WO2024232334A1 - Projector and head-up display system - Google Patents

Abstract

With regard to a projector for a head-up display using a light guide plate and a diffraction element, the present invention addresses the problem of preventing interference light, due to sunlight or the like, from being visible. The present invention solves the problem by providing: an image display device; a light guide plate that guides image light generated by the image display device; a diffraction element that causes the image light to be emitted from the light guide plate; and a selective reflection layer that is provided on the image light emission-side of the diffraction element and reflects linearly polarized light.

Description

Projectors and Head-Up Display Systems

The present invention relates to a projector used in a head-up display and a head-up display system that uses this projector.

A so-called head-up display (head-up display system) is known that projects an image onto the windshield (windshield glass) of a vehicle or the like to provide information to the driver. In the following description, the head-up display is also referred to as "HUD." HUD is an abbreviation for "Head up Display."
With a HUD, the driver can obtain various information such as maps, driving speed, and vehicle status without moving their line of sight significantly while looking at the outside world ahead, which is expected to enable safer driving while obtaining a variety of information.

FIG. 5 conceptually illustrates an example of a typical HUD.
The HUD 100 in the illustrated example includes an image display unit 112, a reflecting mirror 116, a concave mirror 118, and a windshield 126 that acts as a reflecting member for reflecting and projecting the image light. The image display unit 112 includes an image display device 129 such as a liquid crystal display, and a projection lens 131.
In the HUD 100, the image display unit 112, the reflecting mirror 116, and the concave mirror 118 constitute a projector of the HUD, and are housed under a dashboard 120 of a vehicle such as an automobile.

In the HUD 100, the image displayed by the image display device 129, i.e., the image light, is projected by the projection lens 131, reflected along a predetermined optical path by the reflecting mirror 116 and the concave mirror 118, passes through a transparent window 124 provided in the dashboard 120, enters the windshield 126, is reflected and projected, and is observed by the user O as a projected image.
In such a HUD 100 , the user O observes a virtual image of the image projected onto the windshield 126 .

Here, as described above, the projector used in the HUD 100 is housed under the dashboard of the vehicle, and is therefore subject to significant size restrictions. For this reason, conventional HUD projectors use multiple mirrors to fold the optical path, thereby ensuring the necessary optical path length.
Therefore, the structure of the projector used in the HUD 100 tends to be complicated. Moreover, as described above, the space below the dashboard is small, so the degree of freedom in designing the projector is low.

As a small-sized and simply configured HUD that solves these problems, a HUD that combines a light guide plate (optical waveguide) and a diffraction element (hologram), as described in Patent Document 1, is known.
FIG. 6 conceptually shows one example of this.

As shown in FIG. 6, the HUD 130 includes a light guide plate 132 , an input diffraction element 134 , and an output diffraction element 136 .
In the HUD 130, image light G emitted from an image display device (not shown) is diffracted by an incident diffraction element 134 and enters the light guide plate 132 at an angle at which the light is totally reflected. The image light G that enters the light guide plate 132 is guided (propagated) within the light guide plate 132 by repeating total reflection, as indicated by the arrow pointing leftward in the figure.
Here, an output diffraction element 136 is provided on the upper surface (hereinafter also referred to as the output surface) of the light guide plate 132 in the figure. A part of the light guided within the light guide plate 132 is incident on the output diffraction element 136, diffracted, and output upward from the output surface. As before, the image light G output from the light guide plate 132 is incident on the windshield 126, reflected, and projected, and is observed by the user O as a projected image.

In the HUD 130 using such a light guide plate 132, the light guide plate 132 can be disposed, for example, on the dashboard of an automobile or the like, and size restrictions are significantly reduced compared to the conventional HUD 100 as shown in FIG.
Furthermore, since there is no need to use a mirror to fold the optical path, and projection light can be emitted according to the size of the light guide plate 132, the device configuration of the HUD can be simplified.

Special table 2021-528681 publication

Here, in a HUD 130 using a light guide plate 132 and a diffraction element as shown in Patent Document 1, there is a problem in that external light such as sunlight enters the light guide plate 132, is guided, and is emitted, which is observed by the user O as interfering light.

That is, as conceptually shown in FIG. 7 , in the HUD 130 , external light (dashed line) such as sunlight passes through the windshield 126 , enters the vehicle interior, and reaches the light guide plate 132 .
Here, an output diffraction element 136 is provided on the output surface of the light guide plate 132. Therefore, external light is diffracted by the output diffraction element 136, and depending on the incident direction, the light enters the light guide plate 132 at an angle that causes total reflection.
The external light thus incident on the light guide plate 132 is guided by repeated total reflection within the light guide plate 132 as indicated by the arrow pointing to the right in the figure, in contrast to the image light G indicated by the arrow pointing to the left in the figure, and a part of the light is diffracted by the output diffraction element 136 and emitted from the light guide plate 132 together with the image light. As before, the image light and external light emitted from the light guide plate 132 are incident on the windshield 126, reflected and projected (projected), and observed by the user O as interference light D. As a result, the image observed by the user O is glaring, and the interference light D dazzles the user O.

Furthermore, depending on the angle of incidence on the light guide plate 132, the external light that enters the light guide plate 132, is guided, and then exits may not enter the windshield 126, but may exit the light guide plate 132 and reach the user O directly as interfering light D.

In order to solve this problem, in Patent Document 1, an optical filter 138 (bandpass filter) that absorbs light of wavelengths other than the wavelength of the image light G is provided on the exit surface side of the diffraction element, as shown by the dashed line in Figure 6.
In Patent Document 1, this prevents external light such as sunlight from entering the light guide plate 132, suppresses glare in the image observed by the user O caused by the interfering light D, and also suppresses the user O from being dazzled by the interfering light D.

In addition, in recent years, in head-up display systems, it is desirable to increase the angle of view, which is the angular range available for image display. In other words, in head-up display systems, it is desirable to expand the irradiation area of the projection light on the windshield glass. In consideration of this, the light concentration at the concave mirror 118 and the output diffraction element 136 is stronger than before.
Therefore, when external light such as sunlight enters the inside of the projector via the concave mirror 118 or the output diffraction element 136, measures must be taken to prevent thermal damage to the HUD 100, HUD 130, etc.

The object of the present invention is to provide a projector for a HUD that uses a light guide plate and a diffraction element as in Patent Document 1, which prevents the user from observing interfering light and has measures in place to prevent heat damage, and to provide a HUD that uses this projector.

In order to achieve the above object, the present invention has the following configuration.
[1] A projector for a head-up display,
an image display device that emits image light; and a light guide plate that guides the image light emitted by the image display device;
an emission element that emits the image light guided through the light guide plate from the light guide plate;
a selective reflection film that reflects linearly polarized light and is provided on the image light emission side of the emission element.
[2] The projector according to [1], wherein the output element is a diffraction element.
[3] The projector according to [1] or [2], wherein the selective reflection film that reflects linearly polarized light reflects visible light.
[4] The projector according to any one of [1] to [3], wherein the selective reflection film that reflects linearly polarized light reflects visible light and infrared light.
[5] The projector according to any one of [1] to [4], wherein the image light is P-polarized light, and the selective reflection film that reflects linearly polarized light is a film that reflects S-polarized light.
[6] The projector according to any one of [1] to [4], wherein the image light is S-polarized light, and the selective reflection film that reflects linearly polarized light is a film that reflects P-polarized light.
[7] A projector described in any one of [1] to [6], in which the selective reflection film that reflects linearly polarized light has, in this order, a retardation layer, a cholesteric liquid crystal layer formed by fixing a cholesteric liquid crystal phase, and a retardation layer.
[8] A projector described in any one of [1] to [7], in which the selective reflection film that reflects linearly polarized light has, in this order, a retardation layer, a cholesteric liquid crystal layer formed by fixing a cholesteric liquid crystal phase, and a polarization conversion layer.
[9] A head-up display system including the projector according to any one of [1] to [8] and a windshield glass onto which the projection light emitted from the projector is irradiated.

