Display device for a holographic reconstruction, light modulation device for a display device and method for generating a holographic reconstruction

Abstract

Display device for holographic reconstruction, comprising - a spatial light modulation device (SLM) with combined phase modulation pixels (p) and amplitude modulation pixels (a), - a lighting device suitable for illuminating the spatial light modulation device (SLM), - a reflection plane (RP) arranged to generate sufficiently coherent light emitted by the illumination device which enters the spatial light modulation device (SLM), wherein the light passes through both the phase modulation pixel (p) and the amplitude modulation pixel (a) of the spatial light modulation device (SLM), and wherein the light is reflected from the intermediate reflection plane (RP), wherein the light passing through both the phase modulation pixels (p) and the amplitude modulation pixels (a) is reflected from an intermediate mirror system arranged in the reflection plane (RP), and wherein the mirror segments (M) of the mirror system are arranged relative to the phase modulation pixels (P) and the amplitude modulation pixels (a) such that each mirror segment (M) covers both a part of the phase modulation pixel (p) and a part of the amplitude modulation pixel (a).

Description

[0001] The present invention relates to a display device for holographic reconstruction. In particular, the present invention relates to a display device that utilizes a planar combination of adjacent modulation pixels of a spatial light modulation device. Such display devices are primarily required for mobile applications such as smartphones and tablet computers. However, other applications are also possible.

[0002] Furthermore, the present invention also relates to a method for generating a holographic reconstruction and a spatial light modulation device used in such a display device for generating a two-dimensional and / or three-dimensional representation of a scene or content.

[0003] The present display device is configured to display two-dimensional (2D) and / or three-dimensional (3D) images. It is understood that two-dimensional images or three-dimensional images also include two-dimensional or three-dimensional content or movies.

[0004] The field of application of the present invention preferably includes direct-view display devices (direct-view displays) for the three-dimensional representation of holographic images.

[0005] A commercially available flat-panel display for displaying two-dimensional images or films / videos requires bright and uniform illumination of the entire surface at a high resolution. The spatial light modulation device, which serves as the display panel, is required to emit light over a wide angular range. The information to be displayed is written into the spatial light modulation device of the display device. The light emitted by an illumination device, which has a light source unit, is modulated with the information written into the spatial light modulation device. The spatial light modulation device often also serves as a screen or display panel.It is therefore necessary to ensure strictly parallel incidence of the light rays on the spatial light modulation device and to achieve a high refresh rate of the spatial light modulation device. To achieve a high quality three-dimensional representation of the information inscribed in the spatial light modulation device, a defined collimation of the wavefronts coupled out of the illumination device is required, in addition to homogeneous illumination of the entire surface of the spatial light modulation device. This is particularly important for holographic representations in the form of a reconstruction to be generated. The holographic information, which can be, for example, an object consisting of object points in a three-dimensional scene, is encoded into the pixels of the spatial light modulation device in the form of amplitude and phase values.The coded object points are generated by the wave field emitted by the spatial light modulation device.

[0006] A complex value used to modulate both the phase and amplitude of a wavefront cannot be satisfactorily represented directly in a single pixel of a known spatial light modulation device. However, modulating only one value per pixel, i.e., pure phase modulation or pure amplitude modulation, only leads to an inadequate holographic reconstruction of a preferably moving three-dimensional scene. A direct and thus optimal—in the sense of generalized parameters—representation of the complex values ​​can only be achieved by means of complex-valued modulation, preferably at the same level and at the same time in the spatial light modulation device.

[0007] Depending on the actual type of spatial light modulation device, various methods are known for achieving simultaneous modulation of both parts of the complex values ​​to be displayed.

[0008] For example, two separately controllable spatial light modulators can be combined and placed very close together to simultaneously modulate both the amplitude and phase of coherent light. One spatial light modulator modulates the amplitude, the other the phase of the incident light. Other combinations of modulation features are also possible with such an arrangement.

[0009] The light must first pass through a pixel of the first spatial light modulator and then through the corresponding or associated pixel of the second spatial light modulator. This can be achieved, for example, by imaging the first spatial light modulator onto the second spatial light modulator by a large-area optical element, e.g., a lens, or by imaging the first spatial light modulator onto the second spatial light modulator by an array of small lenses, or by sandwiching the two spatial light modulators.

[0010] These combinations of two spatial light modulators used to achieve complex-valued modulation have the disadvantage that the distance between the two spatial light modulators is much larger than their pixel pitch, i.e. the distance between two pixels.

[0011] A typical pixel pitch of a spatial light modulator for holographic applications, for example, is between 10 µm and 50 µm. In contrast, the spacing between the two spatial light modulation panels in a sandwich arrangement is several hundred µm; in applications where one spatial light modulation panel is imaged onto the other, the spacing is even greater.

[0012] Many types of spatial light modulators, such as liquid crystal (LC) light modulators, typically comprise an addressable liquid crystal layer sandwiched between transparent glass substrates. Alternatively, in a reflective display device, the addressable layer is sandwiched between a transparent glass substrate and a reflective glass substrate. The glass substrates typically have a thickness of, for example, between 500 µm and 700 µm.

[0013] A sandwich structure for complex-valued modulation can be created by arranging a single spatial phase modulator and a single spatial amplitude modulator with their glass substrates arranged one behind the other. If a beam of light coming from the addressable layer of a pixel of the spatial phase modulator, after passing through the glass substrates, falls on the addressable layer of a pixel of the spatial amplitude modulator, it would already be widened at the aperture of that pixel due to diffraction effects, resulting in crosstalk between the beams of light from adjacent pixels.

[0014] When using imaging elements, the challenge is that exactly one pixel of the first spatial light modulator must be imaged onto a pixel of the second spatial light modulator, across the entire surface of the spatial light modulators. This requires optical systems with extremely low distortion. Such requirements are difficult to meet in practice. The reason for this is that crosstalk between adjacent pixels occurs even when an imaging technique is used.

[0015] Crosstalk can be even worse if the two spatial light modulation panels, the optical imaging system, or the light sources are not perfectly aligned.

[0016] Furthermore, when spatial light modulation panels are combined and arranged very close to each other, such arrangements are prone to errors when diffraction of the incident wavefield occurs at the first plane of diffraction apertures, e.g., a spatial phase modulator forming the first plane of the sandwiched complex spatial light modulator. The diffracted light from a pixel provided in the first spatial light modulation panel can propagate to adjacent, unrelated pixels of the second spatial light modulation panel. In other words, for example, 80% of the light propagating behind a phase modulation pixel of the first panel can illuminate the corresponding amplitude modulation pixel of the second panel. A further 10% cannot impinge on a transparent portion of the amplitude panel, meaning that this 10% can be blocked by the absorbing portions of the apertures.And the remaining 10% of the light can illuminate adjacent amplitude pixels, creating crosstalk in the sandwich array. This crosstalk degrades the reconstruction quality of a holographic display, for example, because it corresponds to an incorrect combination of amplitude and phase values ​​when complex values ​​are represented by the spatial light modulators.

[0017] Consequently, the distance between phase and amplitude modulation planes must preferably be less than ten times the smallest pixel pitch present in a coherent direction. For example, when using one-dimensionally encoded sub-holograms, which may be the case for holographic TV, the light illuminating the spatial light modulator is coherent only in one direction, which may be, for example, the vertical direction.

[0018] In display devices used for holographic reconstruction, complex modulation of sufficiently coherent light emitted by the illumination device must be achieved to independently control the amplitude and phase of the pixels of a spatial light modulation device, hereinafter referred to as an SLM. The use of an appropriate spatial light modulation device is therefore required.

[0019] One way to achieve complex modulation using an SLM is to laterally combine adjacent phase modulation pixels and amplitude modulation pixels.

[0020] Such a lateral arrangement of adjacent amplitude modulation pixels and phase modulation pixels is disclosed in WO 2009 / 080576 A1 and US 2010 / 0265558 A1, respectively. This document describes a controllable light modulator having a number of macropixels composed of at least two pixels arranged side by side, with retroreflective elements provided. A retroreflective element has two reflective surfaces that run parallel in the vertical direction. The reflective surfaces are arranged without a gap at a predetermined angle of 90° such that they form a prism with a substrate layer in which the retroreflective element is arranged, and such that they reflect an incident part of a wave field. A controllable light modulator with retroreflective elements is also known from US 2012 / 0092750 A1.US 2011 / 0 096 381 A1 describes a controllable light modulator with an apodization mask. US 2010 / 0 046 049 A1 discloses, among other things, a holographic display device with one or more optically addressable spatial light modulators (OASLMs). WO 2012 / 085 045 A1 describes a display device in which visibility areas of the image information to be displayed can be adjusted to the viewer's eyes using light-influencing means.

[0021] In general, a pixel combination arrangement in an SLM can utilize inclined surfaces, which can be realized, for example, as one-dimensional (1D) or two-dimensional (2D) prism structures, which can be realized, for example, by applying molding or embossing techniques.

[0022] The achievable prism angle precision is significantly less than 1 / 10 of a degree. Even an angle tolerance of 0.5 degrees requires considerable technical effort to achieve a display size range with a prism pitch of < 100 µm.

[0023] It is not sufficient to focus on the combination of a phase modulation pixel and an amplitude modulation pixel arranged sequentially along the optical path of the illumination light, regardless of whether a standard sandwich-type display device is used or adjacent phase modulation pixels and amplitude modulation pixels are combined to form a functionally sequential phase and amplitude SLM sandwich array. This is only a boundary condition. To understand the requirements, the entire function must be understood, which is the generation of real or imaginary object points in the space of a frustum spanned by a viewing window containing an observer's eye and the SLM, and which may even continue behind the SLM, and has a pyramid-like structure.Creating the object points requires collimated illumination. Displaying high-resolution (HD) content requires limiting the angular spectrum of plane waves of the illumination to 1 / 60th of a degree.

[0024] Furthermore, there is a difference between the plane wave spectrum and the angular distribution added by the microprism array used, for example, in WO 2009 / 080576 A1. The plane wave spectrum must be ≤ 1 / 60°, but the angular distribution induced by the prisms does not have to be as small as this value of the plane wave spectrum and can, for example, be > 5°. Individual calibration of local pixels of the SLM is not required if the angular distribution is only 1 / 60°. However, if the angular distribution induced by the prisms significantly exceeds 1 / 60°, individual calibration is required to provide the exact phase values ​​within the entrance plane of the human eye, namely the design values ​​of the complex-valued wavefield. In other words, calibration of all pixels or at least a very large number of sample points must be performed. The measurement plane can be the display plane ordisplay plane or provided close to the display device. The locally measured wavefield, i.e., locally measured phase and amplitude distributions, can propagate to an observer window, e.g., using a Fresnel transform. In other words, this can be done by means of calculations of phase and amplitude distributions. However, the complex-valued wavefield emitted by individual pixels, or rather complex-valued pixels, of the SLM can be measured within the observer window itself without using measured propagation. The observer window is a visibility region in an observer plane. If the position of the observer window in the observer plane coincides with the eye of an observer and then the observer looks through the observer window, they can view the reconstructed scene.

[0025] Therefore, it is an object of the present invention to provide a display device that avoids the disadvantages of prior art display devices. Furthermore, the display device should be designed to be correspondingly flat, while achieving complex modulation of sufficiently coherent light to control the phase and amplitude independently of one another in a spatial light modulation device.

[0026] The invention is further based on the object of designing the display device with a structure that is simple and cost-effective.

[0027] The invention is further based on the object of developing a method with which a complex-valued modulation of light can be achieved in a simple manner.

[0028] According to the invention, the object is achieved by a display device according to claim 1 and a light modulation device according to claim 27.

[0029] The display device according to the invention serves for the holographic reconstruction of a scene, preferably for displaying two-dimensional and / or three-dimensional images. The display device comprises a spatial light modulation device with combined phase modulation pixels and amplitude modulation pixels. An illumination device suitable for illuminating the spatial light modulation device is further provided. The display device also has a reflection plane. All of these components are arranged such that they generate sufficiently coherent light required for holographic reconstruction, which is emitted by the illumination device. This light enters the spatial light modulation device and then passes through both the phase modulation pixels and the amplitude modulation pixels, being reflected by the intermediate reflection plane.

[0030] In this way, combined phase modulation and amplitude modulation pixels of a display panel are provided to realize a complex-valued display device. In other words, it is intended to combine phase modulation (shift) pixels and amplitude modulation pixels that are located close to each other. The spatial light modulation device has an addressable transmissive layer for forming the pixels. This addressable transmissive layer can be a liquid crystal layer. However, other discrete implementations of spatial light modulators are also possible, e.g., electrowetting-based spatial light modulators or magnetophotonic crystal-based spatial light modulators.Typically, the liquid crystal thickness of such an addressable transmissive layer required for 2π phase modulation of phase modulation pixels is twice the liquid crystal thickness required for ON-to-OFF modulation of amplitude modulation pixels. However, in the present invention, the liquid crystal layer thickness of the amplitude modulation pixel may be equivalent to that of the phase modulation (shift) pixel. This can preferably be realized by implementing a double-pass arrangement for the phase modulation pixels. The redirection of the light passing through the first pixels, the phase modulation pixels or the amplitude modulation pixels, is introduced by using the reflection plane provided on one side of the spatial light modulation device.

[0031] Thus, a display device for a holographic reconstruction of a scene is created which is flat in its structure and with which a complex light modulation is achieved.

[0032] A display device with these basic components can be supplemented with additional optical components, or the existing components can be arranged in a different way to create, for example, either a transmissive or a reflective spatial light modulation device. A spatial light modulation device is referred to below as an SLM.

[0033] Further preferred embodiments and improvements of the present invention are set out in the dependent claims.

[0034] In a first preferred embodiment of the invention, the display device can be configured such that the phase modulation pixels and the amplitude modulation pixels of the spatial light modulation device are laterally combined within the same plane. The laterally combined arrangement of the phase modulation pixels and the amplitude modulation pixels in the same plane of the spatial light modulation device has the advantage of using a constant thickness of the addressable transmissive layer and allowing the phase modulation pixels or the amplitude modulation pixels to pass through twice, depending on the layout used.

[0035] Advantageously, oblique illumination of the spatial light modulation device can be provided. In other words, oblique illumination of the SLM plane is preferred. Illumination of the SLM at, for example, 5° to 25° can be used. This angular range may be appropriate for a variety of liquid crystal modes. Some liquid crystal modes, for example, may be illuminated at 45°. Oblique illumination can be provided by several types of illumination devices. However, volume grating-based illumination devices may be preferred. This is due to the angular and spectral selectivity of Bragg diffraction-based volume gratings.

[0036] The lighting device can be designed as a front light lighting device or as a backlight lighting device.

[0037] The illumination used in a holographic display device should provide the required longitudinal and mutual coherence. The approach of using subholograms to represent an object point seen by an observer requires reduced mutual coherence. Using the required minimum coherence means using a small area of ​​spatial coherence, which, in turn, means using a large light source in the illumination device, which is equivalent to a broader angular spectrum of plane waves from the light source.

[0038] Another option is to use speckle overlay to reduce the detectable speckle pattern to within 1 / 60th of a degree. A high-speed phase change results in a change in the speckle pattern, meaning the viewer's eye sees an average of hundreds of speckle patterns, resulting in smoothing.

[0039] When using diffraction elements in the illumination device, the variation in the plane wave spectrum must be taken into account. The plane wave spectrum of light illuminating a grating changes depending on the diffraction angle due to the grating diffraction equation and can only be considered unchanged at small angles. For example, if a reconstruction geometry from -84.26° to 0° is realized, i.e., a diffraction angle of 84.26° is achieved in the case of implementation in a medium with a refractive index of, for example, n = 1.5, a beam expansion factor of 10 is obtained, which allows for a suitably thin collimated backlight illumination unit. Alternatively, a PMMA (polymethyl methacrylate) plate can be illuminated, which has a tailored anti-reflection layer on the light entrance side at 84.26°. In the PMMA plate, the light propagates at 41.55°.A volume grating can be provided on the light exit side, which has a reconstruction geometry from -41.55° to 0°. The exit plane of the backlight device can be glued to the entrance plane of the spatial light modulator (SLM). The arrangement, which operates at 84.26° in air, also provides a beam expansion factor of 10. A beam expansion factor of 20 can be achieved by operating at 87.13° instead of 84.26°. The beam expansion factor is inversely proportional to the compression factor of the plane wave spectrum. Thus, an anamorphic beam expansion with a factor of 10 results in a multiplication of the plane wave spectrum by a factor of 1 / 10. On the other hand, a small diffraction angle of 25° changes the plane wave spectrum by only 10%. In other words, small diffraction angles do not significantly change the plane wave spectrum.

[0040] A point source is not required. M > 1 may be acceptable as long as the plane wave spectrum lies within the required range, where M is the magnification. This means that an extended light source is preferred. And this is the reason why dynamic scattering or beam-shaping elements can generally be mounted on piezoceramic (PZT) or voice coil elements, for example. The use of rotating scattering plates, mounted on a rotating electric motor, for example, has always been implemented in lighting design concepts. The plane wave spectrum of the spatial light modulator with which it must be illuminated is ≤ 1 / 60°.

