Wavefront manipulator for head-up display with holographic element to create a tilted virtual image plane

Abstract

The invention relates to a wavefront manipulator (7) for arrangement in the beam path (8) of a head-up display (10) between an imaging unit (1) and a projection surface (4). The wavefront manipulator (7) comprises a holographic assembly (3) which has at least two holographic elements (11, 12), the at least two holographic elements (11, 12) being arranged directly behind one another in the beam path (8), at least in sections, and being designed to be reflective for at least one fixed wavelength and a fixed irradiation angle range, wherein a first holographic element comprises at least one hologram which is assigned to a hologram of a second holographic element for reflection purposes. The wavefront manipulator (7) is designed for at least one fixed object plane to generate an image plane of a virtual image (6) which is tilted about a fixed tilt angle θ with respect to a plane (14) arranged perpendicular to the optical axis (13) in the region of the image plane of a virtual image (6), wherein the holographic arrangement (3) is designed for at least partial correction of at least one imaging error of a virtual image (6) generated in the tilted image plane.

Description

[0001] Wavefront manipulator for head-up display with holographic element for generating a tilted virtual image plane

[0002] The present invention relates to a wavefront manipulator for placement in the beam path of a head-up display (HUD) between a projection lens and a projection surface, in particular a curved projection surface. The invention further relates to an optical arrangement and a head-up display.

[0003] Head-up displays are now used in a wide variety of applications, including vehicle viewing windows, such as car windshields, windscreens, or aircraft viewing windows. These viewing windows, and especially windshields, typically have a curved surface, which is used as a projection surface for head-up displays.

[0004] A head-up display typically comprises an imaging unit (PGU) or a projector, a projection surface, an eyebox, and an image plane of the virtual image. An image is generated using the imaging unit or projector. The image is projected onto the projection surface and from there into the eyebox. The eyebox is a plane or spatial area in which the projected image is perceivable by a viewer as a virtual image. The image plane of the virtual image, i.e., the plane on or in which the virtual image is generated, is located on or behind the projection surface.The curvature of the projection surface and compact arrangements in a small installation space, with potentially significant tilts of individual components relative to one another and correspondingly complex folded beam paths, lead to imaging errors or aberrations. A windshield can generally be described as an optical freeform surface. If a head-up display is used in conjunction with a curved windshield or a curved viewing window, it is desirable to correct imaging errors caused by the curvature, the aforementioned imaging errors that may occur due to installation space constraints, and any imaging errors in the optical beam path caused by the imaging unit. The imaging errors or aberrations that can occur in this case include distortion, defocus, tilt, astigmatism, curvature of the image plane, spherical aberrations, higher astigmatism, and coma.In connection with head-up displays, especially for vehicle applications, the largest possible field of view, the largest possible eyebox and a uniform, bright and multi-colored image, preferably multi-colored in each pixel, are also desired.

[0005] Documents DE 102007 022 247 A1, DE 102015 101 687 A1, DE 10 2017 212 451 A1, and DE 10 2017 222 621 A1 describe holographic imaging optics for head-up displays, particularly in connection with windshields. In connection with head-up displays, it is increasingly necessary and desirable to display information or multicolored images at different image distances. To reduce costs and maintain system stability, it is also necessary for the components used in the head-up display to be permanently installed or arranged in a fixed manner relative to one another.

[0006] Documents JP 2021-012255 A and JP 2015-18099 A disclose head-up displays with a tilted virtual image plane. Against this background, the object of the present invention is to provide an advantageous wavefront manipulator for arrangement in the beam path of a head-up display between a projection lens and a curved projection surface, which at least partially meets the aforementioned requirements and at least partially corrects the aforementioned aberrations. Further objects include providing an advantageous optical arrangement for a head-up display on a curved projection surface, as well as an advantageous head-up display.

[0007] The first object is achieved by a wavefront manipulator according to claim 1. The further objects are achieved by an optical arrangement according to claim 12 and by a head-up display according to claim 15. The dependent claims contain further advantageous embodiments of the invention.

[0008] The wellfront manipulator according to the invention for arrangement in the beam path of a head-up display between an imaging unit (PGU) or a projection lens and a projection surface, for example, a curved projection surface, comprises a holographic arrangement. The holographic arrangement, which can be transmissive and / or reflective overall, comprises at least two holographic elements. The at least two holographic elements are arranged directly one behind the other in the beam path, at least in sections. In other words, no further optical element or component is arranged between the at least two holographic elements.The at least two holographic elements are furthermore designed to be reflective for at least one specified wavelength, in particular at least one specified wavelength range, and a specified angle of incidence range, wherein a first holographic element comprises at least one hologram which is assigned to a hologram of a second holographic element for reflection. In other words, the at least two holographic elements are designed such that light of at least one wavelength and at least one angle of incidence reflected by a first holographic element is reflected by the second holographic element. Furthermore, the holographic elements are preferably designed to be transmissive. The second holographic element can be arranged in the beam path upstream of the first holographic element.In this variant, the light rays are at least partially transmitted before they are reflected by the first holographic element.

[0009] The wavefront manipulator according to the invention is designed for at least one defined object plane, which can be formed, for example, by the plane of an exit pupil of an imaging unit, to generate an image plane of a virtual image that is tilted by a defined tilt angle θ with respect to a plane arranged perpendicular to the optical axis in the region of the image plane of a virtual image. Tilted means that the tilt angle θ is not equal to 0 degrees. In other words, there are at least two image points in the tilted image plane that have a different distance from the eyebox. The aforementioned optical axis can also be defined as the optical axis or virtual principal ray direction starting from the eyebox in the direction of the image plane of the virtual image.The holographic arrangement is designed to at least partially correct at least one aberration, preferably a plurality of aberrations, of a virtual image generated in the tilted image plane. The aberrations can be caused, for example, by the tilt of the image plane, a curvature of the projection surface, a widening of the beam path, etc.

