Patent Publication Number: US-10317612-B2

Title: Display panel unit and display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of application Ser. No. 15/643,013, filed Jul. 6, 2017, which is a Continuation of application Ser. No. 14/509,425, filed Oct. 8, 2014, now U.S. Pat. No. 9,733,415, issued Aug. 15, 2017, and is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-217478, filed Oct. 18, 2013, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a display panel unit and a display device. 
     BACKGROUND 
     A display device having a prism sheet concentrating a spreading angle of light from a light source upon a certain range and a reflective polarizer facing the prism sheet is used conventionally. When the light passes through the prism sheet, the reflective polarizer transmits, for example, the p-polarized component of the light to a display panel (such as a liquid crystal panel) side and reflects the s-polarized component of the light onto a light source side. 
     The s-polarized component reflected by the reflective polarizer is again reflected by, for example, an optical element such as a reflective plate provided on the light source side, and reaches back the reflective polarizer. Such a reflection process is repeated, and the s-polarized component converts into the p-polarized component by-and-by and passes through the reflective polarizer. When the reflective polarizer is used, the utilization efficiency of the light from the light source increases and thus, the brightness of the display device increases. 
     In a display device with such a structure, interference fringes may occur between the prism sheet and the reflective polarizer. The interference fringe is one of the causes of display quality deterioration in the display device. Therefore, reduction or prevention of the occurrence of interference fringes is demanded. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view schematically showing the structure of a liquid crystal display device of an embodiment. 
         FIG. 2  is a cross-sectional view showing the structure of a reflective polarizing plate of the embodiment. 
         FIG. 3  is a schematic diagram showing an example of now the liquid crystal display device is used. 
         FIG. 4  is a table explaining structures of the liquid crystal display device of the embodiment and their evaluation results. 
         FIG. 5  is a graph showing grades of interference fringes occurring in the liquid crystal display device of the embodiment. 
         FIG. 6  is a table indicating some comparative examples used to avoid interference fringes by methods different from the embodiment. 
         FIG. 7  is a cross-sectional view schematically showing the structure of a conventionally-used liquid crystal display device. 
         FIG. 8  is a view schematically showing an example of interference fringes appearing on a display surface of a liquid crystal display device. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a display panel unit includes a reflective polarizing layer, a transparent layer, and a display panel. The reflective polarizing layer includes an incident surface of light and an exit surface opposed to the incident surface, and the reflective polarizing layer transmits a first polarized component of the light incident on the incident surface to the exit surface and reflects a second polarized component of the light incident on the incident surface which is orthogonal to the first polarized component. The transparent layer includes a first surface adhered to the incident surface of the reflective polarizing layer and a second surface opposed to the first surface, and the transparent layer transmits the first polarized component and the second polarized component of the light incident on the second surface to the first surface and transmits the second polarized component reflected by the reflective polarizing layer to be incident on the first surface to the second surface. The display panel faces the exit surface of the reflective polarizing layer and selectively transmits the light exiting from the exit surface. 
     According to another embodiment, a display device includes a light source, a light guide, a reflective polarizing layer, a transparent layer, a prism sheet, and a display panel. The light guide includes a first incident surface on which light from the light source is incident and a first exit surface from which the light incident on the first incident surface exits. The reflective polarizing layer includes a second incident surface on which the light from the first exit surface is incident and a second exit surface opposed to the second incident surface, and the reflective polarizing layer transmits a first polarized component of the light incident on the second incident surface to the second exit surface and reflects a second polarized component of the light incident on the second incident surface which is orthogonal to the first polarized component. The transparent layer includes a first surface adhered to the second incident surface of the reflective polarizing layer and a second surface opposed to the first surface, and the transparent layer transmits the first polarized component and the second polarized component of the light incident on the second surface to the first surface and transmits the second polarized component reflected by the reflective polarizing layer to be incident on the first surface to the second surface. The prism sheet is interposed between the light guide and the transparent layer, and the prism sheet includes a prism surface on which prism lenses are formed and which faces the second surface. The display panel faces the second exit surface of the reflective polarizing layer, and the display panel selectively transmits the light exiting from the second exit surface. 
