Display device and method

Abstract

Liquid crystal display devices and corresponding methods of displaying a hologram are described. A gray scale value for each pixel of the hologram is received and a pixel voltage is determined based on the gray scale of each pixel of the hologram. During at least one first drive event, the pixels of the display device are driven according to a first representation of the pixel voltage. During at least one second drive event after the at least one first drive event, the pixels of the display device are driven according to a second representation of the pixel voltage. The first representation can be an n-bit representation and the second representation can be an m-bit representation, and n < m. The duration of the at least one first drive event can be shorter than the duration of the at least one second drive event.

Description

Technical Field

[0001] This disclosure relates to display devices. More specifically, this disclosure relates to display devices for displaying holograms, methods for driving display devices to display holograms, and holographic projection systems. Some embodiments relate to head-up displays. Background Technology

[0002] Light scattered from an object contains amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate using well-known interferometry techniques to form a holographic record, or "hologram," including interference fringes. The hologram can be reconstructed by illuminating it with appropriate light to form a two-dimensional or three-dimensional holographic reconstruction or replay image representing the original object.

[0003] Computer-generated holography can numerically simulate interference processes. Computer-generated holograms can be calculated using techniques based on mathematical transformations such as Fresnel or Fourier transforms. These types of holograms are called Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. A Fourier hologram can be considered a Fourier domain / plane representation of an object or a frequency domain / plane representation of an object. For example, computer-generated holograms can also be calculated using coherent ray tracing or point cloud techniques.

[0004] Computer-generated holograms can be encoded on spatial light modulators arranged to modulate the amplitude and / or phase of incident light. For example, optical modulation can be achieved using electrically addressable liquid crystals, optically addressable liquid crystals, or micromirrors.

[0005] Spatial light modulators typically comprise multiple individually addressable pixels, which may also be referred to as cells or elements. The light modulation scheme can be binary, multilevel, or sequential. Alternatively, the device can be sequential (i.e., excluding pixels), so the light modulation can be continuous on the device. Spatial light modulators can be reflective, meaning the modulated light is output as reflected light. Spatial light modulators can also be transmissive, meaning the modulated light is output as transmitted light.

[0006] The system described herein can be used to provide a holographic projector. Such a projector has already been used in head-up displays (HUDs) and head-mounted displays (HMDs), including near-eye devices.

[0007] In devices that use coherent light (such as holographic projectors), a moving diffuser can be used to improve image quality. Summary of the Invention

[0008] Various aspects of this disclosure are defined in the appended independent claims.

[0009] Disclosed herein is a method for driving a liquid crystal display device to display a hologram. The method comprises a first step of receiving a grayscale level value for each pixel of the hologram. The method comprises a second step of determining a pixel voltage based on the grayscale level of each pixel of the hologram. Each pixel voltage value can be determined from the corresponding grayscale level value using a predetermined calibration scheme. The method comprises a third step of driving pixels in a pixel array of the liquid crystal display device according to a first representation of the pixel voltage during at least one first display event. The third step further comprises driving pixels in the pixel array of the liquid crystal display device according to a second representation of the pixel voltage during at least one second driving event. The at least one second driving event is subsequent to the at least one first driving event. That is, the at least one second driving event is temporally later than the at least one first driving event. In the embodiments disclosed herein, the first representation is an n-bit representation. That is, each pixel voltage according to the first representation is an n-digit number. The second representation is an m-bit representation, where n<m. That is, each pixel voltage according to the second representation is an m-digit number.

[0010] According to the present disclosure, a hologram is displayed on a display device, particularly a liquid crystal display device. The display device comprises a plurality of pixels, for example a 2D array of pixels. Each hologram is represented by a plurality of hologram pixel values, for example a 2D array of hologram pixel values. Each hologram pixel value may be a grayscale level converted into a pixel driving voltage according to calibration of the display device. By providing a driving voltage corresponding to a respective hologram pixel value to each pixel in the 2D array of pixels of the display device, each hologram is displayed on the display device. The process of supplying driving voltages to all pixels of a display device for displaying a hologram is referred to herein as a driving event. Each driving event may be considered a complete driving event, since during the event corresponding pixel voltages are written to all pixels in the 2D array of pixels of the display device. Accordingly, a driving event is also referred to herein as a display event.

[0011] The method disclosed herein uses at least one coarse (first) display event to form a holographic reconstruction before pixel illumination. During at least one coarse (first) display event, low bit-depth numbers representing pixel voltages are rapidly transmitted (rapidly written) to a display device to at least initiate liquid crystal movement for each pixel. A low bit-depth number is a number comprising a relatively small number of bits, for example 6 or 8 bits. It may be said that a short display event using low bit-depth numbers provides coarse (or rough) alignment of the liquid crystals in accordance with a hologram. This phase is followed by fine-tuning of the orientation of the liquid crystals using higher bit-depth numbers representing the pixel voltages. A higher bit-depth number is a number having more bits than the low bit-depth number, for example 10 or 12 bits. The technical advance provided by the method is that all pixels in the 2D array of pixels of the display device receive a drive voltage (albeit a low bit-depth drive voltage) in accordance with the hologram more quickly than if higher bit-depth numbers were used for all drive events. It has been found that due to the at least one coarse (first) drive event, the liquid crystals of each pixel reach their final state more quickly. Alternatively, it may be said that the method of the present disclosure allows higher bit-depth numbers to be used to represent pixel voltages without adversely affecting the time taken before holographic reconstruction can commence.

[0012] Accordingly, the proposed display driving method comprises a plurality of display events that together correspond to the display of a display frame (i.e., a hologram). Thus, for display of a video image, the duration of the plurality of display events may correspond to a desired frame rate. In some embodiments, prior to commencement of illumination, the at least one (first) drive event is completed during a ramp-up phase of the display (for each frame), otherwise some pixels of the display device may not be in the correct state and the quality of the holographic reconstruction may be affected. In some embodiments, a time delay is provided between completion of the (temporally first) one of the plurality of drive events to allow the liquid crystals of each pixel to reach their final state.

[0013] The at least one first drive event has a shorter duration than the at least one second drive event because n<m. The time taken to transmit a dataset is proportional to the number of bits per data value / number in the dataset. Accordingly, the at least one first display event is (temporally) shorter than the at least second display event.