[0010] The wavefront manipulator according to the invention has the advantage that different image distances can be created in an image plane and the image plane can be made clearer with regard to the arrangement of information thanks to the depth dimension. For example, information can be projected at different image distances. This is particularly relevant in connection with head-up displays for vehicles. In a tilted image plane in which the upper area of ​​the image plane has a greater image distance from the eyebox than the lower area of ​​the image plane, information can be displayed at a shorter distance in the lower area of ​​the image plane. Information on the vehicle speed, for example, can be placed here. In the upper area of ​​the image plane, information from a viewer or an eyebox can be displayed further away. This could be navigation information or warnings, for example.

[0011] The tilt angle θ can be between 10 degrees and 170 degrees, preferably between 30 degrees and 170 degrees, for example between 40 degrees and 50 degrees or between 130 degrees and 140 degrees.

[0012] The defined object plane can be tilted by a defined tilt angle relative to a plane arranged perpendicular to the optical axis at the object plane or in the region of the object plane.

[0013] In a further variant, the at least two holographic elements are arranged at a distance of a maximum of 50 millimeters, e.g. a maximum of 10 millimeters, preferably a maximum of 1 millimeter, from one another. Additionally or alternatively, the at least two holographic elements can be arranged tilted with respect to one another, wherein they enclose an angle of a maximum of 30 degrees, e.g. a maximum of 10 degrees, preferably a maximum of 5 degrees. In particular, the at least two holographic elements can each have surface normals, wherein the surface normals of two holographic elements enclose an angle of a maximum of 30 degrees, e.g. a maximum of 10 degrees, preferably a maximum of 5 degrees. The at least two holographic elements are preferably arranged parallel to one another and / or directly adjacent to one another.

[0014] The use of two at least partially reflective holographic elements arranged directly one behind the other has the advantage that, particularly in connection with a head-up display, a large field of view (FOV) can be achieved with high efficiency and the image quality can be significantly improved by the individual design of the holographic elements. In addition, the holographic elements take up almost no installation space, so that a significant increase in image quality can be achieved by means of the wavefront manipulator according to the invention even when only limited installation space is available, such as in a head-up display designed for a motor vehicle. The holographic arrangement achieves, in particular, a high refractive power, comparable to the refractive power achieved, for example, by a transmissive optical component without chromatic aberration.Compared to transmission holograms, reflective holograms offer a wider angular spectrum for a defined wavelength with high efficiency and greater wavelength selectivity. This allows the color channels to be separated from each other despite a broad spectrum of angles of incidence, thus avoiding double images. The holographic arrangement thus enables a large field of view (FOV) with simultaneous high efficiency, making it suitable for both VR head-up displays (VR - Virtual Reality) and augmented reality head-up displays (AR-HID) with a large field of view and large numerical aperture. Further possible applications include head-up displays with curved projection surfaces, for example, head-up displays for vehicle windshields, especially motor vehicles, aircraft, or ships, as well as for viewing windows in general.In addition, the holographic arrangement can also be used to correct aberrations caused by the tilt of the image plane, such as differences in brightness and distortions, in particular keystone distortion. In other words, in an advantageous variant, the holographic arrangement is designed to at least partially correct at least one imaging error caused by the tilt of the image plane. The at least one imaging error can be the aberrations mentioned above. These aberrations can, however, also be corrected digitally. A further advantage achieved by the holographic arrangement is that, due to the high diffraction angle of the holographic arrangement, the proportion of light from unused diffraction orders that is reflected into the eyebox is reduced.In addition, high-quality multi-color images can be produced.

[0015] Advantageously, the wavefront manipulator is designed to manipulate a wavefront to generate a multicolored virtual image. This means that a multicolored virtual image can be generated at every point on the tilted image plane. The wavefront manipulator can therefore be designed to manipulate light in the wavelengths or frequencies of at least one specified color space and, within the context of a heads-up display, to convert it into a multicolored virtual image. In other words, the wavefront manipulator can be designed for an imaging unit to generate a multicolored image. The imaging unit can be designed to emit light in the wavelengths or frequencies of at least one specified color space. The color space can be, for example, an RGB color space (RGB - Red Green Blue) or a CMY color space (CMY - Cyan Magenta Yellow).

[0016] The at least two holographic elements are, for example, reflective for at least two specified wavelengths that differ from one another and are designed for a specified angle of incidence. Preferably, the at least two holographic elements are reflective for at least two specified wavelength ranges that differ from one another and do not overlap, and for a specified angle of incidence. Advantageously, the at least two holographic elements are transmissive for specified wavelength ranges and / or at least one specified angle of incidence for which they are not reflective. This reduces or avoids filter effects.In a preferred variant, the wavefront manipulator according to the invention comprises at least one optical element that has a freeform surface, i.e., an optically effective freeform surface, and is designed for placement in the beam path between the imaging unit and the holographic arrangement. The optical element comprising the freeform surface contributes to an improvement in resolution through a corresponding design of the freeform surface and allows for targeted correction of imaging errors. Furthermore, due to the freeform surface, the optical element requires very little installation space. It therefore also contributes significantly to improving the imaging quality of a compact head-up display.