     One of the embodiments will be explained with reference to the accompanying drawings. 
     The present embodiment utilizes a transmissive type liquid crystal display device as an example of a display device. 
     Initially, explained are the structure of a liquid crystal display device used generally and interference fringes occurring in the liquid crystal display device. 
       FIG. 7  is a cross-sectional view showing the schematic structure of a generally-used liquid crystal display device  100 . The liquid crystal display device  100  includes a backlight unit  110  and a display panel unit  130 . The dotted arrows in  FIG. 7  show how a part of the light goes within the liquid crystal display device  100 . 
     The backlight unit  110  includes a light source  111  which may be a linear light source or may be composed of a plurality of spot light sources arranged on a single line, optical elements  120 , light guide  112  which gathers the light from the light source  111  and emits the uniform light toward the optical elements  120 , and a reflective sheet  113  which reflects the light going out of the lower part of the light guide  112  in  FIG. 7  to turn the light back in the light guide  112 . 
     The optical elements  120  include a diffusion sheet  121  which diffuses the light from the light guide  112  and a first prism sheet  122  and a second prism sheet  123  both of which concentrate the spreading angle of the light diffused by the diffusion sheet  121  to a certain angle range. The first prism sheet  122  and the second prism sheet  123  have the surfaces to face the display panel unit  130 . These surfaces are formed as prism surfaces on which a number of prism lenses each having a uniform cross-sectional shape are extending in parallel. Note that the cross sectional shape of the prism lens may not be uniform. 
     The display panel unit  130  includes a reflective polarizing plate  131  which transmits the p-polarized component of the light going out of the optical elements  120  and reflects the s-polarized component of the light orthogonal to the p-polarized component, a liquid crystal panel  132  which selectively transmits the light going out of the reflective polarizing plate  131 , and a polarizing plate  133  which transmits a specific polarized component of the light which has passed through the liquid crystal panel  132 . An air layer is formed between the second prism sheet  123  and the reflective polarizing plate  131 . 
     The s-polarized component reflected by the reflective polarizing plate  131  is reflected by, for example, the prism surface of the second prism sheet  123  and the reflective sheet  113  and reaches the reflective polarizing plate  131  again. Through such repeated reflection processes, the s-polarized component by-and-by converts into the p-polarized component and passes through the reflective polarizing plate  131 . 
     In  FIG. 7 , the optical elements  120  are partly warped. Such a warp is caused by, for example, an external force applied to the liquid crystal display device  100 . The warp in the optical elements  120 , especially, the warp of the second prism sheet  123  causes interference fringes on the display surface of the liquid crystal display device  100 . 
       FIG. 8  schematically shows interference fringes appearing on a display surface. Each of symbols d 1  and d 2  (d 1 &lt;d 2 ) in  FIG. 8  indicates a pitch in a part of the interference fringes. In a part where the warp of the second prism sheet  123  is large, the pitch of the interference fringes becomes narrow as d 1 , and in a part where the warp of the second prism sheet  123  is small, the pitch of the interference fringes becomes wide as d 2 . 
     It is considered that the interference fringes are caused due to the interference between the light irradiated from the light source  111  to reach the second prism sheet  123  and the light reflected by the reflective polarizing plate  131  and again reflected by the reflective sheet  113  or the like to reach the second prism sheet  123 , or the interference between the light and shade seen along the prism lens of the second prism sheet  123  and the reflective image generated by the reflective polarizing plate  131 , or the like. A gap between the second prism sheet  123  and the reflective polarizing plate  131  may become narrow locally when the gap becomes nonuniform. And this local narrowness causes the above interferences. 
     Now, a liquid crystal display device of the present embodiment will be explained. 
       FIG. 1  is a cross-sectional view schematically showing the structure of liquid crystal display device  1  of the present embodiment. The liquid crystal display device  1  includes a backlight unit  10  and a display panel unit  30 . The dotted arrows in  FIG. 1  show how a part of the light goes in the liquid crystal display device  1 . 
     The backlight unit  10  includes a light source  11 , a light guide  12 , a reflective sheet  13 , and optical elements  20 . The light source  11  is, for example, disposed at one end of the light guide  12  as a plurality of spot light sources arranged linearly. The spot light sources are, for example, light emitting diodes (LEDs) or organic electroluminescent devices. The light source may be a linear light source formed of a cold cathode fluorescent tube or a hot cathode fluorescent tube instead. 
