REDUCTION OF LIGHT POWER REQUIREMENTS IN A VEHICLE WAVEGUIDE HUD VIA SWITCHABLE GRILLES

Description

Technical field

[0001] The invention relates to a waveguide-based projection display device, which can be used in particular in a motor vehicle or other type of vehicle. The device is designed to generate a virtual display image projected into the user's field of vision by reflection off an at least partially transparent reflective surface, in particular a windshield of the vehicle, and is based on a waveguide to generate a relatively large virtual display image within a small installation space. The invention also relates to a method for operating the projection display device and to a vehicle equipped with it. Technical background

[0002] Projection display devices are particularly well-known under the name Head-Up Display (HUD). For example, in a motor vehicle, a desired display content, such as information about a speed limit or other useful navigation and vehicle operating instructions, or even entertainment content, can be superimposed in the form of a virtual display image onto the real-world view in front of the vehicle as observed by the driver or another occupant.

[0003] For this purpose, a projection display device of classic design comprises a display located below a top surface of the instrument panel with suitable imaging and projection optics to generate a beam of light rays with a display content and to project it onto a reflective disc transparent to the light incident from the rear - such as the windshield of the vehicle or a combiner disc provided separately in front of it - from which it is reflected towards the user in order to create the virtual display image in his field of vision in front of or behind the reflective disc.

[0004] As an alternative to the classic HUD design, whose imaging and projection optics include a concave mirror in the beam path after the display, the size of which scales linearly with the HUD image, a HUD design with a large-area planar waveguide is known, particularly for AR (Augmented Reality) applications. By using a planar waveguide, the installation space required for the HUD can be significantly reduced relative to the size of the virtual display image compared to the classic HUD design.

[0005] To ensure HUDs are easily readable in all lighting conditions, luminance levels exceeding 10,000 cd / m² must be achieved in the virtual display image during the day. This necessitates providing the corresponding light output in the picture generating unit (PGU). For HUDs based on a waveguide system with holographic structures, only lasers are suitable as light sources. Laser light sources are very expensive compared to LED-based light sources. A conventional waveguide has the characteristic of always illuminating the entire eyebox, which corresponds to the cross-section of a light beam coupled across the entire output grid of the waveguide. This means that light is also emitted into areas of the eyebox where the viewer's eyes are not currently located (cf. Fig. 1 ). Such a system is therefore maximally inefficient.

[0006] Based on Fig. 2The operating principle of a conventional waveguide HUD 100 with two-dimensional pupil expansion for illuminating an eyebox is briefly described (see...). Fig. 1 ), which is intended for the eyes of a driver in the vehicle, described. Fig. 2Figure 1 shows a highly simplified schematic top view of the Waveguide-HUD 100, with a similarly schematic representation of a portion of the beam path within the waveguide. A light beam L generated by an imaging unit 2 enters the waveguide 3, which is designed as a glass plate, via its coupling grating 4. From the grating, it is deflected so that it propagates through the glass by total internal reflection and enters the area of ​​a folding grating 5. In the folding grating 5, the light beam L is successively deflected towards an output grating 6. In the output grating 6, it is deflected so that it exits the glass plate towards a windshield 7 of the vehicle, from which it is reflected to the user 8, in this case, the driver.To achieve homogeneous illumination of both the entire eyebox and the displayed virtual image, a location-dependent gradient is applied to the diffraction efficiency of the folding grating 5, as shown in . Fig. 2 The gradient is shown schematically on the left in a graph parallel to the beam propagation direction S of the folding grid 5. In the prior art, this gradient is not time-varying: it is fixed during the fabrication of the folding grid 5.

[0007] In this context, for example, WO 2019 / 238846 A1 discloses a head-up display for a vehicle, which includes an imaging unit for generating an image and a two-dimensional optical waveguide for dilating an exit pupil. A measuring device allows the position of a viewer's eye to be determined. The head-up display also includes means for adjusting the position of an eyebox within the display. These means are controlled by a control unit depending on the viewer's eye position and either include at least one drive for moving at least one component of the head-up display or a display control for adjusting the position of an image content displayed by a display element of the imaging unit.

[0008] On the other hand, in other applications, switchable gratings based on liquid crystals, in particular switchable Bragg gratings, for example according to US 10,890,707 B2, are known in the prior art for switching between a light-diffracting (light-redirecting) and a non-light-diffracting state. Furthermore, as described, for example, in WO 2018 / 146326 A2, switchable grating segments are sometimes used in displays located close to the eye of a viewer, the so-called near-to-eye displays (e.g., head-mounted displays, HMDs), which use waveguides, in order to illuminate a specific angular range for a fixed pupil position.