[0041] The plane wave spectrum that can be used in a wedge-based backlight device based on a 10x beam expander is ten times the plane wave spectrum required for SLM illumination. This value of 1 / 6° is realistic due to the fact that the plane wave spectrum is reduced by a factor of 10 when using a 10x magnification in the wedge-type backlight device. For example, a plane wave spectrum that is 1 / 6° along the diffraction plane and that is present in front of a wedge backlight device based on a 10x anamorphic beam expander will be reduced to 1 / 60° behind that backlight device.

[0042] Due to the grating equation that can be used to describe the plane wave spectrum, one aspect of a wedge-based backlight device, a slightly asymmetrical reduction of the plane wave spectrum is obtained. For example, an incident angle of light on a grating plane of the wedge-type backlight device of - (84.2608 ± 0.7)° results in an exit angle of the light diffracted in the 0° direction to illuminate the SLM of (0 + 0.0743 - 0.0657)°. This means that the compression of the plane wave spectrum due to the 10x beam expansion is slightly asymmetric, but approximately 1 / 10.At angles slightly larger than 84.2608°, the beam expansion factor is slightly larger than 10, and at angles slightly smaller than 84.2608°, the beam expansion factor is slightly smaller than 10, resulting in a slightly asymmetric compression or reduction factor of the plane wave spectrum of approximately 1 / 10.

[0043] Furthermore, the use of scattering elements and an electronically addressable array type element with variable phase change, e.g., MEMS (microelectro-mechanical systems) and mirror array based, may also be provided to tailor the mutual coherence of the illumination device.

[0044] For example, an element that provides for the rapid statistical randomization of the phase values ​​present in the area of ​​an extended light source must be adapted to the required frame rate (frames per second, fps). For example, four viewer RGB (red, green, blue or red, yellow, blue) for the left and right eye at 60 Hz fps results in 24 x 60 Hz = 1440 Hz, which must be provided by a tracking unit, such as an actively controlled grating with in-plane rotating liquid crystals. A time-sequential RGB SLM must operate at 1440 Hz / 4 = 360 Hz. The time window that can be used for "light source ON" is slightly smaller than 1 / 1440 s, which means, for example, only 1 / 1600 of a second. In general, laser diodes as light sources can be modulated this quickly. However, if lasers are used as light sources, which cannot be modulated so quickly, fast shutters can be used. These elements can, for example,They can be based on liquid crystals (LC), LC-dispersed volume gratings, acousto-optic modulators (AOM), saturation-dependent absorption filters, PZT, or even MEMS technology. For example, PZT elements found in standard tweeters achieve frequencies of 24 kHz, which can also be achieved using voice coil actuators. Only a movement of less than 100 µm may be required to provide sufficient randomization of the light source phase distribution. Discrete values ​​depend on the discrete embodiments.

[0045] The end of a multimode fiber can be positioned in front of a dynamic diffusion plate in the direction of light propagation. Furthermore, to provide color imaging, it is advantageous to image the three ends of three multimode optical fibers onto the dynamic diffusion plate. This offers the possibility of implementing slight shifts between the red, green, and blue light sources, resulting in slightly different exit angles. These are present in the direction of light propagation behind a collimating (achromatic) unit, which can be used for RGB-independent alignment. Thus, for example, the overall diffraction efficiency of a backlight device can be optimized. Known color-combining prism arrangements, such as those used for three CMOS chip camcorders or a set of dichroic filters, can be used in front of the collimating unit (seen in the direction of light propagation).It can be an achromatic lens combined with a beam-shaping element, increasing the overall light output through a so-called flattened-tip intensity distribution. Color-combining prism elements designed as X-cubes do not provide a constant phase value within the range of reasonably constant intensity, which, for example, illuminates the collimation unit (achromatic lens). However, color-combining prisms designed as Philips prisms do. Thus, a non-ideal phase distribution may be present in the exit plane of the collimation unit (achromatic or even apochromatic lens), depending on the optical components used.Several options, such as encoding, calibration, or look-up table calibration, can be used to compensate for unwanted phase distributions that would degrade the image quality of holographic three-dimensional objects.

[0046] It is important here that instead of using a point light source or, for example, single-mode fibers, an enlarged beam diameter, which means a laterally expanded light source, can be used. An angular range of 1 / 60° = ± 1 / 120° can be used to illuminate the SLM of the display device. The maximum angular resolution of the human eye of an observer is 1 / 60°. This means that when using a two-dimensional coding and a holographic three-dimensional display device located at a distance d duis arranged in front of the viewer, the viewer detects a smearing of three-dimensional object points as long as these are not arranged closer to the viewer than the du / 2 (assuming optimal coding and display performance). Due to the fact that the frustum is a pyramid-like structure that runs from infinity across the edges of the display device to the viewer's entrance pupil, the three-dimensional volume located within the du / 2 to the eye, is also very small and not so interesting for the three-dimensional content. This is the reason why it should not be a problem to reduce the three-dimensional volume from infinity to the du / 2. To reconstruct object points that are closer to the observer, a reduced plane wave spectrum is required.

[0047] In cases of significant scattering effects, which can be introduced by imperfect display components, the angular range of the illumination can be reduced. This only applies to small interference scattering angles. This means that, for example, a plane wave spectrum of 1 / 100° may also be appropriate.

[0048] Multiple front light illuminators can be used to illuminate complex-valued reflective SLMs.

[0049] Wedge-based frontlight illuminators can be used for medium-sized displays (≥ 20 inches) and large displays (≥ 60 inches). Grazing incidence frontlight illuminators, zigzag beam frontlight illuminators, and waveguide frontlight illuminators can be used for smaller display sizes. The angular selectivity of a volume grating used in the illuminator creates a separation between the illuminator and the imaging beam path. For example, illuminating the volume grating at 10° results in a propagation angle of -10° for the light entering the viewing space. This results in an angular separation of 20°.This is sufficient to space the beam path incident on the SLM and returning to the illumination device comprising the volume grating after modulation in the SLM.

[0050] Structured lighting can also be used to save optical power and / or to eliminate structured polarization filters or a retardation plane in the display device. This requires additional engineering effort and precise alignment. Structured lighting in this case means, for example, strip-shaped lighting.

[0051] It may also be possible to use global illumination, i.e., homogeneous illumination, and a structured aperture introduced behind the front light illuminator in the direction of light propagation, implemented, for example, using a simple amplitude mask. However, this requires a very thin front light illuminator and appropriately large pixels, e.g., a 200 µm thick front light illuminator and a pixel pitch of > 50 µm.

[0052] Backlight devices can be used preferentially for complex-valued transmissive SLMs. In such a case, a fixed illumination is preferred. The base illumination can be extended by using a switchable illumination device that provides, for example, two or even more discrete illumination wavefields on demand. However, a multi-directional scanning illumination device or a continuously angle-scanning illumination device introduces complexity and technical risk. A multi-directional scanning or continuously angle-scanning illumination device results in smaller tolerances, which may be acceptable within the SLM arrangement but results in additional costs.

[0053] This means that, for example, wedge-based, zigzag-based, waveguide-based, frustrated total internal reflection (FTIR)-based, or grazing incidence-based illuminators modified using, for example, a polarization liquid crystal grating with actively controlled electrodes are not preferred for either the complex-valued SLM described in the present invention or the stacked sandwich complex-valued phase + amplitude SLM.

[0054] According to the invention, the light passing through both the phase modulation pixels and the amplitude modulation pixels of the SLM is reflected by an intermediate mirror system arranged in the reflection plane. The mirror system can comprise mirror segments.

[0055] Furthermore, the spatial light modulation device can comprise at least one transparent substrate, wherein the transparent substrate has an addressable transmissive layer with the phase modulation pixels and the amplitude modulation pixels on one side and a plane on the other opposite side, which is the reflection plane, wherein the mirror system is arranged in the reflection plane. In this way, an SLM is created, with which the light beam incident on the entrance plane of the SLM is preferably directed towards the phase modulation element and is phase modulated by this phase modulation element. The light beam is then further directed to a corresponding mirror segment of the mirror system in the reflection plane, where the light beam is reflected by the mirror segment and directed back to the amplitude modulation pixel.The amplitude modulation pixel modulates the intensity of the incident light, with the light then exiting the SLM towards the viewer space of a reconstructed scene.

[0056] The SLM is therefore advantageously designed as a sandwich-type SLM. The amplitude modulation pixels and the phase modulation pixels can thus be provided in the same plane of the SLM, eliminating the need for a separate amplitude SLM and a separate phase SLM to construct a phase-amplitude SLM. The SLM can thus be very flat in its design, allowing the display or display device to also be flat. In other words, a small form factor can be achieved.

[0057] The mirror segments of the mirror system are arranged relative to the phase modulation pixels and the amplitude modulation pixels such that each mirror segment covers both a portion of the phase modulation pixel and a portion of the amplitude modulation pixel. In this way, the light can be directed, preferably in a zigzag pattern, from the phase modulation pixel / amplitude modulation pixel via the mirror system to the amplitude modulation pixel / phase modulation pixel of the SLM, so that the amplitude modulation pixels and the phase modulation pixels can be arranged in the same plane of the SLM.

[0058] In a further advantageous embodiment of the present invention, the display device can comprise a polarization-selective element, preferably a polarization analyzer or a wire grid polarizer, wherein the polarization-selective element is provided in the light propagation direction at a light exit plane of the spatial light modulation device. Polarization management / polarization application can be used to reduce interfering crosstalk between neighboring modulator pixels. Therefore, an optimal polarization state, which is required for the modulator pixels, must be provided. In addition, a complex-valued profile or, for example, even just an amplitude apodization profile of individual exit openings of the SLM can be provided using an absorption pattern or spatially structured polarization structures, such as, for example,spatially structured retardation, polarization-selective reflection or polarization-selective transmission.

[0059] This means, for example, that the amplitude modulation pixels can be designed for a defined input polarization state that is orthogonal to the input polarization state that first impinges on the phase modulation pixels. This blocks diffraction at the crosstalk-introducing entrance plane. For short propagation distances, which are present in the display device that combines phase and amplitude, this additional modification is not necessary.

[0060] Furthermore, the phase modulation pixels can have reflection means, preferably mirror elements, wherein the reflection means are provided at the rear ends of the phase modulation pixels in the direction of light propagation. This embodiment is preferred when using a backlight device, so that the light, which preferably first impinges on the phase modulation pixel, is reflected toward the reflection plane and from there to the amplitude modulation pixel. Thus, a zigzag-shaped light beam is generated.

[0061] In a further embodiment, a polarization filter can be provided in the plane of the phase modulation pixels and the amplitude modulation pixels, wherein the polarization filter has polarization filter segments. The polarization filter segments can be assigned to the phase modulation pixels and the amplitude modulation pixels, wherein the polarization orientation of adjacent polarization filter segments is orthogonal. The polarization filter segments can be absorbing polarization filter segments provided at light exit openings of the amplitude modulation pixels, or the polarization filter segments can be reflective polarization filter segments, preferably wire grid polarizers, which are provided at the reflective rear ends of the phase modulation pixels in the direction of light propagation.

[0062] Advantageously, an apodization filter, preferably a structured apodization profile layer, and / or a retardation element, preferably an achromatic or apochromatic retardation layer, can be provided. The apodization filter can be a microlens-like structure applied to the mirror system in the reflection plane, or the apodization filter can also be an absorbing alloy structure layer. For example, a microlens-like surface relief profile can be pressed from top to bottom onto a glass plate, which has a thin layer on top, e.g., an absorbing, UV-curing adhesive. Thus, the parts of the surface relief structure in direct contact with the glass plate appear perfectly transparent. At these contact points, an absorption material is now present between, e.g., the microlens-like surface relief structure and the glass plate.The absorption adhesive used is cured by switching on UV (ultraviolet) light.

[0063] An amplitude apodization filter of the SLM pixels used can thus provide a reduced intensity of the higher diffraction orders of the SLM. This is due to a reduction in the spatial frequencies present in the SLM plane. The realized form can be, for example, a so-called Kaiser-Bessel window, a Gaussian window, or simply a cosine function. This prevents the adjacent eye from receiving any disturbing light intended for the viewer's other eye.

[0064] For large pixels of approximately 100 µm for TV applications, a printed amplitude structure, which is a continuous profile, can be used, which should not exhibit a periodic grating. The preferred method is to dip a microlens-like structure with a defined pixel pitch face down into an absorbing ink, which can be applied to a mirror substrate. The contact points in the microlens-like structure are clear and have a nearly 100% reflectivity. The regions between the lenses of this structure are dark. This works very well for a reasonably large pixel pitch.

[0065] Another option for an apodization filter is the use of absorbing alloy compositions, such as chromium oxides, which are already used, for example, for a so-called porthole or anti-Gaussian filter. The absorption is defined by the local thickness of such a composition. A suitably thick layer, such as approximately 200 nm, can be locally thinned using gray-tone lithography methods (also referred to as gray-tone, gray-scale, or halftone lithography). For example, a lithographic gray-tone negative lens array patterned in photoresist (PR) can be etched with a dry or wet chemical. The photoresist (PR) serves as a protective material used to transfer the surface profile into a profile of an absorption layer. It is also possible to create this protective structure using nanoimprint lithography (NIL) or a conventional molding process.The center of the protective layer must be thinner than the outer region to create a more transparent center of the absorption material, which can be, for example, an alloy composition or another absorption material, such as platinum black (Pt) or a suitably strong absorption material.

[0066] Another option may be to use a wire grid-like structure (wire grid polarizer), which can be arranged in front of an absorber, instead of using a mirror plane. Alternatively, an absorbing structured filter can be arranged in front of a mirror segment, e.g., in the reflection plane or in the pixel plane. In both functionally opposite cases, the reflected light has a defined distribution of the polarization state, which can be converted into an intensity profile using a polarization filter acting as an analyzer.

[0067] In summary, several technologies can be used to provide the apodization profiles that may be required in a holographic display device.

[0068] In a further embodiment of the present invention, at least one lens arrangement can be provided, wherein the lens arrangement can be provided in front of the spatial light modulation device in the light propagation direction, wherein the light emitted by the illumination device is bundled and enters into inlet openings of the spatial light modulation device or lens structures for focusing the light are provided in the inlet openings of the spatial light modulation device.

[0069] The lens array can be a two-dimensional (2D) lens array or a cylindrical lens array, also known as a lenticular lens. This lens array can be positioned in front of the SLM (in the direction of light propagation), e.g., between the illuminator and the SLM, to focus the light onto the transparent entrance zones of the SLM. This increases the divergence of the light, but the layout of the lens array can be adapted to the existing illumination. A second lens array can be provided, positioned behind the exit plane of the SLM (in the direction of light propagation), to reduce the divergence of the wavefront segments. This results in a telescopic arrangement using two laterally shifted lens arrays.

[0070] It is also possible to provide structured illumination of the SLM by applying structured light extraction, e.g., in the form of strip-like light extraction, at the exit plane of the illumination device. In this case, precise alignment of the illumination device and the SLM structure must be ensured.

[0071] In addition, additional curvatures, which may be provided, for example, on the mirror segments of the mirror system or the phase modulation pixels, can also be used to increase the amount of light transmitted through the apertures of the spatial phase and amplitude light modulation device or to reduce crosstalk between adjacent pixels. However, this modification might add additional engineering effort and a smaller mismatch tolerance that may be acceptable between aligned components. However, lens structures on flat substrates introduce negligible angular deviations. Thus, lens arrays or lens structures that increase transmission, increase the effective fill factor (FF), or reduce internal SLM crosstalk are preferred.

[0072] Advantageously, the spatial light modulation device of the display device can comprise a beam shift element, preferably a birefringent element, more preferably a Savart plate. The transparent substrate is configured as a beam shift element for shifting the incident light passing through the phase modulation pixel or the amplitude modulation pixel, depending on which pixel, phase modulation pixel, or amplitude modulation pixel the light passes through first in the light propagation direction. A Savart plate as a beam shift element can also be used to implement a serial combination of adjacent phase modulation pixels and amplitude modulation pixels. The light passing through the phase modulation pixel is shifted by the beam shift element. The light shifted by the beam shift element has its polarization changed and is reflected back to the pixel plane.Thus, the phase-modulated light hits the amplitude modulation pixel and can then be modulated in its amplitude.

[0073] The spatial light modulation device of the display device may advantageously comprise a backplane comprising clusters of buried electrodes and additional transistors together with transistors for controlling the pixels, wherein the electrodes and the additional transistors are arranged behind the reflection means of the phase modulation pixels of the spatial light modulation device in the light propagation direction.

[0074] Several modifications may be provided with respect to the electronic design and the construction of the display device.

[0075] For example, the effective fill factor (FF) of a transmissive SLM in known display devices is close to 0.5. This means that there is an area on the SLM that does not need to be transparent and can be used for a variety of modifications. This area covers approximately 50% of the SLM's display panel.

[0076] The design of the backplane, which is the electronic circuitry of the display panel, can benefit from the resulting fill factor available for the transmissive SLM.

