Patent Description:
Recently, head-up displays are available for use as display devices for vehicles. A head-up display projects an image display light toward, for example, a windshield of a vehicle, superimposes a virtual image based on the image display light on the scenery outside the vehicle, and displays the resultant image. A windshield has two interfaces inside and outside the vehicle. The image display lights reflected at the respective interfaces and visually perceived may be superimposed with a shift and viewed as double images. To inhibit double images from being produced, there is proposed a mathematical expression for defining a viewing distance to contain the amount of shift between double images within the resolution of the human eyes and obtaining an optical arrangement that realizes the viewing distance (see, for example, <CIT>).

<CIT> discloses a head-up display which includes an image display means and a hologram as reflection means for reflecting display beams emitted from the display. An image is reflected inside a windshield and the image is formed at the front of, and outside, the windshield. Concave mirrors having different focal length in different directions are recorded on the hologram as the reflection means so as to offset distortion of the display image resulting from the radius of curvature of the windshield.

The technology described above requires a large viewing distance to the position of presentation of a virtual image ahead of the user in order to reduce double images. It is preferred to reduce the occurrence of double images suitably, regardless of the viewing distance to a virtual image.

The embodiments address the above-described issue, and a general purpose thereof is to provide a technology for improving the visibility of a virtual image presented.

The invention's problem is solved by the features of independent claim <NUM>. The dependent claims describe specific embodiments of the invention.

According to the invention, the occurrence of double images is reduced and the visibility of the virtual image is improved.

A description will be given of embodiments of the present invention with reference to the drawings. Specific numerical values are shown in the embodiments by way of example only to facilitate the understanding of the invention and should not be construed as limiting the scope of the invention unless specifically indicated as such. Those elements in the specification and drawings that have substantially the identical function and configuration are represented by the same reference symbols, and the description is not duplicated. Elements not directly relevant to the invention are omitted from the illustration.

<FIG> schematically shows a configuration of a virtual image display device <NUM> according to the embodiment. In this embodiment, the virtual image display device <NUM> is installed in a dashboard of a vehicle <NUM>, which exemplifies a moving object. The virtual image display device <NUM> is a so-called head-up display device. The virtual image display device <NUM> presents a virtual image <NUM> in front of the vehicle <NUM> in the direction of travel (rightward direction in <FIG>) by projecting an image display light toward a windshield <NUM>, which is a virtual image presentation plate. The user E (e.g., the driver) can see the virtual image <NUM> superimposed on the actual scenery via the windshield <NUM>. Therefore, the driver E can access information shown in the virtual image <NUM> substantially without moving the line of sight while driving the vehicle. Referring to the <FIG>, the direction of travel of the vehicle <NUM> (longitudinal direction) is defined as the z direction, the vertical direction of the vehicle <NUM> (up/down direction) is defined as the y direction, and the transversal direction of the vehicle <NUM> is defined as the x direction.

The virtual image display device <NUM> includes an illumination unit <NUM>, a display unit <NUM>, a projection optical system <NUM>, and a control unit <NUM>. The illumination unit <NUM> is a light source for generating a display light and generates an illumination light for illuminating the display unit <NUM>. The illumination unit <NUM> includes a light emitting device such as a light emitting diode (LED) and a laser diode (LD), and an optical device for adjusting the light intensity distribution and angle distribution of the light output from the light emitting device. The illumination unit <NUM> provides the display unit <NUM> with a substantially uniform white light. The configuration of the illumination unit <NUM> is not limited to any particular type. For example, a light emitting device such as a light tunnel, a Fresnel lens, and a light diffusion plate may be used to condition the output light from the light emitting device.

The display unit <NUM> modulates an illumination light from the illumination unit <NUM> to generate a display light and forms an intermediate image (real image) corresponding to the display content of the virtual image <NUM>. The display unit <NUM> includes an image display device of transmission type for generating a display light. For example, the display unit <NUM> includes a display device such as a liquid crystal panel of transmission type. The image display device receives an image signal transmitted from the control unit <NUM> and generates an image display light for the display content corresponding to the image signal. The display unit <NUM> may further include an optical device for conditioning the orientation and light distribution angle of the image display light. Further, the display unit <NUM> may be configured by combining an element other than a liquid crystal panel of transmission type and a screen of transmission type such as a microlens array sheet and a light diffusion sheet. The element is exemplified by a projection unit such as a digital micromirror device (DMD), a liquid crystal on silicon (LOCS) laser scanning module (LSM), and a micro electro mechanical system (MEMS) LSM.

