JP7837760B2 - Image projection device - Google Patents

Description

This invention relates to an image projection device, and more particularly to an image projection device that reflects light emitted from an image irradiation unit to a viewpoint.

Traditionally, instrument panels using illuminated icons have been used to display various information within vehicles. Furthermore, with the increasing amount of information to be displayed, proposals have been made to embed image display devices within the instrument panel, or even to construct the entire instrument panel using image display devices.

However, because the instrument panel is located below the vehicle's windshield, the driver needs to shift their gaze downwards while driving to view the information displayed on the instrument panel, which is undesirable. Therefore, image projection devices such as head-up displays (HUDs) have been proposed, which project images onto the windshield, allowing the driver to read the information while looking ahead.

Figure 7 is a schematic diagram showing the configuration of a conventional image projection device. As shown in Figure 7, a conventional image projection device comprises an image projection unit 1 and free-form mirrors 2 and 3. In such an image projection device, the image projection unit 1 emits illumination light L1 containing an image, and the free-form mirrors 2 and 3 reflect the illumination light L1, causing it to reach the viewpoint 5 of the driver or other user so that an image is formed in space via the windshield 4. As a result, the driver or other user can perceive that an image is displayed at the image formation position in the depth direction due to the illumination light L1 incident on their viewpoint 5.

However, in the image projection device shown in Figure 7, when sunlight or other external light (LO) is incident from the outside, the LO is focused onto the surface of the image irradiation unit 1 by the free-form mirrors 2 and 3, potentially causing the temperature of the image irradiation unit 1 to rise and degrade. Therefore, a device has been proposed that uses multiple free-form mirrors 2 and 3 to inter-image the irradiation light L1, and places a shielding section near the inter-image position to reduce the influence of external light (LO) reaching the image irradiation unit from the outside. (See, for example, Patent Document 1)

International Publication No. 2017/195740

However, in the structure using the light-shielding section described in Patent Document 1, it is unavoidable that ambient light reaches the image illumination section from the space used to secure the optical path of the illumination light, and there were limitations to limiting the incidence of ambient light.

Furthermore, in the reflection of light by the windshield 4, the reflectivity of the P-polarized component tends to be low, while the reflectivity of the S-polarized component tends to be high. Therefore, the polarization direction of the illumination light L1 emitted from the image illumination unit 1 is set to be S-polarized relative to the windshield 4. As a result, the illumination light L1 reaching the viewpoint 5 becomes S-polarized light only, without any P-polarization.

However, in environments with strong ambient light (LO) or when driving on snowy roads, passengers may wear polarized sunglasses to view the outside through the windshield 4. In this case, the light reflected by objects outside the vehicle is also S-polarized, and polarized sunglasses are designed to cut S-polarized light and transmit P-polarized light. Therefore, for passengers wearing polarized sunglasses, the S-polarized light of the reflected light L1 from the windshield 4 is cut off by the sunglasses, making it difficult to see the image formed by the light L1.

Therefore, the present invention has been made in view of the above-mentioned conventional problems, and aims to provide an image projection device that effectively suppresses the temperature rise of the image illumination area due to ambient light, while also ensuring visibility even when using polarized sunglasses or the like.

To solve the above problems, the present invention provides an image projection device for projecting a projection image onto a display unit for displaying a virtual image, comprising: an image irradiation unit for irradiating image light; an irradiation optical unit for irradiating the display unit with the image light as the projection image; and a polarization reflection adjustment unit provided on the display unit, wherein the irradiation optical unit includes a polarization unit that transmits light in a predetermined polarization direction of the image light, the image light irradiated from the irradiation optical unit includes P polarization for the display unit, the polarization reflection adjustment unit has a range in which the reflectance of the P polarization component is greater than or equal to the reflectance of the S polarization component in a range of incidence angle of 20 degrees or more and 40 degrees or less, the polarization reflection adjustment unit is attached to the inner surface of the display unit, and the incidence angle of the image light to the polarization reflection adjustment unit is in the range of 20 degrees or more and 40 degrees or less .
Furthermore, in one aspect of the present invention, the polarization reflection adjustment unit has a saturation of 20 or less.