[0077] The fill factor poses a concern with regard to light transmission efficiency. At first glance, this may seem like a general concern, but it is particularly true for displays used close to the viewer, such as mobile tablet-type displays. The critical dimension (CD) of a lithographic process defines the processes to be used and the costs. A technology used to manufacture integrated circuits, which provides a constant and high fill factor even for a reduced pixel size, may not be practical for display devices due to the cost per square meter. This means that the application of lithographic processes that are standard in display manufacturing and that are limited in terms of the critical dimension obtained will result in a reduced fill factor when the pixel dimensions are reduced.Furthermore, more advanced drive schemes, which require additional transistors and electrode lines compared to standard active matrix methods, would further reduce the fill factor. This is why utilizing this additional 50% space is highly advantageous, especially for displays with small pixels (approximately 50 µm), such as mobile holographic displays.

[0078] SLM allocates approximately 50% of the display panel area to non-transparent structures. This means that the non-transparent area can be used for metal electrode lines and transistor structures, for example, which can occupy approximately 50% of the display panel plane. This allows for larger critical dimensions in lithographic processes, or for more complex backplanes.

[0079] The enlarged area, which does not need to be transparent, offers the possibility of using, for example, a value-sorted backplane for mobile holographic displays, as disclosed in WO 2012 / 028678 A1. This backplane separates the signal originally required by all pixels of the display device into a high-frequency signal with low current load for addressing the pixels and a low-frequency rising or falling ramp signal with high current load for driving the pixel capacitances. This determines the final change in the alignment of the liquid crystal modules of the addressable transmissive layer. The value-sorted backplane, also referred to as a cluster backplane, provides for the control of a high number of pixels at high frame rates with low energy loss and at a bit depth of, for example, 8 bits, 10 bits, or even more. The complexity is greater than that of a standard active pixel matrix.Additional electrodes and additional transistors, intended for implementing address decoders, are used compared to the standard active pixel matrix method. This is not a problem for TV displays and can also be handled for desktop displays, but is not practical for a pixel pitch of, for example, 12 µm due to the small fill factor that would be present if, for example, a critical dimension of CD = 2 µm were used. This means that the more advanced drive scheme provided by the value-sorted backplane is not practical for very small pixels with an extension of, for example, 10 µm to 15 µm.

[0080] Hidden electrodes and additional transistors can advantageously be arranged behind the reflective parts associated with the phase modulation pixels in the direction of light propagation. Thus, by using the non-transparent zones, relaxed design rules can be achieved even for a small pixel pitch of, for example, 12 µm, which can be used, for example, for a two-dimensionally encoded mobile holographic tablet display. In summary, the use of non-transparent zones is very advantageous in mobile holographic displays.

[0081] Furthermore, to reduce the amount of transparent electrode material, it can advantageously be provided that the transparent electrodes of the free-standing electrodes of the backplane can be combined with metal edge electrodes. Thus, only the used portion of the free opening needs to have a free opening. The amount of transparent electrode material is thus reduced to a minimum. A further effect is that the electrode mobility of the metal electrodes can provide reduced electrical energy loss and also a slightly higher switching speed.

[0082] For example, an ITO (indium tin oxide) layer can only occupy an inner area equivalent to the free exit aperture of a pixel. The outer edge of a rectangular or square pixel electrode can be formed using metal lines. This reduces the amount of, for example, ITO layer used rather than contributing to a significant switching speed. This is due to the fact that the propagation length of the electrons in the ITO layer is relatively small. Only in those embodiments where electrode lines in the range of >100 mm are used can a significantly reduced switching speed be achieved.

[0083] In a further preferred embodiment of the present invention, a light-emitting device, preferably an organic light-emitting diode array, can be provided behind the pixels of the spatial light modulation device in the light propagation direction, preferably on top of the transparent regions of the phase modulation pixels, in order to generate a two-dimensional functionality of the display device, wherein the light-emitting device, preferably the organic light-emitting diode array, can be formed from clusters with light-emitting zones, wherein a cluster can cover or comprise a specific number of pixels of the spatial light modulation device.

[0084] A light-emitting device or an organic light-emitting diode (OLED) array can preferably be arranged on top of the non-transparent regions on the backside of the phase-modulation pixels. Although the pixel pitch is as small as defined by the complex-valued spatial phase and amplitude light modulation device, clusters or blocks of, e.g., 5 x 5 light-emitting zones can be addressed to create a two-dimensional (2D) image pixel. An OLED emission provides light with a broad spectral and angular range. The majority of this light, i.e., > 90%, is passed through volume gratings, which can be provided downstream of the SLM in the light propagation direction and which can, e.g., realize the function of a field lens without deflection. Additional scattering structures can be provided within the OLED emission regions.

[0085] The non-transparent area of ​​the display panel thus provides the possibility of integrating a backplane of a light-emitting device designed as an OLED and an OLED cluster pixel structure without affecting a holographic three-dimensional process.

[0086] Using transparent OLEDs in the transparent areas of the display panel, e.g., in the direction of light propagation in front of or behind the SLM, can introduce significant scattering. However, this is not preferred.

[0087] In general, this modification is not limited to the use of OLEDs. Light conversion based on an LED (light-emitting diode) or, for example, a quantum dot (Q-dot) can also be used in a plane of light-emitting pixels, contributing to the complex-valued SLM that modulates the phase and amplitude values ​​of partially coherent light.

[0088] Advantageously, the spatial light modulation device of the display device may be a liquid crystal (LC)-based light modulation device or a micro electro mirror system (MEMS)-based spatial light modulation device.

[0089] Alternative SLM configurations are possible. For example, the spatial light modulator of the display device can be a multi-quantum well (MQW)-based spatial light modulator. This type of modulator, which can operate in reflection or transmission, can be configured to modulate amplitude and / or phase. In general, the principle of serially combining adjacent phase modulation pixels and amplitude modulation pixels can also be adapted to a non-liquid crystal-based phase and amplitude modulation SLM. For example, an electrowetting (EW)-based SLM and a magnetophotonic crystal (MPC)-based SLM can be used.Electrowetting (EW) based amplitude modulation can be realized using an absorption fluid, and phase shift / modulation can be introduced using a tilt of a plane-parallel segment, which can be implemented using an electrowetting cell filled with three fluids as disclosed in WO 2010 / 084200 A1.

[0090] Further SLM alternatives are also described. Microelectro-mirror system (MEMS)-based spatial light modulation devices and electrowetting-based spatial light modulation devices can be used to provide amplitude modulation and / or phase modulation. A variety of different MEMS-based and electrowetting-based spatial light modulation devices known in the art can be used in the display device according to the invention.

[0091] A preferred MEMS-based SLM embodiment is described below. Phase modulation pixels operating in transmission can be realized using sweep mirrors capable of operating at frame rates of up to 10 kHz. Amplitude modulation can be achieved using, for example, interference-based MEMS arrays, which can be optimized separately for red, green, and blue light to realize an SLM having a pixel pattern related to these three colors. It is also possible to use one type of SLM based on an amplitude modulation MEMS for all colors in a time-sequential manner.

[0092] A pixel size suitable for the MEMS-based SLM described in this document is, for example, 8 µm × 16 µm, 10 µm × 10 µm, or 12 µm × 15 µm. The desired shift is defined by the wavelength and angle of incidence of the light incident on a MEMS mirror. A suitable shift value for red light is in the range of 0.3 µm to 0.33 µm. The shift value for blue and green light is smaller than that for red light.

[0093] Reflective mirror arrays represent the state of the art. This means that the transfer of this technology, which is currently available for mirror array sizes typically ranging from 10 mm x 10 mm to 100 mm x 100 mm, is practical for some applications, such as mobile displays, which can be as large as 300 mm x 200 mm. This is the size of the entire array, not that of an individual mirror. On the other hand, reflective amplitude modulation panels based on a destructive interference effect have already been implemented at much larger sizes.

[0094] Amplitude modulation pixels based on destructive interference effects can be used for transmissive and reflective SLMs. For example, a metal interference filter (MIF)-based approach can be used as an amplitude modulator to generate variable amplitude levels by introducing a customized spacing between two metallic layers. In more advanced embodiments, dielectric multilayer stacks can also be used. For example, a wave entering a double metal interference filter is partially transmitted through a first layer and further partially transmitted through a second layer. The back-and-forth propagating light forms a standing wave if the wavelength and angle of incidence match, which means meeting the λ / 2 modulo 2π criterion.If the λ / 2 modulo 2π criterion is met, the nodes of the standing wave between reflecting planes are located on a metal plane and contain no significant energy, meaning that transmission through the dual metal layer interference filter is maximized and, for example, t = 0.8. Detuning such a dual metal layer interference filter means detuning the optical distance between the metal layers. This results in a change in the output energy. Thus, amplitude modulation is obtained.

[0095] Pixels based on metal interference filters (MIF) can also be designed for an SLM that operates in reflection.

[0096] Higher spectral selectivity can be obtained by increasing the number of metal layers to > 2. Thus, multiple resonators, i.e., multiple optical configurations thereof, must be considered, leading to a narrowed response function.

[0097] So far, we have described sweep mirrors for modulating the phase and interference-based resonators (interference-based MEMS arrays) for modulating the amplitude. Both can be realized using MEMS technology, which can achieve high frame rates.

[0098] Interference-based resonators for amplitude modulation can also be realized using liquid crystal (LC) material arranged between metal layers. A small detuning of the refractive index leads to a sufficient change in the amplitude leaving the pixel. Furthermore, materials with Δn(U), Δn(I), or, for example, Δn(hv), where Δn is the refractive index deviation, U is the voltage, I is the current, and hv is the energy of a photon, can be provided between metal layers to realize an interference-based amplitude modulation pixel. Polymers, crystals, and doped variants thereof are known in the art and can be adapted to the embodiments according to the invention.

[0099] MEMS-based rotation of a suitably thin birefringent film is possible. This would be equivalent to the well-known Hariharan phase modulation and could be used for phase shift / modulation and amplitude modulation, which also requires at least one additional polarizer. Due to the fact that tilting mirror arrays are already known, implementing this modulation in a reflective pixel may be a faster way than implementing it in a transmissive pixel using this type of modulation. For example, rotary joints can provide the rotation required here. Thus, a tilting mirror embodiment using rotary joints can be used for this purpose. Additional phase shifts, e.g.in a reflective structure can be provided, and the associated phase shift of a phase modulation pixel can be addressed dependently to ultimately realize the required complex value. Further MEMS modifications can be used here to implement, for example, Hariharan or other phase modulation principles, i.e., in the amplitude or phase modulation embodiments, or even in embodiments where both modulations are combined in one pixel.

[0100] According to the invention, it can further be provided that the illumination device has at least one volume grating for coupling the light out of the illumination device in the direction of the spatial light modulation device.

[0101] The output volume grating can realize the required exit angles of the lighting.

[0102] Advantageously, the display device may comprise a field lens, wherein the field lens is in particular a combined field lens having at least one volume grating.

[0103] For example, a reflective complex-valued SLM according to the invention can also be adapted to a volume grating-based field lens used in the display device according to the invention. For this purpose, a combined volume grating (VG) field lens comprises a first volume grating (VG) realizing a plane-to-plane reconstruction, which can be, for example, an oblique plane wave from 0° (on-axis) to 30°. A second volume grating (VG) has a plane wave reconstruction of 30° relative to an on-axis field lens. The combination of these two volume gratings forms an on-axis volume grating field lens, which can be referred to as a combined field lens.

[0104] Preferably, oblique illumination, which results in off-axis propagation of the complex-valued light, can be used to eliminate the first volume grating of the combined field lens. For example, the SLM plane containing the phase modulation pixels and amplitude modulation pixels to be combined can be illuminated at 30°, resulting in an off-axis propagation of the modulated wavefield of -30°. Thus, the pre-diffraction volume grating, which was previously used as the first element of the combined field lens, is no longer required. In this way, the number of components used can be reduced.

[0105] Furthermore, according to the invention, it can be provided that the display device has a vertical tracking unit and / or a horizontal tracking unit, wherein the vertical tracking unit and / or the horizontal tracking unit preferably has (at least) one liquid crystal grating.

[0106] The vertical tracking unit and / or the horizontal tracking unit are preferably provided for angular fine tracking in the light propagation direction behind a complex-valued SLM.

[0107] In addition to the plane used to introduce angle tracking, the angle tracking unit(s) must meet conditions related to the coherent nature of the illumination used.

[0108] Thus, segmentation of the tracking unit(s) that is larger than the pixel pitch used is not preferred. A lens arrangement that can also be used in combination with, for example, ±15° angular tracking is also not preferred. The disadvantage here is not the added thickness, but the segmentation, which, for example, has a 5 mm to 10 mm pixel pitch. This also applies to electrowetting-based prism cells that can be used in the tracking unit and can be arranged upstream or downstream of the SLM in the direction of light propagation. The use of liquid crystal polarization gratings with individually controlled electrode lines is preferred.

[0109] For example, liquid crystal gratings electrically controlled by individually controlled and one-dimensionally arranged electrode lines can generate one-dimensional phase profiles. These controllable one-dimensional phase profiles can provide a wedge function, which is required for fine tracking of the light to be transmitted to a viewer's eye. Furthermore, cylindrical phase functions can be generated. Thus, in addition to providing fine tracking, by generating different grating periods and a different local phase tilt, cylindrical lens functions can be generated that reduce overall aberrations of tilted lens functions. Multiple gratings can be used in series. Thus, for example, two gratings can be stacked on top of each other with an angular offset of 90°.It is also possible to use a grid with vertically oriented electrodes and to provide horizontal fine tracking in combination with two further grids with electrodes oriented parallel to the two crossed display diagonals.

[0110] An advantage of using controllable phase gratings is the provision of fine tracking without adding significant thickness or moiré pattern.

[0111] One option is to apply out-of-plane crystal rotation to create sawtooth-like phase gratings that are subject to rounding effects at 2π to 0 phase steps, reducing the diffraction efficiency to an unsuitable value of less than 0.5 for small phase grating periods, e.g., < 3 µm.

[0112] An in-plane rotating liquid crystal mode, such as the so-called HAN mode, can provide a controllable phase grating that is not subject to rounding effects from 2π to 0 phase steps. This is due to the fact that the absolute value of an angle of an in-plane rotating liquid crystal, which corresponds to a phase value of, for example, 1.9 times π, and an angle corresponding to a phase value of 2.1 times π, which is equivalent to 0.1 times π, differ by only approximately 10%. Consequently, phase gratings based on a periodically applied in-plane rotation of a birefringent material can provide a high diffraction efficiency of ≥ 0.9 even for a grating period of 2 µm. Discrete values ​​depend on the absolute value of the birefringence and thus on the period-to-thickness ratio of the phase grating, which e.g.by applying a mode of a liquid crystal rotating in the plane.

[0113] The object of the invention is further achieved by a method according to claim 29.

[0114] The method according to the invention is intended for generating a holographic reconstruction using a display device according to the invention, the method comprising the following steps: - illuminating phase modulation pixels and amplitude modulation pixels of a spatial light modulation device by means of an illumination device with sufficiently coherent light, - passing the light through both the phase modulation pixels and the amplitude modulation pixels of the spatial light modulation device and - Reflecting the light by means of an intermediate reflection plane, wherein the reflection plane is provided between the phase modulation pixels and the amplitude modulation pixels in the direction of light propagation.

[0115] The invention will now be described in more detail below with reference to embodiments and in conjunction with the accompanying schematic drawings. Generally preferred embodiments and further developments of the teaching will also be explained in conjunction with the explanation of the preferred exemplary embodiments of the invention with reference to the drawings. In the drawings, the figures are side views unless otherwise stated, whereby the drawing is in each case a schematic representation: Fig. Figure 1 shows a planar waveguide based illumination device for a display device according to the invention, Fig. 2 shows a front light illumination device based on a planar waveguide, Fig. 3 shows another front light illumination device based on a planar waveguide, Fig. 4 shows a wedge-based front light illumination device for a display device according to the invention, Fig. Figure 5 shows an embodiment of a display device according to the invention, wherein only a part of the display panel is shown, wherein a combination of phase modulation pixels and amplitude modulation pixels is provided in the same plane, Fig. 6 shows an embodiment of a complex-valued spatial light modulator (SLM) of the reflection type according to the invention, Fig. Figure 7 shows an embodiment of another complex-valued spatial light modulator (SLM) of the reflection type using a beam displacement element in the form of a birefringent plate, Fig. Fig. 8 shows an embodiment of a complex-valued spatial light modulation device of the transmission type according to the invention, Fig. Figure 9 shows in detail the design of the complex-valued spatial light modulation device of the transmission type according to Fig. 8, Fig. 10 shows an illumination angle as a function of the thickness of the transparent plate used between the plane of the phase modulation pixels and the amplitude modulation pixels and an entrance plane of the spatial light modulation device, and as a function of the pixel pitch of the complex-valued pixel, Fig. Figure 11 shows an illumination angle as a function of the thickness of the transparent plate used between the plane of the phase modulation pixels and the amplitude modulation pixels and an entrance plane of the spatial light modulator, and as a function of the pixel pitch of the complex-valued pixel, with an extended upper limit of the useful inner SLM propagation distance being up to 15 times the pixel pitch of the complex-valued pixel. Fig. 12a, Fig. 12b show an embodiment of a MEMS-based complex-valued spatial light modulation device of the reflection type, Fig. 13 shows an embodiment of a display device according to the invention, wherein illumination of a spatial light modulation device is provided in different controllable directions, Fig. Figure 14 shows a diagram of the diffraction efficiency of a volume grating based on Bragg diffraction, which realizes a diffraction geometry of 84.26° in air / 0° in PMMA, which is -41.55° / 0° in a medium having an average refractive index of n0 = 1.5 at a thickness of the holographic recording film of d = 16 µm, as a function of the difference to the intended Bragg angle, which is the intended angle of incidence of the grating, Fig. 15 shows a general structure of a compact collimation unit of the illumination device.