The projection optical system <NUM> projects the image display light generated by the display unit <NUM> toward the windshield <NUM>. The projection optical system <NUM> includes an optical device of transmission type such as a convex lens and an optical device of reflection type such as a concave mirror. The specific configuration of the projection optical system <NUM> will be described separately.

The control unit <NUM> generates an image for display and causes the illumination unit <NUM> and the display unit <NUM> to operate to present the virtual image <NUM> corresponding to the image for display. The control unit <NUM> is connected to an external device <NUM> and generates the image for display based on the information from the external device <NUM>.

The external device <NUM> is a device for generating original data for an image displayed as the virtual image <NUM>. For example, the external device <NUM> may be an Electronic Control Unit (ECU) for the vehicle <NUM>, a navigation device, or a mobile device such as a cell phone, smartphone, and tablet. The external device <NUM> transmits, to the control unit <NUM>, image data necessary to display the virtual image <NUM>, information indicating the content and type of the image data, and information related to the vehicle <NUM> such as the speed and current position of the vehicle <NUM>.

Before describing the optical arrangement according to the embodiment in detail, a description will be given of the occurrence of double images with reference to a comparative example. One factor that causes the virtual image <NUM> to be viewed as double images is that the image display lights reflected at the two interfaces of the windshield <NUM> inside and outside the vehicle and visually perceived are presented with a shift.

<FIG> schematically shows the occurrence of double images induced by the virtual image presentation plate <NUM>. In <FIG>, an optical device such as a concave mirror provided between the virtual image presentation plate <NUM> and a display unit <NUM> is omitted for simplify the description. The virtual image presentation plate <NUM> has a predetermined thickness t and has a first surface <NUM> and a second surface <NUM>. The first surface <NUM> corresponds to the interface of the windshield <NUM> inside the vehicle, and the second surface <NUM> corresponds to the interface of the windshield <NUM> outside the vehicle.

The image display light arriving at the user E from an arbitrary point E of the display unit <NUM> mainly travels on two light paths L1, L2. The first light path L1 is a light path in which the light is reflected by the first surface <NUM> and travels toward the user E. The second light path L2 is a light path in which the light is refracted by the first surface <NUM>, reflected by the second surface <NUM>, and then re-refracted by the first surface <NUM> and travels toward the user E. If there is an angular difference Δθ between the first light path L1 and the second light path L2 leading toward the user E, the image display lights traveling on the two light paths L1, L2 respectively are visually perceived as being shifted from each other according to the angular difference Δθ, producing double images in a virtual image <NUM>. While it is possible to assume a light path in which the light is reflected multiple times between the first surface <NUM> and the second surface <NUM> and travels toward the user E, the component of the image display light reflected multiple times and traveling toward the user E is small and can be neglected in a normal mode of usage.

<FIG> schematically shows inhibition of double images effected by a wedge glass. A virtual image presentation plate <NUM> shown in <FIG> is a so-called "wedge glass" and is configured such that the thickness of the virtual image presentation plate <NUM> varies depending on the location. This results in a first surface <NUM> and a second surface <NUM> of the virtual image presentation plate <NUM> having mutually different angles of inclination relative to a direction of line of sight of the user E, and an angular difference δ is provided between the surfaces. By using a wedge glass in which the angular difference δ is provided between the two surfaces <NUM>, <NUM>, the angular difference Δθ between the first light path L1 and the second light path L2 as shown in <FIG> is corrected, and a virtual image <NUM> in which double images are reduced is presented.

However, a "wedge glass" like this need be formed by controlling the angular difference δ with high precision and so is more expensive than an ordinary glass having a uniform thickness t. Further, forming the windshield <NUM> of the vehicle <NUM> by using a wedge glass not only requires a dedicated wedge glass adapted to the shape of the vehicle <NUM> but also requires replacing the entirety of the windshield <NUM> so that a heavy cost will be incurred. It is therefore preferred to reduce the occurrence of double images without using a special wedge glass.

