Display device, mobile body, and display panel-housing device

Abstract

A display device not to be worn by a user, the display device comprising: a housing which has a viewing part; a display panel which is positioned inside the housing and has a planar display surface disposed so as to face the viewing part and through which display light is emitted; and an optical system which is positioned inside the housing and which forms an image from the display light, wherein the optical system has a first semi-transmissive mirror that reflects light toward the display panel and has the function of dispersing, condensing, or converging light, and a second semi-transmissive mirror that is positioned between the display panel and the first semi-transmissive mirror, reflects light toward the first semi-transmissive mirror, and has the function of condensing or converging light.

Description

Display device, mobile unit, and display panel housing device

[0001] This disclosure relates to a display device, a mobile device, and a display panel housing device.

[0002] Conventionally, a display device described in, for example, Patent Document 1 is known.

[0003] Japanese Patent Application Publication No. 2022-63533

[0004] The display device of this disclosure comprises a housing having a viewing section; a display panel located inside the housing and having a planar display surface facing the viewing section, for emitting display light; and an optical system located inside the housing and forming an image of the display light, wherein the optical system includes a first semi-transparent mirror that reflects light toward the display panel and has the function of diverging, concentrating, or focusing light; and a second semi-transparent mirror located between the display panel and the first semi-transparent mirror and having the function of reflecting light toward the first semi-transparent mirror and having the function of concentrating or focusing light, and is a non-attachable display device for the user.

[0005] The mobile device of this disclosure includes the display device.

[0006] The display panel housing device of the present disclosure comprises a housing having a display panel mounting section on which a display panel having a display surface can be installed, and a viewing section arranged opposite to the display surface; and an optical system located inside the housing and forming a planar image of display light emitted from the display surface, wherein the optical system comprises a first semi-transparent mirror that reflects light toward the display panel mounting section and has the function of diverging, concentrating, or focusing light; and a second semi-transparent mirror located between the display panel mounting section and the first semi-transparent mirror and having the function of reflecting light toward the first semi-transparent mirror and having the function of concentrating or focusing light, and is a display panel housing device that is not attached to the user.

[0007] The display device of this disclosure is installable within a first housing having a first viewing section and comprises a second housing having a second viewing section, a display panel located within the second housing and having a planar display surface arranged to face the first viewing section and the second viewing section, for emitting display light, and an optical system located within the second housing and forming an image of the display light, wherein the optical system comprises a first semi-transparent mirror that reflects light toward the display panel and has the function of diverging, concentrating or focusing light, and a second semi-transparent mirror located between the display panel and the first semi-transparent mirror and has the function of reflecting light toward the first semi-transparent mirror and having the function of concentrating or focusing light.

[0008] The display device of this disclosure comprises: a first housing having a first viewing section; a second housing installed inside the first housing and having a second viewing section; a display panel located inside the second housing and having a planar display surface arranged to face the first and second viewing sections, for emitting display light; and an optical system located inside the second housing and forming an image of the display light, wherein the optical system includes: a first semi-transparent mirror that reflects light toward the display panel and has the function of diverging, concentrating, or focusing light; and a second semi-transparent mirror located between the display panel and the first semi-transparent mirror and having the function of reflecting light toward the first semi-transparent mirror and having the function of concentrating or focusing light.

[0009] The purposes, features, and advantages of this disclosure will become clearer from the detailed description and drawings below. This is a schematic diagram showing the configuration of the display device of this disclosure. This is a cross-sectional view showing an example of the main components of a display device according to one embodiment of this disclosure. This is a cross-sectional view showing another example of the main components of a display device according to one embodiment of this disclosure. This is a cross-sectional view showing an example of the main components of a display device according to another embodiment of this disclosure. This is a cross-sectional view showing another example of the main components of a display device according to another embodiment of this disclosure. This is a diagram illustrating the projection of a virtual image in the display device of Figure 4. This is a diagram illustrating the projection of a virtual image in the display device of Figure 5. This is a diagram illustrating the design of the optical system in the display device of Figure 5. This is a cross-sectional view showing an example of the main components of a display device according to yet another embodiment of this disclosure. This is a diagram showing an example of the configuration of an imaging device according to one embodiment of this disclosure. This is a diagram showing another example of the configuration of an imaging device according to one embodiment of this disclosure. This is a diagram showing another example of the configuration of an imaging device according to one embodiment of this disclosure. This is a cross-sectional view illustrating another example of a display device. This is a cross-sectional view illustrating another example of a display device. This is a diagram illustrating an optical system in This is a diagram illustrating the optical system in another example of a display device. This is a diagram illustrating the optical system in another example of a display device. This is a graph illustrating the optical system in another example of a display device. This is a cross-sectional view illustrating showing an example of the configuration of a second semi-transparent mirror. This is a cross-sectional view illustrating another example of the configuration of a main component of a display device according to one embodiment of this disclosure. This is a cross-sectional view showing another example of the configuration of a main component of a display device according to yet another embodiment of this disclosure. This is a perspective view showing a cross-section of another example of a display device according to one embodiment of this disclosure.This is a cross-sectional view showing another example of a display device according to one embodiment of the present disclosure. This is a diagram showing the optical path of the display light in the display device of Figure 2. This is a cross-sectional view showing an example of the main components of a display device according to an embodiment of the present disclosure. This is a cross-sectional view showing an example of the main components of a display device according to an embodiment of the present disclosure. This is a cross-sectional view showing an example of the main components of a display device according to an embodiment of the present disclosure. This is a diagram illustrating the optical functions of the display devices according to Figures 29 to 32, respectively. This is a diagram illustrating the optical functions of the display devices according to Figures 29 to 32, respectively. This shows an example of various dimensions of the display device shown in Figure 27. This shows a first example in which a first semi-transparent mirror images the display light to form a first image. In Figure 35, a first example is shown in which a second semi-transparent mirror images the first image to form a second image. In Figure 35, a second example is shown in which a second semi-transparent mirror images the first image to form a second image. This shows a second example in which a first semi-transparent mirror images the display light to form a first image. In Figure 38, a first example is shown in which a second semi-transparent mirror images the first image to form a second image. Figure 38 shows a second example in which the second semi-transparent mirror forms the first image to create the second image. A third example shows which the first semi-transparent mirror forms the display light to create the first image. Figure 41 shows a first example in which the second semi-transparent mirror forms the first image to create the second image. Figure 41 shows a second example in which the second semi-transparent mirror forms the first image to create the second image. Figure 41 shows a third example in which the second semi-transparent mirror forms the first image to create the second image. This figure explains the conditions related to the example in Figure 43 and the example in Figure 44. A first arrangement example of the phase difference members is shown. A second arrangement example of the phase difference members is shown. A third arrangement example of the phase difference members is shown. An arrangement example of the third phase difference plate and the fourth phase difference plate is shown.

[0010] Conventionally, various small display devices have been proposed for use in digital rearview mirrors placed inside the vehicle's interior, head-mounted displays worn on the user's head, and so on. The display device described in Patent Document 1 is configured to emit display light from a display panel through a plurality of optical components such as a phase difference plate and a reflective polarizing plate.

[0011] Conventional display devices were prone to deformation of optical components and misalignment of optical components, which could lead to a decrease in display quality.

[0012] Embodiments of the present disclosure will be described below with reference to the drawings. Some of the figures used in the following description are schematic. The figures used in the following description show the main components of the display device and virtual image display device of the present disclosure. The display device and virtual image display device of the present disclosure may include well-known components not shown, such as optical system holders and housings. In this specification, in some drawings, a Cartesian coordinate system XYZ is defined for convenience. The X-axis direction is also referred to as the left-right direction. The Y-axis direction is also referred to as the height direction or up-down direction. The Z-axis direction is also referred to as the output direction or depth direction. In this disclosure, the Y-axis direction may be the direction along the vertical direction of the image of the virtual image V, and the X-axis direction may be the direction along the left-right direction of the image of the virtual image V.

[0013] Figures 1 to 28 are diagrams or graphs illustrating the display device, imaging device, display system, and vehicle of the present disclosure. In Figures 2 to 5, 9, 13, 14, 18 to 20, and 22 to 25, the optical path of light incident on a light-reflective optical member and the optical path of light reflected by the optical member are shown shifted in the height direction (Y-axis direction) for ease of illustration.

[0014] A display device 1 in one embodiment of the present disclosure comprises a display panel 2 and an optical system 3, as shown in Figure 1. The display device 1 directs a portion of the display light emitted from the display panel 2 into the eyes of a user 22, allowing the user 22 to view it as an image, picture, or aerial image. The display device 1 can cause the user 22 to view the display on the display panel 2 at a position different from the position of the display panel 2. In the positional embodiment of the present disclosure, the display device 1 allows the user 22 to view it as a virtual image V. The virtual image V may be formed on the side of the display device 1 that is further away from the user 22. The virtual image V may be an upright virtual image that is an enlarged version of the display image displayed on the display panel 2. If the display device 1 comprises a housing (see Figures 26-27) that houses the display panel 2 and the optical system 3, the virtual image V may be formed inside the housing or outside the housing. The virtual image V may be formed on the side of the user 22 that is further away from the display panel 2 or on the side that is closer to the display panel 2. If the housing has a window 37 (see Figures 26-27) that transmits the display light emitted from the optical system 3, the virtual image V may be formed on the side farther from the window 37 (light-transmitting plate 38) or on the side closer to the window, as viewed from the user 22.

[0015] The display device 1 in one embodiment of this disclosure may be a non-attachable device to the user 22. That is, it may not be attached to the user 22 but may be fixed to the environment. The display device 1 may be fixed to, for example, a wall, column, or ceiling. The display device 1 may also be fixed to the interior of a vehicle. The display device 1 may be attached to the user 22. When attached to the user 22, the display device 1 may have a mounting part (not shown) so that the window 37 is fixed at the position of the user 22's eyes.

[0016] The display device 1 may be configured to direct a portion of the display light emitted from the display panel 2 into the eyes of the user 22, allowing the user 22 to perceive it as a real image. The real image may be formed on the side closer to the display device 1 as seen from the user 22. If the display device 1 includes a housing 36 (see Figures 26-27) that houses the display panel 2 and the optical system 3, the real image may be formed inside the housing 36 or outside the housing 36. The real image may be formed on the side farther from the display panel 2 as seen from the user 22, or on the side closer to the display panel 2. If the housing 36 has a window 37 (see Figures 26-27) that transmits the display light emitted from the optical system 3, the real image may be formed on the side farther from the window 37 (light-transmitting plate 38) as seen from the user 22, or on the side closer to the window 37 (light-transmitting plate 38).

[0017] The display panel 2 has a display surface 2a, on which a display image is displayed. In other words, the display panel 2 emits display light of the display image from the display surface 2a. The display panel 2 may be configured to emit linearly polarized display light. The following description will focus on, but is not limited to, the case in which the display panel 2 emits S-wave polarized display light. For example, if the display panel 2 emits P-wave polarized display light, then S-wave polarization in the following description may be read as P-wave polarization, and P-wave polarization may be read as S-wave polarization.

[0018] The display panel 2 may be a liquid crystal panel. The liquid crystal panel may have a known liquid crystal panel configuration. Known liquid crystal panels may be, for example, IPS (In-Plane Switching), FFS (Fringe Field Switching), VA (Vertical Alignment), ECB (Electrically Controlled Birefringence), and the like.

[0019] The display device 1 may include an irradiator 4 that illuminates the display panel 2 in a planar manner. The irradiator 4 is also called a backlight. The irradiator 4 may be an edge-lit backlight or a direct-lit backlight. An edge-lit backlight has one or more light sources arranged on the outer periphery of the display panel 2, and guides the light emitted from the light sources to the entire back surface of the display panel 2 by a light guide plate for uniform dispersion. A direct-lit backlight has multiple light sources arranged on the back side of the display panel 2, and illuminates the display panel 2 with light emitted from the multiple light sources. The light sources of the irradiator 4 may be cold cathode fluorescent lamps, halogen lamps, or xenon lamps, or they may be light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), or semiconductor lasers (LDs). When the light source of the irradiator 4 is an LD with excellent monochromaticity, the design of the optical system 3, in particular the design of optical components whose optical properties are wavelength-dependent, becomes easier.

[0020] The display panel 2 is not limited to a liquid crystal panel (transmissive display panel). The display panel 2 may be a self-emissive display panel that includes self-emissive elements such as a light-emitting diode (LED), an organic light-emitting diode (OLED), or a semiconductor laser (LD).

[0021] The optical system 3 projects the display light emitted from the display panel 2 as a virtual image V within the user's field of view 22. As shown in Figure 2, the optical system 3 may include a first phase difference plate 5, a semi-transparent mirror 6, a second phase difference plate 7, and a reflective polarizer 8. The first phase difference plate 5, the semi-transparent mirror 6, the second phase difference plate 7, and the reflective polarizer 8 are arranged in this order in the direction of emission of the display light from the display panel 2 (positive direction in the Z-axis direction).

[0022] The first phase difference plate 5 may face the display surface 2a of the display panel 2, or it may be positioned at a distance from the display panel 2. The first phase difference plate 5 may be positioned at a distance from the display surface 2a in the direction of emission of display light from the display panel 2. The second phase difference plate 7 may be positioned at a distance from the first phase difference plate 5 in the direction of emission of display light from the display panel 2. The first phase difference plate 5 and the second phase difference plate 7 may be quarter-wave plates. The first phase difference plate 5 and the second phase difference plate 7 impart a phase difference of 1 / 4 wavelength to the polarization plane (polarization plane in the direction of electric field vibration) of the incident light. This makes it possible to reflect a portion of the display light emitted from the display panel 2 with the reflective polarizer plate 8 and direct it into the semi-transparent mirror 6. The positional relationship between the first phase difference plate 5 and the second phase difference plate 7 may be defined such that, when viewed along the Z-axis direction, the retard axis of the second phase difference plate 7 is perpendicular to the retard axis of the first phase difference plate 5. Alternatively, the positional relationship between the first phase difference plate 5 and the second phase difference plate 7 may be defined such that, when viewed along the Z-axis direction, the retard axis of the second phase difference plate 7 is parallel to the retard axis of the first phase difference plate 5.

[0023] The first phase difference plate 5 and the second phase difference plate 7 only need to be able to provide the necessary phase difference to the light transmitted through them so that the light transmitted through them is reflected by the reflective polarizer 8. That is, for example, when the polarization obtained by transmitting through the first phase difference plate 5 and the second phase difference plate 7 is defined as the second polarization, the first phase difference plate 5 and the second phase difference plate 7 may be other wavelength plates or combinations thereof, rather than quarter-wave plates, as long as the second polarization is obtained. In this disclosure, the case where the first phase difference plate 5 and the second phase difference plate 7 are quarter-wave plates will be explained as an example. Furthermore, the first phase difference plate 5 and the second phase difference plate 7 may be film-like members.

[0024] Furthermore, the second phase difference plate 7 only needs to be able to provide the necessary phase difference to the light that has passed through the second phase difference plate 7 so that the light that has been reflected by the reflective polarizer 8 and passed through the second phase difference plate 7 passes through the reflective polarizer 8 again when it reaches the reflective polarizer 8. In other words, for example, if the polarization obtained after being reflected by the reflective polarizer 8 and passed through the second phase difference plate 7 is taken as the first polarization, the second phase difference plate 7 may be a wave plate other than a quarter wave plate, as long as the first polarization can be obtained.

[0025] The first phase difference plate 5 may be integrated with the display panel 2, as shown in Figure 23. "Integration" may mean that the two members are arranged in contact with each other, or that the two members are joined to each other by an optically transparent adhesive such as OCR (Optical Clear Resin) or OCA (Optical Clear Adhesive). However, the first phase difference plate 5 may be positioned away from the display surface 2a in the direction of emission of display light from the display panel 2.

[0026] The semi-transparent mirror 6 may be positioned between the first phase difference plate 5 and the second phase difference plate 7. The semi-transparent mirror 6 may transmit a portion of the incident light (for example, approximately 50%) and reflect the remainder (for example, approximately 50%). The transmittance and reflectance of light incident on the semi-transparent mirror 6 are not limited to 50%. The semi-transparent mirror 6 reflects a portion of the display light reflected by the reflective polarizing plate 8 and directs it into the eyes of the user 22. This makes it possible for the user 22 to see the virtual image V. The semi-transparent mirror 6 may have a function to collect or focus light. Specifically, the semi-transparent mirror 6 may have a function to collect or focus light that has been incident on and reflected by the semi-transparent mirror 6. In this embodiment, the semi-transparent mirror 6 can collect or focus light more effectively than other members of the optical system 3. In other words, the semi-transparent mirror 6 has a larger value for indicators such as the degree of collection, degree of convergence, or the reciprocal of the focal length than other members of the optical system 3.

[0027] Furthermore, in this embodiment, the optical system 3 may have only a semi-transparent mirror 6 as a member having a light-gathering or focusing function. The semi-transparent mirror 6 may be a concave mirror having a concave reflective surface 6a, as shown in Figure 2. The semi-transparent mirror 6 is located on the side of the second phase difference plate 7, and more specifically, the reflective surface 6a of the semi-transparent mirror 6 may be located on the side of the second phase difference plate 7. In this embodiment, the semi-transparent mirror 6 can gather or focus light by having a concave reflective surface 6a. The reflective surface 6a of the semi-transparent mirror 6 has a greater curvature than the other members of the optical system 3. The semi-transparent mirror 6 may include a spherical shape, an aspherical shape, or a free-form shape in at least a part of the reflective surface 6a. The transmittance of the semi-transparent mirror 6 is not limited to 50%. Also, the semi-transparent mirror 6 may be composed of a holographic optical element (HOE), or its surface shape may have a Fresnel shape.

