Device for generating a virtual image with field-point dependent aperture

Description

[0001] The present invention relates to an apparatus for generating a virtual image.

[0002] A head-up display, also known as a HUD, is a display system that allows the viewer to maintain their line of sight by projecting the content into their field of vision. While such systems were originally used primarily in aviation due to their complexity and cost, they are now also being installed in large-scale production in the automotive sector.

[0003] Head-up displays generally consist of an image generator, an optical unit, and a mirror unit. The image generator generates the image. The optical unit directs the image to the mirror unit. The image generator is often also referred to as an imaging unit or PGU (Picture Generating Unit). The mirror unit is a partially reflective, translucent screen. The viewer therefore sees the content displayed by the image generator as a virtual image and, at the same time, the real world behind the screen. In the automotive sector, the windshield is often used as the mirror unit, and its curved shape must be taken into account in the display. Due to the interaction of the optical unit and the mirror unit, the virtual image is an enlarged representation of the image generated by the image generator.

[0004] The viewer can only view the virtual image from the position of the so-called eyebox. An eyebox is an area whose height and width correspond to a theoretical viewing window. As long as one eye of the viewer is within the eyebox, all elements of the virtual image are visible to the viewer. If, however, the eye is outside the eyebox, the virtual image is only partially visible to the viewer or not visible at all. The larger the eyebox, the less restricted the viewer is in choosing their seating position.

[0005] The size of the eyebox of conventional head-up displays is limited by the size of the optical unit. One approach to enlarging the eyebox is to couple the light coming from the imaging unit into an optical fiber. The light coupled into the optical fiber is totally reflected at its interfaces and is thus guided within the optical fiber. In addition, a portion of the light is coupled out at numerous positions along the propagation direction. In this way, the optical fiber dilates the exit pupil. The effective exit pupil is composed of images of the aperture of the imaging system.

[0006] Against this background, US 2016 / 0124223 A1 describes a display device for virtual images. The display device comprises an optical waveguide that causes light coming from an imaging unit, which is incident through a first light incidence surface, to undergo repeated internal reflection to move in a first direction away from the first light incidence surface. The optical waveguide also causes a portion of the light guided in the optical waveguide to exit to the outside through regions of a first light exit surface extending in the first direction. The display device further comprises a first light-incident-side diffraction grating that diffracts incident light to cause the diffracted light to enter the optical waveguide, and a first light-outgoing diffraction grating that diffracts light incident from the optical waveguide.

[0007] A typical imager for head-up displays with holographic fiber optics features a scanning projector with LED-based light sources. The spectral width and wide range of beam angles ensure good aperture coverage in LED-based systems. Such systems are therefore less susceptible to interference effects such as banding.

[0008] To increase the efficiency and functionally expand head-up displays with holographic optical fibers, it makes sense to use lasers as light sources instead of LEDs. However, the use of lasers as light sources has the disadvantage that the systems are significantly less tolerant. The output aperture must be assembled very precisely to avoid disruptive effects. Currently, complex systems are used to create a variable aperture for uniformly filled composite apertures. These systems consist of various components and are complex to design. Each component is associated with tolerances and light loss. These systems also require some of the already limited installation space and increase both the weight and cost of the head-up display.

[0009] WO 2014 / 009717 A1 discloses a device according to the preamble of claim 1. DE 10 2016 220 742 A1 discloses a scanning projector transmission screen.

[0010] It is an object of the present invention to propose an improved device for generating a virtual image in which a field point-dependent aperture is realized in a simple manner.

[0011] This object is achieved by a device having the features of claim 1. Preferred embodiments of the invention are the subject of the dependent claims.

[0012] According to a first aspect of the invention, an apparatus for generating a virtual image comprises: at least one light source for generating a light beam; an imaging unit for generating an image, the imaging unit comprising a microscanner; and an optical fiber for expanding an exit pupil; wherein an arrangement comprising a first optical element with a first focal length, an optical component for realizing a field point-dependent aperture and a second optical element with a second focal length is arranged in front of a coupling-in region of the optical waveguide, wherein the optical component is configured to direct the light beam into different solid angle ranges for different impact positions of a light beam incident from the microscanner after passing through the first optical element, wherein the optical component has a microlens array with lens spacings and focal lengths that are variable according to a locally resolved diffuser function and lens function, and wherein the locally resolved diffuser function and lens function are selected such that an output beam diameter of the light beam after passing through the second optical element is equal to a period of the coupling-out of the light beam.

