Patent Publication Number: US-2022236597-A1

Title: Display device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of priority under 35 U.S.C. § 120 from U.S. application Ser. No. 17/120,478 filed Dec. 14, 2020, which is a continuation of U.S. application Ser. No. 16/360,651 filed Mar. 21, 2019 (now U.S. Pat. No. 10,908,446 issued Feb. 2, 2021), and claims the benefit of priority under 35 U.S.C. § 119 from Japanese Patent Application No. 2018-059855 filed Mar. 27, 2018, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a display device. 
     BACKGROUND 
     Recently, various types of illumination devices using polymer dispersed liquid crystal (hereinafter referred to also as “PDLC”) capable of switching between a scattering state of scattering incident light and a transmitting state of transmitting incident light have been proposed. 
     Meanwhile, display devices using PDLC have been required to suppress degradation of display quality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a configuration example of a display device DSP according to the present embodiment. 
         FIG. 2  is a diagram showing the relationship between an extension direction ED of a polymer  31  and a transmission axis TA of a polarizer PL in an X-Y plane parallel to a first main surface M 1 . 
         FIG. 3  is a diagram showing the relationship between applied voltage and transmittance in a first display panel PNL 1 . 
         FIG. 4  is a plan view showing a configuration example of the first display panel PNL 1  shown in  FIG. 1 . 
         FIG. 5  is a cross-sectional view of the first display panel PNL 1  shown in  FIG. 4 . 
         FIG. 6  is an enlarged cross-sectional view of the first display panel PNL 1  shown in  FIG. 4 . 
         FIG. 7  is a diagram schematically showing a first liquid crystal layer  30  in an off state. 
         FIG. 8  is a diagram schematically showing the first liquid crystal layer  30  in an on state. 
         FIG. 9  is a cross-sectional view showing the first display panel PNL 1  in a case where the first liquid crystal layer  30  is in the off state. 
         FIG. 10  is a cross-sectional view showing the first display panel PNL 1  in a case where the first display panel PNL 1  includes an area in which the first liquid crystal layer  30  is in the on state. 
         FIG. 11  is a diagram schematically showing the way the display light from a second display panel PNL 2  is transmitted through the first display panel PNL 1 . 
         FIG. 12  is a cross-sectional view showing a configuration example of the display device DSP of the present embodiment. 
         FIG. 13A  is an explanatory diagram showing a display mode of the display device DSP shown in  FIG. 12 . 
         FIG. 13B  is an explanatory diagram showing a display mode of the display device DSP shown in  FIG. 12 . 
         FIG. 13C  is an explanatory diagram showing a display mode of the display device DSP shown in  FIG. 12 . 
         FIG. 14  is a perspective view showing another configuration example of the display device DSP of the present embodiment. 
         FIG. 15  is a perspective view showing another configuration example of the display device DSP of the present embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a display device comprising a first display panel and a polarizer opposed to the first display panel and having a transmission axis for transmitting linearly polarized light is provided. The first display panel comprises a first substrate, a second substrate opposed to the first substrate, and a first liquid crystal layer held between the first substrate and the second substrate and including streak-like polymers and liquid crystal molecules. An extension direction of the polymers is substantially orthogonal to the transmission axis. 
     According to another embodiment, a display device comprising a first display panel and a polarizer opposed to the first display panel is provided. The first display panel comprises a first substrate, a second substrate opposed to the first substrate, and a first liquid crystal layer held between the first substrate and the second substrate and including streak-like polymers and liquid crystal molecules. Linearly polarized light transmitted through the polarizer is transmitted through the first display panel and a polarization state thereof is maintained. 
     Embodiments will be described hereinafter with reference to the accompanying drawings. The disclosure is merely an example, and proper changes in keeping with the spirit of the invention, which are easily conceivable by a person of ordinary skill in the art, come within the scope of the invention as a matter of course. In addition, in some cases, in order to make the description clearer, the widths, thicknesses, shapes, etc., of the respective parts are illustrated schematically in the drawings, rather than as an accurate representation of what is implemented. However, such schematic illustration is merely exemplary, and in no way restricts the interpretation of the invention. In addition, in the specification and drawings, structural elements which function in the same or a similar manner to those described in connection with preceding drawings are denoted by like reference numbers, detailed description thereof being omitted unless necessary. 