[0014] In some embodiments, each first driving event includes addressing each pixel of the liquid crystal display device within a first maximum number of clock counts, and ramping up the voltage on each pixel within a plurality of clock counts corresponding to a first representation of the respective pixel voltage. According to the first representation, the first maximum number of clock counts corresponds to a maximum pixel voltage. Each second driving event includes addressing each pixel of the liquid crystal display device within a second maximum number of clock counts, and ramping up the voltage on each pixel within a plurality of clock counts corresponding to a second representation of the respective pixel voltage. According to the second representation, the second maximum number of clock counts corresponds to a maximum pixel voltage. The first maximum number of clock pulses is less than the second maximum number of clock pulses.

[0015] In these embodiments, during a driving event, each pixel of the liquid crystal display device is addressed within an addressing cycle having a duration corresponding to a maximum number of clock pulses. During the addressing cycle, the pixel is driven by a ramp voltage across the pixel for a ramp cycle whose duration is defined by a plurality of clock pulses corresponding to the desired pixel voltage, up to a maximum number of clock pulses corresponding to the maximum pixel voltage. Therefore, the addressing cycle for each pixel during the first display event is shorter (in time) than the addressing cycle for each pixel during the second display event. Furthermore, when used with a low-bit-depth driving voltage for the first display event, the ramp cycle for each pixel is shorter (in time) for the first display event than for the second display event. Reducing the pixel addressing and / or ramp cycle results in faster pixel driving.

[0016] In the following description, the term "frame" refers to an image in a video rate image sequence. The frame rate is typically 60Hz, meaning that the duration of one frame is 16ms. The next "frame" corresponds to the next image in the video sequence. Therefore, the 16ms "frame interval" corresponds to the time period during which pixel voltages are written and a holographic reconstruction corresponding to the input image must be formed before the next frame can be processed.

[0017] In this specification, the term "drive event" refers to all pixels in a display device on which pixel voltages are transmitted or written. In this context, "all pixels" refers to all pixels in a 2D array of pixels in a display device that are encoded to display a light modulation distribution including a hologram. Those skilled in the art will understand that during each drive event, each pixel in the 2D array of pixels is addressed to display a hologram thereon. Therefore, the term "all addressable pixels" is also used herein to refer to 2D array pixels encoded to display a particular hologram. Thus, the duration of a drive event is the shortest time in which pixel voltages are written to all (addressable) pixels in the display device. As described herein, a frame interval comprises multiple drive events. There may be 10-20 drive events per frame. Each drive event writes a "subframe" to the display device within a "subframe interval" (i.e., the duration of the drive event). The frame interval is typically fixed depending on the frame rate. According to this disclosure, the subframe interval (and therefore the subframe rate) can vary for a given frame's subframes.

[0018] The term "hologram" is used to refer to a record containing amplitude or phase information about an object, or some combination thereof. The term "holographic reconstruction" is used to refer to the optical reconstruction of an object formed by illuminating a hologram. The system disclosed herein is described as a "holographic projector" because the holographic reconstruction is a real image and spatially separate from the hologram. The term "reproduced field" is used to refer to the 2D region within which the holographic reconstruction is formed and fully focused. If the hologram is displayed on a spatial light modulator including pixels, the reproduced field will be repeated in the form of multiple diffraction orders, where each diffraction order is a copy of the zero-order reproduced field. The zero-order reproduced field generally corresponds to the preferred or master reproduced field because it is the brightest reproduced field. Unless otherwise explicitly stated, the term "reproduced field" should be considered to refer to the zero-order reproduced field. The term "reproduced plane" is used to refer to a plane in space containing all reproduced fields. The terms "image," "reproduced image," and "image region" refer to the region of the reproduced field illuminated by the light reconstructed by the hologram. In some embodiments, an “image” may include discrete points, which may be referred to as “image points” or simply as “image pixels” for convenience.

[0019] The terms “encoding,” “writing,” and “addressing” are used to describe the process of providing multiple corresponding control values ​​(or pixel voltages) to multiple pixels of an SLM, each determining the modulation level of the individual pixel. In essence, the pixels of an SLM are configured to “display” a distribution of optical modulation in response to the receipt of multiple control values. Therefore, it can be said that an SLM “displays” a hologram, and a hologram can be considered an array of optical modulation values ​​or levels.

[0020] It has been discovered that acceptable-quality holographic reconstructions can be formed from "holograms" containing only phase information related to the Fourier transform of the original object. Such holographic records can be referred to as pure phase holograms. The embodiments relate to pure phase holograms, but this disclosure is equally applicable to pure amplitude holograms.

[0021] This disclosure is equally applicable to forming holographic reconstructions using amplitude and phase information associated with the Fourier transform of the original object. In some embodiments, this is achieved by using complex modulation of a so-called fully complex hologram containing amplitude and phase information associated with the original object. Because the value (gray level) assigned to each pixel of the hologram has amplitude and phase components, such a hologram may be called a fully complex hologram. The value (gray level) assigned to each pixel can be represented as a complex number with amplitude and phase components. In some embodiments, a fully complex computer-generated hologram is computed.

[0022] The term "phase delay" can be a shorthand for the phase value, phase component, phase information, or simply phase of a pixel in a computer-generated hologram or spatial light modulator. That is, any phase value described is actually a number representing the amount of phase delay provided by that pixel (e.g., in the range of 0 to 2π). For example, a spatial light modulator is described as a pixel having a π / 2 phase value causing a π / 2 radian phase delay in the received light. In some embodiments, each pixel of a spatial light modulator can operate on one of a plurality of possible modulation values ​​(e.g., phase delay values). The term "gray level" can be used to refer to a plurality of available modulation levels. For example, the term "gray level" can be used for convenience to refer to a plurality of available phase levels in a pure phase modulator, even if different phase levels do not provide different shades of gray. For convenience, the term "gray level" can also be used to refer to a plurality of available complex modulation levels in a complex modulator.

