Ghost-free head-up display

Abstract

An image generation system for providing a head-up display without ghost images, the system comprising: a display screen having a front and a back surface; a picture generation unit that projects an image toward the display screen to be reflected toward a predetermined eyebox; and a field lens, wherein the picture generation unit is configured to project light through the field lens such that the light is incident on the front surface of the display screen and forms a first virtual image, and a portion of the light is transmitted through the display screen and is incident on the back surface of the display screen and forms a second virtual image, the first virtual image and the second virtual image having an offset, and the field lens is configured such that the offset is less than a threshold value and the first virtual image and the second virtual image substantially overlap as viewed from the eyebox.

Description

[Technical field]

[0001] The present disclosure relates to display systems, particularly, but not exclusively, to devices for displaying images on a screen in a manner free of ghost images. [Background technology]

[0002] A head-up display (HUD) is a well-known display in which an image is projected onto a transparent surface, such as a windshield or visor. Such displays are well-known in many different environments, including in vehicles.

[0003] In the automotive industry, most commercially available HUDs utilize a separate optical screen (combiner HUD) or display directly on the windshield (windshield HUD). Combiner HUDs can be installed in most vehicle models without significant modification. Combiner UHD utilizes a transparent plastic projector screen to combine the real driving environment with a projected virtual image. Although relatively inexpensive to install, the plastic screen partially obscures the driver's field of view, and furthermore, its performance is poor when the unit is subjected to vibration.

[0004] A windshield HUD utilizes the vehicle's windshield to combine the virtual image with the real environment. Because the windshield has a finite thickness (much larger than the screen typically used in a combiner HUD), the projected image is reflected off both the front and back of the windshield, resulting in a primary image and a secondary offset "ghost" image. The term ghost image is used in the art and throughout this specification to describe the secondary offset image. To counter this effect, special films can be applied to the windshield that enhance the reflection at the front (thus reducing the later reflection at the back, reducing the ghost image). Such films are relatively inexpensive, but can be visually unattractive, adversely affect image quality, and may become less effective with prolonged exposure to sunlight. Another solution is to provide a special multi-layer wedge-shaped windshield that effectively overlaps the two reflected images to present a single sharp image to the driver. A schematic diagram of a wedge-shaped windshield is shown in FIG. 1. Such specially manufactured windshields are expensive to produce because they must be specifically designed for each application.

[0005] An alternative approach is to design a windshield with one or more layers made from emissive / scattering nanoparticles. Each layer produces visible radiation in one of the R / G / B wavebands when excited by a scanning laser projector. A color image is displayed on the windshield by overlapping images of multiple wavebands. This so-called full windshield HUD has unlimited viewing angle and display position, and is free of laser speckle, but is still expensive and complex to manufacture. Furthermore, because the driver must focus on the windshield to see the displayed information, multi-depth images (i.e., images with elements at different apparent depths) cannot be experienced.

[0006] Additional challenges arise when designing HUDs for applications with large windshield tilt angles, such as those found on trains and trucks. For small cars, the windshield tilt angle is typically between 30° and 45°, and the HUD system is located directly under the dashboard, making the design of the HUD system easier. However, for larger vehicles, the windshield tilt angle can vary over a wide range. For example, most train windshields have a tilt angle of 60° to 80°. For trucks and buses, the tilt angle can be even larger, up to 90°. The larger tilt angle, combined with the thicker windshields used on larger vehicles, not only makes the ghost image problem more pronounced due to the larger offset of the images reflected from the front and back of the windshield, but also increases the cost of the traditional mitigation techniques mentioned above.

[0007] Therefore, existing methods are unable to reduce ghost images across multiple different systems.

[0008] SUMMARY OF THE PRESENT EMBODIMENT It is an object of the present invention to alleviate some of the deficiencies of the prior art mentioned above. Summary of the Invention

[0009] Aspects and embodiments of the present invention provide apparatus as claimed in the accompanying claims.

[0010] According to a first aspect of the present invention, there is provided an image generation system for providing a head-up display without ghost images, the system comprising: a display screen having a front and a back surface; a picture generation unit configured to project an image towards the display screen for reflection towards a predetermined eyebox; and a field lens, the picture generation unit configured to project light through the field lens such that the light is incident on the front surface of the display screen to form a first virtual image, and a portion of the light is transmitted through the display screen and incident on the back surface of the display screen to form a second virtual image, the first virtual image and the second virtual image having an offset, the field lens configured such that the offset is less than a threshold magnitude and the first virtual image and the second virtual image substantially overlap as viewed from the eyebox.

[0011] This approach enables ghost-image-free projection without the need for any modifications to the surface or internal structure of the display screen. Moreover, the technology is applicable to display screens with a wide range of tilt angles up to 90°.

[0012] Optionally, the picture generation unit and the field lens are arranged such that light incident on the front surface of the display screen is collimated.

[0013] Optionally, the first virtual image is projected onto a first virtual image plane at a first projection distance from the eyebox, and the second virtual image is projected onto a second virtual image plane at a second projection distance from the eyebox.

[0014] Optionally, the field lens is configured to project the first virtual image and the second virtual image at the first projection distance and the second projection distance such that an offset between the first virtual image and the second virtual image is less than a threshold angular resolution.

[0015] Optionally, the threshold angular resolution is equal to the dimension of a pixel.

[0016] Optionally, the threshold angular resolution is equal to the angular resolution of the human eye. By reducing the offset below the limit that the display and / or the human eye can resolve, ghost images are effectively eliminated.

[0017] Optionally, the first virtual image and the second virtual image are offset in a plane perpendicular to the optical axis.

[0018] Optionally, the field lens is provided by one of a concave mirror, a freeform surface, a Fresnel lens, a wave guide, a diffractive optical element, a holographic optical element, or one or more tapered optical fibers, particularly tapered optical fibers that can expand the fiber space of the projected image without a lens, thereby shortening the optical path of the light and reducing the overall spatial footprint of the system.

