Image projection device

Abstract

The purpose of the present invention is to provide an image projection device that makes it possible to project a plurality of images while suppressing an increase in the number of components of a projection optical member and achieving a reduction in the size of the optical member.　This image projection device (100) projects a projection image onto a display part (WS) for displaying virtual images (P1, P2), and comprises: an image emitting unit (10) that emits first image light, which is circularly polarized light of a first rotation direction, and second image light, which is circularly polarized light rotating in a reverse direction to the first rotation direction; a first polarization holographic optical element (50) that has a refractive index distribution which varies with the polarization direction; a projection optical unit (30) on which the first image light and the second image light are incident via the first polarization holographic optical element (50), and that projects the first image light and the second image light onto the display part (WS); and a multiplexing member (20) that multiplexes the first image light and the second image light at the position of the first polarization holographic optical element (50).

Description

Image Projection Device

[0001] The present invention relates to an image projection device, and more particularly to an image projection device for projecting multiple images.

[0002] Conventionally, dashboards that illuminate icons have been used to display various types of information inside vehicles. As the amount of information to be displayed increases, it has been proposed to embed an image display device in the dashboard or to configure the entire dashboard with an image display device.

[0003] However, since the instrument panel is located below the windshield of the vehicle, in order for a driver or other passenger to view the information displayed on the instrument panel, the driver or other passenger must move his or her eyes downward while driving, which is undesirable. Therefore, image projection devices such as head-up displays (hereinafter referred to as HUDs) have been proposed that project images onto the windshield so that passengers can read information when they view the area ahead of the vehicle (see, for example, Patent Documents 1 and 2).

[0004] In conventional image projection devices, an image projection unit emits light containing an image, which is reflected by a free-form mirror or the like, and the light reaches the viewpoint of the occupant so that the image is formed in space via a display unit such as a windshield. This allows the occupant to perceive the image as being displayed at the imaging position in the depth direction due to the light incident on the viewpoint. It has also been proposed to project multiple images onto the windshield using a driving assistance HUD device in order to present more information.

[0005] However, in order to project and form multiple images as virtual images at different distances, it is necessary to provide multiple image projection units and projection optical systems, which limits the degree of freedom in design when fitting them within an instrument panel. Therefore, the applicant of the present application has proposed an image projection device that saves space by displaying multiple images within a single image projection unit and branching the optical paths of each image.

[0006] JP 2019-119248 A JP 2019-119262 A

[0007] However, in conventional image projection devices that project multiple images, the optical path of each image is changed to separate the imaging positions of the virtual images, which increases the number of required optical components.In addition, optical components must be positioned to avoid the optical paths for projecting other images, which makes it difficult to reduce the size of the device.

[0008] Therefore, the present invention has been made in consideration of the above-mentioned conventional problems, and aims to provide an image projection device that is capable of projecting multiple images while suppressing an increase in the number of parts in the projection optical components and achieving miniaturization of the optical components.

[0009] In order to solve the above problem, the image projection device of the present invention is an image projection device that projects a projection image onto a display unit for displaying a virtual image, and is characterized by having an image irradiation unit that irradiates first image light that is circularly polarized in a first rotation direction and second image light that is circularly polarized in the opposite rotation direction to the first rotation direction, a first polarizing holographic optical element having a refractive index distribution that differs depending on the polarization direction, a projection optical unit into which the first image light and the second image light are incident via the first polarizing holographic optical element and that projects the first image light and the second image light onto the display unit, and a combining member that combines the first image light and the second image light at the position of the first polarizing holographic optical element.

[0010] In the image projection device of the present invention, the first image light and the second image light are irradiated as counter-rotating circularly polarized light and are combined at the position of the first polarizing holographic optical element.Therefore, the paths of the first image light and the second image light can be designed according to the grating period of the first polarizing holographic optical element, making it possible to project multiple images while suppressing an increase in the number of parts and miniaturizing the optical components.

[0011] In one aspect of the present invention, the first polarizing holographic optical element has a lens function and a diffraction grating function.

[0012] In one aspect of the invention, the combining member is a reflecting mirror that reflects the first image light and the second image light irradiated from the image irradiating unit.

[0013] In one aspect of the present invention, the reflecting mirror forms an intermediate image of the first image light and the second image light at an intermediate imaging position, and the first polarizing holographic optical element is disposed at the intermediate imaging position.

[0014] In one aspect of the present invention, the beam combining member is a second polarizing holographic optical element having a refractive index distribution that varies depending on the polarization direction.

[0015] In one aspect of the present invention, the second polarizing holographic optical element has the same grating period as the first polarizing holographic optical element.

[0016] In one aspect of the present invention, the beam combining member is a prism.