In the image projection apparatus of the present invention, the illumination optical unit is equipped with a polarization unit, and since it illuminates the display unit with image light containing P-polarization, a portion of the ambient light is cut off by the polarization unit, effectively suppressing the temperature rise of the image illumination unit, and ensuring visibility even when using polarized sunglasses or the like.

Furthermore, in one aspect of the present invention, the illumination optical unit includes a phase difference plate in the optical path of the image light.

Furthermore, in one aspect of the present invention, the phase difference plate is positioned closer to the display unit than the polarization unit.

Furthermore, in one aspect of the present invention, the polarization unit is positioned closer to the display unit than the phase difference plate.

Furthermore, in one aspect of the present invention, the phase difference plate is a quarter-wave plate.

Furthermore, in one aspect of the present invention, the image irradiation unit irradiates the display unit with image light containing S-polarization, and the phase difference plate is a half-wave plate.

Furthermore, in one aspect of the present invention, the image irradiation unit irradiates the display unit with image light containing P-polarized light, and the polarization unit transmits the P-polarized light.

Furthermore, in one aspect of the present invention, the illumination optical unit includes a reflecting unit that reflects the image light emitted from the image illumination unit, and the polarization unit is arranged between the reflecting unit and the display unit.

This invention provides an image projection device that effectively suppresses the temperature rise of the image illumination area due to ambient light, while also ensuring visibility even when using polarized sunglasses or the like.

This is a schematic diagram showing the configuration of the image projection device 100 according to the first embodiment. This graph schematically shows an example of the reflection characteristics of P-polarized and S-polarized light at the polarization reflection adjustment unit 15. This is a schematic diagram showing the configuration of the image projection device 110 according to the second embodiment. This is a schematic diagram showing the configuration of the image projection device 120 according to the third embodiment. This is a schematic diagram showing the configuration of an image projection device 120 according to a modified example of the third embodiment. This is a schematic diagram showing the configuration of the image projection device 130 according to the fourth embodiment. This is a schematic diagram showing the configuration of a conventional image projection device.

(First Embodiment)
Embodiments of the present invention will be described in detail below 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 explanations will be omitted as appropriate. Figure 1 is a schematic diagram showing the configuration of the image projection device 100 according to this embodiment.

As shown in Figure 1, the image projection device 100 comprises an image illumination unit 11, free-form surface mirrors 12 and 13, and a polarization unit 16. The free-form surface mirrors 12 and 13 and the polarization unit 16 constitute the illumination optical unit 20 in this invention. In Figure 1, the typical optical path of the illumination light L1 emitted from the image illumination unit 11 is schematically shown by a solid arrow, and the typical optical path of ambient light L1, such as sunlight, is shown by a dashed arrow. Furthermore, a vehicle windshield 14 and a polarization reflection adjustment unit 15 are provided outside the image projection device 100, and the driver or other user views the image produced by the illumination light L1 from the viewpoint 5 via the polarization reflection adjustment unit 15.

The image illumination unit 11 is a device that emits illumination light (image light) L1 containing image information when a signal containing image information is supplied from the information processing unit (not shown). The illumination light L1 emitted from the image illumination unit 11 is incident on the free-form surface mirror 12. Examples of the image illumination unit 11 include liquid crystal display devices, organic EL display devices, micro-LED display devices, DMDs (Digital Micro-mirror Devices), and projector devices using laser light sources. The illumination light L1 emitted from the image illumination unit 11 is configured to include light that is P-polarized with respect to the windshield 14, as shown by the double-headed arrow in the figure. If the illumination light L1 from the image illumination unit 11 has a specific polarization, the orientation of the display surface of the image illumination unit 11 is set so that its polarization plane is P-polarized with respect to the windshield 14. Examples of image illumination units 11 that emit illumination light L1 with a specific polarization include liquid crystal display devices, projector devices using laser light sources, and reflective liquid crystal projector devices.

The free-form surface mirror 12 is a mirror that receives the incident light L1 emitted from the image illumination unit 11 and reflects it in the direction of the free-form surface mirror 13. The reflective surface shape of the free-form surface mirror 12 is composed of a free-form surface whose curvature is not constant but changes in two dimensions. While Figure 1 shows a convex mirror as the shape of the free-form surface mirror 12, a concave mirror or a plane mirror may also be used.