<FIG> shows an optical arrangement in a virtual image display device <NUM> according to the comparative embodiment in detail, not covered by the subject-matter of the claims. The comparative example differs from the configuration of <FIG> in that a convex lens <NUM> is provided between the virtual image presentation plate <NUM> and the display unit <NUM>. Thus, according to the comparative example, the angular difference between the first light path L1, in which the light leaving an arbitrary point on the display unit <NUM> and reflected by the first surface <NUM> of the virtual image presentation plate <NUM>, and the second light path L2, in which the light is reflected by the second surface <NUM> of the virtual image presentation plate <NUM>, is reduced by providing the convex lens <NUM>. In particular, by providing the display unit <NUM> at the focal point of the composite optical system formed by the virtual image presentation plate <NUM> and the convex lens <NUM>, the angular difference between the first light path L1 and the second light path L2 is removed and double images are eliminated.

In the comparative example of <FIG>, the image display light L is caused to be diagonally incident on the virtual image presentation plate <NUM> so that astigmatism could be produced if the virtual image presentation plate <NUM> includes a curved surface. The windshield <NUM> of automobiles in general includes a curved surface and is configured such that the first surface <NUM> is concavely curved so that diagonal incidence of the image display light L on the concavely curved surface produces astigmatism. "Astigmatism" is defined here as non-coincidence of the focal point of the composite optical system within the meridional plane and the focal point within the sagittal plane. Astigmatism produces a shift between the imaging position of the virtual image <NUM> in the horizontal direction (x direction) and the imaging position in the vertical direction (y direction) and results in reduction of the imaging performance. The term "meridional plane" refers to a plane that includes the light axis of the composite optical system and the principal ray of the image display light L. The yz plane in <FIG> represents the meridional plane. Meanwhile, the term "sagittal plane" refers to a plane that includes the light axis of the composite optical system and is a plane perpendicular to the meridional plane. The xz plane in <FIG> represents the sagittal plane.

<FIG> schematically show the astigmatism of parallel beams incident on a partial region <NUM> on a concavely curved surface <NUM> and show views from different points of view. <FIG> shows light beams within the meridional plane (yz plane) of the concavely curved surface <NUM>, and <FIG> shows light beams within the sagittal plane (xz plane) of the concavely curved surface <NUM>. As illustrated, the points of convergence Fm and Fs of the parallel beams differ between the meridional plane and the sagittal plane. The focal point Fs within the sagittal plane is located farther from the concavely curved surface <NUM> than the focal point Fm within the meridional plane. This is because of the fact that, when parallel light beams are caused to be diagonally incident on a concave mirror, the distance to the position of convergence of light (i.e., the focal distance) could change in accordance with the angle of incidence. Denoting the focal distance of the concave mirror by f and the angle of incidence and reflection of light on the concave mirror by φ, the focal distance of diagonally incident light is given by f·cosφ. The larger the angle of incidence and reflection φ, the smaller the focal distance f·cosφ. In other words, the focal distance within the meridional plane on which the light beam is diagonally incident will be shortened to f·cosφ. Meanwhile, the focal distance within the sagittal plane will be enlarged to f/cosφ.

In order to reduce the astigmatism As defined as a shift between the focal point Fm within the meridional plane and the focal point Fs within the sagittal plane, the angle of incidence and reflection φ on the concavely curved surface <NUM> may be reduced. However, it is not realistic to reduce the angle of incidence and reflection φ of the image display light L on the virtual image presentation plate <NUM> significantly. This is addressed in this embodiment by configuring the projection optical system <NUM> to reduce the astigmatism in the composite optical system formed by the virtual image presentation plate <NUM> and the projection optical system <NUM> as a whole. More specifically, mitigation of the occurrence of double images and reduction of the astigmatism in the composite optical system are both achieved by using one or more of the features listed in (<NUM>)-(<NUM>) below.

<FIG> show a configuration of the virtual image display device <NUM> according to the first embodiment in detail and show views from different points of view. <FIG> corresponds to <FIG> and shows a configuration viewed on the yz plane. <FIG> shows a configuration viewed on the xz plane, and <FIG> shows a configuration viewed on the xy plane. The embodiment uses the feature (<NUM>) above, the concave mirror <NUM> included in the projection optical system <NUM> is arranged at a position twisted with respect to the virtual image presentation plate <NUM>.