[0023] Therefore, a hologram comprises an array of gray levels, i.e., an array of optical modulation values, such as phase delay values ​​or complex modulation values. A hologram is also considered a diffraction pattern because it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light whose wavelength is relative to (typically less than) the pixel spacing of the spatial light modulator. Reference is made herein to combining holograms with other diffraction patterns, such as diffraction patterns used as lenses or gratings. For example, a diffraction pattern used as a grating can be combined with a hologram to translate the playback field on the playback plane, or a diffraction pattern used as a lens can be combined with a hologram to focus the holographic reconstruction onto the playback plane in the near field.

[0024] Although different embodiments and groups of embodiments may be disclosed separately in the detailed description below, any feature of any embodiment or group of embodiments may be combined with any other feature or combination of features of any embodiment or group of embodiments. That is, all possible combinations and substitutions of the features disclosed in this disclosure are contemplated. Attached Figure Description

[0025] The following figures illustrate specific embodiments by way of example only:

[0026] Figure 1 This is a schematic diagram illustrating a reflective SLM that generates holographic reconstruction on a screen;

[0027] Figure 2A The first iteration of the example Gerchberg-Saxton type algorithm is shown;

[0028] Figure 2B The second and subsequent iterations of the example Gerchberg-Saxton type algorithm are shown;

[0029] Figure 2C Alternative second and subsequent iterations of the example Gerchberg-Saxton type algorithm are shown;

[0030] Figure 3 This is a schematic diagram of a reflective LCOS SLM;

[0031] Figure 4 An example scheme for displaying holograms using a DC balanced drive display device is shown;

[0032] Figure 5A -C illustrates an example driving voltage applied to the pixels and common electrode of a liquid crystal cell in a DC balancing scheme to provide corresponding positive and negative electric fields;

[0033] Figure 6A An example scheme for driving a display device to display holographic frames is shown;

[0034] Figure 6B Another example scheme for driving a display device to display holographic frames is shown;

[0035] Figure 6C An example scheme for driving a display device to display holographic frames according to an embodiment is shown, and...

[0036] Figure 7 This is a schematic block diagram illustrating a holographic projection system according to an embodiment.

[0037] In all the accompanying drawings, the same reference numerals will be used to refer to the same or similar parts. Detailed Implementation

[0038] This invention is not limited to the embodiments described below, but extends to the full scope of the appended claims. That is, the invention may be implemented in different forms and should not be construed as limited to the described embodiments, which are illustrated for illustrative purposes.

[0039] Unless otherwise stated, singular terms may include plural forms.

[0040] A structure described as being formed above or below another structure should be interpreted as including situations where the structures are in contact with each other, and also including situations where a third structure is placed between them.

[0041] When describing temporal relationships, such as when the chronological order of events is described as “after,” “following,” “next,” “before,” etc., this disclosure should be considered to include both consecutive and discontinuous events, unless otherwise stated. For example, unless terms such as “exactly,” “immediately,” or “directly” are used, the description should be considered to include discontinuous cases.

[0042] Although the terms “first,” “second,” etc., may be used herein to describe various elements, these elements are not limited by these terms. These terms are used only to distinguish individual elements. For example, without departing from the scope of the appended claims, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element.

[0043] Features of different embodiments may be coupled or combined with each other in part or in whole, and may interoperate differently with each other. Some embodiments may be implemented independently of each other, or may be implemented together in a mutually dependent relationship.

[0044] Optical configuration

[0045] Figure 1 An embodiment is shown in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is a Fourier transform of the object used for reconstruction. Therefore, a hologram can be described as a Fourier domain, frequency domain, or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon (LCOS) device. The hologram is encoded on the spatial light modulator, and a holographic reconstruction is formed at the playback field, such as a light-receiving surface like a screen or diffuser.

[0046] A light source 110 (e.g., a laser or laser diode) is configured to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a generally plane wavefront of the light to be incident on the SLM. Figure 1In this embodiment, the wavefront is oriented off-normally (e.g., two or three degrees away from a plane that is truly orthogonal to the transparent layer). However, in other embodiments, the approximately planar wavefront is provided with normal incidence, and beam splitters are arranged to separate the input and output optical paths. Figure 1 In the illustrated embodiment, the arrangement is such that light from the light source is reflected from the mirrored rear surface of the SLM and interacts with the light modulation layer to form an outgoing wavefront 112. The outgoing wavefront 112 is applied to an optics device including a Fourier transform lens 120, the focal point of which is located at screen 125. More specifically, the Fourier transform lens 120 receives the modulated beam from the SLM 140 and performs a frequency-space transformation to produce a holographic reconstruction at screen 125.

[0047] It is worth noting that in this type of hologram, each pixel of the hologram contributes to the overall reconstruction. There is no one-to-one correlation between a specific point (or image pixel) on the playback field and a specific optical modulation element (or hologram pixel). In other words, the modulated light leaving the optical modulation layer is distributed across the entire playback field.

[0048] In these embodiments, the spatial position of the holographic reconstruction is determined by the refractive power (focusing) of the Fourier transform lens. Figure 1 In the illustrated embodiment, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and performs a Fourier transform optically. Any lens can act as a Fourier transform lens, but the lens's performance will limit the accuracy of the Fourier transform it performs. Those skilled in the art will understand how to use lenses to perform optical Fourier transforms.

[0049] Holographic computation

[0050] In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or a Fourier-based hologram, wherein the image is reconstructed in the far field by utilizing the Fourier transform properties of a positive lens. The Fourier hologram is computed by Fourier transforming the desired light field in the reproduction plane back to the lens plane. The Fourier transform can be used to compute computer-generated Fourier holograms.

[0051] Algorithms such as the Gerchberg-Saxton algorithm can be used to compute Fourier transform holograms. Furthermore, the Gerchberg-Saxton algorithm can be used to compute holograms (i.e., Fourier transform holograms) in the Fourier domain based on pure amplitude information in the spatial domain (e.g., photographs). This effectively "retrieves" phase information related to the object from pure amplitude information in the spatial domain. In some embodiments, the Gerchberg-Saxton algorithm or its variants are used to compute computer-generated holograms from pure amplitude information.