[0017] In one aspect of the present invention, the polarization holographic optical element is a transmission-type liquid crystal holographic element.

[0018] In one aspect of the present invention, the image irradiation unit has an image display unit that displays a first image and a second image and emits polarized light in a predetermined direction, and the image display unit has a first region that displays the first image and a second region that displays the second image, and has a first circular polarization conversion unit arranged opposite the first region and a second circular polarization conversion unit arranged opposite the second region.

[0019] In one aspect of the present invention, the image display unit is a liquid crystal display device, and the first circular polarization conversion unit and the second circular polarization conversion unit are each a quarter-wave plate or a three-quarter-wave plate, and are arranged so that their fast axis and slow axis intersect the specified direction.

[0020] The present invention can provide an image projection device that can project a plurality of images while suppressing an increase in the number of parts in the projection optical members and achieving miniaturization of the optical members.

[0021] 2A and 2B are schematic cross-sectional views illustrating the projection of virtual images P1 and P2 using the image projection device 100 according to the first embodiment.

[0033] FIG. 2A is a schematic plan view of the image display unit 12, and FIG. 2B is a schematic plan view of the first circular polarization conversion unit 13a and the second circular polarization conversion unit 13b, showing an example of the positional relationship between the image display area in the image display unit 12 and the first and second circular polarization conversion units 13a and 13b.

[0034] FIG. 3A is a schematic plan view of the image display unit 12, and FIG. 3B is a schematic plan view of the first and second circular polarization conversion units 13a and 13b.

[0035] FIG. 3B is a schematic plan view of the polarized holographic optical element 50, showing the direction of propagation of circularly polarized light.

[0036] FIG. 3C is a schematic view illustrating the lens function of left-handed circularly polarized light, FIG. 3B is a lens function of right-handed circularly polarized light, FIG. 3C is a diffraction grating function of left-handed circularly polarized light, and FIG. 3D is a diffraction grating function of right-handed circularly polarized light.

[0037] FIG. 3D is a schematic view illustrating the combination of first and second image light by the reflecting mirror 20 in the image projection device 100 according to the first embodiment. 1 is a schematic diagram illustrating the focal lengths of the first and second image lights transmitted through a polarization holographic optical element 50. FIG. 2 is a schematic diagram illustrating the focal lengths of the first and second image lights transmitted through a polarization holographic optical element 50 in a modified example of the first embodiment. FIG. 3 is a schematic diagram illustrating the multiplexing of the first and second image lights by a polarization holographic optical element 60 in an image projection device 100 according to a second embodiment. FIG. 4 is a schematic diagram illustrating the multiplexing of the first and second image lights by prisms 70 a, 70 b in an image projection device 100 according to a third embodiment.

[0022] First Embodiment Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. The same or equivalent components, members, and processes shown in each drawing will be denoted by the same reference numerals, and redundant description will be omitted as appropriate. FIG. 1 is a schematic cross-sectional view illustrating the projection of virtual images P1 and P2 using an image projection device 100. The dashed lines shown in FIG. 1 indicate the optical path of a first image light, which will be described later, and the dashed-dotted lines indicate the optical path of a second image light.

[0023] As shown in Fig. 1, the image projection device 100 includes an image projection unit 10, a reflection mirror 20, a projection mirror 30, an external light cut filter 40, and a polarization holographic optical element 50. As shown in Fig. 1, the first image light and the second image light projected from the image projection device 100 are reflected by a windshield (display unit) WS and irradiated onto the driver's viewpoint. The driver visually recognizes virtual images P1 and P2 formed on extensions of the optical paths along which the first image light and the second image light are incident.

[0024] 1, the optical paths of the first image light and the second image light are depicted as a single straight line. However, the actual first image light and the second image light are displayed in a predetermined area in the image projection unit 10, and have a predetermined area in the direction perpendicular to the traveling direction. Alternatively, the first image light and the second image light may be reflected by the projection mirror 30, reducing their light diameters as they travel, and may form an image between the projection mirror 30 and the windshield WS.

[0025] 1, each part is controlled by a control unit connected to the other parts so as to be able to communicate information with the other parts. The configuration of the control unit is not limited, but an example includes a CPU (Central Processing Unit) for information processing, a memory device, a recording medium, an information communication device, etc. The control unit controls the operation of each part according to a predetermined program and sends information including an image (image information) to the image projection unit 10.

[0026] The image projection unit 10 is a unit that projects light containing an image based on image information from the control unit. In the example shown in FIG. 1 , the image projection unit 10 includes a backlight 11, an image display unit 12, a first circular polarization conversion unit 13a, and a second circular polarization conversion unit 13b. The backlight 11 projects light onto the image display unit 12, and may be, for example, a unit that projects light using a light-emitting diode (LED). The light projected by the backlight 11 is preferably white, but may also be a unit that emits a single color such as blue, green, or red.