[0028] The semi-transparent mirror 6 is composed of, for example, a substrate and a semi-transparent reflective layer located on the surface of the substrate. The substrate may have a transmittance of 100% or close to 100% for light in the visible light band. The substrate may be made of, for example, a resin material, a glass material, etc. The resin material may be, for example, an acrylic resin, a polycarbonate resin, etc. The semi-transparent reflective layer may be a thin metal film. The thin metal film may be made of, for example, a metal material such as aluminum or chromium. The semi-transparent reflective layer is not limited to a thin metal film, and may be, for example, a dielectric multilayer film, etc. The semi-transparent mirror 6 may be configured to reflect light with the semi-transparent reflective layer. The semi-transparent reflective layer may be formed on the surface of the substrate on the side of the second phase difference plate 7.

[0029] The reflective polarizer 8 may be positioned on the opposite side of the second phase difference plate 7 from the first phase difference plate 5. In other words, the reflective polarizer 8 may be positioned downstream of the second phase difference plate 7 in the direction of emission of display light from the display panel 2. The reflective polarizer 8 may transmit a portion of the incident light and reflect the remainder. In this embodiment, the reflective polarizer 8 may be configured to reflect polarized light having a polarization axis perpendicular to the polarization axis of the display light (also called P-wave polarized light or second polarized light) and transmit polarized light having a polarization axis parallel to the polarization axis of the display light (also called S-wave polarized light or first polarized light). In this case, the positional relationship between the first phase difference plate 5 and the second phase difference plate 7 may be defined such that when the first phase difference plate 5 and the second phase difference plate 7 are viewed along the Z-axis direction, the lagging axis of the second phase difference plate 7 and the lagging axis of the first phase difference plate 5 are parallel. Furthermore, the reflective polarizer 8 may be configured to transmit polarized light having a polarization axis perpendicular to the polarization axis of the display light and to reflect polarized light having a polarization axis parallel to the polarization axis of the display light. In this case, the positional relationship between the first phase difference plate 5 and the second phase difference plate 7 may be defined such that when the first phase difference plate 5 and the second phase difference plate 7 are viewed along the Z-axis direction, the lagging axis of the second phase difference plate 7 is perpendicular to the lagging axis of the first phase difference plate 5. This makes it possible for the user 22 to view the virtual image V. The reflective polarizer 8 may be integrated with the second phase difference plate 7, as shown in Figure 23. The reflective polarizer 8 may be flat, have a concave shape on the display panel 2 side, or have a convex shape on the display panel 2 side. Furthermore, the reflective polarizer 8 may be composed of a holographic optical element (HOE), or its surface shape may have a Fresnel shape.

[0030] The reflective polarizer 8 may be a wire grid polarizer comprising, for example, a substrate and a plurality of metal nanowires (also called a metal nanowire grid) located on the surface of the substrate. The substrate may have a transmittance of 100% or nearly 100% for light in the visible light band. The substrate may be made of, for example, a resin material, a glass material, etc. The metal nanowires may be made of, for example, a metal material such as aluminum, chromium, or titanium oxide. The metal nanowires may be arranged along one direction. The reflective polarizer 8 can transmit light components vibrating in a direction perpendicular to the grid and can reflect light components vibrating in a direction parallel to the grid.

[0031] The optical function of the optical system 3 will now be described. The display panel 2 emits display light of first linear polarization L1. The display light of first linear polarization L1 emitted from the display panel 2 passes through the first phase difference plate 5 and is converted into light of first circular polarization C1. A portion of the first circular polarization C1 that has passed through the first phase difference plate 5 (for example, approximately 50%) passes through the semitransparent mirror 6. The first circular polarization C1 that has passed through the semitransparent mirror 6 passes through the second phase difference plate 7 and is converted into light of second linear polarization L2, whose polarization direction is parallel to that of first linear polarization L1. The light of second linear polarization L2 is incident on the reflective polarizer 8. The light of second linear polarization L2 that has been incident on the reflective polarizer 8 is reflected by the reflective polarizer 8 and converted into light of third linear polarization L3. The light of third linear polarization L3 is transmitted through the second phase difference plate 7 and converted into light of second circular polarization C2. A portion of the light of the second circularly polarized light C2 that has passed through the second phase difference plate 7 (for example, about 50%) is reflected by the semi-transparent mirror 6 and converted into light of the third circularly polarized light C3. The light of the third circularly polarized light C3 passes through the second phase difference plate 7 and is converted into light of the fourth linearly polarized light L4, whose polarization direction is perpendicular to that of the first linearly polarized light L1. The light of the fourth linearly polarized light L4 passes through the reflective polarizer 8 and is emitted to the outside. The amount of light (luminance) emitted from the display device 1 is, for example, about 25% of the amount of light (luminance) of the display light emitted from the display panel 2.

[0032] In one embodiment of the present disclosure, the first to third linearly polarized elements L1 to L3 described above may be S-wave polarized elements, and the fourth linearly polarized element L4 may be P-wave polarized elements.

[0033] Another optical function of the optical system 3 will now be described. The display panel 2 emits display light that is S-wave polarized (first linearly polarized light L1). The display light of the first linearly polarized light L1 emitted from the display panel 2 passes through the first phase difference plate 5 and is converted into light of the first circularly polarized light C1. A portion of the first circularly polarized light C1 that has passed through the first phase difference plate 5 (for example, approximately 50%) passes through the semi-transparent mirror 6. The first circularly polarized light C1 that has passed through the semi-transparent mirror 6 passes through the second phase difference plate 7 and is converted into light of the second linearly polarized light L2, whose polarization direction is perpendicular to the first linearly polarized light L1 (i.e., it is P-wave polarized). The light of the second linearly polarized light L2 is incident on the reflective polarizer 8. As described above, the reflective polarizer 8 may reflect P-wave polarized light and transmit S-wave polarized light. The light of the second linearly polarized light L2 incident on the reflective polarizer 8 is reflected by the reflective polarizer 8 and converted into light of the third linearly polarized light L3. The third linearly polarized light L3 passes through the second phase difference plate 7 and is converted into the second circularly polarized light C2. A portion of the second circularly polarized light C2 that has passed through the second phase difference plate 7 (for example, about 50%) is reflected by the semitransparent mirror 6 and converted into the third circularly polarized light C3. The third circularly polarized light C3 passes through the second phase difference plate 7 and is converted into the fourth linearly polarized light L4, whose polarization direction is parallel to the first linearly polarized light L1 (i.e., it is S-wave polarized). The fourth linearly polarized light L4 passes through the reflective polarizer 8 and is emitted to the outside. The amount of light (luminance) emitted from the display device 1 is, for example, about 25% of the amount of display light (luminance) emitted from the display panel 2.

[0034] Since the optical system 3 is an on-axis type optical system in which the optical axis of the incident light and the optical axis of the emitted light substantially coincide, the space occupied by the optical system 3 can be reduced, and as a result, the display device 1 can be miniaturized. In addition, because the optical system 3 is on-axis, distortion and brightness unevenness of the virtual image V seen by the user 22 can be reduced, and the design of the optical system 3 is simplified.

[0035] The display device 1 emits light from the display panel 2, passes through the semi-transmissive mirror 6, is reflected by the reflective polarizing plate 8, and the optical path length of the light reaching the semi-transmissive mirror 6 may be smaller than the focal length of the semi-transmissive mirror 6. In this case, the user 22 can visually recognize the virtual image V. The display device 1 emits light from the display panel 2, passes through the semi-transmissive mirror 6, is reflected by the reflective polarizing plate 8, and the optical path length of the light reaching the semi-transmissive mirror 6 may be larger than the focal length of the semi-transmissive mirror 6. In this case, the user 22 can visually recognize a real image.

[0036] In FIG. 2, for ease of illustration, the optical path of the light incident on the reflective polarizing plate 8 and the optical path of the light reflected by the reflective polarizing plate 8 are shown shifted in the height direction (Y-axis direction), and the optical path of the light incident on the semi-transmissive mirror 6 and the optical path of the light reflected by the semi-transmissive mirror 6 are shown shifted in the height direction (Y-axis direction). However, actually, the display light emitted from the display panel 2 propagates substantially on a single axis as shown in FIG. 28. This is the same for the optical paths shown in FIGS. 3 to 5, 9, 13, 14, 18 to 20, 22 to 25.

[0037] Next, a display device according to another embodiment of the present disclosure will be described. The display device of this embodiment has a different optical system configuration from the display device of the above embodiment, and other configurations are the same. Therefore, the same configurations are denoted by the same reference numerals, and detailed descriptions thereof are omitted.

[0038] As shown in FIG. 4, the display device 1A of this embodiment includes a display panel 2 and an optical system 10. The display panel 2 has a display surface 2a and displays a display image on the display surface 2a. The optical system 10 projects the display light emitted from the display panel 2 as a virtual image V within the visual field of the user 22.

[0039] The optical system 10 includes a first semi-transmissive mirror 11, a first retardation plate 12, a second semi-transmissive mirror 13, a second retardation plate 14, and a polarizing plate 15. That is, the optical system 10 has a pair of semi-transmissive mirrors (the first semi-transmissive mirror 11 and the second semi-transmissive mirror 13). The first semi-transmissive mirror 11, the first retardation plate 12, the second semi-transmissive mirror 13, the second retardation plate 14, and the polarizing plate 15 are arranged in this order in the emission direction of the display light from the display panel 2.

[0040] The first semi-transparent mirror 11 has a reflective surface 11a that reflects light. The first phase difference plate 12 may be located on the opposite side of the first semi-transparent mirror 11 from the display panel 2, and more specifically, on the side of the reflective surface 11a of the first semi-transparent mirror 11. The first phase difference plate 12 may be located away from the display surface 2a in the direction of emission of display light from the display panel 2. The second phase difference plate 14 may be located away from the first phase difference plate 12 in the direction of emission of display light. The first semi-transparent mirror 11 may have a function to collect or converge light. Specifically, the first semi-transparent mirror 11 may have a function to collect or converge light that is incident on and reflected by the reflective surface 11a of the first semi-transparent mirror 11. In this embodiment, the first semi-transparent mirror 11 has, for example, an index that is larger than the other members of the optical system 10, such as a degree of collection, degree of convergence, or index expressed as the reciprocal of the focal length. The first phase difference plate 12 and the second phase difference plate 14 may be quarter-wave plates. The positional relationship between the first phase difference plate 12 and the second phase difference plate 14 may be defined such that, when viewed along the Z-axis direction, the lagging axis of the second phase difference plate 14 is perpendicular to the lagging axis of the first phase difference plate 12. Alternatively, the positional relationship between the first phase difference plate 12 and the second phase difference plate 14 may be defined such that, when viewed along the Z-axis direction, the lagging axis of the second phase difference plate 14 is parallel to the lagging axis of the first phase difference plate 12.

[0041] The first semi-transparent mirror 11 is located between the display panel 2 and the first phase difference plate 12. The first semi-transparent mirror 11 may transmit a portion of the incident light and reflect the remainder. As shown in Figure 4, the first semi-transparent mirror 11 is a concave mirror having a concave reflective surface 11a located on the side of the first phase difference plate 12. In this embodiment, the first semi-transparent mirror 11 may be configured to transmit polarized light having a polarization axis parallel to the polarization axis of the display light and to reflect polarized light having a polarization axis perpendicular to the polarization axis of the display light. The first semi-transparent mirror 11 may be configured to transmit S-wave polarized light and reflect P-wave polarized light. Alternatively, the first semi-transparent mirror 11 may be configured to reflect S-wave polarized light and transmit P-wave polarized light. At least a portion of the reflective surface 11a of the first semi-transparent mirror 11 may include a spherical shape, an aspherical shape, or a free-form surface shape.

[0042] The first semi-transparent mirror 11 may be composed of, for example, a substrate and a plurality of metal nanowires (metal nanowire grids) located on the surface of the substrate. The substrate may have a transmittance of 100% or close to 100% for light in the visible light band. The substrate may be composed of, for example, a resin material, a glass material, etc. The resin material may be, for example, an acrylic resin, a polycarbonate resin, etc. The metal nanowires may be composed of, for example, a metal material such as aluminum, chromium, or titanium oxide. The metal nanowires may be arranged along one direction. The first semi-transparent mirror 11 can transmit light components vibrating in a direction perpendicular to the grid and can reflect light components vibrating in a direction parallel to the grid. The metal nanowire grid may be formed on the surface of the substrate on the side of the first phase difference plate 12. In this example, the metal nanowire grid is used to impart a reflective polarization function to the first semi-transparent mirror 11, but the first semi-transparent mirror 11 may be used as a simple half-mirror and a separate reflective polarizing plate may be provided.

[0043] The second semi-transparent mirror 13 is located between the first phase difference plate 12 and the second phase difference plate 14. The second semi-transparent mirror 13 may transmit a portion of the incident light (for example, approximately 50%) and reflect the remainder (for example, approximately 50%). The transmittance and reflectance of the light incident on the second semi-transparent mirror 13 are not limited to 50%. The second semi-transparent mirror 13 has a reflective surface 13a. The second semi-transparent mirror 13 may be located on the opposite side of the first semi-transparent mirror 11 from the first phase difference plate 12, and more specifically, as shown in Figure 4, the reflective surface 13a may be located on the side of the first phase difference plate 12. The second semi-transparent mirror 13 may be a plane mirror. The second semi-transparent mirror 13 is also called a plane half-mirror. The second semi-transparent mirror 13 may be integrated with at least one of the first phase difference plate 12 and the second phase difference plate 14, as shown in Figure 24.

[0044] The second semi-transparent mirror 13 may be composed of, for example, a substrate and a semi-transparent reflective layer located on the surface of the substrate. The substrate may have a transmittance of 100% or close to 100% for light in the visible light band. The substrate may be made of, for example, inorganic glass, a resin material, etc. The resin material may be, for example, acrylic resin, polycarbonate resin, etc. The semi-transparent reflective layer may be a thin metal film. The thin metal film may be made of, for example, a metal material such as aluminum or chromium. The semi-transparent reflective layer is not limited to a thin metal film, and may be, for example, a dielectric multilayer film, etc.

[0045] The polarizing plate 15 may be positioned on the opposite side of the second semitransparent mirror 13 from the second phase difference plate 14. In other words, the polarizing plate 15 is positioned downstream of the second phase difference plate 14 in the direction of emission of display light from the display panel 2. The polarizing plate 15 may transmit a portion of the incident light and absorb the remainder. The polarizing plate 15 may transmit a portion of the incident light and reflect the remainder. In this embodiment, the polarizing plate 15 may be configured to absorb or reflect polarization having a polarization axis parallel to the polarization axis of the display light (for example, S-wave polarization, also called third polarization) and transmit polarization having a polarization axis perpendicular to the polarization axis of the display light (for example, P-wave polarization, also called fourth polarization). In this case, the positional relationship between the first phase difference plate 12 and the second phase difference plate 14 may be defined such that when the first phase difference plate 12 and the second phase difference plate 14 are viewed along the Z-axis direction, the lagging axis of the second phase difference plate 14 is perpendicular to the lagging axis of the first phase difference plate 12. Furthermore, the polarizing plate 15 may be configured to absorb or reflect polarized light having a polarization axis perpendicular to the polarization axis of the display light, and to transmit polarized light having a polarization axis parallel to the polarization axis of the display light. In this case, the positional relationship between the first phase difference plate 12 and the second phase difference plate 14 may be defined such that when the first phase difference plate 12 and the second phase difference plate 14 are viewed along the Z-axis direction, the lagging axis of the second phase difference plate 14 and the lagging axis of the first phase difference plate 12 are parallel. The polarizing plate 15 may be integrated with the second phase difference plate 14, as shown in Figure 24.

[0046] The polarizing plate 15 may have the configuration of a known absorption polarizing plate. Known absorption polarizing plates may be, for example, an iodine-based polarizing plate in which an iodine compound is adsorbed and oriented on a polyvinyl alcohol (PVA) film, or a dye-based polarizing plate in which a dichroic organic dye is adsorbed and oriented on a PVA film. Alternatively, the polarizing plate 15 may have the configuration of a reflective polarizing plate.

[0047] The optical function of the optical system 10 will now be described. The display light, which is S-wave polarized (first linearly polarized) light emitted from the display panel 2, passes through the first semi-transparent mirror 11. The display light of the first linearly polarized L1 passes through the first phase difference plate 12 and is converted into light of the first circularly polarized C1. The light of the first circularly polarized C1 is incident on the second semi-transparent mirror 13. A portion of the light of the first circularly polarized C1 (for example, approximately 50%) is reflected by the second semi-transparent mirror 13 and converted into light of the second circularly polarized C2. The light of the second circularly polarized C2 passes through the first phase difference plate 12 and is converted into light of the second linearly polarized L2, whose polarization direction is perpendicular to that of the first linearly polarized L1 (i.e., it is P-wave polarized). The light of the second linearly polarized L2 is reflected by the first semi-transparent mirror 11 and is converted into light of the third linearly polarized L3, whose polarization direction is perpendicular to that of the first linearly polarized L1. The third linearly polarized light L3 passes through the first phase difference plate 12 and is converted into the third circularly polarized light C3. A portion of the third circularly polarized light C3 (for example, approximately 50%) passes through the second semi-transparent mirror 13. The third circularly polarized light C3 that has passed through the second semi-transparent mirror 13 passes through the second phase difference plate 14 and is converted into the fourth linearly polarized light L4, whose polarization direction is perpendicular to the first linearly polarized light L1 (i.e., it is P-wave polarized). The fourth linearly polarized light L4 passes through the polarizer plate 15 and is emitted to the outside.