[0052] The Gerchberg-Saxton algorithm takes into account the fact that the intensity cross section I of the beams in planes A and B is known. A (x,y) and I B (x,y) and I A (x,y) and I B The case where (x,y) is correlated via a single Fourier transform. For a given intensity cross section, the approximate phase distribution Ψ in planes A and B is obtained. A (x,y) and Ψ B (x,y). The Gerchberg-Saxton algorithm finds a solution to the problem by following an iterative process. More specifically, the Gerchberg-Saxton algorithm iteratively applies spatial and spectral constraints while repeatedly transferring the representation I between the spatial and Fourier (spectral or frequency) domains. A (x,y) and I B A dataset (x, y) containing amplitude and phase. A corresponding computer-generated hologram in the spectral domain is obtained through at least one iteration of the algorithm. The algorithm is convergent and arranged to produce a hologram representing the input image. The hologram can be a pure amplitude hologram, a pure phase hologram, or a fully complex hologram.

[0053] In some embodiments, the pure phase hologram is computed using an algorithm based on the Gerchberg-Saxton algorithm, such as the algorithm described in British Patents 2498170 or 2501112, the entire contents of which are incorporated herein by reference. However, the embodiments disclosed herein are described by way of example only when calculating pure phase holograms. In these embodiments, the Gerchberg-Saxton algorithm retrieves the phase information Ψ[u,v] of the Fourier transform of a dataset, which produces known amplitude information T[x,y], where the amplitude information T[x,y] represents the target image (e.g., a photograph). Since amplitude and phase are inherently combined in the Fourier transform, the transformed amplitude and phase contain useful information about the accuracy of the computed dataset. Therefore, the algorithm can be used iteratively with feedback of amplitude and phase information. However, in these embodiments, the pure phase information Ψ[u,v] is used as a hologram to form a holographic representation of the target image at the image plane. The hologram is a dataset of phase values ​​(e.g., a 2D array).

[0054] In other embodiments, an algorithm based on the Gerchberg-Saxton algorithm is used to compute a fully complex hologram. A fully complex hologram is a hologram having amplitude and phase components. A hologram is a dataset (e.g., a 2D array) comprising an array of complex data values, where each complex data value includes an amplitude component and a phase component.

[0055] In some embodiments, the algorithm processes complex data, and the Fourier transform is a complex Fourier transform. Complex data can be viewed as comprising (i) real and imaginary components, or (ii) amplitude and phase components. In some embodiments, the two components of the complex data are processed differently at different stages of the algorithm.

[0056] Figure 2A A first iteration of an algorithm for computing a pure phase hologram, according to some embodiments, is shown. The input to the algorithm is an input image 210 comprising a 2D array of pixel or data values, where each pixel or data value is an amplitude or oscillation value. That is, each pixel or data value of the input image 210 does not have a phase component. Therefore, the input image 210 can be considered as a pure amplitude or pure oscillation or pure intensity distribution. An example of such an input image 210 is a photograph or a frame of a video comprising a time series of frames. The first iteration of the algorithm begins with a data formation step 202A, which includes assigning random phase values ​​to each pixel of the input image using a random phase distribution (or random phase seed) 230 to form an initial complex dataset, where each data element of the dataset includes both amplitude and phase. In other words, the initial complex dataset represents the input image in the spatial domain.

[0057] First processing block 250 receives an initial complex dataset and performs a complex Fourier transform to form a complex dataset of Fourier transforms. Second processing block 253 receives the complex dataset of Fourier transforms and outputs a hologram 280A. In some embodiments, hologram 280A is a pure phase hologram. In these embodiments, second processing block 253 quantizes each phase value and sets each amplitude value to 1 to form hologram 280A. Each phase value is quantized according to the phase level that can be represented on the pixel of the spatial light modulator that will be used to "display" the pure phase hologram. For example, if each pixel of the spatial light modulator provides 256 different phase levels, each phase value of the hologram is quantized to one of the 256 possible phase levels. Hologram 280A is a pure phase Fourier hologram representing an input image. In other embodiments, hologram 280A is a fully complex hologram comprising an array of complex data values ​​(each including an amplitude component and a phase component) derived from the received complex dataset of Fourier transforms. In some embodiments, the second processing block 253 constrains each complex data value to one of a plurality of permissible complex modulation levels to form a hologram 280A. The constraint step may include setting each complex data value to the closest permissible complex modulation level in the complex plane. The hologram 280A can be said to represent an input image in the spectral, Fourier, or frequency domain. In some embodiments, the algorithm stops at this point.

[0058] However, in other embodiments, the algorithm continues, such as Figure 2AAs shown by the dashed arrow in the image. In other words, follow... Figure 2A The steps indicated by the dashed arrows are optional (i.e., not essential for all embodiments).

[0059] The third processing block 256 receives the modified complex dataset from the second processing block 253 and performs an inverse Fourier transform to form a complex dataset with an inverse Fourier transform. The complex dataset with the inverse Fourier transform can be said to represent the input image in the spatial domain.

[0060] The fourth processing block 259 receives the complex dataset of the inverse Fourier transform and extracts the distribution of amplitude values ​​211A and the distribution of phase values ​​213A. Optionally, the fourth processing block 259 evaluates the distribution of amplitude values ​​211A. Specifically, the fourth processing block 259 can compare the distribution of amplitude values ​​211A of the complex dataset of the inverse Fourier transform with the input image 510, which itself is, of course, the distribution of amplitude values. If the difference between the distribution of amplitude values ​​211A and the input image 210 is sufficiently small, the fourth processing block 259 can determine that the hologram 280A is acceptable. That is, if the difference between the distribution of amplitude values ​​211A and the input image 210 is sufficiently small, the fourth processing block 259 can determine that the hologram 280A is a sufficiently accurate representation of the input image 210. In some embodiments, for comparison purposes, the distribution of phase values ​​213A of the complex dataset of the inverse Fourier transform is ignored. It will be understood that any number of different methods can be used to compare the distribution of amplitude values ​​211A with the input image 210, and this disclosure is not limited to any particular method. In some embodiments, the mean squared error is calculated, and if the mean squared error is less than a threshold, the hologram 280A is considered acceptable. If the fourth processing block 259 determines that the hologram 280A is unacceptable, further iterations of the algorithm can be performed. However, this comparison step is not required, and in other embodiments, the number of iterations of the algorithm performed is predetermined, preset, or user-defined.