[0027] The image display unit 12 is a part that displays a projected image in response to an image signal from the control unit. Light from the backlight 11 is irradiated onto the projected image displayed on the image display unit 12, causing the image display unit 12 to irradiate first and second image lights. The specific configuration of the image display unit 12 is not limited, and a liquid crystal display device or the like can be used, for example. As will be described later, the image display unit 12 includes a far display area (first area) 12a and a near display area (second area) 12b that display a far image and a near image, respectively.

[0028] The first circular polarization conversion unit 13a and the second circular polarization conversion unit 13b are units that transmit the first image light and the second image light irradiated from the image display unit 12 and convert them into circularly polarized light. The first circular polarization conversion unit 13a and the second circular polarization conversion unit 13b are configured to convert the light into circularly polarized light with opposite rotations.

[0029] When the first image light and the second image light from the image display unit 12 are not linearly polarized, the first circular polarization conversion unit 13a and the second circular polarization conversion unit 13b are configured by combining a polarizing plate that transmits only linearly polarized light in a predetermined direction with a quarter-wave plate or a three-quarter-wave plate. When a liquid crystal display device is used as the image display unit 12, both the first image light and the second image light irradiated from the image display unit 12 are linearly polarized in a predetermined direction, so that the first circular polarization conversion unit 13a and the second circular polarization conversion unit 13b are configured with a quarter-wave plate or a three-quarter-wave plate without using a separate polarizing plate.

[0030] As an example, quarter-wave plates may be used for the first circular polarization conversion unit 13 a and the second circular polarization conversion unit 13 b, and the fast and slow axes of the quarter-wave plates may be arranged to intersect at an angle of 45 degrees and −45 degrees, respectively, with respect to the predetermined polarization direction. Alternatively, a quarter-wave plate may be used for one of the first circular polarization conversion unit 13 a and the second circular polarization conversion unit 13 b, and a three-quarter-wave plate may be used for the other, and the fast and slow axes may be arranged to intersect at an angle of 45 degrees with respect to the predetermined polarization direction.

[0031] The reflecting mirror 20 is an optical element that receives the first image light that has passed through the first circular polarization conversion unit 13a and the second image light that has passed through the second circular polarization conversion unit 13b, and reflects the first image light and the second image light toward the projection mirror 30 via the polarization holographic optical element 50. In the example shown in FIG. 1 , a free-form concave mirror is shown as the reflecting mirror 20. The reflecting mirror 20 has a reflective surface shape that combines the first image light and the second image light that have entered different regions so that they overlap in the same region at the position of the polarization holographic optical element 50. Although an example using one reflecting mirror 20 is shown here, a combination of multiple reflecting mirrors 20 may also be used.

[0032] Furthermore, the reflecting mirror 20 may have a predetermined focal length and form an intermediate image of the first image light and the second image light at a predetermined intermediate image position. In this case, by disposing the polarizing holographic optical element 50 at the intermediate image position, the area of ​​the polarizing holographic optical element 50 can be reduced.

[0033] The projection mirror 30 is an optical element that receives the first image light and the second image light that have passed through the polarizing holographic optical element 50 and reflects the first image light and the second image light toward the windshield WS via the external light cut filter 40. The projection mirror 30 corresponds to the projection optical unit in this invention. While an example in which the projection mirror 30 is used as the projection optical unit has been shown here, a combination of multiple projection mirrors 30 may also be used, and optical elements such as lenses and prisms may also be included.

[0034] 1, a concave free-form mirror is used as the projection mirror 30. The reflective surface of the projection mirror 30 is designed to expand the light diameter in the driver's viewing direction in order to project the first and second image lights as virtual images P1 and P2 through the windshield WS. Here, "expanding the light diameter in the viewing direction" includes not only the case where the light diameter expands consistently after reflection, but also the case where the light diameter shrinks and expands after forming an image at an intermediate point.

[0035] The external light cut filter 40 is disposed between the projection mirror 30 and the windshield WS and cuts out part of the external light that reaches the inside of the image projection device 100. In particular, to prevent sunlight from reaching the image projection unit 10 from above the windshield WS and causing the temperature of the image display unit 12 to rise, it is preferable to use a wavelength filter that transmits visible light but cuts infrared light and ultraviolet light.