The present invention uses a new method to prevent interference light from being observed by the user in a HUD projector that uses a light guide plate and a diffraction element, and to suppress thermal damage caused by temperature rise inside the projector.

FIG. 1 is a diagram conceptually showing an example of a projector and HUD of the present invention. FIG. 2 is a schematic perspective view for explaining the projector shown in FIG. FIG. 3 is a perspective view conceptually showing another example of a projector according to the invention. FIG. 4 is a diagram showing an example of a selective reflection film that reflects linearly polarized light and is used in the projector shown in FIG. FIG. 5 is a diagram conceptually illustrating an example of a typical projector. FIG. 6 is a diagram conceptually illustrating an example of a conventional projector. FIG. 7 is a conceptual diagram for explaining the operation of the projector shown in FIG.

Below, the projector and head-up display system (HUD) of the present invention will be described in detail based on the preferred embodiment shown in the attached drawings.

In this specification, "~" is used to mean that the numbers before and after it are included as lower and upper limits.

In this specification, visible light refers to electromagnetic waves having wavelengths visible to the human eye, in the wavelength range of 380 to 780 nm, while non-visible light refers to light having a wavelength range of less than 380 nm or more than 780 nm.
In addition, although not limited thereto, light in the visible light wavelength range of 420 to 490 nm is blue light (B light), light in the wavelength range of 495 to 570 nm is green light (G light), and light in the wavelength range of 620 to 750 nm is red light (R light). Furthermore, although not limited thereto, infrared light refers to a wavelength range of non-visible light exceeding 780 nm and not exceeding 2000 nm.
In the present invention, the terms parallel, perpendicular, and orthogonal include the error range generally accepted in the technical field. Specifically, the terms parallel, perpendicular, and orthogonal mean that the difference from strict agreement is within a range of less than 5°. The difference from strict agreement is preferably less than 4°, and more preferably less than 3°.

FIG. 1 conceptually shows an example of a HUD of the present invention that uses an example of a projector of the present invention.
In the example shown in FIG. 1, the projector of the present invention and a windshield glass 30 constitute a HUD 10 of the present invention.
The projector shown in Fig. 1 is a projector for a HUD, and includes a light guide plate 12, an output diffraction element 14, a selective reflection film 16 that reflects linearly polarized light, an image display device 18, and an input diffraction element 20. As an example, the projector uses a windshield 30 of a vehicle such as an automobile to display (project) an image toward a user O. The output diffraction element 14 is the output element of the present invention. Although not shown in Fig. 1, the projector in the illustrated example further includes an intermediate diffraction element 24 on the rear side of the input diffraction element 20 in the figure (see Fig. 2).

This projector is basically the same as the virtual image display device (a device that generates a virtual image (VB)) described in Patent Document 1, which is conceptually shown in Figure 6, except that it has a selective reflection film 16 that reflects linearly polarized light instead of an optical filter 138 that suppresses interfering light by absorbing external light such as sunlight.
6 described above, this projector also diffracts the image light G emitted by the image display device 18 by the incident diffraction element 20 and causes the image light G to enter the light guide plate 12 at an angle at which the image light G is totally reflected. The image light G that enters the light guide plate 12 is diffused by the intermediate diffraction element 24 and, as indicated by the arrow pointing left in the figure, is guided (propagated) through the light guide plate 12 by repeating total reflection.
An output diffraction element 14 is attached to the output surface (top surface in the figure) of the light guide plate 12. A portion of the light guided within the light guide plate 12 enters the output diffraction element 14, is diffracted, and is output from the light guide plate 12. The image light output from the light guide plate 12 enters the windshield 30, is reflected and projected, and is observed by the user O. The user O observes a virtual image of the image projected on the windshield 30, similar to a known HUD (projector).
1 as well, similarly to the HUD 130 shown in FIG. 6 (FIG. 7) described above, external light such as sunlight passes through the windshield 30 and enters the vehicle interior, is diffracted by the output diffraction element 136, and enters the light guide plate 12. As indicated by the arrow pointing to the right in the figure, the light is guided by repeated total reflection within the light guide plate 12, causing glare in the image, interfering light, and thermal damage inside the projector.
However, the projector of the present invention has a selective reflection film 16 on the output side of the output diffraction element 14 from which the image light G is output. Therefore, in the projector (HUD) of the present invention, it is possible to reduce external light incident on the light guide plate 12, thereby reducing glare on the image, disturbing light, and thermal damage inside the projector. This will be described in detail later.

FIG. 2 shows a schematic perspective view of the light guide plate 12. As shown in FIG.
2, the light guide plate 12 is a plate-like object with a rectangular main surface, and an incident diffraction element 20 is provided near one corner of one of the main surfaces. An intermediate diffraction element 24 that is elongated in the short side direction is provided near an end in the longitudinal direction, close to the incident diffraction element 20. Furthermore, in the light guide plate 12, an exit diffraction element 14 is provided so as to cover substantially the entire surface of an area of the main surface where the incident diffraction element 20 and the intermediate diffraction element 24 are not provided.
The main surface refers to the largest surface of a plate-like object (sheet, film, layer), and usually refers to both surfaces in the thickness direction of the plate-like object.
In the following description, for convenience, the vertical direction of the light guide plate 12 on the paper surface is defined as the Y direction, and the horizontal direction on the paper surface is defined as the X direction. Therefore, in Fig. 1, the left direction in the figure is the X direction, and the direction perpendicular to the paper surface and toward the back is the Y direction.

Similar to the HUD 130 shown in FIG. 6 , the incident diffraction element 20 is a diffraction element for making the image light G emitted from the image display device 18 incident on the light guide plate 12 .
The incident diffraction element 20 diffracts the image light G emitted from the image display device 18 at an angle that causes total reflection within the light guide plate 12 and guides the light in the Y direction.

The image light G incident on the light guide plate 12 by the incident diffraction element 20 is guided in the Y direction near the end in the X direction while repeatedly undergoing total reflection within the light guide plate 12, as indicated by the arrow, and a part of the light is incident on the intermediate diffraction element 24. By guiding this image light G in the Y direction, the image light (image) emitted by the image display device 18 is expanded in the Y direction.
The intermediate diffraction element 24 is a diffraction element for deflecting the light guide direction of the image light G in the light guide plate 12. A part of the image light G guided in the Y direction near the end in the X direction is incident on the intermediate diffraction element 24 and diffracted at an angle that causes total reflection in the light guide plate 12 toward the X direction of the light guide plate 12 (the left direction in FIG. 1), i.e., the output diffraction element 14, as shown by the arrow. Note that although only one arrow is shown in FIG. 2, the diffraction of the image light G from the intermediate diffraction element 24 toward the output diffraction element 14 is performed over the entire longitudinal area of the intermediate diffraction element 24 that is elongated in the Y direction of the light guide plate 12.