     The light guide  12  includes an incident surface  12   a  which faces the light source  11  and on which the light from the light source  11  is incident and an exit surface  12   b  which faces the display panel unit  30  and from which the light incident upon the incident surface  12   a  exits. That is, the light guide  12  functions as, for example, a plane light source having a rectangular exit surface. The reflective sheet  13  is provided on the lower surface of the light guide  12  in  FIG. 1  (that is, provided on the surface of the light guide  12  opposite to the exit surface  12   b  side), and reflects the light going out of the lower surface of the light guide  12  to turn back to the light guide  12 . 
     The optical elements  20  include a diffusion sheet  21 , a first prism sheet  22 , and a second prism sheet  23  arranged in this order from the light guide  12  side. The diffusion sheet  21 , first prism sheet  22 , and second prism sheet  23  are all shaped substantially the same as the exit surface  12   b  of the light guide  12  when viewed in a plane view and disposed on the exit surface  12   b  in this order. 
     The diffusion sheet  21  diffuses the light from the exit surface  12   b  of the light guide  12  and uniforms the brightness of the light incident upon the first prism sheet  22 . The diffusion sheet  21  may include, for example, a light-diffusing structure on its surface which diffuses the light, or may include microparticles or the like to have a refractive index different from that of the main structural material. Each of the first prism sheet  22  and the second prism sheet  23  has a prism surface on which a plurality of prism lenses each having a uniform cross-sectional shape are extending in parallel. The cross sectional shape of the prism lens is, for example, a triangle whose vertex angle is substantially 90 degrees. The first prism sheet  22  and the second prism sheet  23  are layered such that the ridgelines of the prism lenses are orthogonal to each other, for example. Note that the cross sectional shape of the prism lens of the first prism sheet  22  and the second prism sheet  23  may not be uniform. 
     Using the work of prism lens, the first prism sheet  22  and the second prism sheet  23  concentrate the spreading angle of the light diffused by the diffusion sheet  21  upon a certain angle range. The ends of the light source  11 , the light guide  12 , the reflective sheet  13 , the diffusion sheet  21 , the first prism sheet  22 , and the second prism sheet  23  are held by, for example, a frame-shaped bezel. 
     The display panel unit  30  faces the exit surface  12   b  of the light guide  12 . Furthermore, the ends of the display panel unit  30  are supported by a frame or a bezel. The display panel unit  30  includes a reflective polarizing plate  31  as a reflective polarizer, a liquid crystal panel  32  as a display panel, and a polarizing plate  33 . 
     The liquid crystal panel  32  includes, for example, an array substrate  32   a  with a large number of pixel electrodes, a counter substrate  32   b  facing the array substrate  32   a  interposing a microgap therebetween, and a liquid crystal layer composed of liquid crystal components sealed between the array substrate  32   a  and the counter substrate  32   b . The counter substrate  32   b  includes color filters corresponding to each of the colors red, green, and blue, or the like. 
     The reflective polarizing plate  31  is adhered to the rear surface of the liquid crystal panel  32 , that is, one of the surfaces of the array substrate  32   a  which faces the backlight unit  10 . The polarizing plate  33  is adhered to the display surface of the liquid crystal panel  32 , that is, one of the surfaces of the counter substrate  32   b  which is opposite to the surface facing the array substrate  32   a . The reflective polarizing plate  31  and the polarizing plate  33  face the display area of the liquid crystal panel  32 . 
     The liquid crystal panel  32  selectively applies voltage to each of the pixel electrodes to switch liquid crystal molecules in the liquid crystal layer for controlling the transmission (transmissive/nontransmissive) of the light exiting the reflective polarizing plate  31  on a pixel-by-pixel manner in the display area. The unnecessary polarized light component passing through the liquid crystal panel  32  is removed by the polarizing plate  33 , and a user can recognize display images on the display area. 