[0061] Figure 2B This represents the second iteration of the algorithm and any further iterations of the algorithm. The distribution of the phase values ​​213A from previous iterations is fed back through the algorithm's processing block. Distributions of amplitude values ​​211A are rejected, favoring the distribution of amplitude values ​​of the input image 210. In the first iteration, data formation step 202A forms a first complex dataset by combining the distribution of amplitude values ​​of the input image 210 with the random phase distribution 230. However, in the second and subsequent iterations, data formation step 202B includes forming a complex dataset by combining (i) the distribution of phase values ​​213A from previous iterations of the algorithm with (ii) the distribution of amplitude values ​​of the input image 210.

[0062] Then, with reference Figure 2AThe same method described is handled by Figure 2B The complex dataset formed in step 202B is used to form the second iterative hologram 280B. Therefore, the description of this process will not be repeated here. The algorithm can stop when the second iterative hologram 280B has been computed. However, any number of further iterations of the algorithm can be performed. It will be understood that the third processing block 256 is only needed if a fourth processing block 259 is required or further iterations are needed. The output hologram 280B generally improves with each iteration. However, in practice, a point is often reached where measurable improvement is no longer observable, or the positive benefits of performing further iterations are offset by the negative impact of the additional processing time. Therefore, the algorithm is described as iterative and convergent.

[0063] Figure 2C This represents an alternative embodiment for the second and subsequent iterations. The distribution of the phase value 213A from the previous iteration is fed back through the algorithm's processing block. The distribution of the amplitude value 211A is rejected, favoring an alternative distribution of the amplitude value. In this alternative embodiment, the alternative distribution of the amplitude value is derived from the distribution of the amplitude value 211 from the previous iteration. Specifically, processing block 258 subtracts the distribution of the amplitude value of the input image 210 from the distribution of the amplitude value 211 from the previous iteration, scales the difference by a gain factor α, and subtracts the scaled difference from the input image 210. This is mathematically expressed by the following equation, where the subscript text and numbers represent the iteration number:

[0064] R n+1 [x,y]=F'{exp(iψ n [u,v])}

[0065] ψ n [u,v]=∠F{η·exp(i∠R n [x,y])}

[0066] η=T[x,y]-α(|R n [x,y]|-T[x,y])

[0067] in:

[0068] F' is the inverse Fourier transform;

[0069] F is the forward Fourier transform;

[0070] R[x,y] is the complex number dataset output by the third processing block 256;

[0071] T[x,y] is the input or target image;

[0072] ∠ is the phase component;

[0073] Ψ is a pure phase hologram 280B;

[0074] η is a new distribution of amplitude value 211B; and

[0075] α is the gain factor.

[0076] The gain factor α can be fixed or variable. In some embodiments, the gain factor α is determined based on the size and rate of the input target image data. In some embodiments, the gain factor α depends on the number of iterations. In some embodiments, the gain factor α is only a function of the number of iterations.

[0077] In all other respects, Figure 2C Implementation examples and Figure 2A and Figure 2B The implementation is the same. It can be said that a pure phase hologram Ψ(u,v) includes the phase distribution in the frequency or Fourier domain.

[0078] In some embodiments, a spatial light modulator is used to perform a Fourier transform. Specifically, holographic data is combined with second data that provides optical power. That is, the data written into the spatial light modulator includes holographic data representing an object and lens data representing a lens. When displayed on the spatial light modulator and illuminated with light, the lens data simulates a physical lens—that is, it focuses light in the same way as a corresponding physical optical element. Therefore, the lens data provides optical power or focusing power. In these embodiments, [the following can be omitted] Figure 1A physical Fourier transform lens 120 is used. Data representing the lens is known. This data can be referred to as a software lens. For example, a purely phase lens can be formed by calculating the phase delay caused by the optical path length at each point of the lens due to its refractive index and spatial variation. For example, the optical path length at the center of a convex lens is greater than the optical path length at the edge of the lens. A purely amplitude lens can be formed from Fresnel zone plates. In the field of computer-generated holography, it is also known how to combine data representing the lens with a hologram to perform a Fourier transform of the hologram without requiring a physical Fourier lens. In some embodiments, the lensed data is combined with the hologram by simple addition, such as simple vector addition. In some embodiments, a physical lens is used in combination with a software lens to perform the Fourier transform. Alternatively, in other embodiments, the Fourier transform lens is omitted entirely, allowing holographic reconstruction to occur in the far field. In further embodiments, the hologram can be combined with grating data—i.e., data arranged to perform grating functions such as image steering—in the same manner. Again, how to calculate such data is known in the art. For example, a pure phase grating can be formed by modeling the phase delay caused by each point on the surface of the blazed grating. A pure amplitude grating can be simply superimposed on a pure amplitude hologram to provide angular steering for holographic reconstruction. The second data providing lensing and / or steering may be referred to as an optical processing function or optical processing pattern to distinguish it from the holographic data, which may be referred to as an image forming function or image forming pattern.

[0079] In some embodiments, the Fourier transform is performed jointly by a physical Fourier transform lens and a software lens. That is, the software lens provides some of the optical power that contributes to the Fourier transform, while one or more physical optics provide the remaining optical power that contributes to the Fourier transform.

[0080] Hologram calculators, such as hologram engines, can be configured to compute each hologram. A hologram can be a computer-generated hologram retrieved from a memory containing multiple holograms, or it can be computed from a target image during real-time processing of a video stream, such as an image. Holograms can be generated by a computer using an iterative phase retrieval algorithm, which can be based on the Gerchberg-Saxton algorithm as described above. The iterative process, including forward and inverse Fourier transforms, results in substantially uniform use of available / allowed gray levels. The hologram computation process can include at least one forward Fourier transform and at least one inverse Fourier transform. The algorithm can iterate more than three times. Therefore, a hologram is a phase hologram, meaning that each gray level includes a phase delay value. The number of gray levels (e.g., phase) can be 2^n, where n is an integer, optionally greater than 3.

[0081] In some embodiments, a real-time engine is provided, arranged to receive image data using an algorithm and compute holograms in real time. In some embodiments, the image data is video comprising a sequence of image frames. In other embodiments, the holograms are pre-computed, stored in computer memory, and retrieved as needed for display on an SLM. That is, in some embodiments, a library of predetermined holograms is provided.

