Time multiplexing for the correction of chromatic aberrations in a transparent screen unit

Abstract

Various examples relate to techniques for the synchronized controlling of a multicolor light source and a multipixel display device of a picture generating unit for a transparent screen unit, e.g. a head-up display system. This allows chromatic aberrations to be reduced, e.g. chromatic aberrations occurring due to the use of holographic optical elements.

Description

[0001] DESCRIPTION

[0002] Time-division multiplexing for color aberration correction in a transparent display unit

[0003] PRIORITY

[0004] This application claims priority from German patent application 102023 121 900.6 filed on August 16, 2023, the disclosure of which is incorporated herein by reference.

[0005] TECHNICAL FIELD

[0006] Various examples of the invention relate to techniques for synchronously driving a multi-color light source and a multi-pixel display device of an image forming unit for a transparent display unit.

[0007] BACKGROUND

[0008] Transparent display units are used in various application scenarios. For example, transparent display units are used in HUD (Head-Up Display) systems in motor vehicles. A HUD system creates a virtual image so that the driver does not have to take their eyes off the road and can still perceive the displayed information. Unlike real images, which are displayed on a physical surface, such as a transparent area, the virtual image is created in a virtual image plane. In HUD systems, the virtual image plane is located outside the vehicle. The so-called eye box is the area within which the driver can perceive the virtual image.

[0009] Transparent display units have a picture generating unit (PGU). The PGU comprises a multi-pixel display device, such as a liquid crystal display or a micromirror array. The multi-pixel display device is illuminated by a light source. Narrowband or monochromatic light sources are often used. This is particularly the case when one or more holographic optical elements (HOEs) are used in an optical imaging system of the HUD system. The reason for this lies in the wavelength-selective diffraction characteristics of an HOE. The diffraction efficiency of the HOE is a function of the wavelength.

[0010] HOEs are specifically manufactured for a specific wavelength.

[0011] It is known to use multiple monochromatic light sources to serve multiple color channels of the optical imaging system. For example, a red light source, a green light source, and a blue light source can be used to illuminate corresponding color channels of a liquid crystal display. Mixed-color images, in particular, can be generated in this way.

[0012] It is observed that the images associated with the different color channels exhibit an undesirable offset from one another. This can occur due to color aberrations in the optical imaging system of the transparent display unit. To reduce or compensate for such undesirable offsets between the individual color images, it is known to apply pre-warping to the individual images. This pre-warping counteracts the observed offset.

[0013] However, it has been observed that in existing systems it is not possible to completely compensate for an offset between the individual color images by means of pre-distortion.

[0014] BRIEF SUMMARY OF THE INVENTION

[0015] There is a need for improved image generation units with multiple color channels for transparent display units. In particular, there is a need for image generation units that address the above-mentioned limitations, i.e., better reduce or fully compensate for offsets between color frames.

[0016] This problem is solved by the features of the independent patent claims.

[0017] The features of the dependent claims define embodiments. A method for controlling an image generation unit for a transparent display unit is disclosed.

[0018] The transparent display unit can, for example, be a HUD system, for example, for projecting information into the field of vision of a car driver. The transparent display unit can also be a holographic display, for example, which displays information on a transparent surface.

[0019] The picture generating unit (PGU) comprises a multi-pixel display device and a multi-color light source. The multi-color light source is configured to emit coherent light in multiple source color channels. This illuminates the multi-pixel display device. Examples of source color channels include red, green, blue, or other complementary color systems.

[0020] For example, lasers or laser diodes can be used. Narrowband light-emitting diodes (LEDs) can also be used.

[0021] The method involves controlling the multicolor light source to emit the coherent light in time slots of a time-division multiplexing scheme. Each time slot is assigned to a single corresponding source color channel of the multiple source color channels. For example, time slot 1 <> red; time slot 2 <> green; time slot 3 <> blue; time slot 4 <> red; and so on.

[0022] The method also includes controlling the multi-pixel display device to display a corresponding display image in each of the time slots, which is associated with the respective source color channel. The display images are thus each assigned to exactly one source color channel. For example, time slot 1 <> red display image; time slot 2 <> green display image; time slot 3 <> blue display image; time slot 4 <> red display image; and so on.

[0023] The display images exhibit a pre-distortion that reduces the color aberrations of an optical imaging system of the transparent screen unit for the source color channel associated with the respective display image. The optical imaging system can, for example, comprise one or more holographic optical elements and / or mirrors and / or lenses. By applying the time-division multiplexing scheme, it is thus possible to correct the color aberrations separately in time through digital pre-processing of the corresponding display images. Typically, the color aberrations exhibit a wavelength dependence, meaning, for example, the direction or strength of an offset depends on the respective light of the source color channel.By applying the pre-distortions separately, a higher accuracy in correcting the color aberrations can be achieved with respect to reference implementations where multiple source color channels are used simultaneously to illuminate the multi-pixel display device (which then also has multiple display color channels).

[0024] Various techniques are based on the realization that mixing of display color channels can occur in typical multi-pixel display devices. This means that pixels that primarily process light from a first source color channel also exhibit a certain sensitivity to light from a second source color channel. In reference implementations that do not use time-division multiplexing but instead pre-distort multiple source color channels in parallel with multiple display color channels, this results in a mixing of channel-specific pre-distortions; this reduces the quality of the color aberration correction. By activating the various source color channels separately, this mixing is avoided, and the color aberrations can be better compensated.

