignment was formed by the following parameters:
     Liquid crystal physical property . . . Δn=0.2   Liquid crystal gap d: 30 μm   Panel size: 3.2″   Number of pixels . . . 480×854   Pixel pitch P: 79.5 μm   Lens pitch Q: 158.8058 μm   Electrode width: 10 μm   

     However, in the liquid crystal lens described above, the ratio between the liquid crystal gap d 1  and the electrode width w 2 , (d/w 2 ), is as large as  3 , so that the electric field is extended in the substrate in-plane direction. From this it is found that sufficient vertical electric filed is not generated. As a result, the light-shielding effect may not be obtained sufficiently above the second electrode  21 . 
       FIGS. 18A and 18B  are an example of the transmission distribution of the liquid crystal lens of TN alignment. In  FIGS. 18A and 18B , the horizontal axis is the position and the vertical axis is the transmission. In the ideal transmission distribution shown in  FIG. 18A , the transmission is approximately zero in the vicinity of the second electrode  21 . However, in the actual sample, the transmission is not sufficiently reduced in the vicinity of the second electrode  21  as shown in  FIG. 18B , for the reasons described above. In other words, the desired light-shielding effect may not be obtained. 
     In a common TN type liquid crystal display device, the liquid crystal gap is about 4 μm, while the electrode width is several tens to hundreds of μm. In other words, the ratio between the gap and the electrode width is very small. 
     SUMMARY 
     The problem to be solved by the present invention is to sufficiently reduce the transmission above the second electrode in the liquid crystal lens of TN alignment, to prevent the appearance of disclination to prevent the crosstalk. 
     The principal solutions of the present invention for this problem are as follows. 
     (1) There is provided a display device with a liquid crystal lens disposed on a display panel. The liquid crystal lens includes a TN type liquid crystal with a twist angle of 90 degrees between a first substrate and a second substrate. A first electrode having a flat surface with a slit is formed on the liquid crystal side of the first substrate. A second electrode having a comb electrode, as seen in a plan view, is formed on the liquid crystal side of the second substrate. The slit formed in the first substrate extends in the same direction as the comb electrode of the second electrode. The slit is located at the center between the comb electrodes of the second electrodes as seen in a plan view. 
     (2) In the display device described in (1), a film of a material with a dielectric constant greater than the dielectric constant of the liquid crystal is formed in a portion of the first electrode corresponding to the comb electrode of the second electrode as seen in a plan view. 
     (3) In the display device described in (1), a film of a material with a dielectric constant smaller than the dielectric constant of the liquid crystal is formed so as to cover the slit in the first substrate between the comb electrodes of the second electrodes as seen in a plan view. 
     (4) There is provided a display device with a liquid crystal lens disposed on a display panel. The liquid crystal lens is formed by placing a TN type liquid crystal with a twist angle of 90 degrees between a first substrate and a second substrate. A first electrode is formed in a planar solid-state on the liquid crystal side of the first substrate. A second electrode with a comb-like shape as seen in a plan view is formed on the liquid crystal side of the second substrate. A film of a material with a dielectric constant greater than the dielectric constant of the liquid crystal is formed in a portion of the first electrode corresponding to the comb electrode of the second electrode as seen in a plan view. 
     (5) There is provided a display device with a liquid crystal lens disposed on a display panel. The liquid crystal lens is formed by placing a TN type liquid crystal with a twist angle of 90 degrees between a first substrate and a second substrate. A first electrode is formed in a planar solid-state on the liquid crystal side of the first substrate. A second electrode with a comb-like shape, as seen in a plan view, is formed on the liquid crystal side of the second substrate. A film of a material with a dielectric constant smaller than the dielectric constant of the liquid crystal is formed on the first substrate between the comb electrodes of the second electrodes as seen in a plan view. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a liquid crystal lens according to a first embodiment of the present invention; 
         FIGS. 2A and 2B  are plan views of first and second electrodes according to the first embodiment of the present invention; 
         FIG. 3  is a cross-sectional view of the first embodiment of the present invention, in which the position is changed; 
         FIG. 4  is a graph showing the relationship between the ratio between the first electrode width and the second electrode width, and the transmission directly above the second electrode with cell gap as a parameter, according to the first embodiment; 
         FIG. 5  is a graph showing the relationship between the cell gap and the ratio between the first electrode width and the second electrode width in which the transmission is zero above the second electrode; 
         FIG. 6  is a graph showing the relationship between the cell gap and the ratio between the first electrode width and the second electrode width in which the transmission is started to decrease above the second electrode; 
         FIG. 7  is a cross-sectional view of a liquid crystal lens according to a second embodiment of the present invention; 
         FIG. 8  is a cross-sectional view of a liquid crystal lens according to a third embodiment of the present invention; 
         FIG. 9  is a cross-sectional view of a liquid crystal lent according to a fourth embodiment of the present invention; 
         FIG. 10  is a cross-sectional view of the structure of a conventional liquid crystal lens; 
         FIG. 11  is a schematic view showing the principle of 3D display using a liquid crystal lens; 
         FIG. 12  is a cross-sectional view of the structure of a 3D image display device using a liquid crystal lens; 
         FIGS. 13A and 13B  are plan views of the first and second electrodes in a liquid crystal lens; 
         FIGS. 14A, 14B, and 14C  are an example of the alignment of the liquid crystal molecules in the conventional liquid crystal lens; 
         FIGS. 15A and 15B  are cross-sectional views of the behavior of the liquid crystal lens of TN alignment; 
         FIG. 16  is a schematic view showing the operation when no voltage is applied in the liquid crystal lens of TN alignment; 
         FIG. 17  is a schematic view showing the operation when a voltage is applied in the liquid crystal lens of TN alignment; and 
         FIGS. 18A and 18B  are comparative views of an ideal transmission distribution and the actual transmission distribution in the conventional example, in the liquid crystal lens of TN alignment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter the present invention will be described in detail with reference to preferred embodiments. Note that parameters described in the following embodiments are based on the structure of the liquid crystal lens formed in the Background of the invention. 