The free-form surface mirror 13 is a concave mirror that receives incident light L1 reflected by the free-form surface mirror 12 and reflects it towards the windshield 14 via the polarizing section 16. The reflective surface shape of the free-form surface mirror 13 is composed of a free-form surface whose curvature is not constant but changes two-dimensionally. While Figure 1 shows a concave mirror as the shape of the free-form surface mirror 13, a convex mirror or a plane mirror may also be used. The free-form surface mirrors 12 and 13 correspond to the reflective sections in this invention.

The polarizing section 16 is an optical component having optical properties that transmit polarized light in the direction of the transmission axis and block polarized light perpendicular to the transmission axis. A known polarizing plate or polarizing film can be used. The polarizing section 16 is positioned between the free-form mirror 13 and the windshield 14. Figure 1 shows an example where the polarizing section 16 is positioned between the free-form mirror 13 and the windshield 14, but the position of the polarizing section 16 is not limited to any position on the optical path of the irradiated light L1 from the polarization reflection adjustment section 15 to the windshield 14. Furthermore, the transmission axis of the polarizing section 16 is positioned to transmit P-polarized light to the windshield 14.

The windshield 14 is located in front of the driver's seat of the vehicle and transmits light from outside the vehicle in the direction of viewpoint 5. Furthermore, since the windshield 14 transmits at least visible light from outside the vehicle, even when external light LO, such as sunlight, enters the vehicle from above, the external light LO reaches the polarizing section 16 and the free-form mirror 13, as shown by the dashed arrow in Figure 1. Additionally, a polarization reflection adjustment section 15 is provided on the inner surface of the windshield 14.

The polarization reflection adjustment section 15 is an optical element provided on the inner surface of the windshield 14 and has optical properties that reflect the S-polarized and P-polarized components of incident light to roughly the same degree. Furthermore, the polarization reflection adjustment section 15 has optical properties that change reflectivity according to the angle of incidence, which is the inclination from the direction perpendicular to the incident surface. It is also preferable that the saturation of the polarization reflection adjustment section 15 be 20 or less. By satisfying this condition, the color of the irradiated light L1 is not degraded, and the deterioration of the quality of the projected virtual image can be suppressed. In the example shown in Figure 1, the polarization reflection adjustment section 15 is configured as a substantially flat film shape and is attached along the curvature of the inner surface of the windshield 14. While Figure 1 shows an example where the polarization reflection adjustment section 15 is provided on a part of the windshield 14, it may also be attached to the front surface of the windshield 14.

Figure 2 is a schematic graph showing an example of the reflection characteristics of P-polarized and S-polarized light in the polarization reflection adjustment unit 15. The horizontal axis of the graph represents the angle of incidence, with the direction perpendicular to the surface of the polarization reflection adjustment unit 15 being 0 degrees. The vertical axis of the graph represents the reflectance of polarized light (P-polarized light) in the in-plane direction including the 0-degree direction and the direction of light incidence, and the reflectance of S-polarized light perpendicular to the P-polarized light. As shown in Figure 2, the polarization reflection adjustment unit 15 has optical characteristics in which the reflectance is low (transmittance is high) for light incident at small incident angles and close to the perpendicular direction, the reflectance increases (transmittance decreases) as the incident angle increases, and the reflectance becomes high above a certain incident angle. Furthermore, as shown in Figure 2, the reflectance of P-polarized and S-polarized light is approximately the same in the incident angle range of 20 to 40 degrees.

As the polarization reflection adjustment unit 15 having the optical properties shown in Figure 2, a laminated film manufactured by Toray Industries, Inc. (product name "PICASUS® VT") or a laminated film described in Japanese Patent Publication No. 2021-54061, etc., can be used. The example shown in Figure 2 illustrates a case where the reflectance approaches its maximum value at an incident angle of around 40 degrees, but the maximum reflectance and the incident angle at which the maximum value is reached are not limited to this.

Furthermore, while Figure 1 shows an example where the polarization reflection adjustment unit 15 is attached to the windshield 14 as the display unit, it is also possible to prepare a combiner as a separate display unit from the windshield 14, attach the polarization reflection adjustment unit 15 to the inner surface of the combiner, and reflect the light from the free-form mirror 13 in the direction of the viewpoint. Also, the display unit is not limited to being located at the front of the vehicle; it may be positioned to the side or rear as long as it projects an image to the occupant's viewpoint 5. The viewpoint 5 is the eye (eyebox) of the vehicle's driver or occupant, and when the irradiated light L1 enters the eyebox and reaches the retina, the driver or occupant sees the formed virtual image.