[0009] For example, US 9,400,395 B2 discloses image uniformity within the eyebox pupil by synchronizing diffraction efficiency with the coupling of different components (i.e., the angular components of an image) of the display content. To this end, US 9,400,395 B2 discloses a near-eye display comprising: a waveguide that transmits angle-connected rays over a limited first range of angles by internal reflection; an image generator that generates angle-connected rays over a second, further angular range to produce a virtual image;an input aperture of the waveguide having a plurality of controllable components, each of which can be operated as diffractive optics to couple a subset of the angle-connected rays over a limited section of the second angular range in order to transmit the subset of the angle-connected rays along the waveguide within the first angular range; an output aperture of the waveguide having a plurality of controllable components, each of which can be selectively operated as diffractive optics to couple a corresponding subset of the angle-connected rays out of the waveguide towards an eyebox;and a control system that synchronizes the operation of the controllable components of the output aperture with the transmission of various subsets of angle-connected beams along the waveguide, in order to couple the subsets of angle-connected beams out of the waveguide to display the virtual image within the eyebox over the second, further angle range. Furthermore, a ; diffractive intermediate optics with controllable sections between the input aperture and the output aperture, to spatially distribute the angle-related rays over different areas of the controllable output aperture, and sections of the controllable output aperture can be selectively controlled in accordance with both controllable diffractive input components and controllable diffractive intermediate optics. to ensure an overlap of the different angle-related rays within the eyebox.

[0010] In this context, US 2017 / 0131546 A1 further discloses a device for use in replicating an image associated with an input pupil to an output pupil. The device comprises a planar optical waveguide containing a bulk substrate, an input coupler, an intermediate component, and an output coupler. The input coupler is configured to couple light corresponding to the image associated with the input pupil into the bulk substrate of the waveguide and toward the intermediate component. The intermediate component is configured to conduct light from a horizontal or vertical pupil dilation and direct the light corresponding to the image toward the output coupler.The output coupler is configured to conduct light from a horizontal or vertical pupil dilation and couple the image-corresponding light, which travels in the planar optical waveguide from the input coupler to the output coupler, out of the waveguide so that the light is emitted from the output pupil and imaged. One or more of the input couplers, the intermediate component, or the output coupler includes a surface relief grating formed in a liquid crystal polymer coating. Specifically, the waveguide can also include switchable Bragg volume gratings that can be pixelated and partially switched on to optimize diffraction efficiency across the grating surface. This can be used in conjunction with an eye tracker to optimize the diffraction efficiency profile for a specific eye position.Additionally, if the switchable Bragg grating is fast enough, it can fluctuate in time to improve brightness uniformity over a pixel switching time.

[0011] The object of the present invention is to provide a waveguide-based projection display device that makes it possible to increase the efficiency of light utilization and thus also of electrical energy consumption. In particular, this can improve the suitability of the projection display device for use in a motor vehicle or other land, air, or water vehicle for the purposes mentioned above. Disclosure of the invention

[0012] This problem is solved by a waveguide-based projection display device according to claim 1, an associated operating method, a corresponding control unit, and a vehicle equipped therewith according to the dependent claims. Further embodiments are specified in the dependent claims. All further features and effects mentioned in the claims and the following description for the projection display device also apply to its operating method, the control unit, and the vehicle, and vice versa.

[0013] According to a first aspect, a waveguide-based projection display device (hereinafter also referred to as "device") is provided, which may be designed in particular for use in a motor vehicle or any other land, air, or water vehicle. The projection display device may, in particular, be a head-up display (HUD).

[0014] The projection display device comprises an imaging unit (also called a picture generating unit, PGU) designed to generate a beam of light rays with a display content to be shown. Furthermore, the device comprises a planar waveguide, in particular a flat or flat surface, designed to guide optical waves by means of total internal reflection. In this case, the waveguide is designed to expand the pupil in two dimensions. For this purpose, the waveguide has a large-area output coupling element on one of its two surfaces, an input coupling element arranged laterally thereto, designed to couple the beam of light generated by the imaging unit into the waveguide, and a folding element arranged between these elements in the light path within the waveguide, designed to successively deflect the coupled light towards the output coupling element.As a rule, the coupling element, the folding element, and the output coupling element are optical gratings, so that in the following, only "coupling grating," "output grating," and "folding grating" will be referred to. (However, this is not restrictive; i.e., an optical element other than a grating can also be suitable for the described function of one of these waveguide elements.)