[0116] Like reference numerals denote like components in the individual figures and in the accompanying description. In the following, the terms "in front of" and "behind"—e.g., in front of the spatial light modulation device—refer to the light as seen with respect to the propagation of the light.

[0117] The illumination device may have several specific modifications that are preferably used in a holographic display device. The illumination device can be used for coherent light and for light that exhibits only reduced spatial and / or temporal coherence. Amplitude apodization and phase apodization can be used to optimize the intensity profile propagating behind the entrance plane of the illumination device. Color filters offer the possibility of separately optimizing it for different colors. The specifications depend on the discrete embodiment.

[0118] Now shows with regard to an optical structure of a lighting device Fig. 1 shows a first embodiment of a flat illumination device for a display device, preferably a holographic display device. The illumination device is a planar waveguide-based illumination device that has sufficient separation between an illumination beam path and an imaging beam path, and is used as a frontlight illumination device. This frontlight illumination device can be used to illuminate a complex-valued reflective spatial light modulation device (SLM), which is used, for example, in Fig. 6 shown.

[0119] Fig. Figure 1 shows a general structure of the frontlight illumination device based on a planar waveguide, which can be used for comparatively small display devices with diagonals of, for example, up to 300 mm. This embodiment can also be used as a backlight device. The illumination device has a laser diode as the light source device LS and an optical element in the form of a lens L for collimating light emitted from the light source LS to a light-guiding element in the form of a substrate S. The substrate S has, on a light exit plane, a light-outcoupling element VG, which can be a volume grating or another type of grating or periodic structure, which can be based, for example, on plasmon resonance. The volume grating VG is provided for coupling out the light propagating in the substrate S in a waveguide mode WGM, for example, a basic Gaussian mode with a mode number m = 0.Furthermore, the substrate has a waveguide layer WL and a cladding layer CL, which is provided between the substrate S and the volume grating VG. The diffraction geometry of the output volume grating VG approximates a polarization beam splitter geometry to provide sufficient separation of an illumination beam path and an imaging beam path, which means, for example, a geometry of 80° illumination beam path / 0° imaging beam path instead of 90° illumination beam path / 0° imaging beam path. This is one of several diffraction geometries that provide the separation of orthogonally polarized light. Furthermore, the output volume grating VG can realize the required exit angle of the illumination from the substrate S, whereby the exit light with the required propagation angle will later illuminate the spatial light modulation device.The required exit angle depends on the thickness of a substrate within the spatial light modulation device (hereinafter referred to as SLM), which is provided between an addressable transmissive layer and a reflection plane within the SLM. The E-field of the light is preferably in the plane of the substrate and is therefore transversely electrically (TE) polarized. The direction of light propagation is indicated by the arrow.

[0120] The lighting device according to Fig. 1 works as follows: The light propagating in the substrate S as a light-guiding element travels parallel through the light-guiding element, to the interface of the cladding and to the outcoupling grating. Part of the light leaves the light-guiding element, travels through the cladding layer CL, and enters the light-outcoupling element, here in the form of a volume grating VG, while the remaining part of the light propagates further in the light-guiding element. The light present behind the cladding layer CL and entering the volume grating at an angle of, for example, almost 80° is coupled out by the volume grating VG in such a way that it falls as a collimated wave field WF onto a spatial light modulation device SLM (not shown). This is indicated by the dashed arrows.After modulation by the reflective pixels of the spatial light modulation device (hereafter referred to as SLM), the light passes through a λ / 4 layer, also known as a quarter-wave plate, of the SLM and again through the illuminator. After passing through the λ / 4 layer for the second time, the light's initial transverse electric polarization (TE) is rotated by 90 degrees. The light now has a transverse magnetic (TM) polarization and is modulated by the pixels with values ​​for presenting information.

[0121] The Fig. The structure shown in Figure 1 can be used not only as a front light illumination device but also as a backlight illumination device.

[0122] Another front light illumination device based on a planar waveguide is shown in a side view in Fig. 2. This front light illumination device uses polarization-selective beam path separation. This figure shows additional optical and electronic elements in addition to the optical elements shown in Fig. 1. Here, a reflection-type spatial light modulator (RSLM) is shown, with a backplane BP for the pixel circuitry arranged behind the RSLM. A λ / 4 plate QWP is provided in front of the RSLM to rotate the polarization of the light. The light propagating in the substrate S first exhibits a waveguide mode WGM. m=0 which means that this mode is a fundamental mode or a Gaussian mode. Later, in the substrate S as a light guide element, the light has a waveguide mode WGM m>0 with higher modes. The principle of the illumination device of Fig. 2 is essentially the same as in Fig. 1. The layer referred to here as the conversion layer COL is a layer that provides light extraction, which is essentially a conversion of a guided mode of a wavefield into a leaky mode. This functionality can be provided by using a volume grating VG. Thus, the notations COL and VG can be considered equivalent here. To explain the function of successively extracting light, the longitudinal variation in the optical thickness of the cladding layer CL, which is n times d, where n is the refractive index and d is the thickness, is exaggerated.

[0123] When using volume gratings as light extraction elements, a set of discrete diffraction angles can be used to provide a polarization beam splitter (PBS) as a beam separation device. This means that several discrete diffraction angles can be used for this polarization beam splitter application. However, not all angles can be chosen for this purpose. Volume gratings can realize a polarization beam splitter application at diffraction angles of, for example, 90°, 60°, or 48.2°. Setting a limit on a few discrete angles to be used exclusively is equivalent to limiting the freedom of the optical design.

[0124] In Fig. 3, another front light illumination device based on a planar waveguide is described, wherein angle-selective diffraction of volume gratings is used to provide beam path separation. The front light illumination device is referred to here as FLU. The light source LS is designed as a color light source, which has three individual light sources, one each for the colors red, green, and blue. A collimation unit CU is provided behind the light source LS. The collimation unit CU has a reflection element, preferably a parabolic mirror PM, which, instead of a lens, which in Fig. 1 and Fig. 2 is used for collimation. A λ / 4 retarder can be provided on an exit plane EXP of the illumination device FLU. A field lens FL is provided in the light propagation direction for focusing the modulated light ML onto an observer space. The field lens can be a volume grating of a volume grating stack. Behind this field lens FL, a vertical tracking unit and / or a horizontal tracking unit is / are arranged for tracking the light in a required direction. The use of angular selectivity offers greater design freedom with regard to the angular geometry, which can also be used. However, the use of angular selectivity is preferred.

[0125] Another alternative lighting device is used in Fig. 4. This wedge-based front light illumination device uses angle- or polarization-selective beam path separation. A wedge-based illumination device may be preferred for medium-sized displays, such as a display with a diagonal of 24 inches to 32 inches, and larger displays, such as a display the size of a TV (e.g., 1 m diagonal), due to reduced complexity compared to, for example, a waveguide or zigzag light-guiding illumination device used in the Fig. 1 to 3.

[0126] The front light lighting device according to Fig. 4 comprises a light source device LS for red, green, and blue light. A gradient index lens (GRINL), e.g., a glass rod with a stepped index profile, is provided at an exit plane of the light source LS. The stepped index profile has—for a positive focal length f—a high refractive index along the optical axis and a lower refractive index in the outer region. Using an optical fiber OF, the light emitted by the light source is guided to a collimation unit CU.The collimation unit CU has a primary collimation lens pCL, a focusing microlens array fML, a PZT element PZT 1D for a one-dimensional movement of a one-dimensional scattering foil sPS 1D, a structured periodic scattering plane sPS 1D for one-dimensional scattering, a structured periodic scattering plane sPS 2D for two-dimensional scattering, a first piezo adjustment element PZT 2D 1 for a one-dimensional horizontal movement, which ultimately results in a two-dimensional movement of a scattering plane, a second piezo adjustment element PZT 2D 2 for a vertical movement, which ultimately results in a two-dimensional movement of a scattering plane, wherein both PZT 2D 1 and PZT 2D 2 together enable a two-dimensional movement.The collimation unit CL further comprises an array of aperture stops serving as a secondary light source array, a collimating microlens array cML, and a volume grating filter assembly cVGF for tailoring the plane wave spectrum (ASPW). For example, a first, thin (e.g., 5 µm thick) volume grating with an angular acceptance of, for example, 5° can diffract stray light out of the beam path, while a second, thick (e.g., 150 µm thick) volume grating with an angular acceptance of, for example, 1 / 6° can diffract the light to be used back into the beam path, thus allowing angular filtering to be applied. Behind the collimation unit CU, collimated wave fields of red, green, and blue light t(x,y,RGB), emitted time-sequentially from the light source LS, are provided.The light then enters a wedge-based substrate S with a first volume grating VG1, used for a first one-dimensional 10x magnification, and a second volume grating VG2, used for a second orthogonal one-dimensional 10x magnification, with the diffraction angle chosen to achieve a ten-fold magnification. The light is then guided via a λ / 4 plate QWP to a spatial light modulator SLM with a backplane BP. After modulation at the SLM, the light is guided a second time through the second volume grating VG2 to a combined field lens cFL, which is an on-axis diffractive lens based on two volume gratings, e.g., a first pre-deflection of 0° / after 45°, and a second 45° after on-axis lens with a focal point at a mean observer distance.Finally, the light coming from the combined field lens cFL is guided to a vertical tracking unit and a horizontal tracking unit, which are designed as a vertical liquid crystal grating vLCG and a horizontal liquid crystal grating hLCG.

[0127] The collimation unit CU provides a wavefield with a defined plane wave spectrum, which is used to illuminate the wedge-based illumination device. The basic principle is the use of an extended monochromatic light source LS with separate RGB (red, green, blue) that exhibits a rapidly varying randomized phase distribution and is collimated by a lens. This is also the basic principle of the Fig. 14, which can also be used instead of the collimation unit CU in front of the first volume grating VG1.

[0128] The wedge-based lighting device used in Fig. 4, illuminates a reflective SLM at the desired calculated angle. Thus, perpendicular illumination, e.g., illumination at 10° or 25°, of the SLM is achieved. Therefore, the use of angular selectivity is preferred.

[0129] In Fig. Figure 5 shows a top view of an embodiment of a display device according to the invention, showing only a detail of a display panel or spatial light modulator (SLM). A combination of phase modulation pixels and amplitude modulation pixels is provided in the same plane of the SLM to realize an in-plane complex-valued SLM. Fig. Figure 5 shows an amplitude modulation pixel a and a phase modulation pixel p arranged side by side in the same plane. The amplitude modulation pixel a is located at the light entrance, and the phase modulation / shift pixel p is located behind the amplitude modulation pixel a in the light propagation direction. This offers the advantage of using a constant thickness of an addressable transmissive layer in the form of a liquid crystal layer, which can be achieved by passing the phase modulation pixel p twice.

[0130] In other words, the basic design consists of combining phase modulation pixels p and amplitude modulation pixels a that are located close to each other. One phase modulation pixel and one amplitude modulation pixel form a complex-valued pixel of the SLM. Typically, the liquid crystal layer thickness required for 2π phase modulation of phase modulation pixels is twice the liquid crystal thickness required for ON-to-OFF modulation of amplitude modulation pixels a. As shown in Fig. As shown in Figure 5, the liquid crystal thickness of the amplitude modulation pixel a can be equivalent to that of the phase modulation pixel p. This is realized by implementing a double-pass array for the phase modulation pixel p. The entrance polarization state of the light emitted by the illuminator is rotated using segments of a λ / 4 plate QWP arranged in front of the phase modulation pixels p. An apodization profile APO is used to reduce the intensity of higher diffraction orders associated with the combined complex-valued pixel. The apodization profile APO, in combination with an absorber A, is arranged behind the phase modulation pixel p. The redirection of the light passing through the amplitude modulation pixel is introduced using a prism structure arranged on one side of the pixel array.The light exit plane of the amplitude modulation pixel a is structured using a prism array PS, which can be realized by applying a molding process. The prism array PS has two reflective surfaces. This means that an inclined mirror plane M is provided behind the amplitude modulation pixel, which in this case is inclined at an angle of 45°. Behind the phase modulation pixel p, a polarization beam splitter PBS is provided in the form of a coating on an inclined plane, preferably inclined at an angle of 45°, with both inclined planes M and PBS being inclined to each other at an angle of approximately 90°. The polarization beam splitter coating PBS can, for example, be a dielectric mirror stack or a wire grid polarizer structure used to redirect the beam path towards the phase modulation pixel p.A polarization filter is provided in the light exit plane of the amplitude modulation pixel. This filter blocks the light reflected by the prism array PS, thus suppressing crosstalk between adjacent pixels a and p by introducing orthogonal polarization. After the phase of the light is modulated, the light is guided to the apodization profile APO at the rear end of the phase modulation pixel p. An in-plane mirror plane MP is provided in the plane of the apodization profile APO to reflect the incident light back to the phase modulation pixel p. The λ / 4 plate QWP behind the phase modulation pixel p changes the polarization state TE of the light to the polarization state TM (transverse magnetic), so that the TM-polarized light can propagate through the polarization beam splitter coating PBS of the prism array PS toward a subsequent field lens not shown in the display device.

[0131] A basic design of a reflective complex-valued spatial light modulator SLM is shown in Fig. 6, which uses a serial combination of laterally adjacent phase modulation pixels and amplitude modulation pixels arranged in the same addressable transmissive layer plane. The addressable transmissive layer for forming the complex-valued pixels is a liquid crystal layer, although other types of layers can also be used. The phase modulation pixels p and the amplitude modulation pixels a are arranged side by side.

[0132] As in Fig. As can be seen in Figure 6, oblique illumination of the SLM is provided. The illumination angle of the light incident on the SLM is, for example, 25°. This illumination angle is appropriate for multiple liquid crystal modes. The light incident on the SLM has a defined polarization state. The light first passes through the phase modulation pixels p, where it is blocked before reaching the amplitude modulation pixels a using a structured polarization filter / analyzer pPF. Structured wire-grid polarizer structures can be provided as a structured polarization filter, but they are not preferred on the amplitude modulation pixels. This is due to the reflected light adding background noise. However, structured wire-grid polarizer structures can be used in a reflection plane RP that reflects back a defined portion of the incident light. Fig. 6, structured wire-grid polarizer structures can be used in front of a non-structured absorber plane provided in the reflection plane RP. The absorber plane corresponds to a mirror plane M. Structured retardation can also be added to this reflection plane using a λ / 4 plate. However, the use of embodiments closer to known standard technologies, such as microlenses immersed face-down in an absorbing, UV-curing adhesive, may be preferred.

[0133] An apodization profile (APO) in the form of a structured absorber is also provided in the reflection plane (RP). This apodization profile (APO) can be suitably thin, e.g., 3 µm to 5 µm.

[0134] One option to obtain a thin configuration of an apodization profile is to use a combination of a retarder and an analyzer.

[0135] A structured retarder can be used in combination with an analyzer that is a polarization filter. The term "structured" is used in this specific context to refer to a structure or pattern present on the inside of the pixel. This combination can provide a defined amplitude apodization profile in the aperture of a pixel. A structured analyzer that is a structured polarization filter can be used in combination with a non-structured retarder. This combination can provide an apodization profile provided on the inside of the pixel. Furthermore, another option is to use a structure present in the pixel for both the analyzer and the retarder structure.

[0136] Thus, even an SLM with a small pixel pitch such as 12.5 µm, which can be used for mobile devices, can be equipped with these structured analyzers, which serve as thin implementations of structured absorbers with an aperture present in the pixel.

[0137] However, thick structured apodization, which provides absorber structures of, for example, 20 µm or more, or high aspect ratios, are not suitable for use in holographic displays. This is due to shadowing effects when using oblique illumination and to diffraction effects. In other words, appropriately thin absorber structures, based, for example, on structured analyzers, must be used.

[0138] In other words, the reflection plane RP contains the mirror plane M, on which the λ / 4 plate QWP is arranged. The apodization profile APO is placed on the λ / 4 plate QWP, whereby all optical elements in the reflection plane can be combined with each other.