[0048] The remaining portion of the light from the first circularly polarized light C1 (for example, approximately 50%) passes through the second semi-transparent mirror 13, then through the second phase difference plate 14, and is converted into fifth linearly polarized light L5, whose polarization direction is parallel to the first linearly polarized light L1 (i.e., S-wave polarized light). The fifth linearly polarized light L5 is absorbed or reflected by the polarizer plate 15 and is therefore not emitted to the outside. In other words, the fifth linearly polarized light L5 is light that is not transmitted through the polarizer plate 15. Therefore, the amount of light (luminance) emitted from the display device 1A is, for example, approximately 25% of the amount of display light (luminance) emitted from the display panel 2.

[0049] In the above example, the first phase difference plate 12 and the second phase difference plate 14 are described as quarter-wave plates. However, the first phase difference plate 12 and the second phase difference plate 14 may be other wave plates or combinations thereof, as long as some of the light is absorbed by the polarizer plate 15 and other light is transmitted through the polarizer plate 15. For example, the first phase difference plate 12 and the second phase difference plate 14 should be designed to provide the necessary phase difference to the light that is transmitted through the first phase difference plate 12 and the second phase difference plate 14 without being reflected by the second semi-transparent mirror 13, so that some of the light that is transmitted through the first phase difference plate 12 and the second phase difference plate 14 without being reflected by the second semi-transparent mirror 13 is absorbed or reflected by the polarizer plate 15. That is, for example, when the polarization obtained by passing through the first phase difference plate 12 and the second phase difference plate 14 without being reflected by the second semi-transparent mirror 13 is taken as the third polarization, the first phase difference plate 12 and the second phase difference plate 14 may be other wave plates or combinations thereof, rather than quarter-wave plates, as long as the third polarization is obtained. In this disclosure, the case where the first phase difference plate 12 and the second phase difference plate 14 are quarter-wave plates will be explained as an example. Furthermore, the first phase difference plate 12 and the second phase difference plate 14 should be able to provide the necessary phase difference to the light that is reflected by the second semi-transparent mirror 13 and the first semi-transparent mirror 11 and passed through the first phase difference plate 12 and the second phase difference plate 14, so that the light that is reflected by the second semi-transparent mirror 13 and the first semi-transparent mirror 11 and passed through the first phase difference plate 12 and the second phase difference plate 14 is able to pass through the polarizer plate 15. In other words, for example, when the polarization obtained by reflecting light from the second semi-transparent mirror 13 and the first semi-transparent mirror 11 and passing through the first phase difference plate 12 and the second phase difference plate 14 is defined as the fourth polarization, the first phase difference plate 12 and the second phase difference plate 14 may be other wave plates instead of quarter-wave plates, as long as the fourth polarization is obtained. Also, the first phase difference plate 12 and the second phase difference plate 14 may be other wave plates or a combination thereof, instead of quarter-wave plates, as long as some of the light is reflected by the first semi-transparent mirror 11 and the other light is passed through the first semi-transparent mirror 11. The first phase difference plate 12 should be designed to give the light that has passed through the first phase difference plate 12 the necessary phase difference so that the light that has passed through the first phase difference plate 12, reflected by the second semi-transparent mirror 13, and then passed through the first phase difference plate 12 again is reflected by the first semi-transparent mirror 11.In other words, for example, if the polarization obtained by passing through the first phase difference plate 12, reflecting off the second semi-transparent mirror 13, and then passing through the first phase difference plate 12 again is defined as the fifth polarization, then the first phase difference plate 12 and the fifth polarization can be obtained, and other wave plates can be used instead of quarter-wave plates. Also, the first phase difference plate 12 and the second phase difference plate 14 may be film-like materials.

[0050] Since the optical system 10 is an on-axis type optical system in which the optical axis of the incident light and the optical axis of the emitted light substantially coincide, the space occupied by the optical system 10 can be reduced, and as a result, the display device 1A can be miniaturized. In addition, because the optical system 10 is on-axis, distortion and brightness unevenness of the virtual image V seen by the user 22 can be reduced, and the design of the optical system 10 is simplified.

[0051] Next, another example of the display device 1A will be described. The display device 1A' in this example differs from the display device 1A described above in the configuration (shape) of the second semi-transparent mirror, but otherwise has the same configuration. Therefore, the same reference numerals are used for the same components, and detailed explanations are omitted.

[0052] The display device 1A' in this example comprises a display panel 2 and an optical system 10, as shown in Figure 5. The optical system 10 is composed of a first semi-transparent mirror 11, a first phase difference plate 12, a second semi-transparent mirror 13', a second phase difference plate 14, and a polarizing plate 15. That is, the optical system 10 consists of a pair of semi-transparent mirrors (first semi-transparent mirror 11, second semi-transparent mirror 13'), the first semi-transparent mirror 11, the first phase difference plate 12, the second semi-transparent mirror 13', the second phase difference plate 14, and the polarizing plate 15, which are arranged in this order in the direction of emission of display light from the display panel 2.

[0053] The second semi-transparent mirror 13' has a convex reflective surface 13'a. The second semi-transparent mirror 13' may be located on the opposite side of the first semi-transparent mirror 11 from the first phase difference plate 12, and more specifically, the reflective surface 13'a may be located on the side of the first phase difference plate 12. The second semi-transparent mirror 13' is also called a convex half-mirror. The second semi-transparent mirror 13' may transmit a portion of the incident light (for example, approximately 50%) and reflect the remainder (for example, approximately 50%). The transmittance and reflectance of light incident on the second semi-transparent mirror 13' are not limited to 50%. In this case, the reflective surface of the second semi-transparent mirror 13' diverges the light incident from the first semi-transparent mirror 11 side and reflects it, while the reflective surface of the first semi-transparent mirror 11 converges the light incident from the second semi-transparent mirror 13 side and reflects it.

[0054] The second semi-transparent mirror 13' may be composed of, for example, a substrate and a semi-transparent reflective layer located on the surface of the substrate. The substrate may have a transmittance of 100% or close to 100% for light in the visible light band. The substrate may be made of, for example, inorganic glass, a resin material, etc. The resin material may be, for example, acrylic resin, polycarbonate resin, etc. The semi-transparent reflective layer may be a thin metal film. The thin metal film may be made of, for example, a metal material such as aluminum or chromium. The semi-transparent reflective layer is not limited to a thin metal film, and may be, for example, a dielectric multilayer film, etc.

[0055] The optical system 10 may be configured such that the focal length of the second semi-transparent mirror 13' is greater than the distance between the display panel 2 and the second semi-transparent mirror 13'. In other words, the optical system 10 may be configured such that the second semi-transparent mirror 13' projects a reduced virtual image Q' (see Figure 7) of the object (i.e., the display surface 2a). Furthermore, the optical system 10 may be configured such that the focal length of the first semi-transparent mirror 11 is greater than the distance between the virtual image Q' and the first semi-transparent mirror 11. In other words, the optical system 10 may be configured such that the first semi-transparent mirror 11 projects an enlarged virtual image V of the object (i.e., the virtual image Q'). In this case, it becomes possible to adjust the magnification ratio and projection distance of the virtual image V while reducing the thickness of the optical system 10 in the depth direction (Z-axis direction).

[0056] Since the optical system 10 is an on-axis type optical system in which the optical axis of the incident light and the optical axis of the emitted light substantially coincide, the space occupied by the optical system 10 can be reduced, and as a result, the display device 1A' can be miniaturized. In addition, because the optical system 10 is on-axis, distortion and brightness unevenness of the virtual image V seen by the user 22 can be reduced, and the design of the optical system 10 is simplified.

[0057] Furthermore, according to the display device 1A' of this example, the optical system 10 can be made thinner in the depth direction (Z-axis direction), thus providing a thin display device. The thinning of the optical system 10 will be explained below with reference to Figures 6 and 7. The first semi-transparent mirror 11 of the display devices 1A and 1A' has a concave reflective surface 11a that reflects the display light emitted to the outside, so below, the first semi-transparent mirror 11 may be referred to as a concave mirror. The second semi-transparent mirror 13 of the display device 1A has a planar reflective surface 13a that reflects the display light emitted to the outside, so below, the second semi-transparent mirror 13 may be referred to as a planar mirror. The second semi-transparent mirror 13' of the display device 1A' has a convex reflective surface 13'a that reflects the display light emitted to the outside, so below, the second semi-transparent mirror 13' may be referred to as a convex mirror. The dimension of the optical system 10 in the depth direction (Z-axis direction) may be referred to as the thickness of the optical system 10.

[0058] Figure 6 illustrates the projection of the virtual image V in the display device 1A. In Figure 6, the illuminator 4 and optical components that do not contribute to the projection distance (virtual image distance) and magnification of the virtual image V (first phase difference plate 12, second phase difference plate 14, and polarizer plate 15) are omitted. Also, the concave mirror 11 is positioned in contact with the display panel 2 so that the distance between the display panel 2 and the concave mirror 11 can be considered as "0". In the following explanation, the focal length of the concave mirror 11 is denoted as f, and the distance between the concave mirror 11 and the plane mirror 13 is denoted as a / 2. The distance a / 2 corresponds to the thickness of the optical system 10 of the display device 1A.

[0059] The display device 1A is configured to project a virtual image V by magnifying the virtual image Q of the display surface 2a, which is formed by the plane mirror 13, using the concave mirror 11. As shown in Figure 6, the virtual image Q is located on the opposite side of the concave mirror 11 from the plane mirror 13, and its distance from the plane mirror 13 is a / 2. The virtual image Q is an image of the display surface 2a magnified to 1x.

[0060] The virtual image distance b and virtual image magnification m of the virtual image V are expressed by the following equations (1) and (2), respectively. The virtual image distance b is the distance between the virtual image V and the concave mirror 11, and the virtual image magnification m is the magnification ratio of the virtual image V relative to the display surface 2a.

[0061] b=1 / (1 / a-1 / f)...(1) m=b / a...(2)

[0062] Table 1 shows configuration examples 1 and 2 of the display device 1A. The units for focal length f, thickness a / 2, and virtual image distance b shown in Table 1 are "mm". Configuration examples 1 and 2 are configured so that the virtual image distance b is 200 mm and the virtual image magnification m is 2 or 3. As shown in Table 1, when the optical system 10 includes a plane mirror 13, in order to set the virtual image distance b to 200 mm and the virtual image magnification m to 2, the thickness a / 2 of the optical system 10 must be 50 mm (see Configuration Example 1), and in order to set the virtual image distance b to 200 mm and the virtual image magnification m to 3, the thickness a / 2 of the optical system 10 must be 33.5 mm (see Configuration Example 2).

[0063] Figure 7 illustrates the projection of the virtual image V in the display device 1A'. In Figure 7, the illuminator 4 and optical components that do not contribute to the projection distance (virtual image distance) and magnification of the virtual image V (first phase difference plate 12, second phase difference plate 14, and polarizer plate 15) are omitted. Also, the concave mirror 11 is positioned in contact with the display panel 2 so that the distance between the display panel 2 and the concave mirror 11 can be considered as "0". In the following description, the focal length of the convex mirror 13' is denoted as f', the focal length of the concave mirror 11 as f'', and the distance between the concave mirror 11 and the convex mirror 13' is denoted as a' / 2. The distance a' / 2 corresponds to the thickness of the optical system 10 of the display device 1A'.

[0064] The display device 1A' is configured to project a virtual image Q' of the display surface 2a, formed by a convex mirror 13', as a virtual image V, magnified by a concave mirror 11. As shown in Figure 7, the virtual image Q' is located on the opposite side of the concave mirror 11 from the convex mirror 13'. The distance b' between the virtual image Q' and the convex mirror 13' is expressed by the following equation (3). The magnification factor m' of the virtual image Q' relative to the display surface 2a is expressed by the following equation (4). As is clear from equation (3), since b' < a' / 2, the magnification factor m' of the virtual image Q' is less than 1. Therefore, the virtual image Q' is a reduced virtual image of the display surface 2a.

[0065] b' = 1 / {1 / f' + 1 / (a' / 2)} ... (3) m' = b' / (a' / 2) ... (4) The virtual image distance b'' and virtual image magnification m'' of the virtual image V are expressed by the following equations (5) and (6). The virtual image distance b'' is the distance between the virtual image V and the concave mirror 11, and the virtual image magnification m'' is the magnification ratio of the virtual image V relative to the display surface 2a.

[0066] b'' = 1 / {1 / (a' / 2 + b') - 1 / f''} ... (5) m'' = (b' / (a' / 2)) × b'' / (a' / 2 + b') ... (6) Table 2 shows configuration examples 3 and 4 of the display device 1A'. The units of the focal length f', f'', thickness a' / 2 and virtual image distance b'' shown in Table 2 are "mm". Configuration examples 3 and 4 are configured to have a virtual image distance b'' of 200 mm and a virtual image magnification m'' of 2 or 3, just like configuration examples 1 and 2. As shown in Table 2, when the optical system 10 includes a convex mirror 13', an optical system 10 with a thickness a' / 2 of 32 mm can achieve a virtual image distance b'' of 200 mm and a virtual image magnification m'' of 2, similar to Configuration Example 1 (see Configuration Example 3), and an optical system 10 with a thickness a' / 2 of 25.5 mm can achieve a virtual image distance b'' of 200 mm and a virtual image magnification m'' of 3, similar to Configuration Example 2 (see Configuration Example 4). Therefore, with the display device 1A', the optical system 10 can be made thinner, and as a result, a thin display device can be provided.

[0067] Given the values ​​of the virtual image distance b'', virtual image magnification m'', and thickness a' / 2, the display device 1A' can be configured to achieve these values ​​by designing the optical system 10 accordingly.

[0068] The design of the optical system 10 of the display device 1A' will be described below with reference to Figure 8. In Figure 8, as in Figure 7, the illuminator 4, the first phase difference plate 12, the second phase difference plate 14, and the polarizing plate 15 are omitted. Also, the concave mirror 11 is positioned in contact with the display panel 2 so that the distance between the display panel 2 and the concave mirror 11 can be considered as "0". In the following description, the thickness of the optical system 10 will be a1, the distance between the convex mirror 13' and the virtual image Q' will be b1, and the distance between the concave mirror 11 and the virtual image V will be b2. Furthermore, the magnification ratio of the virtual image Q' relative to the display surface 2a will be m1, and the magnification ratio of the virtual image V relative to the virtual image Q' will be m2. In addition, the focal length of the convex mirror 13' will be f1, and the focal length of the concave mirror 11 will be f2.

[0069] The magnification M of the virtual image V relative to the display surface 2a is expressed as the product of magnification m1 and magnification m2, as shown in equation (7) below. Also, the distance a2 between the concave mirror 11 and the virtual image Q' is expressed as the sum of thickness a1 and distance b1, as shown in equation (8) below.

[0070] M = m1 × m2 …(7) a2 = a1 + b1 …(8) If we define the thickness a1 of the optical system 10 as T and the virtual image distance (i.e., the distance b1 between the concave mirror 11 and the virtual image V) as D, then the magnification M is expressed by the following equation (9).

[0071] M = m1 × m2 = (b1 / a1) × (b2 / a2) = (b1 / T) × (D / a2) ... (9) Substituting the following equation (10), which holds true for the distance b1 between the convex mirror 13' and the virtual image Q', into equation (9), we obtain the following equation (11).

[0072] 1 / a1 = 1 / b1 + 1 / f1 ... (10) M = f1 × (1 + D / f2) / (T + f1) ... (11) Also, by substituting the following equation (12), which holds true for the distance b2 between the concave mirror 11 and the virtual image V, into equation (8), we obtain the following equation (13).

[0073] 1 / a² = 1 / b² + 1 / f² …(12) D × f² / (D + f²) = T + T × f1 / (T + f1) …(13) From equations (9) and (13), the focal length f1 of the convex mirror 13' and the focal length f2 of the concave mirror 11 can be determined as shown in equations (14) and (15) below. A in equation (15) is expressed by the following equation (16).

[0074] f1 = M × T × T / (D - 2 × M × T) ... (14) f2 = D × A / (M - A) ... (15) A = f1 / (T + f1) ... (16) As can be seen from the above calculations, the display device 1A' can determine the focal lengths f1 and f2 (i.e., design the optical system 10) to realize the magnification M, thickness T, and virtual image distance D, respectively, when these values ​​are given.

[0075] In the display devices 1A and 1A', the optical path length of the light emitted from the display panel 2, passing through the first semi-transparent mirror 11, reflected by the second semi-transparent mirrors 13 and 13', and returning to the first semi-transparent mirror 11 may be smaller than the focal length of the first semi-transparent mirror 11. In this case, a virtual image V can be perceived by the user 22. In the display devices 1A and 1A', the optical path length of the light emitted from the display panel 2, passing through the first semi-transparent mirror 11, reflected by the second semi-transparent mirrors 13 and 13', and returning to the first semi-transparent mirror 11 may be larger than the focal length of the first semi-transparent mirror 11. In this case, a real image can be perceived by the user 22.