[0082] The embodiments described herein are by way of example only and involve Fourier holography and Gerchberg-Saxton type algorithms. This disclosure is equally applicable to Fresnel holography and Fresnel holograms that can be computed using similar methods. This disclosure is also applicable to holograms computed using other techniques, such as point cloud-based methods.

[0083] Optical modulation

[0084] Spatial light modulators can be used to display diffraction patterns, including computer-generated holograms. If the hologram is a pure phase hologram, a spatial light modulator is needed to modulate the phase. If the hologram is a fully complex hologram, a spatial light modulator that modulates both the phase and amplitude can be used, or a first spatial light modulator that modulates the phase and a second spatial light modulator that modulates the amplitude can be used.

[0085] In some embodiments, the light modulation element (i.e., pixel) of the spatial light modulator is a cell comprising liquid crystal. That is, in some embodiments, the spatial light modulator is a liquid crystal device in which the optically active component is liquid crystal. Each liquid crystal cell is configured to selectively provide multiple light modulation levels. That is, each liquid crystal cell is configured at any time to operate at one light modulation level selected from multiple possible light modulation levels. Each liquid crystal cell can be dynamically reconfigured to a light modulation level different from the multiple light modulation levels. In some embodiments, the spatial light modulator is a reflective liquid crystal on silicon (LCOS) spatial light modulator, but this disclosure is not limited to this type of spatial light modulator.

[0086] LCOS devices provide a dense array of light-modulating elements or pixels within a small aperture (e.g., a few centimeters wide). Pixels are typically about 10 micrometers or smaller, resulting in a diffraction angle of a few degrees, meaning the optical system can be compact. The small aperture of an LCOS SLM is much easier to fully illuminate than the larger apertures of other liquid crystal devices. LCOS devices are typically reflective, meaning the circuitry driving the LCOS SLM pixels can be buried beneath the reflective surface. This results in a higher aperture ratio. In other words, the pixels are densely packed, meaning there are virtually no dead zones between pixels. This is advantageous because it reduces optical noise in the playback field. LCOS SLMs use a silicon substrate, which has the advantage of optically flat pixels. This is particularly important for phase modulation devices.

[0087] The following are just examples for reference. Figure 3 To describe a suitable LCOS SLM, an LCOS device is formed using a single-crystal silicon substrate 302. It has a 2D array of square planar aluminum electrodes 301, spaced apart by gaps 301a, arranged on the upper surface of the substrate. Each electrode 301 can be addressed by circuitry 302a buried in the substrate 302. Each electrode forms its own planar mirror. An alignment layer 303 is disposed on the electrode array, and a liquid crystal layer 304 is disposed on the alignment layer 303. A second alignment layer 305 is disposed on a planar transparent layer 306, for example, made of glass. A single transparent electrode 307, for example made of ITO, is disposed between the transparent layer 306 and the second alignment layer 305.

[0088] Each square electrode 301, together with the covered area of ​​the transparent electrode 307 and the intermediate liquid crystal material, defines a controllable phase modulation element 308, commonly referred to as a pixel. Taking into account the space between pixels 301a, the effective pixel area, or fill factor, is the percentage of the total number of optically active pixels. By controlling the voltage applied to each electrode 301 relative to the transparent electrode 307, the properties of the liquid crystal material of the individual phase modulation elements can be altered, thereby providing a variable delay for light incident upon them. The effect is to provide pure phase modulation to the wavefront, i.e., without amplitude effects. As described above, it is known in the art how to calibrate liquid crystal display devices, such as the described LCOS SLM, to provide multiple quantized phase levels corresponding to permissible modulation levels / grayscale levels. In particular, a calibration process is used to determine the control voltage to be applied to each (pixel) electrode 301 relative to the (common) transparent electrode 307 to achieve each permissible phase level.

[0089] The described LCOS SLM outputs spatially modulated light in a reflective manner. The advantage of a reflective LCOS SLM is that the signal lines, grating lines, and transistors are located below the mirror, resulting in a high fill factor (typically greater than 90%) and high resolution. Another advantage of using a reflective LCOS spatial light modulator is that the thickness of the liquid crystal layer can be half that required when using a transmissive device. This significantly improves the switching speed of the liquid crystal (a key advantage for projecting moving video images). However, the teachings of this disclosure can also be implemented using a transmissive LCOS SLM.

[0090] Drive LCD display devices

[0091] In the field of liquid crystal displays (LCDs), it is well known that "field inversion" is crucial for maintaining the performance characteristics of liquid crystal cells. Specifically, it is common practice to repeatedly reverse the polarity of the voltage applied to the liquid crystal cell to repeatedly reverse (i.e., invert) the direction of the electric field. For example, the voltage between the common electrode and the pixel electrode can be positive in the first frame and negative in the second frame. Equal but opposite electric fields in both frames result in the same grayscale level, but ensure that the liquid crystal molecules are "DC balanced" and do not "stick together." In this example, the same image is displayed twice: once with a positive electric field and once with a negative electric field. The number of times the image is displayed using a positive field should equal the number of times it is displayed using a negative field to achieve DC balance. There is a clear bias in this field regarding deviations from this rule.

[0092] Each pixel of a liquid crystal display device can be a Freedericks cell comprising nematic liquid crystal. In other words, each pixel can include nematic liquid crystal arranged to perform Freedericks transitions in response to a driving signal. When a voltage (i.e., a potential difference) is applied to the cell, the liquid crystal molecules arrange themselves to transition from a planar state to a vertically aligned state. The voltage can be positive or negative. The liquid crystal molecules respond to positive or negative voltages in the same way. Therefore, field inversion can be achieved simply by reversing the voltage. Using a common electrode voltage V... COM Many methods for achieving this are known. The driving signal consists of the voltage difference for each pixel, where the voltage difference is the corresponding pixel electrode voltage (V). PIXEL Subtract the common electrode voltage (V) COM If the driving signal is positive, then each voltage difference is positive, resulting in a positive electric field. If the driving voltage is negative, then each voltage difference is negative, resulting in a negative electric field.