The image light G diffracted by the intermediate diffraction element 24 is totally reflected within the light guide plate 12 and guided in the X direction (the left direction in FIG. 1 ), and a part of it enters the output diffraction element 14 .
6, the output diffraction element 14 is a diffraction element for outputting image light G from the light guide plate 12. A part of the image light G that is totally reflected within the light guide plate 12 and guided in the X direction is incident on the output diffraction element 14 and diffracted, and as shown by the arrow pointing upward in the figure and as shown in Fig. 1 described above, the image light G is output from the light guide plate 12. Furthermore, by guiding this image light G in the X direction, the image light (image) output from the image display device 18 is expanded in the X direction.
As described above, the image light G emitted from the light guide plate 12 enters the windshield 30, is reflected and projected, and is observed by the user O. Note that, although only one arrow of the image light is shown in Fig. 2, the image light is emitted from the light guide plate 12 by the output diffraction element 14 from the entire surface of the output diffraction element 14.

In the example shown in FIG. 2, one light guide plate 12 is provided.
However, the present invention is not limited to this. For example, in the case where the HUD 10 displays a full-color image of red (R), green (G), and blue (B), a light guide plate 12R that guides the R image light, a light guide plate 12G that guides the G image light, and a light guide plate 12B that guides the B image light may be provided according to the respective colors, as conceptually shown in FIG. 3 .
The stacking order of the light guide plates is not limited to the example shown in FIG.

Light guide plate 12R, light guide plate 12G and light guide plate 12B are provided with an input diffraction element 20, an intermediate diffraction element 24 and an output diffraction element 14, respectively.
Among the diffraction elements provided in each light guide plate, at least the input diffraction element 20 and the output diffraction element 14 have wavelength dependency (wavelength selectivity) and diffract only the image light of the corresponding color to be incident on the light guide plate. Furthermore, it is preferable that the intermediate diffraction element 24 also has wavelength dependency and diffracts only the image light of the corresponding color. In this case, the diffraction element that receives monochromatic light does not need to have wavelength dependency. That is, in this example, the diffraction element provided in the light guide plate 12B does not need to have wavelength dependency.
However, in a configuration having a plurality of light guide plates, it is preferable that the diffraction elements provided on each light guide plate all have wavelength dependency.

In the example shown in FIG. 3, the image light emitted from the image display device 18 is deflected by the mirror 34 toward the light guide plate 12R.
Of the image light deflected by the mirror 34, the R image light is diffracted by the incident diffraction element 20 provided in the light guide plate 12 R and enters the light guide plate 12 R. The G image light and the B image light pass through this incident diffraction element 20.
The G image light is diffracted by the incident diffraction element 20 provided in the light guide plate 12G and enters the light guide plate 12G. The B image light passes through this incident diffraction element.
Furthermore, the B image light is diffracted by the incident diffraction element 20 provided in the light guide plate 12B, and enters the light guide plate 12B.

The image light incident on each corresponding light guide plate is guided in the Y direction (short direction of the light guide plate) as in FIG. 2, deflected in the light guide direction in the X direction (longitudinal direction of the light guide plate) by the intermediate diffraction element 24, and emitted from the light guide plate toward the windshield 30 by the emission diffraction element 14.
Here, the B image light emitted from the light guide plate 12B passes through the emission diffraction element 14 provided on the light guide plate 12G and the light guide plate 12R, and is emitted toward the windshield 30. The G image light emitted from the light guide plate 12G passes through the emission diffraction element 14 provided on the light guide plate 12R, and is emitted toward the windshield 30.

In the HUD 10 (projector) of the present invention, there are no limitations on the image display device 18, the light guide plate 12, and each diffraction element, and various known elements can be used.
For example, various known image display devices such as a liquid crystal display device, a liquid crystal on silicon (LCOS), an organic electroluminescence (organic EL) display, and a digital light processing (DLP) using a digital micromirror device (DMD) can be used as the image display device 18. A projection lens may be combined with these image display devices as necessary.

For the light guide plate 12, various types of well-known light guide plates such as those used in backlight units of liquid crystal display devices and AR glasses can be used, such as light guide plates made of glass or plastic such as acrylic resin.

Furthermore, as for the diffraction element, various known diffraction elements such as a liquid crystal diffraction element, a holographic diffraction element (hologram), a volume hologram diffraction element, and a surface relief diffraction element can be used.
The diffraction element may be of a reflective type or a transmissive type. In the illustrated example, the diffraction element is of a transmissive type. When the diffraction element is of a reflective type, the diffraction element is disposed on the surface of the light guide plate opposite to the transmissive diffraction element.
Moreover, each diffraction element may be attached to the light guide plate 12 by a known method, such as a method using an OCA (Optical Clear Adhesive).

In the illustrated example of the HUD 10, the diffraction element 14 is used as an output element for outputting the image light G from the light guide plate 12, but the present invention is not limited to this.
For example, as shown in FIG. 4 of Patent Document 1, a plurality of half mirrors inclined with respect to the main surface may be provided in the light guide plate 12 in the light guide direction of the image light G, and these half mirrors may be used as emission elements that emit the image light G from the light guide plate.

The HUD 10 in the illustrated example causes image light G to be incident on the windshield 30 and then reflected and projected, but the present invention is not limited to this.
That is, the HUD 10 (projector) of the present invention may use a combiner such as a half mirror for reflecting an image, and the image light G emitted from the light guide plate 12 may be incident on the combiner, reflected, and projected. Also, a combiner for reflecting the image light G may be provided inside the windshield 30.

In the HUD 10 of the present invention, there is no limitation on the image light G incident on the windshield 30, i.e., the image light emitted by the image display device 18, and the image light may be unpolarized or polarized, but is preferably linearly polarized. Furthermore, when the image light G is linearly polarized, the linearly polarized light is preferably S-polarized or P-polarized.
Here, S-polarized light and P-polarized light refer to the polarization directions of linearly polarized light that is incident on the windshield 30.

1, the HUD 10 of the present invention (the projector of the present invention) has a selective reflection film 16 that reflects linearly polarized light on the output side of the image light G of the output diffraction element 14. The selective reflection film 16 that reflects linearly polarized light is a reflective layer having polarization selectivity that transmits the linearly polarized light output from the image display device 18 and reflects the linearly polarized light orthogonal to the linearly polarized light output from the image display device 18.
Therefore, when the image light G emitted by the image display device 18 is P-polarized light, the selective reflection film 16 is configured to reflect S-polarized light and transmit P-polarized light.
When the image light G emitted from the image display device 18 is S-polarized light, the selective reflection film 16 is configured to reflect P-polarized light and transmit S-polarized light.

The HUD 10 of the present invention has a selective reflection film 16 that reflects such linearly polarized light on the output side of the image light G of the output diffraction element 14, thereby preventing external light such as sunlight from entering the light guide plate 12 and preventing the user O from observing the external light as interfering light.
Moreover, the selective reflection film 16 prevents external light from entering the inside of the projector as interfering light, and solves heat problems such as burning of the image display device.
Furthermore, the selective reflection film 16 transmits the polarized light emitted from the image display device 18, and therefore does not impair the brightness of the image display.