[0017] The optical element having the freeform surface can be reflective and / or transmissive. In connection with an application for compact head-up displays, a reflective design is particularly advantageous because in this way the optical element can simultaneously contribute to the beam deflection that is required anyway, even at high angles of incidence, without inducing additional image defects such as, in particular, chromatic aberrations. The freeform surface is preferably designed to at least partially correct at least one aberration or imaging error. This can be at least one of the imaging errors mentioned above.The aberrations can be caused by the tilt of the image plane and / or by the projection surface, especially in the case of a curved projection surface, and / or by the imaging unit and / or by the geometry of the beam path, for example, in a head-up display. Furthermore, the freeform surface can be used to optimize the resolution and thus the image quality.

[0018] Preferably, the freeform surface has a surface geometry derived from an imaging function dependent on at least one specified parameter. The at least one specified parameter can result from an intended application of the wavefront manipulator. The optical element can have multiple freeform surfaces, in particular to enable corrections of aberrations adapted to the respective application geometry.

[0019] Preferably, each of the at least two holographic elements comprises a number of holograms. Each hologram is recorded or generated with at least one specified wavelength. A holographic element can, for example, comprise several holograms, which can be arranged on top of one another as a stack. For example, a holographic element can have a number, preferably a plurality, of monochromatic holograms. Alternatively, a holographic element can comprise at least one hologram recorded or generated with at least two specified wavelengths. Such a hologram is preferably recorded with three different wavelengths of a specified color space, for example, as an RGB hologram or CMY hologram or as a hologram formed from a number of individual wavelengths of another color space.In the examples mentioned, R stands for red, G for green, B for blue, C for cyan, M for magenta and Y for yellow.

[0020] Thus, at least one, preferably two, of the at least two holographic elements can comprise at least two, preferably three, holograms which are designed to be reflective for wavelengths that differ from one another. Additionally or alternatively, at least one, preferably two, of the at least two holographic elements can comprise at least one hologram that is designed to be reflective for at least two, preferably three, wavelengths that differ from one another. In other words, the said holograms have been recorded with correspondingly different wavelengths. For example, of the at least two holographic elements, a first holographic element can comprise at least one hologram designed or efficient for a first color, preferably three holograms, each of which is designed for one of the three colors of a color space, and a second holographic element can comprise at least one hologram designed or efficient for the first color.efficient hologram, preferably three holograms, each designed for one of the three colors of the color space. The two holographic elements can be arranged next to one another such that a stack of holograms of the first holographic element is arranged next to a stack of holograms of the second holographic element. However, associated holograms can also be arranged directly adjacent to one another. In this case, only individual sections of the holographic elements are arranged directly one behind the other.For example, the hologram of the first holographic element designed for a first color can be arranged directly adjacent to the hologram of the second holographic element designed for the first color, the hologram of the first holographic element designed for a second color can be arranged directly adjacent to the hologram of the second holographic element designed for the second color, etc.

[0021] The arrangement of the individual holograms of a holographic element or of the entire hologram array can be used as a degree of freedom to avoid filtering effects between the holograms. The individual, differing holograms of a holographic element can be arranged side by side and / or behind each other with respect to a center line or central axis, which may coincide with the optical axis, or with respect to another specified geometric parameter of the holographic element.

[0022] For example, the first holographic element can be arranged mirror-symmetrically to the second holographic element with respect to the arrangement of the individual holograms. For example, the first holographic element can comprise a hologram recorded with red light, a hologram recorded with green light, and a hologram recorded with blue light, which are arranged one above the other in the stated order. The second holographic element can also comprise a hologram recorded with red light, a hologram recorded with green light, and a hologram recorded with blue light, which are also arranged one above the other in this order.In the case of a mirror-symmetrical arrangement, the first holographic element and the second holographic element are arranged one above the other or adjacent to each other such that, for example, the hologram of the first holographic element recorded with red light is arranged directly adjacent to the hologram of the second holographic element recorded with red light. Alternatively, the arrangement of the holograms of the first holographic element can be identical to the arrangement of the holograms of the second holographic element with respect to a specified direction.For example, both holographic elements can have holograms arranged in the RGB sequence (R - hologram recorded with red light, G - hologram recorded with green light, B - hologram recorded with blue light) with respect to a specified direction, arranged so that the R hologram of one holographic element is adjacent to the B hologram of the other holographic element. Any other, different arrangements are also possible, for example, RGB adjacent to GBR or R adjacent to R, G adjacent to G, and B adjacent to B, etc.

[0023] In the simplest case, the holographic arrangement can comprise a first holographic element and a second holographic element, wherein several of the holograms or all of the holograms of the respective holographic element are identical or identically configured except for the wavelength for which they are designed. In other words, several or all of the holograms of the first holographic element can be identically configured and differ from each other only with respect to the wavelength for which they are designed. Analogously, several or all of the holograms of the second holographic element can be identically configured and differ from each other only with respect to the wavelength for which they are designed.

[0024] In a further variant, a plurality of holograms of at least one of the holographic elements are recorded with two design wavefronts. Of these, at least one design wavefront of at least one hologram of the holographic elements is identical in terms of wavelength and angle of incidence to at least one design wavefront of another hologram of one of the holographic elements, in particular of the first and / or second holographic element. The use of identical design wavefronts for different wavelengths has the advantage that the required holograms can be produced with little effort and high precision. However, it is also possible for the design wavefronts to differ slightly from one another in terms of wavelength and / or angle of incidence. For example, the angles of incidence can differ by 1 to 2 degrees.The deviation can be used to compensate for material shrinkage and to optimize the homogeneity of efficiency.