[0036] The polarizing holographic optical element 50 is an optical member having a refractive index distribution that varies depending on the polarization direction. The polarizing holographic optical element 50 of this embodiment corresponds to the first polarizing holographic optical element of the present invention. Specific examples of the polarizing holographic optical element 50 are not limited, and a known liquid crystal holographic element such as that described in APPLIED PHYSICS LETTERS Vol. 82, No. 3, pp. 328-330, can be used as an example. While FIG. 1 shows an example in which a transmission-type liquid crystal holographic element is used as the polarizing holographic optical element 50, a reflection-type liquid crystal holographic element may also be used. Circularly polarized light that has passed through the first circular polarization conversion unit 13a and the second circular polarization conversion unit 13b is incident on the polarizing holographic optical element 50, and the first and second image lights interfere with each other due to the refractive indices corresponding to their polarization directions, thereby changing their propagation directions. The change in the propagation directions of the first and second image lights by the polarizing holographic optical element 50 will be described in detail below.

[0037] The windshield WS is a part of the vehicle that is provided in front of the driver's seat and transmits visible light. The windshield WS corresponds to the display unit of the present invention because the windshield WS, on the inside surface of the vehicle, reflects the first image light and the second image light incident from the projection mirror 30 toward the viewpoint and transmits light from outside the vehicle toward the viewpoint. While an example in which the windshield WS is used as the display unit is shown here, a combiner may be provided as a display unit separate from the windshield WS and reflect light from the projection mirror 30 toward the viewpoint. Furthermore, the display unit is not limited to being located at the front of the vehicle, and may be located to the side or rear as long as it projects an image toward the viewpoint of the passenger.

[0038] The virtual images P1 and P2 are images that are displayed as if they were formed in space when the first image light and the second image light reflected by the windshield WS reach the viewpoint (eyebox) of the driver, etc. The positions at which the virtual images P1 and P2 are formed are determined by the combined focal length of the light irradiated from the image irradiator 10, the polarizing holographic optical element 50, the projection mirror 30, and the windshield WS.

[0039] In the image projection device 100, a far image displayed in the far display area 12a of the image display unit 12 is irradiated as a first image light, and a near image displayed in the near display area 12b is irradiated as a second image light. Examples of the far image displayed in the far display area 12a include auxiliary information related to driving, such as an image calling attention or emergency information. Examples of the near image displayed in the near display area 12b include a speed and volume indicator, a driving direction guide, etc.

[0040] 2A and 2B are diagrams showing an example of the positional relationship between the image display area of ​​the image display unit 12 and the first and second circular polarization conversion units 13a and 13b. FIG. 2A is a schematic plan view of the image display unit 12, and FIG. 2B is a schematic plan view of the first and second circular polarization conversion units 13a and 13b. As shown in FIG. 2A, the image display unit 12 displays a far image (first image) and a near image (second image) in a far display area (first area) 12a and a near display area (second area) 12b, respectively, over the entire display area capable of displaying images. Also, as shown in FIGS. 1 and 2B, the first and second circular polarization conversion units 13a and 13b are arranged to overlap the far display area 12a and the near display area 12b of the image display unit 12, respectively.

[0041] Here, arranging the first circular polarization conversion unit 13 a and the second circular polarization conversion unit 13 b so as to overlap with the image display unit 12 means that the area where the first circular polarization conversion unit 13 a and the second circular polarization conversion unit 13 b are arranged overlaps with the image display area of ​​the image display unit 12 in a planar view. The overlapping arrangement also includes cases where the first circular polarization conversion unit 13 a and the second circular polarization conversion unit 13 b are in contact with the image display unit 12 and cases where they are not in contact with the image display unit 12. The overlapping arrangement also includes cases where a light-transmitting optical member or a holding member for maintaining the distance between the first circular polarization conversion unit 13 a and the second circular polarization conversion unit 13 b and the image display unit 12 is interposed between the first circular polarization conversion unit 13 a and the second circular polarization conversion unit 13 b and the image display unit 12.

[0042] In this embodiment, an element having a lens function and a diffraction grating function is used as the polarizing holographic optical element 50. A polarizing holographic optical element 50 having a lens function has a refractive index distribution in which the refractive index varies depending on the polarization direction, and this refractive index distribution causes light to be diffracted at various points within the plane, resulting in the light diameter being converted into a traveling direction that expands or contracts.

[0043] A polarization holographic optical element 50 with a diffraction grating function has a refractive index distribution in which the refractive index varies depending on the polarization direction, and this refractive index distribution causes light to be diffracted at various points within the plane, resulting in a change in the direction of travel of the light. In such a polarization holographic optical element 50 with a diffraction grating function, if the direction of travel of the light emitted when right-handed circularly polarized light is incident perpendicular to the plane is +θ, then the direction of travel of the light emitted when left-handed circularly polarized light is incident is −θ. Conversely, left-handed circularly polarized light incident from the +θ direction and right-handed circularly polarized light incident from the −θ direction will be emitted along the same optical path.