An example of a selective reflection film 16 that reflects linearly polarized light is shown in Fig. 4. The selective reflection film 16 that reflects linearly polarized light shown in Fig. 4 has, from the bottom in the figure, a polarization conversion layer 60, a selective reflection layer 62, a retardation layer 64, and a transparent substrate 66, in this order.
In addition, the selective reflection film 16 in the illustrated example may be disposed with the transparent substrate 66 facing the light guide plate 12, or the transparent substrate 66 may be disposed facing the windshield glass 30. From the viewpoint of protecting the selective reflection film 16, it is preferable to dispose the transparent substrate 66 facing the windshield glass 30. In this case, the transparent substrate 66 may contain an ultraviolet absorbing agent or the like. This point is the same in the configuration described later.

In the HUD (projector) of the present invention, the selective reflection film that reflects linearly polarized light is not limited to the configuration shown in FIG.
For example, the selective reflection film may have, in this order, a retardation layer 64, a selective reflection layer 62, a retardation layer 64, and a transparent substrate 66. Alternatively, the selective reflection film may have, in this order, a polarization conversion layer 60, a selective reflection layer 62, a polarization conversion layer 60, and a transparent substrate 66.
In addition, the selective reflection film may have, in addition to the layers (plates) shown in the figures, an alignment film, an attachment layer (adhesive layer/sticky layer), a support plate (transparent substrate) that supports each layer, an anti-reflection layer (to improve the intensity of image light), and a hard coat layer, etc., as necessary.

In the HUD 10 (projector) of the present invention, the selective reflection film 16 may be arranged so as to cover a portion of the light guide plate 12, but it is preferable that the selective reflection film 16 is arranged so as to cover at least the entire surface of the output diffraction element 14, more preferable that the selective reflection film 16 is arranged so as to cover the entire surfaces of the output diffraction element 14 and the intermediate diffraction element 24, even more preferable that the selective reflection film 16 is arranged so as to cover all of the diffraction elements, and it is particularly preferable that the selective reflection film 16 is arranged so as to cover the entire surface of the light guide plate 12.
Furthermore, the image light G emitted from the image display device 18 may be transmitted through the selective reflection film 16 and then incident on the incident diffraction element 20 , or may be incident on the incident diffraction element 20 without transmitting through the selective reflection film 16 .

The members constituting the selective reflection film 16 that reflects linearly polarized light will be described in detail below.
(Polarization conversion layer)
The polarization conversion layer 60 exhibits optical rotation and birefringence for visible light, and converts the polarization state of incident light.
The polarization conversion layer 60 has a function of converting linearly polarized light orthogonal to the image light G, i.e., the linearly polarized light emitted by the image display device 18, into circularly polarized light that is reflected by the cholesteric liquid crystal layer of the selective reflection layer 62, which will be described later. In other words, the polarization conversion layer 60 converts the image light G, i.e., the linearly polarized light emitted by the image display device 18, into circularly polarized light that passes through the cholesteric liquid crystal layer of the selective reflection layer 62.
In addition, the polarization conversion layer 60 converts the circularly polarized light that has passed through the cholesteric liquid crystal layer of the selective reflection layer 62 into linearly polarized light that is the same as the image light G, and converts the circularly polarized light reflected by the cholesteric liquid crystal layer of the selective reflection layer 62 into linearly polarized light that is perpendicular to the image light G.
For example, when the image light G is P-polarized light, the polarization conversion layer 60 converts S-polarized light into circularly polarized light that is reflected by the cholesteric liquid crystal layer of the selective reflection layer 62, and converts P-polarized light into circularly polarized light that is transmitted through the cholesteric liquid crystal layer of the selective reflection layer 62. The polarization conversion layer 60 also converts circularly polarized light that is reflected by the cholesteric liquid crystal layer of the selective reflection layer 62 into S-polarized light, and converts circularly polarized light that is transmitted through the cholesteric liquid crystal layer of the selective reflection layer 62 into P-polarized light.

The polarization conversion layer 60 is preferably a layer in which the helical alignment structure of a liquid crystal compound is fixed.
In particular, the polarization conversion layer 60 is a layer in which a helical orientation structure of a liquid crystal compound is fixed in the thickness direction, and it is preferable that the pitch number x of the helical orientation structure and the film thickness y (unit: μm) of the polarization conversion layer satisfy all of the following relationship formulas (a) to (c):
0.1≦x≦1.0... Formula (a)
0.5≦y≦3.0... Formula (b)
3000≦(1560×y)/x≦50000... Formula (c)
One pitch of the helical structure of a liquid crystal compound is one turn of the helix of the liquid crystal compound. That is, one pitch is defined as a state in which the director of the helically aligned liquid crystal compound (the long axis direction in the case of a rod-shaped liquid crystal) rotates 360°.

When the polarization conversion layer 60 has a helical structure of a liquid crystal compound, it exhibits optical rotation and birefringence for visible light, which has a shorter wavelength than the reflection peak wavelength in the infrared range. This allows for control of polarization in the visible range. By setting the pitch number x of the helical orientation structure of the polarization conversion layer 60 and the film thickness y of the polarization conversion layer 60 within the above ranges, it is possible to impart the function of converting linearly polarized light incident on the selective reflection film 16 into circularly polarized light.

The liquid crystal compound has a helical structure that satisfies the relational expressions (a) to (c), so that the polarization conversion layer 60 exhibits optical rotation and birefringence for visible light. In particular, by setting the pitch P of the helical structure of the polarization conversion layer 60 to a length that corresponds to the pitch P of the cholesteric liquid crystal layer, whose selective reflection center wavelength is in the long-wavelength infrared range, the layer exhibits high optical rotation and birefringence for visible light, which has a short wavelength.

The pitch number x of the helical structure of the polarization conversion layer 60 is more preferably 0.1 to 0.8, and the film thickness y is more preferably 0.6 to 2.6 μm. In addition, "(1560×y)/x" is more preferably 5000 to 13000.

Such a polarization conversion layer 60 can basically be formed in the same way as a known cholesteric liquid crystal layer.

A layer in which a helical orientation structure (helical structure) of a liquid crystal compound is fixed is a so-called cholesteric liquid crystal layer, which means a layer in which a cholesteric liquid crystal phase is fixed.
The cholesteric liquid crystal layer may be a layer in which the orientation of the liquid crystal compound in the cholesteric liquid crystal phase is maintained. The cholesteric liquid crystal layer may be a layer in which the polymerizable liquid crystal compound is oriented in the cholesteric liquid crystal phase, polymerized and cured by ultraviolet irradiation and heating, etc., to form a layer with no fluidity, and at the same time, changed to a state in which the orientation form is not changed by an external field or external force. In the cholesteric liquid crystal layer, it is sufficient that the optical properties of the cholesteric liquid crystal phase are maintained in the layer, and the liquid crystal compound in the layer does not need to exhibit liquid crystallinity anymore. For example, the polymerizable liquid crystal compound may be polymerized by a curing reaction and no longer have liquid crystallinity.