     The reflective polarizing plate  31  includes a transparent layer  300  and a reflective polarizing layer  310  arranged in this order from the backlight unit  10  side. The reflective polarizing layer  310  has an incident surface  310   a  on which the light from the exit surface  12   b  of the light guide  12  is incident, and an exit surface  310   b  opposed to the incident surface  310   a . The transparent layer  300  is adhered closely to the incident surface  310   a  of the reflective polarizing layer  310 . 
     The reflective polarizing layer  310  passes a first polarized component of the light incident on the incident surface  310   a  through the exit surface  310   b  and reflects back a second polarized component of the light incident on the incident surface  310   a  to the backlight unit  10  side. The transparent layer  300  has a first surface  300   a  adhered closely to the incident surface  310   a  of the reflective polarizing layer  310  and a second surface  300   b  opposed to the first surface  300   a . The transparent layer  300  passes the first polarized component and the second polarized component of the light coming from the backlight unit  10  and incident on the second surface  300   b  through the first surface  300   a  and passes the light coming from the reflective polarizing layer  310  and incident on the first surface  300   a  (for example, the light having the second polarized component reflected by the reflective polarizing layer  310  and incident on the first surface  300   a ) through the second surface  300   b . In the present embodiment, the first polarized component is given the p-polarized component and the second polarized component is given the s-polarized component which is orthogonal to the p-polarized component. Note that no limitation is intended by the above, that is, the reflective polarizing plate  31  or the liquid crystal display device  1  may be structured given that the first polarized component is the s-polarized component and the second polarized component is the p-polarized component. 
     With reference to the cross-sectional view of  FIG. 2 , the structure of the reflective polarizing plate  31  is explained in detail. The transparent layer  300  includes a hardcoat (HC) layer  301 , a transparent base material  302 , and a transparent adhesive layer  303  arranged in this order from the backlight unit  10  side. The reflective polarizing layer  310  includes a reflective layer  311 , a transparent adhesive layer  312 , and a polarizing layer  313  arranged in this order from the backlight unit  10  side. 
     In the example of  FIG. 2 , the incident surface  310   a  of the reflective polarizing layer  310  corresponds to one of the surfaces of the reflective layer  311  which faces the transparent layer  300 , and the exit surface  310   b  corresponds to one of the surfaces of the polarizing layer  313  which faces the display panel unit  30 . Furthermore, the first surface  300   a  of the transparent layer  300  corresponds to one of the surfaces of the transparent adhesive layer  303  which faces the reflective polarizing layer  310 , and the second surface  300   b  corresponds to one of the surfaces of the HC layer  301  which faces the second prism sheet  23 . 
     Each structural element of the transparent layer  300  is formed of, for example, a material which suppresses an optical loss and an optical phase difference between the elements as low as possible. Specifically, the optical loss of the transparent layer  300  should preferably be lowered to such an extent that the light transparency exceeds 95%. Furthermore, substantially no optical phase difference should preferably be found within the transparent layer  300 , that is, substantially no optical phase difference should preferably be found between the light before passing through the transparent layer  300  and the light after passing through the transparent layer  300 . 
     The HC layer  301  protects the surface of the transparent base material  302  in case of, for example, contacting the prism lens of the second prism sheet  23 . The HC layer  301  is formed by applying, for example, acrylic resin to the surface of the transparent base material  302 . The material of the transparent base material  302  is formed of, for example, polyethylene terephthalate (PET), triacetylcellulose (TAC), cycloolefinpolymer (COP), or polymethyl methacrylate (PMMA) as acrylic resin. The transparent adhesive layer  303  is formed by bonding the transparent base material  302  to the incident surface  310   a . The transparent adhesive layer  303  is, for example, an adhesive material of acrylic resin. 
