Optical system and head-up display system equipped therewith

Abstract

In the present invention, a luminous flux incident on an optical waveguide is reproduced in a first direction corresponding to the horizontal direction of an image, or a second direction corresponding to the vertical direction, by diffraction due to a diffractive structure of an expansion region. The following relational expressions are satisfied, where the viewing angle of the image is ±F degrees, the angle between the diffractive structure and the travel direction of the luminous flux is α degrees, the inclination angle of the diffractive structure is β degrees, the angle between a Z-axis and the central light beam of the luminous flux incident on the expansion region is θA degrees, the angle between the Z-axis and the central light beam of a luminous flux diffracted by the expansion region and emitted is θB degrees, T [µm] is the thickness in the Z-direction of the diffractive structure, Ts [µm] is the smaller of the intervals between the diffractive structure and the front surface of the optical waveguide and the back surface of the optical waveguide, and L [µm] is the coherence length in the optical waveguide of the luminous flux diffracted by the expansion region and emitted. |θA – θB| < |F| / 2, |β| × 2 × cos(α) ≤ |F| - |θA – θB|, and Ts > L / 2.

Description

Technical Field

[0001] The present disclosure relates to an optical system used for image display and a head-up display system including the same.

Background Art

[0002] Conventionally, a vehicle information projection system for performing augmented reality (AR) display using a head-up display device has been disclosed. The head-up display device projects light representing a virtual image onto, for example, the windshield of a vehicle, allowing a driver to visually recognize the virtual image together with the actual scene outside the vehicle.

[0003] As a device for displaying a virtual image, Patent Document 1 describes an optical element including a waveguide (light guide) for expanding an exit pupil in two directions. The optical element can expand the exit pupil using a diffractive optical element. Further, Document 2 describes a head-mounted display that performs augmented reality (AR) display using a volume hologram diffraction grating.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, for example, when realizing a pupil-expanding hologram used in a head-mounted display in a head-up display, if a transmissive diffraction structure is used, for example, the diffraction efficiency in the diffraction structure is low.

[0006] An object of the present disclosure is to provide an optical system and a head-up display system with improved diffraction efficiency. [Means for solving the problem]

[0007] The optical system of this disclosure comprises a display unit that emits a light beam that is visible to an observer as an image, and a light guide that replicates the light beam. The light guide has an incident surface to which the light beam from the display unit is incident, and an exit surface from which the light beam is emitted. The central ray of the light beam emitted from the display unit is incident on the incident surface of the light guide. The light beam incident on the incident surface of the light guide has its direction of propagation changed by diffraction due to the diffraction structure of the coupling region within the light guide. The light beam whose direction of propagation has been changed is replicated in a first direction corresponding to the horizontal direction of the image visible to the observer, a second direction corresponding to the vertical direction of the image, or both, by diffraction due to the diffraction structure of the expanded region within the light guide, and is then emitted from the exit surface. The direction normal to the surface of the light guide at the center or centroid of the expanded region is the Z-axis direction, and the tangent plane is the XY plane, and the diffraction structure of the expanded region is located inside the light guide with respect to the Z-axis direction. In the XY plane, when the direction of propagation of the central ray of a light beam incident on the extended region is the X-axis, and the direction perpendicular to the X-axis is the Y-axis, the light beam incident on the extended region is duplicated when it passes through the XY plane of the extended region from the positive Z-axis direction, and the light beam duplicated when it passes through from the negative Z-axis direction combines and is emitted from the extended region. When the field of view of the image seen by the observer is ±F degrees, the angle between the diffraction structure of the extended region in the XY plane and the direction of propagation of the light beam incident on the extended region is α degrees, the tilt angle between the diffraction structure and the Z axis is β degrees, the angle between the central ray of the light beam incident on the extended region and the Z axis is θA degrees, the angle between the central ray of the light beam diffracted and emitted in the extended region and the Z axis is θB degrees, the thickness of the diffraction structure in the Z direction is T [μm], the shorter of the distances between the diffraction structure and the front and back surfaces of the light guide is Ts [μm], and the coherence length of the light beam diffracted and emitted in the extended region within the light guide is L [μm], then the following relationship is satisfied. |θA-θB| < |F| / 2, and, |β|×2×cos(α) ≦ |F|-|θA-θB|, and Ts > L / 2

[0008] Furthermore, the head-up display system of this disclosure comprises the optical system described above and a light-transmitting member that reflects the light beam emitted from the light guide, and displays an image superimposed as a virtual image on a real scene visible through the light-transmitting member. [Effects of the Invention]

[0009] The optical system and head-up display system of this disclosure can improve diffraction efficiency. [Brief explanation of the drawing]

[0010] [Figure 1] Schematic perspective view showing the configuration of the light guide. [Figure 2] An explanatory diagram showing the direction of incident and outgoing light to the light guide of a head-mounted display. [Figure 3] An explanatory diagram showing the direction of incident and outgoing light to the light guide of a head-up display. [Figure 4] Cross-sectional view of a vehicle equipped with the head-up display system of the embodiment, Y1Z1 plane. [Figure 5A] An explanatory diagram showing the optical path of the light beam emitted from the display unit. [Figure 5B] An explanatory diagram showing the horizontal field of view of a virtual image. [Figure 5C] Diagram illustrating the vertical field of view of a virtual image. [Figure 6] Perspective view showing the configuration of the light guide in the embodiment. [Figure 7] An explanatory diagram showing the optical path at the center of the light beam emitted from the display unit. [Figure 8] Plan view of the first expansion area [Figure 9] Cross-sectional view taken along the line IX-IX in Figure 8. [Figure 10] Plan view of the first expansion area [Figure 11] Diagram illustrating the light beam incident on the diffraction structure and the replicated light beam. [Figure 12] Cross-sectional view of the light guide [Figure 13] Table showing the numerical values ​​in each example and comparative example. [Figure 14]Explanatory drawing showing the virtual image visual field area of Example 1 [Figure 15] Explanatory drawing showing the virtual image visual field area of Comparative Example 1 [Figure 16] Explanatory drawing showing the virtual image visual field area of Example 2 [Figure 17] Explanatory drawing showing the virtual image visual field area of Comparative Example 2 [Figure 18] Explanatory drawing showing the virtual image visual field area of Example 3 [Figure 19] Graph showing the relationship between the visual field angle and the normalized diffraction efficiency [Figure 20] Graph showing the relationship between the visual field angle and the normalized diffraction efficiency

