Image projection apparatus

Abstract

To provide an image projection device capable of enhancing visibility of a virtual image by suppressing stray light due to external light from reaching a viewpoint position.SOLUTION: An image projection device that projects a projection image onto a display unit for displaying a virtual image includes an image irradiation unit (10) that irradiates image light, a projection optical unit (20,30) that forms an image of the image light at a first distance from a viewpoint position via the display unit, and a cover (40) that is disposed between the projection optical unit (20,30) and the display unit and transmits the image light, in which the cover (40) is formed in a curved surface shape and is disposed such that a front side thereof is lower than a rear side thereof.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an image projection device. [Background technology]

[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, because the instrument panel is located below the vehicle's windshield, passengers such as the driver must undesirably move their eyes downward while driving in order to view the information displayed on the instrument panel. 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 the image projection devices disclosed in Patent Documents 1 and 2, an image projection unit emits light containing an image, which is reflected by a free-form mirror or the like, and 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 emit multiple beams of image light to form multiple virtual images at different distances from the windshield. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2019-119248 [Patent Document 2] Japanese Patent Application Publication No. 2019-119262 Summary of the Invention [Problem to be solved by the invention]

[0006] In conventional image projection devices, image light is projected upward from a projection optical unit such as a free-form mirror, and a light-transmitting cover is placed on the optical path of the image light to prevent dust and other particles from inside the vehicle cabin from entering the device. However, external light such as sunlight reaches the cover from above the windshield, and some of the external light is reflected by the cover, creating stray light that reaches the viewpoint (eye box), potentially reducing the visibility of the virtual image at the viewpoint.

[0007] Therefore, the present invention has been developed in consideration of the above-mentioned conventional problems, and aims to provide an image projection device that can prevent stray light from external light from reaching the viewpoint position and improve the visibility of virtual images. [Means for solving the problem]

[0008] 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 equipped with an image irradiation unit that irradiates image light, a projection optical unit that forms an image of the image light at a first distance from a viewpoint position via the display unit, and a cover unit that is arranged between the projection optical unit and the display unit and transmits the image light, and is characterized in that the cover unit is configured in a curved shape and is arranged so that the front side is lower than the rear side.

[0009] In the image projection device of the present invention, the cover portion is configured in a curved shape and the front side is positioned lower than the rear side, so even if some of the external light that reaches the cover portion becomes stray light, the stray light is reflected toward the front of the vehicle, thereby preventing stray light from external light from reaching the viewpoint and improving the visibility of the virtual image.

[0010] In one aspect of the present invention, the cover portion has a small inclination angle from the horizontal direction on the front side and a large inclination angle on the rear side.

[0011] In one aspect of the present invention, the cover portion has a lowest point midway between the front end and the rear end.

[0012] In one aspect of the present invention, the lowest point is located forward of the middle of the cover portion in the front-rear direction.

[0013] In one aspect of the present invention, the cover portion functions as a quarter-wave plate.

[0014] In one aspect of the present invention, the projection optical unit includes a first mirror that reflects the image light irradiated from the image irradiation unit, and a polarizing plate arranged between the image irradiation unit and the first mirror, and includes another quarter-wave plate between the polarizing plate and the first mirror.

[0015] In one aspect of the present invention, the projection optical unit includes a second mirror disposed below the cover unit.

[0016] 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 in that it comprises an image irradiation unit that irradiates image light, and a projection optical unit that forms an image of the image light at a first distance from a viewpoint position via the display unit, and the projection optical unit comprises a first mirror that reflects the image light irradiated from the image irradiation unit, a polarizing plate arranged between the image irradiation unit and the first mirror, and a quarter-wave plate arranged between the polarizing plate and the first mirror. [Effects of the Invention]

[0017] The present invention can provide an image projection device that can prevent stray light from external light from reaching the viewpoint position and improve the visibility of a virtual image. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a schematic diagram showing projection of a virtual image P using the image projection device 100 according to the first embodiment. [Figure 2] FIG. 1 is a schematic cross-sectional view illustrating an overview of an image projection device 100 according to a first embodiment. [Figure 3] 3A and 3B are schematic diagrams illustrating the curved surface and inclination of the cover portion 40. FIG. [Figure 4] FIG. 10 is a schematic cross-sectional view illustrating an overview of an image projection device 100 according to a second embodiment. [Figure 5] FIG. 10 is a schematic cross-sectional view illustrating an overview of an image projection device 100 according to a third embodiment. [Figure 6] FIG. 10 is a schematic cross-sectional view illustrating an overview of an image projection device 100 according to a fourth embodiment. [Figure 7] 10A and 10B are schematic diagrams illustrating the curved surface and inclination of the cover portion 40 according to the fifth embodiment.