[0044] 3A and 3B are diagrams illustrating the direction of propagation of circularly polarized light in the polarization holographic optical element 50, with Fig. 3A showing the lens function for left-handed circularly polarized light, Fig. 3B showing the lens function for right-handed circularly polarized light, Fig. 3C showing the diffraction grating function for left-handed circularly polarized light, and Fig. 3D showing the diffraction grating function for right-handed circularly polarized light. As shown in Fig. 3A, when left-handed circularly polarized light is incident on the polarization holographic optical element 50, the light diameter of the transmitted circularly polarized light decreases, and after being focused at the focal point F2, the light diameter increases and a real image is formed. In other words, the polarization holographic optical element 50 is equivalent to one that imparts positive optical power to left-handed circularly polarized light.

[0045] 3(b), when right-handed circularly polarized light is incident on the polarization holographic optical element 50, the beam diameter of the transmitted circularly polarized light expands. In this case, the light is focused at a virtual focal point F1 before passing through the polarization holographic optical element 50, and a virtual image of the light emitted from the focal point F1 is formed. In other words, the polarization holographic optical element 50 is equivalent to one that imparts negative optical power to right-handed circularly polarized light.

[0046] 3(a) and 3(b), the polarization holographic optical element 50 is shown to impart positive optical power to left-handed circularly polarized light and negative optical power to right-handed circularly polarized light, but the direction of rotation of the circularly polarized light and the positive and negative of the power may be reversed. Also, while right-handed and left-handed circularly polarized light are incident on separate polarization holographic optical elements 50 in Figures 3(a) and 3(b), right-handed and left-handed circularly polarized light may be incident on different regions of a common polarization holographic optical element 50.

[0047] As shown in Figure 3(c), by making left-handed circularly polarized light incident on the polarizing holographic optical element 50 from the direction of +θ, the transmitted light is converted to right-handed circularly polarized light and travels in a direction perpendicular to the surface. Also, as shown in Figure 3(d), by making right-handed circularly polarized light incident on the polarizing holographic optical element 50 from the direction of -θ, the transmitted light is converted to left-handed circularly polarized light and travels in a direction perpendicular to the surface.

[0048] By incorporating both a lens function and a diffraction grating function into the polarizing holographic optical element 50, it is possible to achieve, with a single polarizing holographic optical element 50, the effect of creating a difference in the imaging position due to the lens function and the effect of overlapping optical paths due to the diffraction grating function.

[0049] 4 is a schematic diagram illustrating the combination of the first image light and the second image light by the reflecting mirror 20 in the image projection device 100 according to this embodiment. In FIG. 4, the width of the area through which the first image light passes is schematically indicated by a dashed line, and the width of the area through which the second image light passes is schematically indicated by a dashed line.

[0050] In this embodiment, the first image light and the second image light are emitted from different regions of the image display unit 12, namely the far display region 12a and the near display region 12b, and are converted into circularly polarized light with opposite rotations to each other by passing through the first circular polarization converter 13a and the second circular polarization converter 13b, respectively. The first image light and the second image light converted into circularly polarized light are reflected from different regions on the reflecting surface of the reflecting mirror 20 and combined so as to overlap at the same region at the position of the polarizing holographic optical element 50. At this time, the first image light and the second image light are incident on the polarizing holographic optical element 50 at different angles of incidence.

[0051] The polarizing holographic optical element 50 has a refractive index distribution that varies depending on the polarization direction and has the diffraction grating function described in Figures 3(c) and (d). Therefore, by appropriately setting the refractive index distribution of the polarizing holographic optical element 50, it is possible to transmit the first image light and the second image light that are incident at different angles in the same direction. The first image light and the second image light that are combined at the position of the polarizing holographic optical element 50 are reflected by the projection mirror 30 and travel toward the windshield WS. The first image light and the second image light that reach the windshield WS are reflected toward the driver's viewpoint, and the driver visually recognizes virtual images P1 and P2 on an extension of the direction in which the first image light and the second image light arrived.

[0052] Fig. 5 is a schematic diagram illustrating the focal lengths of the first image light and the second image light that have passed through the polarizing holographic optical element 50. Fig. 5 shows the paths of the first image light and the second image light when the reflecting mirror 20, the polarizing holographic optical element 50, and the projection mirror 30 are viewed in plan from above. In Fig. 5, the width of the area through which the first image light passes is schematically indicated by a dashed line, and the width of the area through which the second image light passes is schematically indicated by a dashed line.