The virtual image is displayed as if it were formed in space when the illuminated light L1, reflected by the polarization reflection adjustment unit 15, reaches the driver's or passenger's viewpoint (eyebox) 5. The position where the virtual image is formed is determined by the spread angle of the light emitted from the image illumination unit 11 as it travels in the direction of the viewpoint after being reflected by the free-form mirrors 12, 13 and the polarization reflection adjustment unit 15. At this time, the driver or passenger perceives the virtual image as being located at a position farther than the windshield 14. Here, the position of the virtual image formation primarily depends on the combined focal length of the free-form mirrors 12 and 13. Even if the windshield 14 is a curved surface rather than a flat surface, the influence of the optical power from the windshield 14 is negligible because its radius of curvature is larger than that of the free-form mirrors 12 and 13.

As shown in Figure 1, the light L1 emitted from the image illumination unit 11 contains P-polarized light for the windshield 14, and the light L1 reflected by the free-form mirrors 12 and 13 also contains P-polarized light. Furthermore, since the polarization unit 16 is positioned to transmit P-polarized light for the windshield 14, the light L1 passes through the polarization unit 16 and reaches the polarization reflection adjustment unit 15. As shown in Figure 2, the polarization reflection adjustment unit 15 has approximately equal reflectivity for S-polarized and P-polarized light, so the P-polarized component of the light L1 is also well reflected and reaches the viewpoint 5. Therefore, the virtual image is projected by P-polarized light, making it easily visible even when the rider is wearing polarized sunglasses. Here, as shown in Figure 2, the polarization reflection adjustment unit 15 has a small difference in reflectivity between S-polarized and P-polarized light in the range of incident angles of 20 to 40 degrees; therefore, it is preferable to set the incident angle of the light L1 to the polarization reflection adjustment unit 15 to 20 to 40 degrees.

As shown in Figure 1, ambient light LO, such as sunlight, enters from above the windshield 14. A portion of it passes through the polarization reflection adjustment section 15 and the polarization section 16, is reflected by the free-form mirror 13 and the free-form mirror 12, and reaches the image illumination section 11. Here, ambient light LO entering from outside the vehicle is unpolarized, containing components of all polarization directions. However, only the P-polarized component can pass through the polarization section 16 relative to the windshield 14. Therefore, the S-polarized component of the ambient light LO is cut off by the polarization section 16, suppressing the amount of light reaching the image illumination section 11. This suppresses the temperature rise caused by ambient light LO reaching the image illumination section 11, thus preventing deterioration.

As described above, in the image projection device 100 of this embodiment, the illumination optical unit 20 is equipped with a polarization unit 16 and projects illumination light L1 containing P-polarization onto the display unit. Therefore, a portion of the ambient light LO is cut off by the polarization unit 16, effectively suppressing the temperature rise of the image illumination unit 11, while also ensuring visibility even when using polarized sunglasses or the like.

(Second Embodiment)
Next, a second embodiment of the present invention will be described with reference to Figure 3. Details that overlap with the first embodiment will be omitted. Figure 3 is a schematic diagram showing the configuration of the image projection device 110 according to this embodiment. As shown in Figure 3, the image projection device 110 includes an image irradiation unit 11, free-form surface mirrors 12 and 13, a polarizing unit 16, and a quarter-wave plate 17. The free-form surface mirrors 12 and 13, the polarizing unit 16, and the quarter-wave plate 17 constitute the irradiation optical unit 20 in the present invention.

The quarter-wave plate 17 is an optical element positioned on the optical path of the irradiated light (image light) L1, generating a phase difference of one-quarter wavelength between the mutually orthogonal fast axis and slow axis, and corresponds to the phase difference plate in this invention. The quarter-wave plate 17 is preferably positioned on the display side of the polarization unit 16; in the example shown in Figure 3, it is positioned between the polarization unit 16 and the polarization reflection adjustment unit 15. Furthermore, the fast axis of the quarter-wave plate 17 is positioned so as to be 45 degrees different from the S-polarization component and P-polarization component of the irradiated light L1.