[0076] Next, an imaging device according to one embodiment of the present disclosure will be described. The imaging device 100 of this embodiment includes display devices 1, 1A, 1A', and 1B. The imaging device 100 causes the user 22 to view the display light emitted from the display panel 2 as a virtual image V. Since the imaging device 100 includes display devices 1, 1A, 1A', and 1B, a compact imaging device can be realized, and the user 22 can view a virtual image V with improved display quality. In particular, when the imaging device 100 includes the display device 1A', a thin imaging device can be realized. The imaging device 100 may also cause the user 22 to view the display light emitted from the display panel 2 as a real image.

[0077] The imaging device 100 may be mounted on the mobile body 23, as shown in Figure 10. The mobile body 23 may be a vehicle. Although Figure 10 shows the case where the vehicle is a passenger car, the vehicle is not limited to a passenger car and may be an automobile such as a truck, bus, or trolleybus. The positions of the display devices 1, 1A, 1A', and 1B are arbitrary within the mobile body 23. The display devices 1, 1A, 1A', and 1B may be located on the dashboard (instrument panel), inside the dashboard, on the ceiling of the vehicle compartment, on the A-pillar, etc. The imaging device 100 may share some of its components with other devices and parts provided by the mobile body 23.

[0078] The imaging device 100 may include a camera 102 that captures the surrounding scenery of the moving object 23, as shown in Figure 10. Here, the surroundings of the moving object 23 may be at least one of the front, rear, side, above, and below the moving object 23. The camera 102 may include, for example, a CCD (Charge Coupled Device) image sensor or a CMOS (Complementary Metal Oxide Semiconductor) image sensor. The imaging device 100 and the camera 102 are connected by at least one of wired communication and wireless communication. If the moving object 23 is a vehicle, the imaging device 100 and the camera 102 may be connected via a vehicle network such as a CAN (Control Area Network).

[0079] The imaging device 100 may be configured to display at least a portion of the captured image captured by the camera 102 on the display panel 2. In this case, the imaging device 100 can allow the user 22 (driver of the mobile vehicle 23) to view the surrounding scenery of the mobile vehicle 23 as a virtual image V formed on a side farther from the imaging device 100. As a result, the user 22 can view the surrounding scenery of the mobile vehicle 23 without significantly changing the viewing distance (point of gaze) while driving the mobile vehicle 23, making it easier to see the virtual image V and improving driving safety. Furthermore, since the imaging device 100 is a small imaging device, even if it is placed in the driver's cab of the mobile vehicle 23, it does not occupy a large volume in the driver's cab and is less likely to interfere with driving. The imaging device 100, mounted on the mobile vehicle 23 and configured to allow the user 22 to view the surrounding scenery of the mobile vehicle 23 (for example, the scenery behind the vehicle 23) as a virtual image V, is also called a digital rearview mirror.

[0080] The imaging device 100 may be applied to a digital side mirror. In this case, as shown in Figure 12, the imaging device 100 may include a display device 1, 1A, 1A', 1B (hereinafter also referred to as the left-side display device 1L) located on the left A-pillar of the mobile body 23, a camera 102 (hereinafter also referred to as the left-side camera 102L) that images the left rear of the mobile body 23, a display device 1, 1A, 1A', 1B (hereinafter also referred to as the right-side display device 1R) located on the right A-pillar of the mobile body 23, and a camera 102 (hereinafter also referred to as the right-side camera 102R) that images the right rear of the mobile body 23. The left-side display device 1L may allow the user 22 to view the image of the left rear of the mobile body 23 captured by the left-side camera 102L as a virtual image V (hereinafter also referred to as the virtual image V2). The right-side display device 1R may allow the user 22 to view a virtual image V (hereinafter also referred to as virtual image V3) of the right rear of the moving object 23, which has been captured by the right-side camera 102R. The image may be a moving image (also called a video image) or a still image.

[0081] The imaging device 100 may be applied to the cluster 29 in the dashboard of the mobile body 23 (see Figure 12). In this case, the display devices 1, 1A, 1A', and 1B may allow the user 22 to view images showing driving-related information such as vehicle speed, engine rotation speed, and fuel level as virtual images V (hereinafter also referred to as virtual images V4).

[0082] The imaging device 100 may be applied to a CID (Center Information Display) 30 (see Figure 12). In this case, the display devices 1, 1A, 1A', and 1B are arranged in the center cluster of the mobile body 23, and images showing information related to navigation, the in-vehicle environment (for example, settings for the air conditioning system, audio system, etc.) may be viewed by the user 22 as virtual images V (hereinafter also referred to as virtual image V5).

[0083] The imaging device 100 may be applied to a PID (Passenger Information Display) 31 (see Figure 12). In this case, the display devices 1, 1A, 1A', and 1B are positioned near the passenger seat on the dashboard, and may display images of entertainment content and images showing information about audio equipment, air conditioning equipment, etc., as virtual images V for the passenger.

[0084] The imaging device 100 may also be applied to the RSE (Rear Seat Entertainment) system 32 (see Figure 10). In this case, the display devices 1, 1A, 1A', and 1B are positioned on the backs of the front seats, and images of entertainment content and images showing information about audio equipment, air conditioning equipment, etc., can be viewed as virtual images V by passengers seated in the rear seats of the mobile unit 23.

[0085] Next, other examples of display devices 1, 1A, and 1A' will be described. Figures 13 and 14 are cross-sectional views illustrating other examples of display devices, Figures 15A to 15D and 16A to 16D are diagrams illustrating the optical system in other examples of display devices, and Figure 17 is a graph illustrating the optical system in other examples of display devices. In the following explanation, display device 1 will be used as an example, but the same applies to display devices 1A and 1A'.

[0086] The display device 1 is configured such that when the user 22 is positioned in front of the display device 1, the light of the second linearly polarized element L2 is reflected by the reflective polarizer 8 and not emitted from the display device 1 (see Figures 2 and 3). In other words, when viewed from the front of the display device 1, the display device 1 is configured such that the transmission axis of the polarizer on the front side (user 22 side) of the display panel 2 (liquid crystal panel) and the transmission axis of the reflective polarizer 8 are orthogonal (in a crossed nicol arrangement). As a result, as shown in Figures 2 and 3, the light of the second linearly polarized element L2 is not emitted from the display device 1, and the light of the fourth linearly polarized element L4 is emitted from the display device 1. In other words, the user 22 does not directly view the display panel 2, but views the reflected image reflected by the semitransparent mirror 6 as a virtual image V.

[0087] If the user 22 is not positioned directly in front of the display device 1, the crossed nicol arrangement of the transmission axis of the front polarizer plate of the display panel 2 and the transmission axis of the reflective polarizer plate 8 may be disrupted, causing some of the light of the second linearly polarized L2 to pass through the reflective polarizer plate 8. As a result, the user 22 may be able to see both the real image directly on the display panel 2 and the virtual image V reflected by the semitransparent mirror 6, which may degrade the display quality of the display device 1.

[0088] As shown in Figures 13 and 14, the display device 1 in this example has a third phase difference plate 25 located between the display panel 2 and the reflective polarizing plate 8. This allows the relative angle between the transmission axis of the front polarizing plate of the display panel 2 and the transmission axis of the reflective polarizing plate 8 to be brought closer to a crossed nicol arrangement, even when the user 22 is not positioned in front of the display device 1, thereby reducing the degradation of the display quality of the display device 1. The third phase difference plate 25 may be a half-wave plate, a quarter-wave plate, an eighth-wave plate, a sixteenth-wave plate, or any other wave plate that imparts a phase difference. The optical axis of the third phase difference plate 25 may be substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizing plate 8. The third phase difference plate 25 may be located between the second phase difference plate 7 and the reflective polarizing plate 8, or between the display panel 2 and the first phase difference plate 5.

[0089] The display device 1 in this example may further have a fourth phase difference plate 26 located between the display panel 2 and the reflective polarizing plate 8, as shown in Figures 13 and 14. In this case, even when the user 22 is not positioned in front of the display device 1, the relative angle between the transmission axis of the front polarizing plate of the display panel 2 and the transmission axis of the reflective polarizing plate 8 can be brought closer by the crossed nicol arrangement, further reducing the deterioration of the display quality of the display device 1. The fourth phase difference plate 26 may be a half-wave plate, a quarter-wave plate, an eighth-wave plate, a sixteenth-wave plate, etc., or any other wave plate that imparts a phase difference. The optical axis of the fourth phase difference plate 26 may be substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizing plate 8. The fourth phase difference plate 26 may be located between the second phase difference plate 7 and the reflective polarizing plate 8, or between the display panel 2 and the first phase difference plate 5.

[0090] The third phase difference plate 25 and the fourth phase difference plate 26 only need to be positioned between the display panel 2 and the reflective polarizer 8, and their positions are arbitrary. If no other optical elements are positioned between the third phase difference plate 25 and the fourth phase difference plate 26, the third phase difference plate 25 and the fourth phase difference plate 26 may be in contact with each other. In this case, the thickness of the optical system 3 in the depth direction can be reduced. The optical axes of the third phase difference plate 25 and the fourth phase difference plate 26 are substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizer 8, and the optical axis of the third phase difference plate 25 may be substantially perpendicular to the optical axis of the fourth phase difference plate 26.

[0091] The third phase difference plate 25 and the fourth phase difference plate 26 may be a quarter-wave plate and the other a half-wave plate. In this case, the deterioration of the display quality of the display device 1 can be effectively reduced. The third phase difference plate 25 and the fourth phase difference plate 26 may both be half-wave plates. In this case, the deterioration of the display quality of the display device 1 can be reduced more effectively.

[0092] FIGS. 15A, 15B, 15C, and 15D are Poincare spheres showing the optical functions (effects on the polarization state of light) of the third retardation plate 25 and the fourth retardation plate 26 when the third retardation plate 25 and the fourth retardation plate 26 are half-wave plates. FIGS. 15A and 15B are diagrams for explaining the optical function of the third retardation plate 25, and FIGS. 15C and 15D are diagrams for explaining the optical function of the fourth retardation plate 26. FIGS. 15A and 15C show views of the Poincare sphere seen from the north pole (S3-axis direction), and FIGS. 15B and 15D show views of the Poincare sphere seen from the side (S1-axis direction). In FIGS. 15A, 15B, 15C, and 15D, S ＬＣＤ indicates the polarization state of the light immediately after exiting the display panel 2. S ２５ indicates the polarization state of the light that has passed through the third retardation plate 25, and S ２６ indicates the polarization state of the light that has passed through the fourth retardation plate 26. It can be said that S ２６ indicates the polarization state of the light immediately before entering the reflective polarizing plate 8. S ＲＰ indicates the polarization state of the light that passes through the reflective polarizing plate 8 with a substantially 100% transmittance, and S ＡＰ is S ＲＰ 's antipodal point (a point symmetric with respect to the center of the Poincare sphere). S ２６ is located at S ＡＰ , or when located in the vicinity of S ＡＰ , it is possible to reduce the risk that the light emitted from the display panel 2 and passing through the third retardation plate 25 and the fourth retardation plate 26 passes through the reflective polarizing plate 8. As a result, it is possible to reduce the risk that the user 22 visually recognizes the real image directly viewing the display panel 2, and it is possible to reduce the degradation of the display quality of the display device 1.

[0093] As shown in FIGS. 15C and 15D, when the third retardation plate 25 and the fourth retardation plate 26 are half-wave plates, S ２６ is substantially located at S ＡＰ . Therefore, it is possible to reduce the risk that the user 22 visually recognizes the real image directly viewing the display panel 2, and it is possible to reduce the degradation of the display quality of the display device 1.

[0094] Figures 16A, 16B, 16C, and 16D show a Poincaré sphere illustrating the optical functions of the third phase difference plate 25 and the fourth phase difference plate 26 when the third phase difference plate 25 is a quarter-wave plate and the fourth phase difference plate 26 is a half-wave plate. Figures 16A and 16B illustrate the optical function of the third phase difference plate 25, and Figures 16C and 16D illustrate the optical function of the fourth phase difference plate 26. Figures 16A and 16C show the Poincaré sphere viewed from the north pole (S3 axis direction), and Figures 16B and 16D show the Poincaré sphere viewed from the side (S1 axis direction). ＬＣＤ S ２５ S ２６ S ＲＰ and S ＡＰ This is as stated above.

[0095] As shown in Figures 16C and 16D, when the third phase difference plate 25 is a quarter-wave plate and the fourth phase difference plate 26 is a half-wave plate, S ２６ is, S ＡＰ It is located in the vicinity of [the display panel]. Therefore, the risk of the user 22 directly viewing the actual image on the display panel 2 can be reduced, and the deterioration of the display quality of the display device 1 can be reduced.

[0096] Figure 17 is a graph showing the relationship between the light transmittance of an optical system formed by inserting a third phase difference plate 25 and a fourth phase difference plate 26 between polarizers PP1 and PP2 whose transmission axes are mutually orthogonal, and the phase difference between the third phase difference plate 25 and the fourth phase difference plate 26. Figure 17 shows the results obtained by simulation. The incident light was green light with a wavelength λ of 550 nm. Polarizers PP1, PP2, and PP1 are arranged in this order in the direction of propagation of the incident light. Polarizer PP1 is modeled after the front polarizer of the display panel 2, and polarizer PP2 is modeled after the reflective polarizer 8.

[0097] The solid line in the graph of Figure 17 shows the transmittance when the phase difference of the fourth phase difference plate 26 is fixed at 0 nm and the phase difference of the third phase difference plate 25 is varied, with the transmittance being minimum when the phase difference of the third phase difference plate 25 is approximately 275 nm (half the wavelength λ of the incident light). The dashed line in the graph of Figure 17 shows the transmittance when the phase difference of the third phase difference plate 25 is fixed at 270 nm and the phase difference of the fourth phase difference plate 26 is varied, with the transmittance being minimum when the phase difference of the fourth phase difference plate 26 is approximately 275 nm (half the wavelength of the incident light).

[0098] From the simulation results shown in the graph of Figure 17, it can be seen that when the third phase difference plate 25 and the fourth phase difference plate 26 of the display device 1 are half-wave plates, the risk of the user 22 directly viewing the real image on the display panel 2 can be effectively reduced, and the deterioration of the display quality of the display device 1 can be effectively reduced. Furthermore, even when the phase difference of the fourth phase difference plate 26 of the display device 1 is fixed at 0 nm (i.e., when only the third phase difference plate 25 is present), if the third phase difference plate 25 can impart a phase difference greater than 0 nm (i.e., non-zero) to the light incident on the third phase difference plate 25, the risk of the user 22 directly viewing the real image on the display panel 2 can be effectively reduced, and the deterioration of the display quality of the display device 1 can be effectively reduced.

[0099] The same applies to the display devices 1A and 1A'. The display devices 1A and 1A' may have a third phase difference plate 25 located between the display panel 2 and the polarizing plate 15. In this case, the risk of the user 22 directly viewing the real image on the display panel 2 can be reduced, and the deterioration of the display quality of the display devices 1A and 1A' can be reduced. The display devices 1A and 1A' may further have a fourth phase difference plate 26 located between the display panel 2 and the polarizing plate 15. In this case, the risk of the user 22 directly viewing the real image on the display panel 2 can be further reduced, and the deterioration of the display quality of the display devices 1A and 1A' can be further reduced. The third phase difference plate 25 and the fourth phase difference plate 26 may be a half-wave plate, a quarter-wave plate, an eighth-wave plate, a sixteenth-wave plate, etc., or other wave plates that impart a phase difference. The third phase difference plate 25 and the fourth phase difference plate 26 may be a quarter-wave plate and the other a half-wave plate. In this case, the degradation of the display quality of the display device 1 can be effectively reduced. The third phase difference plate 25 and the fourth phase difference plate 26 may both be half-wave plates. In this case, the degradation of the display quality of the display device 1 can be reduced more effectively. The third phase difference plate 25 and the fourth phase difference plate 26 may be located between the display panel 2 and the polarizing plate 15, and their positions are arbitrary. The third phase difference plate 25 and the fourth phase difference plate 26 may be located between the second phase difference plate 14 and the polarizing plate 15, or between the display panel 2 and the first phase difference plate 12. The optical axes of the third phase difference plate 25 and the fourth phase difference plate 26 may be substantially parallel or substantially perpendicular to the transmission axis of the polarizing plate 15. The third phase difference plate 25 and the fourth phase difference plate 26 have optical axes that are substantially parallel or substantially perpendicular to the transmission axis of the polarizing plate 15, and the optical axis of the third phase difference plate 25 may be substantially perpendicular to the optical axis of the fourth phase difference plate 26.

[0100] Next, other examples of the display devices 1, 1A, 1A', and 1B will be described. Figure 18 is a cross-sectional view showing another example of the display device 1A', and Figure 19 is a cross-sectional view showing another example of the display device 1A. Descriptions of the configurations or parts described in the above embodiments may be omitted.

[0101] The second semi-transparent mirror 13' of the display device 1A' may include a holographic optical element (HOE). In this case, as shown in Figure 18, the optical function of the second semi-transparent mirror 13' can be realized by a flat optical element, and the thickness of the second semi-transparent mirror 13' in the depth direction (Z-axis direction) can be reduced. As a result, the display device 1A' can be made smaller in the depth direction. Furthermore, because the second semi-transparent mirror 13' is a flat optical element, the distance between the second semi-transparent mirror 13' and the second phase difference plate 14 can be reduced, or the second semi-transparent mirror 13' and the second phase difference plate 14 can be brought into contact with each other, making it possible to make the display device 1A' even smaller in the depth direction.