[0093] As described above, a "driving event" corresponds to a shortest time window within which pixel voltages can be written to all (addressable) pixels of the display device to display a light modulation distribution including a specific hologram. The duration of the driving event is less than the frame rate required to display video (i.e., an image sequence at the video frame rate). Therefore, each liquid crystal pixel of the display device can be driven multiple times during the display of the same image frame. Each image frame constituting the image frame sequence of the video can be divided into multiple subframes, where each subframe relates to a different representation of the same image frame. According to this disclosure, the driving events of each frame are grouped into subframes. Based on each driving event of a subframe, each subframe is effectively written to pixels multiple times during the subframe period. Each subframe can be DC balanced by providing an equal number of positive and negative field driving events during the subframe period. In some embodiments, the field polarity of the driving events is continuously reversed to achieve DC balance of the subframe. In some embodiments, each subframe includes an even number of driving events. Each driving event can be equal to and opposite to the subframe.

[0094] DC Balance

[0095] Figure 4 A scheme for driving a pixel array in a liquid crystal display device to display an image, hologram, or frame is shown to illustrate the process of DC balancing. The x-axis represents time, and the y-axis represents the potential difference across the liquid crystal, i.e., the potential difference between the common electrode and the pixel electrode. The potential difference can be positive (represented here by the voltage above the x-axis) or negative (represented here by the voltage below the x-axis). Four subframes are shown, each comprising six driving events. Each subframe corresponds to an image representation of an image sequence for display (e.g., projection from a holographic projector). The image sequence can be a video-rate image sequence forming a moving image. Figure 4 Each shaded rectangle represents a driving event. Each driving (or display) event involves displaying a pattern on a pixel on the display device. Therefore, each display event involves applying a separate pixel voltage to each pixel of the display device. Each pixel voltage determines the local behavior of the liquid crystal, such as orientation. Because liquid crystal is birefringent, each pixel voltage corresponds to a light modulation value, such as a phase modulation value. (For illustration only, in...) Figure 4 The diagram shows a larger time interval between subframes than between display events within the same subframe. In reality, the time interval between display events may be uniform.

[0096] In this example, each subframe comprises six display events. Each display event in the subframe corresponds to the same pattern used for display (e.g., projection from a holographic projector). In effect, the same pattern is displayed six times within the subframe interval. It can be said that the pixels of the display device are refreshed five times per subframe to form a total of six display events per pattern (or per subframe). It will be noted that the polarity of the potential difference alternates in each display event. In this example, each subframe includes three display events using a positive potential difference, interleaved with three display events using a negative potential difference. The concept of displaying each pattern using a positive or negative potential difference is further described below with reference to FIG5. To avoid confusion, the polarity of the potential difference applied to each pixel in the pixel array during the display event is the same. That is, during the driving event, all (addressable) pixels are either subjected to a positive potential difference or all (addressable) pixels are subjected to a negative potential difference. The terms positive and negative are used here primarily to reflect that the direction of the potential difference is reversed. As those skilled in the art will understand, for successive driving events, it is not necessary for the polarity of the potential difference, and therefore the polarity of the electric field, to continuously reverse in order to achieve DC balance of the liquid crystal. For example, each subframe may include three consecutive positive field driving events, followed by three consecutive negative field display events. A key principle accepted in the art is that the number of positive field display events in each frame must be equal to the number of negative field display events in each frame—regardless of the order of the positive and negative field display events in subframes or frames. This ensures so-called DC balance and prevents liquid crystal adhesion.

[0097] Each image can be displayed using either a positive or negative electric field. According to a scheme previously determined through calibration, image pixel values ​​(e.g., light modulation values) are converted into pixel voltages, as described above and known in the art. Each light modulation value can be achieved by applying a positive or negative voltage to the pixel. In other words, using a positive electric field ( Figure 5A or negative electric field Figure 5B It can achieve specific liquid crystal orientation (i.e., light modulation value). Figure 5A and 5B The common electrode voltage V is shown. COM and pixel electrode voltage V PIXEL The direction of the potential difference between the common electrode and the pixel electrode determines the direction of the electric field on the liquid crystal, as shown by arrow E. The liquid crystal director responds to the electric field, but the magnitude of the electric field determines the orientation of the liquid crystal director, not its polarity. Since the potential difference between positive and negative electric fields is the same (i.e., 3V), the response (i.e., orientation) of the liquid crystal molecules in the layer between the common electrode and the pixel electrode is the same, as shown by arrow E. Figure 5A and 5B As shown.

[0098] Figure 5C A common electrode voltage of 6V is shown. If a pixel voltage between 6V and 12V is used to represent a pattern, a positive electric field is induced in the liquid crystal. If a pixel voltage between 0V and 6V is used to represent a pattern, a negative electric field is induced in the liquid crystal. Figure 5 illustrates how the same light modulation level (i.e., the same liquid crystal response / or orientation) can be achieved using either a positive or negative electric field. Many techniques are known in the art for manipulating pixel voltages and / or common voltages to provide positive and negative field display events—some of these methods involve using a fixed V... COM (according to Figure 5A and 5B V COM (6V in both positive and negative field cases), other methods involve using variable V. COM The method of providing complementary positive and negative fields is not important in the context of this disclosure.

[0099] Driving scheme with variable subframe rate

[0100] Some embodiments involve a so-called "ramp pixel driver scheme," in which the voltage on each pixel ramps up over time until the desired gray level is achieved. During a so-called "ramp cycle," the gate of the pixel (transistor) is turned on, and charge is driven to the pixel within a plurality of clock pulses corresponding to the gray level. When the gate is turned off, the ramp cycle ends; however, the charge applied during the ramp cycle remains on the pixel. Conventional schemes for driving liquid crystal display devices to display holograms involve driving events of fixed duration, depending on the number of clock pulses required to achieve the maximum gray level. Therefore, in a ramp pixel driver scheme, each pixel is addressed for a fixed duration (here referred to as an "addressing cycle") corresponding to the maximum number of clock pulses, such that the clock pulses can continue to reach their maximum value even when the gate may be turned off at the end of the ramp cycle. Typically, all pixels in a row in an image (or pixel array) are addressed in parallel, thus sharing a common addressing cycle. However, each pixel in a row has its own ramp period, which is less than or equal to the addressing period (i.e., the gate of each pixel in that row can be turned off at different times), depending on the desired gray level to be written. The total number of clock pulses corresponding to the maximum value defines the so-called "row time," which is the time required to drive all (addressable) pixels in a row. Typically, the duration of the driving event for a subframe is the row time multiplied by the number of rows in the image. In this disclosure, the number of clock pulses required to achieve the maximum gray level (i.e., the maximum pixel voltage) using a ramp pixel driver scheme is referred to as the "maximum number of clock pulses for pixel driving" or simply the "maximum number of clock pulses."