As shown in Figure 3, in the image projection device 110 of this embodiment, the light L1 emitted from the image irradiation unit 11 contains S-polarized light relative to the windshield 14, and the light L1 reflected by the free-form mirrors 12 and 13 also contains S-polarized light. Furthermore, since the polarization unit 16 is positioned to transmit S-polarized light relative to the windshield 14, the light L1 passes through the polarization unit 16, then through the quarter-wave plate 17, and finally reaches the polarization reflection adjustment unit 15. Because the polarization direction of the S-polarized light and the fast axis of the quarter-wave plate 17 are 45 degrees apart, the light L1 emitted after passing through the quarter-wave plate 17 becomes circularly polarized light C. Circularly polarized light C is light in which the P-polarized and S-polarized components are shifted by a quarter wavelength, and therefore contains P-polarized light.

As shown in Figure 2, the polarization reflection adjustment unit 15 has similar reflectivity for S-polarized and P-polarized light. Therefore, the circularly polarized light L1 is reflected well, with both the P-polarized and S-polarized components being reflected to the viewpoint 5. Consequently, the virtual image is projected by the circularly polarized light C. In this case, the light L1 is S-polarized, and the polarization unit 16 transmits S-polarized light. Therefore, even if the light L1 passes through the polarization unit 16 and the quarter-wave plate 17, no specific polarization component is cut off. Since polarized sunglasses transmit circularly polarized light C, the virtual image can be clearly seen even when the passenger is wearing polarized sunglasses.

As shown in Figure 3, ambient light LO, such as sunlight, enters from above the windshield 14. A portion of it passes through the polarization reflection adjustment unit 15, the quarter-wave plate 17, and the polarization unit 16, and is reflected by the free-form mirror 13 and the free-form mirror 12 before reaching the image illumination unit 11. Here, ambient light LO entering from outside the vehicle is unpolarized, containing components of all polarization directions, and remains unpolarized even after passing through the quarter-wave plate 17. Furthermore, the polarization unit 16 allows only the S-polarized component to pass through the windshield 14. Therefore, the P-polarized component of the ambient light LO is cut off by the polarization unit 16, suppressing the amount of light reaching the image illumination unit 11. This suppresses the temperature rise caused by ambient light LO reaching the image illumination unit 11, thus preventing deterioration.

Figure 3 shows an example where S-polarized light L1 is emitted from the image illumination unit 11 and transmitted through the polarization unit 16. However, the polarization direction is not limited as long as the polarization of the emitted light L1 can be converted to circularly polarized light C by the quarter-wave plate 17. For example, even if P-polarized light L1 is emitted from the image illumination unit 11 and transmitted through the polarization unit 16, the emitted light L1 will still be transmitted through the quarter-wave plate 17 to become circularly polarized light C, allowing for clear visualization of the virtual image even through polarized sunglasses.

As described above, in the image projection device 110 of this embodiment, the illumination optical unit 20 is equipped with a polarization unit 16 and a quarter-wave plate 17, and projects the illumination light L1 onto the display unit as circularly polarized light C containing P-polarization. This effectively suppresses the temperature rise of the image illumination unit 11 by cutting off a portion of the ambient light LO with the polarization unit 16, while also ensuring visibility even when using polarized sunglasses or the like.

(Third Embodiment)
Next, a third embodiment of the present invention will be described with reference to Figure 4. Content that overlaps with the first embodiment will be omitted from the explanation. Figure 4 is a schematic diagram showing the configuration of the image projection device 120 according to this embodiment. As shown in Figure 4, the image projection device 120 includes an image irradiation unit 11, free-form surface mirrors 12 and 13, a polarizing unit 16, and a half-wave plate 18. The free-form surface mirrors 12 and 13, the polarizing unit 16, and the half-wave plate 18 constitute the irradiation optical unit 20 in the present invention.