[0019] Optionally, the picture generation unit comprises a light source and a spatial light modulator.

[0020] Optionally, the picture generation unit comprises a projector and a diffuser for realizing the projected image.

[0021] Optionally, the picture generation unit comprises a laser and a 2D (two-dimensional) scanning mirror.

[0022] Optionally, the picture generation unit comprises a holographic unit for generating a computer-generated hologram, and a diffuser for realizing the hologram.

[0023] Optionally, the picture generating unit comprises one or more of an LCD (Liquid Crystal Display) device, an LED (Light Emitting Diode) device, a micro LED (Light Emitting Diode) device, an OLED (Organic Light Emitting Diode) device, or a digital light processing digital micromirror device. Such devices can be activated by application of a current, which can be localized and modulated as desired. These devices can further provide a flexible multi-color display.

[0024] Optionally, the system further comprises optical elements interposed between the picture generation unit and the field lens and / or between the field lens and the display screen.

[0025] Optionally, the intervening optical element comprises one of a folding mirror, a cylindrical mirror, a cylindrical lens, a wave guide, a diffractive optical element, or a holographic optical element. Such intervening optical elements allow the light path to be positioned around the physical confines of the installation environment and can compensate for any optical effects of the windshield itself.

[0026] Optionally, the system further comprises an image processor in communication with the light field display, the image processor being configured to account for distortions caused by the optical configuration so that the image perceived by the observer does not appear distorted, thereby eliminating the need for any post-image correction and bulky corrective optics, and also providing high flexibility to accommodate a variety of display surfaces and optical configurations.

[0027] Optionally, the screen of the head-up display is the windshield of the vehicle.

[0028] Optionally, the first virtual image and the second virtual image are associated with a first image of a stereoscopic picture of images, such that a user can view an object or scene from multiple perspectives depending on which of the superimposed images the user focuses on.

[0029] Optionally, one or more of the field lens, the projection unit and / or intervening optical components (if present) are moveable relative to each other.

[0030] Optionally, the first virtual image and the second virtual image are multi-dimensional images. Projecting three-dimensional images through the HUD system allows such images to be displayed at various appropriate depths to create a convincing representation of a real object.

[0031] Optionally, the first virtual image and the second virtual image constitute a subset of images, each image of the subset being realized at a different distance from the focal length of the field lens such that when projected onto a display screen, each image of the subset has a different apparent depth when viewed from the eyebox.

[0032] Optionally, the system further comprises a photosensitive device arranged to capture light reflected by the display screen through the field lens, thereby enabling the system to function as a camera that may be used to monitor an environment and / or a user within said environment.

[0033] According to a second aspect of the present invention, there is provided a method for providing a head-up display without ghost images, the method comprising the steps of generating with a picture generation unit an image to be rendered on a display screen having a front and a back surface for reflection towards a predetermined eyebox, providing a field lens between the picture generation unit and the display screen, such that a portion of light incident on the front surface of the display screen is reflected to form a first virtual image and a portion of the light is transmitted through the display screen and incident on the back surface to form a second virtual image that is offset from the first virtual image, and configuring the field lens such that the offset is less than a threshold magnitude and the first virtual image and the second virtual image substantially overlap as viewed from the eyebox.

[0034] Other aspects of the invention will become apparent from the accompanying claims set.

[0035] One or more embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings. [Brief description of the drawings]

[0036] [Figure 1] FIG. 1 is a schematic diagram of a known HUD system utilizing a wedge windscreen. [Diagram 2] FIG. 1 is a schematic diagram of a HUD system according to one embodiment of the present invention. [Figure 3A] FIG. 1 is a schematic diagram of a HUD system according to one embodiment of the present invention. [Figure 3B] FIG. 1 is a schematic diagram of a HUD system according to one embodiment of the present invention. [Figure 4] FIG. 1 is a schematic diagram of a HUD system according to one embodiment of the present invention. [Figure 5A] FIG. 1 is a schematic diagram of a HUD system according to one embodiment of the present invention. [Figure 5B] FIG. 1 is a schematic diagram of a HUD system according to one embodiment of the present invention. [Figure 6] FIG. 1 is a schematic diagram of a HUD system according to one embodiment of the present invention. [Figure 7] FIG. 1 is a schematic diagram of a HUD system according to one embodiment of the present invention. [Figure 8] FIG. 1 is a schematic diagram of a HUD system according to one embodiment of the present invention. [Figure 9A] 1 is a graph showing the relationship between incidence angle and vertical displacement for different display screen thicknesses for a given front glass refractive index of 1.5. [Figure 9B] 1 is a graph showing the relationship between incidence angle and lateral displacement for different display screen thicknesses for a given front glass refractive index of 1.5. [Figure 10] 1 is a graph showing the vertical and lateral displacement of a double image over a range of refractive indices of a display screen for a given windshield thickness of 8 mm. [Figure 11] FIG. 2 illustrates the operation of the magnification system according to one embodiment of the present invention. [Figure 12] 1 is a graph showing the relationship between the focal length of a field lens and the distance from the image plane to the focal plane for different projection distances. [Figure 13] FIG. 2 illustrates the operation of a diffuser according to one embodiment of the present invention. [Figure 14] FIG. 1 is a schematic diagram of a HUD system according to one embodiment of the present invention. [Figure 15A] FIG. 1 illustrates a schematic 3D model and prototype of a HUD system according to one embodiment of the present invention. [Figure 15B] FIG. 1 illustrates a schematic 3D model and prototype of a HUD system according to one embodiment of the present invention. [Figure 15C] FIG. 1 illustrates a schematic 3D model and prototype of a HUD system according to one embodiment of the present invention. [Figure 15D] FIG. 1 illustrates a schematic 3D model and prototype of a HUD system according to one embodiment of the present invention. [Figure 16] FIG. 2 is a flow diagram illustrating steps for designing and implementing components of a HUD system according to one embodiment of the present invention. [Figure 17A] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. [Figure 17B] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. [Figure 17C] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. [Figure 18] 1 is a model of an environment in which a HUD is installed according to an embodiment of the present invention. [Figure 19] FIG. 1 is a schematic diagram of a HUD system according to one embodiment of the present invention. [Figure 20A] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. [Figure 20B] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. [Figure 20C] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. [Figure 21A] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. [Figure 21B] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. [Figure 22]FIG. 1 is a schematic diagram of a HUD system according to one embodiment of the present invention. [Figure 23A] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. [Figure 23B] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. [Figure 23C] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. [Figure 24A] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. [Figure 24B] FIG. 2 illustrates a simulation of a HUD system according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] In one aspect of the invention, the device and display are installed in a vehicle, such as an automobile. Although the following description is written in terms of an automobile HUD utilizing the windshield as the display screen, the disclosure and concepts described herein are applicable to other forms of HUDs (e.g., those installed in other forms of vehicles, wearable platforms such as helmets or goggles, or other known types of HUDs), as well as displays in general.