[0139] Now to the principle of Fig. 6. The light provided by a light source LS in the form of laser diodes with different colors propagates along a preferred single-mode fiber SMF. A plurality of fiber ends are arranged at the edge of a planar waveguide structure WG. A one-dimensional array of collimation elements, which here in Fig. 6 geodesic lenses GL are used to provide collimation in the planar waveguide structure WG. The first part of a front light illumination device FLU used is called the light collimation unit LCU. The light propagating along the waveguide structure WG is output using a volume grating VG. The output angle used in this design is 25° to the surface normal axis, although other angles can also be used. Homogeneous illumination requires less technical effort than using segmented output, which means, for example, strip-shaped output, which requires precise adjustment. The output light is, for example, TE-polarized.A polarization filter / analyzer is arranged on top of the SLM layer, with a structured polarization filter sPF on top of the phase modulation pixels p and a structured polarization filter pPF on top of the amplitude modulation pixels a. Phase modulation pixels p allow transmission, and amplitude modulation pixels a allow blocking of the incident light. The light passes through the phase modulation pixels p and is modulated by them. The light passing through the phase modulation pixels p then illuminates a reflection plane RP, which comprises a layer Apo with a structured apodization profile, a non-structured λ / 4 plate / retarder QWP, and a mirror plane M. The order of the layer providing an apodization profile and the λ / 4 plate / retarder can be changed.The light reflected back from the reflection plane RP has a polarization orthogonal to the initial polarization, which here is, for example, TM. This light will be modulated by the amplitude modulation pixels a and transmitted by them. A broadening of the light will occur due to diffraction. However, the light that hits the phase modulation pixels on its path from the mirror plane M back is blocked by the structured polarization filter sPF of the phase modulation pixels. Due to the angular selectivity of the volume grating VG arranged at the exit plane of the front light device FLU, the complex-valued wave field passes through the front light device FLU on its way to a field lens FL. If the light exits the SLM at a sufficiently large oblique angle, e.g., approximately 30°, then an off-axis volume grating-based field lens FL can be used.This means that no additional volume grating is required, arranged in front of the field lens FL. This is only the case when a combined field lens is used. The field lens FL essentially provides an observer window VW at the entrance plane of an observer's eye E. Vertical tracking and / or horizontal tracking of the light can be provided using a vertical tracking unit VT and / or a horizontal tracking unit HT. The tracking units VT and HT preferably comprise liquid crystal (LC) gratings having individually controlled electrode lines and an in-plane rotating liquid crystal (LC) mode.

[0140] Apodization profiles can also be arranged in the SLM plane instead of or in addition to the arrangement of apodization profiles near the mirror plane M. Furthermore, the aperture—or fill factor (FF)—of the exit pixels, which are, for example, the amplitude modulation pixels, can be slightly smaller, e.g., 20% smaller, than the aperture of the entrance pixels, which are, for example, the phase modulation pixels. This reduces the requirements that must be met by a transparent substrate TRS, which serves as a spacer between the addressable transmissive layer with the complex-valued pixels and the reflection plane RP. Thus, higher values ​​of the total thickness deviation of the transparent substrate TRS are acceptable.A slight lateral offset of the arranged displacement can be compensated by applying this approach, thereby reducing the influence of thickness variations of the transparent substrate TRS, which is arranged between the plane containing the addressable transmissive layer and the reflection plane RP. In other words, if a fill factor of an exit aperture or exit aperture stop of, for example, 0.5 is used and a wave segment illuminating this exit aperture and having the same lateral extent as the exit aperture, then a change in the distance to the reflection plane RP will cause a lateral displacement of the wave segment illuminating the exit aperture stop. Thus, the exit aperture is no longer fully illuminated.However, if the exit aperture is slightly smaller than the wave segment illuminating it, then a slight lateral shift of the illuminating wave segment will still provide a fully illuminated exit aperture.

[0141] Another complex-valued spatial light modulation device SLM of the reflection type, which provides a serial combination of phase modulation pixels and amplitude modulation pixels, is described in Fig. 7. In this figure, the SLM uses a beam shifting element SP in the form of a birefringent plate, preferably a Savart plate. A Savart plate can thus also be used to realize the serial combination of adjacent phase modulation pixels and amplitude modulation pixels. A front-light illumination device can provide illumination light that has an exit angle of, for example, 0° or 10° to the surface normal axis. If an exit angle of 0° is used, the light reflected back from the reflection plane must be separated using a polarization beam splitter geometry, which can be realized by means of a volume grating with a diffraction angle of, for example, 60°. However, illuminating the SLM at, for example, 10° offers the possibility of using the Savart plate as a beam shifting element or beam shifting element.The Savart plate is most preferred at an angle of 10°, where additionally the back-reflected light has an off-Bragg angle of 20° to the reconstruction geometry of the illumination light (see volume grating VG in ). Fig. 7), which is sufficient in terms of the angular selectivity of the volume grating for coupling out the light from the illumination device.

[0142] To suppress unwanted light and reduce crosstalk between neighboring pixels, it is preferable to use a structured polarization filter in front of adjacent phase modulation pixels and amplitude modulation pixels, which are to be combined as complex-valued pixels. The functional orientation of adjacent polarization filter segments of the polarization filter is orthogonal.

[0143] In Fig. Figure 7 describes the principle of the display device shown. A complex-valued spatial light modulation device (SLM) of the reflection type provides a serial combination of phase modulation pixels and amplitude modulation pixels that are adjacent to each other and arranged in the same plane. In contrast to the Fig. In the embodiment shown in Figure 6, a beam displacement element SP in the form of a birefringent plate, here a Savart plate, is used. Slight off-axis illumination at, for example, 10° to the surface normal axis is used to be off-Bragg on the path back through a volume grating VG, which couples the light from a front light illuminator FLU. The light passing through the phase modulation pixel p is shifted by the birefringent plate SP with an orientation of its optical axis of, for example, 42° to 45° to the surface normal axis. The double arrow shown in the lower part of the birefringent plate SP marks the orientation of the optical axis of the birefringent plate SP. A λ / 4 retardation plate QWP is arranged on the back of the birefringent plate SP, which is followed by a mirror plane M. The λ / 4 plate QWP and the mirror plane M form the reflection plane RP.The mirror plane M can further exhibit an apodization profile Apo. The λ / 4-wavelength retardation plate (QWP) can be realized using a polymerized liquid crystal (LC) layer, which can be oriented using photoalignment. Photoalignment is advantageous for structured retarders, but is not required for a non-structured layer. In other words, it may also be expedient to laminate only a λ / 4-wavelength retardation plate QWP film, adapted to the design geometry and the wavelengths used on the mirror plane M. Larger angles of incidence onto the λ / 4-wavelength retardation plate QWP require a smaller thickness of the λ / 4-wavelength retardation plate QWP material. Photoalignment is a standard technique that can be applied to structured retardation as well as structured polarization filtration.

[0144] The light shifted by the beam shifting element SP, which can also or alternatively be realized using a non-structured volume grating, is polarized and reflected back to the SLM plane. Thus, the phase-modulated light impinges on the amplitude modulation pixel a and is amplitude-modulated. Crosstalk to adjacent phase modulation pixels p is suppressed by introducing orthogonal polarization.

[0145] The complex-valued wavefield is emitted, for example, at 20° off-axis to the illumination geometry of the volume grating-based front light illumination device FLU, which represents a reasonable deviation from the axis (off-axis), and thus the wavefield propagates towards a field lens FL. For an off-axis angle as small as that in Fig. 7, a combined field lens cFL must be used instead of a single off-axis volume grating-based lens.

[0146] The use of on-axis illumination of the SLM—normal incidence or front-light illumination (FLU) at an illumination angle of 0°—in combination with beam shifting based on a birefringent plate (Savart plate) has a limitation regarding the thickness of the birefringent plate. Calcite (calcite) provides a so-called walk-off angle of slightly more than 6°, and a highly birefringent polymerized liquid crystal (LC) can achieve a walk-off angle of, for example, 7°. This is equivalent to a lateral shift and does not affect the Poynting vector. In other words, the beam impinges on the λ / 4 waveplate and the mirror plane at 0°. The polarization is changed from the λ / 4 waveplate to the orthogonal one, and the reflected light passes through the birefringent plate without any lateral displacement.This means that a maximum walk-off angle of 7° is provided, resulting in a minimum thickness of the birefringent plate greater than eight times the pixel pitch. And this is only one path, i.e., half of the total beam path. There is no shift on the return path from the mirror plane. Thus, the effective spacing between the phase-modulation pixels and the amplitude-modulation pixels is greater than sixteen times the pixel pitch, which is impractical. Due to crosstalk between adjacent pixels introduced by diffraction, the limiting spacing is approximately ten times the pixel pitch present in the coherent direction.In other words, using a birefringent plate with an illumination light incidence of 0° does not provide for a sufficiently short distance between diffraction apertures and thus does not work adequately here. This is why additional, slight off-axis illumination can be used.

[0147] Fig. 8 describes an alternative embodiment of a display device which operates in transmission and laterally combines adjacent amplitude modulation pixels and phase modulation pixels of a display panel of a spatial light modulation device SLM.

[0148] A backlight device BLU illuminates the SLM array at an angular offset of 25° from normal incidence. The discrete value of the oblique incidence depends on the discrete design of the display device and can be, for example, 10° to 45°. Preferred values ​​of the oblique incidence also depend on the liquid crystal (LC) modes used in the addressable transmissive layer of the SLM, which can be, for example, in-plane or out-of-plane LC modes. A significant engineering effort would be required to use different LC compositions for different subsets of pixels. Thus, the LC mixture used for both types of subpixels (amplitude modulation pixels, phase modulation pixels) can be the same rather than different.Patterned photoalignment is not a problem, and thus different orientations of alignment layers are feasible for different subsets of pixels. In other words, a preferred embodiment uses a single LC composition, e.g., having more than 20 different chemical components, for the phase modulation pixels and the amplitude modulation pixels. However, the orientation of the phase modulation pixels and the amplitude modulation pixels can be different and thus optimized to meet the different requirements of the two subsets of pixels used. The same LC composition can be used for different types of operation, i.e., for in-plane or out-of-plane operation. Thus, the electrode arrangement used for the phase modulation pixels and for the amplitude modulation pixels can be different.

[0149] A structured polarization filter (pPF) plane with segments is not required. Polarization filter elements can be used to prevent crosstalk between adjacent pixels. The simplest approach (with respect to the pPF element shown) is to not use the plane of polarization filter elements (pPF), with the segments interleaved according to the order of the phase modulation pixels and the amplitude modulation pixels.

[0150] The next practical step (regarding the pPF element) is the use of polarization filter segments pPF only at the exit aperture of the amplitude modulation pixels a and mirror segments MS at the reflective rear end of the phase modulation pixels p. This also means that a non-structured polarization filter plane can be used behind the exit plane of the SLM array. Absorbing polarization filters are preferred to provide a high contrast of the holographically generated image, e.g., 1000:1.

[0151] The third practical step (regarding the pPF element) is the use of a pattern of interleaved polarization filter elements that provide orthogonal operation. Polarization rotation due to the LC mode must also be taken into account here.

[0152] Finally, the preferred embodiment involves the use of reflective polarization filter segments, such as wire-grid polarizer segments, at the reflective rear end of the phase modulation pixels p. Wire-grid polarizer segments can be coated with a patterned absorbing film that serves as a black mask for the polarization to be suppressed. In the preferred embodiment, absorbing polarization filter segments are arranged at the exit apertures of the amplitude modulation pixels.

[0153] The phase modulation pixels are based on a mirror, while the amplitude modulation pixels have a structured polarization filter to suppress crosstalk.

[0154] According to Fig. 8, an illumination device configured as a backlight device BLU provides collimated light with a plane wave spectrum of ≤ 1 / 60°, which is present in the coherent direction. The collimated light is emitted by the illumination device BLU at a defined illumination angle, which is, for example, 25°. Furthermore, the exit polarization of the illumination device BLU is defined, and a high degree of polarization is present. Thus, the Fig. 8 shown SLM illuminated under defined conditions.

[0155] The light then passes through the optical elements shown in a defined order. The light first hits the phase modulation pixels p. At a rear end of the phase modulation pixels p, which have a phase shift φ iA reflective element is provided. The provision of a mirror having mirror segments MS is sufficient. A structured wire grid polarizer can also be used. The use of mirror segments MS covering the rear end of the phase modulation pixels may be sufficient in terms of lower manufacturing costs versus retained image contrast. At the exit plane of the SLM, the mirror segments MS are arranged adjacent to the polarization filter segments pPF. The mirror segments MS and the polarization filter segments pPF, which are designed as transmissive polarization filter segments, are interleaved, which are provided, for example, using an alternating arrangement of mirror segments MS and polarization filter segments pPF. In other words, an array of mirror segments is interleaved within an array of polarization filter segments.The light incident on the phase modulation pixels p is reflected back by the mirror segments, thus creating a double-path arrangement within the phase modulation pixels p. This reduces the thickness of the addressable transmissive layer, preferably the LC layer, which is required for a 2π phase shift that must be provided by the phase modulation pixels p. Doubling the thickness of the LC layer means reducing the switching speed for controlling the SLM pixels by a factor of four. This applies to most of the LC modes and a voltage that is kept constant. The hybrid aligned nematic liquid crystal (HAN LC) mode and, alternatively, in-plane LC modes behave differently. The reduced thickness of the LC layer provides a higher refresh rate at a suitable voltage. This means that there is no need to significantly increase the voltage.An increased voltage causes significant technical complexity regarding the implementation of a backplane, which is the electrical circuitry that provides electrical control of the SLM's display panel. Therefore, the dual-path arrangement within the SLM, which is implemented for the phase modulation pixels p, is very advantageous.

[0156] Furthermore, the light, which passes through the phase modulation pixels p twice, propagates to a reflection plane RP, which provides a pattern or structure of structured reflectors in the form of mirror elements M. The reflection plane RP has an apodization profile Apo, which causes apodization. This apodization profile is designed as a segmented amplitude apodization profile and can, for example, be a Kaiser-Bessel window or a Gaussian distribution. The effect of apodization of the finally obtained complex-valued pixels is to reduce the spatial frequencies present in the exit plane of the complex-valued spatial phase and amplitude light modulator SLM, and thus to reduce the intensity of the higher diffraction orders of the SLM grating, which can otherwise disturb the neighboring eye of an observer viewing the reconstructed scene through an observer window VW.The apodization profile can also be provided in front of the phase modulation pixels p or even behind the amplitude modulation pixels a in order to provide the inner pixel intensity distribution with which the intensity of the higher diffraction orders can be reduced.

[0157] The light passing through the apodization profile Apo encounters a structured retarder in the form of a λ / 4 plate QWP comprising λ / 4 retarder segments QWPS, which introduces a λ / 4 retarder for a single pass. These segments QWPS of the λ / 4 retarder QWP are arranged on top of the mirror elements M, creating a segmented mirror plane. The light is then reflected back and passes through the λ / 4 retarder QWP again. This creates a polarization state that is orthogonal to the input polarization. This means, for example, TM (transverse magnetic) polarization instead of TE (transverse electric) polarization, or LCP (left circular) polarization instead of RCP (right circular) polarization. This change in polarization is used to ultimately reduce crosstalk between adjacent pixels, ultimately resulting in increased image contrast.It can also be used to provide the entrance polarization required for the amplitude modulation pixels a that follow later in the optical path.

[0158] The light reflected back in the forward direction by the mirror elements M strikes the amplitude modulation pixels a. A structured polarization filter pPF is arranged at the exit plane of the amplitude modulation pixels a. For the amplitude modulation pixels a, the structured polarization filter pPF is a plane of structured polarization filter elements pPF2, which form the second polarization filter segments as seen in the propagation direction of the light passing through the SLM. At the amplitude modulation pixels a, absorption polarization filter elements are preferred, which are oriented orthogonally to the other group of interleaved structured polarization filter elements pPF1 associated with the phase modulation pixels and which are used at the reflective rear end of the phase modulation pixels p.These structured polarization filter segments ppPF1 are the first on the path through the SLM, viewed in the direction of light propagation. For example, a structured wire-grid polarizer can be used. Alternatively, an absorbing polarization filter element arranged in front of the mirror segment MS can also provide polarization-selective reflection, which is required at the rear end of the phase modulation pixels p. However, higher absorption will be present compared to a structured wire-grid polarizer.

[0159] This structured polarization filter (pPF) suppresses crosstalk between adjacent pixels. An absorbing structured polarization filter (pPF) is preferred for the amplitude modulation pixels (a), as discussed above. The structured polarization filter (pPF) can also be arranged at the entrance plane of the amplitude modulation pixels (a).

[0160] After passing through the spatial phase and amplitude light modulator SLM, the light propagates to a field lens FL, which can be, for example, a volume grating or a stack of volume gratings. The field lens FL focuses the wavefield, which contains all wavefield segments of three-dimensional object points to be reconstructed, onto a focal plane that can be equivalent to the mean average observer distance. After passing through the field lens FL, vertical diffraction or vertical tracking is introduced, e.g., using a vertical tracking unit VT, which has an in-plane liquid crystal (LC) grating with individually controlled electrode lines. A horizontal tracking unit HT, which has an in-plane LC grating, can be used for horizontal tracking.