[0015] Furthermore, the device comprises a reflective disc which is arranged and designed to reflect a beam of light rays coupled from the waveguide into an eyebox predetermined for the user's eyes, such that a virtual display image is created behind the reflective disc in the user's field of vision. When used in a vehicle, the reflective disc can be formed, for example, by a windshield or a combiner disc arranged in front of it on the vehicle's interior side. It is at least partially transparent to ambient light incident from the rear in order to superimpose the virtual display image onto a real environment observed by the user through the windshield, etc.

[0016] The eyebox is understood here as a two-dimensional spatial area perpendicular to the beam propagation direction. This area can extend, in particular, perpendicular to the beam propagation direction and / or vertically within the vehicle. The eyebox position in the beam propagation direction can be defined, for example, by the distance from the reflector, ensuring that the user can still see the virtual display image in consistently good quality even when moving their head approximately 15 cm towards or away from the reflector.

[0017] Furthermore, the projection display device includes an eye-tracking unit designed to determine the eyebox window of a predetermined size currently occupied by the user's eyes within the eyebox, i.e., a predetermined sub-area within the total eyebox area. While the total area of ​​the eyebox corresponds, for example, to the cross-section of a beam of light rays coupled out across the entire folding and coupling grid and reflected to the user via the reflection disc, the currently active eyebox window represents only a sub-area of ​​this eyebox, covering only a fraction (such as about half, a third, a quarter, or less) of the total height (and thus also the total area) of the eyebox, and its position may depend, among other things, on the user's height and / or pose.Depending on the accuracy of the eye-tracking, the predetermined size of the eyebox window can vary and, for example, be defined by a window height of approximately 20 mm. The eye-tracking device can, for example, include at least one camera designed to optically capture the eyebox and the user's eyes.

[0018] To solve the efficiency problem mentioned at the beginning, the folding grating is aligned in the direction of beam propagation, which, after passing the coupling grating and reflection at the reflecting disk, corresponds to a vertical direction of the eyebox (see...). Fig. 3The projection display device is divided into several segments that can be switched independently into a light-reflecting (diffracting) state. Depending on the currently determined eyebox window, only a subset of these segments is activated, with diffraction efficiencies varying to generate a (time-varying) spatial gradient in that direction. This ensures that the emitted light beam is limited to the determined eyebox window in the vertical direction and illuminates it as uniformly as possible.

[0019] For this purpose, the projection display device can, for example, include a suitable control unit designed and configured to receive and evaluate an eye-tracking signal from the eye-tracking device by determining the eyebox window currently occupied by the user's eyes, the segments of the folding grating required for its illumination, and the associated diffraction efficiencies. The control unit is further designed and configured to control the segments accordingly.

[0020] For example, successive segments can be switched to the light-reflecting state, which, depending on the size of the determined eyebox window, can represent a fraction—approximately half, one, two-thirds, or one-quarter—of the total extent of the folded grating in the specified direction. The remaining segments in the grating remain switched off and thus do not deflect any light. By appropriately controlling the switched-on segments, the diffraction efficiency can be set to be spatially constant (i.e., uniform) within each segment and different from segment to segment. In this way, a gradient of diffraction efficiency in the specified beam propagation direction of the folded grating can be generated that is adaptable to the determined eyebox window—i.e.,, in contrast to the prior art, time-varying.

[0021] One aspect of the invention is to illuminate only the vertical area of ​​the eyebox where the viewer's eyes are located by dividing the folded grating into independently switchable grating segments. By appropriately controlling the activated segments, the respective diffraction efficiency can be adjusted differently from segment to segment. This is used to generate a temporally variable spatial gradient of diffraction efficiency in the folded grating, adapting to the current eyebox window and thus achieving the most uniform illumination of the eyebox window possible.

[0022] This allows the light requirement of the projection display device, and thus the power requirements of its light sources, to be reduced by at least 50% compared to a conventional waveguide HUD, which has a time-invariant diffraction efficiency in the pleated grating. In this way, not only can significant cost savings be achieved in the waveguide HUD itself, but also considerable savings in the device's electrical energy consumption. Given the increasing electrification of vehicles, the latter point may be even more important than the pure cost savings.