Modes for Carrying Out the Invention

[0011] (Summary of the Present Disclosure) Referring to FIG. 1, the summary of the present disclosure will be first described. FIG. 1 is a schematic diagram showing the configuration of a light guide 13. In an optical system used in a head-mounted display (hereinafter referred to as an HMD) or the like, a so-called pupil-expanding type light guide 13 is used. The pupil-expanding type light guide 13 includes a coupling region 21 that receives image light from the display unit 11 and changes the traveling direction, a first expansion region 23 that expands in a first direction, and a second expansion region 25 that expands in a second direction. The first direction and the second direction intersect each other and may be, for example, orthogonal to each other.

[0012] The coupling region 21, the first expansion region 23, and the second expansion region 25 each have a diffraction power for diffracting image light, and an embossed hologram or a volume hologram is formed. The embossed hologram is, for example, a diffraction grating. The volume hologram is, for example, a periodic refractive index distribution in a dielectric film. The coupling region 21 changes the traveling direction of the image light incident from the outside so as to be directed toward the first expansion region 23 by the diffraction power.

[0013] The first extended region 23, for example, has diffractive structural elements arranged therein, which duplicate the image light by splitting the incident image light into image light that travels in a first direction and image light that travels to the second extended region 25 due to diffraction power. For example, in Figure 1, in the first extended region 23, diffractive structural elements are arranged at four points 23p aligned in the direction in which the image light travels through repeated total internal reflection. At each point 23p, the diffractive structural element splits the image light and propagates the split image light to the second extended region 25. As a result, the beam of incident image light is duplicated into four beams of image light in the first direction, thereby extending the image light.

[0014] The second extended region 25, for example, has a diffractive structure element, which duplicates the image light by splitting the incident image light into image light that propagates in a second direction due to diffraction power and image light that exits the second extended region 25 to the outside. For example, in Figure 1, in the second extended region 25, there are three points 25p arranged in each row in the direction in which the image light propagates through repeated total internal reflection, and a diffractive structure element is placed at each of the 12 points 25p in the four rows. The image light is split at each point 25p, and the split image light is emitted to the outside. As a result, the light beam of the incident image light in each of the four rows is duplicated into three light beams of image light in the second direction, thereby expanding the field of view. In this way, the light guide 13 can duplicate 12 light beams of image light from one light beam of incident image light, and can duplicate the light beams in the first and second directions respectively to expand the field of view. The observer can perceive each of these 12 image beams as a virtual image, thereby widening the viewing area in which the observer can perceive the image light.

[0015] Next, we will explain the difference between pupil-extended HMDs and head-up displays (hereinafter referred to as HUDs) with reference to Figures 2 and 3. Figure 2 is an explanatory diagram showing the incident and outgoing light of an HMD. Figure 3 is an explanatory diagram showing the incident and outgoing light of a HUD.

[0016] As shown in Figure 2, the light guide 13 in the HMD is almost directly facing the viewing area Ac in which the observer can see the virtual image. Image light incident perpendicularly from the display unit 11 is divided within the light guide 13, and the divided image light is emitted perpendicularly from the emission surface 27 of the light guide 13 toward the viewing area Ac.

[0017] In contrast, as shown in Figure 3, in the case of a HUD, the image light emitted from the light guide 13 is reflected, for example, by the windshield 5 and incident on the viewing area Ac, so the divided image light is emitted obliquely from the emission surface 27 of the light guide 13. The optical system for the HUD will be described below.

[0018] (Embodiment) The embodiments will be described below with reference to Figures 4 to 6. Components having the same function as those described above are denoted by the same reference numerals. Also, the inclination angles of the windshields in the figures are shown for ease of understanding and may differ from figure to figure. [1-1. Structure] [1-1-1. Overall Configuration of the Optical System and Head-Up Display System] A specific embodiment of the head-up display system 1 (hereinafter referred to as HUD system 1) of this disclosure will be described. Figure 4 is a cross-sectional view of a vehicle 3 equipped with the HUD system 1 according to this disclosure. Figure 5A is an explanatory diagram showing the optical path of the light beam emitted from the display unit. In the embodiment, the HUD system 1 mounted on the vehicle 3 will be described as an example.

[0019] In the following, the directions of the HUD system 1 will be described based on the X1, Y1, and Z1 axes shown in Figure 4. The Z1 axis direction is the direction in which the observer views the virtual image IV from the viewing area Ac where the virtual image IV can be seen by the observer. The X1 axis direction is the horizontal direction perpendicular to the Z1 axis. The Y1 axis direction is the direction perpendicular to the X1Z1 plane formed by the X1 and Z1 axes. Therefore, the X1 axis direction corresponds to the horizontal direction of the vehicle 3, the Y1 axis direction corresponds to the approximately vertical direction of the vehicle 3, and the Z1 axis direction corresponds to the approximately forward direction of the vehicle 3.

[0020] As shown in Figure 4, the optical system 2 is located inside the dashboard (not shown) below the windshield 5 of the vehicle 3. Observer D, seated in the driver's seat of the vehicle 3, recognizes the image projected from the HUD system 1 as a virtual image IV. In this way, the HUD system 1 overlays the virtual image IV onto the real scene visible through the windshield 5. Since multiple duplicated images are projected into the viewing area Ac, the virtual image IV can be viewed even if the position of observer D's eyes shifts in the Y1 axis and X1 axis directions, as long as the viewing area Ac is within the viewing area Ac. Regarding the range of the virtual image IV that observer D can see, the angle θh indicating the horizontal field of view of the virtual image IV as seen by observer D is shown in Figure 5B, and the angle θv indicating the vertical field of view of the virtual image IV is shown in Figure 5C. Observer D is a passenger riding in a moving vehicle such as the vehicle 3, for example, the driver or a passenger sitting in the front passenger seat.