The half-wave plate 18 is an optical element positioned on the optical path of the irradiated light (image light) L1, generating a phase difference of half a wavelength between the mutually orthogonal fast axis and slow axis, and corresponds to the phase difference plate in this invention. The half-wave plate 18 is preferably positioned on the display side of the polarization unit 16; in the example shown in Figure 4, it is positioned between the polarization unit 16 and the polarization reflection adjustment unit 15. Furthermore, the fast axis of the half-wave plate 18 is positioned so as to be 45 degrees different from the S-polarization component and P-polarization component of the irradiated light L1.

As shown in Figure 4, in the image projection device 120 of this embodiment, the light L1 emitted from the image irradiation unit 11 contains S-polarized light for the windshield 14, and the light L1 reflected by the free-form mirrors 12 and 13 also contains S-polarized light. Furthermore, since the polarization unit 16 is positioned to transmit S-polarized light for the windshield 14, the light L1 passes through the polarization unit 16, then through the half-wave plate 18, and finally reaches the polarization reflection adjustment unit 15. Because the polarization direction of the S-polarized light and the fast axis of the half-wave plate 18 are 45 degrees apart, the light L1 emitted after passing through the half-wave plate 18 becomes P-polarized light.

As shown in Figure 2, the polarization reflection adjustment unit 15 has approximately equal reflectivity for S-polarized and P-polarized light. Therefore, the P-polarized component of the irradiated light L1 is well reflected and reaches the viewpoint 5. Consequently, the virtual image is projected by P-polarized light. Since polarized sunglasses transmit P-polarized light, the virtual image can be clearly seen even when the passenger is wearing polarized sunglasses.

As shown in Figure 4, ambient light LO, such as sunlight, enters from above the windshield 14. A portion of it passes through the polarization reflection adjustment unit 15, the half-wave plate 18, and the polarization unit 16, and is reflected by the free-form mirror 13 and the free-form mirror 12 before reaching the image illumination unit 11. Here, ambient light LO entering from outside the vehicle is unpolarized, containing components of all polarization directions, and remains unpolarized even after passing through the half-wave plate 18. Furthermore, the polarization unit 16 allows only the S-polarized component to pass through the windshield 14. Therefore, the P-polarized component of the ambient light LO is cut off by the polarization unit 16, suppressing the amount of light reaching the image illumination unit 11. This suppresses the temperature rise caused by ambient light LO reaching the image illumination unit 11, thus preventing deterioration.

As described above, in the image projection device 120 of this embodiment, the illumination optical unit 20 is equipped with a polarization unit 16 and a half-wave plate 18, and projects P-polarized illumination light L1 onto the display unit. This effectively suppresses the temperature rise of the image illumination unit 11 by cutting off a portion of the ambient light LO with the polarization unit 16, while also ensuring visibility even when using polarized sunglasses or the like.

(Modification of the third embodiment)
Next, a modified example of the third embodiment of the present invention will be described with reference to Figure 5. Figure 5 is a schematic diagram showing the configuration of the image projection device 120 according to this modified example. In this modified example, the half-wave plate 18 is placed between the polarizing section 16 and the free-form mirror 13, and the polarizing section 16 is arranged to transmit P-polarized light to the windshield 14, which is different from the third embodiment.

As shown in Figure 5, in the image projection device 120 of this embodiment, the light L1 emitted from the image irradiation unit 11 contains S-polarized light relative to the windshield 14, and the light L1 reflected by the free-form mirrors 12 and 13 also contains S-polarized light. Since the polarization direction of the S-polarized light and the fast axis of the half-wave plate 18 are 45 degrees apart, the light L1 transmitted through the half-wave plate 18 becomes P-polarized light. Because the polarization unit 16 is positioned to transmit P-polarized light relative to the windshield 14, the light L1 passes through the polarization unit 16 before reaching the polarization reflection adjustment unit 15.

As shown in Figure 2, the polarization reflection adjustment unit 15 has approximately equal reflectivity for S-polarized and P-polarized light. Therefore, the P-polarized component of the irradiated light L1 is well reflected and reaches the viewpoint 5. Consequently, the virtual image is projected by P-polarized light. Since polarized sunglasses transmit P-polarized light, the virtual image can be clearly seen even when the passenger is wearing polarized sunglasses.