[0025] The jointly used design wavefront is preferably defined as a plane wave, which results in minimal filtering between different wavelengths. The direction of incidence of the design wavefront for the at least two holographic elements of the holographic arrangement can be used as a degree of freedom to avoid filtering between different wavelengths. The direction of incidence can also be chosen differently for each wavelength. In a simple case, the design wavefronts for the at least two wavelengths, preferably the three wavelengths, are the same design wavefronts for each holographic element and differ only in the wavelength used.In principle, the at least two holographic elements can comprise reflection holograms recorded with two construction wavefronts, of which at least one construction wavefront is a plane wavefront or a spherical wavefront or a freeform wavefront.

[0026] In connection with a correction of a plurality of aberrations in a tilted image plane, it is advantageous if the at least two holographic elements comprise reflection holograms, wherein at least one reflection hologram is recorded or written with two design wavefronts, wherein at least one of the design wavefronts is designed such that it is generated according to a function or formation rule, which comprises, for example, polynomials, which has a plurality of degrees of freedom, i.e. a plurality of independently adjustable parameters. To effectively correct as many imaging errors as possible, the highest possible number of degrees of freedom is required. This can be achieved by appropriate design wavefronts using a single compact component.

[0027] The at least two holographic elements are preferably configured such that a first holographic element comprises at least one hologram associated with a hologram of a second holographic element, wherein mutually associated holograms are configured to be point-by-point diffraction-efficient with respect to one another. To determine the diffraction efficiency, either the intensity of the first diffraction order is set in relation to the sum of the intensity of the first diffraction order and the intensity of the zero diffraction order, or the intensity of the first diffraction order is set in relation to the total incident beam intensity.In other words, point-wise diffraction efficiency means that at least one point of the first holographic element is designed to diffract light of at least one specified wavelength and within a specified irradiation angle range to a point of the second holographic element, which in turn diffracts the light diffracted by the first holographic element. The efficiency is preferably above 90 percent.

[0028] The spacing and thickness of the holograms are negligible compared to the dimensions of the wavefront manipulator or an optical arrangement encompassing the wavefront manipulator. The holographic arrangement is therefore free of aberrations potentially caused by expansion along an optical axis. Furthermore, the design wavefronts of the holographic elements can be used as a degree of freedom to compensate for material tolerances, for example, to compensate for material shrinkage. In this case, the overall design wavefronts differ slightly from each other.

[0029] Preferably, the at least two holographic elements are arranged at a distance of less than one millimeter from each other, in particular less than 0.5 millimeters, preferably less than 0.1 millimeters. The distance is preferably zero or negligible. This achieves high imaging quality, while also eliminating the need for subsequent adjustment of the individual holographic elements with respect to their relative positions.

[0030] The holographic arrangement can be designed in the form of a layer or a film or a substrate, for example in the form of a volume hologram, or a plate. Additionally or alternatively, the holographic arrangement can have a flat surface or a curved surface. The holographic arrangement can, for example, be arranged on or on a surface of a cover glass or another already present optical component. In this way, no additional installation space is required. For example, the wavefront manipulator can comprise a transmissive optical component designed to be arranged in the beam path between the holographic arrangement and the projection surface. In this case, the holographic arrangement can preferably be arranged on a surface of the transmissive optical component facing away from the projection surfaces.Both the transmissive optical component and the holographic arrangement can be curved, preferably with the same curvature. The transmissive optical component can, for example, be a so-called glare trap, which is typically arranged between a windshield and a head-up display and is designed to reflect sunlight in a specified direction so that it is not reflected via the head-up display toward the eyebox. In this embodiment, the holographic arrangement and the glare trap are preferably designed with the same curvature and arranged directly adjacent to one another.

[0031] Overall, the wavefront manipulator according to the invention enables significantly stronger or more extreme redirection of the light used through the holographic elements than is possible with conventional refractive optical components. Furthermore, high-quality multicolor images can be projected into a tilted image plane.

[0032] The optical arrangement according to the invention for a head-up display on a projection surface, for example, a curved projection surface, comprises an imaging unit and a previously described wavefront manipulator. The imaging unit advantageously comprises an object plane, i.e., is spatially extended, wherein the object plane is designed to emit light within a specified radiation angle range and with a specified maximum bandwidth with respect to the wavelengths of the emitted light. The object plane can be determined or defined by the exit pupil of the imaging unit. Preferably, the imaging unit is designed to generate a multicolor image. For example, each light-emitting point of the object plane emits light in the form of a scattering lobe or within a specified angular range. This can be achieved, for example, by using a diffuser.The imaging unit is preferably designed to emit laser light, in particular laser beams. Advantageously, the imaging unit is designed to emit laser light in at least two, preferably at least three, different wavelengths. These are preferably three different wavelengths of a defined color space, for example, red, green, and blue or cyan, magenta, and yellow. Since the holographic elements are more sensitive to the bandwidth of each wavelength than other optical components, such as mirrors and lenses, it is advantageous if the imaging unit is designed as a laser scanner with a sharp bandwidth for each color.

[0033] The optical arrangement according to the invention preferably has a volume of less than 15 liters, e.g., less than 10 liters, i.e., it occupies a space of less than 10 liters. The optical arrangement according to the invention has the features and advantages already mentioned above in connection with the wavefront manipulator according to the invention. In particular, it offers a head-up display that is very compact, thus requiring only a small amount of space, while simultaneously ensuring very high image quality.

[0034] Both the wavefront manipulator according to the invention and the optical arrangement according to the invention are suitable for retrofitting in, for example, motor vehicles, aircraft or VR arrangements, for example VR glasses.