[0038] In particular, the present invention is envisioned to be installed for use in constrained environments such as vehicles operable on land (on road / off road or on track), under or at sea, in the air or in space. Examples include, but are not limited to, automobiles, buses, trucks, excavators, heavy duty exoskeleton suits, motorcycles, trains, theme park rides, submarines, ships, boats, yachts, jet skis and other types of sea vehicles, airplanes, gliders and other types of air vehicles, space vehicles and shuttles for spacecraft. Furthermore, the present technology can also be installed / integrated into moving platforms such as driver / operator head / eye protection devices such as helmets or goggles. Thus, any activity involving the wearing of protective helmets / goggles can utilize the invention described herein. These protective helmets / goggles can be worn by, but are not limited to, motorcyclists / cyclists, skiers, astronauts, exoskeleton operators, military personnel, miners, scuba divers, and construction workers. Furthermore, it can be used in a standalone environment for game consoles, arcade machines, and can also be used as a simulation platform by combining with an external 2D / 3D display. It may also be used in research institutes and museums for education and entertainment purposes.

[0039] FIG. 1 shows a conventional HUD system that uses a wedge-shaped windshield.

[0040] A picture generating unit (PGU) 100a is shown projecting an image onto a windshield 1a, where the image is reflected by both the front surface 2a and the back surface 3a of the windshield 1a. The windshield 1a is formed of three layers: a first glass layer 4a, a second glass layer 5a, and an intervening wedge-shaped layer 6a that biases the first and second glass layers at an angle α. As a result of the wedge layer 6a, the front surface 2a and the back surface 3a of the windshield 1a are at different angles to the incident light such that the images reflected by each surface overlap each other and are perceived as a single image by an observer 200a, thereby effectively eliminating any ghost images. However, this type of wedge-shaped windshield is expensive, difficult to manufacture and install, and generally is not as flexible or robust as a conventional windshield.

[0041] FIG. 2 illustrates a HUD system 10 according to one embodiment of the present invention.

[0042] The HUD system 10 is comprised of a PGU 100, a diffuser 110, a field lens 120, and a conventional windshield 1. The PGU 100 is provided by a projector, but those skilled in the art will appreciate that any suitable light source and imaging means may be used, provided that it is operable as described below. Thus, in one embodiment, the PGU 100 is formed from a laser and a 2D (two-dimensional) scanning mirror, or a holographic unit that generates a computer-generated hologram for forming on the diffuser 110. In an alternative embodiment, the PGU 100 is a light field unit that generates a three-dimensional light field image for forming on the diffuser 110. Digital micromirror devices (DMDs), liquid crystal display (LCD) devices, LCoS (Liquid Crystal on Silicon) displays, laser projectors, light emitting diode (LED) displays, organic light emitting diode (OLED) displays, quantum dot light emitting diode (QLED) displays, and micro light emitting diode (μLED) displays may also be used in or as the PGU 100. Those skilled in the art will understand that in DMD, LCoS and LCD (liquid crystal display) embodiments, PGU 100 further comprises an initial light source. In contrast, PGU 100 with LED (light emitting diode) will not require additional light emitting components. Furthermore, in DMD, LCoS, LCD (liquid crystal display) or LED (light emitting diode), OLED (organic light emitting diode), QLED, μLED embodiments, no external image realization surface is required, such that diffuser 110 is not present.

[0043] In one embodiment, the PGU 100 is able to account for distortions due to the transmission of light through components used to manipulate the optical path so that the final image seen by the user appears correct. In one embodiment, this is accomplished by a software-based distortion correction module within the PGU 100 (or in other ways in communication with the PGU 100) that applies pre-compensated inverse distortion to the image in the digital domain before it is projected. In one embodiment, the distortion correction module calculates the expected distortion from the projection optics and the display optics, and determines the inverse distortion that must be applied so that the final image seen by the user is not distorted. This allows the PGU to account for asymmetries in the optical path of each image. Such pre-compensated distortions can be determined by software, as is known. This eliminates the need for any post-image correction and bulky correction optics. Furthermore, this also provides a high degree of flexibility to accommodate a variety of display surfaces and optical configurations.

[0044] In use, PGU 100 projects light onto diffuser 110 to form an image. This image is then projected through field lens 120, which focuses the projected image onto windshield 1 where it is reflected towards observer 200. Those skilled in the art will appreciate that any suitable focusing and magnifying optics may be used, provided that they meet the following requirements: In one embodiment, field lens 120 is provided by a Fresnel lens. In one embodiment, field lens 120 is configured to change the diverging beam from PGU 100 into a nearly parallel beam.