The virtual image display device <NUM> includes an illumination unit <NUM>, a display unit <NUM>, and a projection optical system <NUM>. The projection optical system <NUM> includes a concave mirror <NUM> and a convex lens <NUM>. The illumination unit <NUM>, the display unit <NUM>, the convex lens <NUM>, and the concave mirror <NUM> are arranged on the light axis extending in the x direction. The concave mirror <NUM> reflects, toward the virtual image presentation plate <NUM> and in the y direction, the image display light L incident in the x direction. The virtual image presentation plate <NUM> reflects, toward the user E and in the z direction, the image display light L incident in the y direction.

The virtual image presentation plate <NUM> is configured such that the first surface <NUM> is concavely curved. The first surface <NUM> of the virtual image presentation plate <NUM> is a first concavely curved surface on which the image display light L is incident and reflected diagonally. The virtual image presentation plate <NUM> is configured to have a uniform thickness t, and the curved surfaces of the first surface <NUM> and the second surface <NUM> are identically shaped. The virtual image presentation plate <NUM> is arranged such that the direction (axis A) orthogonal to both the direction of incidence and the direction of reflection of the image display light L on the first surface <NUM> (first concavely curved surface) is the x direction.

The concave mirror <NUM> is a projection mirror that projects the image display light L toward the virtual image presentation plate <NUM>. The convex lens <NUM> is provided between the display unit <NUM> and the concave mirror <NUM>. The concave mirror <NUM> has a second concavely curved surface on which the image display light L is incident and reflected diagonally and is arranged such that the direction (axis B) orthogonal to both the direction of incidence and the direction of reflection of the image display light L on the second concavely curved surface is the z direction. Therefore, the orientation of the axis B of the concave mirror <NUM> is orthogonal to the orientation of the axis A of the virtual image presentation plate <NUM>, and the concave mirror <NUM> and the virtual image presentation plate <NUM> are in a twisted arrangement.

In this embodiment, the virtual image presentation plate <NUM> and the concave mirror <NUM> are in a twisted arrangement so that the astigmatism produced in the virtual image presentation plate <NUM> and the astigmatism produced in the concave mirror <NUM> occur in the opposite directions. Assuming, for example, that parallel beams are incident on the meridional plane (yz plane) of the virtual image presentation plate <NUM>, the parallel beams are reflected by the virtual image presentation plate <NUM> and the concave mirror <NUM> and are transmitted through the convex lens <NUM>, before being converged on the light axis of the display unit <NUM> in the x direction within the xz plane. Therefore, the virtual image presentation plate <NUM> operates, on the light axis of the display unit <NUM>, to shorten the focal distance of the light beam within the xz plane and enlarge the focal distance of the light beam within the xy plane. Meanwhile, the concave mirror <NUM> operates to shorten the focal distance of the light beam within the xy plane and shorten the focal distance of the light beam within the xz plane. Therefore, by combining the virtual image presentation plate <NUM> and the concave mirror <NUM> in a twisted arrangement like this, the astigmatism is reduced as compared with a case where the concave mirror <NUM> in a twisted arrangement is not provided.

For reduction of the astigmatism in the composite optical system <NUM> in which the concave mirror <NUM> and the virtual image presentation plate <NUM> are combined, it will be necessary to ensure that the astigmatism in the concave mirror <NUM> and that of the virtual image presentation plate <NUM> are substantially equal. More specifically, this requires ensuring that the product fa·cosφa of the focal distance fa of the concave mirror <NUM> and the cosine cosφa of the angle of incidence and reflection of the image display light L on the concave mirror <NUM> is substantially equal to the product fb·cosφb of the focal distance fb of the virtual image presentation plate <NUM> and the cosine cosφb of the angle of incidence and reflection of the image display light L on the virtual image presentation plate <NUM>. For example, reduction of the imaging performance caused by astigmatism is suitably prevented by designing the device such that the focal distance fa·cosφa of the concave mirror <NUM> within the meridional pane is not less than <NUM> times and not more than twice the focal distance fb·cosφb of the virtual image presentation plate <NUM> within the meridional plane.