[0102] The first semi-transparent mirror 11 of the display device 1A' may be configured to include a HOE (Hardware Overlay). In this case, as shown in Figure 18, the optical function of the first semi-transparent mirror 11 can be realized by a flat optical element, and the thickness of the first semi-transparent mirror 11 in the depth direction can be reduced. As a result, the display device 1A' can be made smaller in the depth direction. Furthermore, because the first semi-transparent mirror 11 is a flat optical element, the distance between the first semi-transparent mirror 11 and the display panel 2 can be reduced, or the first semi-transparent mirror 11 and the display panel 2 can be brought into contact with each other, making it possible to make the display device 1A' even smaller in the depth direction.

[0103] If the first semi-transparent mirror 11 does not have polarization selectivity, the amount of light emitted from the display device 1A' decreases, and the brightness of the virtual image V seen by the user 22 decreases. Therefore, the first semi-transparent mirror 11, including the HOE, may be configured to have polarization selectivity. For example, the first semi-transparent mirror 11, including the HOE, may have a plurality of metal nanowires (metal nanowire grids) formed on the surface located on the display panel 2 side or the surface located on the first phase difference plate 12 side, which transmit S-wave polarized light and reflect P-wave polarized light, thereby achieving polarization selectivity. In this case, the decrease in brightness of the virtual image V seen by the user 22 can be reduced.

[0104] The first semi-transparent mirror 11 of the display device 1A may be configured to include a HOE (Heat Edge Enclosure). In this case, as shown in Figure 19, the optical function of the first semi-transparent mirror 11 can be realized by a flat optical element, and the thickness of the first semi-transparent mirror 11 in the depth direction can be reduced. As a result, the display device 1A can be made smaller in the depth direction. Furthermore, because the first semi-transparent mirror 11 is a flat optical element, the distance between the first semi-transparent mirror 11 and the display panel 2 can be reduced, or the first semi-transparent mirror 11 and the display panel 2 can be brought into contact with each other, making it possible to further miniaturize the display device 1A in the depth direction. The first semi-transparent mirror 11 including the HOE may have polarization selectivity. For example, the first semi-transparent mirror 11 including the HOE may have a plurality of fine metal wires formed on the surface located on the display panel 2 side or the surface located on the first phase difference plate 12 side, which transmit S-wave polarized light and reflect P-wave polarized light, thereby achieving polarization selectivity. In this case, the reduction in brightness of the virtual image V seen by the user 22 can be reduced.

[0105] The semi-transparent mirror 6 of the display device 1 may be configured to include a HOE (Hardware Overlay). In this case, the optical function of the semi-transparent mirror 6 can be realized by a flat optical element, and the thickness of the semi-transparent mirror 6 in the depth direction can be reduced. As a result, the display device 1 can be made smaller in the depth direction. Furthermore, because the semi-transparent mirror 6 is a flat optical element, the distance between the semi-transparent mirror 6 and the first phase difference plate 5 can be reduced, or the semi-transparent mirror 6 and the first phase difference plate 5 can be brought into contact with each other, making it possible to make the display device 1 even smaller in the depth direction.

[0106] A holographic optical element may, for example, have an interference fringe pattern and be configured to diffract incident light in a predetermined direction.

[0107] The display device 1A' may be configured such that the second semi-transparent mirror 13' includes a Fresnel lens. In other words, the display device 1A' may have a Fresnel shape on the surface of the second semi-transparent mirror 13'. In this case, as shown in Figure 20, the optical function of the second semi-transparent mirror 13' can be realized by a substantially flat optical element with a reduced thickness (dimension in the depth direction) compared to a convex half-mirror, thereby reducing the thickness of the second semi-transparent mirror 13' in the depth direction. As a result, the display device 1A' can be made smaller in the depth direction. Furthermore, because the second semi-transparent mirror 13' is substantially flat, the distance between the second semi-transparent mirror 13' and the second phase difference plate 14 can be reduced, or the second semi-transparent mirror 13' and the second phase difference plate 14 can be brought into contact with each other, making it possible to further miniaturize the display device 1A' in the depth direction. The second semi-transparent mirror 13' including a Fresnel lens is also called a Fresnel half-mirror 13'.

[0108] As shown in Figure 21, the Fresnel half-mirror 13' may be composed of a Fresnel lens (Fresnel convex lens) 33 having a planar first surface 33a located on the second phase difference plate 14 side and a Fresnel-shaped second surface 33b located on the first phase difference plate 12 side, and a semi-transparent reflective layer 34 located on the second surface 33b. The Fresnel shape has concentric grooves centered on a reference point 33c. The grooves include a surface substantially perpendicular to the first surface 33a and an inclined surface inclined with respect to the first surface 33a. The inclined surface may be a curved surface or a flat surface. The semi-transparent reflective layer 34 may be located on the inclined surface of the Fresnel shape. The semi-transparent reflective layer 34 may transmit a portion of the incident light (e.g., approximately 50%) and reflect the remainder (e.g., approximately 50%). The transmittance and reflectance of light incident on the semi-transparent reflective layer 34 are not limited to 50%. The semi-transparent reflective layer 34 may be a thin metal film. The metal thin film may be composed of a metallic material such as aluminum or chromium. The metal thin film may be formed by a vapor deposition method such as CVD (Chemical Vapor Deposition) or PVD (Physical Vapor Deposition).

[0109] The Fresnel half-mirror 13' has both an optical function as a lens and an optical function as a half-mirror. The optical function as a lens (e.g., focal length) is determined by the curvature and angle of inclination of the inclined surface, the refractive index of the material constituting the Fresnel lens 33, etc. The optical function as a half-mirror (e.g., focal length, transmittance, etc.) is determined by the curvature and angle of inclination of the inclined surface, the transmittance of the semi-transparent reflective layer 34, etc.

[0110] The Fresnel half-mirror 13' may have its surface facing the second phase difference plate 14 flattened by a transparent material layer formed on the second surface 33b of the Fresnel lens 33. The transparent material layer may be made of a material having substantially the same refractive index as the material constituting the Fresnel lens 33. The transparent material layer may be made of the same material as the material constituting the Fresnel lens 33.

[0111] The display device 1A' may be configured such that the first semi-transparent mirror 11 includes a Fresnel lens. In other words, the display device 1A' may have a Fresnel shape on the surface of the first semi-transparent mirror 11. In this case, as shown in Figure 20, the thickness of the first semi-transparent mirror 11 can be reduced, and as a result, the display device 1A' can be made smaller in the depth direction. Furthermore, since the first semi-transparent mirror 11 including the Fresnel lens is substantially flat, the distance between the first semi-transparent mirror 11 and the display panel 2 can be reduced, or the first semi-transparent mirror 11 and the display panel 2 can be brought into contact with each other, making it possible to make the display device 1A' even smaller in the depth direction. The first semi-transparent mirror 11 including the Fresnel lens is also called a Fresnel half mirror 11. The Fresnel half mirror 11 may have the same configuration as the Fresnel half mirror 13'. The Fresnel half mirror 11 may be configured including a Fresnel concave lens.

[0112] If the first semi-transparent mirror 11 is replaced with a Fresnel half-mirror 11 that does not have polarization selectivity, the amount of light emitted from the display device 1A' will decrease, and the brightness of the virtual image V seen by the user 22 will decrease. Therefore, the Fresnel half-mirror 11 may be configured to have polarization selectivity. For example, the Fresnel half-mirror 11 may have multiple metal nanowires (metal nanowire grids) formed on the surface facing the display panel 2 or the surface facing the first phase difference plate 12, which transmit S-wave polarized light and reflect P-wave polarized light, thereby achieving polarization selectivity. This can reduce the decrease in brightness of the virtual image V seen by the user 22.

[0113] The first semi-transparent mirror 11 of the display device 1A may be configured to include a Fresnel lens, as shown in Figure 21. In other words, the display device 1A may have a Fresnel shape on the surface of the first semi-transparent mirror 11. In this case, the thickness of the first semi-transparent mirror 11 can be reduced, and as a result, the display device 1A can be made smaller in the depth direction. Furthermore, since the first semi-transparent mirror 11 including the Fresnel lens is substantially flat, the distance between the first semi-transparent mirror 11 and the display panel 2 can be reduced, or the first semi-transparent mirror 11 and the display panel 2 can be brought into contact with each other, making it possible to further miniaturize the display device 1A in the depth direction. The first semi-transparent mirror 11 including the Fresnel lens may be configured to have polarization selectivity. For example, the first semi-transparent mirror 11 including the Fresnel lens may have a plurality of fine metal wires formed on the surface located on the display panel 2 side or the surface located on the first phase difference plate 12 side to achieve polarization selectivity that transmits S-wave polarized light and reflects P-wave polarized light. In this case, the decrease in brightness of the virtual image V seen by the user 22 can be reduced.

[0114] The semi-transparent mirror 6 of the display device 1 may include a Fresnel lens. In other words, the display device 1 may have a Fresnel shape on the surface of the semi-transparent mirror 6. In this case, the thickness of the semi-transparent mirror 6 can be reduced, and as a result, the display device 1 can be made smaller in the depth direction. Furthermore, since the semi-transparent mirror 6 including the Fresnel lens is substantially flat, the distance between the semi-transparent mirror 6 and the first phase difference plate 5 can be reduced, or the semi-transparent mirror 6 and the first phase difference plate 5 can be brought into contact with each other, making it possible to make the display device 1 even smaller in the depth direction.

[0115] Other examples of the display devices 1, 1A, 1A', and 1B will be described. Components with the same configuration as those in the display devices 1, 1A, 1A', and 1B are given the same reference numerals, and detailed explanations are omitted. The display device 1C in this example comprises a display panel 2, an optical system 35, and a housing 36, as shown in Figure 26.

[0116] The display panel 2 has a display surface 2a, on which a display image is displayed. The optical system 35 projects the display light emitted from the display panel 2 as a virtual image V into the user's field of view 22. The optical system 35 may be optical system 3 (see Figures 2, 3, 23), optical system 10 (see Figures 4, 5, 24), or optical system 16 (see Figures 9, 25). Figures 26-27 show the case where the optical system 35 is the optical system 3 shown in Figure 23.

[0117] The housing 36 houses the display panel 2 and the optical system 35. The housing 36 may hold the display panel 2 and the optical system 35. If the display device 1C includes an irradiator 4, the housing 36 may house and hold the irradiator 4. The housing 36 may have a viewing section that allows the inside of the housing 36 to be seen from the outside of the housing 36. The housing 36 may have a window (aperture) 37 that transmits light emitted from the optical system 35. The window (aperture) 37 may function as a viewing section. The display device 1C may be arranged such that when the window 37 of the housing 36 is viewed, the window 37 and the display panel 2 overlap. The display device 1C may also be arranged such that when the window 37 of the housing 36 is viewed, the window 37 and the optical system 35 overlap. The display device 1C may also be arranged such that when the window 37 of the housing 36 is viewed, the display panel 2 and the optical system 35 overlap. In this case, the space occupied by the display device 1C can be reduced, and as a result, the display device 1C can be miniaturized. Furthermore, in the display device 1C, the display light emitted from the display panel 2 propagates substantially along one axis and is formed as a virtual image V. Therefore, distortion, brightness unevenness, etc. of the virtual image V seen by the user 22 can be reduced, and the design of the optical system 35 becomes easier.

[0118] The housing 36 may have a component that allows the interior of the housing 36, located in the window (opening) 37, to be visible from the outside of the housing 36. The housing 36 may have a light-transmitting plate 38 located in the window 37 (opening), as shown in Figures 26 and 27. The light-transmitting plate 38 may transmit light emitted from the optical system 35. The light-transmitting plate 38 may at least partially block the window (opening) 37. The light-transmitting plate 38 may be made of, for example, glass, resin, or the like. The component that allows the interior of the housing 36, located in the window (opening) 37, to be visible from the outside of the housing 36 (for example, the light-transmitting plate 38) may function as a viewing section.

[0119] The optical system 35 (optical system 3) may have a third phase difference plate 25 and a fourth phase difference plate 26. The third phase difference plate 25 may be located on the side of the second phase difference plate 7 that faces the semi-transparent mirror 6. The fourth phase difference plate 26 may be located on the side of the third phase difference plate 25 that faces the semi-transparent mirror 6. This makes it possible to bring the relative angle between the transmission axis of the front polarizing plate of the display panel 2 and the transmission axis of the reflective polarizing plate 8 closer to a cross-nicol arrangement, even when the user 22 is not positioned in front of the display device 1C, thereby reducing the deterioration of the display quality of the display device 1C. The third phase difference plate 25 and the fourth phase difference plate 26 may be half-wave plates, but are not limited to them. The third phase difference plate 25 and the fourth phase difference plate 26 may be quarter-wave plates, eighth-wave plates, sixteenth-wave plates, etc., or other wave plates that impart phase difference. The third phase difference plate 25 and the fourth phase difference plate 26 may be waveplates that impart the same phase difference, or they may be waveplates that impart different phase differences. The optical axis of the third phase difference plate 25 may be substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizer 8.

[0120] According to this disclosure, it is possible to reduce the degradation of display quality in small display devices and improve light utilization efficiency. Furthermore, according to this disclosure, it is possible to provide a small imaging device that can allow users to clearly perceive virtual images.

[0121] Figures 29 and 30 are cross-sectional views showing examples of the configuration of a display device according to the present disclosure. In Figures 29 and 30, some components not relevant to the description may be omitted. Also, the description of components or parts equivalent to those in the above examples may be omitted.

[0122] The display device 501 comprises a display panel 2 and an optical system 3. The optical system 3 may include a first phase difference plate 5, a semi-transparent mirror 6, a second phase difference plate 7, and a reflective polarizing plate 8. Alternatively, the optical system 3 may be arranged in the order of the first phase difference plate 5, the semi-transparent mirror 6, the second phase difference plate 7, and the reflective polarizing plate 8 from the display panel 2 side.

[0123] The functions of the optical systems 3 in Figures 29 and 30 are the same as those of the optical system 3 in Figure 2, from the time the indicator light of the first linearly polarized light L1 passes through the first phase difference plate 5 until the light of the fourth linearly polarized light L4 passes through the reflective polarizer plate 8 and is emitted to the outside.

[0124] In this embodiment, the reflective polarizing plate 8 can collect or focus the light that is incident on and reflected by the reflective polarizing plate 8. Specifically, the reflective polarizing plate 8 has a concave shape located on the display panel 2 side. As a result, the reflective polarizing plate 8 can collect or focus the light. It can also be said that the reflective polarizing plate 8 has a concave shape that opens towards the display panel 2 side. Alternatively, it can be said that the reflective polarizing plate 8 has a concave shape that is recessed toward the opposite side of the display panel 2. Furthermore, it can be said that the reflective polarizing plate 8 has a convex shape that protrudes toward the opposite side of the display panel 2 (towards the user 22 side). As shown in Figure 29, the reflective polarizing plate 8 may be arranged on the inner surface of the base material 424 which has a concave shape toward the display panel 2 side.

[0125] In this embodiment, the semi-transparent mirror 6 can collect or focus the light that is incident on and reflected by the semi-transparent mirror 6. Specifically, the semi-transparent mirror 6 has a concave shape located on the opposite side from the display panel 2 (the user 22 side). As a result, the semi-transparent mirror 6 can collect or focus the light. It can also be said that the semi-transparent mirror 6 has a concave shape that opens towards the user 22 side. Alternatively, it can be said that the semi-transparent mirror 6 has a concave shape that is recessed toward the display panel side. Furthermore, it can also be said that the semi-transparent 6 has a convex shape that protrudes toward the display panel 2 side.

[0126] As shown in Figure 30, at least one of the reflective polarizing plate 8 and the semi-transparent mirror 6 may be flat. In this case, at least one of the reflective polarizing plate 8 and the semi-transparent mirror 6 may be a holographic optical element, or the surface shape of at least one of the reflective polarizing plate 8 and the semi-transparent mirror 6 may have a Fresnel shape. In this case, the reflective polarizing plate 8 may be placed on the surface of a flat substrate 424. Glass is one example of the material of the substrate 424.

[0127] Each component of the optical system 3 in Figures 29 and 30 may correspond to each component of the optical system 3 in Figure 2. Each component of the optical system 10 in Figures 31 and 32 may correspond to each component of the optical system 10 in Figure 4, including the corresponding relationship between the reflective polarizer 425 and the first semi-transparent mirror 11.

[0128] Figures 31 and 32 are cross-sectional views showing examples of the configuration of a display device according to the present disclosure. In Figures 31 and 32, some components not relevant to the description may be omitted. Also, the description of components or parts equivalent to those in the above examples may be omitted.

[0129] The display device 501 comprises a display panel 2 and an optical system 10. The optical system 10 may include a reflective polarizing plate 425, a first phase difference plate 12, a second semi-transparent mirror 13, a second phase difference plate 14, and a polarizing plate 15. Alternatively, the optical system 10 may be arranged in the following order from the display panel 2 side: reflective polarizing plate 425, first phase difference plate 12, second semi-transparent mirror 13, second phase difference plate 14, and polarizing plate 15. The reflective polarizing plate 425 may have substantially the same function as the first semi-transparent mirror 11 (see Figure 4).