[0045] The path of light from PGU 100 transmitted from field lens 120 to windshield 1 is referred to as the optical path. Those skilled in the art will appreciate that any number of intervening reflectors / lenses or other optical components may be placed along the optical path between PGU 100 and field lens 120, or between field lens 120 and windshield 1, to manipulate the optical path as desired (e.g., to minimize the overall size of HUD system 10).

[0046] An aspect of the present invention is to enable a flexible and configurable system, resulting in reduced ghost images, that is installation-independent. As described in more detail below, variations in user height, display angle, size, etc. can be taken into account by requiring that the angular separation between the primary image and the ghost images be less than the angular resolution of the human eye, and implementing the HUD system accordingly.

[0047] 3A, 3B, 4 and 5A, 5B, for convenience, neither the field lens 120 nor the windshield 1 are shown, but illustrate embodiments including such intervening optical elements.

[0048] 3A shows a HUD system 10 comprised of a PGU 100, a diffuser 110, a first mirror 111, a second mirror 112, a third mirror 113, and a field lens 114. As discussed above in relation to FIG. 2, the PGU 100 may be provided by any suitable image generating means, including one that eliminates the need for a diffuser 110. A field lens 120 and a windshield 114 are not shown, but are positioned further along the optical path, similar to the embodiment of FIG.

[0049] In use, PGU 100 and diffuser 110 (if present) are arranged to project an image onto mirror 111, which reflects the image onto mirror 112. Mirror 112 is rotatable about the z-axis as indicated by the curved arrow. The image is then reflected onto mirror 113, which redirects the image through field lens 114.

[0050] Beneficially, this arrangement is flexible so that ghost-free images can be projected onto the display for a number of different conditions (e.g., different windshield thicknesses or tilt angles, driver heights, etc.) In this embodiment, the PGU 100 and mirror 112 can be adjusted to ensure a ghost-free projection.

[0051] Figure 3B shows an alternative embodiment of a HUD system 10 consisting of a PGU 100, a diffuser 110, a first mirror 111 and a second mirror 112, and a field lens 113a. As with the embodiment of Figure 3A, any suitable image generation and realization means may be employed. Again, a windshield 1 is not shown, but would be positioned along the optical path as in Figure 2.

[0052] In use, PGU 100 and diffuser 110 are arranged to project an image onto mirror 111 and mirror 112, as described above in relation to Figure 3A. In this embodiment, mirror 112 is fixed and is used to redirect the image onto field lens 113a.

[0053] In this embodiment, the adjustability of the system comes from a folding mirror that can be rotated to compensate for differential system variations.

[0054] All of these folding mirrors can be replaced with concave mirrors or freeform surfaces to compensate for imaging distortion or to act as magnifying components, thereby reducing the magnification of the field lens.

[0055] 4 shows yet another alternative embodiment of HUD system 10, comprised of PGU 100, diffuser 110, first mirror 111b, second mirror 112b, and third mirror 113b. As with the embodiment of FIGS. 3A and 3B, any suitable image generation and realization means may be employed. Not shown are field lens 120 and windshield, which are positioned further along the optical path as in FIG. 2.

[0056] In use, PGU 100 and mirror 111b are positioned to reflect light from PGU 100 to diffuser 110, which reflects the image to mirror 112b and finally to mirror 113b. Those skilled in the art will appreciate that diffuser 110 may be positioned at any point along the optical path before field lens 120. Both mirror 112b and mirror 113b are rotatable about the x-axis to precisely control the direction of the projected image as well as to accommodate any variations in the system, resulting in a more tunable system.

[0057] FIG. 5A illustrates an embodiment in which a waveguide 115a is placed after the field lens 120 and is used to redirect light emitted from the PGU 100 to the windshield 1 (not shown). FIG. 5B illustrates an alternative embodiment in which a waveguide 115b is used to redirect and diverge light from the PGU 100 to magnify the image. Although not shown, it is envisioned that the waveguides 115a and 115b are implemented as part of any of the HUD systems 10 illustrated in FIGS. 2-4. In further embodiments, the waveguides 115a and 115b are replaced with diffractive optical elements (DOEs) or holographic optical elements (HOEs). Those skilled in the art will appreciate that any suitably reflective or refractive optical components may be used, provided they operate as described.

[0058] The arrangement of the optical elements of the present invention shown in Figures 3A, 3B, 4, 5A and 5B allows the HUD system 10 to adapt to windshields with different thicknesses or tilt angles, or to observers of different heights. This can also be achieved by varying the tilt of the picture generation unit 100 relative to the field lens 120. The area where the user's eyes must be located to see the image reflected by the windshield 1 is called the eyebox. In each of Figures 3A-5B, it is envisaged that the rotatable mirror comprises one or more actuators in communication with the control means, so that its rotation angle can be changed. In a further embodiment, additional actuators are provided on each of the mirror, the diffuser 110 and the PGU 10 to change their relative positions and angular components in each of the x-, y- and z-planes.

[0059] In one embodiment, the HUD system 10 is further configured to provide a 3D effect through autostereoscopy, in which binocular disparity is used to create the perception of depth.

[0060] Figure 6 shows an embodiment of a HUD system 10 comprised of two PGUs 100a, 100b and a pair of field lenses 120a, 120b. Each of the PGUs 100a, 100b may be provided by any suitable image generating means, as described above in connection with at least Figure 2. For simplicity, the diffuser 110, field lens 120 and windshield 1 are not shown, but would be positioned further along the optical path, similar to the embodiment of Figure 2.

[0061] In use, PGU 100a, PGU 100b each projects one image of a stereogram of images via a dedicated field lens 120a, 120b towards one of the observer's eyes 200a, 200b. The observer therefore sees two nearly identical images with different binocular disparities. For each observer's eye, a ghost-free image is generated according to principles described below.

[0062] FIG. 7 shows an alternative embodiment of a HUD system 10 consisting of a PGU 100, a field lens 120, a beam splitting mirror 116, a first mirror 112a and a second mirror 112b, and an additional pair of field lenses 120a, 120b.

