Patent Description:
Patent Literature (PTL) <NUM> discloses a display device including a first reflector whose reflective surface has a convex profile and a second reflector whose reflective surface has a concave profile. <CIT> relates to a head-up display having at least two mirrors that can be moved into the light rays. The curvature of the mirrors is adapted to different windshield curvatures and the mirror can be chosen such that the curvature of the mirror is adapted to the windshield curvature. Furthermore, the distance between two mirrors can be adapted such that the distance of the virtual image behind the windshield can be changed. <CIT> relates to a projection optical system using concave mirrors which can be installed in a motor vehicle. <CIT> relates to a head-up display in a vehicle wherein the head-up display uses mirrors comprising an array of alternating convex and concave areas to reduce external light shining through the windshield being reflected back from the projector towards the windshield.

PTL <NUM>: International Publication <CIT>.

With a so-called telephoto type display device including a reflector whose reflective surface has a convex profile and a reflector whose reflective surface has a concave profile, when the power (refractive power) of the windshield is high, an increased screen size leads to a difficulty in reducing interference between a ray of light and structural components of a head-up display.

A head-up display according to the present disclosure is a head-up display which projects an image on a windshield and forms a virtual image visually recognizable by a viewer. The head-up display includes a display element which displays the image; and a projection optical system which guides the image displayed by the display element to the windshield and forms the virtual image. The projection optical system includes at least two mirrors each having a reflective surface with a concave profile near a center of the reflective surface, wherein the head-up display satisfies: <MAT> where LM denotes a distance between a first mirror and a second mirror in a central optical path of the projection optical system from the display element to the windshield, and LD denotes a distance from the display element to the first mirror in the central optical path of the projection optical system from the display element to the windshield,given that the first mirror is one of the two mirrors which is located closer to the display element in an optical path from the display element to the windshield, and the second mirror is another one of the two mirrors which is located closer to the windshield in the optical path from the display element to the windshield.

According to the present disclosure, a head-up display having a large-sized display can be provided while reducing interference between a ray of light and structural components of the head-up display.

Hereinafter, embodiments will be described in detail with reference to the drawings as necessary. However, unnecessarily detailed description may be omitted. For example, detailed descriptions of well-known aspects or repetitive descriptions of essentially the same configurations may be omitted. This is to avoid unnecessary redundancy and make the following description easier for a person skilled in the art to understand.

Note that the accompanying drawings and the following description are provided not to limit the subject matter recited in the claims but to aid a person skilled in the art to adequately understand the present disclosure.

A head-up display according to Embodiment <NUM> will be described with reference to the drawings.

Note that the specific numerical values related to the head-up display according to Embodiment <NUM> will be described later in Numerical Examples <NUM> through <NUM>.

<FIG> is a schematic diagram illustrating a cross section of a vehicle including the head-up display according to Embodiment <NUM>.

Head-up display <NUM> which projects an image is provided inside dashboard <NUM> of vehicle <NUM>. Head-up display <NUM> projects an image to windshield <NUM> that is a light-transmissive component which is provided in vehicle <NUM> and on which an image is to be projected. This way, virtual image <NUM> of the image projected by head-up display <NUM> is formed on a side of windshield <NUM> opposite the side thereof where viewer <NUM> is present. Viewer <NUM> can visually recognize, via windshield <NUM>, virtual image <NUM> of the image projected by head-up display <NUM>. Two-dot chain line illustrated in <FIG> represents central optical path L of the image projected. Throughout the description, central optical path L is illustrated with a two-dot chain line.

<FIG> is a schematic diagram of head-up display <NUM> according to Embodiment <NUM>.

Head-up display <NUM> projects an image to windshield <NUM> which is a component on which an image is projected. Head-up display <NUM> projects an image to windshield <NUM> to form virtual image <NUM> on a side of windshield <NUM> opposite the side thereof where viewer <NUM> is present. Head-up display <NUM> includes: display element <NUM> which is a projection component; and a projection optical system which guides an image displayed by display element <NUM> to windshield <NUM> and forms virtual image <NUM>.

The projection optical system includes first mirror M1 and second mirror M2. Second mirror M2 is disposed approximately perpendicularly downward with respect to windshield <NUM>, and guides a ray of light from display element <NUM> to windshield <NUM> via first mirror M1. The reflective surface of first mirror M1 has a concave profile. The reflective surface of second mirror M2 has a concave profile. This allows viewer <NUM> to visually recognize virtual image <NUM> of the image through windshield <NUM>. Virtual image <NUM> of the image projected on windshield <NUM> can be visually recognized by viewer <NUM> from eye box <NUM> that is a viewing region supposed in advance. Note that plural optical elements such as a lens element and a reflector may be disposed in the optical path of a ray of light from display element <NUM> to second mirror M2.