     First Embodiment 
       FIG. 1  is a cross-sectional view of a first embodiment of the present invention.  FIG. 1  is different from  FIGS. 15A and 15B  in that a slit  15  is formed in a first electrode  11  in the center of a lens, namely, in the center between a second electrode  21  and a second electrode  21  as seen in a plan view. The slit  15  extends in the direction perpendicular to the paper. Both the first electrode  11  and the second electrode  21  are formed by a transparent conductive film such as ITO. Hereinafter, the gap between the second electrode  21  and the second electrode  21  means the gap between comb electrodes of the second electrodes  21 . Note that the second electrode  21  is the same meaning as the comb electrode of the second electrode  21 . 
       FIG. 2A  is a plan view of the first electrode  11  in the first substrate  10 . The slit  15  is formed within the first electrode  11 .  FIG. 2B  is a plan view of the second electrode  21  in the second substrate  20 . The shape of the second electrode  21  is the same as  FIG. 13B  which is the conventional example. The extending direction of the slit  15  formed in the first electrode  11  in  FIG. 2A  and the extending direction of the comb electrode of the second electrode  21  in  FIG. 2  are the same.  FIG. 1  corresponds to the C-C′ cross section in  FIGS. 2A and 2B . 
       FIG. 1  is a schematic view showing the state in which no voltage is applied between the first electrode  11  and the second electrode  21 . In  FIG. 1 , when a voltage is applied between the first electrode  11  and the second electrode  21 , it is possible to prevent the electric lines of force, which are directed from directly above the second electrode  21  to the first electrode  11 , from spreading in the plane in the vicinity of the first electrode  11  by the influence of the slit  15  formed in the first electrode  11 . Thus, it is possible to prevent the light from the backlight from transmitting directly above the second electrode  21 . 
     Note that in the liquid crystal lens, the liquid crystal is hardly affected by the electric field in the vicinity of the slit  15 , so that the optical rotation of the liquid crystal is maintained without the use of the electrode, which has little influence on the characteristics of the liquid crystal lens. 
       FIG. 3  is a cross-sectional view in which the position is changed from  FIG. 1  in the liquid crystal lens according to the first embodiment. In  FIG. 3 , the first electrode width is w 1 , the second electrode width is w 2 , and the cell gap is d. The pitch Q between the second electrodes in  FIG. 3 , namely, the lens pitch is the same as the dimension of the sample described above: Q=158.8058 μm. 
       FIG. 4  is a graph showing the ratio w 1 /w 2  between the width of the first electrode  11 , w 1 , and the width of the second electrode  21 , w 2 , with respect to the transmission directly above the second electrode, when the cell gap d is set to 15 μm, 20 μm, and 30 μm. In  FIG. 4 , the horizontal axis is the ratio w 1 /w 2  between the first electrode width w 1  and the second electrode width w 2 , and the vertical axis is the transmission directly above the second electrode. In  FIG. 4 , when the value of w 1 /w 2  is below a certain value, the transmission directly above the second electrode is zero. This is because the spread of the electric lines of force over the plane in the vicinity of the first substrate is reduced as w 1 /w 2  decreases, and the optical rotation of the liquid crystal does not occur in this area. The ratio w 1 /w 2 , in which the transmission is actually zero, varies depending on the cell gap. 
       FIG. 5  is a graph showing the relationship between the cell gap and w 1 /w 2  in which the transmission is zero. In  FIG. 5 , the horizontal axis is the cell gap d, and the vertical axis is w 1 /w 2  in which the transmission is zero. The grater the cell gap d is, the greater the ratio w 1 /w 2  in which the transmission is zero. 