[0021] Refer to Figure 4. The HUD system 1 comprises an optical system 2 and a windshield 5. The optical system 2 comprises a display unit 11, a light guide 13, and a control unit 15. The display unit 11 emits a light beam L1 that forms an image visible to the observer as a virtual image Iv. The light guide 13 divides and duplicates the light beam L1 emitted from the display unit 11 and guides the duplicated light beam L2 to the windshield 5. The light beam L2 reflected by the windshield 5 is displayed as a virtual image Iv, superimposed on the real scene visible through the windshield 5.

[0022] The display unit 11 displays an image based on control from an external control unit. For example, the display unit 11 can be a backlit liquid crystal display, an organic light-emitting diode display, or a plasma display. Alternatively, the display unit 11 may generate images using a screen that diffuses or reflects light, and a projector or scanning laser. The display unit 11 can display image content containing various information, such as road guidance, distance to the vehicle ahead, car battery level, and current vehicle speed. In this way, the display unit 11 emits a light beam L1 containing image content that is perceived by observer D as a virtual image Iv.

[0023] The control unit 15 can be implemented using a circuit composed of semiconductor elements or the like. For example, the control unit 15 can be composed of a microcontroller, CPU, MPU, GPU, DSP, FPGA, or ASIC. The control unit 15 reads data and programs stored in its built-in memory unit (not shown) and performs various arithmetic operations to realize predetermined functions. The control unit 15 also includes a storage device 17.

[0024] The storage device 17 is a storage medium that stores the programs and data necessary to realize the functions of the control unit 15. The storage device 17 can be implemented, for example, by a hard disk drive (HDD), SSD, RAM, DRAM, ferroelectric memory, flash memory, magnetic disk, or a combination thereof. Multiple image data representing virtual image Iv are stored in the storage device 17. The control unit 15 determines the virtual image Iv to display based on vehicle-related information acquired from an external source. The control unit 15 reads the image data of the determined virtual image Iv from the storage unit and outputs it to the display unit 11.

[0025] [1-1-2. Light guide] The configuration of the light guide 13 will be explained with reference to Figure 6. Figure 6 is a perspective view showing the configuration of the light guide 13. Below, the directions of the extension region of the light guide 13 will be explained based on the X, Y, and Z axes shown in Figure 6. The direction of the normal to the surface of the light guide 13 at the center or centroid of the first extension region 23 is the Z-axis direction, and the tangent plane is the XY plane. In the XY plane, the direction of propagation of the central ray of the light beam incident on the first extension region 23 is the X-axis direction, and the direction perpendicular to the X-axis direction is the Y-axis direction. Similarly, the direction of propagation of the central ray of the light guide 13 at the center or centroid of the second extension region 25 is the Za-axis direction, and the tangent plane is the XaYa plane. In the XaYa plane, the direction of propagation of the central ray of the light beam incident on the second extension region is the Xa-axis direction, and the direction perpendicular to the Xa-axis direction is the Ya-axis direction.

[0026] The light guide 13 has a first main surface 13a and a second main surface 13b, which are its front surfaces. The first main surface 13a and the second main surface 13b face each other. The light guide 13 has an incident surface 20, a coupling region 21, a first extended region 23, a second extended region 25, and an exit surface 27. The incident surface 20, the coupling region 21, the first extended region 23, and the second extended region 25 are included in the second main surface 13b, and the exit surface 27 is included in the first main surface 13a. The exit surface 27 faces the second extended region 25. Note that the coupling region 21, the first extended region 23, and the second extended region 25 may be located between the first main surface 13a and the second main surface 13b. The first main surface 13a faces the windshield 5. In this embodiment, the incident surface 20 is included in the coupling region 21, but it may be a surface facing the coupling region 21 and included in the first main surface 13a. Furthermore, the ejection surface 27 may be included in the second extended region 25.

[0027] The coupling region 21, the first extended region 23, and the second extended region 25 each have different diffraction powers, and diffractive structural elements are formed in each of them. The coupling region 21, the first extended region 23, and the second extended region 25 each have different diffraction angles of image light. Furthermore, the light guide 13 is configured such that the incident light beam undergoes total internal reflection. The light guide 13 is made of, for example, a glass or resin plate with a mirror-finished surface. The light guide 13 is not limited to a planar shape but may also have a curved shape. Thus, the light guide 13 includes a diffractive structural element that diffracts light in part, such as a volume hologram. When the coupling region 21, the first extended region 23, and the second extended region 25 include a volume hologram, they become three-dimensional regions.

[0028] The coupling region 21 is a region that receives the light beam L1 emitted from the display unit 11 from the incident surface 20 and changes the direction of propagation of the light beam L1. The coupling region 21 has diffraction power and changes the direction of propagation of the incident light beam L1 to the direction of the first extended region 23, and emits it as light beam L1A. In this embodiment, coupling refers to the state in which light propagates within the light guide 13 under total internal reflection conditions.

[0029] The first extended region 23 extends the luminous beam L1A in a first direction corresponding to the horizontal direction of the virtual image Iv, and emits it into the second extended region in a second direction intersecting the first direction. In the first extended region 23 that extends the luminous beam L1A in the first direction, the length in the first direction is greater than the length in the second direction. In this embodiment, the light guide 13 is arranged so that the first direction is the horizontal direction (direction of the X1 axis), but this is not limited to this, and the first direction does not have to perfectly coincide with the horizontal direction. The luminous beam L1A propagated from the coupling region 21 propagates in the first direction while undergoing repeated total internal reflection at the first principal surface 13a and the second principal surface 13b, and the luminous beam L1 is replicated by the diffraction structure of the first extended region 23 formed on the second principal surface 13b and emitted into the second extended region 25.

[0030] The second extended region 25 extends the luminous beam L1B in a second direction corresponding to the vertical direction of the virtual image Iv, and emits the extended luminous beam L2 from the emission surface 27. The second direction is, for example, perpendicular to the first direction. The light guide 13 is positioned so that the second direction is in the Z1 axis direction. The luminous beam L1B propagating from the first extended region 23 propagates in the second direction while undergoing repeated total internal reflection at the first principal surface 13a and the second principal surface 13b, and is duplicated by the diffraction structure of the second extended region 25 formed on the second principal surface 13b before being emitted to the outside of the light guide 13 via the emission surface 27.