[0161] The Fig. The discrete embodiment of this alternative of the invention shown in Figure 8 can be modified. For example, the oblique angle of incidence on the SLM can be modified. A modified oblique angle of incidence results in a modified thickness of a transparent substrate TRS used between the spatial phase and amplitude light modulator plane and the reflection plane RP, which includes the structured mirror plane arranged between the backlight device BLU and the spatial phase and amplitude light modulator plane. A thin glass film can be used as the transparent substrate TRS. Several glass films are already available in rolls with a thickness of, for example, 100 µm, 75 µm, 50 µm, or even 25 µm.It is important to limit the path length between the first structured structure in the diffraction-adding reflection plane (RP) and the last structured structure at the SLM exit plane to 10 times the pixel pitch used. This applies to the coherent direction of the light, i.e., to two directions in the case of two-dimensional (2D) encoding of subholograms in the SLM. This defines appropriate geometries. More specifically, the path length from the plane of the first array-like apertures, which is the plane in front of the spatial phase and amplitude light modulator (SLM), to the exit plane, which is the plane of the spatial phase and amplitude light modulator (SLM), must preferably be limited to 10 times the pixel pitch used. This means 125 µm for a pixel pitch of 12.5 µm (e.g. for a mobile holographic tablet display), 250 µm for a pixel pitch of 25 µm (e.g.for a holographic desktop application) and to 0.8 mm for a pixel pitch of 80 µm (e.g., for a holographic TV application). The apodization structure can also be located at the entrance plane, i.e., in front of the phase modulation pixels p. Optionally, multiple apodization planes can be used within the SLM structure, which can be used, for example, to extend the confinement to 10 times the pixel pitch of the optical path used for the combination.

[0162] In Fig. Figure 9 describes a preferred embodiment of the display device according to the invention. This display device can be used with the ECB (electrically controlled birefringence) mode of liquid crystals (LC). A liquid crystal (LC) layer is used as the addressable transparent layer LCL with the complex-valued pixels. Due to the required modulation of the refractive index of the LC layer LCL, the use of TE-polarized light for large diffraction angles of Bragg diffraction-based volume gratings is preferred. The polarization state of the light can be, for example,by changing the orientation of an illumination device, here in the form of a backlight device BLU, or by adding another achromatic retarder plane, which can be arranged between the backlight device BLU and the entrance plane of the spatial light modulator SLM. For example, an illumination device emitting TE (transverse electric) polarized light can be rotated 90° with respect to the implemented optical design in order to illuminate the SLM with TM (transverse magnetic) polarized light instead of TE polarized light. Thus, for example, TE, TM, CLP, or CRP light can be provided, as required for the discrete design. Achromatic retarders can be produced cost-effectively, for example, using polymerized LC compounds.The backlight device BLU (side view) shown is rotated by 90° relative to the SLM (top view) for better understanding.

[0163] As in Fig. As can be seen in Figure 9, linearly polarized light, which is TM-polarized here (here in a top view instead of a side view), enters the SLM, which is sandwiched between the LC layer LCL and a reflection plane RP provided behind the backlight device BLU and which forms the entrance plane of the SLM. A transmissive substrate TRS acts as a spacer between the LC layer LCL and the reflection plane RP. When using the ECB mode in the LC layer LCL, the phase modulation pixel p changes the phase without changing the polarization state. This is due to the orientation of the liquid crystals in the LC layer LCL. The optical axis of the phase modulation pixel p lies in the plane of the electric field of the TE-polarized light.

[0164] The light reflected back from the phase modulation pixel p by the mirror segments MS at the rear end of the phase modulation pixel p is guided rearward to the backlight device BLU and impinges on the reflection plane RP as a structured plane comprising reflective segments M, apodization profile segments Apo, and λ / 4 element segments QWPS, which introduce a retardation of 2 x π / 4 = π / 2 (2 x λ / 8 = λ / 4). Thus, circularly polarized light is obtained, which can be, for example, LCP light or RCP light.

[0165] The mirror segments MS at the rear end of the phase modulation pixels p and the mirror elements M of the entrance plane of the SLM or the reflection plane RP can be manufactured using metallic or dielectric mirror stacks, or even a combination thereof. Chromium (Cr) or aluminum (Al) can be used for the metal electrodes, as is already known.

[0166] The light reflected back from the reflection plane RP, which is also the entrance plane of the SLM, can be formed on demand and can thus exhibit a TE, TM, LCP or RCP polarization state depending on the LC and LC mode used.

[0167] As in Fig. As shown in Figure 9, the amplitude modulation pixels a are illuminated with circularly polarized light, which is, for example, RCP light or LCP light. The amplitude modulation pixels a, which operate here in ECB mode, change the polarization state by introducing a controllable delay. Thus, for example, LCP light can be changed into TE light. The amplitude modulation pixels a use transparent ITO (tin-doped indium oxide) electrodes. A variety of LC modes can be used, such as TN (twisted nematic) modes or in-plane rotating LC modes, which realize an in-plane LC rotation, for example, on the side of the LC layer, such as the HAN mode or LC modes that realize an in-plane rotation of the LC in the center of the LC layer.

[0168] A polarization filter PF, which is provided behind the complex-valued pixels and is, for example, an absorbing wire-grid polarizer, blocks the non-TE-polarized part of the light and thus converts the retardation of the amplitude modulation pixel a, the second controlled pixel, into a real amplitude modulation. The polarization of the entrance plane of the SLM can be adjusted using an achromatic or apochromatic retardation layer, which can be arranged at the exit plane of the backlight device BLU. Another, e.g., achromatic or apochromatic, retardation layer / λ / 4 plate QWP is provided later in the beam path to provide the correct polarization state, e.g., for polarization LC gratings used for fine angular tracking by means of the vertical tracking unit VT and / or the horizontal tracking unit HT.

[0169] The pixel plane may include color filters CF for RGB, which may be used, for example, for a one-dimensionally coded TV display device with vertical parallax. Fig. 9, the amplitude modulation pixels a are provided with the color filters CF. For a two-dimensionally coded display device, a time-division multiplexing method can be used without using structured color filters CF.

[0170] Structured polarization filters / analyzers, designed as a checkerboard or striped pattern, are provided for at least one channel to prevent crosstalk between adjacent pixels. Furthermore, the input channels CH, where a channel CH corresponds to the beam path of a complex-valued pixel and is indicated by a dashed line in Fig. 9, since it is a virtual channel, it can also be equipped with structured analyzers that provide transmission for orthogonally polarized light. Thus, the signal-to-noise ratio can be increased. The term "channel" is used with respect to the optical function, meaning that the light passes through the SLM along separate channels related to the complex value that provides a combination of phase modulation pixels p and amplitude modulation pixels a.

[0171] In the following section, the propagation distance of the light within the SLM is described with reference to the Fig. 10 and Fig. 11. The propagation distance of the light present within the SLM must be limited to keep the level of pixel crosstalk as low as required, which means, for example, so low that an image contrast of > 1:100 or even > 1:1000 can be achieved.

[0172] Fig. Figure 10 shows the suitable thickness range d of the transparent substrate used between the plane of the phase modulation pixels and the amplitude modulation pixels and the entrance plane of the SLM, which has mirror segments that can be provided with additional apodization profiles. The thickness range d depends on the pitch of the complex-valued pixel Λ. φ+a dependent. For a two-dimensional (2D) encoding of the SLM, the smallest pitch must be used to define the maximum thickness d. The angular range used for the illumination of the SLM is limited to 25°, which is acceptable for several LC modes and, for example, for the ECB mode. Thus, the lower left part of the diagram is Fig. 10 is cut away to mark this area as unsuitable because the angular range is larger than 25°. This part is shown in white. The further cutaway part on the right side of the diagram is defined by propagation distances that exceed the limit of 10 times the pitch of the complex-valued pixel Λ φ+a This limit of 10 times the pixel pitch is a result of simulations that take the optical near field into account.

[0173] Diffraction occurring at an entrance plane of the SLM causes crosstalk between the pixels in a pixel plane of the SLM located behind the entrance plane. The longer the wavelength used and the smaller the pixel aperture, the greater the crosstalk present in the pixel plane. Thus, the propagation distance that can be introduced between the two structured planes mentioned above must be limited. Simulations have shown a limit of 10 times the pixel pitch. This is why pixel pitch values ​​larger than this value are Fig. 10 are cut away.

[0174] Polarization management was not considered for the simulations mentioned above. However, due to polarization management, diffraction at the entrance plane of the SLM, which introduces crosstalk, is blocked later in the display device. Thus, the crosstalk that may exist between adjacent pixels is reduced. This means that a wider range of thicknesses can be used due to polarization management.

[0175] This makes the difference between Fig. 10 and Fig. 11. As defined in Fig. As shown in Figure 11, the angular range used for the illumination of the SLM is also limited to 25°, which is acceptable for several LC modes. Thus, the lower left part of the diagram is Fig. 11 is also cut away. This defines the unsuitable area with regard to the angle of incidence of the light coming from the illuminator and incident on the SLM. The further part, which is cut away on the right side of the diagram, is defined by the propagation distances, which limit the 15-fold pitch of the complex-valued pixel Λ φ+a exceed.

[0176] Thus, it is advantageous that the thickness d of the transparent substrate or the thickness d of a cover substrate / glass can be increased due to polarization management.

[0177] Lateral shifts of individual wave segments due to thickness deviations of a cover glass, which is provided on top of the addressable transmissive layer in the direction of light propagation and which contains the mirror segments of the entrance plane of the SLM, are less critical if a larger thickness d can be used. This is due to the relationship between the thickness d and the thickness tolerance Δd (d / Δd). For example, achieving a total thickness deviation of ±2 µm for a substrate thickness of 100 µm is less expensive in terms of production costs than achieving a total thickness deviation of ±1 µm for a substrate thickness of 50 µm.

[0178] As in the Fig. 10 and Fig. As shown in Figure 11, LC modes are preferred that can be used in such a way that a reduced number of components is achieved. Thus, one option, for example, is to avoid the structured retardation used in the entrance plane of the transmissive complex-valued, in-plane SLM. This further means that a change in the polarization state, which may be introduced by a phase shift or a pixel used to change the phase in a controlled manner, can be compensated by the second amplitude modulation pixel ultimately used to modulate the amplitude, i.e., together with a polarization filter used later in the optical path behind the SLM.

[0179] This means that the complex-valued modulation, which is preferably carried out using a phase shift φ that does not change the amplitude a or the polarization state, can also be realized otherwise using pixels that provide a complex-valued modulation as a combination of a phase shift and a change in the polarization state, which ultimately leads to a modulation of the amplitude.

[0180] The advantageous way to provide a complex value c is c = axe iφ , where a is the amplitude and φ is the phase. This means using a first phase-modulating pixel that does not change the amplitude value, and a second amplitude-modulating pixel that does not change the phase value. In this situation, the amplitude values ​​must be transferred to the amplitude-modulating pixels and the phase values ​​to the phase-modulating pixels.

[0181] Alternatively, it is also possible to use pixel embodiments and associated LC modes that realize the complex value only as a combination. This means that, for example, the state of both pixels must be changed to change only the amplitude or only the phase of the output plane of the complex-valued pixel. In this situation, the amplitude and phase values ​​can be transferred to a look-up table (LUT) to obtain the electrical control signals that must be transmitted to the two pixels that ultimately form the complex-valued pixel.

[0182] In the Fig. 12a and Fig. 12b, an embodiment is described in which an alternative SLM configuration is used for laterally combining the phase modulation pixels and the amplitude modulation pixels in a display panel. However, the phase modulation pixels and the amplitude modulation pixels are not provided in the same plane, but are arranged laterally and form complex-valued pixels of a complex-valued MEMS (micro-electro-mechanical system)-based SLM. This MEMS-based SLM has a MEMS arrangement with several mirror elements that are slightly offset from one another. A phase modulation pixel p is formed using a lifting mirror element PME with a reflection plane RP, which has a reflection surface RS, and an actuator A for moving the lifting mirror element PME in the direction indicated by the arrow in Fig. 12b. The amplitude modulation pixel a is realized using a mirror element ME, which is fixed and can be controlled by an actuator A. The mirror element ME has a reflection surface RS facing the reflection surface RS of the lifting mirror element PME. Furthermore, by controlling the mirror element ME and thus the reflection surface RS by means of the actuator A, the reflection surface RS can be changed in its shape so that the incident light can be directed in a predetermined direction. As a result, the reflection surface can be changed from a sufficiently flat surface in the OFF state, as in Fig. 12a, to a curved plane, e.g., a convex or concave plane, or to a plane formed at an angle in the ON state, as in Fig. 12b. The hub mirror element PME and the mirror element ME are shown only in a simplified manner.

[0183] Fig. 12a shows the complex-valued pixel in an OFF state, while in Fig. 12b, the complex-valued pixel is shown in an ON state. In the OFF state, the light is first guided to the amplitude modulation pixel a in the form of the mirror element ME. Since in the OFF state the mirror element ME is not controlled by the actuator A, the incident light is reflected back in its direction of incidence, as indicated by the double arrow in Fig. 12a. In the ON state, the mirror element ME is controlled by applying a voltage to the actuator A, whereby the reflection surface RS is changed such that the light incident on the mirror element ME is modulated in intensity and then directed to the lifting mirror element PME to modulate the phase of the light. The phase- and amplitude-modulated light is then used to focus the light onto an observer space, e.g., to a (in Fig. 12b not shown) field lens, as in Fig. 8 or Fig. 9 is shown.

[0184] The Fig. 12a and Fig. 12b show only a general embodiment. The tilting mirror PME can be used for a direct and straightforward implementation of phase modulation. The tilting mirror element ME leads to a more indirect amplitude modulation by tilting a wave segment away from an aperture stop AS, which is later Fig. 12b. This aperture stop AS, which can stop or prevent further propagation of the light into the viewing space, can be formed by an array of exit aperture stops of the SLM. If there is an asymmetric intensity distribution of the exit aperture stops of the complex-valued pixels provided by the SLM, a correction can be taken into account during the hologram encoding process.

[0185] In Fig. Figure 13 shows a further embodiment of the display device according to the invention. In this embodiment, an SLM that combines the phase modulation pixels and the amplitude modulation pixels is illuminated in different, individually controllable directions. This means that the SLM is illuminated, for example, along two or four different entrance angles, which can be switched ON or OFF on demand. This is generally Fig. 13 shown.

[0186] The SLM, shown here only in a simplified form, is arranged to provide complex-valued modulation when illuminated from several discrete directions. Fig. Figure 13 shows an SLM illuminated from two directions, D1 and D2. The SLM can be, for example, an SLM as shown in the Fig. 6, Fig. 7, Fig. 8 and Fig. 9. An illumination device, not shown here, provides different exit angles of the light illuminating the SLM. This light is controlled by switching ON or OFF different light sources or illumination directions in the illumination device, e.g., a wedge-based backlight device.

[0187] Multiplexed volume gratings with different field lens functions can be provided behind the SLM. These can be, for example, a field lens function 1 and a field lens function 2. Field lens function 1 can have a lateral focal point offset of +10°, and the second field lens function 2 can have a lateral focal point offset of -10°. Thus, the tracking range can be increased through this coarse tracking. Thus, the light exit angles of the SLM are adapted to several discrete field lens geometries.

[0188] Fig. 14 concerns the plane wave spectrum, although only one example is described here. The diagram of Fig. Figure 14 shows the diffraction efficiency of a Bragg-based volume grating, which achieves a diffraction geometry of 84.26° in air / 0° in PMMA, which is -41.55° / 0° in a medium with an average refractive index of n0 = 1.5, as a function of the difference to the specific Bragg angle, which is the designed angle of incidence of the grating. The thickness of the volume grating used is D HOE = 16 µm.

[0189] The plane wave spectrum, which is up to 1 / 60° degree, can be used in the coherent direction of illumination (in a holographic display device) to provide high resolution (high definition, HD) viewing up to a distance from the display device that is half the distance to the viewer's eye.

[0190] Understanding the operating principles of holographic three-dimensional (3D) display devices with viewer window tracking, it can be seen that this also means the use of an extended light source instead of a point light source and does not require single-mode optical fibers. Consequently, laser light sources with a so-called beam quality factor M 2 which is significantly larger than 1. Using a point light source that has a high degree of absolute value of mutual coherence |µ 12 | would not make sense for the application described here.

[0191] The incoherent direction in the display device can, for example, use 0.5° to 1° to create a sweet spot. Tenfold beam expansion can be achieved, for example, using volume gratings based on Bragg diffraction. Surface relief gratings or polarization gratings can also be used for this purpose. The plane wave spectrum must be designed as an incoherent superposition of plane waves. This is what the plane wave spectrum physically is. And this means that the curvatures of the wavefronts or wave segments are not problematic here.

[0192] The advantage here is that the plane wave spectrum can be designed as a combination of dynamic scattering along at least one direction and static, expanding scattering along at least one direction. However, the plane wave spectrum cannot be designed solely as a static scattering function. That would not make sense here.