[0019] (First embodiment) Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. The same or equivalent components, parts, and processes shown in each drawing will be given the same reference numerals, and redundant explanations will be omitted as appropriate. In the following explanation, an image projection device 100 according to the present invention will be described by way of example in which it is applied to a HUD mounted on a vehicle or the like.

[0020] FIG. 1 is a schematic diagram showing the projection of a virtual image P using an image projection device 100 according to this embodiment. As shown in FIG. 1, image light projected from the image projection device 100 is irradiated toward a windshield (display unit) WS through an opening provided in a dashboard DB, reflected by the windshield WS, and irradiated at a driver's viewpoint E. The driver visually recognizes a virtual image P formed on an extension of the optical path of the incident image light. The solid and dashed lines shown in FIG. 1 schematically represent the luminous flux of the image light irradiated from the image projection device 100, the optical path of the image light reflected by the windshield WS and reaching the viewpoint E, and its extension. The actual image light is displayed in a predetermined area by the image projection unit 10, and has a predetermined area in a direction perpendicular to the traveling direction.

[0021] 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 image light L incident from the image projection device 100 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 second 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.

[0022] The virtual image P is an image that appears as if it were formed in space when the image light L reflected by the windshield WS reaches the viewpoint E (eyebox) of the driver or the like. The position at which the virtual image P is formed is determined by the spread angle of the image light L irradiated from the image projection device 100 as it travels toward the viewpoint after being reflected by the projection optical unit (not shown in FIG. 1) and the windshield WS. The display content of the image projected as the virtual image P may include auxiliary information related to driving such as a warning image or emergency information, a volume indicator, a driving direction guide, etc.

[0023] FIG. 2 is a schematic cross-sectional view illustrating an image projection device 100 according to this embodiment. As shown in FIG. 2, the image projection device 100 includes an image projection unit 10, a first mirror 20, a second mirror 30, and a cover unit 40. Although not shown in FIG. 2, the image projection device 100 also includes a housing unit, in which the image projection unit 10, the first mirror 20, and the second mirror 30 are housed. An opening is provided at the top of the housing unit, and the cover unit 40 is provided to cover the opening and seal the interior. The shape and material of the housing unit are not limited, and light-blocking resin or metal materials can be used. A vertical wall 50 is provided around the periphery of the opening in the dashboard DB.

[0024] 2, each part is controlled by a control unit connected to the other parts so as to be able to communicate information with them. The configuration of the control unit is not limited, but an example includes a CPU (Central Processing Unit) for processing information, a memory device, a recording medium, an information communication device, etc. The control unit controls the operation of each part in accordance with a predetermined program, and sends information including an image (image information) to the image projection unit 10.

[0025] The image projection unit 10 is a part that projects image light L containing an image based on image information from the control unit. The specific configuration of the image projection unit 10 is not limited, and a conventionally known device such as a liquid crystal display device or an organic EL display device can be used. As an example, a device that projects light from a light emitting diode (LED) from the rear side of a liquid crystal display device is used.

[0026] The first mirror 20 is an optical member that receives the image light emitted from the image projection unit 10 and reflects the image light in the direction of the second mirror 30. In the example shown in Fig. 2, a flat mirror is shown as the first mirror 20. Here, an example of a flat mirror is shown as the first mirror 20, but a concave or convex reflecting mirror or a free-form surface mirror may also be used.

[0027] The second mirror 30 is an optical member that receives the image light L reflected by the first mirror 20 and reflects the image light L in the direction of the windshield WS. In the example shown in FIG. 2, the second mirror 30 is shown as a free-form surface mirror that has an optical design necessary for projecting the image light L as a virtual image P. In the example shown in FIG. 2, the second mirror 30 is disposed below the cover unit 40. The second mirror 30 may be configured so that its inclination angle with respect to the horizontal direction can be changed, and the projection direction of the image light L can be changed to move the projection position of the virtual image P in the vertical direction.