[0025] In particular, a translational offset between the images perceived by the user (e.g. virtual images in a HUD system) can be compensated for by shifting the display images in the multi-pixel display device in opposite directions. Such a translational offset is typically created by using a so-called HOE stack in the optical imaging system of the transparent screen unit. This involves using multiple HOEs. The light from each source color channel is assigned a corresponding HOE element. Due to the diffractive properties of the holographic optical elements, they are highly wavelength-sensitive; accordingly, it is necessary to provide separate holographic optical elements for each color channel (e.g. source color channel). The HOEs of the (source) color channels are arranged in a stack structure. Each HOE has a certain thickness.This creates a corresponding offset of the light beam paths of the different channels. Different multi-pixel display devices can be used. In particular, it would be possible to use monochromatic multi-pixel display devices that have a single display color channel. This single display color channel can be broadband, i.e., configured to influence light from all source color channels in the relevant spectrum (e.g., to switch, redirect, or modulate it appropriately). An example would be a switchable digital micromirror array (DMD) without a color filter. By separating the color channels in time, a color image can still be displayed if the repetition rate of the time slots is selected to be sufficiently fast. The time-division multiplexing scheme can, for example, have a switching frequency of time slots of no less than N • 25 Hz, where N is the number of color channels.Typically (for red-green-blue channels) N = 3. Color mixing is thus achieved in the viewer's eye by the rapid succession of different displayed color images (e.g. virtual images in the HUD system).

[0026] In other examples, however, a multi-pixel display device with multiple display color channels corresponding to the source color channels could also be used. For example, a liquid crystal display with red-green-blue display channels could be used. Appropriate color filters can be provided for this purpose. Despite any overlap between the display color channels, it is possible to achieve a good level of color aberration control due to the sequential activation of the corresponding source color channels according to the time-division multiplexing scheme.

[0027] In the various examples, the multi-pixel display device is illuminated with phase-coherent light. Accordingly, speckle noise may occur. A system for illuminating a pixel plane of a multi-pixel display device (illumination system) is disclosed, which allows the speckle noise to be suppressed or at least reduced.

[0028] The system includes a laser light source. For example, the laser light source could comprise a single light emitter, such as a laser diode. A multicolor laser light source could also be used; such a multicolor laser light source could, for example, comprise three light emitters providing light of the wavelengths red, green, and blue. Aspects related to a multicolor light source have already been described above. The laser light source is generally configured to provide phase-coherent light. This can then cause speckle noise.

[0029] The illumination system also includes a speckle noise suppression unit. As a general rule, different types of speckle noise suppression units can be used. One implementation involves the use of a moving diffuser, which "averages out" the speckle pattern. However, other types of speckle noise suppression units are also available, for example, a liquid crystal light modulator element. Using the speckle noise suppression unit, the phase profile of the laser light can be modified, in particular, randomized.

[0030] The speckle noise suppression unit can be designed, for example, as a diffuser that is moved in the kHz range.

[0031] The illumination system also includes an optical waveguide that guides the coherent light emitted by the light source to the speckle noise suppression unit. An optical fiber can be used as the optical waveguide. For example, a multimode fiber could be used. An output lens could be provided at one end of the optical waveguide facing the speckle noise suppression unit.

[0032] The illumination system also includes an optical homogenization plate. This plate has a top side, a bottom side, and (at least) one side surface. The side surface is thus arranged at the edge between the top side and the bottom side. The thickness of the optical homogenization plate can be defined as the distance between the top side and the bottom side. In a system integration into a PGU of a HUD system, the top side can face a multi-pixel display device (corresponding aspects in connection with the control of a multi-pixel display device were described above). The homogenization plate can be made of Plexiglas or glass. The homogenization plate has a plate-shaped, optically transparent substrate. The illumination system includes a coupling structure. The coupling structure is formed with a speckle noise suppression unit adjacent to the side surface of the homogenization plate.The coupling structure is configured to couple light coming from the speckle noise suppression unit into the homogenization plate. The coupling structure can be glued or otherwise applied to the side surface of the homogenization plate as a separate component. However, the coupling structure could also be designed as a surface topography variation of the side surface of the homogenization plate.

[0033] The illumination system further comprises an output coupling structure. This is formed on the upper side of the homogenization plate. The output coupling structure is configured to output light from the homogenization plate, distributed over the entire surface of the output coupling structure, to illuminate the multi-pixel display device. This means that a specific light field, i.e., a specific intensity distribution as a function of the positions along the surface of the homogenization plate, can be achieved via the output coupling structure. This allows suitable illumination of the pixel plane of the multi-pixel display device to be achieved.

[0034] By using the homogenization plate with coupling-out structure, a particularly compact illumination unit can be implemented, e.g. in particular in comparison to a free-beam beam path.

[0035] The output coupling structure can be implemented, in particular, as a diffuser. The diffuser scatters the light, thus achieving a specific radiation characteristic.

[0036] As a general rule, diffusers can be implemented in various ways, including microstructured diffusers, holographic diffusers, volumetric diffusers, and surface-relief-based diffusers. Diffusers can be made of various materials, including plastic, glass, and possibly coated.

[0037] As described above, it is possible to achieve a desired illumination of the pixel plane by using multiple diffusers (speckle noise suppression unit and output coupling structure). Accordingly, another illumination system for illuminating a pixel plane of a multi-pixel display device comprises a laser light source. The laser light source is configured to emit light along a beam path. The illumination system also comprises a speckle noise suppression unit, implemented, for example, as a moving diffuser. This is arranged in the beam path. The illumination system further comprises a diffuser. The diffuser is arranged in the beam path emanating from the laser light source behind the speckle noise suppression unit and is configured to emit the light with a radiation characteristic toward the multi-pixel display device.

[0038] The diffuser positioned first in the beam path thus enables the suppression of speckle noise. The diffuser positioned adjacent to the pixel plane of the multi-pixel display device in the beam path thus enables the desired radiation pattern for, first, the illumination of the pixel plane and, second, the illumination of an entrance pupil of an optical imaging system positioned behind the multi-pixel display device in the beam path.

[0039] Optionally, a further diffuser could also be provided, arranged between the speckle noise suppression unit and the diffuser (near the display device). This diffuser is configured to radiate the light with a further radiation characteristic toward the diffuser arranged adjacent to the display device. This further radiation characteristic can be configured to achieve full-surface illumination of the aperture of the diffuser arranged adjacent to the display device.