Further, by providing the display unit <NUM> at the focal point within the meridional plane of the composite optical system <NUM> formed by the convex lens <NUM> and the virtual image presentation plate <NUM>, the occurrence of double images caused by the virtual image presentation plate <NUM> having the two surfaces <NUM> and <NUM> is eliminated. The focal point of the composite optical system <NUM> within the meridional plane is the position of convergence of parallel beams assumed to be incident from the user E toward the virtual image presentation plate <NUM> along the meridional plane (yz plane) of the virtual image presentation plate <NUM>.

According to this embodiment, the occurrence of double images is mitigated without using a virtual presentation image of a customized specification such as a wedge glass, by providing the display unit <NUM> at the focal point of the composite optical system <NUM> within the meridional plane. Further, by providing the concave mirror <NUM> in a twisted arrangement with respect to the virtual image presentation plate <NUM>, the astigmatism in the composite optical system <NUM> is reduced and reduction of the imaging performance caused by astigmatism is prevented. This can enhance the visibility of the virtual image <NUM> presented to the user E.

<FIG> shows a configuration of a virtual image display device <NUM> according to the second embodiment in detail. This embodiment differs from the first embodiment in that the axis A of the virtual image presentation plate <NUM> and the axis B of a concave mirror <NUM> are aligned in the same direction (x direction), and the concave mirror <NUM> is not in a twisted arrangement with respect to the virtual image presentation plate <NUM>.

The virtual image display device <NUM> includes an illumination unit <NUM>, a display unit <NUM>, and a projection optical system <NUM>. The projection optical system <NUM> includes a concave mirror <NUM> and a convex lens <NUM>. The concave mirror <NUM> is a projection mirror having a concavely curved surface on which the image display light L is incident and reflected diagonally. The illumination unit <NUM>, the display unit <NUM>, the convex lens <NUM>, and the concave mirror <NUM> are arranged on the light axis extending in the z direction. The concave mirror <NUM> reflects the image display light L incident in the z direction toward the virtual image presentation plate <NUM>. The virtual image presentation plate <NUM> reflects the image display light L from the concave mirror <NUM> toward the user E.

The concave mirror <NUM> is configured such that the curvature within the meridional plane (first cross section) and the curvature within the sagittal plane (second cross section) are different in order to reduce the astigmatism in the composite optical system <NUM> formed by the projection optical system <NUM> and the virtual image presentation plate <NUM>. The meridional plane of the concave mirror <NUM> is a plane (yz plane) that includes both the direction of incidence and the direction of reflection of the image display light diagonally incident on the concave mirror <NUM> and that intersects the concavely curved surface of the concave mirror <NUM>. Meanwhile, the sagittal plane of the concave mirror <NUM> is a plane orthogonal to the meridional plane and is a plane that is along the direction (x direction) orthogonal to both the direction of incidence and the direction of reflection of the image display light and that intersects the concavely curved surface of the concave mirror <NUM>. The curvature within the meridional plane is related to the focal distance of parallel beams incident along the meridional plane, i.e., related to the focal distance within the meridional plane. Meanwhile, the curvature within the sagittal plane is related to the focal distance within the sagittal plane.

In this embodiment, the focal point of the composite optical system <NUM> within the meridional plane and the focal point within the sagittal plane are made to coincide by appropriately setting the curvature of the concave mirror <NUM> within the meridional plane and the curvature within the sagittal plane. For example, the difference between the focal distance within the meridional plane and the focal distance within the sagittal plane is reduced by configuring the curvature of the concave mirror <NUM> within the meridional plane to be smaller than the curvature within the sagittal plane. It is preferred to set the specific curvature of the concave mirror <NUM> in accordance with the curvature of the virtual image presentation plate <NUM> within the meridional plane and the curvature thereof within the sagittal plane, the angle of incidence and reflection φa of the image display light L on the virtual image presentation plate <NUM>, and the angle of incidence and reflection φb of the image display light L on the concave mirror <NUM>.

According to this embodiment, the astigmatism in the composite optical system <NUM> is reduced and reduction of the imaging performance caused by astigmatism is prevented by configuring the curvature of the concave mirror <NUM> within the meridional plane and the curvature thereof within the sagittal plane to be different. Further, the occurrence of double images is inhibited by providing the display unit <NUM> at the focal point of the composite optical system <NUM> within the meridional plane. In this way, a highly visible virtual image <NUM> is presented to the user.