[0031] Therefore, from the perspective of observer D, the light guide 13 extends the light beam L1, which has been incident on the incident surface 20 and whose direction of propagation has been changed, in the horizontal direction (direction of the X1 axis) of the virtual image Iv seen by observer D, and then further extends it in the vertical direction (direction of the Y1 axis) of the virtual image Iv, and emits the light beam L2 from the exit surface 27. Here, horizontal replication of the image is not limited to replication in the perfectly horizontal direction, but also includes replication in the approximately horizontal direction. Similarly, vertical replication of the image is not limited to replication in the perfectly vertical direction, but also includes replication in the approximately vertical direction.

[0032] [1-1-3. Order of pupil dilation] In the light guide 13 arranged as described above, the magnitude of the wave vectors of the first expansion region 23 and the second expansion region 25 differs depending on the order of pupil expansion of the image light beam L1 in the HUD system 1. The order of pupil expansion in the embodiment will be explained with reference to Figure 7. Figure 7 is an explanatory diagram showing the optical path of the center of the light beam emitted from the display unit.

[0033] The luminous beam L1 of image light incident on the light guide 13 changes its propagation direction to the first expansion region 23, which expands the pupil horizontally (X-axis direction) as the first direction, due to the diffraction structure formed in the coupling region 21. Therefore, after the luminous beam L1 is incident on the coupling region 21 at an oblique angle, it propagates in the direction of the first expansion region 23 as a luminous beam L1A under the influence of the wave vector k1 shown in Figure 7.

[0034] The luminous beam L1A propagating into the first extended region 23 extending in the first direction is divided by the diffraction structure formed in the first extended region 23 while undergoing repeated total internal reflection into a luminous beam L1A propagating in the first direction and a luminous beam L1B which is duplicated and changes its propagation direction to the second extended region 25. At this time, the duplicated luminous beam L1B propagates in the direction of the second extended region 25 under the influence of the wave vector k2 shown in Figure 7.

[0035] The light beam L1B, whose propagation direction has been changed to a second extended region 25 extending along the negative direction of the Z1 axis as a second direction, is split by the diffraction structure formed in the second extended region 25 into a light beam L1B that propagates in the second direction and a light beam L2 that is duplicated and emitted from the second extended region 25 to the outside of the light guide 13 via the emission surface 27. At this time, the duplicated light beam L2 propagates in the direction of the emission surface 27 under the influence of the wave vector k3 shown in Figure 7.

[0036] [1-1-4. Diffraction Structure] Next, the diffraction structure of the first extended region 23 will be described with reference to Figures 8 and 9. Figure 8 is a plan view of the first extended region 23, and Figure 9 is a cross-sectional view taken along the line IX-IX in Figure 8.

[0037] If the diffraction structure of the first extended region 23 is, for example, a volumetric hologram, then interference fringes 31 are formed as the diffraction structure of the first extended region 23. In the first extended region 23, α is the angle between the direction in which the interference fringes 31 extend in the XY plane and the direction of propagation of the light beam L1A. Furthermore, β is the inclination angle of the interference fringes 31 with respect to the vertical in a cross-sectional view of the diffraction structure perpendicular to the vertical, i.e., in the cross-sectional view taken along the IX-IX arrow in Figure 8.

[0038] As shown in Figure 10, the light beam L1A propagating to the first extended region 23 extending in the first direction is divided by the diffraction structure formed in the first extended region 23 while undergoing repeated total internal reflection into a light beam L1A propagating in the first direction and a light beam L1B which is replicated and changes its propagation direction to the second extended region 25.

[0039] Figure 11 shows the replicated luminous beam L1B in spherical coordinates when luminous beam L1A is transmitted through the XY plane of the first extended region 23 from the negative direction to the positive direction of the Z axis. Let the field of view angle of the virtual image Iv seen by observer D be ±F degrees, let the angle of the central ray of luminous beam L1A with respect to the Z axis be θA degrees, and let the angle of the central ray of luminous beam L1B with respect to the Z axis be θB degrees, satisfying the following equations (1) and (2). |θA-θB| < |F| / 2 ···(1) |β|×2×cos(α) ≦ |F|-|θA-θB| ···(2) formula

[0040] The horizontal field of view of the virtual image IV is given by 2 × |F| = θh, and the vertical field of view of the virtual image IV is given by 2 × |F| = θv (see Figures 5B and 5C). The horizontal field of view will be explained below, but the same relationship holds for the vertical field of view as well.

[0041] Figure 12 is a cross-sectional view of the light guide 13. In Figure 12, the diffracted light L1B1 obtained when the diffraction structure was transmitted from the negative Z-axis direction to the positive Z-axis direction, and the diffracted light L1B2 obtained when the diffraction structure was transmitted from the positive Z-axis direction to the negative Z-axis direction, are shown from the diffracted light L1B obtained in the first extended region 23. When the thickness of the diffraction structure in the Z-direction is T [μm], and the shorter of the distances between the diffraction structure and the front surface (first main surface 13a) and back surface (second main surface 13b) of the light guide 13 is Ts [μm], the coherence length L satisfies the following equation (3). Ts > L / 2 ···(3) formula

[0042] In Figure 12, for ease of understanding, the diffracted light L1B1 diffracted in the first extended region 23 is shown to be reflected by the first principal surface 13a along the Z-axis and propagating toward the second principal surface 13b along the Z-axis; however, it may also be tilted with respect to the Z-axis. The same applies to the diffracted light L1B2.