As shown in Figure 5, ambient light LO, such as sunlight, enters from above the windshield 14. A portion of it passes through the polarization reflection adjustment unit 15, the polarization unit 16, and the half-wave plate 18, and is reflected by the free-form mirror 13 and the free-form mirror 12 before reaching the image illumination unit 11. Here, the polarization unit 16 allows only the P-polarized component to pass through the windshield 14. Therefore, the S-polarized component of the ambient light LO is cut off by the polarization unit 16, suppressing the amount of light reaching the image illumination unit 11. The P-polarized light that passes through the polarization unit 16 is converted to S-polarized light after passing through the half-wave plate 18, but the amount of light does not change significantly. This suppresses the temperature rise caused by ambient light LO reaching the image illumination unit 11, thus preventing degradation.

As described above, in this modified image projection device 120, the illumination optical unit 20 is equipped with a half-wave plate 18 and a polarization unit 16, projecting P-polarized illumination light L1 onto the display unit. This effectively suppresses the temperature rise of the image illumination unit 11 by cutting off a portion of the ambient light LO with the polarization unit 16, while also ensuring visibility even when using polarized sunglasses.

(Fourth Embodiment)
Next, a fourth embodiment of the present invention will be described with reference to Figure 6. Content that overlaps with the first embodiment will be omitted from the explanation. Figure 6 is a schematic diagram showing the configuration of the image projection device 130 according to this embodiment. As shown in Figure 6, the image projection device 130 includes an image irradiation unit 11, free-form surface mirrors 12 and 13, a polarization unit 16, and a phase difference plate 19. The free-form surface mirrors 12 and 13, the polarization unit 16, and the phase difference plate 19 constitute the irradiation optical unit 20 in the present invention.

The phase difference plate 19 is an optical element positioned on the optical path of the irradiated light (image light) L1, and generates a phase difference between the mutually orthogonal fast axis and slow axis. Preferably, the phase difference plate 19 is positioned closer to the display unit than the polarization unit 16; in the example shown in Figure 6, it is positioned between the polarization unit 16 and the polarization reflection adjustment unit 15. Furthermore, it is preferable that the fast axis of the phase difference plate 19 is positioned so that it differs by 45 degrees from the S-polarization component and P-polarization component of the irradiated light L1, but the angle is not limited.

As shown in Figure 6, in the image projection device 130 of this embodiment, the light L1 emitted from the image irradiation unit 11 contains S-polarized light relative to the windshield 14, and the light L1 reflected by the free-form mirrors 12 and 13 also contains S-polarized light. Furthermore, since the polarization unit 16 is positioned to transmit S-polarized light relative to the windshield 14, the light L1 passes through the polarization unit 16, then through the phase difference plate 19, and finally reaches the polarization reflection adjustment unit 15. Because the polarization direction and fast axis of the S-polarized light are different in the phase difference plate 19, the light L1 transmitted through the phase difference plate 19 becomes elliptically polarized light E. Elliptically polarized light E is light in which the phases of the P-polarized and S-polarized components are shifted, and therefore contains P-polarized light.

As shown in Figure 2, the polarization reflection adjustment unit 15 has approximately equal reflectivity for S-polarized and P-polarized light. Therefore, the elliptically polarized light L1 is reflected well in both its P-polarized and S-polarized components and reaches the viewpoint 5. Consequently, the virtual image is projected by the elliptically polarized light E. Since polarized sunglasses transmit elliptically polarized light E, the virtual image can be clearly seen even when the passenger is wearing polarized sunglasses.

As shown in Figure 6, ambient light LO, such as sunlight, enters from above the windshield 14. A portion of it passes through the polarization reflection adjustment unit 15, the phase difference plate 19, and the polarization unit 16, and is reflected by the free-form mirror 13 and the free-form mirror 12 before reaching the image illumination unit 11. Here, ambient light LO entering from outside the vehicle is unpolarized, containing components of all polarization directions, and remains unpolarized even after passing through the phase difference plate 19. Furthermore, the polarization unit 16 allows only the S-polarized component to pass through the windshield 14. Therefore, the P-polarized component of the ambient light LO is cut off by the polarization unit 16, suppressing the amount of light reaching the image illumination unit 11. This suppresses the temperature rise caused by ambient light LO reaching the image illumination unit 11, thus preventing deterioration.