<FIG> illustrates an Xc-Yc cross section of windshield <NUM> according to Embodiments <NUM> to <NUM>, and illustrates local curvature Rx of windshield <NUM> in the right-left direction viewed from the driver. <FIG> also illustrates a local coordinate system (xl, yl, zl coordinates) of windshield <NUM>.

<FIG> illustrates an Xc-Zc cross section of windshield <NUM> according to Embodiments <NUM> to <NUM>, and illustrates local curvature Ry of windshield <NUM> in the up-down direction viewed from the driver. <FIG> also illustrates a local coordinate system (xl, yl, zl coordinates) of windshield <NUM>.

<FIG> illustrates inclined angle θz of windshield <NUM> according to Embodiments <NUM> to <NUM>. Ol is the origin of a definition expression that defines the shape of the reflective surface of the windshield; N1 is the normal vector to the reflective surface at origin Ol; N2 is the normal vector to the reflective surface at a given point; and θz is the inclined angle formed by normal vector N1 and normal vector N2.

<FIG> illustrates a coordinate system (Xm1, Ym1, Zm1 coordinates) of first mirror M1 and origin Om1 of the definition expression of first mirror M1. Zm1, which is the Z axis of the coordinate system of first mirror M1, is defined in the direction of the normal to the reflective surface passing through origin Om1 of the definition expression of first mirror M1. Xm1 and Ym1 are components orthogonal to Zm1. Xm1 is a direction parallel to the longer side of first mirror M1 illustrated in <FIG>, and is generally the right-left direction viewed from viewer <NUM> such as the driver. Ym1 is a direction parallel to the shorter side of first mirror M1 illustrated in <FIG>, and is generally the up-down direction viewed from viewer <NUM>. <FIG> also illustrates: local curvature RxM1 of first mirror M1 according to Embodiments <NUM> to <NUM> in the X direction defined in a plane parallel to the Xm1-Zm1 plane; and local curvature RyM1 of first mirror M1 in the Y direction defined in a plane parallel to the Ym1-Zm1 plane.

<FIG> illustrates a coordinate system (Xm2, Ym2, Zm2 coordinates) of second mirror M2 and origin Om2 of the definition expression of second mirror M2. Zm2, which is the Z axis of the coordinate system of second mirror M2, is defined in the direction of the normal to the reflective surface passing through origin Om2 of the definition expression of second mirror M2. Xm2 and Ym2 are components orthogonal to Zm2. Xm2 is a direction parallel to the longer side of second mirror M2 illustrated in <FIG>, and is generally the right-left direction viewed from viewer <NUM>. Ym2 is a direction parallel to the shorter side of second mirror M2 illustrated in <FIG>, and is generally the up-down direction viewed from viewer <NUM>. <FIG> also illustrates: local curvature RxM2 of second mirror M2 according to Embodiments <NUM> to <NUM>, in the X direction defined in a plane parallel to the Xm2-Zm2 plane; and local curvature RyM2 of second mirror M2 in the Y direction defined in a plane parallel to the Ym2-Zm2 plane.

<FIG> is a schematic diagram illustrating a coordinate system of the display element according to Embodiment <NUM>.

Embodiment <NUM> has described a liquid crystal display (LCD) as an example of display element <NUM> which is a projection component. The display element may be a display device such as an organic light-emitting diode (electroluminescent display), a fluorescent display device (seven-segment display), or a plasma display. Moreover, the display element may be a projector or a scanning laser. Thus, the display element is not limited to the LCD.

Head-up display <NUM> projects an image to windshield <NUM> which is a component on which an image is projected. Head-up display <NUM> projects an image to windshield <NUM> to form virtual image <NUM> on a side of windshield <NUM> opposite the side thereof where viewer <NUM> is present. Head-up display <NUM> includes: display element <NUM> which is a projection component that projects an image; and a projection optical system which guides the image to the component on which an image is projected and forms virtual image <NUM>. The projection optical system includes at least two mirrors each having a reflective surface with a concave profile. Windshield <NUM> has a reflective surface which satisfies the following condition (<NUM>) or (<NUM>) or (<NUM>):
<MAT>.

Below the lower limit of condition (<NUM>) or (<NUM>), the power (refractive power) of windshield <NUM> weakens and the optical path length allowed for the projection optical system increases. Thus, in order to achieve a decrease in the size of the head-up display and an increase in the virtual image size, it is better to have a so-called telephoto configuration in which the reflective surface of first mirror M1 has a convex profile and the reflective surface of second mirror M2 has a concave profile. Above the upper limit of condition (<NUM>) or (<NUM>), the power of windshield <NUM> increases and the optical path length allowed for the projection optical system decreases. Thus, unless the configuration is a so-called retrofocus configuration in which the reflective surface of first mirror M1 has a concave profile and the reflective surface of second mirror M2 has a convex profile, interference occurs between a ray of light and structural components of the head-up display such as the mirrors, thereby not allowing for a sufficient eye box size (size of eye box <NUM>).