[0040] Such lighting systems as described above can be part of a PGU of a HUD system. The HUD system can have one or more HOEs in the light beam path. The multi-pixel display device can be, for example, a liquid crystal display or a micromirror array.

[0041] The one or more HOEs can perform different functions. For example, it would be conceivable for the one or more HOEs to be part of or implement a wavefront manipulator, as described in WO 2022 / 189275 A1 (referred to therein as holographic elements), the corresponding disclosure content being incorporated herein by cross-reference. A control device for controlling an image generation unit for a transparent screen unit is disclosed. The image generation unit comprises a multi-pixel display device and a multi-color light source. The multi-color light source is configured to emit coherent light in multiple source color channels to thus illuminate the multi-pixel display device. The control device is configured to control the multi-color light source to emit the coherent light in time slots of a time-division multiplexing scheme.Each magazine is assigned a single corresponding source color channel of the multiple source color channels. The control unit is also configured to control the multi-pixel display device to display a corresponding display image associated with the respective source color channel in each of the time slots. The display images have pre-distortion to reduce color aberrations of an optical imaging system of the transparent display unit for the respective source color channel associated with the respective display image.

[0042] It also describes program code that can be executed by a processor, whereby the processor can perform a method as described above.

[0043] Variations of the above-mentioned techniques are possible. For example, it would be conceivable to activate more than a single source color channel at least in some time slots and to display more than a single display image in these time slots. In such a variation, the displayed display images can each reduce or compensate for color aberrations for each of the activated source color channels. In particular, it is possible to activate source color channels and reproduce the associated display images that are "orthogonal" to one another, i.e. for which no channel mixing occurs. Channel mixing does not occur for a first and a second source color channel if the first source color channel has an associated display color channel whose spectrum does not overlap with the spectrum of a second display color channel assigned to the second source color channel.

[0044] The features set forth above and the features described below may be used not only in the corresponding explicitly set forth combinations, but also in further combinations or in isolation, without departing from the scope of the present invention. BRIEF DESCRIPTION OF THE FIGURES

[0045] FIG. 1 schematically illustrates a HUD system with an image generation unit according to various examples.

[0046] FIG. 2 illustrates a stacked arrangement of multiple HOEs assigned to different source color channels and forming part of an optical imaging system of the HUD system of FIG. 1.

[0047] FIG. 3 schematically illustrates a lateral displacement of virtual images corresponding to multiple color channels due to color aberrations induced by the stacked HOEs.

[0048] FIG. 4 schematically illustrates the spectra of several source color channels as well as the spectra of several display color channels.

[0049] FIG. 5A schematically illustrates a sequence of time slots of a time-division multiplexing scheme for driving the multicolor light source of the multi-pixel display device according to various examples.

[0050] FIG. 5B schematically illustrates a sequence of time slots of a time-division multiplexing scheme for driving the multicolor light source of the multi-pixel display device according to various examples.

[0051] FIG. 6 is a flowchart of an exemplary method.

[0052] FIG. 7 is a schematic view of an exemplary illumination system for illuminating a multi-pixel display device.

[0053] FIG. 8 is another side view of the exemplary system of FIG. 7.

[0054] FIG. 9 is a perspective view of the exemplary system of FIG. 7.

[0055] FIG. 10 schematically illustrates a speckle noise suppression unit.

[0056] FIG. 11 is a schematic view of an exemplary illumination system for illuminating a multi-pixel display device.

[0057] FIG. 12 is a perspective view of the exemplary system of FIG. 11. DETAILED DESCRIPTION

[0058] The above-described properties, features and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more clearly understood in connection with the following description of the embodiments, which are explained in more detail in connection with the drawings.

[0059] The present invention is explained in more detail below using preferred embodiments with reference to the drawings. In the figures, identical reference numerals designate identical or similar elements. The figures are schematic representations of various embodiments of the invention. Elements shown in the figures are not necessarily drawn to scale. Rather, the various elements shown in the figures are depicted in such a way that their function and general purpose will be understood by those skilled in the art. Connections and couplings between functional units and elements shown in the figures can also be implemented as an indirect connection or coupling. A connection or coupling can be implemented wired or wirelessly. Functional units can be implemented as hardware, software, or a combination of hardware and software.

[0060] FIG. 1 schematically illustrates a HUD system 100 according to various examples. The system 100 includes an image generation unit 115. The image generation unit 115 includes a multicolor light source 111 configured to emit red light, blue light, and green light. In particular, coherent, narrowband light is emitted. The multicolor light source 111 is thus configured to emit narrowband, coherent light in three source color channels 301, 302, 303 (hereinafter simply referred to as color channels).

[0061] The corresponding optical paths of the three corresponding color channels 301, 302, 303 (red - green - blue) of the HUD system 100 are shown with solid, dotted, and dashed arrows. In addition, the image generation unit 115 also includes a multi-pixel display device 112. Generally, a monochromatic multi-pixel display device 112 or a multi-pixel display device with multiple display color channels can be used. An exemplary implementation would be a liquid crystal display with a red display color channel, a green display color channel, and a blue display color channel. An alternative would be, for example, an implementation of the multi-pixel display device 112 using a DMD.

[0062] In particular, techniques are described below in which the multi-pixel display device 112 has multiple display color channels. Therefore, a monochromatic multi-pixel display device 112 is not used in these examples. For example, the multi-pixel display device 112 could have three color channels, such as red, green, and blue. Such multi-pixel display devices 112 are relatively widespread, for example in the form of conventional liquid crystal thin-film displays ("TFT displays"). Accordingly, such multi-pixel display devices 112 are relatively inexpensive. In particular, multi-pixel display devices with multiple color channels are more widely available than monochromatic multi-pixel display devices.