     The reflective layer  311  passes the p-polarized component of the light incident on the incident surface  310   a  to the polarizing layer  313  side and reflects the s-polarized component to the second prism sheet  23  side. The transparent adhesive layer  312  is formed by bonding the reflective layer  311  to the polarizing layer  313 . The transparent adhesive layer  312  does not substantially block the transmission of any polarized component of the light. The polarizing layer  313  passes the p-polarized component of the light which has passed through the reflective layer  311  and the transparent adhesive layer  312  to the exit surface  310   b  side and absorbs the other polarizing components. The s-polarized component reflected by the reflective layer  311  again heads for the backlight  10  side and is again reflected by, for example, the prism surface of the second prism sheet  23  or the reflective sheet  13 . The s-polarized component then reaches the reflective polarizing plate  31  again. The s-polarized component is reflected repeatedly between the reflective polarizing plate  31  and the backlight  10  and is soon converted into the p-polarized component, and eventually, passes through the reflective polarizing layer  310 . As can be understood from the above, when the s-polarized component and the p-polarized component contained in the light from the backlight unit  10  are used effectively, the brightness of the liquid crystal display device  1  can be improved. 
     The backlight unit  10  and the display panel unit  30  are assembled by fixing their ends with, for example, an adhesive material, a bonding material, or a spacer, or by fixing their ends by a bezel. For example, the assembled liquid crystal display device  1  is used in such a state that it is installed in a casing  201  of a smartphone  200  as in  FIG. 3 . The liquid crystal display device  1  can be used, instead in such a smartphone, as a display of a tablet computer, a mobile phone, a notebook type personal computer, an electronic dictionary, or a various electronic or audio visual devices such as a television device. 
     Here, as shown in  FIG. 2  by a chain double-dashed line, a plane including the ridgelines of the prism lenses formed on the second prism sheet  23  is defined as a prism surface  23   a . The prism surface  23   a  is a surface facing the second surface  300   b  of the transparent layer  300 . Furthermore, the gap between the incident surface  310   a  of the reflective polarizing layer  310  and the prism surface  23   a  is defined as G, and the thickness of the transparent layer  300  is defined as H. 
     Here, since the air layer is formed between the transparent layer  300  and the prism surface  23   a , H≤G. Furthermore, the thickness of the air layer, that is, a gap between the prism surface  23   a  and the second surface  300   b  of the transparent layer  300  should preferably be less than the thickness of the transparent layer  300 . 
     The gap G may be narrowed locally by warpage caused by an external force applied to the second prism sheet  23  or the reflective polarizing plate  31 , or warpage or curving of the second prism sheet  23  or the reflective polarizing plate  31  themselves. If the gap G is locally narrowed beyond a certain extent, interference fringes explained with reference to  FIG. 8  will occur. However, the reflective polarizing plate  31  of the present embodiment includes the transparent layer  300  between the reflective polarizing layer  310  and the second prism sheet  23 . Therefore, even if the second prism sheet  23  or the like is warped by, for example, an external force, the gap G can be kept greater than the thickness H. Accordingly, the occurrence of interference fringes can be reduced or prevented. 
     EXAMPLES 
     In the following examples, the structure and function of the liquid crystal display device  1  are explained in detail. Note that the scope of the invention is not limited to the following disclosure of these examples. 
     Liquid crystal display devices  1  with transparent layers  300  having varying thickness H were prepared for visual evaluation of grades of interference fringes. Results were obtained as indicated in the table of  FIG. 4 . The targets of the visual evaluation were five liquid crystal display devices  1  with the transparent layers  300  having a thickness of (1) none (H=0 μm); (2) H=10 μm; (3) H=25 μm; (4) H=37 μm; and (5) H=59 μm. In each device of (1) to (5), the first prism sheet  22  was prepared as a prism sheet composed of a polyester base material and acrylic resin prism lenses with 24 μm pitches, with a smoothed rear surface of the base material and a total thickness of 65 μm. Furthermore, in each device of (1) to (5), the second prism sheet  23  was prepared as a prism sheet composed of a polyester base material and acrylic resin prism lenses with a pitch of 24 μm, with a rear surface of the base material provided with a mat layer and a total thickness of 70 μm. The gap G was the same in each device of (1) to (5). 
     In the device of (2), the transparent layer  300  was prepared as a film including a transparent base material  302  formed of PET and having a thickness of 5 μm, and a transparent adhesive layer  303  (glue) formed of acrylic resin and having a thickness of 5 μm. In the device of (2), the transparent layer  300  does not include HC layer  301 . 
     In the device of (3), the transparent layer  300  was prepared as a film including a transparent base material  302  formed of PMMA and having a thickness of 20 μm, and a transparent adhesive layer  303  (glue) formed of acrylic resin and having a thickness of 5 μm. In the device of (3), the transparent layer  300  does not include HC layer  301 . 