It should be noted that this embodiment is applicable to a case where the first surface <NUM> of the virtual image presentation plate <NUM> is a flat surface. In this case, the astigmatism in the virtual image presentation plate <NUM> is negligible. It therefore suffices to set the curvature of the concave mirror <NUM> within the meridional plane and the curvature thereof within the sagittal plane to have different values so as to reduce the astigmatism in the concave mirror <NUM>.

<FIG> shows a configuration of a virtual image display device <NUM> according to the third embodiment in detail. This embodiment differs from the foregoing embodiments in that a plane parallel plate <NUM> provided at an angle between the display unit <NUM> and a convex lens <NUM> is added.

The virtual image display device <NUM> includes an illumination unit <NUM>, a display unit <NUM>, and a projection optical system <NUM>. The projection optical system <NUM> includes a concave mirror <NUM>, a convex lens <NUM>, and the plane parallel plate <NUM>. The illumination unit <NUM>, the display unit <NUM>, the plane parallel plate <NUM>, the convex lens <NUM>, and the concave mirror <NUM> are arranged on the light axis extending in the z direction.

The plane parallel plate <NUM> is a transparent member having a uniform thickness and is made of glass or a resin material. The plane parallel plate <NUM> is provided at an angle to the light path of the projection optical system <NUM> such that the rotational axis C is aligned with the direction (x direction) orthogonal to the meridional plate (yz plane). Inserting the plane parallel plate <NUM> at an angle enlarges the focal distance of a composite optical system <NUM> formed by the virtual image presentation plate <NUM> and the projection optical system <NUM>. In particular, configuring the direction of the rotational axis C of the plane parallel plate <NUM> to be orthogonal to the meridional plane enlarges the focal distance within the meridional plane as compared with the focal distance within the sagittal plane. This reduces the difference between the focal distance within the meridional plane and the focal distance within the sagittal plane and mitigates the astigmatism as compared with a case where the plane parallel plate <NUM> is not inserted. The amount by which the focal distance increases by providing the plane parallel plate <NUM> depends on the angle of inclination φc of the plane parallel plate <NUM> so that the astigmatism can be adjusted by changing the angle of inclination φc.

According to this embodiment, the astigmatism in the composite optical system <NUM> is reduced by inserting the plane parallel plate <NUM>. Further, the occurrence of double images is inhibited by providing the display unit <NUM> at the focal point of the composite optical system <NUM> within the meridional plane. This can enhance the visibility of a virtual image <NUM>.

<FIG> show a configuration of a virtual image display device <NUM> according to the fourth embodiment in detail and show views from different points of view. <FIG> corresponds to <FIG> and shows a configuration viewed on the yz plane, and <FIG> shows a configuration viewed on the xz plane. This embodiment differs from the foregoing embodiments in that the projection optical system <NUM> includes two concave mirrors <NUM> and <NUM>, and the two concave mirrors <NUM> and <NUM> are in a twisted arrangement.

The virtual image display device <NUM> includes an illumination unit <NUM>, a display unit <NUM>, and a projection optical system <NUM>. The projection optical system <NUM> includes a first concave mirror <NUM> and a second concave mirror <NUM>. The illumination unit <NUM>, the display unit <NUM>, and the second concave mirror <NUM> are arranged on the light axis extending in the x direction. The second concave mirror <NUM> is arranged such that the direction (axis D) orthogonal to both the direction of incidence and the direction of reflection of the image display light L is the y direction and reflects, in the z direction, the image display light L incident in the x direction. The first concave mirror <NUM> is arranged such that the direction (axis B) orthogonal to both the direction of incidence and the direction of reflection of the image display light L is the x direction and reflects the image display light L incident in the z direction toward the virtual image presentation plate <NUM>.

Since the first concave mirror <NUM> and the second concave mirror <NUM> are in a twisted arrangement according to the embodiment, the astigmatism produced in the respective concave mirrors is made to occur in the opposite directions. The mechanism to mitigate the astigmatism by combining two concavely curved surfaces is as described in the first embodiment. Therefore, this embodiment can also reduce the astigmatism in the composite optical system <NUM> formed by the virtual image presentation plate <NUM> and the projection optical system <NUM> and enhance the imaging performance. Further, the occurrence of double images is inhibited by providing the display unit <NUM> at the focal point of the composite optical system <NUM> within the meridional plane. This can enhance the visibility of the virtual image <NUM>.