[0013] To achieve the field-point-dependent aperture, the optical component directs the light beam into different solid angle ranges for different incidence positions. The optical component can also incorporate additional optical functions, such as a Fresnelized lens function.

[0014] Due to the propagation of light in the optical fiber, small-aperture light sources, such as lasers, do not achieve illumination of the entire eyebox from every single point of image generation. The optical component provided according to the invention, which is specifically designed for the respective image generation area, generates a spatially resolved, adjusted aperture. This enables a more homogeneous filling of the output aperture of the optical fiber and thus of the eyebox.

[0015] According to the invention, the optical component has a locally resolved diffuser function and a lens function. The optical component can, for example, be designed as a flat plastic part with locally resolved diffuser and lens functions according to the desired spatial function for deflecting the incident light rays into the different solid angle ranges.

[0016] According to the invention, the optical component comprises a microlens array. When implemented as a microlens array, variable lens spacings and focal lengths can be used, depending on the desired spatial function.

[0017] According to the invention, the imaging unit comprises a microscanner. The inventive solution harmonizes particularly well with such a scanner, since the scanner ensures a defined relationship between the position of an incident light beam on the optical component and the associated angle of incidence. At the same time, a microscanner enables the realization of a particularly simple and compact imaging unit. The microscanner can, for example, be a MEMS scanner (MEMS: microelectromechanical system).

[0018] According to one aspect of the invention, light rays emitted by the light source are focused onto the optical component by the first optical element. This focusing ensures that light rays emitted by the light source impinge on a desired, spatially narrowly defined area of ​​the optical component. The first optical element, for example, a lens, can optionally also reduce image aberrations, such as field curvature.

[0019] According to one aspect of the invention, the second optical element collimates light rays emanating from the optical component, i.e., the second optical element parallelizes the divergent light beams formed by the optical component. In the process, the solid angle ranges are transformed into spatial aperture areas.

[0020] A device according to the invention is preferably used in a vehicle, in particular a motor vehicle.

[0021] Further features of the present invention will become apparent from the following description and the appended claims taken in conjunction with the figures. Figure overview

[0022] Fig. 1 schematically shows a head-up display according to the prior art for a motor vehicle; Fig. 2 shows an optical fiber with two-dimensional magnification; Fig. 3 schematically shows a head-up display with an optical fiber; Fig. 4 schematically shows a head-up display with an optical fiber in a motor vehicle; Fig. 5 schematically shows a head-up display according to the invention with an imaging unit, a coupling unit and an optical fiber; Fig. 6 illustrates the functioning of an optical component arranged in the coupling unit; Fig. 7 schematically shows a perspective view of an optical fiber of a head-up display according to the invention. Fig. 8 shows an example of the course of a collimated beam in an optical fiber; Fig. 9 shows this course for two different angles in comparison; and Fig. 10 shows an example of beam shaping according to the invention. Character description

[0023] To better understand the principles of the present invention, embodiments of the invention are explained in more detail below with reference to the figures. Like reference numerals are used in the figures for like or equivalent elements and are not necessarily described again for each figure. It is understood that the invention is not limited to the illustrated embodiments and that the described features can also be combined or modified without departing from the scope of the invention as defined in the appended claims.

[0024] First, based on the Figures 1 to 4 the basic idea of ​​a head-up display with fiber optic cable is explained.

[0025] Fig. 1shows a schematic diagram of a head-up display according to the prior art for a motor vehicle. The head-up display has an image generator 1, an optical unit 2, and a mirror unit 3. A beam SB1 emanates from a display element 11, which is reflected by a folding mirror 21 onto a curved mirror 22, which reflects it toward the mirror unit 3. The mirror unit 3 is depicted here as the windshield 31 of a motor vehicle. From there, the beam SB2 travels toward an eye 61 of a viewer.