     In the device of (4), the transparent layer  300  was prepared as a film including a HC layer  301  formed of acrylic resin and having a thickness of 7 μm, a transparent base material  302  formed of TAC and having a thickness of 25 μm, and a transparent adhesive layer  303  (glue) formed of acrylic resin and having a thickness of 5 μm. 
     In the device of (5), the transparent layer  300  was prepared as a film including a HC layer  301  formed of acrylic resin and having a thickness of 7 μm, a transparent base material  302  formed of TAC and having a thickness of 40 μm, and a transparent adhesive layer  303  (glue) formed of acrylic resin and having a thickness of 12 μm. 
     In the table of  FIG. 4 , the item “transparency” indicates the transparency of the transparent layer  300 . The device of (1) lacks the transparent layer  300  and thus, the transparency is 100%. The devices of (2) to (5) exert substantially 100% transparency. 
     In the table of  FIG. 4 , the item “N/R” indicates the result of the visual evaluation of interference fringes generated by applying an external force to the liquid crystal display device  1  of each of (1) to (5). In (1) to (3), the occurrence of interference fringes was not fully prevented (results were NG). On the other hand, in (4) and (5), interference fringes were almost unrecognizable and an excellent preventive effect was obtained (results were OK). 
     The interference fringe state was graded from 1 (bad) to 5 (light) and grades below 4 were evaluated “NG” and grades 4 to 5 were evaluated “OK”.  FIG. 5  is a graph showing a relationship between the interference fringe grades and the thickness H of the transparent layer  300 . The sampling points used for the graph include the thickness H of each of (1) to (5) shown in  FIG. 4 . The curve in the graph is an approximate curve going along grade evaluated at each of the sampling points. The interference fringe grade evaluated when the thickness H=0 μm was set to 1. The interference fringe grade gradually improved when the thickness H approached 10 μm showing a steeple rise, exceeded grade 4 when the thickness H approached 30 μm, and then almost reached grade 5. 
     The graph shows that interference fringes are suppressed better as the thickness H increases. Furthermore, around a thickness of 30 μm, the interference fringe grade exceeds 4 which is a given criterion for the evaluation. The above proves that the present embodiment with the transparent layer  300  can achieve at least reduction or prevention of the occurrence of interference fringes, and can achieve a better result in suppression of interference fringes if the thickness H of the transparent layer  300  is approximately 30 μm or more. 
     If the thickness H is near 50 μm, the interference fringe grade reaches 5 which is the best condition in this evaluation. This proves that the present embodiment can achieve an excellent result in suppression of interference fringes if the thickness H of the transparent layer  300  is approximately 50 μm or more. 
     [Comparison with Other Methods] 
     Now, the structure of the present embodiment is compared with the other methods used for reduction or prevention of interference fringes with reference to  FIG. 6 . 
     As the other methods, the following (I) to (IV) were adopted. 
     (I) In this method, liquid crystal display device  1  was prepared without a transparent layer  300  but with a diffusion material added to the transparent adhesive layer  312  of the reflective polarizing layer  310 . The transparent adhesive layer  312  with the diffusion material added thereto diffuses the light and shade of interference fringes, and the visibility of the interference fringes decreases. 
     (II) In this method, the liquid crystal display device  1  was prepared without a transparent layer  300  but with the polarizing plate  33  whose surface has been subjected to a diffusion treatment (for example, an antiglare treatment) to be a diffusion layer. The diffusion layer diffuses the light and shade of interference fringes, and the visibility of the interference fringes decreases. 
     (III) In this method, the liquid crystal display device  1  was prepared without a transparent layer  300  but with the reflective polarizing layer  310  whose surface at the second prism sheet  23  side (that is, incident surface  310   a ) has been subjected to a diffusion treatment (for example, an antiglare treatment) to be a diffusion layer. The diffusion layer diffuses the light passing through and reflected upon the reflective layer  311 , and the occurrence of interference fringe itself can be prevented. 