       FIG. 1  is a perspective view showing a configuration example of a display device DSP according to the present embodiment. A first direction X, a second direction Y and a third direction Z are orthogonal to each other in the drawing but may intersect at an angle other than 90°. In some cases, a position on the leading end side of an arrow indicating the third direction Z may be referred to as “above” and a position on the side opposite to the leading end of the arrow may be referred to as “below” in the present specification. In the case of “a second member above a first member” and the case of “a second member below a first member”, the second member may be in contact with the first member or may be away from the first member. In addition, an observation position OV at which the display device DSP is observed is assumed to be located on the leading end side of the arrow indicating the third direction Z, and a view from the observation position OV toward an X-Y plane defined by the first direction X and the second direction Y is referred to as planar view. 
     The display device DSP comprises a first display panel PNL 1 , a second display panel PNL 2 , a polarizer PL, a light source unit LU and a controller CNT. 
     The first display panel PNL 1 , the second display panel PNL 2  and the polarizer PL are in the form of flat plates parallel to the X-Y plane. The display panel PNL 2  is opposed to the first display panel PNL 1 . The polarizer PL is located between the first display panel PNL 1  and the second display panel PNL 2 . The second display panel PNL 2 , the polarizer PL and the first display panel PNL 1  are arranged in this order in the third direction Z. The light source unit LU is opposed to an end portion PNLE of the first display panel PNL 1 . The end portion PNLE extends in the first direction X. The first display panel PNL 1  and the light source unit LU are arranged in this order in the second direction Y. 
     The light source unit LU comprises, for example, light-emitting elements LS as light sources. The light-emitting elements LS are, for example, light-emitting diodes. The light-emitting elements LS are arranged in the first direction X. The light emitted from each of the light-emitting elements LS travels in a direction substantially opposite to an arrow indicating the second direction Y and enters the first display panel PNL 1  from the end portion PNLE. 
     The first display panel PNL 1  is a liquid crystal display panel using PDLC and comprises a first main surface M 1 , a second main surface M 2  and a first liquid crystal layer  30 . The first main surface M 1  and the second main surface M 2  are surfaces parallel to the X-Y plane, for example. The first main surface M 1  is opposed to the polarizer PL, and the second main surface M 2  is located on the observation position OV side. The first liquid crystal layer  30  is located between the first main surface M 1  and the second main surface M 2 . 
     The first liquid crystal layer  30  comprises polymer dispersed liquid crystal which contains polymers  31  and liquid crystal molecules  32  as shown in an enlarged view in  FIG. 1 . In one example, the polymers  31  are liquid crystalline polymers. The polymers  31  are in the form of streaks extending in the first direction X. An extension direction ED of the polymers  31  is parallel to the first direction X as indicated by a dashed line in the drawing. The liquid crystal molecules  32  are dispersed in the gaps between the polymers  31  and are aligned such that major axes thereof extend in the first direction X. The polymers  31  and the liquid crystal molecules  32  have optical anisotropy or refractive anisotropy. The liquid crystal molecules  32  may be positive liquid crystal molecules having positive dielectric anisotropy or may be negative liquid crystal molecules having negative dielectric anisotropy. The polymers  31  and the liquid crystal molecules  32  differ from each other in responsivity to an electric field. The responsivity of the polymers  31  to an electric field is lower than the responsivity of the liquid crystal molecules  32  to an electric field. As will be described later, the first display panel PNL 1  has a transparent state in which the first display panel PNL 1  transmits the light emitted from the light source unit LU in the first liquid crystal layer  30  and a scattering state in which the first display panel PNL 1  scatters the light emitted from the light source unit LU in the first liquid crystal layer  30 . For example, the transparent state is formed in a state where voltage is not applied to the first liquid crystal layer  30 , and the scattering state is formed in a state where voltage is applied to the first liquid crystal layer  30 . 
     The second display panel PNL 2  emits display light regardless of whether light is emitted from the light source unit LU or not. For example, the second display panel PNL 2  may be a liquid crystal display panel which selectively reflects or selectively transmits illumination light from an illumination device or may be a self-luminous display panel which comprises an organic electroluminescent (EL) element, etc. 
     The polarizer PL has a transmission axis TA for transmitting linearly polarized light. In the example illustrated, the transmission axis TA is parallel to the second direction Y. That is, the display light from the second display panel PNL 2  is the transmitted light of the polarizer PL and linearly polarized light having a vibration direction parallel to the second direction Y. Here, an incidence plane IP in which the display light enters the first display panel PNL 1  is shown by a dashed line in the drawing. The incidence plane IP is assumed to be parallel to an X-Z plane defined by the first direction X and the third direction Z. In this case, linearly polarized light which is the transmitted light of the polarizer PL is called s-polarized light which is perpendicular to the incidence plane IP. The linearly polarized light parallel to the incidence plane IP is called p-polarized light. The transmitted light of the polarizer PL hardly includes p-polarized light. The extension direction ED is orthogonal to the transmission axis TA. The extension direction ED is not necessarily parallel to the first direction X and only needs to be parallel to the incidence plane IP or the X-Z plane containing the normal to the first display panel PNL 1 . 