(Selective Reflection Layer)
In the illustrated selective reflection film 16, the selective reflection layer 62 has three cholesteric liquid crystal layers (68R, 68G, 68B).
The three cholesteric liquid crystal layers have different selective reflection center wavelengths. In the illustrated example, the cholesteric liquid crystal layer 68R having a selective reflection center wavelength in the red wavelength region, the cholesteric liquid crystal layer 68G having a selective reflection center wavelength in the green wavelength region, and the cholesteric liquid crystal layer 68B having a selective reflection center wavelength in the blue wavelength region are arranged in this order. In the illustrated example, each cholesteric liquid crystal layer is in direct contact with any of the other cholesteric liquid crystal layers.

A cholesteric liquid crystal layer is a layer in which liquid crystal compounds are fixed in an oriented state of a helical structure of a cholesteric liquid crystal phase, and it reflects circularly polarized light of a wavelength corresponding to the pitch of the helical structure and transmits light of other wavelength ranges.
Furthermore, the cholesteric liquid crystal layer exhibits selective reflectivity for either left-handed or right-handed circularly polarized light at a specific wavelength.

In addition, in the illustrated example, the selective reflection layer 62 has a configuration including three cholesteric liquid crystal layers having different selective reflection center wavelengths, but this is not limited to this, and the selective reflection layer 62 may have one cholesteric liquid crystal layer, or two or four or more cholesteric liquid crystal layers.
For example, by adding a cholesteric liquid crystal layer that selectively reflects infrared light to a selective reflection layer, it is possible to block sunlight over an even wider wavelength range.

The thicker the selective reflection layer 62 (cholesteric liquid crystal layer), the higher the reflectance and the easier it is to block sunlight. On the other hand, if the layer is too thick, the polarization state of light that is obliquely incident on the selective reflection layer 62 changes, so it is preferable that the layer is not too thick.
From this viewpoint, the total thickness of the selective reflection layer 62 is preferably from 1 to 50 μm, more preferably from 1.5 to 40 μm, and even more preferably from 2 to 30 μm.

(Retardation Layer)
The retardation layer 64 changes the state of the incident polarized light by imparting a phase difference (optical path difference) to two orthogonal polarized light components.
In the case where the retardation layer 64 is disposed on the outer side of the vehicle and provides optical compensation, the front retardation of the retardation layer 64 may be set to a retardation that provides optical compensation.
In this case, the retardation layer 64 preferably has a front retardation of 50 to 160 nm at a wavelength of 550 nm.

In addition, when the retardation layer 64 converts linearly polarized light into circularly polarized light, the retardation layer 64 is preferably configured to provide an in-plane retardation of λ/4, and may be configured to provide an in-plane retardation of 3λ/4. The angle of the slow axis of the retardation layer 64 may be arranged so as to be oriented in such a way that it converts the incident linearly polarized light into circularly polarized light.

In this case, the retardation layer 64 preferably has an in-plane retardation in the range of 100 to 450 nm at a wavelength of 550 nm, more preferably in the range of 120 to 200 nm or 300 to 400 nm.

In the selective reflection film 16 that reflects linearly polarized light, the retardation layer 64 basically exhibits the same function as the polarization conversion layer 60 described above.
Specifically, the retardation layer 64 converts the linearly polarized light orthogonal to the image light G, i.e., the linearly polarized light emitted by the image display device 18, into circularly polarized light that is reflected by the cholesteric liquid crystal layer of the selective reflection layer 62. In other words, the polarization conversion layer 60 converts the image light G, i.e., the linearly polarized light emitted by the image display device 18, into circularly polarized light that passes through the cholesteric liquid crystal layer of the selective reflection layer 62.
In addition, the retardation layer 64 converts the circularly polarized light that has passed through the cholesteric liquid crystal layer of the selective reflection layer 62 into linearly polarized light that is the same as the image light G, and converts the circularly polarized light reflected by the cholesteric liquid crystal layer of the selective reflection layer 62 into linearly polarized light that is perpendicular to the image light G.
For example, when the image light G is P-polarized light, the retardation layer 64 converts S-polarized light into circularly polarized light reflected by the cholesteric liquid crystal layer of the selective reflection layer 62, and converts P-polarized light into circularly polarized light transmitted through the cholesteric liquid crystal layer of the selective reflection layer 62. In addition, the retardation layer 64 converts circularly polarized light reflected by the cholesteric liquid crystal layer of the selective reflection layer 62 into S-polarized light, and converts circularly polarized light transmitted through the cholesteric liquid crystal layer of the selective reflection layer 62 into P-polarized light.

The type of the retardation layer 64 is not particularly limited and can be appropriately selected depending on the purpose.
Examples of the retardation layer 64 include a stretched polycarbonate film, a stretched norbornene-based polymer film, a transparent film containing and oriented inorganic particles having birefringence such as strontium carbonate, a thin film formed by obliquely depositing an inorganic dielectric onto a support, a film in which a polymerizable liquid crystal compound is uniaxially oriented and fixed in orientation, and a film in which a liquid crystal compound is uniaxially oriented and fixed in orientation.

The thickness of the retardation layer 64 is not particularly limited, but is preferably 0.2 to 300 μm, more preferably 0.5 to 150 μm, and even more preferably 1.0 to 80 μm.

(Transparent substrate)
In the selective reflection film 16 that reflects linearly polarized light, the transparent substrate 66 supports each layer of the selective reflection film 16. The transparent substrate 66 can also be used as a substrate when the selective reflection layer 62 is formed.
The transparent substrate 66 used for forming the selective reflection layer 62 may be a temporary support that is peeled off after the selective reflection layer 62 is formed.

The material of the transparent substrate 66 is not particularly limited, and examples include polyesters such as polyethylene terephthalate (PET), polycarbonate, acrylic resins, epoxy resins, polyurethanes, polyamides, polyolefins, cellulose derivatives, and plastic films such as silicone.

There are no restrictions on the thickness of the transparent substrate 66, but it should be approximately 5.0 to 1000 μm, preferably 10 to 250 μm, and more preferably 15 to 90 μm.

Hereinafter, the projector and HUD of the present invention will be described in detail by explaining the function of the selective reflection film that reflects linearly polarized light.
In the following example, it is assumed that the image light G, that is, the image light irradiated by the image display device 18, is P-polarized light. Therefore, the selective reflection film 16 selectively reflects S-polarized light.
Moreover, each cholesteric liquid crystal layer of the selective reflection layer 62 selectively reflects right-handed circularly polarized light in a specific wavelength range and transmits left-handed circularly polarized light. Therefore, the polarization conversion layer 60 and the retardation layer 64 convert P-polarized light into left-handed circularly polarized light and S-polarized light into right-handed circularly polarized light. That is, the polarization conversion layer 60 and the retardation layer 64 convert left-handed circularly polarized light into P-polarized light and right-handed circularly polarized light into S-polarized light.
Furthermore, in this example, the selective reflection film 16 is disposed with the polarization conversion layer 60 facing the output diffraction element 14. Therefore, the image light G is incident on the selective reflection film 16 from the polarization conversion layer 60 side, and external light such as sunlight is incident on the selective reflection film 16 from the transparent substrate 66 side.