[0063] In use, PGU 100 generates both views of a stereogram of an image by projecting a single image through field lens 120 onto beam splitting mirror 111. The image is then directed onto each of mirrors 112a and 112b and then forward through field lenses 120a and 120b to the corresponding eyes 200a, 200b of an observer.

[0064] The optical system of the HUD can be divided into three parts based on their different functions and logical order: combiner system, magnification system, and intermediate optical elements. The combiner system defines the overall relationship between the HUD and the installation environment. For example, as shown in FIG. 8, the installation position of the HUD system 10, the incidence angle of the light beam from the HUD to the windshield 1, and the angle of the light beam reflected back from the windshield 1 to the observer / driver 200. Once the geometry of the combiner system is defined, the magnification system can be designed to magnify the projected image to an appropriate size so that the driver can comfortably observe the information. It is possible to design the magnification system so that the angular offset between the images reflected from the front 2 and back 3 of the windshield 1 is close to the minimum resolving power of the human eye, thereby solving the problem of ghost images. As mentioned above, an advantage of the present invention is that if the geometry of the system changes, the magnifier system and / or the combiner system can be adjusted to ensure that a ghost-free image is generated.

[0065] When designing the magnification system, there are several important parameters of the HUD that need to be considered, such as resolution, field of view (FoV) and eyebox. After the combiner system and the magnification system are defined, the requirements of the intermediate optical elements are known, such as the size and position of the mirrors, and the diffusion angle of the diffuser 110, if used.

[0066] Combiner System To design a combiner system, the system requirements and constraints of the installation environment need to be known. An example environment is shown in FIG.

[0067] 8 shows a HUD system 10 that utilizes a vehicle windshield 1 as a combiner device. A PGU 100, diffuser 110 and field lens 120 (not shown) are arranged to project an image from either location (x1,y1) or (x2,y2). Image p0 is projected from both the front 2 and rear 3 of the windshield 1, L x ,L y 2, resulting in a primary image p1 in the first image plane and a ghost image p2 in the second image plane. The windshield 1 has a refractive index n of the windshield, a thickness d c , and the inclination angle α of the windshield. These factors define the lateral displacement δ between the primary image reflected by the front surface 2 of the windshield 1 and the ghost image reflected by the rear surface 3 of the windshield 1. y and the longitudinal displacement δ x This relationship is given by the following equation:

number

[0068] As can be seen from FIG. 9B, for the same incidence angle, the lateral displacement increases as the windshield thickness increases. From FIG. 9A, it can be seen that for different windshield thicknesses, the lateral displacement first increases with increasing incidence angle, and then decreases with increasing incidence angle. y It can be seen that increases, reaches its maximum value around 50°, and then decreases until it reaches 0°. In practical situations, γ is less than 50°, and in this case, the longitudinal displacement δ y To reduce or eliminate , a small angle of incidence is required.

[0069] 10 shows the variation of lateral and vertical displacement as a function of incidence angle over a range of refractive indices between 1.49 and 1.54, indicating that the refractive index does not have a significant effect and is therefore not a critical factor when designing the HUD system 10.

[0070] For a given vehicle model, the refractive index, thickness, and inclination angle α of the windshield 1 are set. Meanwhile, the position of the observer is also usually fixed, so this is L x and L y This means that the values ​​of n and d are known. c , α, L x and L y For , the position (x,y) of the HUD can be adjusted to obtain different angles of incidence according to the following relationship:

number

[0071] From equation (3), it can be seen that the angle of incidence on the windshield 1 depends on the inclination angle of the windshield 1 and the position of the observer 200. To reduce the lateral displacement of the double image, the magnitude of the angle of incidence must be reduced by increasing the value of y and decreasing the value of x. For example, in Figure 8, position (x2,y2) gives a smaller value of γ than position (x1,y1).

[0072] Magnification System Once the installation environment parameters and the position of the HUD system 10 are determined, δ y We can then calculate the value of , which defines the throw distance required to achieve a ghost-free projection.

[0073] The minimum angular resolution that the human eye can distinguish is 0.02 degrees, or 0.0003 radians, which is determined by the distance between the image and the observer 200, δ y If d is smaller than 0.0003 radians, the observer 200 will interpret the two images as one. Let d be the distance between the image and the observer 200. ei Thus, the condition for removing the ghost image can be written as follows:

number

[0074] Therefore, d ei = δ y A value of 0.0003 or greater is desirable and can be achieved by modifying the magnification system.

[0075] Figure 11 shows the main features of a magnification system abstracted. For ease of understanding, Figure 11 shows a system with a single field lens 120 with focal length f. An image of size h is projected at a distance d from the field lens 120. o , which is formed on the image plane 300 and passes through the lens 120 to form an image of size H on the image plane 400. ei are the distances between the projected image and the left L and right R eyes of the observer 200, as given by equation (4). The environmental installation parameters include the distance d between the virtual image plane 400 and the lens, as well as the magnification factor M=H / h. i =d ei -d v is known, the distance d between the exit aperture and the observer is v The imaging equation can be written as follows:

number

[0076] The magnification system produces a virtual image, so d i To keep d positive i The sign before becomes negative.

[0077] Rearranging equation (5) gives the following:

number

[0078] When choosing the focal length of a lens, the effect of distortion must also be considered. When the image plane is close to the focal plane, one may observe a barrel distortion of the target image, which increases in proportion to the square of the distance from the center. One solution to this problem is to choose a lens with a larger aperture or a longer focal length. For a fixed projection distance, the relationship between the focal length and the distance between the image plane and the focal plane is shown in Figure 12. As can be seen, for a given projection distance, the longer the focal length, the smaller the fd o The value of θ can be increased to reduce distortion. However, a longer focal length increases the overall optical path of the system, which can make it bulky. Therefore, it is necessary to choose a lens with a focal length that balances image quality with the overall size of the HUD.