[0026] The viewer sees a virtual image VB, which is located outside the motor vehicle above the hood or even in front of the motor vehicle. Due to the interaction of optical unit 2 and mirror unit 3, the virtual image VB is an enlarged representation of the image displayed by display element 11. A speed limit, the current vehicle speed, and navigation instructions are symbolically displayed here. As long as the eye 61 is within the eyebox 62 indicated by a rectangle, all elements of the virtual image are visible to the eye 61. If the eye 61 is outside the eyebox 62, the virtual image VB is only partially visible to the viewer or not visible at all. The larger the eyebox 62, the less restricted the viewer is in choosing their seating position.

[0027] The curvature of the curved mirror 22 serves, on the one hand, to prepare the beam path and thus ensure a larger image and a larger eyebox 62. On the other hand, the curvature compensates for a curvature of the windshield 31, so that the virtual image VB corresponds to an enlarged reproduction of the image displayed by the display element 11. The curved mirror 22 is rotatably mounted by means of a bearing 221. The resulting rotation of the curved mirror 22 enables the eyebox 62 to be moved and thus the position of the eyebox 62 to be adjusted to the position of the eye 61. The folding mirror 21 ensures that the path traveled by the beam SB1 between the display element 11 and the curved mirror 22 is long, while at the same time the optical unit 2 remains compact. The optical unit 2 is separated from the surroundings by a transparent cover 23.The optical elements of the optical unit 2 are thus protected, for example, against dust present in the interior of the vehicle. Furthermore, an optical film 24 or a coating is located on the cover 23, which is intended to prevent incoming sunlight SL from reaching the display element 11 via the mirrors 21, 22. This could otherwise be temporarily or permanently damaged by the heat generated during this process. To prevent this, an infrared component of the sunlight SL, for example, is filtered out using the optical film 24. A glare shield 25 serves to shade light entering from the front so that it is not reflected by the cover 23 toward the windshield 31, which could dazzle the viewer. In addition to the sunlight SL, the light from another interfering light source 64 can also reach the display element 11.

[0028] Fig. 2shows a schematic spatial representation of an optical waveguide 5 with two-dimensional magnification. In the lower left area, a coupling hologram 53 can be seen, by means of which light L1 coming from an imaging unit (not shown) is coupled into the optical waveguide 5. In this area, it propagates to the top right in the drawing, according to arrow L2. In this area of ​​the optical waveguide 5 is a folded hologram 51, which acts similarly to many partially transparent mirrors arranged one behind the other, and generates a light beam that is broadened in the Y direction and propagates in the X direction. This is indicated by three arrows L3.In the part of the optical waveguide 5 extending to the right in the figure, there is an output hologram 52, which also functions similarly to many partially transparent mirrors arranged one behind the other and, indicated by arrows L4, outputs light upwards in the Z direction from the optical waveguide 5. This results in a broadening in the X direction, so that the original incident light beam L1 leaves the optical waveguide 5 as a light beam L4 enlarged in two dimensions.

[0029] Fig. 3shows a spatial representation of a head-up display with three optical waveguides 5R, 5G, 5B, which are arranged one above the other and each represent an elementary color red, green, and blue. Together, they form the optical waveguide 5. The holograms 51, 52, 53 present in the optical waveguide 5 are wavelength-dependent, so that one optical waveguide 5R, 5G, 5B is used for each of the elementary colors. An image generator 1 and an optical unit 2 are shown above the optical waveguide 5. The optical unit 2 has a mirror 20, by means of which the light generated by the image generator 1 and shaped by the optical unit 2 is deflected in the direction of the respective input hologram 53. The image generator 1 has three light sources 14R, 14G, 14B for the three elementary colors. It can be seen that the entire unit shown has a low overall height compared to its light-emitting surface.

[0030] Fig. 4shows a head-up display in a motor vehicle similar to Fig. 1 , but here in a spatial representation and with an optical fiber 5. The schematically indicated image generator 1 can be seen, which generates a parallel beam SB1, which is coupled into the optical fiber 5 by means of the mirror plane 523. The optical unit is not shown for the sake of simplicity. Several mirror planes 522 each reflect a portion of the light incident on them towards the windshield 31, the mirror unit 3. From this, the light is reflected towards the eye 61. The observer sees a virtual image VB above the hood or at an even greater distance in front of the vehicle.