[0053] The polarizing holographic optical element 50 has a refractive index distribution that varies depending on the polarization direction and has the lens function described in FIGS. 3A and 3B . Therefore, by appropriately setting the refractive index distribution of the polarizing holographic optical element 50, it is possible to differentiate the optical power acting on the first image light and the second image light, thereby focusing or expanding the light diameter so that the respective focal positions are different. In the example shown in FIG. 6 , a negative optical power acts on the first image light, and the light diameter expands as the first image light reaches the projection mirror 30 so that the virtual focal position (not shown) is located between the reflecting mirror 20 and the polarizing holographic optical element 50. A positive optical power acts on the second image light, and the second image light is focused so that the focal position is located between the polarizing holographic optical element 50 and the projection mirror 30. After intermediate image formation, the light diameter expands as the second image light reaches the projection mirror 30.

[0054] The first and second image lights transmitted through the polarizing holographic optical element 50 are reflected by the projection mirror 30 and the windshield WS, and the driver views virtual images P1 and P2 on an extension of the direction in which the first and second image lights reached. At this time, the focal positions of the first and second image lights are different due to the lens function of the polarizing holographic optical element 50, and therefore the virtual images P1 and P2 are viewed as being formed at different positions. Specifically, as shown in Figures 1 and 2, the first image and the second image are formed at positions farther from the viewpoint than the second image.

[0055] 4 and 5 show an example in which only the diffraction grating function of the polarization holographic optical element 50 is applied to the height direction of the first image light and the second image light, but the lens function may also be applied in the height direction. Also, while an example in which the first image light and the second image light are applied to different regions in the height direction of the reflecting mirror 20 is shown, the first image light and the second image light may also be applied to different regions in the width direction, so that the diffraction grating function is applied in the width direction of the polarization holographic optical element 50.

[0056] As described above, in the image projection device 100 of this embodiment, the first image light and the second image light are converted into counter-rotating circularly polarized light and then incident on the same region of the polarizing holographic optical element 50 at different angles using the reflecting mirror 20. Because the polarizing holographic optical element 50 has a diffraction grating function, the optical paths of the transmitted first image light and the second image light can be overlapped. Furthermore, the first image light and the second image light that have passed through the polarizing holographic optical element 50 reach the same region of the projection mirror 30 and are reflected while their optical paths remain overlapped. This allows the reflective surface of the projection mirror 30 to be made smaller, thereby achieving a more compact design, compared to when the first image light and the second image light are reflected from different regions of the projection mirror 30. Furthermore, because the polarizing holographic optical element 50 has a lens function, it is possible to differentiate the focal positions of the first image light and the second image light, thereby creating an optical path difference and differentiating the imaging positions of the virtual images P1 and P2, without using a separate lens or mirror. This makes it possible to project multiple images while suppressing an increase in the number of parts and reducing the size of optical components.

[0057] (Modification of First Embodiment) Next, a modification of the first embodiment of the present invention will be described with reference to FIG. 6. Description of content that overlaps with the first embodiment will be omitted. FIG. 6 is a schematic diagram illustrating the focal lengths of the first image light and the second image light that have passed through the polarization holographic optical element 50 in this modification. FIG. 6 shows the paths of the first image light and the second image light when the reflecting mirror 20, the polarization holographic optical element 50, and the projection mirror 30 are viewed in plan from above. In FIG. 6, the width of the region through which the first image light passes is schematically indicated by a dashed line, and the width of the region through which the second image light passes is schematically indicated by a dashed line.

[0058] This modification differs from the example shown in Fig. 5 in that different positive optical powers are applied to the first image light and the second image light. As shown in Fig. 6, a positive optical power is applied to the first image light, and the light is condensed to form an intermediate image, and then reaches the projection mirror 30 while expanding in diameter, so that the focal position is located between the polarization holographic optical element 50 and the projection mirror 30. A positive optical power is also applied to the second image light, and the light is reduced in diameter as it reaches the projection mirror 30, so that the virtual focal position (not shown) is located between the projection mirror 30 and the windshield WS.

[0059] In this modification, the driver also views the virtual images P1 and P2 on an extension of the direction in which the first image light and the second image light arrive. At this time, due to the lens function of the polarizing holographic optical element 50, the focal positions of the first image light and the second image light are different, and therefore the virtual images P1 and P2 are perceived as being formed at different positions. Specifically, as shown in FIGS. 1 and 2 , the positions at which the virtual images P1 and P2 are formed are such that the first image is farther from the viewpoint than the second image. While different positive optical powers are applied to the first image light and the second image light in FIG. 6 , different negative optical powers may be applied to cause the focal positions of the first image light and the second image light to differ, thereby causing the virtual images P1 and P2 to be formed at different positions.