     The controller CNT controls the first display panel PNL 1 , the second display panel PNL 2  and the light source unit LU. 
       FIG. 2  is a diagram showing the relationship between the extension direction ED of the polymers  31  and the transmission axis TA of the polarizer PL in the X-Y plane parallel to the first main surface M 1 . As described above, the extension direction ED is parallel to the first direction X, the transmission axis TA is parallel to the second direction Y, and the extension direction ED and the transmission axis TA are orthogonal to each other. An angle θ formed by the extension direction ED and the transmission axis TA is not limited to 90° and is acceptable as long as the angle θ is in the range of 90°±10°. 
       FIG. 3  is a diagram showing the relationship between applied voltage and transmittance in the first display panel PNL 1 . The horizontal axis in the drawing indicates applied voltage (V) which is applied to the first liquid crystal layer  30  and the vertical axis in the drawing indicates transmittance (%). The transmittance corresponds to the ratio of the light transmitted from the second main surface M 2  to the light entering from the first main surface M 1  regarding the light of a wavelength of 550 nm which travels along the normal to the first display panel PNL 1  shown in  FIG. 1  (in the third direction Z). 
     The transmittance of s-polarized light is substantially constant regardless of the magnitude of the applied voltage. On the other hand, the transmittance of p-polarized light decreases as the applied voltage increases, and becomes less than or equal to half the transmittance of s-polarized light. The transmittance of n-polarized light in the drawing corresponds to the average of the transmittance of s-polarized light and the transmittance of p-polarized light. 
       FIG. 4  is a plan view showing a configuration example of the first display panel PNL 1  shown in  FIG. 1 . The first display panel PNL 1  comprises a first substrate SUB 1  and a second substrate SUB 2 . The first substrate SUB 1  and the second substrate SUB 2  overlap each other in planar view. The first display panel PNL 1  comprises a display area DA on which an image is displayed and a frame-like non-display area NDA which surrounds the display area DA. The display area DA is located in an area in which the first substrate SUB 1  and the second substrate SUB 2  overlap each other. The first display panel PNL 1  comprises n scanning lines G (G 1  to Gn) and m signal lines S (S 1  to Sm) in the display area DA. Each of n and m is a positive integer, and n may be equal to or different from m. The scanning lines G extend in the first direction X and are spaced apart and arranged in the second direction Y. The signal lines S extend in the second direction Y and are spaced apart and arranged in the first direction X. 
     The first substrate SUB 1  has end portions E 11  and E 12  extending in the first direction X and end portions E 13  and E 14  extending in the second direction Y. The second substrate SUB 2  has end portions E 21  and E 22  extending in the first direction X and end portions E 23  and E 24  extending in the second direction Y. In the example illustrated, the end portion E 11  and the end portion E 21 , the end portion E 13  and the end portion E 23 , and the end portion E 14  and the end portion E 24  overlap, respectively, in planar view. However, these end portions do not necessarily overlap. The end portion E 22  is located between the end portion E 12  and the display area DA in planar view. The first substrate SUB 1  has an extension portion Ex between the end portion E 12  and the end portion E 22 . 
     Wiring substrates F 1  to F 3  are each connected to the extension portion Ex and are arranged in this order in the first direction X. The wiring substrate F 1  is provided with a gate driver GD 1 . The wiring substrate F 2  is provided with a source driver SD. The wiring substrate F 3  is provided with a gate driver GD 2 . The wiring substrates F 1  to F 3  may be replaced with a single wiring substrate. 
     The signal lines S are drawn to the non-display area NDA and are connected to the source driver SD. The scanning lines G are drawn to the non-display area NDA and are connected to the gate drivers GD 1  and GD 2 . In the example illustrated, odd-numbered scanning lines G are drawn between the end portion E 14  and the display area DA and are connected to the gate driver GD 2 . In addition, even-numbered scanning lines G are drawn between the end portion E 13  and the display area DA and are connected to the gate driver GD 1 . The relationship in connection between the gate drivers GD 1  and GD 2  and the scanning lines G is not limited to the example illustrated. 