[0193] The volume gratings, which, for example, provide a 10-fold beam expansion within the (background) illumination device, may have an angular acceptance that is smaller than required. This means that for a one-dimensional coding, the angular acceptance should be < 1 / 60° for the coherent direction and < 0.5° to 1° for the incoherent direction. In this case, the primary illumination can provide the plane wave spectrum that is accepted by the volume gratings. For example, with reference to Fig. 14 a volume grid with a thickness of D HOE= 16 µm, which realizes a geometry from -84.26° in air to 0° in PMMA, which is 41.554° / 0° in PMMA, with ±0.2° (in PMMA) with a diffraction efficiency of η ≥ 0.9. In air, this range is transmitted from (41.554 ±0.2)° (PMMA) to (82.33 to 87.31)° (air). This range can be diffracted with low loss using a dielectric anti-reflection stack, meaning this angular range can be diffracted by ≥ 90%. Thus, the plane wave spectrum of ±0.2° present behind the SLM can be transmitted with an efficiency of > 0.81 (1 is equivalent to 100%). At a viewing distance of 3 m, this plane wave spectrum of ±0.2° provides a sweet spot of ±9.42 mm, which is a total of 18.85 mm and much larger than required.

[0194] The entrance pupil of the human eye has a diameter of 2 mm to 3.5 mm. This must be taken into account, especially in TV applications. The xyz resolution of a tracking unit in a simulated display device is approximately ±1 mm. This means that for a TV application, a plane wave spectrum of even less than ±0.1° is sufficient.

[0195] However, large screens, such as TVs, and shorter viewing distances may require a larger plane wave spectrum spanning the sweet spot. In this case, the dynamic part of the plane wave spectrum, which is, for example, ±0.2° or ±0.1°, can be increased by adding another static or dynamic one-dimensional (1D) scatter function, adding, for example, another ±0.2°.

[0196] Purely one-dimensional static scattering may not make sense in the described embodiments. However, in combination with a dynamic base part, it can be useful in some cases. In other words, a plane wave spectrum of ±0.4° can be generated using a first dynamic base part of ±0.2°, which is expanded using a second or secondary static diffraction-based part.

[0197] A wedge lighting device, such as that used in Fig. 4, but can be slightly modified to provide a larger plane wave spectrum of, for example, ±0.4° degrees even without the use of a static diffraction element or even a dynamic scattering element. The first small strip-shaped volume grating (VG1 in Fig. 4) can be modified to operate at a reduced beam expansion factor, e.g., only 7 times. Thus, a broader plane wave spectrum, which meets the requirements of one-dimensional sub-hologram coding, which means, for example, that it is usable in TV display devices, can be transmitted through the first Bragg diffraction-based volume grating VG1. The second volume grating VG2 (VG2 in Fig. 4) can still operate at 10x or 20x beam expansion. For this example, one-dimensional (1D) sub-hologram encoding of the SLM along the vertical display direction is provided.

[0198] In the following, an illumination device for an in-plane complex-valued spatial light modulator SLM is described.

[0199] State-of-the-art SLMs using birefringence-based polarization state modification are capable of operating with a plane-wave spectrum significantly exceeding a range of ±10° without losing the high image contrast of, for example, 10,000:1. In other words, accurate amplitude values ​​can be obtained even when an expanded plane-wave spectrum is present.

[0200] To obtain precise phase values, a sufficiently small plane wave spectrum, such as 1 / 60°, must be used. As long as the plane wave spectrum propagating in the viewing space is less than or equal to 1 / 60° (±1 / 120°), the wave field can be used to generate three-dimensional (3D) holographic images that meet the high resolution (HD) standard.

[0201] A preferred lighting device, in particular a background lighting device, is described with reference to Fig. 15 described.

[0202] The beam expansion factor of a backlighting device is M (used for the magnification of optical systems) or M_x and M_y, and defines the change in the plane wave spectrum, which is the reciprocal of the beam expansion factor. Thus, a backlighting device that requires a plane wave spectrum at its exit plane of ≤ ±1 / 120° and a beam expansion factor of M_x = M_y = 10 can be illuminated with a plane wave spectrum of ≤ ±1 / 12°.

[0203] This applies to the coherent direction of the encoding, which can be, for example, the vertical direction only for vertical parallax, or both the horizontal and vertical directions if a two-dimensional encoding is used. The incoherent direction can operate with a much broader plane wave spectrum, spanning the required sweet spot, which is, for example, ±0.35° degrees, or even a smaller value, such as ±(0.1 - 0.2)° degrees for TV applications.

[0204] It is advantageous to use polarization-maintaining single-mode fibers and beam-forming parameters, also known as beam quality factors, M 2 close to 1. The symbol used to define beam quality is M 2 A value close to 1 represents a beam that is close to a Gaussian beam, which is the theoretical optimum of M 2 = 1. To sufficiently couple light into a single-mode fiber, a factor M2 close to 1 is required. This is due to the fact that light that does not propagate in a TE00 Gaussian mode is not guided within a single-mode fiber. A larger beam parameter product M 2 leads to a lower coupling efficiency.

[0205] When using an extended light source, beam parameters of M 2 > 1.2 is suitable. An extended light source can be used that is monochromatic and exhibits dynamic randomized phase modulation within the light source area. A suitable embodiment is a moving scattering plane, which provides dynamic and randomized phase modulation and the formation of an adapted intensity distribution, which is used, for example, to illuminate the collimation unit and ultimately the entire spatial light modulator (SLM) plane.

[0206] Fig. 15 shows a structure of a compact collimation unit that can be used within holographic one-dimensional (1D) or two-dimensional (2D) encoded three-dimensional (3D) displays.

[0207] The primary colors RGB (red, green, blue) can be provided by a light source LS as a light source with compact laser modules, which, for example, have an optical power of 1 W within a few cm 2 realize.

[0208] This description provides a holographic display that projects a light source into the plane of the entrance pupil of a human eye. For two-dimensional (2D) coding, a round configuration of the initial beams is preferred, which

[0209] Diameter, for example, within a range of 0.5 mm to 2 mm.

[0210] A polarization filter / analyzer can be provided, for example, behind a collimating lens COL of the illumination module / light source LS. An absorbent wire grid polarizer is preferred as a polarization filter. Absorbent wire grid polarizers are realized by applying an additional oxidation process to standard wire grid polarizers. Laser modules that achieve a polarization ratio of TE / TM > 1000:1 are commercially available standard components. A polarization filter is not a required component.

[0211] Reflection elements M in the form of mirrors are used to direct the primary RGB beams onto controlled random phase-generating elements designed as engineered diffusers ED-R, ED-G, and ED-B, which are optimized for the corresponding color and for a discrete beam diameter. The engineered diffusers ED-R, ED-G, and ED-B are located on three planes that serve as light source planes in the sense that they are ultimately imaged at the plane of the entrance pupil of the human eye. The phase profiles of the engineered diffusers ED-R, ED-G, and ED-B create a flat upper intensity distribution at the focal lengths of the collimating lens COL. Thus, the collimating lens COL is illuminated with a homogeneous light intensity distribution. Using three optimized engineered diffusers ED-R, ED-G, and ED-B, an optimized result can be obtained separately for each RGB color in a separate way.

[0212] Using the ED-R, ED-G, and ED-B engineered diffusers, a multitude of different phase values ​​can be provided with a spatially randomized distribution. This provides reasonably fast dynamic phase randomization. This means that a multitude of differently randomized phase distributions are used for each individual image (frame). A master with, for example, 16 different phase levels can be copied using a UV-curing adhesive. This is a cost-effective, standard process.

[0213] Binary phase plates can also be used alternatively as random phase generating elements or engineered diffusers ED-R, ED-G, and ED-B, using a spatially randomized design, to provide flat top beam shaping in a dynamic motion mode. These types of engineered diffusers are commercially available standard components. Several companies offer customized functionality, which involves tailoring the parameters to discrete requirements to achieve the optimized intensity distribution without losing a significant amount of optical power.

[0214] A more advanced random phase generating element can be realized using Bragg diffraction-based volume grating films or a single multiplexed Bragg diffraction-based volume grating film featuring three geometries. This random phase generating element can be realized, for example, as a reflective hologram or as a transmissive hologram, operating at an incident angle of (30 or 45) degrees and a normal exit angle. A multiplexed grating allows for individual and separate optimization for each color and a single-foil setup (Bragg volume grating in a single film) that can operate with a single PZT (piezo) element.

[0215] A PZT element that moves a film and displays individual images or frames is used in commercially available cameras when switched on to provide dust removal in front of a CCD or CMOS imaging chip by applying vibration. Feature sizes of > 10 µm can be used, meaning that lateral dynamic motion within a range of less than 0.5 mm can be sufficient for an engineered diffuser / dynamic random phase generating element. Using two PZT elements with a phase offset, rotational motions and Lissajous figures can be realized.

[0216] The reflection elements M or mirrors of Fig. 15, which are used for adjustment, provide a small angular offset with respect to the optical axis of the collimation unit. These can be used, for example, to compensate for small angular offsets that may be present in the Bragg diffraction-based volume gratings used in a backlighting device. A backlighting arrangement, for example, is a double wedge configuration that uses a first volume grating strip, which introduces a first, e.g., 10x to 15x horizontal beam expansion, and a second, display-sized volume grating, which realizes a second, e.g., 10x to 15x beam expansion in the vertical direction.

[0217] By shifting the light spot on the ED-R, ED-G, and ED-B engineered diffusers, which can be achieved by mirror adjustment, the average output angle for each primary color RGB is changed separately. Piezo translation elements PZT-R, PZT-G, and PZT-B provide lateral movement of the ED-R, ED-G, and ED-B engineered diffusers, for example, at a frequency above 20 kHz. This ensures dynamic phase randomization in each individual image that hits the human eye. The light of an individual image, which is a single frame displayed by the display device to a viewer's eye, of a single color that hits the eye, can have a time window of, for example, only 1 ms to 5 ms. Therefore, the dynamic phase randomization of the light source plane must be sufficiently fast.

[0218] A color beam combining element (CBC) in the form of a color beam combining prism with two dichroic coatings is used to generate a white light source located in the front focal plane of the collimating lens (COL). Prism types that do not have segmentation within the clear aperture, such as the so-called Philips color combining prism array, are preferred. The use of X-cubes in holographic displays is discouraged due to the arbitrary phase step between different parts of the clear aperture.

[0219] A backlight device providing 10x anamorphic magnification along two directions changes a plane wave spectrum of 1 / 6° degree, which is present at the entrance plane of the illuminator, to 1 / 60° degree, with which the SLM is illuminated.

[0220] Using a collimating lens with a focal length f_c allows the use of a round light source with a diameter of d_LS = f_c * tan(1 / 6° degree). Thus, f = 400 mm allows the use of d_LS = 1.16 mm, realizing a plane wave spectrum of 1 / 60° degree that will illuminate the SLM. Two-dimensional (2D) encoding of subholograms in the SLM requires a symmetrical light source design.

[0221] The maximum lateral extent of spatial coherence required to illuminate the SLM depends on the maximum positive value of the z-position (object point to the SLM) of the reconstructed object points. Thus, limiting the maximum z-value to half the distance from the plane of the display device to the observer means limiting the maximum "usable" size of the subholograms to a size equal to or smaller than the observer window (VW), which spanned between the 0th and 1st diffraction orders in an observer plane.

[0222] The maximum diameter of the entrance pupil of the human eye, which must be taken into account when defining the maximum size of the subholograms, is d EP = 5 mm. The entrance pupil of the human eye depends on the luminance, and the size of the subholograms d EP = 5 mm is only achieved if the luminance is in the range of only a few cd / m2 , such as 10 cd / m 2 , lies.

[0223] Within the SLM plane, the region of spatial coherence is defined by the maximum size of the subholograms encoded in the SLM. This size depends on the maximum distance z max of the reconstructed object points. For a maximum distance of z max = ½ times the distance to the observer is the maximum size of the subholograms d sub-hologram-max the same as the size of the observer window provided in the observer plane and is therefore, for example, 10 mm.

[0224] The size of a human pupil Ø EP depends on the luminance of the reconstructed scene. Typical values ​​for the human pupil are Ø EP = 2.5 mm for a luminance of 300 cd / m 2 and Ø EP = 3.5 mm for a luminance of 30 cd / m 2 . Thus, a maximum size of a subhologram of d sub-hologram-max= 5 mm is sufficient to reconstruct object points that are below the resolution limit of the human eye. Therefore, a spatial coherence range of 5 mm can be selected.

[0225] As mentioned above, a sub-hologram size of 5 mm is sufficient. Three-dimensional (3D) scenes with a luminance of more than 30 cd / m 2 which is most likely for a standard holographic display device used, but result in a pupil of Ø EP = 3.5 mm and can therefore be used with a spatial coherence area of ​​only Ø c = 3.5 mm. Therefore, for z max = ½ times the distance from the plane of the display device to the observer an area of ​​spatial coherence of Ø c = 5 mm more than enough.

[0226] More precisely, the computational effort increases due to the larger size of the subholograms that must be superimposed. This functional relationship is stronger than a linear one. This means that instead of using a (two-dimensional) subhologram size of 10 mm × 10 mm = 100 mm 2 using a size of 5 mm × 5 mm = 25 mm 2 will result in a reduction in computational effort considerably greater than four. Using circular subholograms will further reduce the size occupied in the SLM plane, thus further reducing computational power.

[0227] Two-dimensional (2D) coded displays can simply use a beam expander that provides the light source size required in front of the collimating lens COL. Experimental simulation:

[0228] The exit plane of a multimode fiber with a core diameter of d core= 400 µm and a numerical aperture of NA = 0.22 was placed on the front focal plane of a collimating achromatic lens with f coll = 400 mm and Ø coll = 50 mm using a mounted pair of two achromatic lenses with an image size of 1:3.3 (f1 = 30 mm, f2 = 100 mm). Thus, the size of the light source to be collimated was 1.33 mm. A test resulted in 1.6 mm instead of 1.33 mm. The numerical aperture NA of the light source plane was set to NA LS = 0.066, which is slightly above the numerical aperture NA of the collimating lens, the NA coll = 0.624.

[0229] A plane wave spectrum of 1 / 6°, which is ±1 / 12°, is equivalent to a light source size of 1.16 mm to be collimated. The size realized in practice was slightly too large. Therefore, an aperture stop with a diameter of 0.9 mm was installed. The collimated wavefield was used to illuminate a 14-inch wedge backlight, which was realized using Bragg-based volume gratings with twice the 10x anamorphic magnification. Increasing the beam diameter causes a reduction in the plane wave spectrum. Thus, the plane wave spectrum in front of the backlight, which is ±1 / 12°, is converted into a plane wave spectrum of ±1 / 120°, which is used to illuminate a 14-inch two-panel phase + amplitude SLM. A good reconstruction was obtained.

[0230] However, the reconstruction was tested with a light source size of 0.9 mm (aperture size to be used) and using optimized synchronization of the pulsed laser illumination. Furthermore, a loudspeaker used to implement dynamically randomized phase modulation of the light source in the form of fiber shaking was replaced with a voice coil array. For an initial test, a frequency of 50 Hz and rectangular pulse modulation were used for the fiber shaking.

[0231] The following description concerns a one-dimensional (1D) encoding of subholograms in a spatial light modulation device SLM.

[0232] The application of one-dimensional (1D) encoding of subholograms involves changing the design features of an illumination device used in the direction in which the so-called sweet spot is later formed. The sweet spot, which exists in the plane of an observer's eye, cannot exceed the size of the coherently formed observer window, which exists in the orthogonal direction.

[0233] For example, a lateral deviation of ±5 mm of a beam at a viewing distance of 1 m is equivalent to an angular deviation of ±0.3°, and a lateral deviation of ±5 mm at a viewing distance of 3 m is equivalent to an angular deviation of ±0.1°. This is the angular range that must be spanned by the sweet spot. Thus, the asymmetric size of a light source used is 18:1 for a viewing distance of 1 m and 6:1 for a viewing distance of 3 m.

[0234] One possibility for maintaining the numerical aperture of the light source while providing a line-like configuration is to use an optical fiber array as the light source, which, for example, has a round or even an elliptical shape adapted to the laser diode in the entrance plane and line-like segments at the exit plane. Experimentally tested example:

[0235] A round to linear 7-core multimode fiber was used as the light source arrangement, which had a core diameter of d core = 200 µm and a numerical aperture of NA = 0.22. Due to a 30 µm thick cladding layer, which results in a fiber diameter of 260 µm, a light source line segment, which is an arrangement of several optical fibers along a line segment, with a size of 0.2 mm x 1.58 mm is provided for a one-dimensional (1D) encoded color phase + amplitude SLM, which is implemented using an off-axis parabolic mirror with a focal length of f coll_OAPM = 1 m and a diameter of Ø OAPM= 200 mm. The off-axis parabolic mirror has a numerical aperture of 0.1, which means that the numerical aperture of the round to linear fiber bundle is too large and thus causes a significant loss of energy. Therefore, an enlargement of the exit plane of the optical fiber is necessary. A magnification of M = 2 reduces the numerical aperture of the light source to 0.11 and expands the size to 0.4 mm x 3.16 mm. Within this configuration, using f coll_OAPM= 1 m, where no further beam expansion is applied, a plane wave spectrum of 1 / 60° is equivalent to a light source expansion of 0.29 mm. This means that a slit-like aperture stop is required that does not exceed 0.29 mm in the spatially coherent direction. Using a magnification of M = 2 provides a light source size along the sweet spot direction that is 3.16 mm, which is still slightly too small. However, fiber bundle assemblies tailored to customer requirements are already available. Thus, the sweet spot direction can easily be extended to a size of 8 mm, for example. This further means that, for example,B a round to linear bundle containing 23 fibers and having a core diameter of 150 µm and a cladding layer of 30 µm, with a magnification of M = 2, can be used to produce a light source size of 4.11 mm x 0.3 mm at a numerical aperture of 0.11, which is only slightly larger than that of a collimation unit used.