[0028] The reflective surfaces of the first mirror 20 and the second mirror 30 are designed to expand the light diameter in the driver's line of sight in order to project the image light L as a virtual image P through the windshield WS. Here, "expanding the light diameter in the line of sight" 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. The combination of the first mirror 20 and the second mirror 30 has the function of projecting the image light L through the windshield WS and corresponds to the projection optical unit in this invention.

[0029] The cover 40 is made of a material that transmits the image light L, and is a member disposed to cover the opening of the housing. Although not shown in Fig. 2, the cover 40 is fixed to the housing in a structure that leaves no gap between them, preventing dust and dirt from entering the housing. There are no limitations on the material that can be used to make the cover 40, and known resin materials or glass that transmit the image light L can be used.

[0030] The standing wall 50 is a wall-like portion provided on the periphery of the opening of the dashboard DB, and stands upright from the opening downward. In the example shown in Fig. 2, the image light L travels diagonally upward toward the rear of the vehicle, so the standing wall 50 is inclined diagonally downward toward the front of the vehicle in the traveling direction of the image light L. Here, an example is shown in which the standing wall 50 is provided on the dashboard DB, but the standing wall 50 may also be provided upright from a housing portion of the image projection device 100. The material constituting the standing wall 50 is not limited, but it is preferably a dark color that blocks and absorbs the image light L.

[0031] FIG. 3 is a schematic diagram illustrating the curved surface and inclination of the cover portion 40. The left-right direction in the figure indicates the front-rear direction of the vehicle, the up-down direction indicates the up-down direction of the vehicle, and the direction perpendicular to the paper surface indicates the left-right direction of the vehicle. Among the dashed lines shown in the figure, the left-right direction indicates the horizontal direction of the vehicle, and the up-down direction indicates the vertical direction of the vehicle. The diagonal dashed line tangent to the cover portion 40 indicates a tangent at the rear end portion 41a. The inclination angles of the cover portion 40 relative to the horizontal direction are θ1, θ2, and θ3 at the rear end portion 41a, the middle portion 41b, and the front end portion 41d, respectively.

[0032] As shown in Figure 3, the cover portion 40 has a downwardly convex curved surface in a cross section along the longitudinal direction of the vehicle. The cover portion 40 has a rear end portion 41a, an intermediate portion 41b, a lowest point 41c, and a front end portion 41d. The rear end portion 41a and the front end portion 41d are the ends of the cover portion 40 at the rear and front of the vehicle, respectively. The intermediate portion 41b is midway between the rear end portion 41a and the front end portion 41d in the longitudinal direction of the vehicle. The lowest point 41c is the lowest position of the cover portion 40 in the vertical direction of the vehicle.

[0033] 3, the front side of the cover part 40 is positioned lower than the rear side, and the inclination angle θ1 at the rear end side from the horizontal is larger than θ2 and θ3 at the front end side. The cover part 40 has a lowest point 41c midway between the front end part 41d and the rear end part 41a, and the front end part 41d is located higher than the lowest point 41c. The lowest point 41c is located forward of the intermediate part 41b of the cover part 40 in the front-to-rear direction.

[0034] As shown in FIGS. 2 and 3 , the cover unit 40 is disposed at an angle such that the front side of the vehicle farther from the first mirror 20 is lower than the rear side of the vehicle closer to the first mirror 20. As a result, even if external light L0 reaches the cover unit 40 from the windshield WS located above the image projection device 100 and part of the external light L0 is reflected by the cover unit 40 and becomes stray light, as indicated by the dashed line in FIG. 2 , the stray light is reflected forward of the vehicle, preventing it from reaching the viewpoint E. Furthermore, by allowing the stray light reflected by the cover unit 40 to reach the standing wall 50, the stray light is absorbed by the standing wall 50, further reducing the stray light reaching the viewpoint E. Furthermore, since the standing wall 50 is inclined obliquely downward toward the front of the vehicle, even if the stray light is partially reflected by the standing wall 50, the stray light traveling toward the viewpoint E can be further reduced.