[0060] Second Embodiment Next, a second embodiment of the present invention will be described with reference to FIG. 7. Description of content that overlaps with the first embodiment will be omitted. FIG. 7 is a schematic diagram illustrating the combining of first image light and second image light by a polarization holographic optical element 60 in an image projection device 100 according to the second embodiment. As shown in FIG. 7, the image projection device 100 includes an image irradiation unit 10, a reflection mirror 20, a projection mirror 30, an external light cut filter 40, and polarization holographic optical elements 50 and 60.

[0061] The polarizing holographic optical element 60 is an optical member having a refractive index distribution that varies depending on the polarization direction. The polarizing holographic optical element 60 has at least a diffraction grating function. Furthermore, the grating period of the polarizing holographic optical element 60 is preferably the same as that of the polarizing holographic optical element 50. The polarizing holographic optical element 60 of this embodiment corresponds to the beam-combining member and second polarizing holographic optical element of the present invention. While FIG. 7 shows an example in which the polarizing holographic optical element 60 is disposed in contact with the first circular polarization conversion unit 13 a and the second circular polarization conversion unit 13 b, the position of the polarizing holographic optical element 60 is not limited as long as it is located on the path of the first image light and the second image light that have passed through the first circular polarization conversion unit 13 a and the second circular polarization conversion unit 13 b.

[0062] In this embodiment, the first image light and the second image light are emitted from different regions of the image display unit 12, namely the far display region 12a and the near display region 12b, and are converted into circularly polarized light with opposite rotations to each other by passing through the first circular polarization converter 13a and the second circular polarization converter 13b, respectively. The first image light and the second image light converted into circularly polarized light are bent in opposite directions by the diffraction grating function of the polarizing holographic optical element 60, as shown in Figures 3(c) and 3(d), and are combined so as to overlap in the same region at the position of the polarizing holographic optical element 50. Here, the first image light and the second image light that have passed through the polarizing holographic optical element 60 have become circularly polarized light with opposite rotations to the direction of rotation before entering the polarizing holographic optical element 60.

[0063] Furthermore, the first and second image lights bent by the polarizing holographic optical element 60 enter the same region of the polarizing holographic optical element 50 at different angles of incidence. As shown in Figures 3(c) and 3(d), the bending direction is reversed depending on the direction of rotation of the circularly polarized light, so the first and second image lights incident on the polarizing holographic optical element 50 are bent in opposite directions from the polarizing holographic optical element 60. When the grating period of the polarizing holographic optical element 60 is the same, the diffraction angle of the same wavelength is the same, so the first and second image lights combined by the polarizing holographic optical element 60 travel in the same direction. The first and second image lights combined by the polarizing holographic optical element 50 are reflected by the projection mirror 30 and travel toward the windshield WS. The first and second image lights that reach the windshield WS are reflected toward the driver's viewpoint, and the driver views virtual images P1 and P2 on extensions of the directions in which the first and second image lights arrived.

[0064] In this embodiment, the polarizing holographic optical element 60 is used as the beam-combining member, which allows the beam-combining member to be made smaller and lighter. Furthermore, by making the grating periods of the polarizing holographic optical elements 50 and 60 the same and thus making the diffraction angles the same, it becomes easy to overlap the paths of the first image light and the second image light that have passed through the polarizing holographic optical element 50.

[0065] Furthermore, when the first image light and the second image light contain light of multiple wavelengths, the diffraction angles at the polarizing holographic optical elements 50, 60 differ for each wavelength. However, the circular polarization of the first image light and the second image light rotates in opposite directions when they pass through the polarizing holographic optical element 60, so the diffraction direction at the polarizing holographic optical element 50 is opposite to that of the polarizing holographic optical element 60. As a result, even if the diffraction angles differ for each wavelength, the difference in diffraction angle due to wavelength is canceled out by bending in opposite directions at the polarizing holographic optical elements 50, 60, and color shift can be corrected.

[0066] In the image projection device 100 of this embodiment, the reflection surface of the projection mirror 30 can be made smaller and the device can be made more compact than when the first image light and the second image light are reflected in different areas of the projection mirror 30. Furthermore, it is possible to project multiple images while suppressing an increase in the number of parts and making the optical members smaller.

[0067] Third Embodiment Next, a third embodiment of the present invention will be described with reference to FIG. 8. Description of content that overlaps with the first embodiment will be omitted. FIG. 8 is a schematic diagram illustrating the multiplexing of first image light and second image light by prisms 70a and 70b in an image projection device 100 according to the third embodiment. As shown in FIG. 8, the image projection device 100 includes an image irradiation unit 10, a reflecting mirror 20, a projection mirror 30, an external light cut filter 40, a polarization holographic optical element 50, and prisms 70a and 70b.