[0236] A circular to linear beam can be tailored to the customer's requirements to provide a line-shaped light source with an aspect ratio of, for example, 6:1 to 18:1 or even up to 25:1, while providing approximately the same numerical aperture in two directions. Simply using standard cylindrical optical arrays or anamorphic prism pairs can provide line-shaped beam shaping, but this will increase the numerical aperture along the coherent direction, resulting in an avoidable loss of optical power.

[0237] Using a round to linear fiber bundle is advantageous. If a so-called 4f setup is used to image the exit plane of the linear fiber array onto the front focal plane of a collimator, polarization recycling can be provided between two lenses of the 2f setup, which is more like a 2f1 + 2f2 setup when the magnification M ≠ 1.

[0238] Polarization recycling can be achieved using a so-called polarization beam splitter array, which is already used in projectors. The polarization beam splitter is designed as a one-dimensional (1D) segmented polarization beam splitter array arranged between a telescopic array of two parallel cylindrical lens arrays with the same pitch as the polarization beam splitter strips. The structure is organized as follows: line-like fiber end | f1 | achromatic lens 1 | cylindrical lens array 1 | strip-shaped polarization beam splitter array | cylindrical lens array 2 | achromatic lens 2 | f2 | light source plane to be collimated by a collimation unit.

[0239] As an alternative to a strip-shaped polarization beam splitter polarization recycling device, a birefringent plate can be provided at the center of a telescopic array formed by two identical cylindrical lens arrays. A first cylindrical lens array creates a strip-shaped illumination pattern at the entrance plane of the birefringent plate. The exit plane of the birefringent plate has strip-shaped achromatic retarders that provide a π / 2 retarder. The pitch of the strip-shaped so-called π / 2 or λ / 4 retarders is the same as that of the cylindrical lens array. Thus, a single polarization state and a reasonably homogeneous intensity distribution can be achieved at the exit plane of the birefringent plate.

[0240] The polarization recycling array can be shifted to one of the two achromatic lenses used to image the exit plane of the line fiber array onto the front focal plane of the collimation unit. This prevents details or non-homogeneous intensity distributions from being subsequently imaged onto the SLM plane.

[0241] A reduced-complexity arrangement, such as the one described above, utilizes a one-dimensional (1D) engineered diffuser located in the front focal plane of a collimating lens. This engineered diffuser, designed as an engineered diffuser, produces a line-like, flat upper light distribution at the position where the collimating lens is located. The scattering angle of the one-dimensional, line-generating engineered diffuser and the focal length of the collimating lens are selected to provide a beam shaping ratio of, for example, 18:1. A dynamic phase-shifting diffuser is used in the plane of the light source to be collimated by a lens called a collimating lens. The collimating lens must be illuminated to provide a homogeneous intensity distribution. Thus, the diffuser must provide a customized or tailored scattering behavior, which means defined scattering angles along defined directions.

[0242] For example, an initial beam diameter of 1 mm may be provided. When using a collimating lens with a focal length of f BS = 25 mm, the one-dimensional (1D) line-generating engineered diffuser must operate at a scattering angle of ±19.8° and provide a homogeneous intensity distribution, which in this case is an 18 mm wide line at a pitch of 25 mm. For a two-dimensional (2D) encoding, the angular dispersion function used in the plane of the extended light source to be collimated can be close to a symmetric dispersion function, but this is not the case for a one-dimensional (1D) encoding.

[0243] Viewed in the direction of light propagation, a line measuring 18 mm x 1 mm is generated behind the collimating lens used, formed by collimated light. Consequently, a second engineered diffuser, which provides the dynamic statistically randomized phase distribution of the extended light source, is placed behind this collimating lens. If the line generated behind the collimating lens and which illuminates the randomization plane with the dynamic phase of the light source to be collimated is not homogeneous enough, then the first one-dimensional (1D) line-generating engineered diffuser must also be moved at an appropriate speed. This can easily be achieved by mounting the one-dimensional (1D) diffuser on a piezoelectric element (PZT) operating, for example, at > 20 kHz. This can be done for each of the primary light sources to optimize the illumination for each primary color RGB separately.

[0244] A practical solution is to arrange three line-generating light sources in front of three RGB-related entrance planes of a color combining prism, which is provided in front of the achromatic lens that realizes the collimation, as in Fig. 14. Polarization recycling is not required in this arrangement.

[0245] Therefore, homogeneous and cost-effective arrays can be used to provide the tailored plane wave spectrum used to illuminate the SLM.

[0246] The following describes how a high fill factor of the spatial light modulation device SLM can be achieved.

[0247] The fill factor can be increased, for example, by adding beam-shaping functionality to the different planes of the SLM. A direct approach is the use of spherical or aspherical lens or mirror structuring in front of the SLM panel, within the SLM panel, or at the exit plane of the SLM panel, which also includes, for example, the mm range of ≤ 3 mm behind the SLM panel.

[0248] An exemplary embodiment is the use of a microlens array or an array of cylindrical lenses having the pitch of the complex-valued pixel formed by the combination of the phase modulation pixel and the amplitude modulation pixel in front of the SLM. The fill factor FF of the entrance plane of the SLM can be, for example, FF = 0.5. Light that would be reflected back or absorbed without the use of a lens array is transmitted to the apertures of the entrance plane of the SLM. Thus, the energy loss at that point is halved, thereby increasing the energy transfer. Focusing the light increases its divergence, that is, the divergence present behind the focal plane, which in this case is the entrance plane of the SLM. Increased divergence will increase the crosstalk present in the complex-valued SLM.Thus, a polarization application that can reduce crosstalk is advantageous. The reflection plane or mirror plane, which, for example, has a common complex-valued apodization profile or an amplitude apodization profile, can also be provided with a spherical lens function or an aspherical phase function to increase energy transfer. However, this can add excessive complexity in terms of manufacturability, yield, and cost.

[0249] Another possibility is to use a second microlens array behind the exit plane of the complex-valued SLM in addition to the microlens array described above. This increases the energy transfer and also the fill factor. A higher fill factor will provide less energy at the higher far-field diffraction orders of the SLM pattern than a lower fill factor, which will lead to an increase in light intensity outside a viewing window, e.g., the region between the 0th, +1st y, and +1st x diffraction orders. An increase in energy transfer and an increase in the effective fill factor are interrelated.

[0250] The following description concerns a far-field calibration.

[0251] A far-field calibration process can be advantageously applied to a holographic three-dimensional (3D) display device. Far-field calibration is an application that considers the discrete functionality of all components in the display device through which light passes on its path from a primary light source to an entrance plane of a viewer's eye. This means measuring and calibrating the amplitude and phase values ​​of the light reaching the viewing window, or the location where a viewer's eye is located.

[0252] Consequently, the phase and amplitude distributions can be obtained within the viewing window provided by each individual pixel of the SLM. Generally, it is not necessary to calibrate the complex-valued field distribution present in the plane of the observer's entrance pupil and generated by each individual pixel. Choosing characteristic sampling points, which can be, for example, ≤ 1% of the total pixel count, is advantageous due to the fact that most of the characteristics in the display device exhibit only slightly fluctuating distributions.

[0253] As an alternative to far-field calibration, near-field calibration can also be applied. It is also possible to map the exit plane of the complex-valued SLM onto a detector plane for calibration processing. The obtained results can be used directly or as calibration data that propagate numerically to the plane of the viewing window.

[0254] Not only the phase distribution and amplitude distribution are of interest. The values ​​of Δx and Δy can also be measured, where Δx is the lateral displacement of the nominal pixel grid in the x-direction and Δy is the lateral displacement of the nominal pixel grid in the y-direction. A lateral deviation with respect to the nominal pixel grid will produce incorrect phase values ​​in the viewing window.

[0255] These values ​​can be stored in a look-up table (LUT). Individual phase shifts Δφ ijdepending on the values ​​recorded in the look-up table. This means that individual lateral offsets of the nominal pixel grid Δx and Δy of the SLM can be compensated for with respect to their influence on the phase values ​​that must be generated in the plane of the viewing window.

[0256] During far-field calibrations, these local lateral offsets can also be taken into account to provide correction data captured in the look-up table, which is used to correct the SLM's hologram encoding in such a way that the difference between theoretical three-dimensional (3D) holographic images and real, aberration-affected images is reduced. Thus, real values ​​can be considered instead of purely theoretical ones.

[0257] Finally, it should be noted that the above-described embodiments, the embodiments of a complex-valued spatial light modulation device, the embodiments of the illumination device, and the embodiments of the display device according to the invention are to be understood exclusively as illustrating the claimed teaching, but the claimed teaching is not intended to be limited to these embodiments. Combinations of embodiments are possible.

Claims

[1] Display device for a holographic reconstruction, comprising - a spatial light modulation device (SLM) with combined phase modulation pixels (p) and amplitude modulation pixels (a), - a lighting device suitable for illuminating the spatial light modulation device (SLM), - a reflection plane (RP) arranged to generate sufficiently coherent light emitted by the illumination device, which enters the spatial light modulation device (SLM), wherein the light passes through both the phase modulation pixel (p) and the amplitude modulation pixel (a) of the spatial light modulation device (SLM), and wherein the light is reflected by the intermediate reflection plane (RP), wherein the light passing through both the phase modulation pixels (p) and the amplitude modulation pixels (a) is reflected by an intermediate mirror system arranged in the reflection plane (RP), and wherein the mirror segments (M) of the mirror system are arranged opposite the phase modulation pixels (P) and the amplitude modulation pixels (a) in such a way that each mirror segment (M) covers both a part of the phase modulation pixel (p) and a part of the amplitude modulation pixel (a). [2] Display device according to claim 1, characterized by that the phase modulation pixels (p) and the amplitude modulation pixels (a) of the spatial light modulation device (SLM) are laterally combined in the same plane. [3] Display device according to claim 1 or 2, characterized by that oblique illumination of the spatial light modulation device (SLM) is provided. [4] Display device according to one of claims 1 to 3, characterized by that the lighting device is designed as a front light lighting device or a backlight lighting device. [5] Display device according to claim 1, characterized by that the mirror system has mirror segments (M). [6] Display device according to claim 1 or 5, characterized byin that the spatial light modulation device (SLM) has at least one transparent substrate (TRS), wherein the transparent substrate (TRS) has on one side an addressable transmissive layer with the phase modulation pixels (p) and the amplitude modulation pixels (a) and on the other, opposite side a plane which is the reflection plane (RP), wherein the mirror system is arranged in the reflection plane (RP). [7] Display device according to one of claims 1 to 6, characterized by that a polarization-selective element is provided, wherein the polarization-selective element is provided in the light propagation direction at a light exit plane of the spatial light modulation device (SLM). [8] Display device according to claim 7, characterized by that the polarization-selective element is designed as a polarization analyzer or wire grid polarizer. [9] Display device according to one of claims 1 to 8, characterized by that the phase modulation pixels (p) have reflection means (MS), preferably mirror elements, wherein the reflection means (MS) are provided at rear ends of the phase modulation pixels (p) in the direction of light propagation. [10] Display device according to one of claims 1 to 9, characterized by that a polarization filter (pPF) is provided in the plane of the phase modulation pixels (p) and the amplitude modulation pixels (a), wherein the polarization filter (pPF) has polarization filter segments (pPF1, pPF2). [11] Display device according to claim 10, characterized by that the polarization filter segments (pPF1, pPF2) are assigned to the phase modulation pixels (p) and the amplitude modulation pixels (a), wherein the polarization direction of adjacent polarization filter segments (pPF1, pPF2) is orthogonal. [12] Display device according to claim 10, characterized bythat the polarization filter segments (pPF2) are absorbing polarization filter segments provided at light exit openings of the amplitude modulation pixels, or the polarization filter segments (pPF1) are reflective polarization filter segments, preferably wire grid polarizers, provided at the reflective rear ends of the phase modulation pixels (p), seen in the light propagation direction. [13] Display device according to one of claims 1 to 12, characterized by that an apodization filter (Apo), preferably a structured apodization profile layer, and / or a delay element (QWP), preferably an achromatic or apochromatic delay layer, is provided. [14] Display device according to claim 13, characterized bythat the apodization filter (Apo) is a microlens-like structure applied to the mirror system in the reflection plane (RP), or the apodization filter (Apo) is an absorbing alloy composition layer. [15] Display device according to one of claims 1 to 14, characterized by that at least one lens arrangement is provided, wherein the lens arrangement is provided in front of the spatial light modulation device (SLM) in the light propagation direction, wherein the light emitted by the illumination device is bundled and enters inlet openings of the spatial light modulation device (SLM). [16] Display device according to one of claims 1 to 14, characterized by that lens structures for focusing the light are provided in the entrance openings of the spatial light modulation device (SLM). [17] Display device according to one of claims 1 to 16, characterized byin that the spatial light modulation device (SLM) comprises a beam displacement element (SP), preferably a birefringent element, preferably a Savart plate, wherein the transparent substrate (TRS) is designed as the beam displacement element (SP) for displacing the incident light passing through the phase modulation pixels (p) or the amplitude modulation pixels (a). [18] Display device according to one of claims 1 to 17, characterized by that the spatial light modulation device (SLM) is illuminated along different, individually controllable directions (D1, D2). [19] Display device according to one of claims 1 to 18, characterized bythat the spatial light modulation device (SLM) has a backplane (BP) which has clusters with hidden electrodes and additional transistors together with transistors for controlling the pixels, wherein in the light propagation direction the electrodes and the additional transistors are arranged behind the reflection means (MS) of the phase modulation pixels (p) of the spatial light modulation device (SLM). [20] Display device according to one of claims 1 to 19, characterized by that a light-emitting device, preferably an organic light-emitting diode array, is provided in the light propagation direction behind the pixels (p, a) of the spatial light modulation device (SLM), preferably on the non-transparent regions of the phase modulation pixels (p) for generating a two-dimensional functionality of the display device. [21] Display device according to claim 20, characterized bythat the light-emitting device, preferably the organic light-emitting diode array, is formed from clusters of light-emitting zones, wherein a cluster comprises a specific number of pixels (p, a) of the spatial light modulation device (SLM). [22] Display device according to claim 1, characterized by that the spatial light modulation device (SLM) is designed as a liquid crystal (LC)-based spatial light modulation device, a microelectromechanical system (MEMS)-based spatial light modulation device or a multi-quantum well (MQW)-based spatial light modulation device. [23] Display device according to claims 1, 3 or 4, characterized by that the illumination device has at least one volume grating (VG, VG1, VG2) for coupling the light out of the illumination device in the direction of the spatial light modulation device (SLM). [24] Display device according to claim 1, characterized by that the illumination device has at least one controllable, random phase generating element for generating a homogeneous intensity distribution of the light incident on a subsequent collimation element. [25] Display device according to one of claims 1 to 24, characterized by that a field lens (FL) is provided, wherein the field lens (FL) is in particular a combined field lens (cFL) which has at least one volume grating. [26] Display device according to one of claims 1 to 25, characterized by that a vertical tracking unit (VT) and / or a horizontal tracking unit (HT) is provided, wherein the vertical tracking unit (VT) and / or the horizontal tracking unit (HT) preferably has at least one liquid crystal grating. [27] Light modulation device for a display device for generating at least one two-dimensional and / or one three-dimensional representation of a scene or content, comprising phase modulation pixels (p) and amplitude modulation pixels (a), wherein the spatial light modulation device is designed according to a spatial light modulation device (SLM) according to one of claims 1 to 22. [28] Light modulation device according to claim 27, characterized by that at least one transparent substrate (TRS), an addressable transmissive layer having the phase modulation pixels (p) and the amplitude modulation pixels (a), and a reflection plane (RP) are provided. [29] A method for generating a holographic reconstruction using a display device according to any one of claims 1 to 26, characterized by - illuminating phase modulation pixels (p) and amplitude modulation pixels (a) of a spatial light modulation device (SLM) by means of an illumination device with sufficiently coherent light, - passing the light through both the phase modulation pixels (p) and the amplitude modulation pixels (a) of the spatial light modulation device (SLM), and - Reflecting the light by means of an intermediate reflection plane (RP), wherein the reflection plane (RP) is provided in the light propagation direction between the phase modulation pixels (p) and the amplitude modulation pixels (a). [30] Method according to claim 29, characterized by that the light passes through the phase modulation pixels (p) twice.

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