[0043] In equation (1), |θA-θB| determines the midpoint between the two peaks of diffraction efficiency: one for light transmitted from the positive to the negative direction along the Z-axis, and another for light transmitted from the negative to the positive direction. In equation (2), |β|×2×cos(α) determines the distance between the two peaks of diffraction efficiency: one for light transmitted from the positive to the negative direction along the Z-axis, and another for light transmitted from the negative to the positive direction. The relationship in equation (3) defines the range in which the diffracted light L1B1, obtained when the light beam is transmitted from one side to the other within the extended region, and the diffracted light L1B2, obtained when the light beam is transmitted from the other side to the one side, do not interfere with each other.

[0044] Next, each example and comparative example will be described with reference to Figures 13 to 18. Figure 13 is a table of the parameters for each example and comparative example. Figures 14 to 18 show the diffraction efficiency at the field of view for each example and comparative example. Figures 14(a) to 18(a) show the diffraction efficiency of the luminous beam L1B1 that is replicated when the luminous beam L1A is transmitted through the first extended region 23 from the negative direction to the positive direction of the Z axis under each condition. Figures 14(b) to 18(b) show the diffraction efficiency of the luminous beam L1B2 that is replicated when the luminous beam L1A is transmitted through the first extended region 23 from the positive direction to the negative direction of the Z axis under each condition.

[0045] In Examples 1 to Comparative Example 2, the field of view F is always 3.50 degrees. The field of view F in Examples 1 to Comparative Example 2 represents the horizontal field of view (left-right direction). The same relationship holds for the vertical field of view (vertical field of view). In Example 1, shown in Figure 14, angles θA and θB, explained in relation to equations (1) and (2), are 50.00 degrees, angle α is 45.00 degrees, and tilt angle β is 0.00 degrees. Figure 14(c) shows the diffraction efficiency of the light beam L1B after passing through the first extended region 23 once along the Z axis. That is, Figure 14(c) shows the diffraction efficiency when the light beam L1B1, which is duplicated when transmitted through the first extended region 23 from the negative direction to the positive direction of the Z axis, and the light beam L1B2, which is duplicated when transmitted from the positive direction to the negative direction of the Z axis, interfere with each other. Diffraction efficiency is shown in stages from level A1 to A6, with the diffraction efficiency increasing as you move from level A1 to A6. Level A1 shows a diffraction efficiency of 3% to less than 4%, level A2 shows a diffraction efficiency of 4% to less than 5%, level A3 shows a diffraction efficiency of 5% to less than 6%, level A3a shows a diffraction efficiency of 5% to less than 7%, level A4 shows a diffraction efficiency of 7% to less than 9%, level A5 shows a diffraction efficiency of 9% to less than 11%, and level A6 shows a diffraction efficiency of 11% to less than 13%.

[0046] In Example 1, the wavelength λ of the luminous beam is 520 nm, and the linewidth Δλ, which indicates the wavelength band of the light source, is 5 nm, so the coherence length L is 24 μm. As a result, the spacing Ts is 1000 μm, so the diffracted light shown in Figures 14(a) and (b) does not interfere with each other. Therefore, the diffraction efficiency is simply the sum of the diffraction efficiency shown in Figure 14(a) and the diffraction efficiency shown in Figure 14(b). The maximum diffraction efficiency, which was conventionally 5%, was improved to a maximum of 11% in Example 1.

[0047] In contrast, in Comparative Example 1 shown in Figure 15, the angles θA, θB, α, β, thickness T, and wavelength λ are the same as in Example 1, and the line width Δλ of the light source is 0.1 nm, so the coherence length L is 1202 μm. As a result, the spacing Ts is 0.1 μm, and the diffracted light shown in Figures 15(a) and (b) interferes with each other. Due to this interference, as shown in Figure 15(c), the diffraction efficiency is reduced to near zero in the range to the right of the center of the field of view where diffraction efficiency was originally obtained. Level C1 in Figure 15(c) shows a diffraction efficiency of 0% or more and less than 5%, level C2 shows a diffraction efficiency of 5% or more and less than 10%, level C3 shows a diffraction efficiency of 10% or more and less than 15%, and level C4 shows a diffraction efficiency of 15% or more and less than 20%.

[0048] In Example 2 shown in Figure 16, the angles θA, θB, α and thickness T are the same as in Example 1, with an angle β of 1.24 degrees, a wavelength λ of 450 μm, and a linewidth Δλ of the light source of 2 nm. Therefore, the coherence length L is 45 μm and the spacing Ts is 100 μm, so the diffracted light shown in Figures 16(a) and (b) does not interfere with each other. As a result, as shown in Figure 16(c), the diffraction efficiency improves from 6% to 13% at the center of the field of view and from 7% to 10% in the peripheral part of the field of view. As shown in Figure 16, level D1 indicates a diffraction efficiency of 0% to less than 2%, level D2 indicates a diffraction efficiency of 2% to less than 4%, level D3 indicates a diffraction efficiency of 4% to less than 6%, level D4 indicates a diffraction efficiency of 6% to less than 8%, level D5 indicates a diffraction efficiency of 6% to less than 9%, level D6 indicates a diffraction efficiency of 9% to less than 12%, and level D7 indicates a diffraction efficiency of 12% to less than 15%.

[0049] In Comparative Example 2 shown in Figure 17, the angles θA, θB, α, β, thickness T, wavelength λ, and linewidth Δλ of the light source are the same as in Example 2, and the coherence length L is also the same at 45 μm. However, since the spacing Ts is 1 μm, the diffracted light shown in Figures 17(a) and (b) interferes with each other. Due to this interference, as shown in Figure 17(c), the diffraction efficiency in the central part of the field of view, where diffraction efficiency was originally obtained, is reduced to near zero. Levels C1 to C4 shown in Figure 17(c) are the same as in Figure 15, and level C5 shows a diffraction efficiency of 20% or more and less than 25%.