       FIG. 5  is a cross-sectional view of the first display panel PNL 1  shown in  FIG. 4 . Only main portions in the cross-section of the first display device panel PNL 1  in a Y-Z plane defined by the second direction Y and the third direction Z will be described here. The first display panel PNL 1  comprises the first liquid crystal layer  30  held between the first substrate SUB 1  and the second substrate SUB 2 . The first substrate SUB 1  and the second substrate SUB 2  are bonded together by a sealant  40 . 
     The light-emitting element LS in the light source unit LU is connected to the wiring substrate F 4 . In the example illustrated, the light-emitting element LS is located between the extension portion Ex and the wiring substrate F 4  in the third direction Z. In addition, the light-emitting element LS is located between the wiring substrates F 1  to F 3  and the second substrate SUB 2  in the second direction Y. The light-emitting element LS has an emission portion EM opposed to the end portion E 22 . The light-emitting element LS emits light from the emission portion EM to the end portion E 22 . The emission portion EM may be in contact with the end portion E 22 . In addition, an air layer, an optical element or the like may be interposed between the emission portion EM and the end portion E 22 . The end portion E 22  corresponds to an entrance portion which the light emitted from the emission portion EM enters. That is, the end portion E 22  corresponds to the end portion PNLE of the first display panel PNL 1  shown in  FIG. 1 . The light which has entered from the end portion E 22  propagates through the first display panel PNL 1  in the direction opposite to the arrow indicating the second direction Y as will be described later. The light-emitting element LS may be opposed to the end portions of both the first substrate SUB 1  and the second substrate SUB 2  and may be opposed to, for example, the end portions E 11  and E 21 . 
       FIG. 6  is an enlarged cross-sectional view of the first display panel PNL 1  shown in  FIG. 4 . The first substrate SUB 1  comprises a transparent substrate  10 , wiring lines  11 , an insulating layer  12 , pixel electrodes  13  and an alignment film  14 . The second substrate SUB 2  comprises a transparent substrate  20 , a common electrode  21  and an alignment film  22 . The second substrate SUB 2  does not comprise a light-shielding layer which overlaps the wiring lines  11 . The transparent substrates  10  and  20  are insulating substrates such as glass substrates or plastic substrates. The wiring lines  11  are formed of a nontransparent metal material such as molybdenum, tungsten, aluminum, titanium or silver. The illustrated wiring lines  11  extend in the first direction X but may extend in the second direction Y. The insulating layer  12  is formed of a transparent insulating material. The pixel electrodes  13  and the common electrode  21  are formed of a transparent conductive material such as indium tin oxide (ITO) or indium zinc oxide (IZO). The pixel electrodes  13  are disposed in pixels PX, respectively. The common electrode  21  is disposed across the pixels PX. The alignment films  14  and  22  may be horizontal alignment films having an alignment restriction force substantially parallel to the X-Y plane or may be vertical alignment films having an alignment restriction force substantially parallel to the third direction Z. 
     The first liquid crystal layer  30  is located between the alignment film  14  and the alignment film  22 . In one example, the alignment treatment direction of the alignment films  14  and  22  is parallel to the first direction X, and the alignment films  14  and  22  have an alignment restriction force in the first direction X. The polymers  31  shown in  FIG. 1  are obtained in the form of streaks extending in the first direction X, for example, by the polymerization of liquid crystal monomers in a state of being aligned in the first direction X by the alignment restriction force of the alignment films  14  and  22 . The first liquid crystal layer  30  is located between the pixel electrodes  13  and the common electrode  21 . The applied voltage shown in  FIG. 3  corresponds to the potential difference between the pixel electrode  13  and the common electrode  21 . 
       FIG. 7  is a diagram schematically showing the first liquid crystal layer  30  in an off state. The drawing shows a cross-section of the first liquid crystal layer  30  in the X-Z plane intersecting the second direction Y which is the traveling direction of the light from the liquid source unit LU. The off state corresponds to a state in which voltage is not applied to the first liquid crystal layer  30  (for example, a state in which the potential difference between the pixel electrode  13  and the common electrode  21  is approximately zero). An optical axis Ax 1  of the polymer  31  and an optical axis Ax 2  of the liquid crystal molecule  32  are parallel to each other. In the example illustrated, the optical axis Ax 1  and the optical axis Ax 2  are parallel to the first direction X. The polymer  31  and the liquid crystal molecule  32  have substantially equal refractive anisotropy. That is, the ordinary refractive index of the polymer  31  and the ordinary refractive index of the liquid crystal molecule  32  are substantially equal to each other, and the extraordinary refractive index of the polymer  31  and the extraordinary refractive index of the liquid crystal molecule  32  are substantially equal to each other. For this reason, there is hardly any refractive index difference between the polymer  31  and the liquid crystal molecule  32  in all directions including the first direction X, the second direction Y and the third direction Z. 