[0079] Another important consideration is the lens aperture. A HUD has a certain field of view (FoV) and projects an image towards an eyebox. The FoV is the angular size of the virtual image seen by the observer 200, while the eyebox is usually defined as a rectangular box within which the observer 200 can move his / her head and still see the virtual image. The larger the FoV, the larger the image the observer 200 can see, and the larger the eyebox defines the effective zone within which the driver can see the HUD content. The theoretical FoV of the HUD system 10 is determined by the size of the target image and the magnification of the lens, while the eyebox of the HUD system 10 is determined by the lens aperture. The FOV value can be expressed as follows:

number

[0080] Consider three different aperture sizes W1-W3, with reference to the model in Figure 11. W1 is the minimum aperture size that achieves the theoretical FoV, W2 is the aperture size that allows both eyes to see the virtual image, and W3 is the aperture size that allows the driver to move his head within the eyebox. Then, the following identity is given:

number

[0081] In this way, system requirements are driven and determined by theoretical considerations, ensuring that the images produced are always ghost-free.

[0082] Intermediate Optical Element The previous section details the determination of the HUD installation location, the corresponding projection distance, and the selection of the appropriate lens. Intermediate optical elements are used to further adjust the light path and can be used to achieve high efficiency in a small overall system volume. Intermediate optical elements can include additional lenses, diffusers, mirrors, waveguides, diffractive optical elements, holographic optical elements, or transparent glass.

[0083] The field lens 120 is the main optical element of the system, which directly affects the quality of the projected image. Considering the eyebox of the HUD system 10, it is usually required to make the required aperture size quite large. Therefore, a Fresnel lens is selected as the field lens 120. However, the imaging quality of the Fresnel lens is not as good as that of a normal optical lens, which will bring distortion and aberration to the projected image. Therefore, in one embodiment, in order to optimize the imaging quality, a lens set for improving the imaging quality may be used instead of a single Fresnel lens.

[0084] Another important optical component in one embodiment is the diffuser 110. In a conventional HUD, a PGU 100 such as an LED (light emitting diode) panel or an LCD (liquid crystal display) screen is used as the image source, and the diffuser 110 is not necessary. However, one significant drawback of LED (light emitting diode) panels and LCD (liquid crystal display) screens is that the brightness and contrast of the images generated by these types of PGUs are rather low, making it difficult to see the projected image during the day. To improve the brightness and contrast of the HUD system 10, a digital micromirror device (DMD) projector may be used as the PGU 100. This requires that a diffuser 110 is placed after the projector to diffuse the light from the DMD so that the observer 200 can see the full image.

[0085] There are three important factors when selecting a diffuser 110: light transmittance, diffusion angle, and size. For light transmittance, the higher the better. For diffusion angle and size, it is determined by the magnification system mentioned above.

[0086] 13 shows a portion of a HUD system 10 consisting of a PGU 100, a diffuser 110 and a field lens 120. For simplicity, the diffuser 110 and the field lens 120 are represented by single lines indicating their positions and orientations, referred to as the diffuser plane and the lens plane, respectively. The diffuser 110 is located a distance d from the PGU 100. d and the diffuser 110 and the field lens are disposed at a distance d l W a is the aperture size of the field lens 120.

[0087] In use, the PGU 100 projects light onto the diffuser 110 to form an image, which is directed through the field lens 120. The diffuser plane is therefore equivalent to the image plane described in the magnification system shown in FIG.

[0088] In Fig. 13, η is the divergence angle of PGU 100 and ε is the diffusion angle of diffuser 110. Looking back to Fig. 11, the size of the image is defined as h, which is determined by the magnification system to meet the requirements of FoV and eyebox. To obtain the correct value of h, the distance between PGU 100 and diffuser 110 must satisfy:

number

number

[0089] By selecting a small diffusion angle, the effective viewing angle is restricted, but by selecting a large diffusion angle, the intensity of the light of the display decreases. In addition, the aperture size must be as large as possible in order to at least realize the required eye box. Therefore, the diffusion angle must satisfy the condition of W a <It must satisfy the condition of W3.

[0090] The mirror is another important optical element of the system, which is used to reduce the overall volume of the system or to change the optical path for the convenience of system design. When the main structure of the magnification system is defined, the size and position of the mirror added to the system can be calculated through the established optical path, ensuring that the mirror functions as an aperture pattern and does not adversely affect the performance of the HUD system 10.

[0091] System Configuration An exemplary installation environment is shown in FIG. 14. The HUD system 10 is arranged in the same way as in FIGS. 2 and 8, and the individual components are shown in FIG. 15A. The tilt angle of the front glass is 72°, the refractive index of the front glass is 1.5, and the thickness of the front glass is 8 mm. L x is 1.22 m, and L y is 70 mm. The distance d between the observer 200 and the Fresnel lens vis about 1.5 m. According to the approach described in the above section, 29° is selected as the minimum incidence angle, and the vertical displacement of the double image can be calculated to be 4.78 mm. Therefore, the ideal projection distance is about 14 m. To achieve a projection distance of 14 meters, a Fresnel lens with a focal length of 400 mm and an aperture size of 300 mm x 300 mm is selected. According to equation (6), the image plane must be located 20 mm away from the focal plane of the Fresnel length. The target FoV is 8° x 4°, and the eyebox is set to 130 mm x 50 mm. Therefore, the theoretically required aperture size of the Fresnel lens is 344 mm x 172 mm. Therefore, if a Fresnel lens with an aperture size of 300 mm x 300 mm is used, the actual eyebox will be about 100 mm x 50 mm. For the diffuser 110, a holographic diffuser with a transmittance of 90% and a diffusion angle of 30° is selected.