[0050] In Example 3 shown in Figure 18, the angle θB and wavelength λ are the same as in Example 1, with angle θA being 49.00 degrees, angle α being 44.57 degrees, angle β being 0.71 degrees, thickness T being 1 μm, and the linewidth Δλ of the light source being 2 nm. Therefore, the coherence length L is 60 μm and the spacing Ts is 400 μm, so the diffracted light shown in Figures 18(a) and (b) does not interfere with each other. As a result, the diffraction efficiency improves from 10% to 19% at the center of the field of view, as shown in Figure 18(c). Level E1 in Figure 18 shows a diffraction efficiency of 8% or more and less than 9%, level E2 shows a diffraction efficiency of 9% or more and less than 10%, level E3 shows a diffraction efficiency of 10% or more and less than 11%, level E4 shows a diffraction efficiency of 7% or more and less than 8%, and level E5 shows a diffraction efficiency of 8% or more and less than 9%. Furthermore, level E6 indicates a diffraction efficiency of 9% or more but less than 10%, level E7 indicates a diffraction efficiency of 10% or more but less than 11%, level E8 indicates a diffraction efficiency of 10% or more but less than 11%, and level E9 indicates a diffraction efficiency of 10% or more but less than 11%. Additionally, level E10 indicates a diffraction efficiency of 10% or more but less than 11%, level E11 indicates a diffraction efficiency of 10% or more but less than 11%, level E12 indicates a diffraction efficiency of 10% or more but less than 11%, and level E13 indicates a diffraction efficiency of 10% or more but less than 11%.

[0051] Furthermore, if the thickness T [μm] in the Z direction of the volume hologram shown in Figure 9 and the wavelength λ [μm] of the luminous flux L1A satisfy the following relationship, the diffraction efficiency is improved by this embodiment. T < (-2.3576×λ+0.0952)×|F|+(22.3540×λ-0.9125) ···(4) formula

[0052] Figure 19 is a graph showing an example of normalized diffraction efficiency when the thickness T in equation (4) is near the upper limit. When the thickness T is greater than the value on the right side of equation (4), the diffraction efficiency may become zero within the field of view. This causes part of the image to be lost and the quality to deteriorate, but in this embodiment, the diffraction efficiency is improved, so a volume hologram thickness that satisfies the relationship in equation (4) can be adopted.

[0053] Furthermore, if the thickness T in the Z direction of the volume hologram satisfies the following relationship, the diffraction efficiency is improved by this embodiment. T < (-0.9805×λ-0.0487)×|F|+(9.0771×λ+0.4032) ···(5) formula

[0054] Figure 20 is a graph showing an example of normalized diffraction efficiency when the thickness T in equation (5) is near the upper limit. When the thickness T is greater than the value on the right side of equation (5), the diffraction efficiency within the viewing angle range reaches 50% of the peak. When the diffraction efficiency falls below 50%, the virtual image becomes dark and the image quality deteriorates, but in this embodiment, the diffraction efficiency is improved, so a volume hologram thickness that satisfies the relationship in equation 5 can be adopted.

[0055] In this embodiment, the second extended region 25 also has a structure similar to the diffraction structure of the first extended region 23. Such a structure may be present in only one of the extended regions, the first extended region 23 or the second extended region 25, or the optical system 2 may further include another extended region, and this other extended region may have such a diffraction structure. Alternatively, the functions of the first extended region 23 and the second extended region 25 may be realized in a single extended region, and this single extended region may, for example, have two-dimensional interference fringes, thereby replicating the incident light beam in the horizontal and vertical directions.

[0056] [1-2. Effects, etc.] The optical system 2 of this disclosure includes a display unit 11 that emits a light beam L1 that is visible to observer D as a virtual image Iv, and a light guide 13 that replicates the light beam L1. The light guide 13 has an incident surface 20 into which the light beam L1 from the display unit 11 is incident, and an exit surface 27 from which the light beam L2 is emitted from the light guide 13. The central ray of the light beam L1 emitted from the display unit 11 is incident on the incident surface 20 of the light guide 13. The light beam L1 incident on the incident surface 20 of the light guide 13 has its direction of propagation changed by diffraction due to the diffraction structure of the coupling region within the light guide 13. The light beam whose direction of propagation has been changed is replicated by diffraction due to the diffraction structure of the expanded region within the light guide 13 in a first direction corresponding to the horizontal direction of the virtual image Iv visible to observer D, or in a second direction corresponding to the vertical direction of the virtual image Iv, or in both directions, and is then expanded before being emitted from the exit surface 27. The direction normal to the surface of the light guide 13 at the center or centroid of the extended region is the Z-axis direction, and the tangent plane is the XY plane. The diffraction structure of the extended region exists inside the light guide 13 with respect to the Z-axis direction. Let the light beam incident on the extended region be light beam L1A, and the light beam diffracted and emitted from the extended region be light beam L1B. In the XY plane, when the direction of propagation of the central ray of light beam L1A is the X-axis, and the direction perpendicular to the X-axis is the Y-axis, when light beam L1A is transmitted through the XY plane of the extended region from the positive Z-axis direction, the replicated light beam L1B and when it is transmitted from the negative Z-axis direction combine and are emitted from the extended region. The field of view angle of the virtual image Iv seen by observer D is ±F degrees. The diffraction structure of the extended region in the XY plane and When the angle between the direction of propagation of the luminous beam L1A and the light guide is α degrees, the inclination angle between the diffraction structure and the Z axis is β degrees, the angle between the central ray of the luminous beam L1A and the Z axis is θA degrees, the angle between the central ray of the luminous beam L1B and the Z axis is θB degrees, the thickness of the diffraction structure in the Z direction is T [μm], the shorter of the distances between the diffraction structure and the first principal surface 13a and the second principal surface 13b of the light guide 13 is Ts [μm], and the coherence length of the luminous beam LB1 within the light guide 13 is L [μm], then the following relationship is satisfied. |θA-θB| < |F| / 2, and, |β|×2×cos(α) ≦ |F|-|θA-θB|, and Ts > L / 2

[0057] Since the coherence length L of the diffracted light beam LB1 in the extended region is smaller than twice the distance Ts between the diffracting structure and the light guide 13, it is possible to prevent interference between the replicated light beam L1B when transmitted through the XY plane of the extended region from the positive Z-axis direction and the replicated light beam L1B when transmitted from the negative Z-axis direction. This prevents a decrease in diffraction efficiency, and furthermore, since the replicated light beam L1B when transmitted through the XY plane of the extended region from the positive Z-axis direction and the replicated light beam L1B when transmitted from the negative Z-axis direction can be emitted together from the extended region, an optical system with improved diffraction efficiency can be provided.