[0023] In particular, the waveguide – for example, its coupling grating and / or folded grating and / or output grating – can exhibit holographic structures. In this case, only lasers are suitable as the light source for the imaging unit. The efficiency increase of the device described herein allows the light power requirements for the laser source to be significantly reduced compared to the prior art.

[0024] The switchable segments of the folded grating can, for example, be designed as gratings based on liquid crystals, in particular as switchable Bragg gratings, that can be switched between a light-deflectoring and a non-light-deflectoring state. Alternative designs are also possible, for example, passive polarization-sensitive grating segments in combination with switchable polarization filters.

[0025] As mentioned above, the diffraction efficiency of each individual segment of the convoluted grating can be set uniformly for the entire segment and varied over time independently of the other segments. This allows for the adjustment of a suitable spatial gradient within the entire convoluted grating section formed by the activated segments, ensuring uniform illumination of the current eyebox window.

[0026] In particular, each switchable segment of the folded grating can have a width of a few millimeters to approximately 20 mm in the aforementioned beam propagation direction. The finer this subdivision of the folded grating into segments, the more precisely the spatial gradient of the diffraction efficiency can be adjusted by switching individual segments.

[0027] According to one embodiment, the projection display device is designed and configured to generate the respective diffraction efficiency of each individual activated segment of the folded grating by pulse-width modulation. This means that an individual segment is not continuously activated during the display duration of a single image (unless 100% diffraction efficiency is desired in that segment). Each individual segment is only activated for the duration necessary to deflect only as much light as is required to produce a homogeneous display image for the current position of the user's eyes (eyebox window) in conjunction with the other activated segments. It is important to note that the pulse widths should be significantly shorter than the temporal resolution of the human eye.They should generally have a maximum pulse width significantly shorter than 25 ms to avoid causing flickering effects for the viewer. One possible pulse sequence is shown in . Fig. 4 Two different diffraction efficiencies are shown as examples.

[0028] As an alternative to this embodiment, the segments can be switched not only between the two limiting states "light-deflectoring" and "non-light-deflectoring," but also continuously or discretely into intermediate states to adjust the respective diffraction efficiency. This can be implemented, for example, in switchable gratings based on liquid crystals by a corresponding variation of the applied electrical voltage.

[0029] According to another aspect, a method for operating a projection display device of the type set forth herein is provided. The method comprises the following steps: Generating a beam of light by the imaging unit and coupling this beam into the waveguide; receiving an eye-tracking signal from the eye-tracking device and determining from this an eyebox window currently occupied by the user's eyes within the eyebox; determining those segments of the convoluted grating that contribute to illuminating the determined eyebox window; determining for these segments a diffraction efficiency that is different from each other in order to generate a spatial gradient in the beam propagation direction of the convoluted grating, corresponding to the eyebox vertical direction, such that the beam of light coupled from the waveguide in the eyebox vertical direction is confined to the determined eyebox window and illuminates it as uniformly as possible; and switching on the segments of the convoluted grating thus determined with the diffraction efficiency determined for the respective segment.

[0030] In particular, in the latter step, the diffraction efficiency of each individual segment of the folded grating can be generated by pulse width modulation.

[0031] According to another aspect, a control unit already mentioned above is provided for operating a projection display device of the type set out herein, which is designed and equipped to automatically execute such a procedure.

[0032] According to a further aspect, a vehicle, in particular a motor vehicle or any other land, air, or water vehicle, is provided. The vehicle comprises a windshield and an instrument panel arranged below it. Furthermore, the vehicle comprises a projection display device of the type described herein, the reflector of which is formed by the windshield or a combiner panel arranged in front of it on the inside of the vehicle, and the waveguide of which extends with its surface, in which the output grid is formed, into or along a top surface of the instrument panel (in particular, flush with it). The vehicle also comprises a control unit of the type described herein. Brief description of the drawings