       FIG. 8  is a diagram schematically showing the first liquid crystal layer  30  in an on state. The on state corresponds to a state in which voltage is applied to the first liquid crystal layer  30  (for example, a state in which the potential difference between the pixel electrode  13  and the common electrode  21  is greater than or equal to a threshold value). As described above, the responsivity of the polymer  31  to an electric field is lower than the responsivity of the liquid crystal molecule  32  to an electric field. In one example, the alignment direction of the polymer  31  (the extension direction ED shown in  FIG. 1 ) hardly changes regardless of the presence of an electric field. On the other hand, the alignment direction of the liquid crystal molecule  32  changes in accordance with an electric field when high voltage which is greater than or equal to the threshold value is applied to the first liquid crystal layer  30 . That is, as illustrated in the drawing, the optical axis Ax 1  is substantially parallel to the first direction X, whereas the optical axis Ax 2  is inclined with respect to the first direction X. If the liquid crystal molecule  32  is a positive liquid crystal molecule, the liquid crystal molecule  32  is aligned such that a major axis thereof extends along an electric field. The electric field between the pixel electrode  13  and the common electrode  21  is formed in the third direction Z. Therefore, the liquid crystal molecule  32  is aligned such that the major axis thereof or the optical axis Ax 2  extends in the third direction Z. That is, the optical axis Ax 1  and the optical axis Ax 2  intersect each other. Therefore, there is a large refractive index difference between the polymer  31  and the liquid crystal molecule  32  in all directions including the first direction X, the second direction Y and the third direction Z. 
       FIG. 9  is a cross-sectional view showing the first display panel PNL 1  in a case where the first liquid crystal layer  30  is in the off state. A light beam L 11  emitted from the light-emitting element LS enters the first display panel PNL 1  from the end portion E 22  and propagates through the transparent substrate  20 , the first liquid crystal layer  30 , the transparent substrate  10  and the like. If the first liquid crystal layer  30  is in the off state, the light beam L 11  is transmitted and hardly scattered in the first liquid crystal layer  30 . The light beam L 11  propagates through the first display panel PNL 1  and hardly leaks from the first main surface M 1  which is the lower surface of the transparent substrate  10  and the second main surface M 2  which is the upper surface of the transparent substrate  20 . That is, the first liquid crystal layer  30  is in a transparent state. 
       FIG. 10  is a cross-sectional view showing the first display panel PNL 1  in a case where the first display panel PNL 1  includes an area in which the first liquid crystal layer  30  is in the on state. A light beam L 21  emitted from the light-emitting element LS enters the first display panel PNL 1  from the end portion E 22  and propagates through the transparent substrate  20 , the first liquid crystal layer  30 , the transparent substrate  10  and the like. In the example illustrated, the first liquid crystal layer  30  overlapping a pixel electrode  13 A is in the off state, and the first liquid crystal layer  30  overlapping a pixel electrode  13 B is in the on state. For this reason, the light beam L 21  is transmitted and hardly scattered in an area of the first liquid crystal layer  30  which overlaps the pixel electrode  13 A, while the light beam L 21  is scattered in an area of the first liquid crystal layer  30  which overlaps the pixel electrode  13 B. Of the light beam L 21 , some scattered light beams L 211  are transmitted through the first main surface M 1 , some scattered light beams L 212  are transmitted through the second main surface M 2 , and the other scattered light beams propagate through the first display panel PNL 1 . These scattered light beams L 211  and L 212  correspond to the display light from the first display panel PNL 1  and form the display image of the first display panel PNL 1 . 
       FIG. 11  is a diagram schematically showing the way the display light from the second display panel PNL 2  is transmitted through the first display panel PNL 1 . The drawing shows a cross-section of the first display panel PNL 1  in the Y-Z plane containing the third direction Z, which is the traveling direction of display light beams D 21  and D 22  from the second display panel PNL 2 , and the transmission axis TA of the polarizer PL. The display light beams D 21  and D 22  transmitted through the polarizer PL are s-polarized light as described above and are linearly polarized light having a vibration direction parallel to the second direction Y. 