[0035] 2 and 3, the cover portion 40 is curved downward in the left-right direction in the figures, and the angle of inclination from the horizontal direction is greater and steeper at the rear end portion 41a (rear side) than at the front end portion 41d (front side). The rear end portion 41a of the cover portion 40 is far from the standing wall 50, but is positioned higher than the front end portion 41d and forms a steeper slope, so that stray light reflected on the rear end portion 41a side can easily reach the standing wall 50, and stray light reaching the viewpoint position E can be effectively suppressed.

[0036] As described above, in the image projection device 100 of this embodiment, the cover portion 40 is configured in a curved shape and is positioned lower at the front than the rear. Therefore, even if some of the external light L0 that reaches the cover portion 40 becomes stray light, the stray light is reflected toward the front of the vehicle, thereby preventing the stray light from the external light L0 from reaching the viewpoint position E and making it possible to improve the visibility of the virtual image P.

[0037] (Second embodiment) Next, a second embodiment of the present invention will be described with reference to FIG. 4. Description of content that overlaps with the first embodiment will be omitted. FIG. 4 is a schematic cross-sectional view illustrating an overview of an image projection device 100 according to this embodiment. As shown in FIG. 4, the image projection device 100 of this embodiment also includes an image projection unit 10, a first mirror 20, a second mirror 30, and a cover unit 40. A standing wall 50 is provided around the periphery of the opening of the dashboard DB.

[0038] This embodiment differs from the first embodiment in that the second mirror 30 is located below the cover portion 40 near the rear of the opening in the dashboard DB. Furthermore, the image light L reflected by the second mirror 30 travels obliquely upward toward the front of the vehicle. The standing wall 50 is inclined obliquely downward toward the rear of the vehicle along the traveling direction of the image light L. Furthermore, the cover portion 40 has a downwardly convex curved surface in a cross section along the longitudinal direction of the vehicle, with the front side being lower than the rear side. Furthermore, the cover portion 40 has a larger inclination angle from the horizontal at the rear side than at the front side, resulting in a steeper inclination.

[0039] In this embodiment, the rear end 41a of the cover unit 40 is also far from the standing wall 50, but is positioned higher than the front end 41d and has a steep slope. This makes it easier for stray light reflected at the rear end 41a to reach the standing wall 50, effectively preventing stray light from reaching the viewpoint E. Furthermore, as shown by the dashed line in FIG. 4 , even if external light L0 reaches the standing wall 50 and is reflected by the standing wall 50, causing stray light, the stray light is more likely to reach the front side of the cover unit 40 because the standing wall 50 is inclined diagonally downward toward the rear. Therefore, it is possible to prevent stray light re-reflected at the gently sloping front side of the cover unit 40 from traveling toward the windshield WS or the viewpoint E.

[0040] As described above, in the image projection device 100 of this embodiment, the cover portion 40 is configured in a curved shape and is positioned lower at the front than the rear, so even if some of the external light L0 that reaches the cover portion 40 becomes stray light, the stray light is reflected toward the front of the vehicle, thereby preventing the stray light from the external light L0 from reaching the viewpoint position E and improving the visibility of the virtual image P.

[0041] (Third embodiment) Next, a third embodiment of the present invention will be described with reference to FIG. 5. Description of content that overlaps with the first embodiment will be omitted. FIG. 5 is a schematic cross-sectional view illustrating an overview of an image projection device 100 according to this embodiment. As shown in FIG. 5, the image projection device 100 of this embodiment includes an image projection unit 10, a first mirror 20, a second mirror 30, a cover unit 40, a polarizing plate 60, and a quarter-wave plate 70. In FIG. 5, the dashboard DB and the standing wall 50 are not shown. This embodiment differs from the first embodiment in that the polarizing plate 60 and the quarter-wave plate 70 are arranged on the optical path of the image light L between the image projection unit 10 and the first mirror 20.

[0042] The polarizing plate 60 is an optical element having the optical property of transmitting polarized light in the direction of its transmission axis and reflecting or absorbing polarized light perpendicular to the direction of the transmission axis, and a known polarizing plate or polarizing film can be used. It is preferable that the transmittance of the transmission axis is uniform within the plane of the polarizing plate 60. It is also preferable that the transmission axis of the polarizing plate 60 is arranged so as to transmit polarized light of the image light L irradiated from the image irradiation unit 10. For example, when a liquid crystal display device is used as the image display unit of the image irradiation unit 10, the polarization direction of the image light L irradiated by the liquid crystal display device is set to be the same as the direction of the transmission axis of the polarizing plate 60.