[0063] A multi-pixel display device 112 may comprise an array, where each array unit cell has a number of pixels corresponding to the number of display color channels. Each of the pixels is assigned to a corresponding display color channel. For example, a typical multi-pixel display device 112 could comprise an array where each array unit cell has three pixels, for the red, green, and blue color channels. Each pixel may have a switchable liquid crystal cell and a corresponding color filter arranged in front of or behind it. The color filters of the different pixels have different filter curves. These filter curves define the spectrum of the respective display color channel. Typically, the filter curves of available multi-pixel display devices are not particularly narrowband. In particular, the filter curves are often wider than required for the efficient reconstruction of a hologram using illumination from an HOE.Therefore, in the various examples, display color channels are used whose spectra have only a fraction of the width of the corresponding filter curves. An illumination system 30 can optionally be provided between the multicolor light source 111 and the multi-pixel display device 112, which is configured to improve the illumination of the multi-pixel display device 112.

[0064] The multicolor light source 111 and the display device 112 are controlled by a control unit 119. The control unit 119 can be implemented, for example, as a processor with a memory, whereby the processor can load and execute program code from the memory. It would be possible for the control unit 119 to be implemented as an application-specific integrated circuit or as a field-programmable gate array (FPGA). A combination of corresponding logic circuits is also conceivable.

[0065] The control unit 119 can, for example, activate or deactivate different color channels of the multi-color light source 111. The control unit 119 can display different displays on the multi-pixel display device 112, e.g.—if the display device 112 has multiple color channels—on one or more color channels. The control unit 119 can deactivate individual color channels on the multi-pixel display device 112; in such a case, the corresponding pixels assigned to the respective deactivated display color channel are controlled so that light is not transmitted.

[0066] The HUD system 100 also includes an optical imaging system 113. This is configured to generate a virtual image of the display images of the display device 112, which is visible from an eyebox 114. The optical imaging system 113 can comprise one or more lenses and / or mirrors. Alternatively or additionally, the optical imaging system 113 can comprise one or more HOEs, which are used, for example, for beam shaping. If particularly narrowband and coherent light is generated by the multi-color light source 111, the optical imaging system 113 and / or image generation unit 115 can comprise a speckle noise suppression unit. For example, a moving diffuser can be used to reduce speckle noise. For example, the speckle noise suppression unit could be part of the system 30 for illuminating the multi-pixel display device 112.As described above, it is possible for the optical imaging system 113 to include one or more HOEs. Details are discussed in connection with FIG. 2.

[0067] FIG. 2 illustrates that the optical imaging system 113 can have a stack structure 150 having a plurality of HOEs 151, 152, 153. Each of the HOEs 151, 152, 153 can be designed, for example, as a volume hologram in which a transparent substrate is provided with a refractive index modulation. The spatial frequency of the respective refractive index modulation is tuned to the wavelength or the narrowband spectrum of a corresponding source color channel 301, 302, 303, so that the light from the other source color channels passes through the respective holographic optical element h unaffected. The stack structure with a finite thickness of the holographic optical elements 151, 152, 153 results in a splitting of the beam paths of the color channels 301, 302, 303, as shown in FIG. 2. As a result, the virtual images 180 of the color channels 301, 302, 303 shift relative to one another, as shown in FIG. 3.This lateral offset is reduced in reference implementations by pre-distorting the respective display images of the multi-pixel display device 112, which are assigned to the various color channels 301, 302, 303. However, in conventional multi-pixel display devices, the display color channels are mixed, so that the pre-distortion actually intended for the red source color channel 301 would also affect the green source color channel 302. This is shown in FIG. 4.

[0068] FIG. 4 shows the spectra of the multicolor light source 111 for the source color channels 301, 302, and 303 (dashed-dotted lines). These emission spectra are narrowband, meaning they typically have a width of no more than 5 nm. This is helpful for ensuring good reconstruction of holograms with reduced stray light.

[0069] Furthermore, FIG. 4 shows the spectra of the display color channels 391, 392, 393. From FIG. 4 it can be seen that, for example, the spectrum of the display color channel 391 has a maximum approximately at the position of the maximum of the spectrum of the source color channel 301 (the display color channel 391 is therefore assigned to the source color channel 301 because they have a maximum overlap), but the spectrum of the source color channel 302 is arranged on an edge of the spectrum of the display color channel 391. The spectrum of the display color channel 391 therefore overlaps with the spectrum of the display color channel 392; likewise, the spectrum of the display color channel 391 overlaps with both the spectrum of the source color channel 301 and the spectrum of the source color channel 302. In FIG. 4 also shows that the spectrum of the display color channel 391 does not overlap with the spectrum of the source color channel 303. Thus, one can say that the source color channels 301 and 303 respectively.the display color channels 391, 393 are orthogonal to each other because they can be operated simultaneously without significant color mixing occurring.

[0070] Due to the spectral overlap of the display color channel 391 with the source color channels 301, 302, any predistortion applied to the display image for the source color channel 301 also affects the source color channel 302 to some extent. Therefore, the lateral offset between the source color channels 301, 302, 303 perceived in the virtual images 180 (see FIG. 3) cannot be fully compensated. To resolve or at least mitigate this problem, a time-division multiplexing scheme can be used. This is illustrated in FIG. 5A.

[0071] FIG. 5A illustrates time slots 501-506 of a time-division multiplexing scheme 500. Time slot 501 and time slot 504 are assigned to source color channel 301; time slot 502 and time slot 505 to source color channel 302; and time slot 503 and time slot 506 to source color channel 303.

[0072] During time slots 501 and 504, the multicolor light source 111 is controlled to emit only light according to the source color channel 301. This means that only the source color channel 301 is activated; the other source color channels 302, 303 are deactivated. At the same time, the multi-pixel display device 112 is controlled to display a corresponding display image 511 associated with the source color channel 301 during time slots 501, 504. During time slots 502, 505, the source color channel 302 is then activated; in addition, a display image 512 is activated which has a predistortion appropriate for the source color channel 302. During time slots 503, 506, the source color channel 303 is activated. Additionally, the multi-pixel display device 112 is controlled to display the display image 513 having a predistortion associated with the source color channel 303.In other words, a sequential and discrete-color multiplexing of the individual RGB color channels occurs. For each display image associated with a single source color channel, the corresponding light emitter associated with the same source color channel is also activated in the corresponding time slot. Thus, different color channel individual images are displayed one after the other on the multi-pixel display device.