     The first liquid crystal layer  30  has an off area (first area)  30 A in the off state which is indicated by a dashed line on the left side of the drawing and an on area (second area)  30 B in the on state which is indicated by a dashed line on the right side of the drawing. In the off area  30 A, the optical axis Ax 1  of the polymer  31  and the optical axis Ax 2  of the liquid crystal molecule  32  are parallel to the first direction X as described above. In other words, the polymer  31  and the liquid crystal molecule  32  have an extraordinary refractive index in the first direction X. In the on area  30 B, the optical axis Ax 1  of the polymer  31  is parallel to the first direction X and the optical axis Ax 2  of the liquid crystal molecule  32  is parallel to the third direction Z as described above. In other words, the polymer  31  has an extraordinary refractive index in the first direction X and the liquid crystal molecule  32  has an extraordinary refractive index in the third direction Z. 
     The display light beam D 21  is transmitted through the off area  30 A and the display light beam D 22  is transmitted through the on area  30 B in the first display panel PNL 1 . Since the display light beams D 21  and D 22  are s-polarized light, the display light beams D 21  and D 22  is transmitted through the second main surface M 2  while polarization states thereof are being maintained and are hardly influenced by the extraordinary refractive indexes of the polymer  31  and the liquid crystal molecule  32  in the first display panel PNL 1 . That is, as described with reference to  FIG. 3 , the transmittance of s-polarized light is constant regardless of the magnitude of the applied voltage of the first liquid crystal layer  30 . In other words, the display light beams D 21  and D 22  (s-polarized light) which enter the first display panel PNL 1  from the second display panel PNL 2  are transmitted through the first display panel PNL 1  while the polarization state of s-polarized light is being maintained and are hardly scattered in the first display panel PNL 1 . The display light beams D 21  and D 22  form the display image of the second display panel PNL 2 . 
     Next, a more specific configuration example will be described. 
       FIG. 12  is a cross-sectional view showing a configuration example of the display device DSP of the present embodiment. The display device DSP comprises the first display panel PNL 1 , the light source unit LU, the second display panel PNL 2 , an illumination device IL, polarizers PL 1  and PL 2  and the controller CNT. The polarizer PL 1  is located between the illumination device IL and the second display panel PNL 2 . The polarizer PL 2  is located between the first display panel PNL 1  and the second display panel PNL 2 . The second display panel PNL 2  is located between the polarizer PL 2  and the illumination device IL. 
     The second display panel PNL 2  comprises a third substrate SUB 3 , a fourth substrate SUB 4  and a second liquid crystal layer LC. The fourth substrate SUB 4  is opposed to the third substrate SUB 3 . The second liquid crystal layer LC is held between the third substrate SUB 3  and the fourth substrate SUB 4 . The polarizer PL 1  is bonded to the third substrate SUB 3 . The polarizer PL 2  is bonded to the fourth substrate SUB 4  in one example but may be bonded to the first main surface M 1  of the first substrate SUB 1 . The transmission axis of the polarizer PL 2  is, for example, orthogonal to the transmission axis of the polarizer PL 1  in the X-Y plane. The polarizer PL 2  corresponds to the polarizer PL shown in  FIG. 1 , etc. 
     Regarding the configuration of the second display panel PNL 2 , detailed description thereof will be omitted here, but the second display panel PNL 2  may be configured in conformity with a display mode using a lateral electric field along a substrate main surface, a display mode using a longitudinal electric field along the normal to a substrate main surface, a display mode using an inclined electric field which is inclined with respect to a substrate main surface, or a display mode using an appropriate combination of the lateral electric field, the longitudinal electric field and the inclined electric field. The substrate main surface here is a surface parallel to the X-Y plane. 
     The controller CNT controls the first display panel PNL 1 , the light source unit LU, the second display panel PNL 2  and the illumination device IL. For example, the controller CNT supplies a first control signal including a first video signal to the first display panel PNL 1 . In addition, the controller CNT supplies a light source control signal to the light source unit LU in synchronization with the supply of the first control signal to the first display panel PNL 1 . On the other hand, the controller CNT supplies a second control signal including a second video signal to the second display panel PNL 2 . In addition, the controller CNT supplies an illumination control signal to the illumination device IL. The second video signal is, for example, a signal different from the first video signal. As a result, a display image based on the first video signal is displayed on the first display panel PNL 1  and a display image based on the second video signal is displayed on the second display panel PNL 2 . 