[0092] To achieve a large projection angle and reduce installation size, two mirrors 111, 112 are placed between the diffuser 110 and the field lens 120 as shown in Figures 15A-15D. Figures 15A-15D show a HUD system 10 consisting of a PGU 100, a diffuser 110, two mirrors 111, 112, and a field lens 120. The system 10 operates at least in a similar manner as described above in connection with Figures 2-4.

[0093] FIG. 16 summarizes the process for designing the HUD system 10 described above.

[0094] In step S100, environmental conditions are input, including the tilt angle, thickness and refractive index of the windshield 1, the position of the observer 200, and the available installation volume, which are then used to calculate the vertical displacement of the ghost image along with the virtual image distance.

[0095] In step S110, the calculated virtual image distance and the desired FoV and eyebox size are used to define the required focal length of the field lens 120.

[0096] In step S120, the outputted field lens 120 and PGU 100 characteristics are used to calculate the required position and parameters of the diffuser 110 (if used), as well as the aperture of any mirrors employed in the system 10. In this manner, the outputted parameters are adjusted according to environmental conditions.

[0097] 17A and 17B show a simulation of one embodiment of the present invention utilizing a PGU 100, a diffuser 110, two reflectors 111, 112, and a field lens 120. The simulation is based on a windshield with a tilt angle of 78°, a thickness of 8 mm, and a refractive index of 1.5. In this case, the image is projected at a distance of 15 m, achieving a ghost-free image.

[0098] FIG. 17C shows the projected light footprint of field lens 120 to demonstrate the absence of vignetting effects if each component is sized appropriately.

[0099] Applications involving asymmetric or highly curved windshields The thickness of windshields used in high-speed trains is much larger than that of conventional automobile windshields, as is their curvature. In addition, these windshields are often much more expensive than those for conventional road vehicles. Therefore, it is not particularly economically practical to manufacture wedge-shaped windshields for high-speed trains. However, based on the principles of the present invention, a ghost image-free HUD is provided that does not require modifications to the windshield.

[0100] A representative model of a high-speed train driver's seat is shown in Figure 18, which has a cylindrical outer shape in the target area (i.e., the area of ​​the windshield inner surface where the image is projected) with a windshield glass inclination angle of 20°, a windshield refractive index of 1.5, a windshield thickness of 28 mm, and a focal length of 714 mm. A typical installation position of the HUD results in an incidence angle of 59° from the HUD to the windshield. According to equations (1) and (2), to achieve a projection without ghost images, the projection distance needs to be more than 66.67 meters. However, to achieve such a long distance projection, the precision of the optics must be very high and the image size must be very large. As a compromise, a target projection distance of 20 meters was used.

[0101] The design goal of a high-speed train HUD is therefore to project an image 20 meters ahead with a field of view of H7° x V4° onto a 130mm x 50mm eyebox. The windshield, due to its curvature and thickness, acts as an anisotropic optic with different magnifications along the horizontal (secondary direction) and vertical (primary direction where ghost images are primarily seen) directions. The rest of the HUD system needs to be designed to accommodate this.

[0102] So, if a single lens is used to project an image 20 meters away, the windshield acts as a cylindrical mirror to re-image the image according to the off-axis imaging rules, so that the final image is presented at a very short projection distance (e.g., less than 1 m) in front of the driver. Therefore, the curvature of the windshield must be compensated by using an additional optical component, e.g., a second anisotropic optical component such as a cylindrical lens or a cylindrical mirror. In one embodiment, the additional optical component can be a freeform lens or freeform mirror for more complex windshield profiles.

[0103] 19 shows an embodiment of a HUD system 10 formed by a folding mirror 211, one isotropic field lens 220, and one concave cylindrical lens 221 configured to project an image onto a windshield 201 to be seen by an observer 200. For simplicity, no picture generation unit or diffuser is shown, although any suitable image generation and realization means may be employed.

[0104] In use, the generated optical object is projected by the PGU and first imaged by the field lens 220 such that the image is located within one focal length of the windscreen 2011, which then images the image 20 meters away. Since the windscreen 201 acts like a cylindrical mirror, ghost images (i.e. δ x ) is generated. Along the primary direction, the image has an apparent projection distance of only 1.5 meters from the observer, so ghost-free projection is not achieved. To achieve ghost-free projection, a cylindrical lens 221 is used, which has refractive power only along the primary direction. The cylindrical lens 221 is placed 10 mm behind the normal lens and is configured such that the final projection distance along the primary direction is 20 meters to match the final projection distance in the secondary direction. In conclusion, the image is projected 20 meters in both directions.

[0105] 2D and 3D simulation results for the optical configuration shown in Figure 19 are shown in Figures 20A, 20B, 20C, 21A and 21B. Figure 21A is a spot diagram across the eyebox showing nine fields of view of the generated optical object. Figure 21B shows the resulting image distortion of the optics including the windshield 201.

[0106] To further optimize the imaging quality, more optical elements may be introduced into the system along the optical path, if necessary. In addition, as shown in Figure 21B, the image seen by observer 201 in the above embodiment appears squashed along the horizontal direction due to the magnification difference along the horizontal and vertical viewing directions caused by the illustrated configuration. In one embodiment, this is corrected using pre-compensated image generation means, as is known.

[0107] Figure 22 shows an alternative embodiment identical to Figure 19, except that a cylindrical mirror 212 is used instead of mirror 211 and cylindrical lens 221. Following the same principle as in Figure 19, lens 220 is used to image the generated optical object, which is then further imaged 20 meters away along the secondary direction by windshield 201. Cylindrical mirror 212 is then used to add refractive power along the primary direction that affects the ghost image to match the projection distance of 20 meters along the primary direction, achieving a ghost image-free projection.

[0108] 2D and 3D simulation results of the optical configuration shown in Figure 22 are shown in Figures 23A, 23B, 23C, 24A and 24B. As can be seen, the use of a cylindrical mirror 212 can reduce achromatic dispersion in the full color image, although this can adversely affect absolute distortion and overall image quality.