[0073] If display image 511 is activated in time slot 501, it can be displayed on display color channel 391. This means that the pixels of multi-pixel display device 112 assigned to display color channel 391 are controlled based on image information from display image 511. Different variants are conceivable for the remaining pixels of multi-pixel display device 112—those pixels that are not assigned to display color channel 391, but rather to display color channel 392 or display color channel 393.

[0074] In a first variant, pixels of the multi-pixel display device 112 associated with the display color channels 392, 393 are switched off in time slot 501. This means that such pixels no longer transmit light significantly. More generally, this means that in a time slot assigned to a specific source color channel, only a single corresponding display color channel is switched on, and all other display color channels are switched off. The switched-on display color channel reproduces the display image associated with the source color channel of the respective time slot.

[0075] In a second variant, more than a single display color channel is switched on. For example, generally speaking, two or more display color channels are switched on that overlap with a respective active source color channel. In the second variant, pixels associated with at least one of the display color channels 392, 393 are switched on in time slot 501. Such pixels are also controlled based on image information from the display image 511 associated with the source color channel 301. In other words, this means that the display image 511 is displayed not only on the display color channel 391 in time slot 501, but also on the display color channel 392 and / or on the display color channel 393.More generally, this means that in the second variant, in a time slot assigned to a specific source color channel, the corresponding display color channel is switched on, and at least one of the remaining display color channels is also switched on. The switched-on display color channels reproduce the corresponding display image associated with the respective source color channel. This second variant has the advantage that more light overall leaves the multi-pixel display device 112, so that greater brightness can be achieved.

[0076] FIG. 5B illustrates time slots 591-594 of a time-division multiplexing scheme 590. In contrast to the variant of FIG. 5A, or in contrast to the time-division multiplexing scheme 500, the time-division multiplexing scheme 590 includes time slots 591, 593 that are assigned to more than one source color channel. In the example of FIG. 5B, the time slots 591, 593 are assigned to both the source color channel 301 and the source color channel 303. This takes advantage of the fact that the source color channels 301, 303 are orthogonal to one another.

[0077] In detail: The simultaneous assignment of time slots 591, 593 to both source color channels 301, 303 is possible because the display color channel 391 (assigned to the source color channel 301) has a spectrum that does not overlap with the spectrum of the source color channel 303; furthermore, the spectrum of the display color channel 393 (assigned to the source color channel 303) does not overlap with the spectrum of the source color channel 301.

[0078] On the other hand, the spectrum of the source color channel 302 overlaps with the spectra of all display color channels 391, 392, 393.

[0079] In the variant of FIG. 5B, there are two types of time slots: on the one hand, time slots 591, 593, to which two source color channels 301, 303 are assigned; and on the other hand, time slots 592, 594, to which only a single source color channel 302 is assigned in order to avoid mixing the color correction.

[0080] It is therefore possible to control the multi-pixel display device 112 in time slot 591 so that it displays the display image 511 on the display color channel 391 in time slot 591; and in the same time slot 591, the display image 513 on the display color channel 393. At the same time, both the source color channel 301 and the source color channel 303 are activated. The source color channel 302 is activated, and the display color channel 392 is deactivated. The display color channel 392 and the source color channel 302 are then addressed in the subsequent time slot 592. In the subsequent time slot 592, the source color channels 301, 303 are deactivated, and the display color channels 391, 393 are deactivated. The time slot 592 is assigned to the source color channel 302 and the multi-pixel display device 112 is controlled so that in the time slot 302 the display image 512 is reproduced on the display color channel 392.Time slot 593 then corresponds to time slot 591, and time slot 594 corresponds to time slot 592. The advantage of such a technique according to FIG. 5B is that the time interval between the repeated addressing of the same source color channel or display color channel is reduced compared to the variant of FIG. 5A. This enables the use of multi-pixel display devices that provide a lower refresh rate; without a simultaneous loss of quality in the cognitive color mixing by the viewer.

[0081] FIG. 6 is a flowchart of an exemplary method. The method of FIG. 6 is computer-implemented. The method of FIG. 6 may be applied by a controller of an image generation unit for a transparent display unit. For example, the method of FIG. 6 could be applied by the controller 119 of the HUD system 100 of FIG. 1.

[0082] In box 905, it is checked whether the next time slot of a sequence of time slots of a time-division multiplexing scheme has started or is starting.

[0083] If this is the case, in box 910 a source color channel assigned to the respective time slot according to a predefined scheme (for example, "round robin", ie toggling through all color channels one after the other, see FIG. 5A) is selected as the current source color channel (see color channels 301, 302, 303 in FIG. 1).

[0084] In box 915, a current display image for the activated color channel is obtained, and if necessary, in box 920, a specific pre-distortion is calculated for this current display image. The pre-distortion can also be pre-calculated. Techniques for determining the pre-distortion are known in the art. For example, a lateral offset due to a translational color aberration can be corrected by appropriately translating pixels in the display image. In box 925, a multi-color light source is then controlled to activate one or more light emitters of the current source color channel (see box 910).

[0085] In box 930, the current image with pre-distortion (see box 915, box 920) is displayed on the multi-pixel display device by controlling the multi-pixel display device accordingly. If the multi-pixel display device has multiple color channels, the current image is displayed on the display color channel associated with the current color channel, and the other display color channels are turned off or also display the current image. For a multi-pixel display device without multiple color channels, the current image is displayed on the single channel.