[0035] The head-up display according to the invention comprises a curved projection surface and an optical arrangement according to the invention as described above. The curved projection surface is, for example, a windshield of a vehicle, for example, a motor vehicle, an aircraft, or a ship. However, the curved projection surface can also be another viewing window, for example, a viewing window of VR glasses. The curved projection surface can, for example, be viewed as a freeform surface. By means of the wavefront manipulator according to the invention, imaging errors or aberrations caused thereby are compensated and, in addition, a tilted image plane of a virtual image is generated.

[0036] The head-up display according to the invention enables the generation of a virtual image in a tilted image plane with a large field of view. For example, a rectangular virtual image can be generated that has a field of view of, for example, at least 10 degrees, preferably at least 15 degrees by 5 degrees (FOV: 15° x 5°), and is observable at a specific distance from the eyebox, for example, at a distance between 6 meters and 12 meters. The eyebox can have dimensions of up to 150 mm x 150 mm.

[0037] By appropriately designing the holographic elements, the brightness and uniformity of the virtual image can be optimized. Furthermore, by adjusting the color mixing factor, for example, the RGB color space, in the imaging unit, the uniformity of the white level can be adjusted.

[0038] The invention will be explained in more detail below using exemplary embodiments with reference to the accompanying figures. Although the invention is illustrated and described in more detail by the preferred embodiments, the invention is not limited to the disclosed examples, and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention.

[0039] The figures are not necessarily detailed or to scale and may be enlarged or reduced to provide a better overview. Therefore, the functional details disclosed herein are not to be interpreted in a limiting sense, but merely as an illustrative basis for teaching one skilled in the art how to variously employ the present invention.

[0040] As used herein, the term "and / or," when used in a series of two or more elements, means that any of the listed elements may be used alone, or any combination of two or more of the listed elements may be used. For example, if a composition is described containing components A, B, and / or C, the composition may contain A alone; B alone; C alone; A and B in combination; A and C in combination; B and C in combination; or A, B, and C in combination.

[0041] Figure 1 shows schematically the beam path of a head-up display for a windshield of a motor vehicle in a side view.

[0042] Figure 2 shows schematically the beam path of a head-up display according to the invention for a windshield of a motor vehicle in a side view.

[0043] Figure 3 shows schematically the Scheimpflug principle for generating a tilted image plane.

[0044] Figures 4-8 show schematic examples of two mutually associated reflection holograms with their design wavefronts.

[0045] Figure 9 schematically shows the beam path within a hologram stack. Figure 10 schematically shows the beam path of the head-up display shown in Figure 2, including a generated image in a top view.

[0046] Figure 11 shows schematically an optical arrangement according to the invention with a wavefront manipulator according to the invention in the form of a block diagram.

[0047] Figure 1 schematically shows the beam path of a head-up display 10. The head-up display 10 comprises an imaging unit 1, a projection surface 4, for example in the form of a windshield of a motor vehicle, and a wavefront manipulator 7. The projection surface 4, for example the windshield, can be curved. In the case of an application for a vehicle, the imaging unit 1 and the wavefront manipulator 7 are preferably arranged integrated into a fitting (not shown). The head-up display 10 is designed such that it generates a virtual image 6 on or behind the projection surface 4, in particular on or behind the surface of the windshield, i.e. in the outer area of ​​the vehicle, for example behind the surface of the windshield in the direction of travel.The imaged object output by the imaging unit 1 or the exit pupil of the imaging unit 1 is indicated by an arrow with the reference number 9.

[0048] In the embodiment shown, the wavefront manipulator 7 comprises a holographic arrangement 3 and a reflective optical element 2, which has a freeform surface and is arranged in the beam path 8 extending from the imaging unit 1 between the imaging unit 1 and the holographic arrangement 3. The optical element 2 is preferably designed as a freeform mirror.

[0049] The imaging unit 1 emits light waves in the direction of the wavefront manipulator 7. The wavefront manipulator 7 corrects imaging errors and, if necessary, widens the beam path. The wavefront manipulator 7 directs light waves toward the projection surface 4, in particular the curved projection surface. At the projection surface 4, the light waves are reflected toward an eyebox 5. The eyebox 5 forms the area in which a user must or can be located in order to perceive the virtual image 6 generated by the head-up display 10. The conventional head-up display 10 shown has a virtual image plane 6, which runs perpendicular to an optical axis 13 in the area of ​​the image plane and which has a fixed image distance that is identical for all pixels.

[0050] Figure 2 schematically shows the beam path of a head-up display 10 according to the invention. The head-up display 10 according to the invention is designed such that it is tilted by an angle 6 with respect to a plane 14 running perpendicular to the optical axis 13 in the region of the image plane 6. The angle 6 lies between 10 degrees and 170 degrees, preferably between 30 degrees and 150 degrees, in particular between 40 degrees and 50 degrees or between 130 degrees and 140 degrees. In this way, different image distances are generated with one image plane 6. This has the advantage that information can be projected at different image distances. This is particularly relevant in connection with head-up displays for vehicles.In the case of a tilt, as shown in Figure 2, in which the upper area of ​​the image plane 6 has a greater image distance from the eyebox 5 than the lower area of ​​the image plane 6, information can be displayed in the lower area of ​​the image plane 6 at a shorter distance from the eyebox 5. Information on the vehicle speed, for example, can be placed here. Information that is displayed further away by a viewer or an eyebox 5 can be displayed in the upper area of ​​the image plane 6. This can be information on navigation or warnings, for example. In the variant shown in Figure 2, the holographic arrangement comprises a first holographic element 11 and a second holographic element 12, which are arranged directly one behind the other in sections.Each of the holographic elements 11, 12 comprises three holograms 15, 16, 17, which are designed as reflection holograms and are each diffraction-efficient in reflection for at least one specified wavelength or frequency and a specified angle of incidence range. In the example shown, for a specified angle of incidence range, the holograms 15 are efficient for at least one wavelength of a first color of a specified color space, for example, for blue light, the holograms 16 are efficient for at least one wavelength of a second color of a specified color space, for example, for green light, and the holograms 17 are efficient for at least one wavelength of a third color of a specified color space, for example, for red light.