     Here, when w 1 /w 2  in which the transmission is zero is represented by y and d is represented by x, the curve shown in  FIG. 5  can be approximated by y=3.30391n(x)−6.2973. 
       FIG. 6  is a graph showing the relationship between the cell gap d and w 1 /w 2 in which the transmission is started to decrease in  FIG. 4 . In  FIG. 6 , the horizontal axis is the cell gap d, and the vertical axis w 1 /w 2  in which the transmission is started to decrease directly above the second electrode. The greater the cell gap d is, the grater the ratio w 1 /w 2  in which the transmission is started to decrease. 
     Here, when w 1 /w 2  in which the transmission is zero is represented by y and d is represented x, the curve shown in  FIG. 5  can be approximated by y=12.2671n(x)−25.732. 
     Second Embodiment 
       FIG. 7  is a cross-sectional view of a liquid crystal lens according to a second embodiment of the present invention.  FIG. 7  is different from the conventional example of  FIG. 15  in that a high dielectric constant film  16  of a material with a dielectric constant greater than the dielectric constant of the liquid crystal is provided in the portion of the first electrode  11  corresponding to the second electrode  21 . Because the high dielectric constant film  16  formed in the first substrate  10  is present, the electric lines of force from directly above the second electrode  21  are collected on the high dielectric constant film  16 . Thus, it is possible to prevent the electric lines of force from spreading in the plan direction in the vicinity of the first substrate  10 . As a result, it is possible to prevent the light from rotating directly above the second electrode, and prevent the light from the backlight from transmitting directly above the second electrode. 
     The liquid crystal molecule has a bar-like shape, in which the dielectric constant is different in the short diameter direction and the long diameter direction. In this case, the dielectric constant of the liquid crystal is the average in the short diameter direction and in the long diameter direction, and for example, 10. Thus, the dielectric constant of the high dielectric constant film is preferably 10 or more. 
     As described above, also in the present embodiment, it is possible to prevent the transmission directly above the second electrode, and prevent crosstalk due to disclination. 
     Third Embodiment 
       FIG. 8  is a cross-sectional view of a liquid crystal lens according to a third embodiment of the present invention.  FIG. 8  is different from  FIG. 15 , which is the conventional example, in that a low dielectric constant film  17  of a material with a dielectric constant lower than the dielectric constant of the liquid crystal is formed in the portion of the first electrode  11  corresponding between the second electrodes  21 , as seen in a plan view. Because the low dielectric constant film  17  formed in the first substrate  10  is present, the electric lines of force from directly above the second electrode  21  are distributed so as to avoid the low dielectric constant film  17 . Thus, it is possible to prevent the electric lines of force from spreading in the plan direction in the vicinity of the first substrate  10 . As a result, it is possible to prevent the light from rotating directly above the second electrode, and prevent the light from the backlight from transmitting directly above the second electrode. 
     As described above, the dielectric constant of the liquid crystal is, for example, about 10, so that the dielectric constant of the low dielectric constant film  17  is preferably 10 or less. Such a low dielectric constant material can be formed by a photosensitive resin such as epoxy resin or silicone resin. 
     As described above, also in the present embodiment, it is possible to prevent the transmission directly above the second electrode, and prevent crosstalk due to disclination. 
     Fourth Embodiment 
       FIG. 9  is a cross-sectional view of a liquid crystal lens according to a fourth embodiment of the present invention.  FIG. 9  is a combination of the structure in which the slit  15  is formed in the first electrode  11  according to the first embodiment, and the structure in which the high dielectric constant film  16  is formed in the portion of the first substrate  10  corresponding to the second electrode  21  according to the second embodiment. 
     As described in the first embodiment, the electric lines of force from directly above the second electrode  21  is prevented from spreading in the plan direction in the vicinity of the first substrate  10  by the influence of the slit  15  formed in the first electrode  11 . In addition, as described in the second embodiment, due to the presence of the high dielectric constant film  16  formed in the first electrode  11 , the electric lines of force generated from directly above the second electrode are collected by the high dielectric constant film  16 . Thus, it is possible to prevent the electric lines of force from further spreading in the plan direction in the vicinity of the first substrate  10 . In this way, the present embodiment has the effect of both the first and second embodiments. As a result, it is possible to further reduce the transmission of the light from the backlight directly above the second electrode. 
     Although not shown, another aspect of the present embodiment is a structure in which the slit  15  is formed in the first electrode  11  as described in the first embodiment, and the low dielectric constant film  17  of a low dielectric constant material in the third embodiment is formed in the first substrate  10  so as to cover the slit  15 . In this way, it is possible to form a liquid crystal lens having the effect of both the first and third embodiments. In this case, the low dielectric constant film  17  is formed in the first substrate  10  between the second electrodes  21 , as seen in a plan view.