[0086] Then, box 905 is executed again for the next magazine. By executing the repeated iterations of box 905 quickly enough, i.e., by keeping the time slots short enough (switching frequency of the time slots), color mixing of the virtual images generated by the images displayed in box 930 occurs in the viewer's eye.

[0087] Various variations of the method shown in FIG. 6 are conceivable.

[0088] For example, it would be particularly conceivable for more than one source color channel with its associated display image to be assigned to a specific time slot. In particular, orthogonal source color channels can be assigned to a single, common time slot. Then, different display images can be reproduced on the different display color channels, each compensating for color aberration for the respective source color channel.

[0089] FIG. 7 schematically illustrates an illumination system 7030 for illuminating a multi-pixel display device 112 (cf. FIG. 1). The illumination system 7030 can, for example, implement the illumination system 30 from FIG. 1. Laser light (e.g., red-green-blue channels, or fewer or more channels; cf. FIG. 1: channels 301, 302, 303) from a laser light source 111 is coupled into a speckle noise suppression unit 3—implemented here as a dynamically moving diffuser—by means of an optical waveguide 4. For this purpose, a lens, e.g., a GRIN lens, can optionally be provided at the corresponding end of the optical waveguide 4. The radiation characteristic of such a lens can be varied depending on the application. The lens can provide collimation. For example, a lens with a collimation characteristic can be provided that is adapted to an input angle range of the speckle noise suppression unit 3.Dispersion, in particular, must be taken into account. Alternatively or additionally, a parabolic mirror can be used for beam expansion and collimation between the end of the optical fiber 4 and the speckle noise suppression unit 3.

[0090] Speckle noise (also light granulation or laser granulation or speckle for short) refers to the granular interference phenomena that can be observed with sufficiently coherent illumination of optically rough object surfaces (unevenness in the order of the wavelength).

[0091] The diffuser implementing the speckle noise suppression unit 3 is moved randomly, for example, in both lateral directions perpendicular to the optical fiber axis, both relative to the optical fiber 4 and to the rest of the structure. The movement frequency can be adjusted, for example. This allows speckle noise to be effectively suppressed.

[0092] The light is then coupled into the transparent substrate of an optical homogenization plate 1 (e.g., made of glass or plastic) in a coupling structure 2 (also referred to as a light redistribution structure). The coupling structure 2 is arranged on a side surface 23 of the homogenization plate 1, directly adjacent to the speckle noise suppression unit 3. This enables a compact design; additional collimation lenses or similar devices are unnecessary.

[0093] The coupling structure 2 can be applied as a separate component to the homogenization plate 1. The coupling structure 2 can be designed, for example, as a lenticular array.

[0094] A lenticular array generally comprises a 1D or 2D array of lens-shaped elements (lenticules) that are configured to refract light in different directions. For example, a 1D array can be used in which the lenticules are spaced apart in a first direction and the lenticules are extended in a second direction perpendicular thereto. The coupling structure 2 can, for example, be designed as a film, for example a lenticular array film. However, it would also be possible for the coupling structure 2 to be formed integrally with the homogenization plate 1, i.e., as a monolithic component. For example, the side surface 23 can be designed with a corresponding surface topography (e.g., as a lenticular array in the form of indentations on the side surface 23).

[0095] The light-conducting substrate of the optical homogenization plate 1 is provided on its underside 22 with a further transparent light redistribution structure 5, which redistributes the light field according to the desired design specifications, e.g. by multiple scattering and reflection 9.

[0096] The light redistribution structure 5 can be applied to the homogenization plate 1 as a separate component. The light redistribution structure 5 is realized, for example, in the form of (hemispherical) or ellipsoidal elevations or depressions directly in the substrate, but can also be achieved by gluing a structural film adapted to the refractive index. The light redistribution structure 5 can, for example, alternatively be designed as a film, for example a lenticular array film. However, it would also be possible for the light redistribution structure 5 to be formed integrally with the homogenization plate 1, i.e., as a monolithic component. For example, the underside 22 can be designed with a corresponding surface topography (e.g., as a lenticular array in the form of indentations).

[0097] The design specifications, such as lateral homogeneity and spatial orientation of the light field 8 emitted by the homogenization plate 1 (radiation characteristic of the homogenization plate 1) can be adjusted by the design, arrangement, size and / or number of such light redistribution structures (the main radiation direction 205 - perpendicular to the surface of the homogenization plate 1 - is also provided with a reference symbol).

[0098] To reduce light losses on the underside 22 of the optical homogenization plate 1 below the light redistribution structure 5, a reflector structure 6 is provided, here embodied as a reflective layer. The reflector structure 6 redirects the light back toward the optical homogenization plate 1. The reflector structure 6 can, for example, be glued to the light redistribution structure 5 as a reflective foil. It would also be possible for the reflector structure 6 to be produced as a metallic vapor deposition (thin film), for example, with aluminum.

[0099] To specifically adjust the angular distribution of the light field leaving the homogenization plate, an optional coupling structure 7 can be used on the upper side 21 of the homogenization plate 1. This can be designed as a diffractive diffuser. By means of a suitable design of the coupling structure 7, a pixel plane 11 of the multi-pixel display device 112 can be illuminated in a customized manner. For example, OLED pixels or micromirrors are arranged in the pixel plane 11.

[0100] The coupling-out structure 7 can be applied to the homogenization plate 1 as a separate component. The coupling-out structure 7 can be formed, for example, as a film. However, it would also be possible for the coupling-out structure 7 to be formed integrally with the homogenization plate 1, i.e., as a monolithic component. For example, the upper side 21 can be formed with a corresponding surface topography.

[0101] In particular, in various examples, it would be possible for the optical homogenization plate 1, together with the outcoupling structure 7, the incoupling structure 2, and the light redistribution structure 5, to be manufactured as a monolithic component. For example, this monolithic component could be made from a single plastic block. Possible manufacturing techniques include 3D printing or injection molding.