     An air layer or a transparent member having a refractive index similar to that of the first substrate SUB 1 , etc., may be interposed between the first display panel PNL 1  and the second display panel PNL 2 , between the polarizer PL 2  and the first display panel PNL 1  or between the polarizer PL 2  and the second display panel PNL 2 . In addition, the first display panel PNL 1  may be turned upside down, that is, the second substrate SUB 2  may be located between the first substrate SUB 1  and the polarizer PL 2 . In addition, a light guide which propagates the light emitted from the light source unit LU in the X-Y plane may be disposed between the first display panel PNL 1  and the second display panel PNL 2 . 
     If the display device DSP is required to give a three-dimensional appearance (or an appearance of depth) by the display image of the first display panel PNL 1  and the display image of the second display panel PNL 2 , the first display panel PNL 1  and the second display panel PNL 2  should preferably be located at a predetermined distance from each other in the third direction Z. If the display device DSP is required to give an appearance of depth particularly, the edge of the display image of the first display panel PNL 1  and the edge of the display image of the second display panel PNL 2  should preferably be shifted from each other. 
       FIGS. 13A to 13C  are explanatory diagrams showing the display modes of the display device DSP shown in  FIG. 12 . 
       FIG. 13A  shows the first mode in which only the second display panel PNL 2  emits the display light beam D 21 . The light source unit LU is in a non-lighting state in which the light source unit LU does not emit light. The video signal is not supplied from the controller CNT to the first display panel PNL 1 , and the entire area of the first liquid crystal layer  30  is the off area  30 A. The illumination device IL and the second display panel PNL 2  are controlled by the controller CNT. The illumination device IL is in a lighting state in which the illumination device IL emits illumination light. The second display panel PNL 2  is controlled based on the video signal from the controller CNT, selectively transmits the illumination light from the illumination device IL, and thereby emits the display light beam D 21  which is s-polarized light. 
     In the first mode, the display light beam D 21  is transmitted through the first display panel PNL 1  while the polarization state is being maintained. Therefore, if the user observes the display device DSP from the observation position OV, the user can observe the display image displayed on the second display panel PNL 2  via the first display panel PNL 1 . 
       FIG. 13B  shows the second mode in which only the first display panel PNL 1  emits a display light beam D 11 . The illumination device IL is in a non-lighting state in which the illumination device IL does not emit illumination light. The video signal is not supplied from the controller CNT to the second display panel PNL 2 . The light source unit LU and the first display panel PNL 1  are controlled by the controller CNT. The first display panel PNL 1  is controlled based on the video signal from the controller CNT, transmits the light from the light source unit LU in the off area  30 A and scatters the light from the light source unit LU in the on area  30 B, and thereby emits the display light beam D 11 . 
     In the second mode, if the user observes the display device DSP from the observation position OV, the user can observe the display image displayed on the first display panel PNL 1 . 
       FIG. 13C  shows the third mode in which the first display panel PNL 1  emits the display light beam D 11  and the second display panel PNL 2  emits the display light beam D 21  and the display light D 22 . The light source unit LU and the first display panel PNL 1  are controlled by the controller CNT. In addition, the illumination device IL and the second display panel PNL 2  are controlled by the controller CNT. The first display panel PNL 1  transmits the light from the light source unit LU in the off area  30 A and scatters the light from the light source unit LU in the on area  30 B, and thereby emits the display light beam D 11 , as in the case with the second mode. The second display panel PNL 2  selectively transmits the illumination light from the illumination devise IL, and thereby emits the display light beams D 21  and D 22  which are s-polarized light, as in the case with the first mode. 
     The display light beam D 21  is not scattered but is transmitted through the off area  30 A of the first display panel PNL 1  while the polarization state is being maintained. The display light beam D 22  is not scattered but is transmitted through the on area  30 B of the first display panel PNL 1  while the polarization state is being maintained. That is, the first display panel PNL 1  emits both the display light beam D 11  and the display light D 22  in the on area  30 B. 
     In the third mode, if the user observes the display device DSP from the observation position OV, the user can observe the display image displayed on the first display panel PNL 1 , and the user can also observe the display image displayed on the second display panel PNL 2  via the first display panel PNL 1 . 
     According to the present embodiment described above, the extension direction ED of the streak-like polymers  31  contained in the first display panel PNL 1  is orthogonal to the vibration direction of the linearly polarized light which enters from the first main surface M 1  of the first display panel PNL 1 . This linearly polarized light is transmitted through the second main surface M 2  of the first display panel PNL 1  while the polarization state is being maintained and is hardly scattered in the first liquid crystal layer  30  regardless of the magnitude of the applied voltage of the first liquid crystal layer  30  in the first display panel PNL 1 . That is, undesirable scattering of the display light from the second display panel PNL 2  located on the back side of the first display panel PNL 1  can be suppressed. As a result, degradation of the display quality can be suppressed. When the second display panel PNL 2  is observed via the first display panel PNL 1  in this display device DSP, the display image of the second display panel PNL 2  can be clearly viewed. 