[0130] The functions of the optical systems 10 in Figures 31 and 32 are the same as those of the optical system 10 in Figure 4, from the point when the display light of the first linearly polarized light L1 passes through the reflective polarizer 425, which has substantially the same function as the first semi-transparent mirror 11, until the light of the fourth linearly polarized light L4 passes through the polarizer 15 and is emitted to the outside, and the light of the fifth linearly polarized light L5 is absorbed by the polarizer 15.

[0131] In this embodiment, the reflective polarizing plate 425 can collect or focus the light that is incident on and reflected by the reflective polarizing plate 425. Specifically, the reflective polarizing plate 425 has a concave shape located on the opposite side from the display panel 2 (the user 22 side). As a result, the reflective polarizing plate 425 can collect or focus the light. It can also be said that the reflective polarizing plate 425 has a concave shape that opens towards the user 22 side. It can also be said that the reflective polarizing plate 425 has a concave shape that is recessed toward the display panel 2 side. It can also be said that the reflective polarizing plate 425 has a convex shape that protrudes toward the display panel 2 side. As shown in Figure 31, the reflective polarizing plate 425 may be arranged on the inner surface of the base material 426 which has a concave shape toward the user 22 side.

[0132] In this embodiment, the second semi-transparent mirror 13 can collect or focus the light that is incident on and reflected by the second semi-transparent mirror 13. Specifically, the second semi-transparent mirror 13 has a concave shape located on the display panel 2 side. As a result, the second semi-transparent mirror 13 can collect or focus the light. It can also be said that the second semi-transparent mirror 13 has a concave shape that opens towards the display panel 2 side. Furthermore, it can also be said that the second semi-transparent mirror 13 has a concave shape that is recessed toward the opposite side of the display panel 2 (towards the user 22 side). Furthermore, it can also be said that the second semi-transparent mirror 13 has a convex shape that protrudes toward the user 22 side.

[0133] As shown in Figure 32, at least one of the reflective polarizing plate 425 and the second semi-transparent mirror 13 may be flat. In this case, at least one of the reflective polarizing plate 425 and the second semi-transparent mirror 13 may be a holographic optical element, or the surface shape of at least one of the reflective polarizing plate 425 and the second semi-transparent mirror 13 may have a Fresnel shape. In this case, the reflective polarizing plate 425 may be disposed on the surface of a flat substrate 426. Glass is one example of the material of the substrate 426.

[0134] The display panel 2 and optical system 3 (see Figures 29 and 30) and the display panel 2 and optical system 10 (see Figures 31 and 32) may be interpreted as including a display system 427, a concave mirror section 428, and a concave mirror section 429, respectively. The concave mirror sections 428 and 429 may transmit a portion of the incident light and reflect the remainder. The concave mirror sections 428 and 429 are not limited to concave mirrors themselves, and may have the function of a concave mirror optically regardless of their shape.

[0135] Figures 33A and 33B illustrate the optical functions of the display devices shown in Figures 29 to 32, respectively. As shown in Figure 33A, the concave mirror portion 429 forms an image on the display surface 2a of the display system 427, forming a virtual image V429 that is magnified relative to the display surface 2a. Then, as shown in Figure 33B, the concave mirror portion 428 forms an image on the virtual image V429, forming a virtual image V428 that is magnified relative to the virtual image V429. This makes it easier to adjust the magnification ratio of the virtual image V relative to the display surface 2a.

[0136] According to the display device 501 shown in Figures 29 to 32, the focal length of each concave mirror portion 428 and 429 can be made longer, which is convenient when the spacing between components is narrowed. Also, since the aspect ratio of the Fresnel shape can be set small, molding and deposition are easy. For each of the display panel 2 and optical system 3 (see Figures 29 and 30) and the display panel 2 and optical system 10 (see Figures 31 and 32), two adjacent components may or may not be in contact with each other, at least in part.

[0137] Figure 34 shows an example of various dimensions of the display device shown in Figure 27. All dimensions in Figure 34 may be in the direction of the optical axis of the light incident on the optical system 3 or 10. The inequality sign "<" in Figure 34 means that the indicated dimension may be less than the actual dimension. The spatial dimension (reference numeral 6-40) may be the distance from the part of the semi-transparent mirror 6 that is closest to the display panel 2 on the exit side to the moth-eye structure film 40. The distance between the display panel and the reflective polarizer (including members 2 and 8) (reference numeral 2-8) may be the sum of the thickness of the display panel 2, the distance between the display panel 2 and the reflective polarizer 8, and the thickness of the reflective polarizer 8. The amount of curvature (reference numeral 6-6) may be the dimension from the part of the semi-transparent mirror 6 that is closest to the display panel 2 on the incident side to the semi-transparent mirror 6 to the part of the semi-transparent mirror 6 that is closest to the second phase difference plate 7 on the exit side to the semi-transparent mirror 6.

[0138] Figure 35 shows a first example in which the first semi-transparent mirror 601 images the display light to form a first image 602. The display panel 2 may emit the display light. The first semi-transparent mirror 601 may have the function of reflecting and diverging light toward the display panel 2, and may image the display light to form the first image 602. The first image 602 may be formed on the side of the display panel 2 opposite to the first semi-transparent mirror 601, with the display image of the display panel 2 reduced in size. In this embodiment, the first semi-transparent mirror 601 can reflect and diverge light that is incident on and reflected by the first semi-transparent mirror 601. For example, the first semi-transparent mirror 601 may have a convex shape located toward the display panel 2. As a result, the first semi-transparent mirror 601 can reflect and diverge light. It can also be said that the first semi-transparent mirror 601 has a convex shape that protrudes toward the display panel 2. Furthermore, the first semi-transparent mirror 601 may also be described as having a concave shape located on the opposite side of the display panel 2 (the user 22 side). The first semi-transparent mirror 601 may also be described as having a concave shape that opens towards the user 22 side. The first semi-transparent mirror 601 may also be described as having a concave shape that is recessed toward the display panel side. The first semi-transparent mirror 601 may be flat. In this case, the first semi-transparent mirror 601 may be a holographic optical element or its surface shape may be Fresnel shape. The focal length f601 of the first semi-transparent mirror 601 may be greater than the separation distance d60B between the display panel 2 and the first semi-transparent mirror 601. The focal length f601 of the first semi-transparent mirror 601 may be less than the separation distance d60B between the display panel 2 and the first semi-transparent mirror 601. The first image 602 may be formed on the opposite side of the display panel 2 from the first semi-transparent mirror 601 by reducing the display image of the display panel 2.

[0139] Figure 36 shows a first example in Figure 35 in which the second semi-transparent mirror 603 forms the first image 602 to form the second image 604. The second semi-transparent mirror 603 may have the function of focusing or converging light toward the first image 602, and may form the first image 602 to form the second image 604. In this embodiment, the second semi-transparent mirror 603 can focus or converge light that is incident on and reflected by the second semi-transparent mirror 603. For example, the second semi-transparent mirror 603 has a concave shape located on the opposite side from the display panel 2 (the user 22 side). As a result, the semi-transparent mirror 6 can focus or converge light. It can also be said that the second semi-transparent mirror 603 has a concave shape that opens toward the user 22 side. It can also be said that the second semi-transparent mirror 603 has a concave shape that is recessed toward the first semi-transparent mirror 601 side. Furthermore, it can also be said that the second semi-transparent mirror 603 has a convex shape that protrudes toward the display panel 2 side. The second semi-transparent mirror 603 may be flat. In this case, the second semi-transparent mirror 603 may be a holographic optical element or have a Fresnel shape. The focal length f 603 of the second semi-transparent mirror 603 may be greater than the separation distance d 60A between the first image 602 and the second semi-transparent mirror 603. The second image 604 may be a virtual image formed by magnifying the first image 602 on the opposite side of the second semi-transparent mirror 603 (the side of the display panel 2). The focal point of the second semi-transparent mirror 603 is indicated by reference numeral 603'.

[0140] The definitions of each separation distance d60A and separation distance d60B will now be explained. Each separation distance d60A and separation distance d60B may be a distance in a direction parallel to at least one of the following: the optical axis of incident light to the first semi-transparent mirror 601, the optical axis of outgoing light to the first semi-transparent mirror 601, the optical axis of incident light to the second semi-transparent mirror 603, and the optical axis of outgoing light to the second semi-transparent mirror 603. Alternatively, each separation distance d60A and separation distance d60B may be a distance in a direction parallel to at least one of the following: the optical axis of the first semi-transparent mirror 601, the optical axis of the second semi-transparent mirror 603, the optical axis of the display panel 2 (for example, the normal direction to the display surface of the display panel 2), the normal direction to the reflective surface of the optical component (for example, the first semi-transparent mirror 601 or the second semi-transparent mirror 603). With respect to that direction, each separation distance d60A and separation distance d60B may be a distance that can define whether the optical component forms a real image or a virtual image in comparison to the focal position of the corresponding optical component. Accordingly, at each separation distance d60A and separation distance d60B, the starting point on the optical component side may be the position of the center of the optical component, and the starting point on the image side may be the position of the display surface in the case of the display panel 2, or the image formation position in the case of each first image 602 and second image 604.

[0141] Figure 37 shows a second example in Figure 35 in which the second semi-transparent mirror 603 forms an image of the first image 602 to form the second image 604. The second semi-transparent mirror 603 may have the function of focusing or converging light toward the first image 602, and may form an image of the first image 602 to form the second image 604. The focal length f603 of the second semi-transparent mirror 603 may be smaller than the separation distance d60A between the first image 602 and the second semi-transparent mirror 603. The second image 604 may be a real image formed by enlarging or reducing the first image 602 on the side of the first image 602 (opposite side of the display panel 2) relative to the second semi-transparent mirror 603. The second image 604 may be inverted relative to the first image 602.

[0142] Figure 38 shows a second example in which the first semi-transparent mirror 601 forms an image of the display light to form a first image 602. The display panel 2 may emit display light. The first semi-transparent mirror 601 may have a function to collect or focus light toward the display panel 2 and form an image of the display light to form a first image 602. In this embodiment, the first semi-transparent mirror 601 can collect or focus light that is incident on and reflected by the first semi-transparent mirror 601. For example, the first semi-transparent mirror 601 may have a concave shape located toward the display panel 2. As a result, the first semi-transparent mirror 601 can collect or focus light. It can also be said that the first semi-transparent mirror 601 has a concave shape located toward the display panel 2. Alternatively, it can be said that the first semi-transparent mirror 601 has a convex shape that protrudes toward the opposite side of the display panel 2 (the user 22 side). It can also be said that the first semi-transparent mirror 601 has a concave shape that opens toward the display panel 2. The first semi-transparent mirror 601 may also be described as having a concave shape that is recessed toward the opposite side of the display panel 2 (towards the user 22). The first semi-transparent mirror 601 may also be flat. In this case, the first semi-transparent mirror 601 may be a holographic optical element or may have a Fresnel shape on its surface. The focal length f 601 of the first semi-transparent mirror 601 may be greater than the separation distance d 60B between the display panel 2 and the first semi-transparent mirror 601. The first image 602 may be formed on the opposite side of the display panel 2 from the first semi-transparent mirror 601 by an enlargement of the display image of the display panel 2. The focal point of the first semi-transparent mirror 601 is indicated by reference numeral 601'.

[0143] Figure 39 shows a first example in Figure 38 where the second semi-transparent mirror 603 forms the first image 602 to form the second image 604. The second semi-transparent mirror 603 may have the function of focusing or converging light toward the first image 602, and may form the first image 602 to form the second image 604. In this embodiment, the second semi-transparent mirror 603 can focus or converge light that is incident on and reflected by the second semi-transparent mirror 603. For example, the second semi-transparent mirror 603 has a concave shape located on the opposite side from the display panel 2 (the user 22 side). As a result, the semi-transparent mirror 6 can focus or converge light. It can also be said that the second semi-transparent mirror 603 has a concave shape that opens toward the user 22 side. It can also be said that the second semi-transparent mirror 603 has a concave shape that is recessed toward the first semi-transparent mirror 601 side. Furthermore, it can also be said that the second semi-transparent mirror 603 has a convex shape that protrudes toward the display panel 2 side. The second semi-transparent mirror 603 may be flat. In this case, the second semi-transparent mirror 603 may be a holographic optical element or have a Fresnel shape. The focal length f603 of the second semi-transparent mirror 603 may be greater than the separation distance d60A between the first image 602 and the second semi-transparent mirror 603. The second image 604 may be a virtual image formed by magnifying the first image 602 on the opposite side of the second semi-transparent mirror 603 (the side of the display panel 2).

[0144] Figure 40 shows a second example in Figure 38 in which the second semi-transparent mirror 603 forms the first image 602 to form the second image 604. The second semi-transparent mirror 603 may have the function of focusing or converging light toward the first image 602, and may form the first image 602 to form the second image 604. In this embodiment, the second semi-transparent mirror 603 can focus or converge light that is incident on and reflected by the second semi-transparent mirror 603. For example, the second semi-transparent mirror 603 has a concave shape located on the opposite side from the display panel 2 (the user 22 side). As a result, the semi-transparent mirror 6 can focus or converge light. It can also be said that the second semi-transparent mirror 603 has a concave shape that opens toward the user 22 side. It can also be said that the second semi-transparent mirror 603 has a concave shape that is recessed toward the first semi-transparent mirror 601 side. Furthermore, it can also be said that the second semi-transparent mirror 603 has a convex shape that protrudes toward the display panel 2 side. The second semi-transparent mirror 603 may be flat. In this case, the second semi-transparent mirror 603 may be a holographic optical element or have a Fresnel shape. The focal length f603 of the second semi-transparent mirror 603 may be smaller than the separation distance d60A between the first image 602 and the second semi-transparent mirror 603. The second image 604 may be a real image formed on the side of the first image 602 (opposite the display panel 2) relative to the second semi-transparent mirror 603 by enlarging or reducing the first image 602. The second image 604 may be inverted relative to the first image 602.

[0145] Figure 41 shows a third example in which the first semi-transparent mirror 601 images the display light to form a first image 602. The display panel 2 may emit display light. The first semi-transparent mirror 601 may have the function of focusing or converging light toward the display panel 2, and may form an image of the display light to form a first image 602. In this embodiment, the first semi-transparent mirror 601 can focus or converge the light that is incident on and reflected by the first semi-transparent mirror 601. For example, the first semi-transparent mirror 601 may have a concave shape located toward the display panel 2. As a result, the first semi-transparent mirror 601 can focus or converge the light. It can also be said that the first semi-transparent mirror 601 has a concave shape located toward the display panel 2. Alternatively, it can be said that the first semi-transparent mirror 601 has a convex shape that protrudes toward the opposite side of the display panel 2 (the user 22 side). It can also be said that the first semi-transparent mirror 601 has a concave shape that opens toward the display panel 2. The first semi-transparent mirror 601 may also be described as having a concave shape that is recessed toward the opposite side of the display panel 2 (towards the user 22). The first semi-transparent mirror 601 may also be flat. In this case, the first semi-transparent mirror 601 may be a holographic optical element or may have a Fresnel shape on its surface. The focal length f601 of the first semi-transparent mirror 601 may be smaller than the separation distance d60B between the display panel 2 and the first semi-transparent mirror 601. The first image 602 may be formed on the side of the display panel 2 relative to the first semi-transparent mirror 601 by enlarging or reducing the display image of the display panel 2. The first image 602 may be inverted relative to the display image of the display panel 2.

[0146] Figure 42 shows a first example in Figure 41 in which the second semi-transparent mirror 603 forms the first image 602 to form the second image 604. The second semi-transparent mirror 603 may have the function of focusing or converging light toward the first image 602, and may form the first image 602 to form the second image 604. In this embodiment, the second semi-transparent mirror 603 can focus or converge light that is incident on and reflected by the second semi-transparent mirror 603. For example, the second semi-transparent mirror 603 has a concave shape located on the opposite side from the display panel 2 (the user 22 side). As a result, the semi-transparent mirror 6 can focus or converge light. It can also be said that the second semi-transparent mirror 603 has a concave shape that opens toward the user 22 side. It can also be said that the second semi-transparent mirror 603 has a concave shape that is recessed toward the first semi-transparent mirror 601 side. Furthermore, it can also be said that the second semi-transparent mirror 603 has a convex shape that protrudes toward the display panel 2 side. The second semi-transparent mirror 603 may be flat. In this case, the second semi-transparent mirror 603 may be a holographic optical element or have a Fresnel shape. The first image 602 may be located on the opposite side of the display panel 2 from the second semi-transparent mirror 603. The focal length f 603 of the second semi-transparent mirror 603 may be greater than the separation distance d 60A between the first image 602 and the second semi-transparent mirror 603. The second image 604 may be a virtual image formed by magnifying the first image 602 on the opposite side of the first image 602 (the side of the display panel 2) from the second semi-transparent mirror 603.

[0147] Figure 43 shows a second example in Figure 41 in which the second semi-transparent mirror 603 forms the first image 602 to form the second image 604. The second semi-transparent mirror 603 may have the function of focusing or converging light toward the first image 602, and may form the first image 602 to form the second image 604. In this embodiment, the second semi-transparent mirror 603 can focus or converge light that is incident on and reflected by the second semi-transparent mirror 603. For example, the second semi-transparent mirror 603 has a concave shape located on the opposite side from the display panel 2 (the user 22 side). As a result, the semi-transparent mirror 6 can focus or converge light. It can also be said that the second semi-transparent mirror 603 has a concave shape that opens toward the user 22 side. It can also be said that the second semi-transparent mirror 603 has a concave shape that is recessed toward the first semi-transparent mirror 601 side. Furthermore, it can also be said that the second semi-transparent mirror 603 has a convex shape that protrudes toward the display panel 2 side. The second semi-transparent mirror 603 may be flat. In this case, the second semi-transparent mirror 603 may be a holographic optical element or have a Fresnel shape. The first image 602 may be located on the opposite side of the display panel 2 from the second semi-transparent mirror 603. The focal length f 603 of the second semi-transparent mirror 603 may be smaller than the separation distance d 60A between the first image 602 and the second semi-transparent mirror 603. The second image 604 may be a real image formed by enlarging or reducing the first image 602 on the side of the first image 602 (opposite side of the display panel 2) from the second semi-transparent mirror 603. The second image 604 may be inverted relative to the first image 602, in other words, upright relative to the display image on the display panel 2.