[0102] In order to suppress the visibility of the diffuser features of the output coupling structure 7 for the human eye, it may be useful to decouple the pixel plane 11 from the exit plane or the output coupling structure 7, i.e. to distance it (cf. gap 12). This depends on the system requirements. The gap 12 is preferably larger than 20% of the thickness 13 of the homogenization plate 1. The human eye adapts to the pixel plane 11. If the gap 12 is larger than the depth of field of the eye, the diffuser features of the output coupling structure 7 are only blurred and are cognitively suppressed, for example. Furthermore, it is possible to incline or tilt the pixel plane 11 relative to the homogenization plate 1.

[0103] FIG. 8 is a side view of the illumination system 7030 from FIG. 7. In FIG. 8 it can be seen that the lateral extent of the coupling structure 2 is smaller than the extent of the side surface 23 of the homogenization plate 1, i.e. along the z-axis in FIG. 8 along the long side of the side surface 23. For example, in the example of FIG. 8 the coupling structure 2 covers approximately 70% of the side surface 23. In general it would be conceivable for the lateral extent of the coupling structure 2 to be no greater than 80% or no greater than 70% of a side length of the side surface 23 or no greater than 50% of a side length of the side surface 23. In other examples it would also be conceivable for the coupling structure 2 to cover the entire side 23. The lateral extent of the coupling structure 2 can be equal to the side length of the side surface 23. The aperture of the speckle noise suppression unit 3 is also smaller than the lateral extent of the coupling structure 2.In general, the lateral extent of the speckle noise suppression unit 3, for example, cannot be greater than 20% of the area of ​​the coupling structure 2. By means of the coupling structure 2, a relatively large-area coupling of light into the homogenization plate 1 can take place, although the speckle noise suppression unit 3 typically has a relatively limited extent with respect to the side surface 23. The extent (width across the side surface) and texture (lenticular, prismatic) of the coupling structure 2 can be variably adapted (design degrees of freedom). For example, the coupling structure 2 can comprise one or more prismatic structures; these serve as scattering geometries for already coupled and internally reflected / scattered light. Thus, the radiation characteristics of the homogenization plate 1 - i.e. the light distribution, e.g. homogeneity, of the light at the coupling structure 7 - can be influenced at the location of the coupling structure 2.The light is also further redistributed within the homogenization plate 1, for example, by the light redistribution structure 5. For this reason, it is not absolutely necessary for the lateral extent of the coupling structure 2 to cover the entire length of the side surface 23. This is also evident in the perspective view of the illumination system 7030 in FIG. 9. FIG. 10 schematically shows a speckle noise suppression unit 9300. This can, for example, implement the speckle noise suppression unit 3 from FIG. 7. The speckle noise suppression unit 9300 comprises a diffuser 9301 in an xy plane. Different drive types are conceivable to move the diffuser 9301 in the xy plane. For example, an electrodynamic drive can be used. An electrodynamic drive, also referred to as an electromagnetic drive, uses, for example, a coil to generate an alternating magnetic field by which a magnet then moves.The magnet is connected to the diffuser 9301. Corresponding actuators 9303, which are mounted via a frame in a fixed reference system (attachment 9304), are shown in FIG. 9. The drive frequency of the electrodynamic actuators 9303 can be dynamically adjusted within a relatively wide frequency range, so that, for example, one or more eigenmodes of the mass-spring system (formed by the diffuser 9301 and return springs 9302) are resonantly excited. The best despeckle results can be achieved in such resonant states. This type of drive can be designed as a single- or two-axis system (a two-axis system is shown as an example in FIG. 10). Furthermore, it is also possible to initiate a non-deterministic movement via one or more vibration motors (e.g., a miniature motor with an eccentric flywheel on a motor shaft) connected to the support frame of the diffuser.

[0104] To achieve a suitable statistical averaging z, the motion frequency should not be less than 100 Hz. However, the optimal frequency range depends on the geometry and mass of the diffuser (natural frequency of the mass-spring system). Furthermore, psychoacoustic effects should also be considered during design, so there may be a trade-off between different target variables.

[0105] The scenario in FIG. 10 is just one example of a hardware implementation of a speckle noise suppression unit. In general, the light beam path can be varied over time. This can be achieved by mechanical movement (such as vibration or rotation) of an element in the beam path, such as a diffuse disk (see FIG. 10) or a fiber optic cable. Changing the beam path over time causes the speckles to be "smeared" over time, which reduces the perceived noise. Alternatively, the phase of the incident light could also be varied, e.g., using spatial light modulators (SLMs).

[0106] FIG. 11 schematically illustrates an illumination system 10500. The illumination system 10500 provides functionality corresponding to the illumination system 7030 in FIG. 7. However, in FIG. 11 (unlike FIG. 7), a free-beam beam path is used for the light to illuminate a multi-pixel plane 10506 of a multi-pixel display device 112.

[0107] The light is generated by a laser light source 10501. From the laser light source 10501, the light propagates to a speckle noise suppression unit 10502. This can be configured like the speckle noise suppression unit 9300 in FIG. 10.

[0108] The light can propagate as a free beam between the laser light source 10501 and the speckle noise suppressor of a 10502. It is also conceivable for the light to be guided by a fiber optic cable.

[0109] The speckle noise suppression unit 10502 itself has a moving diffuser that implements a specific radiation characteristic 10507. This illuminates another diffuser 10504, which in turn provides a radiation characteristic 10508. The radiation characteristic 10508 is configured to achieve a homogeneous illumination of another diffuser 10505. The diffuser 10504 thus takes over the functionality of the

[0110] Homogenization plate 1 together with the light redistribution structure 5 in FIG. 7.

[0111] The light can propagate as a free beam between the speckle noise suppression unit and the diffuser 10504 and between the diffuser 10504 and the diffuser 10505.

[0112] The diffuser 10505 has a radiation characteristic 10509. This

[0113] Beam characteristic 10509 can be adjusted to illuminate an entrance pupil of an optical imaging system of a HUD system as desired.