[0033] The above aspects of the invention and its embodiments and specific configurations are explained in more detail below with reference to the examples shown in the accompanying drawings. For the sake of clarity, the drawings are at least partially purely schematic and are therefore not to scale. They show: Figure 1 shows a side cross-sectional view of a vehicle with a prior art waveguide-based HUD, in which an eyebox window currently occupied by the user's eyes corresponds only to a fraction of the total cross-section of a light beam coupled out of the waveguide; Figure 2 shows a top view of the waveguide HUD. Fig. 1 , where the beam path in the waveguide is greatly simplified and a graph is shown on the left in Fig. 2Figure 1 schematically shows a fixed, location-dependent progression of the diffraction efficiency in a folded grating of the waveguide; Figure 3 shows a top view of an embodiment of a waveguide-based projection display device of the type shown herein with a segmentation of the folded grating, wherein a graph is shown on the left. Fig. 3 Figure 4a-4 shows a schematic representation of a pulse width modulation for the time-variable generation of a desired diffraction efficiency in a segment of the folded grating; and Figure 4a-4 shows a schematic representation of a pulse width modulation for the time-variable generation of a desired diffraction efficiency in a segment of the folded grating. Fig. 3 , where a pulse sequence is exemplified for two diffraction efficiencies of 50% ( Fig. 4a ) and 10% ( Fig. 4b ) is shown. Description of embodiments

[0034] Fig. 1Figure 1 shows a highly simplified schematic side cross-sectional view of an example of a motor vehicle 11 with a waveguide-based projection display device according to the prior art. This projection display device is a conventional waveguide HUD 100 mentioned above. The motor vehicle 11 comprises a windshield 7 and an instrument panel 9 arranged below it, in the upper surface of which the waveguide HUD 100 is located.

[0035] Fig. 2 shows a highly simplified schematic representation of a top view of the Waveguide-HUD 100 of the Fig. 1 with a highly simplified representation of part of the beam path in its waveguide 3. Based on the Fig. 2The operating principle of a conventional waveguide HUD 100 with two-dimensional pupil expansion is briefly described: A beam of light L generated by an imaging unit 2 of the waveguide HUD 100 enters the waveguide 3, which is designed, for example, as a glass plate, via its coupling grating 4. The coupling grating 4 deflects the beam of light L so that it propagates through the glass by total internal reflection and enters the area of ​​a folding grating 5. In the folding grating 5, the beam of light is successively deflected towards an output grating 6, and in the output grating 6, it is deflected so that it exits the glass plate towards the windshield 7 of the vehicle 11, from which it is reflected to the user 8, in this case, the driver of the vehicle 11.

[0036] For homogeneous illumination of both an entire eyebox 12 and a displayed virtual image, the diffraction efficiency of the conventional folding grating 5 in Fig. 2 a location-dependent gradient is imposed. This is in Fig. 2 The gradient is shown schematically on the left in a graph parallel to the beam propagation direction S of the folding grid 5. According to the prior art, this gradient is not time-varying and is fixed during the fabrication of the folding grid 5, for example, its holographic structures.

[0037] The front panel 7 of the Waveguide-HUD 100 serves as a reflective panel that reflects a beam of light rays L coupled out of the waveguide 3 to the eyebox 12, so that a virtual display image (not shown) is created in the user's field of vision 8 behind the front panel 7.

[0038] Eyebox 12 is defined here as a two-dimensional spatial region approximately perpendicular to the beam propagation direction, from which the virtual display image is visible to user 8. Even when user 8 moves their head, for example, approximately 10-15 cm forward or backward in the longitudinal direction of the vehicle from the depicted eyebox position, they can still clearly see the virtual display image. A corresponding three-dimensional spatial region around eyebox 12 is shown in Fig. 1 indicated by its contour.

[0039] The Eyebox 12 accommodates different body sizes and seating positions for various users in its vertical direction H, as shown in Fig. 1 This is clearly visible. This means that when operating the conventional waveguide HUD 100, light is always also sent into areas of the eyebox 12 where the user's eyes 8 are not currently located. Thus, in Fig. 1Only a small fraction of the total beam volume GL, which is extracted from the waveguide 3 and illuminates the entire eyebox 12, contributes as useful light NL to illuminate one of the eyebox windows EF corresponding to the current head position of the user 8. Such a system is therefore maximally inefficient.

[0040] Fig. 3 Figure 1 shows a top view of an embodiment of a waveguide-based projection display device 1 of the type described herein. To solve the efficiency problem mentioned above, this device differs from the conventional waveguide HUD 100 described above in that, firstly, the folded grating 5 in its waveguide 3 is divided into several segments 14 that can be switched independently into a light-deflectoring (light-diffusing) state in the beam propagation direction S, which, after passing the output coupling grating 4 and the reflection at the reflection disk 4, corresponds to an eyebox height direction H. Fig. 3Only ten segments 14 are shown, purely as examples or for illustrative purposes; the number of segments may be significantly more than ten, but also less.