[0043] 5 shows an example in which the polarizing plate 60 is made of a single plate-like member, but it may also have a structure in which a polarizing sheet is attached to a light-transmitting substrate such as a resin. Also, a sheet that performs other optical functions may be attached to the polarizing plate 60 to achieve multiple functions, such as attaching an infrared light cut filter or an ultraviolet light cut filter.

[0044] The quarter-wave plate 70 is an optical element made of a birefringent material with different refractive indices in the slow axis and the fast axis. The thickness of the quarter-wave plate 70 is designed so that a phase difference of one-quarter of the wavelength of light occurs between the slow axis and the fast axis before light incident on one surface reaches the opposite surface. The slow axis and the fast axis of the quarter-wave plate 70 are oriented 45 degrees apart from the polarization direction (transmission axis direction) of the image light L transmitted through the polarizing plate 60. While the quarter-wave plate 70 shown here generates a phase difference of one-quarter of the wavelength between the slow axis and the fast axis, the phase difference does not necessarily have to be exactly one-quarter wavelength. Furthermore, a three-quarter-wave plate or the like that generates a phase difference of three-quarters of the wavelength between the slow axis and the fast axis may be used as long as it can convert linearly polarized light into circularly polarized light or elliptically polarized light.

[0045] 5, the polarizing plate 60 and the quarter-wave plate 70 are preferably arranged so as to be inclined at a predetermined angle with respect to the optical path of the image light L. By inclining them with respect to the optical path of the image light L, external light L0 incident from above the cover unit 40 is reflected by the second mirror 30 and the first mirror 20 and reaches the polarizing plate 60 or the quarter-wave plate 70, and even if the external light L0 is reflected again on the surface, it can be prevented from reaching the viewpoint position E via the same optical path as the image light L.

[0046] As shown in FIG. 5, image light L emitted from the image projection unit 10 passes through the polarizing plate 60 and the quarter-wave plate 70, is reflected by the first mirror 20 and the second mirror 30, and is projected toward the windshield WS via the cover unit 40. At this time, the image light L passing through the polarizing plate 60 is linearly polarized light along the transmission axis direction of the polarizing plate 60. Furthermore, the polarization direction of the image light L that reaches the quarter-wave plate 70 is tilted 45 degrees with respect to the slow axis and fast axis of the quarter-wave plate 70, so that the image light L is converted into circularly polarized light (or elliptically polarized light) by passing through the quarter-wave plate 70. As a result, the image light L projected from the image projection device 100 to the viewpoint E via the windshield WS becomes circularly polarized light. When the image light L reaches the viewpoint E as circularly polarized light, polarized sunglasses transmit circularly polarized light, so the virtual image P can be clearly viewed even when the driver is wearing polarized sunglasses.

[0047] A portion of the external light L0 that arrives from above the windshield WS is reflected by the second mirror 30 and reaches the first mirror 20, and is then reflected by the first mirror 20 and reaches the image irradiation unit 10 via the quarter-wave plate 70 and the polarizing plate 60. At this time, although the external light L0 is unpolarized, the polarization direction that passes through the transmission axis of the polarizing plate 60 is limited, and the polarized component perpendicular to the transmission axis is cut. Therefore, the amount of external light L0 that reaches the image irradiation unit 10 is halved, and the temperature rise of the image irradiation unit 10 can be suppressed.

[0048] Furthermore, when stray light of the image light L partially reflected by the first mirror 20, the second mirror 30, the cover unit 40, etc. travels back along the optical path and reaches the quarter-wave plate 70 again, the stray light of the image light L that passes through the quarter-wave plate 70 again becomes linearly polarized light again. At this time, the polarization direction of the image light L becomes perpendicular to the transmission axis of the polarizing plate 60 because it passes through the quarter-wave plate 70 twice, creating a phase difference of half a wavelength between the fast axis and the slow axis. Therefore, the stray light of the image light L does not pass through the polarizing plate 60 and does not reach the image projection unit 10. This makes it possible to suppress the possibility of temperature rise and deterioration of the liquid crystal display device or the like in the image projection unit 10.