[0109] Thus, a HUD system 10 according to an aspect of the present invention is provided.

Claims

1. 1. An image generating system for providing a ghost image-free head-up display, comprising: a display screen having a front surface and a back surface and having a first configuration; a picture generation unit for projecting an image towards the display screen for reflection towards a first predetermined eyebox; a field lens having a predetermined aperture size, the field lens being configured to change a diverging beam from the picture generating unit into a substantially parallel beam; the picture generation unit is configured to project light through the field lens such that light incident on the front side of the display screen is collimated to form a first virtual image, and a portion of the light is transmitted through the display screen and incident on the back side of the display screen to form a second virtual image, the first virtual image and the second virtual image having an offset; the field lens is configured such that the offset is less than a threshold magnitude, the first virtual image and the second virtual image are substantially overlapping when viewed from the eyebox, and the offset between the first virtual image and the second virtual image is less than a threshold angular resolution; the aperture size allows the eyebox to be larger than the interpupillary distance; The image generation system, wherein the picture generation unit and / or the field lens are further configured to adjust the offset between the first virtual image and the second virtual image when one of a configuration of the display screen or a predetermined eyebox changes.

2. 2. The imaging system of claim 1, wherein the first virtual image is projected onto a first virtual image plane at a first projection distance from the eyebox, and the second virtual image is projected onto a second virtual image plane at a second projection distance from the eyebox.

3. 3. The imaging system of claim 1, wherein the field lens is configured to project the first and second virtual images at a first and second projection distance such that the offset between the first and second virtual images is less than a threshold angular resolution.

4. The imaging system of claim 3 , wherein the threshold angular resolution is equal to a pixel dimension.

5. The imaging system of claim 3 , wherein the threshold angular resolution is equal to the angular resolution of the human eye.

6. 6. The image generating system of claim 1, wherein the first virtual image and the second virtual image are offset in a plane perpendicular to an optical axis.

7. 7. The imaging system of claim 1, wherein the field lens is provided by one of a concave mirror, a freeform surface, a Fresnel lens, a waveguide, a diffractive optical element, a holographic optical element, or one or more tapered optical fibers.

8. The image generation system of any one of claims 1 to 7, wherein the picture generation unit comprises a light source and a spatial light modulator.

9. The image generation system of any one of claims 1 to 8, wherein the picture generation unit comprises a projector and a diffuser for realizing a projected image.

10. The image generating system of any one of claims 1 to 8, wherein the picture generating unit comprises a laser and a two-dimensional scanning mirror.

11. 9. An image generation system according to any one of claims 1 to 8, wherein the picture generation unit comprises a holographic unit for generating a computer-generated hologram and a diffuser for realizing said hologram.

12. 9. The image generating system of claim 1, wherein the picture generating unit comprises one or more of a liquid crystal display device, a light emitting diode device, a micro light emitting diode device, an organic light emitting diode device, or a digital light processing digital micromirror device.

13. An image generation system according to any one of the preceding claims, further comprising optical elements interposed between the picture generation unit and the field lens and / or between the field lens and the display screen.

14. The imaging system of claim 13 , wherein the intervening optical element comprises one of a folding mirror, a cylindrical mirror, a cylindrical lens, a waveguide, a diffractive optical element, or a holographic optical element.

15. 15. The image generation system of claim 13 or 14, further comprising an image processor in communication with the picture generation unit, the image processor configured to take into account distortions caused by the intervening optical elements such that the image does not appear distorted on the display screen.

16. 16. An image generation system according to any one of the preceding claims, wherein the display screen of the head-up display is a windshield of a vehicle.

17. An image generation system according to any one of the preceding claims, wherein the first virtual image and the second virtual image relate to a first image of a stereogram of images.

18. An imaging system according to any preceding claim, wherein one or more of the field lens, projection unit and / or intervening optical components (if present) are movable relative to each other.

19. The first virtual image and the second virtual image are multi-dimensional images. An image generating system according to any one of claims 1 to 18.

20. 20. The image generation system of claim 1, wherein the first virtual image and the second virtual image comprise subsets of images, each image of the subset being realized at a different distance from a focal length of the field lens such that, when projected onto the display screen, each image of the subset has a different apparent depth when viewed from the eyebox.

21. 21. An image generating system according to any preceding claim, further comprising a photosensitive device arranged to capture light reflected by the display screen through the field lens.

22. 1. A method for providing a ghost image free head up display, comprising: generating, in a picture generation unit, an image to be rendered on a display screen having a front surface and a back surface for reflection towards a predetermined eyebox; providing a field lens between the picture generation unit and the display screen, the field lens having a predetermined aperture size and configured to change a diverging beam from the picture generating unit into a substantially parallel beam; a portion of the light incident on the front surface of the display screen is collimated and reflected to form a first virtual image, and a portion of the light is transmitted through the display screen and incident on the back surface to form a second virtual image that is offset from the first virtual image; configuring the field lens such that the offset is less than a threshold magnitude and the first virtual image and the second virtual image substantially overlap as viewed from the eyebox, the offset between the first virtual image and the second virtual image being less than a threshold angular resolution and the aperture size allowing the eyebox to be larger than an interpupillary distance; recognizing changes in the eyebox or the display screen and adjusting the images generated by the picture generation unit and / or field of view such that the first virtual image and the second virtual image substantially overlap as viewed from the eyebox; A method comprising:

23. 23. The method of claim 22, further comprising projecting the first virtual image onto a first virtual image plane at a first projection distance from the eyebox and the second virtual image onto a second virtual image plane at a second projection distance from the eyebox.

24. The method of claim 22 , wherein the threshold angular resolution is equal to a dimension of a pixel.

25. The method of claim 22 , wherein the threshold angular resolution is equal to the angular resolution of the human eye.

26. The method of any one of claims 22 to 25, wherein the first virtual image and the second virtual image are offset in a plane perpendicular to the optical axis.

Images