That is to say, it is possible to provide a head-up display smaller in size and larger in display size, while causing no light ray interference.

Below the lower limit of condition (<NUM>), the power (refractive power) of windshield <NUM> weakens and the optical path length allowed for the projection optical system increases. Thus, in order to achieve a decrease in the size of the head-up display, it is better to have a so-called telephoto configuration in which the reflective surface of first mirror M1 has a convex profile and the reflective surface of second mirror M2 has a concave profile. Above the upper limit of condition (<NUM>), the power of windshield <NUM> becomes too strong and the optical path length allowed for the projection optical system decreases. Thus, unless the configuration is a so-called retrofocus configuration in which, for example, the reflective surface of first mirror M1 has a concave profile and the reflective surface of second mirror M2 has a convex profile, interference occurs between a ray of light and structural components of the head-up display, thereby not allowing for a sufficient eye box size.

Head-up display <NUM> may satisfy the following condition (<NUM>):
<MAT>.

Below <NUM>, which is the lower limit of condition (<NUM>), the distance between display element <NUM> and first mirror M1 increases, resulting in an increase in the size of the head-up display. Above <NUM>, which is the upper limit of condition (<NUM>), the distance between display element <NUM> and first mirror M1 becomes too short, resulting in interference between display element <NUM> and a ray of light.

That is to say, it is possible to achieve a decrease in the size of head-up display <NUM> by satisfying condition (<NUM>).

Furthermore, head-up display <NUM> may satisfy the following condition (<NUM>):
<MAT>.

The lower limit of condition (<NUM>) is <NUM> since LD and LM are non-zero finite values. Above <NUM>, which is the upper limit of condition (<NUM>), the distance between display element <NUM> and first mirror <NUM> and the distance between first mirror M1 and second mirror M2 both increase, resulting in an increase in the size of the head-up display.

Head-up display <NUM> may satisfy the following condition (<NUM>) in the case where each of windshield <NUM>, first mirror M1, and second mirror M2 has a positive curvature radius when having a reflective surface with a concave profile and a negative curvature radius when having a reflective surface with a convex profile.

Above <NUM>, which is the upper limit of condition (<NUM>), aberration that occurs at windshield <NUM> cannot be corrected by first mirror M1 and second mirror M2, and various aberrations, particularly curvature of field, astigmatism, and distortion, increase, resulting in deterioration in display quality; for example, the virtual image does not look sharp or becomes distorted. Below the lower limit of <NUM>, the powers of first mirror M1 and second mirror M2 decrease and the optical path length increases, resulting in an increase in the size of the head-up display.

Head-up display <NUM> may satisfy the following condition (<NUM>)':
<MAT>.

By satisfying the above condition (<NUM>) or condition (<NUM>)' not only at at least one of the reflection points of a ray of light passing through first regions A1, second regions A2, and third regions A3 but in all the areas of first regions A1, second regions A2, and third regions A3, the above-described advantageous effect can be further achieved.

First regions A1 to third regions A3 are approximate rectangular regions each having, as opposite angles, one of the four corners and a point at <NUM>% from the center; however, first regions A1 to third regions A3 may be approximate rectangular regions each having, as opposite angles, one of the four corners and a point at <NUM>% from the center. With this, the above-described advantageous effect can be achieved by satisfying above condition (<NUM>) or condition (<NUM>)' at at least one of the reflection points of a ray of light passing through first regions A1, second regions A2, and third regions A3.

The above-described advantageous effect can be further achieved by satisfying above condition (<NUM>) or condition (<NUM>)' in all the areas in third regions A3 of windshield <NUM> on which an image is projected, all the areas in first regions A1 of first mirror M1 where a ray of light passes through, and all the areas in second regions A2 of second mirror M2 where a ray of light passes through.

Head-up display <NUM> may satisfy the following condition (<NUM>) in the case where each of windshield <NUM>, first mirror M1, and second mirror M2 has a positive curvature radius when having a reflective surface with a concave profile and has a negative curvature radius when having a reflective surface with a convex profile.

Above <NUM>, which is the upper limit of condition (<NUM>), aberration that occurs at windshield <NUM> cannot be corrected by first mirror M1 and second mirror M2, and various aberrations, particularly curvature of field, astigmatism, and distortion, increase, resulting in deterioration in display quality; for example, the virtual image does not look sharp or becomes distorted. Below the lower limit of -<NUM>, aberration that occurs at windshield <NUM> cannot be corrected by first mirror M1 and second mirror M2, and various aberrations such as curvature of field increase, resulting in deterioration in display quality; for example, the virtual image does not look sharp or becomes distorted.