[0049] As described above, in the image projection device 100 of this embodiment, the polarizing plate 60 and the quarter-wave plate 70 are disposed between the image projection unit 10 and the first mirror 20, thereby converting the image light L into circularly polarized light, thereby improving the visibility of the virtual image P even when the driver is wearing polarized sunglasses. Furthermore, even when stray light of the image light L travels backward, it is converted back into linearly polarized light by the quarter-wave plate 70 and blocked by the polarizing plate 60, thereby suppressing a temperature rise in the image projection unit 10.

[0050] Furthermore, in the image projection device 100 of this embodiment, the cover portion 40 is configured in a curved shape and positioned lower at the front than at the rear, so even if some of the external light L0 that reaches the cover portion 40 becomes stray light, the stray light is reflected toward the front of the vehicle, thereby preventing the stray light from the external light L0 from reaching the viewpoint position E and making it possible to improve the visibility of the virtual image P.

[0051] (Fourth embodiment) Next, a fourth 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 cross-sectional view illustrating an overview of an image projection device 100 according to this embodiment. As shown in FIG. 6, the image projection device 100 of this embodiment includes an image irradiation unit 10, a first mirror 20, a second mirror 30, a polarizing plate 60, a quarter-wave plate 70, and a cover unit 80. In FIG. 6, the dashboard DB and the standing wall 50 are not shown. This embodiment differs from the third embodiment in that the cover unit 80 also has the function of a quarter-wave plate.

[0052] The cover portion 80 is made of a birefringent material with different refractive indices in the slow axis and the fast axis. The thickness of the cover portion 80 is designed so that light incident on one surface experiences a phase difference between the slow axis and the fast axis of one-quarter of the wavelength of the light before reaching the opposing surface. The cover portion 80 is curved downward in the left-right direction in the figure, with the rear side having a larger inclination angle from the horizontal than the front side, resulting in a steeper inclination. The front side of the cover portion 80 is positioned lower than the rear side, and the inclination angle from the horizontal is larger on the rear side than on the front side.

[0053] As shown in FIG. 6, image light L emitted from the image projection unit 10 passes through the polarizing plate 60 and the quarter-wave plate 70, is reflected by the first mirror 20 and the second mirror 30, and is projected toward the windshield WS via the cover unit 80. At this time, the image light L passing through the polarizing plate 60 is linearly polarized light along the transmission axis direction of the polarizing plate 60. Furthermore, the polarization direction of the image light L that reaches the quarter-wave plate 70 is tilted 45 degrees with respect to the slow axis and fast axis of the quarter-wave plate 70, so that the image light L is converted into circularly polarized light (or elliptically polarized light) by passing through the quarter-wave plate 70. Furthermore, the image light L that passes through the cover unit 80 is further shifted in phase by a quarter wavelength between the slow axis and the fast axis, and is converted back into linearly polarized light by passing through the cover unit 80. As a result, the image light L projected from the image projection device 100 to the viewpoint position E via the windshield WS becomes linearly polarized light.

[0054] Here, if the polarization direction of the image light L that has passed through the cover portion 80 is P-polarized relative to the windshield WS, even if the driver is wearing polarized sunglasses, the image light L will pass through the polarized sunglasses and the virtual image P will be clearly visible. Also, if the polarization direction of the image light L that has passed through the cover portion 80 is S-polarized relative to the windshield WS, the reflectance at the windshield WS will be increased and the brightness of the virtual image P can be improved.

[0055] In the image projection device 100 of this embodiment, the polarizing plate 60 and the quarter-wave plate 70 are also disposed between the image projection unit 10 and the first mirror 20, so that even if stray light of the image light L travels backward, it is converted back into linearly polarized light by the quarter-wave plate 70 and blocked by the polarizing plate 60, thereby suppressing a rise in temperature in the image projection unit 10. Furthermore, because the cover unit 80 functions as a quarter-wave plate, the image light L projected from the image projection device 100 is converted into linearly polarized light, making it possible to appropriately select improved visibility when wearing polarized sunglasses and improved reflectance on the windshield WS.

[0056] (Fifth embodiment) Next, a fifth embodiment of the present invention will be described with reference to Fig. 7. Descriptions of content that overlap with the first embodiment will be omitted. Fig. 7 is a schematic diagram illustrating the curved surface and inclination of the cover portion 40 according to this embodiment. This embodiment differs from the first embodiment in that the front end of the cover portion 40 is the lowest point.