[0031] Fig. 5shows a schematic diagram of a head-up display according to the invention with an imaging unit 1, a coupling unit 80, and an optical fiber 5. In this example, an imaging unit 1 with a light source 14 and a microscanner 73 is used. The light source 14 consists of at least one laser, but a maximum of four lasers for different wavelength ranges. For example, the lasers can implement an RGBY light source and thus emit red, green, blue, and yellow light. Fig. 5Three light sources 14R, 14G, 14B for red, green, and blue light are shown. The three light sources 14R, 14G, 14B are each modulated in time in terms of their intensity. The light L1 from the light source 14 is focused by an optics 74 onto a mirror 730 of the microscanner 73, which oscillates in at least one direction. A beam waist with a diameter of less than 1.5 mm is preferably used. The mirror 730 directs the light L1 onto a coupling unit 80 for the optical waveguide 5 in accordance with its oscillation. According to the invention, a first optical element 70 is located between the microscanner 73 and the coupling unit 80, which first optical element 70 focuses the light L1 onto an optical component 71 of the coupling unit 80 and, in the process, optionally minimizes image errors, e.g., reduces the image field curvature.

[0032] In addition to the optical component 71, the coupling unit 80 comprises a coupling hologram 53 of the optical waveguide 5 and a second optical element 72. The optical component 71 directs an incident light beam L1 into different solid angle ranges for different incidence positions. The second optical element 72 collimates the divergent light beam formed by the optical component 71. The respective solid angle ranges are transformed into spatial aperture areas. The coupling hologram 53 couples the incident collimated light into the optical waveguide 5. The coupling unit 80 of the optical waveguide 5 thus converts a light beam L1 incident at an angle into a propagating light beam in the material of the optical waveguide 5. The temporal modulation of the intensity of the light beam L1 is controlled according to the oscillation for displaying the image content.

[0033] Fig. 6illustrates the functionality of the optical component 71 arranged in the coupling unit. The light beams L1_1, L1_2 deflected by the microscanner strike the optical component 71 in at least one spatial direction according to the oscillation of the mirror in a defined angle-to-position relationship. This fills different solid angle ranges for different impact positions of the incident light beams L1_1, L1_2. As an example, the conversion of incoming light beams L1_1, L1_2, which enter at a defined angle, into outgoing light beams L1_1', L1_2' with a specific aperture, which may emerge at a different angle, is shown. The optical component 71 can also comprise other optical functions, such as a Fresnelized lens function.The optical component 71 can, for example, be designed as a flat plastic part with locally resolved diffuser and lens functions corresponding to the desired spatial function for the deflection of the incident light rays L1_1, L1_2 into the different solid angle ranges.

[0034] According to one embodiment, the optical component 71 comprises a hologram or a microlens array. When implemented as a microlens array, variable lens spacings and focal lengths can be used, adapted to the desired spatial function.

[0035] Fig. 7shows a schematic perspective view of an optical waveguide 5 of a head-up display according to the invention. It can be seen that the light beams L1_1', L1_2 coupled into the optical waveguide 5 at different angles fill different specific apertures A1, A2. Due to the propagation of the light in the optical waveguide 5, laser light sources with a small aperture conventionally do not illuminate the entire eyebox by every single point generated by the scanner. The optical component according to the invention, which is specifically designed for the respective area of ​​image generation, generates a spatially resolved, adapted aperture A1, A2. This enables a more homogeneous filling of the output aperture of the optical waveguide 5 and thus of the eyebox.

[0036] Fig. 8shows a schematic of the path of a collimated beam of light in an optical fiber. The described head-up display typically aims to display a virtual image far (measured in meters) behind the output aperture of the described head-up display. This aperture is preferably located above the radiator of a motor vehicle at the front of the vehicle or - especially for augmented reality applications - further in front of the vehicle. One way to implement special features of the image coupling is shown using Fig.7explained. In a head-up display with fiber optic technology, a pixel on the imager (image point a) is translated by the optics of the imaging unit 1 into a collimated beam with the projected width b (aperture) and the angle α. If the viewer's eye 61 is located in the output coupling area, the imaged point a' corresponding to a is created in the viewer's eye upon accommodation to the distance. The aperture of the optics of the imaging unit 1 is replicated multiple times by reflections during the propagation of the light by means of total internal reflection and by stepwise output coupling through the grating. The period g with which the output couplings are repeated depends on the glass thickness d and the angle at which the beam propagates in the waveguide.One aim of the invention is to achieve the most uniform possible illumination of the output aperture by arranging reflections of the aperture of the optics of the imaging unit 1 in order to avoid image reproduction errors such as stripes. For this purpose, the gap z=gb should be controlled, ie . be eliminated according to the invention (z=0). According to the invention, b=g is the goal.