Figure 6 shows an example where S-polarized light L1 is emitted from the image illumination unit 11 and transmitted through the polarization unit 16. However, the polarization direction is not limited as long as the polarization of the emitted light L1 can be converted to elliptical polarization E by the phase difference plate 19. For example, even if P-polarized light L1 is emitted from the image illumination unit 11 and transmitted through the polarization unit 16, the emitted light L1 will pass through the phase difference plate 19 and become elliptical polarization E, allowing for clear visualization of the virtual image even through polarized sunglasses.

As described above, in the image projection device 130 of this embodiment, the illumination optical unit 20 is equipped with a polarization unit 16 and a phase difference plate 19, and projects the illumination light L1 onto the display unit as elliptical polarization E containing P polarization. This effectively suppresses the temperature rise of the image illumination unit 11 by cutting off a portion of the ambient light LO with the polarization unit 16, while also ensuring visibility even when using polarized sunglasses or the like.

(Fifth Embodiment)
Next, a fifth embodiment of the present invention will be described. In the first to fourth embodiments, the polarization reflection adjustment unit 15 was used as an optical characteristic that reflects the S-polarized and P-polarized components of incident light to an equal degree. However, a polarization reflection adjustment unit 15 having optical characteristics that reflect the P-polarized component with a higher reflectivity than the S-polarized component may also be used.

In the examples shown in Figures 1, 4, and 5, the illumination light projected onto the windshield 14 from the image projection device is P-polarized. In the example shown in Figure 6, elliptical polarization is used as the illumination light, allowing for elliptical polarization where the P-polarization component is larger than the S-polarization component. Therefore, even when using a polarization reflection adjustment unit 15 that reflects the P-polarization component with a higher reflectivity than the S-polarization component, image projection with a highly reflective P-polarization component can be performed, ensuring visibility even when using polarized sunglasses.

The present invention is not limited to the embodiments described above, 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 within the technical scope of the present invention.

100, 110, 120, 130...Image projection device 11...Image irradiation unit 12, 13...Free-form surface mirror 14...Windshield 15...Polarization reflection adjustment unit 16...Polarization unit 17...Quarter wave plate 18...Half wave plate 19...Phase difference plate 20...Irradiation optics unit

Claims (9)

An image projection device that projects a projected image onto a display unit for displaying a virtual image,
An image illumination unit that emits image light,
An illumination optical unit that irradiates the display unit with the image light as the projected image,
The display unit is provided with a polarization reflection adjustment unit,
The illumination optical unit includes a polarization unit that transmits light in a predetermined polarization direction of the image light,
The image light emitted from the illumination optical unit includes P-polarized light for the display unit.
The polarization reflection adjustment unit has a range in which the reflectance of the P polarization component is equal to or greater than the reflectance of the S polarization component in the range of incident angles of 20 degrees to 40 degrees .
The polarization reflection adjustment unit is attached to the inner surface of the display unit.
An image projection device characterized in that the angle of incidence of the image light to the polarization reflection adjustment unit is in the range of 20 degrees to 40 degrees . An image projection device according to claim 1,
The image projection apparatus is characterized in that the polarization reflection adjustment unit has a saturation of 20 or less. An image projection apparatus according to claim 1 or 2,
The image projection device is characterized in that the illumination optical unit includes a phase difference plate in the optical path of the image light. The image projection apparatus according to claim 3,
The image projection apparatus is characterized in that the phase difference plate is positioned on the display side of the polarization unit. The image projection apparatus according to claim 3,
The image projection apparatus is characterized in that the polarization portion is positioned on the display portion side of the phase difference plate. An image projection device according to any one of claims 3 to 5,
The image projection apparatus is characterized in that the phase difference plate is a quarter-wave plate. An image projection device according to any one of claims 3 to 5,
The image illumination unit illuminates the display unit with the image light containing S-polarization,
The image projection apparatus is characterized in that the phase difference plate is a half-wave plate. An image projection apparatus according to claim 1 or 2,
The image illumination unit illuminates the display unit with image light containing P-polarized light,
The image projection device is characterized in that the polarizing portion transmits the P-polarized light. An image projection device according to any one of claims 1 to 8,
The illumination optical unit includes a reflecting unit that reflects the image light emitted from the image illumination unit,
The image projection device is characterized in that the polarization unit is arranged between the reflective unit and the display unit.