[0041] Furthermore, the projection display device 1 includes an eye-tracking device 15, which is designed to determine an eyebox window EF of a predetermined size, i.e., with a predetermined area or height fraction, currently occupied by the eyes of the user 8 within the entire eyebox 12. The projection display device 1 is configured to activate only a subset of the segments 14, depending on the currently determined eyebox window EF, with diffraction efficiencies such that a time-varying spatial gradient in this direction is generated, such that the light beam L in the eyebox height direction H is restricted to the determined eyebox window EF and illuminates it as uniformly as possible.

[0042] For this purpose, the projection display device 1 includes a suitable control unit 16, which is designed and configured to receive an eye-tracking signal from the eye-tracking device 15 and to evaluate it in order to determine the required segments 14 and to control them accordingly.

[0043] The width of the individual segments 14 in the beam propagation direction S can, purely as an example, range from a few millimeters to approximately 20 mm. This allows a time-varying gradient of the diffraction efficiency to be generated in the folded grating 5 in the region B that is responsible for illuminating the current eyebox window. EF The responsible segment is the one in which the user's eyes 8 are currently located. The remaining segments in the folding grating 5 - i.e., for example, holographic structures in a holographic folding grating 5 - are switched off and thus no longer deflect light.

[0044] Thus, in Fig. 3only five middle segments 14 of the folding grating 5 are switched on, while at this moment the first three and the last two segments 14 in the beam propagation direction S remain switched off.

[0045] By means of suitable control, the successively switched-on segments at the depicted time have a diffraction efficiency increasing from segment to segment: 20%, 25%, 33%, 50%, and 100%. In this way, a spatial gradient of the diffraction efficiency in the specified beam propagation direction S of the folded grating 15 can be generated, adapted to the currently determined eyebox window EF – i.e., in contrast to the prior art, a temporally variable gradient – ​​which in Fig. 3 The graph on the left shows a schematic representation.

[0046] Note that in Fig. 3In this example, only half of segments 14 are activated. If the user's eyes 8 move vertically (eyebox height direction H) within eyebox 12, the area B of the activated segments 14 also moves automatically.

[0047] This allows the light requirement of the projection display device 1, and thus the power requirements of its light sources, to be reduced by at least 50% compared to the conventional waveguide HUD 100 according to the Figs. 1 and 2 reduce. In this way, not only significant cost savings but also considerable energy savings in the device's electrical consumption can be achieved. Given the increasing electrification of vehicles, the latter point may be even more important than pure cost savings.

[0048] Furthermore, the projection display device 1 can be designed in a manner similar to the conventional waveguide HUD 100 according to the Figures 1 and 2be trained.

[0049] The time-varying gradients are generated, for example, using pulse-width modulation. This means that a single segment 14 is not continuously switched on during the display duration of a single frame of the virtual display image (unless 100% diffraction efficiency is desired in this segment 14). It is only switched on long enough to deflect only as much light as is needed to create a homogeneous image in the eyebox 12 at the eye's current position over the display duration of the single frame. It is important to note that the pulse widths should be significantly shorter than the temporal resolution of the human eye (generally, the pulse width tp << 25 ms) to avoid causing flickering effects for the viewer.

[0050] This shows Fig. 4a and Fig. 4bEach example shows a possible pulse sequence for two different diffraction efficiencies. This is a schematic representation of pulse width modulation. The diffraction efficiency of the respective segment 14 can be controlled by the number of individual pulses 17 in which the respective segment 14 is switched on during the display duration 18 of a single frame. Thus, in Fig. 4a By switching on five individual pulses 17, which constitute half of the display duration 18 of a single image, a diffraction efficiency of 50% is set in the relevant segment 14. Fig. 4b Only one of ten possible individual pulses 17 is generated in the display duration 18 in order to set a diffraction efficiency of 10% in the relevant segment 14. Reference symbol list

[0051] 1 Waveguide-based projection display device 100 Conventional waveguide HUD 2 Imaging unit 3 Waveguide 4 Input grid 5 Folding grid 6 Output grid 7 Windscreen 8 User 9 Instrument panel 11 Vehicle 12 Eyebox 14 Segment 15 Eye-tracking device 16 Control unit 17 Single pulse 18 Display duration L Light beam bundle E F Eyebox window currently occupied by the user's eyes N L Useful light contributing to the illumination of the current eyebox window G Total beam volume illuminating the entire eyebox S Beam propagation direction in the folding grid H Eyebox vertical direction B Area of ​​the folding grid with segments turned on