[0051] A tilted image plane 6 can be created using the so-called Scheimpflug principle. The Scheimpflug principle is explained below with reference to Figure 3. In Figure 3, an image 20 of an object 19 in a tilted image plane is created using a single lens 18. If the lens 18 is very thin, the arrangement behaves almost like a paraxial or ideal imaging system. The two principal planes (PP' plane) lie together in the lens plane. If the object plane is arranged parallel to the lens plane, all field points in the object plane are at the same distance from the principal planes. Therefore, the magnification is the same for all field points in the object plane. The imaged points all lie on a plane with the same distance from the principal planes. Therefore, the object plane, the lens plane, and the image plane are arranged parallel to one another.

[0052] If the object plane 19 is tilted by an angle 0', the points on the object plane have different distances (e.g., S1, S2, S3) from the principal planes. Accordingly, the image points lie on a tilted image plane 20 at an angle θ. Such an imaging principle with a tilted object plane 19 and a tilted image plane 20 is called the Scheimpflug principle. The angles θ and θ' are related to the magnification of the system. The challenge of a Scheimpflug system is that the aberrations are very different for different image distances S'. It is therefore necessary to correct as many aberrations as possible for different image distances (A to C in Figure 3) in order to achieve good imaging performance. In Figure 3, an object distance Si is given for an object plane A, an object distance S2 for an object plane B, and an object distance S3 for an object plane C.The object plane A is imaged into an image plane A' with an object distance Si', the object plane B into an image plane B' with an image distance S2 and the object plane C into an image plane C' with an image distance Ss'.

[0053] For an augmented reality RGB head-up display (AR-RGB HUD), the eyebox and field of view are already comparatively large. Within a certain installation space with a fixed number of components, the optical components have limited correction capability. An additional challenge for optical correction is achieving good performance for all points in the image plane at multiple image distances. Therefore, either more components or more degrees of freedom of the components are required for correction.

[0054] In order to reduce installation space and offer more degrees of freedom for correction, the invention uses a holographic arrangement 3 in conjunction with a tilted imaging plane. The holographic arrangement 3 can offer many degrees of freedom for manipulating the wavefront. Together with the at least one element 2, which is designed as a freeform component, for example as a freeform mirror, aberrations of different image distances can be corrected. The holographic arrangement 3 between the projection surface 4, for example the windshield, and the freeform component 2 has the additional function of enabling a high refractive power without chromatic aberrations and having a very small volume. This makes it possible to realize an AR-RGB HUD with a much smaller volume compared to a conventional HUD, which uses exclusively freeform components for aberration correction.

[0055] The challenge of a HUD system lies in the specifications, for example, the size of the eyebox, the field of view (FOV), the shift of the image distance, or the installation space. If the installation space and the number of components are fixed, the holographic arrangement 3 can provide additional potential for the realization of efficient aberration correction. The required degrees of freedom can be realized by generating the holograms using design wavefronts that contain or implement the required degrees of freedom.

[0056] Figures 4 to 8 each show holograms designed for a specific wavelength or frequency, or for a specific wavelength range or frequency range. In the following, examples of two holograms 21 and 22, which are used as mutually assigned reflection holograms, along with their design wavefronts, are explained with reference to Figures 4 to 8. The holograms 21 and 22 can, for example, be holograms 14, 15, or 16 in Figure 2. For example, the first holographic element 11 can comprise the hologram 21, and the second holographic element 11 can comprise the hologram 22. Figure 9 shows the arrangement of the mutually assigned holograms 21 and 22 relative to one another and the beam path within a correspondingly constructed hologram stack.

[0057] On the left in Figure 4, a first hologram 21 is shown for reflecting light of a specific wavelength, for example, green light. On the right, a second hologram 22 is shown, which is designed as a reflection hologram for light of the same color as the first hologram 21 and which interacts with the first hologram 21 within the holographic arrangement 3, as shown in Figure 9. The design wavefronts for the first hologram 21 are identified by the reference numerals 31 and 32. The design wavefronts for constructing the second hologram 22 are identified by the reference numerals 33 and 34. In the example shown, the first hologram 21 is exposed, i.e. written, with a spherical wavefront 31 and a planar wavefront 32, and the second hologram 22 is written with two planar wavefronts 33 and 34.Wavefronts 31 and 33 define the directions of the light according to its components, i.e., upon entering and exiting the holographic arrangement, as well as the refractive power of the entire holographic arrangement constructed from these holograms 21 and 22. The design wavefronts 32 and 34 define the reproduction wavefronts between the two holograms 21 and 22 (see wavefront 36 in Figure 9). It is important to ensure that a filtering effect between different colors or wavelengths is avoided. Filtering effects are avoided by appropriate differences in the exposure wavelengths and / or a suitable determination of the exposure angles of wavefronts 32 and 34. In the variant shown in Figure 4, the directions of wavefronts 32 and 34 are identical.

[0058] The wavefront 31 can be formed from the sum of a spherical wavefront and a freeform wavefront. The wavefront can be represented by a polynomial expansion from the sum of Zernike polynomials, where the individual Zernike polynomials Z are multiplied by coefficients c(Z). The following table provides examples of suitable values ​​for coefficients c(Z) of the Zernike polynomials Z5 to Z9 for tilt angles θ between 0 and 80 degrees, particularly for an arrangement shown schematically in Figure 2 with a non-curved windshield. The Zernike polynomial Z5 corrects the astigmatism at 45°, Z6 corrects the astigmatism at 0°, Z7 corrects the coma in the x-direction, Z8 corrects the coma in the y-direction, and Z9 corrects the spherical aberration.