     Next, another configuration example will be described. 
       FIG. 14  is a perspective view showing another configuration example of the display device DSP of the present embodiment. The configuration example shown in  FIG. 14  differs from the configuration example shown in  FIG. 1  in that a retarder RT is disposed on the observation position OV side of the first display panel PNL 1 . The first display panel PNL 1  is located between the polarizer PL and the retarder RT. With respect to the transmitted light of a predetermined wavelength (λ=550 nm), the retarder RT gives a retardation of λ/4. The angle formed by a slow axis D of the retarder RT and the transmission axis TA of the polarizer PL is 45° in the X-Y plane. Therefore, the linearly polarized light (s-polarized light) transmitted through the first display panel PNL 1  is transmitted through the retarder RT and converted into circularly polarized light. 
     In this configuration example also, advantages similar to those of the above-described configuration example can be achieved. In addition, when the user observes the display device DSP while wearing polarized sunglasses from the observation position OV, the user can view both the display image of the first display panel PNL 1  and the display image of the second display panel PNL 2 . 
     In addition, if a viewing angle compensation film is provided in addition to the retarder, the image quality can be improved against the light which enters the first display panel PNL 1  obliquely. This viewing angle compensation film may be used alone without the retarder. 
       FIG. 15  is a perspective view showing another configuration example of the display device DSP of the present embodiment. The configuration example shown in  FIG. 15  differs from the configuration example shown in  FIG. 1  in that the first display panel PNL 1  is a liquid crystal display panel using vertical alignment type PDLC. 
     The first display panel PNL 1  has a cross-section structure similar to the cross-section structure described with reference to  FIG. 6 . However, the alignment films  14  and  22  are vertical alignment films. In the first display panel PNL 1 , the polymer  31  and the liquid crystal molecule  32  are aligned in the third direction Z in the off area  30 A, and the optical axis Ax 1  and the optical axis Ax 2  are parallel to the third direction Z. In the on area  30 B, only the alignment direction of the liquid crystal molecule  32  changes and the optical axis Ax 2  is parallel to the first direction X. The alignment direction of the liquid crystal molecule  32  in the on area  30 B can be controlled, for example, by the following methods. 
     In one example, an electric field formed between the pixel electrode  13  and the common electrode  21  includes an inclined electric field which is inclined with respect to the third direction Z. Such an inclined electric field can be formed, for example, by providing a projection or providing a slit in the pixel electrode  13  and the common electrode  21 . The liquid crystal molecule  32  is aligned in the first direction X in the X-Y plane by the inclined electric field. 
     In another example, if rubbing treatment or the like is applied to the alignment films  14  and  22  which are vertical alignment films beforehand, the liquid crystal molecules  32  located near the alignment films  14  and  22  are aligned in a pretilted manner. The liquid crystal molecules  32  are aligned in the first direction X by the interaction between the pretilt of the liquid crystal molecules  32  and the electric field formed between the pixel electrode  13  and the common electrode  21 . 
     The light which has reached the off area  30 A from the light-emitting element LS is not scattered but propagates through the first display panel PNL 1  in the second direction Y. In addition, the display light beam (s-polarized light) D 21  from the second display panel PNL 2  is not influenced by the extraordinary refractive indexes of the polymer  31  and the liquid crystal molecule  32 , and is not scattered but is transmitted through the first display panel PNL 1  while the polarization state is being maintained in the off area  30 A. 
     The light which has reached the on area  30 B from the light-emitting element LS is scattered and emitted from the first display panel PNL 1 . In addition, the display light beam (s-polarized light) D 22  from the second display panel PNL 2  is not scattered but is transmitted through the first display panel PNL 1  while the polarization state is being maintained in the on area  30 B as is the case with the off area  30 A. 
     In this configuration example also, advantages similar to those of the above-described configuration example can be achieved. 
     As described above, a display device which can suppress degradation of display quality can be provided by the present embodiment. 
     The present invention is not limited to the embodiments described above but the constituent elements of the invention can be modified in various manners without departing from the spirit and scope of the invention. Various aspects of the invention can also be extracted from any appropriate combination of a plurality of constituent elements disclosed in the embodiments. Some constituent elements may be deleted in all of the constituent elements disclosed in the embodiments. The constituent elements described in different embodiments may be combined arbitrarily.