[0148] Figure 44 shows a third example in Figure 41 in which the second semi-transparent mirror 603 forms the first image 602 to form the second image 604. The display panel 2 may emit display light. The first semi-transparent mirror 601 may have the function of focusing or converging light toward the display panel 2 and forming the first image 602 by imaging the display light. In this embodiment, the first semi-transparent mirror 601 can focus or converge light that is incident on and reflected by the first semi-transparent mirror 601. For example, the first semi-transparent mirror 601 may have a concave shape located toward the display panel 2. As a result, the first semi-transparent mirror 601 can focus or converge light. It can also be said that the first semi-transparent mirror 601 has a concave shape located toward the display panel 2. Alternatively, it can be said that the first semi-transparent mirror 601 has a convex shape that protrudes toward the opposite side of the display panel 2 (the user 22 side). The first semi-transparent mirror 601 can also be described as having a concave shape that opens towards the display panel 2 side. The first semi-transparent mirror 601 can also be described as having a concave shape that is recessed toward the opposite side of the display panel 2 (towards the user 22 side). The first semi-transparent mirror 601 may be flat. In this case, the first semi-transparent mirror 601 may be a holographic optical element or have a Fresnel shape as its surface. The second semi-transparent mirror 603 has the function of focusing or converging light toward the opposite side of the first image 602, and may form a second image 604 by imaging the first image 602. In this embodiment, the second semi-transparent mirror 603 can focus or converge light that is incident on and reflected by the second semi-transparent mirror 603. For example, the second semi-transparent mirror 603 has a concave shape located toward the opposite side of the display panel 2 (towards the user 22 side). As a result, the semi-transparent mirror 6 can focus or converge light. The second semi-transparent mirror 603 can also be described as having a concave shape that opens towards the user 22. The second semi-transparent mirror 603 can also be described as having a concave shape that is recessed toward the first semi-transparent mirror 601. Furthermore, the second semi-transparent mirror 603 can also be described as having a convex shape that protrudes toward the display panel 2. The second semi-transparent mirror 603 may also be flat. In this case, the second semi-transparent mirror 603 may be a holographic optical element or its surface shape may be Fresnel-shaped. The focal length f601 of the first semi-transparent mirror 601 may be smaller than the separation distance d60B between the display panel 2 and the first semi-transparent mirror 601 (see Figure 41).The first image 602 may be positioned on the side of the display panel 2 relative to the second semitransparent mirror 603. The second image 604 may be a real image formed by reducing the first image 602 on the opposite side of the second semitransparent mirror 603 (the opposite side of the display panel 2). The first image 602 may be inverted with respect to the display image of the display panel 2. The second image 604 may be upright with respect to the first image 602, in other words, it may be inverted with respect to the display image of the display panel 2.

[0149] Figure 45 is a diagram illustrating the conditions relating to the example in Figure 43 and the example in Figure 44. With respect to the focal length f601, the separation distance d60B, and the separation distance L60 between the first semi-transparent mirror 601 and the first image 602, (X) may be satisfied.

[0150] L60 : d60B = f601 : (d60B - f601) That is, L60 = d60B × f601 / (d60B - f601) ... (X) In (X), if the separation distance L60 is greater than the separation distance d60 between the first semi-transparent mirror 601 and the second semi-transparent mirror 603, that is, if d60B × f601 / (d60B - f601) > d60, then it may correspond to the example in Figure 44. In (X), if the separation distance L60 is less than the separation distance d60, that is, if d60B × f601 / (d60B - f601) < d60, then it may correspond to the example in Figure 43.

[0151] In each example in Figures 35 to 44, the focal length f603 of the second semi-transparent mirror 603 may be greater than the focal length f601 of the first semi-transparent mirror 601. In each example in Figures 35 to 44, the focal length f603 of the second semi-transparent mirror 603 may be less than the focal length f601 of the first semi-transparent mirror 601.

[0152] By applying an example related to any of Figures 36, 37, 39, 40, or 42-44 to the display device 1, the display device 1 can be miniaturized relative to the size of the image by combining two semi-transparent mirrors to form the image seen by the user 22. In each example of Figures 35-44, the first semi-transparent mirror 601 and the second semi-transparent mirror 603 may transmit a portion of the incident light and reflect the remainder. For example, the first semi-transparent mirror 601 and the second semi-transparent mirror 603 may transmit a portion of the incident light (e.g., approximately 50%) and reflect the remainder (e.g., approximately 50%), or they may be given a reflective polarization function. The transmittance and reflectance of light incident on the first semi-transparent mirror 601 or the second semi-transparent mirror 603 are not limited to 50%. That is, for example, the reflective polarizing plate 8 described herein may be considered as the first semi-transparent mirror 601, and the semi-transparent mirror 6 may be considered as the second semi-transparent mirror 603. Furthermore, for example, the second semi-transparent mirror 13 or 13' described herein may be considered as the first semi-transparent mirror 601, and the first semi-transparent mirror 11 may be considered as the second semi-transparent mirror 603.

[0153] Figures 46 to 48 show examples of the arrangement of the phase difference member 605. Figure 49 shows examples of the arrangement of the third phase difference plate 606 and the fourth phase difference plate 607.

[0154] The display device 1 may include a display panel 2 that emits display light, a first phase difference plate 5, a second phase difference plate 7 positioned at a distance from the first phase difference plate 5, a semi-transparent mirror 6 positioned between the first phase difference plate 5 and the second phase difference plate 7 and having a reflective surface capable of reflecting light toward the second phase difference plate 7, and a reflective polarizing plate 8 positioned on the opposite side of the first phase difference plate 5 from the second phase difference plate 7, which transmits first linearly polarized light and reflects second linearly polarized light.

[0155] In the example shown in Figure 46, the display device 1 may include a phase difference member 605 positioned between the display panel 2 and the first phase difference plate 5, with its optical axis positioned to be substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizer 8. In the example shown in Figure 46, the first phase difference plate 5 and the second phase difference plate 7 convert the display light to second linear polarization, and the second phase difference plate 7 may convert the second linear polarization, reflected by the reflective polarizer 8, to first linear polarization before it is reflected by the semitransparent mirror 6 and re-enters the reflective polarizer 8.

[0156] In the example shown in Figure 47, the display device 1 may include a phase difference member 605 positioned between the first phase difference plate 5 and the second phase difference plate 7, with its optical axis positioned to be substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizer 8. In the example shown in Figure 47, the phase difference member 605 may include two phase difference plates (third phase difference plate 606 and fourth phase difference plate 607) that provide the same phase difference to the transmitted light. In that case, the optical axis of the third phase difference plate 606 may be substantially perpendicular to the optical axis of the fourth phase difference plate 607. In the example shown in Figure 47, the first phase difference plate 5 and the second phase difference plate 7 convert the display light to second linear polarization, and the second phase difference plate 7 may convert the second linear polarization, reflected by the reflective polarizer 8, to first linear polarization before it is reflected by the semitransparent mirror 6 and re-enters the reflective polarizer 8. In the example shown in Figure 47, the phase difference member 605 includes a third phase difference plate 606 and a fourth phase difference plate 607, but is not limited thereto. For example, in the example shown in Figure 47, the phase difference member 605 may include one phase difference plate (the third phase difference plate 606 or the fourth phase difference plate 607). In this case, the one phase difference plate (the third phase difference plate 606 or the fourth phase difference plate 607) may be a half-wave plate. Also in this case, the one phase difference plate (the third phase difference plate 606 or the fourth phase difference plate 607) may be located between the first phase difference plate 5 and the semi-transparent mirror 6, or between the semi-transparent mirror 6 and the second phase difference plate 7. In this case, if the reflective polarizing plate 8 is configured to reflect polarized light having a polarization axis perpendicular to the polarization axis of the display light and transmit polarized light having a polarization axis parallel to the polarization axis of the display light, the positional relationship between the first phase difference plate 5 and the second phase difference plate 7 may be defined such that, when the first phase difference plate 5 and the second phase difference plate 7 are viewed along the Z-axis direction, the lagging axis of the second phase difference plate 7 is perpendicular to the lagging axis of the first phase difference plate 5. Also in this case, if the reflective polarizing plate 8 is configured to transmit polarized light having a polarization axis perpendicular to the polarization axis of the display light and reflect polarized light having a polarization axis parallel to the polarization axis of the display light, the positional relationship between the first phase difference plate 5 and the second phase difference plate 7 may be defined such that, when the first phase difference plate 5 and the second phase difference plate 7 are viewed along the Z-axis direction, the lagging axis of the second phase difference plate 7 is parallel to the lagging axis of the first phase difference plate 5.

[0157] In the example shown in Figure 48, the display device 1 may include a phase difference member 605 positioned between the second phase difference plate 7 and the reflective polarizing plate 8, such that its optical axis is substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizing plate 8. In the example shown in Figure 48, the first phase difference plate 5 and the second phase difference plate 7 convert the display light to a second linearly polarized light, and the second phase difference plate 7 may convert the second linearly polarized light, reflected by the reflective polarizing plate 8, to a first linearly polarized light before it is reflected by the semitransparent mirror 6 and re-enters the reflective polarizing plate 8.

[0158] In the example shown in Figure 49, the display device 1 may include a third phase difference plate 606 positioned between the display panel 2 and the first phase difference plate 5, with its optical axis being substantially parallel or perpendicular to the transmission axis of the reflective polarizer 8, and a fourth phase difference plate 607 positioned between the second phase difference plate 7 and the reflective polarizer 8, with its optical axis being substantially parallel or perpendicular to the transmission axis of the reflective polarizer 8. In the example shown in Figure 49, the first phase difference plate 5 and the second phase difference plate 7 convert the display light into second linearly polarized light, and the second phase difference plate 7 may convert the second linearly polarized light, reflected by the reflective polarizer 8, into first linearly polarized light before it is reflected by the semitransparent mirror 6 and re-enters the reflective polarizer 8.

[0159] The phase difference member 605 may include one phase difference plate (the third phase difference plate 606 or the fourth phase difference plate 607). The phase difference member 605 may include two phase difference plates (the third phase difference plate 606 and the fourth phase difference plate 607). In this case, the optical axes of the third phase difference plate 606 and the fourth phase difference plate 607 may be substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizer, and the optical axis of the third phase difference plate 606 may be substantially perpendicular to the optical axis of the fourth phase difference plate 607. Furthermore, the phase difference member 605 may include two phase difference plates (the third phase difference plate 606 and the fourth phase difference plate 607) that provide the same phase difference to the transmitted light. Examples of the third phase difference plate 606 and the fourth phase difference plate 607 include a half-wave plate, a quarter-wave plate, and an eighth-wave plate, respectively. The phase difference member 605 may include a film (phase difference plate) having the relationship refractive index NX = refractive index NY > refractive index nz, which is called a negative c plate or negative c plate. The phase difference member 605 may include a film (phase difference plate) having the relationship refractive index NX = refractive index NY < refractive index nz, which is called a positive c plate or positive c plate. The phase difference member 605 may include a film (phase difference plate) having different refractive indices NX, NY, and nz, which is called a biaxial phase difference plate. The refractive indices NX and NY are refractive indices in the direction within the film plane, where the x-direction corresponding to refractive index NX and the y-direction corresponding to refractive index NY are orthogonal, and the refractive index nz is the refractive index in the z-direction perpendicular to the film plane. The phase difference member 605 includes two phase difference plates (third phase difference plate 606 and fourth phase difference plate 607) that provide the same phase difference to transmitted light, and the optical axis of the third phase difference plate 606 is substantially perpendicular to the optical axis of the fourth phase difference plate 607. In this case, the phase difference member 605 may be replaced with a film (phase difference plate) in which refractive indices nx and ny are the same, and refractive index nz is different from refractive indices nx and ny. In other words, it may be replaced with a film (phase difference plate) having the relationship refractive index nx = refractive index ny > refractive index nz or refractive index nx = refractive index ny < refractive index nz. In this case, the x-direction corresponding to the refractive index nx with respect to the transmission axis of the reflective polarizer and the y-direction corresponding to the refractive index ny with respect to the transmission axis of the reflective polarizer are not particularly limited.In this case, either the x-direction corresponding to refractive index nx or the y-direction corresponding to refractive index ny may be substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizer. The phase difference member 605 may also include two phase difference plates (third phase difference plate 606 and fourth phase difference plate 607) that provide different phase differences to the transmitted light. The phase difference member 605 may also include a biaxial phase difference plate. When the phase difference member 605 includes two phase difference plates (third phase difference plate 606 and fourth phase difference plate 607) that provide different phase differences to the transmitted light, and the optical axis of the third phase difference plate 606 is substantially perpendicular to the optical axis of the fourth phase difference plate 607, the phase difference member 605 may be replaced with a biaxial phase difference plate with different refractive indices nx, ny, and nz. In this case, either the x-direction corresponding to refractive index nx or the y-direction corresponding to refractive index ny may be substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizer. Furthermore, in this case, the relationship between refractive index nx, refractive index ny, and refractive index nz may be any one of the following: refractive index nx > refractive index ny > refractive index nz, refractive index ny > refractive index nx > refractive index nz, refractive index nx > refractive index nz > refractive index ny, refractive index ny > refractive index nz > refractive index nz, refractive index nz > refractive index ny > refractive index nz.

[0160] Regarding the general method of manufacturing a phase difference plate, by pulling the film in one axial direction, the pulled side may have a higher refractive index and become the slow phase axis of the phase difference plate. Regarding the method of manufacturing a negative c plate, by pulling the film equally in two vertical axes (in-plane x and y directions), the pulled x and y axes may have a higher refractive index and the z axis may have a lower refractive index. Regarding the method of manufacturing a biaxial phase difference plate, by pulling the film with a difference in the vertical axes (in-plane x and y directions), the pulled x and y axes will have a higher refractive index, but a refractive index difference may occur between them.

[0161] In the example shown in Figure 47, the third phase difference plate 606 and the fourth phase difference plate 607 are located between the first phase difference plate 5 and the semi-transparent mirror 6, but the example is not limited to this. In the example shown in Figure 47, at least one of the third phase difference plate 606 and the fourth phase difference plate 607 may be located between the semi-transparent mirror 6 and the second phase difference plate 7. That is, in the example shown in Figure 47, one of the third phase difference plate 606 and the fourth phase difference plate 607 may be located between the first phase difference plate 5 and the semi-transparent mirror 6, and the other may be located between the semi-transparent mirror 6 and the second phase difference plate 7. Alternatively, the third phase difference plate 606 and the fourth phase difference plate 607 may be located between the semi-transparent mirror 6 and the second phase difference plate 7. In the example shown in Figure 47, the positional relationship of the third phase difference plate 606 and the fourth phase difference plate 607 with respect to the semi-transparent mirror 6 is not particularly limited. Furthermore, one of the third phase difference plate 606 and the fourth phase difference plate 607 may be located between the first phase difference plate 5 and the semi-transparent mirror 6, or between the semi-transparent mirror 6 and the second phase difference plate 7, while the other may be located between the display panel 2 and the first phase difference plate 5, or between the second phase difference plate 7 and the reflective polarizing plate 8.

[0162] In the third phase difference plate and the fourth phase difference plate described herein, the third phase difference plate may be replaced with the fourth phase difference plate, or the fourth phase difference plate may be replaced with the third phase difference plate.

[0163] The first linearly polarized light and the second linearly polarized light may have complementary polarization characteristics or they may have common polarization characteristics. Of the first and second linearly polarized light, one may be P-wave polarized and the other S-wave polarized.

[0164] By applying an example from any of Figures 46 to 49 to the display device 1, even when the user 22 is not positioned directly in front of the display device 1, the relative angle between the transmission axis of the front polarizing plate of the display panel 2 and the transmission axis of the reflective polarizing plate 8 can be brought closer to a cross-nicol arrangement, thereby reducing the deterioration of the display quality of the display device 1.

[0165] By replacing the first phase difference plate 5 with the first phase difference plate 12, the semi-transparent mirror 6 with the second semi-transparent mirror 13, the second phase difference plate 7 with the second phase difference plate 14, and the reflective polarizing plate 8 with the polarizing plate 15, the example shown in any of Figures 46 to 49 can also be applied to the display device 1A. By applying the example shown in any of Figures 46 to 49 to the display device 1A, even when the user 22 is not positioned directly in front of the display device 1A, the relative angle between the transmission axis of the front polarizing plate of the display panel 2 and the transmission axis of the polarizing plate 15 can be brought closer to a cross-nicol arrangement, thereby reducing the degradation of the display quality of the display device 1'.