[0059]

[0060] In the variant shown in Figure 5, in order to improve the homogeneity of brightness and color homogeneity, two design wavefronts 32 and 34 that differ from each other in their direction of incidence are used.

[0061] In the variant shown in Figure 6, the design wavefronts 32 and 34 are designed as freeform wavefronts. This allows for an even higher level of homogeneity. Wavefronts 32 and 34 can be adjusted locally, in particular, using complex exposure systems. In this way, the shape of the wavefronts and the angle of incidence can be specified.

[0062] In the variant shown in Figure 7, the design wavefronts 31 and 33 are configured as freeform wavefronts, which also differ from each other in terms of their shape and angle of incidence. The design wavefronts 32 and 34 are configured as plane waves with different angles of incidence, as in Figure 5.

[0063] The illustrated designs offer a high number of degrees of freedom. The required degrees of freedom are typically implemented using freeform components. In the variant shown here, corresponding requirements can be realized using the holograms used, with the required degrees of freedom being realized by appropriate freeform exposure using the design wavefronts 31, 32, 33, and 34. For the imaging quality and, in particular, the aberration correction, the wavefronts 31 and 33 in the variant shown in Figure 7 are used. The holograms 21 and 22 constructed or written in this way ultimately carry complex microstructures designed to correct numerous aberrations.

[0064] The variant shown in Figure 8 is suitable for applications with very high requirements. Here, all four wavefronts 31-34 can be designed as freeform wavefronts to achieve a maximum number of degrees of freedom for the hologram stack.

[0065] Figure 9 shows the beam path through a hologram stack constructed from holograms 21 and 22, for example within the framework of an optical arrangement according to the invention or an HUD 10 according to the invention. Light coming from the imaging unit 1 with a corresponding wavefront is guided, for example by further freeform components 2, in the direction of the holographic arrangement 3. The light or the wavefront 35 is first transmitted through the second hologram 22 and is then reflected by the first hologram 21. The wavefront reflected by the first hologram 21 is identified by the reference numeral 36. This wavefront 36 is reflected by the second hologram 22 and then transmitted by the first hologram 21. The corresponding wavefront 37 then leaves the holographic arrangement 3 and is guided in the direction of the projection surface 4.

[0066] Mutually assigned reflection holograms 21 and 22, i.e. holograms which are designed for the reflection of coordinated wavelengths or frequencies, i.e. identical wavelengths or frequencies or at least partially overlapping wavelength ranges or frequency ranges, and / or for coordinated irradiation angle ranges or at least have a point-wise mutual efficiency, can be arranged directly adjacent to one another within the holographic arrangement 3, as shown in Figure 2.However, it can also comprise a first holographic element 11, which comprises a plurality of first holograms, each of which is designed and efficient for different wavelengths or wavelength ranges, and a second holographic element 12, which comprises a plurality of second holograms, each of which is assigned to the first holograms, i.e., is designed or efficient for the same wavelengths or wavelength ranges as the first holograms. The first holographic element 11 and the second holographic element 12 can preferably be arranged directly adjacent to one another. To avoid filtering effects, the holograms are preferably designed to be transmissive for the wavelengths or frequencies of the color space used for which they are not designed or efficient as reflection holograms.

[0067] Freeform wavefronts for hologram construction are not only useful for a Scheimpflug HUD, but also suitable for other HUD systems with high specifications, for example, for implementing a large eyebox and a large FOV. Accordingly, application options arise for the wavefront manipulator and optical arrangement according to the invention.

[0068] When imaging an image generated by an image generator 1 into a tilted or tilted virtual image 6, as shown in Figure 2, the virtual image 25 suffers from keystone distortion if the image 24 is displayed rectangularly on the image generator or imaging unit 1. A corresponding image to be imaged is shown in Figure 10, which otherwise corresponds to Figure 2, next to the imaging unit 1 and is identified by the reference number 24. A corresponding virtual image, which is imaged on the virtual image plane 6, is shown next to the virtual image plane 6 and is identified by the reference number 25. The distortion results from the different magnification of the different image distances. This keystone distortion can be corrected digitally.If the brightness at imaging unit 1 is the same everywhere, the brightness of the virtual image varies due to the different magnifications depending on the image distance of the individual pixel. In the example shown, the brightness of virtual image 25 is greater in the lower area than in the upper area. The brightness can also be digitally adjusted. For example, the image from imaging unit 1 can be brighter in the upper area than in the lower area.

[0069] Figure 7 schematically shows an optical arrangement 23 according to the invention with a wavefront manipulator 7 according to the invention in the form of a block diagram. The optical arrangement 23 according to the invention comprises an imaging unit 1 and a wavefront manipulator 7 according to the invention, which are arranged one behind the other in a beam path 8. The wavefront manipulator 7 comprises a holographic arrangement 3 already described and optionally an optical element 2 already described in connection with Figure 2, which has a freeform surface and is preferably designed as a freeform mirror. The optical element 2 is arranged in a beam path between the imaging unit 1 and the holographic arrangement 3.