The function of the selective reflection film 16 is the same even when the transparent substrate 66 is on the output diffraction element side. The function of the selective reflection film is also the same when the polarization conversion layer 60 is changed to the retardation layer 64, i.e., when two retardation layers 64 are used, and when the retardation layer 64 is changed to the polarization conversion layer 60, i.e., when two polarization conversion layers 60 are used.
Furthermore, even when the image light is S-polarized light and/or each cholesteric liquid crystal layer of the selective reflection layer 62 selectively reflects left-handed circularly polarized light and transmits right-handed circularly polarized light, the function of the selective reflection film will be the same if the polarization conversion layer 60 and/or the phase difference layer 64 converts the image light G into circularly polarized light that transmits through the cholesteric liquid crystal layer and converts linearly polarized light perpendicular to the image light G into circularly polarized light that is reflected by the cholesteric liquid crystal layer.

As described above, the image light G (P-polarized light) emitted from the image display device 18 is diffracted by the incident diffraction element 20, enters the light guide plate 12, and propagates in the direction of the arrow Y inside the light guide plate 12. Furthermore, the image light G propagated in the direction of the arrow Y is diffracted by the intermediate diffraction element 24, and changes its propagation direction to the direction of the arrow X, i.e., to the left in FIG.
The image light G propagating in the direction of the arrow X is diffracted by the output diffraction element 14 , exits the light guide plate 12 , and enters the polarization conversion layer 60 of the selective reflection film 16 .

As described above, the image light G is P-polarized light.
Moreover, the polarization conversion layer 60 converts the image light G, that is, P-polarized light, into left-handed circularly polarized light.
Furthermore, each cholesteric liquid crystal layer of the selective reflection layer 62 selectively reflects right-handed circularly polarized light and transmits left-handed circularly polarized light.
Therefore, the P-polarized image light G that is emitted from the light guide plate 12 and enters the selective reflection film 16 is converted into left-handed circularly polarized light by the polarization conversion layer 60 , passes through the selective reflection layer 62 , and enters the retardation layer 64 .

As described above, like the polarization conversion layer 60, the retardation layer 64 also converts P-polarized light into left-handed circularly polarized light and converts left-handed circularly polarized light into P-polarized light.
Therefore, the left-handed circularly polarized image light incident on the retardation layer 64 is converted into P-polarized light by the retardation layer 64, passes through the transparent substrate 66, and exits from the selective reflection film 16, i.e., the projector, as P-polarized image light G.
The P-polarized image light G emitted from the projector is incident on the windshield glass 30, is reflected, and is observed by the user O as a projected image of the HUD.

On the other hand, when external light such as sunlight passes through the windshield glass 30 and enters the vehicle interior and then enters the selective reflection film 16 , the external light passes through the transparent substrate 66 and enters the retardation layer 64 .
Here, since the external light (sunlight) is unpolarized, it is not converted by the retardation layer 64 and passes through as it is to enter the selective reflection layer 62 .
As described above, each cholesteric liquid crystal layer constituting the selective reflection layer 62 selectively reflects right-handed circularly polarized light. Therefore, of the external light incident on the selective reflection layer 62, the right-handed circularly polarized component is reflected by the selective reflection layer 62, and the left-handed circularly polarized component is transmitted through the selective reflection layer 62.
That is, half of the external light incident on the selective reflection layer 62 is reflected by the selective reflection layer 62 .

The right-handed circularly polarized external light reflected by the selective reflection layer 62 enters the retardation layer 64 again.
As described above, the retardation layer 64 converts left-handed circularly polarized light into P-polarized light and P-polarized light into left-handed circularly polarized light. Therefore, the right-handed circularly polarized external light incident on the retardation layer 64 is converted into S-polarized light by the retardation layer 64, passes through the transparent substrate 66, and is emitted from the selective reflection film 16.
In other words, the selective reflection film 16 selectively reflects the S-polarized component of the incident external light. As a result, the selective reflection film 16 reflects half of the external light that is incident on the selective reflection layer 62.

On the other hand, the left-handed circularly polarized component of the external light that has passed through the selective reflection layer 62 enters the polarization conversion layer 60 which, like the phase difference layer 64, converts left-handed circularly polarized light to P-polarized light and P-polarized light to left-handed circularly polarized light, is converted to P-polarized light, and is emitted from the selective reflection film 16, with a portion of it entering the exit diffraction element 14.
A part of this P-polarized external light is diffracted by the output diffraction element 14 as described above, and enters the light guide plate 12 .

As described above, the external light incident on the light guide plate 12 in this manner can cause glare in the image, disturbing light, and thermal damage inside the projector.
However, as described above, in the present invention, half of the external light is reflected by the selective reflection film 16, and therefore only half of the external light passes through the selective reflection film 16. Therefore, according to the present invention, it is possible to significantly reduce glare on an image, disturbing light, and thermal damage inside the projector caused by external light incident on the light guide plate 12.
In the projector of the present invention, the selective reflection film is a reflective film. Therefore, compared with an absorptive film, heat is not trapped in the selective reflection film, and heat damage to the selective reflection film can be reduced. In addition, the reflective film transmits almost 100% of the non-reflected light, i.e., the image light G, so the selective reflection film can prevent the HUD display image from becoming dark.

Incidentally, in the case of a HUD that displays a projected image using P-polarized light, a reflective polarizer that selectively reflects P-polarized light may be provided on the windshield glass 30. An example of a selective reflective polarizer is a reflective circular polarizer such as a combination of a retardation plate (λ/4 plate) and a cholesteric liquid crystal layer.
External light that passes through such a reflective polarizer that selectively reflects P-polarized light usually becomes S-polarized light. That is, in this case, the external light that is S-polarized is incident on the selective reflection film 16.
As described above, the external light incident on the selective reflection film 16 is incident on the retardation layer 64. The retardation layer 64 converts P-polarized light into left-handed circularly polarized light and converts left-handed circularly polarized light into P-polarized light. The cholesteric liquid crystal layer constituting the selective reflection layer 62 selectively reflects right-handed circularly polarized light.
Therefore, the S-polarized external light incident on the retardation layer 64 is converted into right-handed circularly polarized light, reflected by the cholesteric liquid crystal layer of the selective reflection layer 62, re-incident on the retardation layer 64, converted into S-polarized light, and emitted from the selective reflection film 16. That is, the selective reflection film selectively reflects the S-polarized external light.

In the above example, the image display device 18 emits P-polarized image light G, and the image light G emitted from the selective reflection film 16 that reflects linearly polarized light, i.e., the projection light of the HUD, is also P-polarized light, and the selective reflection film 16 reflects S-polarized light, but the present invention is not limited to this.
For example, in the present invention, the image display device 18 may emit S-polarized image light G, and the image light G emitted from the selective reflection film 16 that reflects linearly polarized light, i.e., the projection light of the HUD, may also be S-polarized light, and the selective reflection film 16 may reflect P-polarized light.

Such a projector of the present invention can be suitably used in HUDs mounted on various types of transportation, such as automobiles, trains, airplanes, and ships.
The projector and HUD of the present invention can be used for a variety of purposes, in addition to such transportation.

The projector and head mounted display system (HUD) of the present invention have been described in detail above, but the present invention is not limited to the above-described embodiments, and various improvements and modifications may of course be made without departing from the spirit of the present invention.

The present invention will be explained in more detail below with reference to examples. The materials, reagents, amounts of substances and their ratios, operations, etc. shown in the following examples, comparative examples, and preparation examples can be modified as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention is not limited to the following examples and reference examples.