[0068] The prisms 70a and 70b are optical components made of a material that transmits light and has a refractive index greater than that of air, and which change the angle of incidence and the angle of emergence of light due to the difference in refractive index with air. As shown in FIG. 8 , the prisms 70a and 70b are disposed opposite the first circular polarization conversion unit 13a and the second circular polarization conversion unit 13b, respectively. The prisms 70a and 70b are disposed so that their light refracting directions are opposite to each other, and the first image light refracted by the prism 70a and the second image light refracted by the prism 70b are combined so as to overlap in the same region of the polarization holographic optical element 50. Therefore, the prisms 70a and 70b in this embodiment correspond to the combining member of the present invention.

[0069] In this embodiment, the first image light and the second image light are emitted from different regions of the image display unit 12, namely the far display region 12a and the near display region 12b, and are converted into circularly polarized light with opposite rotations to each other by passing through the first circular polarization converter 13a and the second circular polarization converter 13b, respectively. The circularly polarized first image light and the second image light are bent in opposite directions by the prisms 70a and 70b, respectively, and combined so as to overlap in the same region at the position of the polarizing holographic optical element 50. At this time, the first image light and the second image light are incident on the polarizing holographic optical element 50 at different incident angles.

[0070] The polarizing holographic optical element 50 has a diffraction grating function, and the first and second image lights combined at the position of the polarizing holographic optical element 50 reach the projection mirror 30 along the same path, are reflected, and travel toward the windshield WS. The first and second image lights that reach the windshield WS are reflected toward the driver's viewpoint, and the driver visually recognizes virtual images P1 and P2 on an extension of the direction in which the first and second image lights arrived.

[0071] In the image projection device 100 of this embodiment, the reflection surface of the projection mirror 30 can be made smaller and the device can be made more compact than when the first image light and the second image light are reflected in different areas of the projection mirror 30. Furthermore, it is possible to project multiple images while suppressing an increase in the number of parts and making the optical members smaller.

[0072] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0073] This international application claims priority based on Japanese Patent Application No. 2023-185113, filed on October 27, 2023, the entire contents of which are incorporated herein by reference.

[0074] The above descriptions of specific embodiments of the present invention have been presented for purposes of illustration. They are not intended to be exhaustive or to limit the invention to the precise forms described. Numerous modifications and variations will be apparent to those skilled in the art in light of the above description.

[0075] REFERENCE SIGNS LIST 100... image projection device 10... image irradiation section 20... reflection mirror 30... projection mirror 40... external light cut filter 50, 60... polarization holographic optical element 70a, 70b... prism 11... backlight 12... image display section 12a... far display area 12b... near display area 13a... first circular polarization conversion section 13b... second circular polarization conversion section

Claims

1. An image projection device which projects a projection image onto a display unit for displaying a virtual image, comprising: an image projection unit which irradiates a first image light which is circularly polarized in a first rotation direction and a second image light which is circularly polarized in the opposite rotation direction to the first rotation direction; a first polarizing holographic optical element which has a refractive index distribution which varies depending on the polarization direction; a projection optical unit which receives the first image light and the second image light via the first polarizing holographic optical element and projects the first image light and the second image light onto the display unit; and a combining member which combines the first image light and the second image light at the position of the first polarizing holographic optical element.

2. An image projection device according to claim 1, wherein the first polarizing holographic optical element has a lens function and a diffraction grating function.

3. An image projection device according to claim 1, characterized in that the combining member is a reflecting mirror that reflects the first image light and the second image light irradiated from the image irradiation unit.

4. An image projection device as described in claim 3, characterized in that the reflecting mirror intermediately images the first image light and the second image light at an intermediate imaging position, and the first polarizing holographic optical element is disposed at the intermediate imaging position.

5. An image projection device according to claim 1, characterized in that the combining member is a second polarizing holographic optical element having a refractive index distribution that varies depending on the polarization direction.

6. An image projection device according to claim 5, characterized in that the second polarizing holographic optical element has the same grating period as the first polarizing holographic optical element.

7. An image projection device according to claim 1, characterized in that the beam combining member is a prism.

8. An image projection device according to claim 1, characterized in that the polarizing holographic optical element is a transmissive liquid crystal holographic element.

9. An image projection device as defined in any one of claims 1 to 8, wherein the image irradiation unit has an image display unit that displays a first image and a second image and emits polarized light in a predetermined direction, the image display unit having a first area that displays the first image and a second area that displays the second image, a first circular polarization conversion unit arranged opposite the first area, and a second circular polarization conversion unit arranged opposite the second area.

10. An image projection device as described in claim 9, wherein the image display unit is a liquid crystal display device, and the first circular polarization conversion unit and the second circular polarization conversion unit are each a quarter-wave plate or a three-quarter-wave plate, and are arranged so that their fast axis and slow axis intersect with the specified direction.

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