[0058] Furthermore, by projecting the light emitted from the optical system 2 onto the windshield 5 of the vehicle 3, a virtual image IV suitable for observer D driving the vehicle 3 can be displayed.

[0059] (Other embodiments) As described above, the above embodiments have been explained as examples of the technology disclosed in this application. However, the technology in this disclosure is not limited to these embodiments and can be applied to embodiments that have been modified, replaced, added, or omitted as appropriate. Therefore, other embodiments will be illustrated below.

[0060] In the above embodiment, the diffraction structure of the extended region was interference fringes, but it is not limited to this. For example, it may be a physical uneven structure filled with resin.

[0061] In the above embodiment, the divided and duplicated light beam L2 was reflected off the windshield 5 to allow observer D to see the virtual image Iv, but this is not the only possible method. A combiner may be used instead of the windshield 5, and the divided and duplicated light beam L2 may be reflected off the combiner to allow observer D to see the virtual image Iv.

[0062] In the above embodiment, the first direction for expanding the luminous flux L1A in the first expansion region 23 and the second direction for expanding the luminous flux L1B in the second expansion region 25 were orthogonal to each other, but this is not limited to the above. As shown in Figure 6, to expand the luminous flux L1A in the first direction in the first expansion region 23, it is sufficient that the component expanding in the horizontal direction is larger than the component expanding in the direction along the Z axis, and to expand the luminous flux L1B in the second direction in the second expansion region 25, it is sufficient that the component expanding in the direction along the Z axis is larger than the component expanding in the horizontal direction.

[0063] The above embodiment describes the case where the HUD system 1 is applied to a vehicle 3 such as an automobile. However, the object to which the HUD system 1 is applied is not limited to a vehicle 3. The object to which the HUD system 1 is applied may be, for example, a train, a motorcycle, a ship, or an aircraft, or an amusement machine that does not move. In the case of an amusement machine, instead of a windshield 5, the light beam emitted from the display unit 11 is reflected by a transparent curved plate which acts as a light-transmitting member that reflects the light beam emitted from the display unit 11. Furthermore, the actual scene that the user can see through the transparent curved plate may be an image displayed from another video display device. That is, a virtual image from the HUD system 1 may be superimposed on an image displayed from another video display device. Thus, any of the windshield 5, a combiner, and a transparent curved plate may be used as the light-transmitting member in this disclosure.

[0064] In the above embodiment, the optical system 2 was used in a HUD system 1 that displays a virtual image Iv, but it is not limited to this. The optical system 2 may also be used in an image display system in which the observer directly observes the light beam emitted from, for example, the emission surface 27, rather than viewing the virtual image through a light-transmitting member. In this case, the observer is the person who directly views the image formed by the emitted light beam, and is not limited to the occupants of a moving vehicle.

[0065] (Summary of the embodiment) (1) The optical system of the present disclosure comprises a display unit that emits a light beam that is visible to an observer as an image, and a light guide that replicates the light beam. The light guide has an incident surface to which the light beam from the display unit is incident, and an exit surface from which the light beam is emitted from the light guide. The central ray of the light beam emitted from the display unit is incident on the incident surface of the light guide. The light beam incident on the incident surface of the light guide has its direction of propagation changed by diffraction due to the diffraction structure of the coupling region within the light guide. The light beam whose direction of propagation has been changed is replicated by diffraction due to the diffraction structure of the expanded region within the light guide in a first direction corresponding to the horizontal direction of the image visible to the observer, or in a second direction corresponding to the vertical direction of the image, or in both directions, and is then emitted from the exit surface. The direction normal to the surface of the light guide at the center or centroid of the expanded region is the Z-axis direction, and the tangent plane is the XY plane, and the diffraction structure of the expanded region is located inside the light guide with respect to the Z-axis direction. In the XY plane, when the direction of propagation of the central ray of a light beam incident on the extended region is the X-axis, and the direction perpendicular to the X-axis is the Y-axis, the light beam incident on the extended region is duplicated when it passes through the XY plane of the extended region from the positive Z-axis direction, and the light beam duplicated when it passes through from the negative Z-axis direction combines and is emitted from the extended region. When the field of view of the image seen by the observer is ±F degrees, the angle between the diffraction structure of the extended region in the XY plane and the direction of propagation of the light beam incident on the extended region is α degrees, the tilt angle between the diffraction structure and the Z axis is β degrees, the angle between the central ray of the light beam incident on the extended region and the Z axis is θA degrees, the angle between the central ray of the light beam diffracted and emitted in the extended region and the Z axis is θB degrees, the thickness of the diffraction structure in the Z direction is T [μm], the shorter of the distances between the diffraction structure and the front and back surfaces of the light guide is Ts [μm], and the coherence length of the light beam diffracted and emitted in the extended region within the light guide is L [μm], then the following relationship is satisfied. |θA-θB| < |F| / 2, and, |β|×2×cos(α) ≦ |F|-|θA-θB|, and Ts > L / 2

[0066] Since the coherence length L of the light beam diffracted in the extended region is less than twice the distance Ts between the diffraction structure and the light guide, interference between the replicated light beam transmitted through the XY plane of the extended region from the positive Z-axis direction and the replicated light beam transmitted from the negative Z-axis direction can be prevented. This prevents a decrease in diffraction efficiency, and furthermore, since the replicated light beam transmitted through the XY plane of the extended region from the positive Z-axis direction and the replicated light beam transmitted from the negative Z-axis direction can be emitted together from the extended region, an optical system with improved diffraction efficiency can be provided.

[0067] (2) In the optical system of (1), the optical system has two extended regions, one of which is extended by replicating the light beam incident on the first extended region in a first direction corresponding to the horizontal direction of the image seen by the observer, and the other extended region is extended by replicating the light beam incident on the other extended region in a second direction corresponding to the vertical direction of the image seen by the observer.

[0068] (3) In the optical system of (3), the above relation is satisfied in the extended region where the diffraction pitch of the diffraction structure is narrower, in the two extended regions.