[0037] The period g depends on the propagation angle in the optical fiber 5 from the field point. Fig.9 shows the path of a collimated light beam for two different angles α in comparison, the left for a larger angle than the right. Note that the light beams L1_1, L1_1', L1_2, and L1_2' each form extended groups of light beams. These groups are also called light beams. These light beams have a certain width.

[0038] The invention achieves the most uniform illumination possible through beam shaping. In the example of the Fig. 10 A beam of diameter d1 is assumed, which scans the image field, e.g., with a microscanner. The angle of incidence ε1 on the plane of the first optical element varies with the focal length f1, which focuses the beam into a small area of ​​the optical component. The point of incidence on this depends directly on ε1 and thus on the field point. The aperture angle α, on the other hand, is given by d1, both assuming a fixed f1. The optical component, e.g., an optical microstructure, deflects and expands the incident beam.

[0039] This means that for each incident area, the angular range γ into which the incident light is distributed, as long as γ≥α, and the orientation δ in which the light lies are individually determined. A second optical element with a focal length of f2 transforms the light back into a collimated beam, whose diameter d2 is determined by γ, and the angle ε2 by the position on the optical component. This allows the appropriate d2, and thus effectively b, to be set for each ε2 for the optical component, so that b=g is achieved.

[0040] The position of the exit aperture can be controlled using δ, which is advantageous for ensuring the coupling grating is aligned evenly for every field point. The ratio of the effective focal lengths can be used to translate the angular range, thus generating the desired image field of the system from a given scan angle.

[0041] The operating principle remains the same when other imaging techniques are used. When imaging with an SLM, such as a DMD (Digital Micromirror Device) projector, the first optical element is typically part of the projector. Due to the parallelism of the optical component to the DMD intermediate image plane, the deflecting effect is referred to as the lens function, and the widening effect is referred to as the diffuser function.

Claims

1. Device for generating a virtual image (VB), comprising: - at least one light source (14, 14R, 14G, 14B) for producing a light beam (L1); - a picture-generating unit (1) for producing an image; and - an optical waveguide (5, 5R, 5G, 5B) for expanding an exit pupil; characterized in that - the picture-generating unit (1) has a microscanner (73), and - an arrangement comprising a first optical element (70) having a first focal length (f1), an optical component (71) for realizing a field-point-dependent aperture and a second optical element (72) having a second focal length (f2) is arranged upstream of a coupling-in region of the optical waveguide (5, 5R, 5G, 5B), wherein the optical component (71) is set up to direct, for different incidence positions of a light beam (L1_1, L1_2) that is incident from the microscanner (73) after passing the first optical element (70), the light beam (L1_1, L1_2) into different solid-angle regions, wherein the optical component (71) has a microlens array having lens distances and focal lengths that are variable according to a locally resolved diffuser function and lens function, and wherein the locally resolved diffuser function and lens function is selected such that the output beam diameter (b, d2) of the light beam (L1_1', L1_2') after passing the second optical element (72) is equal to the period (g) of the coupling-out of the light beam (L1_1', L1_2') from the optical waveguide (5, 5R, 5G, 5B), and wherein light beams (L1_1, L1_2) emanating from the picture-generating unit (1) are focused by the first optical element (70) at the optical component (71) and the second optical element (72) collimates light beams (L1_1', L1_2') emanating from the optical component (71).

2. Device according to the preceding claim, wherein the light source (14, 14R, 14G, 14B) is a laser.

3. Vehicle having a device according to one of Claims 1 to 2 for generating a virtual image (VB) for a driver of the vehicle.