<Preparation of selective reflection film that reflects linearly polarized light>
A 40 μm thick cellulose acylate film was produced by the same production method as in Example 20 of International Publication No. 2014/112575. In addition, UV-531 manufactured by Teisei Kako Co., Ltd. was added to this cellulose acylate film as an ultraviolet absorbent. The amount added was 3 phr (per hundred resin).
The prepared cellulose acylate film was passed through a dielectric heating roll at 60° C. to raise the film surface temperature to 40° C. Then, an alkaline solution having the composition shown below was applied to one side of the film with a coating amount of 14 mL/ ^m2 using a bar coater, and the film was allowed to remain under a steam type far-infrared heater (manufactured by Noritake Co., Ltd.) heated to 110° C. for 10 seconds.
Next, 3 mL/m ² of pure water was applied using the same bar coater.
Next, washing with water using a fountain coater and draining with an air knife were repeated three times, and then the film was allowed to stay in a drying zone at 70° C. for 5 seconds and dried to prepare a saponified cellulose acylate film.
The in-plane retardation of the saponified cellulose acylate film was measured by AxoScan and found to be 1 nm.

- Composition of alkaline solution -
· Potassium hydroxide 4.7 parts by mass · Water 15.7 parts by mass · Isopropanol 64.8 parts by mass · Surfactant ( _{C16H33O ( CH2CH2O} ) _{10H )} 1.0 part by mass · Propylene glycol 14.9 parts by mass

On the saponified surface of the saponified cellulose acylate film (transparent support), a coating solution for forming an alignment layer having the composition shown below was applied by a wire bar coater at 24 mL/m ² , and dried with hot air at 100° C. for 120 seconds.

--Composition of coating solution for forming alignment film--
Modified polyvinyl alcohol shown below: 28 parts by weight Citric acid ester (AS3, manufactured by Sankyo Chemical Co., Ltd.) 1.2 parts by weight Photoinitiator (Irgacure 2959, manufactured by BASF) 0.84 parts by weight Glutaraldehyde 2.8 parts by weight Water 699 parts by weight Methanol 226 parts by weight

(Modified polyvinyl alcohol)

The cellulose acylate film having the alignment layer formed thereon was used as a support (transparent substrate).
One surface of the support facing the alignment film was subjected to a rubbing treatment (rayon cloth, pressure: 0.1 kgf (0.98 N), rotation speed: 1000 rpm (revolutions per minute), conveying speed: 10 m/min, number of reciprocations: 1) in a direction rotated 45° counterclockwise from the long side direction of the support.

The following coating liquid for forming a retardation layer was applied to the rubbed surface of the alignment film on the support using a wire bar, and then dried to obtain a coating film of the coating liquid for forming a retardation layer.
--Composition of Coating Solution for Forming Retardation Layer--
Mixture 1: 100 parts by weight Fluorine-based horizontal alignment agent 1 (alignment control agent 1): 0.05 parts by weight Fluorine-based horizontal alignment agent 2 (alignment control agent 2): 0.01 parts by weight Polymerization initiator IRGACURE OXE01 (manufactured by BASF)
1.0 part by mass of solvent (methyl ethyl ketone) - amount to give a solute concentration of 20% by mass

The support with the coating film thus obtained was then placed on a hot plate at 50° C., and the coating film was irradiated with ultraviolet light for 6 seconds using an electrodeless lamp “D bulb” (60 mW/ ^cm2 ) manufactured by Fusion UV Systems in an environment with an oxygen concentration of 1000 ppm or less, thereby fixing the liquid crystal phase. In this way, a retardation layer with a thickness adjusted to obtain the desired in-plane retardation was obtained.
The in-plane retardation of the produced retardation layer was measured by AxoScan and found to be 138 nm.

The following cholesteric liquid crystal layer forming coating solution (B1) was applied to the surface of the obtained retardation layer at room temperature using a wire bar so that the thickness of the dried film after drying would be 3.0 μm, thereby obtaining a coating film.
The coating film was dried at room temperature for 30 seconds, and then heated for 2 minutes in an atmosphere at 85° C. Thereafter, in an environment with an oxygen concentration of 1000 ppm or less, the coating film was irradiated with ultraviolet light at an output of 60% for 6 to 12 seconds using a D bulb (90 mW/ ^cm2 lamp) manufactured by Fusion Co., Ltd. at 60° C. to fix the cholesteric liquid crystal phase, thereby obtaining a cholesteric liquid crystal layer B1 having a thickness of 2.5 μm.

--Coating solution for forming cholesteric liquid crystal layer (B1)--
The following components were mixed to prepare a coating solution (B1) for forming a cholesteric liquid crystal layer having the following composition: The amount of right-handed chiral dopant LC756 used was adjusted so that the selective reflection center wavelength of the resulting cholesteric liquid crystal layer was 450 nm.
Mixture 1: 100 parts by mass Fluorine-based horizontal alignment agent 1 (alignment control agent 1): 0.05 parts by mass Fluorine-based horizontal alignment agent 2 (alignment control agent 2): 0.02 parts by mass Right-handed chiral agent LC756 (manufactured by BASF)
Polymerization initiator IRGACURE OXE01 (manufactured by BASF)
1.0 part by mass of solvent (methyl ethyl ketone) - amount to give a solute concentration of 30% by mass

Next, the same process was repeated using the following cholesteric liquid crystal layer forming coating liquid (G1) on the surface of the obtained cholesteric liquid crystal layer B1 to form a cholesteric liquid crystal layer G1 having a thickness of 3.0 μm.

--Cholesteric liquid crystal layer forming coating solution (G1)--
A coating solution (G1) for forming a cholesteric liquid crystal layer was prepared by mixing the same components as those in the coating solution (B1) for forming a cholesteric liquid crystal layer, except that the amount of the right-handed chiral dopant LC756 used in the coating solution (B1) for forming a cholesteric liquid crystal layer was adjusted so that the selective reflection center wavelength of the resulting cholesteric liquid crystal layer was 530 nm.

Next, the same process was repeated using the following cholesteric liquid crystal layer forming coating liquid (R1) on the surface of the obtained cholesteric liquid crystal layer G1 to form a cholesteric liquid crystal layer R1 having a thickness of 3.5 μm.

--Cholesteric liquid crystal layer forming coating solution (R1)--
A coating solution (R1) for forming a cholesteric liquid crystal layer was prepared by mixing the same components as those in the coating solution (B1) for forming a cholesteric liquid crystal layer, except that the amount of the right-handed chiral dopant LC756 used in the coating solution (B1) for forming a cholesteric liquid crystal layer was adjusted so that the selective reflection center wavelength of the resulting cholesteric liquid crystal layer was 630 nm.

In this way, a selective reflection layer with three cholesteric liquid crystal layers was obtained on top of the retardation layer. When the reflection spectrum of S-polarized light from the selective reflection layer was measured using a spectrophotometer (V-670, manufactured by JASCO Corporation), a reflection spectrum of 90% was obtained at wavelengths of 450 nm, 530 nm, and 630 nm.

Next, the coating liquid for forming a polarization conversion layer was further applied to the surface of the obtained cholesteric liquid crystal layer so as to have a target film thickness, thereby forming a polarization conversion layer, and thus the selective reflection film of Example 1 was produced.
The polarization conversion layer was formed in the same manner as the above-mentioned cholesteric liquid crystal layer.
In addition, the pitch number, film thickness, and selective reflection center wavelength of the polarization conversion layer were adjusted so that the polarization conversion layer had the same polarization conversion characteristics as the previously formed retardation layer (λ/4 wavelength plate) when light was incident perpendicularly to the substrate.