[0069] (4) In any one of the optical systems described in (1) to (3), the extended region includes a transmission volume hologram.

[0070] In the optical system of (5)(4), the following relationship is satisfied between the thickness T [μm] of the volume hologram in the Z direction and the wavelength λ [μm] of the light beam incident on the volume hologram. T<(-2.3576×λ+0.0952)×|F|+(22.3540×λ-0.9125)

[0071] In the optical system of (6)(5), the following relationship is satisfied between the thickness T [μm] of the volume hologram in the Z direction and the wavelength λ [μm] of the light beam incident on the volume hologram. T < (-0.9805×λ-0.0487)×|F|+(9.0771×λ+0.4032)

[0072] (7) In any one of the optical systems described in (1) to (6), the central ray of the light beam emitted from the display unit is incident at an angle with respect to the direction normal to the incident surface of the light guide, and the central ray of the light beam emitted from the light guide is emitted at an angle with respect to the direction normal to the exit surface of the light guide.

[0073] (8) The head-up display system of the present disclosure comprises one optical system from (1) to (7) and a light-transmitting member that reflects a light beam emitted from a light guide, and displays an image superimposed as a virtual image on a real scene visible through the light-transmitting member.

[0074] In the head-up display system of (9)(8), the light-transmitting member is the windshield of the moving body. [Industrial applicability]

[0075] This disclosure is applicable to optical systems and head-up display systems for duplicating and displaying images. [Explanation of symbols]

[0076] 1. Head-Up Display System 3 vehicles 3a center line 5 Windshield 11 Display section 13 Light guide 13a First main surface 13b Second main surface 15 Control Unit 17 Storage device 20 Entrance plane 21 Combined area 23. First Expansion Area 23a points 25 Second Expansion Area 25a points 27 Ejection surface Ac Visibility Area D Observer IV Illusion k1, k2, k3 wave vectors L1, L1A, L1B, L2 luminous flux

Claims

1. A display unit that emits a beam of light that is visible to the observer as an image, The system comprises a light guide that replicates the aforementioned light beam, The light guide has an incident surface into which the light beam from the display unit is incident, and an exit surface from which the light beam is emitted from the light guide. The light ray at the center of the light beam emitted from the display unit is incident on the incident surface of the light guide, The light beam incident on the incident surface of the light guide has its direction of propagation changed by diffraction due to the diffraction structure of the coupling region within the light guide. The light beam whose direction of propagation has been changed is then expanded by diffraction by the diffraction structure of the expanded region within the light guide body, by being duplicated in a first direction corresponding to the horizontal direction of the image seen by the observer, or a second direction corresponding to the vertical direction of the image, or both, before being emitted from the emission surface. The normal direction to the surface of the light guide at the center or centroid of the extended region is the Z-axis direction, and the tangent plane is the XY plane. The diffraction structure of the extended region is located inside the light guide with respect to the Z-axis direction. In the aforementioned XY plane, when the direction of propagation of the central ray of the light beam incident on the extended region is the X-axis, and the direction perpendicular to the X-axis is the Y-axis, When a light beam incident on the extended region passes through the XY plane of the extended region from the positive direction of the Z axis, the replicated light beam and the replicated light beam when transmitted from the negative direction of the Z axis combine and are emitted from the extended region. The relationship is that the coherence length of the light beam diffracted and emitted in the extended region within the light guide is less than twice the shorter of the distances between the diffraction structure and the front and back surfaces of the light guide. optical system.

2. When the field of view of the image seen by the observer is ±F degrees, the angle between the diffraction structure of the extended region in the XY plane and the direction of propagation of the light beam incident on the extended region is α degrees, the inclination angle between the diffraction structure and the Z axis is β degrees, the angle between the central ray of the light beam incident on the extended region and the Z axis is θA degrees, the angle between the central ray of the light beam diffracted and emitted in the extended region and the Z axis is θB degrees, the shorter of the distances between the diffraction structure and the front and back surfaces of the light guide is Ts [μm], and the coherence length is L [μm], then the following relation satisfies the above relationship: The optical system according to claim 1. |θA - θB| < |F| / 2, and, |β|×2×cos(α) ≦ |F|−|θA−θB|, and Ts > L / 2

3. The optical system has two of the extended regions, One of the extended regions is expanded by duplicating the light beam incident on the one extended region in the first direction corresponding to the horizontal direction of the image seen by the observer. The other extended region is extended by duplicating the light beam incident on the other extended region in the second direction corresponding to the vertical direction of the image seen by the observer. The optical system according to claim 1 or 2.

4. In the two extended regions, the relationship is satisfied in the extended region where the diffraction pitch of the diffraction structure is narrower. The optical system according to claim 3.

5. The aforementioned extended region includes a transmission volume hologram. The optical system according to any one of claims 1 to 4.

6. The field of view of the image seen by the observer is ±F degrees, and the thickness T [μm] of the transmissive volume hologram in the Z direction and the wavelength λ [μm] of the light beam incident on the transmissive volume hologram satisfy the following relationship: The optical system according to claim 5. T<(-2.3576×λ+0.0952)×|F|+(22.3540×λ-0.9125)

7. The field of view of the image seen by the observer is ±F degrees, and the thickness T [μm] of the transmissive volume hologram in the Z direction and the wavelength λ [μm] of the light beam incident on the transmissive volume hologram satisfy the following relationship: The optical system according to claim 6. T < (-0.9805×λ-0.0487)×|F|+(9.0771×λ+0.4032)

8. The central ray of the light beam emitted from the display unit is incident at an angle with respect to the direction normal to the incident surface of the light guide, and the central ray of the light beam emitted from the light guide is emitted at an angle with respect to the direction normal to the exit surface of the light guide. The optical system according to any one of claims 1 to 7.

9. The optical system according to any one of claims 1 to 8, The light guide comprises a light-transmitting member that reflects the light beam emitted from the light guide, The image is superimposed as a virtual image onto the actual scene visible through the light-transmitting member. Head-up display system.

10. The light-transmitting member is a windshield for a moving object. The head-up display system according to claim 9.

Images