[0070] List of reference symbols

[0071] 1 imaging unit

[0072] 2 optical element

[0073] 3 holographic arrangement

[0074] 4 Projection surface

[0075] 5 Eyebox

[0076] 6 virtual image

[0077] 7 Wavefront manipulator

[0078] 8 Beam path

[0079] 9 Object / Object level / Exit pupil

[0080] 10 Head-up display

[0081] 11 first holographic element

[0082] 12 second holographic element

[0083] 13 optical axis

[0084] 14 Plane perpendicular to the optical axis

[0085] 15 Hologram, in reflection diffraction efficient for first wavelength

[0086] 16 Hologram, diffraction efficient in reflection for second wavelength

[0087] 17 Hologram, in reflection diffraction efficient for third wavelength

[0088] 18 lens

[0089] 19 objects

[0090] 20 Figure

[0091] 21 Hologram

[0092] 22 hologram

[0093] 23 optical arrangement

[0094] 24 image to be projected

[0095] 25 Figure

[0096] 31 Construction wave front

[0097] 32 Construction wavefront

[0098] 33 Construction wavefront

[0099] 34 Construction wavefront

[0100] 35 Wavefront

[0101] 36 Wavefront

[0102] 37 Wavefront A Plane

[0103] B Level

[0104] C Level

[0105] A' Level B' Level

[0106] C' Level

[0107] P Main Level

[0108] P' main level

[0109] S Distance to principal plane S' Distance to principal plane θ Tilt angle of image plane θ' Tilt angle of object plane

Claims

Patent claims 1. Wavefront manipulator (7) for arrangement in the beam path (8) of a head-up display (10) between an imaging unit (1) and a projection surface (4), wherein the wavefront manipulator (7) comprises a holographic arrangement (3) which includes at least two holographic elements (11, 12), wherein the at least two holographic elements (11, 12) are arranged at least sectionally directly one behind the other in the beam path (8) and are designed to be reflective for at least one defined wavelength and a defined angle of incidence range, wherein a first holographic element comprises at least one hologram which is assigned to a hologram of a second holographic element for reflection, characterized in that the wavefront manipulator (7) is designed to generate an image plane of a virtual image (6) for at least one defined object plane.which is tilted by a defined tilt angle θ with respect to a plane (14) arranged perpendicular to the optical axis (13) in the region of the image plane of a virtual image (6), wherein the holographic arrangement (3) is designed to at least partially correct at least one imaging error of a virtual image (6) generated in the tilted image plane.

2. Wavefront manipulator (7) according to claim 1 , characterized in that the holographic arrangement (3) is designed to at least partially correct at least one imaging error caused by the tilting of the image plane.

3. Wavefront manipulator (7) according to claim 1 or claim 2, characterized in that the tilt angle θ is between 10 degrees and 170 degrees.

4. Wavefront manipulator (7) according to one of claims 1 to 3, characterized in that the defined object plane is arranged tilted by a defined tilt angle with respect to a plane arranged perpendicular to the optical axis in the region of the object plane.

5. Wavefront manipulator (7) according to one of claims 1 to 4, characterized in that the wavefront manipulator (7) is designed to manipulate a wavefront to generate a multicolored virtual image.

6. Wavefront manipulator (7) according to one of claims 1 to 5, characterized in that the at least two holographic elements (11 , 12) are designed to be reflective for at least two defined wavelengths which differ from each other and a defined angle of incidence range.

7. Wavefront manipulator (7) according to one of claims 1 to 6, characterized in that the at least two holographic elements (11 , 12) are designed to be reflective for at least two defined wavelength ranges which differ from each other and do not overlap, and a defined angle of incidence range.

8. Wavefront manipulator (7) according to one of claims 1 to 7, characterized in that the at least two holographic elements (11 , 12) are designed to be transmissive for defined wavelength ranges and / or at least one defined angle of incidence range, for which they are not designed to be reflective.

9. Wavefront manipulator (7) according to one of claims 1 to 8, characterized in that comprising at least two holographic elements (11, 12) reflection holograms (14, 15, 16, 21, 22) which are recorded with two construction wavefronts (31, 32, 33, 34), of which at least one construction wavefront (31, 32, 33, 34) is a plane wavefront or a spherical wavefront or a freeform wavefront.

10. Wavefront manipulator (7) according to one of claims 1 to 9, characterized in that the at least two holographic elements (11 , 12) comprise reflection holograms (14, 15, 16, 21 , 22), wherein at least one reflection hologram (14, 15, 16, 21 , 22) is received with two construction wavefronts (31 , 32, 33, 34), wherein at least one of the construction wavefronts (31 , 32, 33, 34) is configured such that it is generated according to a function which has a plurality of degrees of freedom.

11. Wavefront manipulator (7) according to one of claims 1 to 10, characterized in that the at least two holographic elements (11 , 12) are configured such that a first holographic element (11 ) comprises at least one hologram (14, 15, 16, 21 ) which is assigned to a hologram (14, 15, 16, 22) of a second holographic element (22), wherein the holograms assigned to each other are configured pointwise diffraction-efficient with respect to each other.

12. Optical arrangement (23) for a head-up display (10) for a projection surface (4), wherein the optical arrangement comprises an imaging unit (1 ), characterized in that the optical arrangement (23) comprises a wavefront manipulator (7) according to any one of claims 1 to 11.

13. Optical arrangement (23) according to claim 12, characterized in that the imaging unit (1 ) comprises an object plane (9) designed to emit light within a specified beam angle range and with a specified maximum bandwidth with respect to the wavelengths of the emitted light.

14. Optical arrangement (23) according to claim 12 or claim 13, characterized in that the imaging unit (1 ) is designed to produce a multicolored image.

15. Head-up display (10) comprising a projection surface (4), characterized in that the head-up display (10) comprises an optical arrangement (23) according to one of claims 12 to 14.

Images