It should be noted that this embodiment is applicable to a case where the first surface <NUM> of the virtual image presentation plate <NUM> is a flat surface. In this case, the astigmatism in the virtual image presentation plate <NUM> is negligible. It therefore suffices to set the curvature and the angles of incidence and reflection φb, φd of the two concave mirrors <NUM>, <NUM> respectively so as to reduce the astigmatism in the combination of the first concave mirror <NUM> and the second concave mirror <NUM>.

In this embodiment, the projection optical system <NUM> is not provided with a convex lens, but an additional convex lens may be provided in the projection optical system <NUM>. For example, an additional convex lens may be provided between the display unit <NUM> and the second concave mirror <NUM>, or an additional convex lens may be provided between the second concave mirror <NUM> and the first concave mirror <NUM>.

A feature of the illustrated examples may be appropriately combined or replaced.

The embodiment described above shows that the astigmatism in the composite optical system is mitigated by using one of the features (<NUM>)-(<NUM>) described above. In one variation, a plurality of features may be combined. For example, the feature (<NUM>) or the feature (<NUM>) may be combined with the feature (<NUM>), or the feature (<NUM>) or the feature (<NUM>) may be combined with the feature (<NUM>). Also, the feature (<NUM>) may be combined with the feature (<NUM>).

The embodiment described above shows that the first surface <NUM> of the virtual image presentation plate <NUM> is formed by a concavely curved surface or a flat surface. In one variation, the composite optical system may be configured to mitigate the astigmatism of the convexly curved first surface <NUM> of the virtual image presentation plate <NUM>. In the case of the feature (<NUM>), curvature within the meridional plane may be larger than the curvature within the sagittal plane.

The embodiment described above shows that a convex lens is included in the projection optical system. In one variation, a convex lens may not be provided in the projection optical system. In the case of the embodiment shown in <FIG>, for example, the convex lens <NUM> may not be included in the projection optical system <NUM>, and the display unit <NUM> may be provided at the focal point within the meridional plane of the composite optical system <NUM> not including the convex lens <NUM>. Similarly, the convex lens <NUM> may not be included in the projection optical system <NUM> of the embodiment shown in <FIG>, and the convex lens <NUM> may not be included in the projection optical system <NUM> of the embodiment shown in <FIG>.

image display light, <NUM>. virtual image display device, <NUM>. display unit, <NUM>. projection optical system, <NUM>. concave mirror (projection mirror), <NUM>. composite optical system, <NUM>. virtual image presentation plate, <NUM>, <NUM>. windshield.

Claim 1:
A virtual image display device (<NUM>) for presenting a virtual image (<NUM>) to a user (E), comprising:
a display unit (<NUM>) configured to generate an image display light (L); and
a virtual image presentation plate (<NUM>, <NUM>) having a first surface (<NUM>) and a second surface (<NUM>), and having a uniform thickness (t) between the first surface (<NUM>) and the second surface (<NUM>);a projection optical system (<NUM>) that includes a projection mirror (<NUM>) configured to project the image display light toward the virtual image presentation plate, wherein
the projection mirror has a concavely curved surface on which the image display light is incident and reflected diagonally and is arranged such that a curvature in a second cross section intersecting the concavely curved surface is larger than a curvature in a first cross section intersecting the concavely curved surface, the first cross section is a plane that includes both the direction of incidence and the direction of reflection of the image display light incident on the concavely curved surface diagonally, and the second cross section is a plane orthogonal to the first cross section and is a plane along a direction orthogonal to both the direction of incidence and the direction of reflection of the image display light incident on the concavely curved surface diagonally,
characterized in that
the display unit (<NUM>) is provided at a focal point of a composite optical system (<NUM>) within a meridional plane, the composite optical system being formed by the virtual image presentation plate and the projection optical system, and
the focal point of the composite optical system within the meridional plane is a position of convergence of parallel beams assumed to be incident from the user toward the virtual image presentation plate along the meridional plane of the virtual image presentation plate.