[0114] Then, the pixel plane 10506 of the multi-pixel detector 112 is illuminated. This, in turn, has a radiation characteristic 10510. FIG. 11 also shows that the diffuser 10505 is arranged at a distance from the pixel plane 10506 by a gap. This suppresses the visibility of diffuser features. This is particularly helpful when the optical imaging system of the HUD system greatly magnifies the pixel plane 10506 of the multi-pixel detector 112. A virtual image of the pixel plane 10506 is perceived by the user as having a fairly large extent, so that diffuser features of the diffuser 10505—if they were visible—would be perceived as particularly disturbing. FIG. 12 is a perspective view of a specific implementation of the system 500 from FIG. 11. Two deflecting mirrors 521 and 522 are also shown.

[0115] Of course, the features of the previously described embodiments and aspects of the invention can be combined with one another. In particular, the features can be used not only in the described combinations, but also in other combinations or on their own, without departing from the scope of the invention.

[0116] For example, various aspects related to a HUD system were discussed above. However, other transparent display units, such as holographic displays, can also benefit from the techniques described herein. Other optical imaging systems, such as those for projectors, can also benefit from the techniques described herein.

[0117] Furthermore, the discussion above primarily focused on color aberrations caused by a stack of HOEs, i.e., a translational offset from the images perceived by the user. However, other types of color aberrations can also be compensated.

Claims

PATENT CLAIMS 1. A method for controlling an image generation unit (115) for a transparent display unit (100), wherein the image generation unit (115) comprises a multi-pixel display device (112) and a multi-color light source (111), wherein the multi-color light source (111) is configured to emit coherent light in a plurality of source color channels (301, 302, 303) to illuminate the multi-pixel display device (112), the method comprising: - controlling (925) the multi-color light source (111) to emit the coherent light in time slots (501, 502, 503, 504, 505, 506) of a time-division multiplexing scheme (500), each time slot (501, 502, 503, 504, 505, 506) being assigned to a single corresponding source color channel (301, 302, 303) of the plurality of source color channels (301, 302, 303), and - controlling (930) the multi-pixel display device (112) in order to display a corresponding display image (511, 512, 513) in each of the time slots (501, 502, 503, 504, 505, 506) which is associated with the respective source color channel (301, 302, 303), wherein the display images (511, 512, 513) have a pre-distortion which reduces the color aberrations of an optical imaging system (113) of the transparent screen unit (115) for the respective source color channel (301, 302, 303) associated with the respective display image (511, 512, 513).

2. The method of claim 1, wherein the color aberrations are caused by a layer thickness of stacked holographic optical elements (151, 152, 153) of the optical imaging system (113), wherein different ones of the stacked holographic optical elements (151, 152, 153) are associated with different ones of the plurality of source color channels (301, 302, 303).

3. The method according to claim 1 or 2, wherein the multi-pixel display device (112) has an associated display color channel (391, 392, 393) for each source color channel (301, 302, 303), wherein Spectra of the display color channels overlap with spectra of several source color channels (301, 302, 303).

4. The method according to claim 3, wherein the multi-pixel display device (112) is controlled in each of the time slots to display the corresponding display image (511, 512, 513) on the display color channel (391, 392, 393) assigned to the respective source color channel (301, 302, 303), and to display the corresponding display image (511, 512, 513) optionally on one or more further display color channels (391, 392, 393).

5. The method according to claim 3 or 4, wherein the multi-pixel display device (112) is controlled in each of the time slots (501, 502, 503, 504, 505, 506) to display the corresponding display image (511, 512, 513) on the display color channel (391, 392, 393) assigned to the respective source color channel (301, 302, 303) and to switch off at least one further display color channel (391, 392, 393).

6. A method according to any one of the preceding claims, wherein the time division multiplexing scheme is a switching frequency of time slots of not less than N*25 Hz, where N is the number of source color channels.

7. A method for controlling an image generation unit (115) for a transparent display unit (100), wherein the image generation unit (115) comprises a multi-pixel display device (112) and a multi-color light source (111), wherein the multi-color light source (111) is configured to emit coherent light in a plurality of source color channels (301, 302, 303) to illuminate the multi-pixel display device (112), the method comprising: - controlling (925) the multi-color light source (111) to emit the coherent light in time slots (591, 592, 593, 594) of a time-division multiplexing scheme (590), wherein first time slots (592, 594) are each assigned to a single corresponding source color channel (302) of the plurality of source color channels (301, 302, 303), wherein second time slots (591, 593) are each assigned to at least two corresponding source color channels (301, 303) of the plurality of source color channels (301, 302, 303), - controlling (930) the multi-pixel display device (112) in order to display, in each of the time slots (591, 592, 593, 594), one or more display images (511, 512, 513) associated with the respective one or more source color channels (301, 302, 303), wherein the display images (511, 512, 513) have a pre-distortion that reduces color aberrations of an optical imaging system (113) of the transparent screen unit (115) for the respective source color channel (301, 302, 303) associated with the respective display image (511, 512, 513), wherein the multi-pixel display device (112) has an associated Display color channel (391, 392, 393), wherein spectra of source color channels associated with the second time slots do not overlap with spectra of display color channels associated with other source color channels associated with the second time slots.

8. Control device (119) of an image generation device (115) for a transparent screen unit (100), which is configured to carry out the method according to one of the preceding claims.

9. System (100) comprising the control device (119) and the image generating device (115).

10. The system (100) of claim 9, wherein the system (100) further comprises an illumination system (30, 7030, 10500) for illuminating the multi-pixel display device (112).

11. The system (100) of claim 10, wherein the illumination system (30, 7030, 10500) further comprises a speckle noise suppression unit (3, 9300, 10502).

12. Program code that can be loaded and executed by a processor, wherein the processor, when executing the program code, carries out methods according to any one of claims 1 to 7.

Images