[0166] By replacing the first phase difference plate 5 with the first phase difference plate 12, the semi-transparent mirror 6 with the second semi-transparent mirror 13', the second phase difference plate 7 with the second phase difference plate 14, and the reflective polarizing plate 8 with the polarizing plate 15, the example shown in any of Figures 46 to 49 can also be applied to the display device 1A'. By applying the example shown in any of Figures 46 to 49 to the display device 1A', even when the user 22 is not positioned directly in front of the display device 1A', the relative angle between the transmission axis of the front polarizing plate of the display panel 2 and the transmission axis of the polarizing plate 15 can be brought closer to a cross-nicol arrangement, thereby reducing the degradation of the display quality of the display device 1A'.

[0167] Although embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the embodiments described above.

[0168] For example, the above description concerns a display device equipped with a display panel, but is not limited to this. For example, this disclosure may describe a device without a display panel that is equipped with an optical system. For example, the housing of a display device may include a display panel mounting section. The display panel mounting section may be capable of mounting a display panel. The display panel mounting section may be located on a part of the wall surface of the housing, or it may be located inside the housing. In this case, the display panel may be located inside the housing. Alternatively, the display panel mounting section may be located outside the housing. That is, the display panel may be located outside the housing. In this case, the housing may have an opening in which a part of the wall surface is cut out. The display panel mounting section may be positioned relative to the housing such that display light emitted from the display panel installed in the display panel mounting section is guided into the inside of the housing through the opening. The display panel mounting section may be connected to the outer wall of the housing, or it may be connected to the outer wall of the housing such that it closes at least a part of the opening. A light-transmitting member may be placed in the opening, and this member may be, for example, glass or resin. For example, Figure 10 shows a display device in which a display panel is installed in the display panel installation section, but the device may not have a display panel installed in the display panel installation section. In this case, the device may be a display panel housing device having a housing that includes a viewing section, an optical system, and a display panel installation section on which a display panel can be installed. The display panel housing device may also realize the configuration of the display device of each embodiment described above. That is, the position of the display panel installation section of the display panel housing device may be defined so that when a display panel is installed in the display panel installation section, it will have the configuration of each embodiment described above. Furthermore, the housing of the display panel housing device may have an opening, and the display panel may be insertable through the opening. In this case, the display panel housing device may have the same configuration as the display device, except that the housing has an opening and the display panel can be inserted from the outside.

[0169] The display device of this disclosure may be installed in a first housing having a first viewing section, and may also have a second housing having a second viewing section. The display device of this disclosure may also have a first housing having a first viewing section, and a second housing installed inside the first housing having a second viewing section. The display device of this disclosure may include a display panel for emitting display light, located inside the second housing and having a planar display surface arranged to face the first and second viewing sections. The display device of this disclosure may include an optical system located inside the second housing that forms an image of the display light. The first viewing section may function to allow the interior of the first housing to be visible from outside the first housing. The second viewing section may function to allow the interior of the second housing to be visible from outside the second housing. The first housing may have a first window (aperture) that transmits light emitted from the optical system. The first window (aperture) may function as the first viewing section. The first housing may have a member positioned in the first window (opening) that makes the interior of the first housing visible from the outside of the first housing. The first housing may have a member positioned in the first window (opening) that transmits at least a portion of the light emitted from the display panel. The first housing may have a light-transmitting plate positioned in the first window (opening). The light-transmitting plate may transmit light emitted from the optical system. The light-transmitting plate may at least partially block the window (opening). The light-transmitting plate may be made of, for example, glass, resin, etc. A member positioned in the first window (opening) that makes the interior of the housing visible from the outside of the housing (for example, a light-transmitting plate or at least one member of the optical system) may function as a first viewing section. A member positioned in the first window (opening) that transmits light emitted from the optical system (for example, a light-transmitting plate) may function as a first viewing section. The second housing may have a second window (opening) that transmits light emitted from the optical system. The second window (opening) may function as a second viewing area. The second housing may have a component that makes the interior of the second housing, located in the second window (opening), visible from the outside of the second housing. The second housing may have a component located in the second window (opening) that transmits at least a portion of the light emitted from the display panel. The second housing may have a light-transmitting plate located in the second window (opening). The light-transmitting plate may transmit light emitted from the optical system.The light-transmitting plate may at least partially block the second window (aperture). The light-transmitting plate may be made of, for example, glass, resin, etc. At least one component of the optical system (for example, a polarizer such as a reflective polarizer or an absorbing polarizer, a phase difference plate, a semi-transparent mirror, etc.) may be placed in the second window (aperture) of the second housing. In this case, both at least one component of the optical system and the light-transmitting plate may be placed in the second window (aperture) of the second housing, or either one or the other may be placed there. A component that makes the inside of the housing visible from the outside of the housing, placed in the second window (aperture) (for example, a light-transmitting plate or at least one component of the optical system), may function as a second viewing section. A component that transmits at least a portion of the light emitted from the display panel, placed in the second window (aperture), may function as a second viewing section.

[0170] The display device of this disclosure may consist only of a second housing in which a display panel and optical system are arranged. Alternatively, the display device of this disclosure may be configured by installing the second housing inside a first housing. For example, a display device such as a digital rearview mirror may be configured by installing the second housing, in which a display panel and optical system are arranged, inside the housing of a digital rearview mirror or the like. Furthermore, the display device of this disclosure may have an opening in the first housing, through which the second housing can be inserted.

[0171] The display device of this disclosure may be installed in a first housing having a first viewing section, and may also include a second housing having a second viewing section, or it may include a first housing having a first viewing section and a second housing installed in the first housing having a second viewing section. The display device of this disclosure may include a display panel for emitting display light, located in the second housing and having a planar display surface arranged to face the first viewing section and the second viewing section, and an optical system located in the second housing and forming an image of the display light. The optical system may include a first semi-transparent mirror that reflects light toward the display panel and has the function of diverging, concentrating or focusing light, and a second semi-transparent mirror located between the display panel and the first semi-transparent mirror and having the function of reflecting light toward the first semi-transparent mirror and having the function of concentrating or focusing light.

[0172] The shapes of the first and second housings are not particularly limited. For example, two housings 36 may be prepared, and the two housings 36 may be designated as the first housing and the second housing, and the windows (openings) 37 of the two housings 36 may be designated as the first viewing section and the second viewing section, respectively. Alternatively, housing 36 may be designated as the second housing, and the windows (openings) 37 of housing 36 may be designated as the second viewing section. In this case, the housing in which housing 36, which is the second housing, can be installed may be designated as the first housing. Alternatively, housing 36 may be designated as the first housing, and the windows (openings) 37 of housing 36 may be designated as the first viewing section. In this case, the housing that can be installed inside housing 36, which is the first housing, may be designated as the second housing. The display panel may be display panel 2, the optical system may be any of optical system 3, optical system 10, optical system 16, or optical system 35, the first semi-transparent mirror may be the first semi-transparent mirror 601, and the second semi-transparent mirror may be the second semi-transparent mirror 603.

[0173] Furthermore, this disclosure is not limited to the embodiments described above, and various modifications and improvements are possible without departing from the gist of this disclosure. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the invention. For example, the functions included in each component can be rearranged in a logically consistent manner, and multiple components can be combined into one or separated. In other words, it should be noted that it is easy for those skilled in the art to make various modifications or alterations based on this disclosure. It should also be noted that these modifications, alterations, or alterations are included in the scope of this disclosure. In addition, the phase difference plate described herein may be a film-like member.

[0174] The display device of this disclosure can be implemented in the following manner.

[0175] (1) A display device comprising: a display panel that emits display light; a first phase difference plate; a second phase difference plate disposed at a distance from the first phase difference plate; a semi-transparent mirror disposed between the first phase difference plate and the second phase difference plate and having a reflective surface capable of reflecting light toward the second phase difference plate; a reflective polarizing plate disposed on the opposite side of the first phase difference plate from the second phase difference plate and transmitting first linearly polarized light and reflecting second linearly polarized light; and a phase difference member disposed between the display panel and the first phase difference plate and having an optical axis substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizing plate, wherein the first phase difference plate and the second phase difference plate convert the display light into the second linearly polarized light, and the second phase difference plate converts the second linearly polarized light, reflected by the reflective polarizing plate, into the first linearly polarized light before it is reflected by the semi-transparent mirror and incident again on the reflective polarizing plate.

[0176] (2) A display device comprising: a display panel that emits display light; a first phase difference plate; a second phase difference plate disposed at a distance from the first phase difference plate; a semi-transparent mirror disposed between the first phase difference plate and the second phase difference plate and having a reflective surface capable of reflecting light toward the second phase difference plate; a reflective polarizing plate disposed on the opposite side of the first phase difference plate from the second phase difference plate and transmitting first linearly polarized light and reflecting second linearly polarized light; and a phase difference member disposed between the first phase difference plate and the second phase difference plate and having an optical axis substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizing plate, wherein the first phase difference plate and the second phase difference plate convert the display light into the second linearly polarized light, and the second phase difference plate converts the second linearly polarized light, reflected by the reflective polarizing plate, into the first linearly polarized light before it is reflected by the semi-transparent mirror and incident again on the reflective polarizing plate.

[0177] (3) The display device according to (2), wherein the phase difference member includes two phase difference plates that provide the same phase difference with respect to transmitted light.

[0178] (4) A display device comprising: a display panel that emits display light; a first phase difference plate; a second phase difference plate disposed at a distance from the first phase difference plate; a semi-transparent mirror disposed between the first phase difference plate and the second phase difference plate and having a reflective surface capable of reflecting light toward the second phase difference plate; a reflective polarizing plate disposed on the opposite side of the first phase difference plate from the second phase difference plate and transmitting first linearly polarized light and reflecting second linearly polarized light; and a phase difference member disposed between the second phase difference plate and the reflective polarizing plate and having an optical axis substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizing plate, wherein the first phase difference plate and the second phase difference plate convert the display light into the second linearly polarized light, and the second phase difference plate converts the second linearly polarized light, reflected by the reflective polarizing plate, into the first linearly polarized light before it is reflected by the semi-transparent mirror and incident again on the reflective polarizing plate.

[0179] (5) A display panel that emits display light; a first phase difference plate; a second phase difference plate positioned at a distance from the first phase difference plate; a semi-transparent mirror positioned between the first phase difference plate and the second phase difference plate and having a reflective surface capable of reflecting light toward the second phase difference plate; a reflective polarizer positioned on the opposite side of the first phase difference plate from the second phase difference plate and transmitting first linearly polarized light and reflecting second linearly polarized light; a third phase difference plate positioned between the display panel and the first phase difference plate and positioned such that its optical axis is substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizer; and a fourth phase difference plate positioned between the second phase difference plate and the reflective polarizer and positioned such that its optical axis is substantially parallel or substantially perpendicular to the transmission axis of the reflective polarizer, wherein the first phase difference plate and the second phase difference plate convert the display light into the second linearly polarized light. The second phase difference plate is a display device that converts the second linearly polarized light, reflected by the reflective polarizer, into the first linearly polarized light before it is reflected by the semitransparent mirror and re-enters the reflective polarizer.

[0180] 1, 1A, 1A', 1B Display device 2 Display panel 2a Display surface 3 Optical system 4 Irradiator 5 First retardation plate 6 Semi-transmissive mirror 6a Reflective surface 7 Second retardation plate 8 Reflective polarizing plate 9 Optical element 10 Optical system 11 First semi-transmissive mirror 11a Reflective surface 12 First retardation plate 13, 13' Second semi-transmissive mirror 13a, 13'a Reflective surface 14 Second retardation plate 15 Polarizing plate 16 Optical system 17 First semi-transmissive mirror 17a Reflective surface 18 First retardation plate 19 Second semi-transmissive mirror 19a Reflective surface 19b Reflective surface 20 Second retardation plate 21 Third semi-transmissive mirror 21a Reflective surface 22 User 22L Left eye 22R Right eye 23 Moving object 24 Windshield 25 Third phase difference plate 26 Fourth phase difference plate 27 Housing 28 Aperture 29 Cluster 30 CID 31 PID 32 RSE 33 Fresnel lens 33a First surface 33b Second surface 33c Reference point 34 Semi-transparent reflective layer 35 Optical system 36 Housing 37 Window 38 Light transmitting plate 39,40 Moth-eye structure film 41 Touch panel 42 Convex lens 43 Controller 100 Imaging device 101 Reflective optical element 102 Camera 200 Display system 201 Camera 401 First region 402 Second region 403 Center of virtual image 404 Normal to the center of virtual image 405 Center of user 406 Part of the second region located above and below the first region 407 Part of the second region located on the side of the first region 408 First end 409 Center of window 410 First straight line 411 Second end 412 Second straight line 413 and 414 End points of the virtual image 415 and 416 End points of the semitransparent mirror 417 and 418 Two straight lines connecting the end points of the virtual image and the end points of the semitransparent mirror, respectively 419 420 Intersection of two lines connecting the endpoints of the virtual image and the endpoints of the semitransparent mirror 421 End of the semitransparent mirror 422 Shielding wall 422 Viewing surface 423 Control unit 424 Base material 425 Reflective polarizing plate 426 Base material 427 Display system 428 Concave mirror section 429 Concave mirror section 501 Display device f6 Focal point of the semitransparent mirror L6 Length of the semitransparent mirror in the longitudinal direction L37 Length along the longitudinal direction of the window Lf6 Focal length of the semitransparent mirror V428 Virtual image V429 Virtual image WL Longitudinal direction of the window 601 First semitransparent mirror 601' Focal point of the first semitransparent mirror 602 First image 603 Second semitransparent mirror 603' Focal point of the second semitransparent mirror 604 Second image 605 Phase difference member 606 Third phase difference plate 607 Fourth phase difference plate d60A Distance between the first image and the second semi-transparent mirror d60B Distance between the display panel and the first semi-transparent mirror f601 Focal length of the first semi-transparent mirror f603 Focal length of the second semi-transparent mirror

Claims

1. A non-attachable display device for the user, comprising: a housing having a viewing section; a display panel located inside the housing and having a planar display surface facing the viewing section, for emitting display light; and an optical system located inside the housing and forming an image of the display light, wherein the optical system includes: a first semi-transparent mirror that reflects light toward the display panel and has the function of diverging, concentrating, or focusing light; and a second semi-transparent mirror located between the display panel and the first semi-transparent mirror and having the function of reflecting light toward the first semi-transparent mirror and having the function of concentrating or focusing light.

2. The display device according to claim 1, wherein the focal length of the first semi-transparent mirror is greater than the distance between the display panel and the first semi-transparent mirror.

3. The display device according to claim 2, wherein the first semi-transparent mirror forms an image of the display light to form a first image, and the focal length of the second semi-transparent mirror is greater than the distance between the first image and the second semi-transparent mirror.

4. The display device according to claim 2, wherein the first semi-transparent mirror forms an image of the display light to form a first image, and the focal length of the second semi-transparent mirror is smaller than the distance between the first image and the second semi-transparent mirror.

5. The display device according to claim 1, wherein the focal length of the first semi-transparent mirror is smaller than the distance between the display panel and the first semi-transparent mirror.

6. The display device according to claim 5, wherein the first semi-transparent mirror forms an image of the display light to form a first image, the first image is located on the opposite side of the display panel from the second semi-transparent mirror, and the focal length of the second semi-transparent mirror is greater than the distance between the first image and the second semi-transparent mirror.

7. The display device according to claim 5, wherein the first semi-transparent mirror forms an image of the display light to form a first image, the first image is located on the opposite side of the display panel from the second semi-transparent mirror, and the focal length of the second semi-transparent mirror is smaller than the distance between the first image and the second semi-transparent mirror.

8. The display device according to any one of claims 3, 4, 6, or 7, wherein the focal length of the second semi-transparent mirror is greater than the focal length of the first semi-transparent mirror.

9. The display device according to any one of claims 3, 4, 6, or 7, wherein the focal length of the second semi-transparent mirror is smaller than the focal length of the first semi-transparent mirror.

10. A mobile body comprising the display device described in claim 1.

11. A non-user-attachable display panel housing device comprising: a housing having a display panel mounting section on which a display panel having a display surface can be installed, and a viewing section arranged opposite to the display surface; and an optical system located inside the housing and forming a planar image of display light emitted from the display surface, wherein the optical system comprises: a first semi-transparent mirror that reflects light toward the display panel mounting section and has the function of diverging, concentrating, or focusing light; and a second semi-transparent mirror located between the display panel mounting section and the first semi-transparent mirror and having the function of reflecting light toward the first semi-transparent mirror and having the function of concentrating or focusing light.

12. A display device comprising: a second housing having a second viewing section and being installable within a first housing having a first viewing section; a display panel for emitting display light, located within the second housing and having a planar display surface arranged to face the first and second viewing sections; and an optical system located within the second housing and forming an image of the display light, wherein the optical system includes: a first semi-transparent mirror having the function of reflecting light toward the display panel and diverging, concentrating, or focusing light; and a second semi-transparent mirror located between the display panel and the first semi-transparent mirror and having the function of reflecting light toward the first semi-transparent mirror and concentrating or focusing light.

13. A display device comprising: a first housing having a first viewing section; a second housing installed inside the first housing and having a second viewing section; a display panel located inside the second housing and having a planar display surface arranged to face the first and second viewing sections, for emitting display light; and an optical system located inside the second housing and forming an image of the display light, wherein the optical system comprises: a first semi-transparent mirror that reflects light toward the display panel and has the function of diverging, concentrating, or focusing light; and a second semi-transparent mirror located between the display panel and the first semi-transparent mirror and having the function of reflecting light toward the first semi-transparent mirror and having the function of concentrating or focusing light.

Images