Claims

1. Waveguide-based projection display device (1), in particular for use in a vehicle, comprising: • an imaging unit (2) and a planar, in particular flat, waveguide (3) which is configured for two-dimensional pupil expansion by having an outcoupling grating (6) formed over a large area in one of its surface sides, an incoupling grating (4) arranged laterally thereof for coupling in a light beam bundle generated by the imaging unit, and a folding grating (5) arranged therebetween for successively redirecting the coupled-in light to the outcoupling grating (6); • an at least partially transparent reflection screen arranged to reflect a light beam bundle coupled out of the waveguide (3) to a two-dimensional eyebox (12) predetermined for the eyes of a user (8) such that a virtual display image is formed behind the reflection screen in the field of view of the user (8); • an eye-tracking device (15) which is configured to determine an eyebox window (EF) of a predetermined size currently occupied by the eyes of the user (8) within the eyebox (12), such that the eyebox window (EF) covers only a fraction of a total height of the eyebox (12); • wherein the folding grating (5) is subdivided in the beam propagation direction (S), which corresponds to an eyebox height direction (H), into a plurality of segments (14) which can be switched into a light-redirecting state independently of one another; and • the device is configured to switch on, depending on the determined eyebox window (EF), only some of these segments (14) and with such mutually different diffraction efficiencies that the coupled-out light beam bundle is limited in the eyebox height direction (H) to the determined eyebox window (EF) and illuminates it substantially uniformly.

2. Projection display device (1) according to claim 1, wherein • the waveguide (3), in particular the incoupling grating (4) and / or the folding grating (5) and / or the outcoupling grating (6), has / have holographic structures; and • the imaging unit (2) has a laser source as a light source.

3. Projection display device (1) according to claim 1 or 2, wherein • the switchable segments (14) of the folding grating (5) are configured as switchable Bragg gratings.

4. Projection display device (1) according to any one of the preceding claims, wherein • the diffraction efficiency of each individual segment (14) of the folding grating (5) is uniformly adjustable for the entire segment (14) and can be varied over time independently of the other segments (14).

5. Projection display device (1) according to any one of the preceding claims, wherein • each switchable segment (14) of the folding grating (5) has a width of a few millimeters to about 20 mm in the beam propagation direction (S) which corresponds to the eyebox height direction (H).

6. Projection display device (1) according to any one of the preceding claims, which • is configured and arranged to generate said mutually different diffraction efficiency of individual switched-on segments (14) of the folding grating (5) by pulse width modulation.

7. Method for operating a projection display device (1) according to any one of the preceding claims, comprising the steps: • generating a light beam bundle (L) by the imaging unit (2) and coupling this light beam bundle (L) into the waveguide (3); • receiving an eye-tracking signal from the eye-tracking device (15) and determining therefrom an eyebox window (EF) currently occupied by the eyes of the user (8) within the eyebox (12); • determining those segments (14) of the folding grating (5) which contribute to illuminating the determined eyebox window (EF), and determining for these segments (14) such mutually different diffraction efficiencies for generating a spatial gradient in the beam propagation direction (S) of the folding grating (5), which corresponds to the eyebox height direction (H), that the light beam bundle coupled out of the waveguide (3) is limited in the eyebox height direction (H) to the determined eyebox window (EF) and illuminates it substantially uniformly; and • switching on the segments (14) of the folding grating (5) thus determined with the diffraction efficiency determined for the respective segment (14).

8. Method according to claim 7, wherein • the respectively determined diffraction efficiency of each individual switched-on segment (14) of the folding grating (5) is generated by pulse width modulation.

9. Control unit (16) for operating a projection display device (1) according to any one of claims 1 to 6, which is configured and arranged by means of automatic execution of a method according to claim 7 or 8.

10. Vehicle, in particular a motor vehicle (11), comprising: • a windscreen (7) and an instrument panel (9) arranged thereunder; • a projection display device (1) according to any one of claims 1 to 6, the reflection screen of which is formed by the windscreen (7) or a combiner screen arranged in front thereof on the vehicle interior side, and the waveguide (3) of which extends with its surface side, in which the outcoupling grating (4) is formed, in or along an upper side of the instrument panel (9); and • a control unit (9) according to claim 9.