First regions A1 to third regions A3 are approximate rectangular regions each having, as a diagonal, a line segment connecting one of the four corners and a point at <NUM>% from the center; however, first regions A1 to third regions A3 may be approximate rectangular regions each having, as a diagonal, a line segment connecting one of the four corners and a point at <NUM>% from the center. By satisfying the above condition (<NUM>) or condition (<NUM>)' at at least one of the reflection points of a ray of light passing through first regions A1, second regions A2, and third regions A3, the above-described advantageous effect can be further achieved.

Embodiment <NUM> has described first mirror M1 and second mirror M2 as each having a reflective surface with a concave profile as one example. First mirror M1 and second mirror M2 may have any reflective surface so long as the reflective surface has an approximately concave profile near the center. Accordingly, the reflective surfaces of first mirror M1 and second mirror M2 are not limited to reflective surfaces entirely having a concave profile. However, the optical path length allowed for the projection optical system can be increased when the reflective surfaces of first mirror M1 and second mirror M2 have a concave profile. This makes it possible to reduce interference between a ray of light and structural components of the head-up display and to secure a sufficient eye box size.

An end portion of the reflective surface of first mirror M1 or second mirror M2 may have a locally convex profile. When an end portion of the reflective surface of first mirror M1 or second mirror M2 has a locally convex profile, it is possible to correct various aberrations of windshield <NUM> that occur in the vicinity of the eye box, in particular, curvature of field, astigmatism, and distortion.

The following describes specific numerical examples of the head-up display according to the present disclosure. Note that the following numerical examples use mm as the unit of length and degree as the unit of angle in the tables below. A free-form surface is defined by the following equations:
<MAT><MAT>.

Here, z is the amount of sag at the position (x, y) from an axis which defines a plane; r is a curvature radius at the origin of the axis defining the plane; c is a curvature at the origin of the axis defining the plane; k is the conic constant; m and n are integers satisfying Equation <NUM>; and Cj is a coefficient of a monomial xmyn.

In each numerical example, the coordinate origin serving as the reference is the center of display image <NUM> of display element <NUM>, and defines the X axis, Y axis, and Z axis as illustrated in <FIG>.

Moreover, in decentering data in each numerical example, ADE is the amount of rotation from the Z-axis direction to the Y-axis direction about the X axis, BDE is the amount of rotation from the X-axis direction to the Z-axis direction about the Y axis, and CDE is the amount of rotation from the X-axis direction to the Y-axis direction about the Z axis.

A projection optical system according to Numerical Example <NUM> is an example of the projection optical system. Table <NUM> shows configuration data of the projection optical system according to Numerical Example <NUM>, and Table <NUM> shows coefficients of polynomial free-form surfaces.

Table <NUM> shows examples of the image display size, the virtual image size, virtual image distance V which is the distance from the center of the viewing region of the viewer to the center of the virtual image, and the eye box size (EBx and EBy). Table <NUM> shows values of conditions (<NUM>) to (<NUM>).

Conditions (<NUM>) and (<NUM>) are satisfied in all the regions of each optical element, namely, windshield <NUM>, first mirror M1, and second mirror M2, rather than being satisfied only when, in each optical element, the approximately rectangular regions each having, as opposite angles, one of four corners and a point at <NUM>% of a line segment from the center of the optical element to the one of the four corners are located near the four corners.

Claim 1:
A head-up display (<NUM>) configured to project an image on a windshield (<NUM>) and to form a virtual image (<NUM>) visually recognizable by a viewer (<NUM>), the head-up display (<NUM>) comprising:
a display element (<NUM>) which displays the image; and
a projection optical system configured to guide the image displayed by the display element (<NUM>) to the windshield (<NUM>) and to form the virtual image (<NUM>), wherein
the projection optical system includes two mirrors (M1,M2) each having a reflective surface with a concave profile near a center of the reflective surface,
characterized in that
the head-up display (<NUM>) satisfies: <MAT>
where LM denotes a distance between a first mirror (M1) and a second mirror (M2) in a central optical path L of the projection optical system from the display element (<NUM>) to the windshield (<NUM>), and
LD denotes a distance from the display element (<NUM>) to the first mirror (M1) in the central optical path L of the projection optical system from the display element (<NUM>) to the windshield (<NUM>),
given that the first mirror (M1) is one of the two mirrors (M1,M2) which is located closer to the display element (<NUM>) in an optical path from the display element (<NUM>) to the windshield (<NUM>), and the second mirror (M2) is another one of the two mirrors (M1,M2) which is located closer to the windshield (<NUM>) in the optical path from the display element (<NUM>) to the windshield (<NUM>).