--Polarization conversion layer forming coating liquid--
The following components were mixed to prepare a coating solution for forming a polarization conversion layer having the following composition: The amount of right-handed chiral agent LC756 used was adjusted so as to obtain a polarization conversion layer with the above characteristics.
Mixture 1: 100 parts by mass Fluorine-based horizontal alignment agent 1 (alignment control agent 1): 0.05 parts by mass Fluorine-based horizontal alignment agent 2 (alignment control agent 2): 0.02 parts by mass Right-handed chiral agent LC756 (manufactured by BASF)
Polymerization initiator IRGACURE OXE01 (manufactured by BASF)
1.0 part by mass of solvent (methyl ethyl ketone) - amount to give a solute concentration of 20% by mass

As described above, the selective reflection film of Example 1 that reflects linearly polarized light was prepared.
The selective reflection film of Example 2 that reflects linearly polarized light was produced by preparing the retardation layer and the selective reflection layer in the same manner as in the examples, and then preparing a retardation layer in which the angle of the rubbing treatment on the above-mentioned alignment film was rotated 45° clockwise, and laminating the retardation layer on the selective reflection layer with an OCA.
The selective reflection film of Example 3 that reflects linearly polarized light was prepared in the same manner as in Example 2, except for the selective reflection layer. The selective reflection layer was prepared by similarly laminating a cholesteric liquid crystal layer having a selective reflection center wavelength of 850 nm on three cholesteric liquid crystal layers (B1, G1, R1).
The selective reflection film of Example 4 that reflects linearly polarized light was produced by changing only the conditions of the retardation layer of Example 3. The retardation layer in contact with the cholesteric liquid crystal layer was produced by rotating the rubbing angle of the alignment film by 45° clockwise, and the retardation layer that was bonded to the cholesteric liquid crystal layer via the OCA was produced by rotating the rubbing angle of the alignment film by 45° counterclockwise. In this example, the image light G emitted from the selective reflection film, i.e., the projection light in the HUD, becomes S-polarized light.
In Comparative Example 1, a selective reflection film that reflects linearly polarized light was not produced.
In Comparative Example 2, only a cholesteric liquid crystal layer was prepared without forming the retardation layer and the polarization conversion layer from Example 1. Table 1 shows the structure of the selective reflection film that reflects linearly polarized light.

[Example]
A light guide plate having an entrance diffraction element, an intermediate diffraction element and an exit diffraction element on one surface as shown in FIG. 2 was prepared.
The light guide plate was made of acrylic resin with a thickness of 10 mm and a size of 200×200 mm.
All the diffraction elements were holographic diffraction elements, with the incident diffraction element being 10 x 5 mm, the intermediate diffraction element being 185 x 10 mm, and the exit diffraction element being 185 x 175 mm, and were arranged according to Fig. 2. The diffraction elements were fixed using an ultraviolet-curing adhesive (UVX-5457, manufactured by Toa Gosei Co., Ltd.). The diffraction characteristics of each diffraction element were as described above.

The light guide plate provided with the diffraction element was placed on the dashboard of a car.
In addition, an image display device was provided so that an image was incident on the light guide plate from the incident diffraction element. A laser scanning projection unit was used as the image display device. The linearly polarized light emitted by the image display device is shown in Table 2 below.
Six types of projectors were produced by providing the selective reflection films for reflecting linearly polarized light of Examples 1 to 4 and Comparative Example 2 on the output diffraction element of this light guide plate. In addition, as Comparative Example 1, a projector having nothing on the output diffraction element was also produced.
The projector was installed in an actual vehicle, and a head-up display system was constructed to project an image onto the windshield glass. The windshield glasses of Examples 1 to 3 and Comparative Examples 1 and 2 were made of windshield glass that reflects P-polarized light, as described in International Publication No. 2022/123946. The windshield glass of Example 4 was made of wedge-shaped glass.

<Evaluation of image glare caused by sunlight>
A white image was displayed on the image display device outdoors under clear skies, and the image projected from the windshield (projected image) was observed and evaluated according to the following criteria.
Evaluation Criteria A: The image had little sunlight noise and was clearly visible.
B: There is sunlight noise in the image, making it difficult to see at many moments.
C: The image has strong sunlight noise and is difficult to see.
The results are shown in Table 2 below.

<Evaluation of temperature rise inside the projector due to sunlight>
A thermocouple was placed in front of the image display device outdoors under a clear sky in midsummer, and the temperature was measured and evaluated according to the following criteria.
Evaluation Criteria A: The temperature at the front of the image display device is 60° C. or less.
B: The temperature at the front of the image display device is 60°C or higher and 80°C or lower.
C: The temperature at the front of the image display device rose to 80° C. or higher (a temperature at which the internal members were damaged).
The results are shown in Table 2 below.

As shown in Table 2, it was confirmed that the projectors of Examples 1 to 4 reduced glare and illusion of images caused by sunlight, and reduced the internal temperature of the projector, compared to the projector of the comparative example.
From the above results, the effects of the present invention are clear.

It can be ideally used in HUDs mounted on vehicles, etc.

10.100,130 HUD (Head-up Display System)
12, 132 Light guide plate 14, 136 Exit diffraction element 16 Selective reflection film 18, 129 Image display device 20, 134 Incident diffraction element 24 Intermediate diffraction element 30, 126 Windshield 34 Mirror 60 Polarization conversion layer 62 Selective reflection layer 64 Retardation layer 66 Transparent substrate 68B Blue light selective reflection layer 68G Green light selective reflection layer 68R Red light selective reflection layer 112 Image display unit 116 Reflection mirror 118 Concave mirror 120 Dashboard 124 Transmissive window 131 Projection lens 138 Optical filter O User

Claims (9)

A projector for a head-up display,
an image display device that emits image light; and a light guide plate that guides the image light emitted by the image display device;
an emission element that emits the image light guided through the light guide plate from the light guide plate;
a selective reflection film that is provided on the output side of the output element and reflects linearly polarized light. The projector of claim 1, wherein the output element is a diffraction element. The projector of claim 1, wherein the selective reflection film that reflects linearly polarized light reflects visible light. The projector of claim 1, wherein the selective reflection film that reflects linearly polarized light reflects visible light and infrared light. The projector according to claim 1, wherein the image light is P-polarized light, and the selective reflection film that reflects the linearly polarized light is a film that reflects S-polarized light. The projector according to claim 1, wherein the image light is S-polarized light, and the selective reflection film that reflects the linearly polarized light is a film that reflects P-polarized light. The projector of claim 1, wherein the selective reflection film that reflects the linearly polarized light has a retardation layer, a cholesteric liquid crystal layer in which a cholesteric liquid crystal phase is fixed, and a retardation layer, in this order. The projector according to claim 1, wherein the selective reflection film that reflects the linearly polarized light has, in this order, a retardation layer, a cholesteric liquid crystal layer in which a cholesteric liquid crystal phase is fixed, and a polarization conversion layer. A head-up display system comprising the projector according to any one of claims 1 to 8 and a windshield glass onto which the projection light emitted from the projector is irradiated.

Images