     (IV) In this method, the liquid crystal display device  1  was prepared without a transparent layer  300  but with a diffusion sheet inserted between the reflective polarizing layer  310  and the second prism sheet  23 . The diffusion sheet diffuses the light from the second prism sheet  23  and incident on the reflective polarizing layer  310  and the light reflected by the reflective polarizing layer  310  and incident on the second prism sheet  23 , and the occurrence of interference fringe itself can be prevented. 
     Liquid crystal display devices of the methods (I) to (IV) and the liquid crystal display device  1  of the present embodiment with the transparent layer  300  on the incident surface  310   a  of the reflective polarizing layer  310  were prepared for evaluating the avoidance of occurrence of interference fringes and the change ratio of display brightness. The liquid crystal display devices were all prepared the same except the parts explained in (I) to (IV) above. The brightness of a liquid crystal display device which was prepared not based on any method of the present embodiment or of above (I) to (IV), that is, the brightness of a liquid crystal display device shown in  FIG. 1  excluding the transparent layer  300  was used as the criterion for the evaluation of the change ratio of the brightness. 
     The evaluation showed that the liquid crystal display devices of the methods (I) and (II) slightly reduced the interference fringes. However, the change ratio of the brightness was −6% or less in the liquid crystal display device of the method (I) and the change ratio of the brightness was −5% or less in the liquid crystal display device of the method (II). 
     Furthermore, the evaluation showed that the liquid crystal display devices of the methods (III) and (IV) almost completely avoided the occurrence of the interference fringes. However, the change ratio of the brightness was −5% or less in the liquid crystal display device of the method (III) and the change ratio of the brightness was −10% or less in the liquid crystal display device of the method (IV). 
     The liquid crystal display device  1  of the method of the present embodiment can perform substantially complete avoidance of interference fringes particularly when the thickness H is 30 μm or more. Or, the present embodiment can reduce the visibility of interference fringes remarkably. In the present embodiment, the transparent layer  300  is interposed between the prism surface  23   a  and the reflective polarizing layer  310  and is adhered closely to the reflective layer  311 . Such a structure always keeps the gap G greater than the thickness H of the transparent layer  300  even if the gap G is reduced by warpage or the like of the prism surface  23   a . As shown in  FIG. 8 , the interference fringes appearing on the display area become larger as the gap G decreases; however, the gap G never becomes less than the thickness H of the transparent layer  300  in the present embodiment and thus, the gap G never becomes excessively narrow in the present embodiment. Furthermore, as can be understood from the above examples, the interference fringes become recognizable when the gap G is less than 30 μm, and thus, the thickness H of the transparent layer  300  is 30 μm or more to maintain a gap G of at least 30 μm. 
     Consequently, the interference fringes do not occur at all, or occur very limitedly. Thus, the visibility of interference fringes can be reduced remarkably. 
     Furthermore, the change ratio of the brightness is substantially 0% (no change in the brightness) in the present embodiment. That is, the liquid crystal display device  1  of the present embodiment can reduce or prevent the occurrence of interference fringes without affecting the brightness. 
     The structure of the above embodiment may be changed into various forms. 
     For example, in the present embodiment, the transmissive type liquid crystal display device  1  is disclosed as an example of the display device including the reflective polarizing plate  31 . However, the reflective polarizing plate  31  can be used as an optical element in any other display devices such as a reflective type liquid crystal display device, a transreflective type liquid crystal display device, or a non-liquid crystal display device. 
     The structure of the reflective polarizing layer  310  is not limited to the disclosure of the above embodiment. The reflective polarizing layer  310  may be structured as a circular polarizing separation type layer or a wire grid type layer. 
     The structure of the transparent layer  300  is not limited to the disclosure of the above embodiment. For example, the transparent layer  300  may not include the HC layer  301 . The advantage of the reduction or prevention of the occurrence of interference fringes mainly depends on the thickness H of the transparent layer  300 . Therefore, the transparent layer  300  without the HC layer  301  can reduce or prevent the occurrence of interference fringes. 
     The optical elements of the liquid crystal display device  1  are not limited to those shown in  FIG. 1 . The liquid crystal display device  1  may include additional optical elements in addition to the optical elements shown in  FIG. 1  or may exclude part of the optical elements therein. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.