[0057] 7, the cover portion 40 of this embodiment has its rear end positioned at the highest point and its front end positioned at the lowest point, with the lowest point 41c coinciding with the front end 41d. The inclination angle of the cover portion 40 is θ1 at the rear end and θ2 at the front end, with θ1 being greater than θ2.

[0058] In the image projection device 100 of this embodiment, the cover portion 40 is also configured in a curved shape, and the front side is positioned lower than the rear side, so even if some of the external light L0 that reaches the cover portion 40 becomes stray light, the stray light is reflected toward the front of the vehicle, and the stray light from the external light L0 reaches the viewpoint position E.

[0059] (Sixth embodiment) Next, a sixth embodiment of the present invention will be described. Details that overlap with the first embodiment will not be described. In the first embodiment, an example was shown in which the first mirror 20 was flat, but the reflective surface of the first mirror 20 may be concave. In this case, the image light L reflected by the first mirror 20 travels while its light diameter is reduced, and at an intermediate imaging position F (not shown) before reaching the second mirror 30, the vertical (height) component in the drawing is intermediately imaged. Because the image light L reflected by the first mirror 20 is intermediately imaged between the first mirror 20 and the second mirror 30, the cross-sectional area through which the image light L passes is minimized at the intermediate imaging position F between the first mirror 20 and the second mirror 30. The image light L intermediately imaged at the intermediate imaging position F reaches the second mirror 30 while its light diameter is expanded, and after being reflected by the second mirror 30, it passes through the cover unit 40 and is irradiated to the outside of the image projection device 100.

[0060] The rear end 41a of the cover portion 40 is located higher than the front end 41d. As described above, the image light L is intermediately imaged at the intermediate image position F between the first mirror 20 and the second mirror 30, and therefore the cross-sectional area through which the image light L passes is minimized near the intermediate image position F. Therefore, even if the rear end 41a of the cover portion 40 is close to the first mirror 20, the image light L is not blocked by the cover portion 40 or the housing portion, which improves design freedom and enables miniaturization.

[0061] 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. [Explanation of symbols]

[0062] 100...Image projection device 10...Image irradiation unit 20...1st mirror 30...Second mirror 40...Cover part 41a...Rear end 41b…middle part 41c…Lowest point 41d...front end 50...Standing wall 60...Polarizing plate 70...quarter-wave plate 80...Cover part

Claims

1. An image projection device that projects a projection image onto a display unit for displaying a virtual image, an image irradiating unit that irradiates image light; a projection optical unit that forms an image of the image light at a first distance from a viewpoint position via the display unit; a cover portion disposed between the projection optical portion and the display portion and transmitting the image light; The image projection device is characterized in that the cover portion has a curved shape and is positioned so that the front side is lower than the rear side.

2. 2. The image projection device according to claim 1, The image projection device, wherein the cover portion has a small inclination angle from the horizontal direction at the front side and a large inclination angle from the horizontal direction at the rear side.

3. 2. The image projection device according to claim 1, The image projection device is characterized in that the cover portion has a lowest point midway between the front end and the rear end.

4. 4. The image projection device according to claim 3, The image projection device according to claim 1, wherein the lowest point is located forward of the center of the cover portion in the front-to-rear direction.

5. 2. The image projection device according to claim 1, The image projection device, wherein the cover portion functions as a quarter-wave plate.

6. 6. The image projection device according to claim 5, the projection optical unit includes a first mirror that reflects the image light emitted from the image irradiation unit; a polarizing plate disposed between the image irradiation unit and the first mirror, an image projection device comprising: another quarter-wave plate between said polarizing plate and said first mirror;

7. 7. The image projection device according to claim 1, The image projection device, wherein the projection optical unit includes a second mirror disposed below the cover unit.

8. An image projection device that projects a projection image onto a display unit for displaying a virtual image, an image irradiating unit that irradiates image light; a projection optical unit that forms an image of the image light at a first distance from a viewpoint position via the display unit, the projection optical unit includes a first mirror that reflects the image light emitted from the image irradiation unit; a polarizing plate disposed between the image irradiation unit and the first mirror; an image projection device comprising a quarter-wave plate disposed between the polarizing plate and the first mirror;

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