[0101] Figure 6A A scheme for driving a liquid crystal display device to display a hologram, according to a first comparative example, is illustrated. Specifically, the scheme comprises a time sequence of six consecutive display events (corresponding to a full frame), wherein each display event writes a value to all pixels of the display device (corresponding to a subframe). In this example, as described above, for DC balancing, the six display events alternate consecutively between positive and negative field display events of the same hologram. Specifically, the first display event 610a, the third display event 610b, and the fifth display event 610c in the sequence each apply a positive potential difference between a pixel and a common electrode to provide a positive field, as described above regarding... Figure 5A As described above, the second display event 620a, the fourth display event 620b, and the sixth display event 620c in this sequence each apply a negative potential difference between the pixel and the common electrode to provide a negative field, as described above. Figure 5BAs described above, each display event (subframe) has the same duration, which depends on the maximum number of clock pulses required to achieve the maximum permissible grayscale level. Illumination event 600 begins at the end of a time interval t following the completion of the first display event 610a to display the hologram. In illumination event 600, the display device is illuminated to form a holographic reconstruction of the displayed hologram. The time interval t is predetermined based on the response time of the liquid crystal.

[0102] Specifically, the time interval t represents the time taken for the liquid crystal to be correctly aligned according to the desired gray level. The time interval t is measured from the end of the first subframe because this is the first moment when all (addressable) pixels of the display device have been written with the hologram / subframe data. Therefore, the time interval t allows the last pixel to be written during the first display event 610a to respond and be correctly aligned according to the applied pixel value (i.e., pixel voltage / gray level).

[0103] Figure 6B Another scheme for driving a liquid crystal display device to display a hologram, according to the second comparative example, is shown. Specifically, this scheme includes a time series of two consecutive display events (corresponding to a full frame), wherein each display event writes a value to all pixels of the display device (corresponding to a subframe). As described above, the display events include a positive display event and a negative display event for DC balancing. Specifically, the first display event 630a can be as follows: Figure 5A The positive display event of the hologram shown is shown, while the second display event 640a can be as follows: Figure 5B The negative display events of the (same) hologram are shown. As described above, each display event (subframe) has the same duration, which depends on the maximum number of clock pulses. Illumination event 600 begins at the end of time interval t after the completion of the first display event 630a to display the hologram. Figure 6B In the second comparison example, the maximum number of clock pulses is greater than Figure 6A The maximum number of clock pulses in the first comparison example. Therefore, Figure 6B The duration of each display event is longer than Figure 6A The duration of each display event is long. However, in Figure 6B In the second comparison example, the time interval t is compared with Figure 6A The example is the same because the time interval t is a function of the liquid crystal, not the driving scheme.

[0104] As mentioned above, Figure 6B In the second comparison example, each display event 630a, 640a has a ratio Figure 6AIn the first comparative example, each display event 610-c, 620a-c has a longer duration. This allows for an increase in the maximum number of clock pulses. The longer subframe interval / display event means that the higher bits of the pixel voltage value represent all pixels that can be written to the display device. Therefore, more precise pixel voltage values ​​corresponding to grayscale levels can be written to the display device, resulting in higher resolution images.

[0105] Figure 6C An example scheme for driving a liquid crystal display device to display a hologram according to an embodiment of the present disclosure is shown. Specifically, the scheme includes a time sequence of four consecutive display events (corresponding to a full frame), wherein each display event writes a value to all pixels of the display device (corresponding to a subframe). As described above, in the example shown, the display events include an equal number of positive and negative field display events for the same hologram used for DC balancing. Specifically, the first display event 650a and the third display event 670a in the sequence can be as follows: Figure 5A The positive display event shown is shown, while the second display event 660a and the fourth display event 680a can be as follows: Figure 5B The negative display event is shown. Compared to the schemes in the first and second comparison examples above, the display event (subframe) can have a variable duration. For DC balancing, the paired positive and negative display events (subframes) should ideally have the same duration. Figure 6C In the scheme, each of the first display event 650a and the second display event 660a has a first duration, which depends on a first maximum number of clock pulses used for pixel driving. Each of the third display event 670a and the fourth display event 680a has a second duration, which depends on a second maximum number of clock pulses used for pixel driving. The first durations of the first and second display events 650a and 660a are less than the second durations of the third and fourth display events 670a and 680a. Therefore, the first maximum number of clock pulses used for pixel driving is less than the second maximum number of clock pulses used for pixel driving. In the example shown, the first maximum number of clock pulses corresponds to... Figure 6A The first comparison example uses the maximum number of clock pulses in the driving event, and the second maximum number of clock pulses corresponds to the maximum number of clock pulses used in the driving event. Figure 6B The second comparative example uses the maximum number of clock pulses in a longer-duration driving event. Illumination event 600 begins at the end of time interval t after the completion of the first display event 650a to display the hologram. Figure 6C In the scheme shown, the time interval t and Figure 6A and 6B The same as in the example.

[0106] according to Figure 6CIn this embodiment, the maximum number of clock pulses used to drive the pixel for the event is varied while maintaining the same clock rate. Therefore, contrary to conventional thinking, the driving event (subframe) can have different durations within the frame interval (full frame), and thus the time spent writing the subframe to the display device is variable. This is because, according to the invention, the resolution of the pixel voltage value (e.g., bit-level representation) of the pixel written to the display device is varied. This leads to several advantages, which are further described below.

[0107] In particular, Figure 6C In the embodiment, the duration of each of the first pair of (DC-balanced) driving events 650a, 660a (subframes) is reduced. This can be achieved, for example, by writing pixel voltage values ​​with a reduced number of bits or a "reduced bit depth" (i.e., a reduced precision / resolution pixel voltage value). For example, the calculated pixel value may have n bits (i.e., an n-bit representation), and can be reduced to m bits (i.e., an m-bit representation) by removing bits (e.g., removing the two least significant bits). By reducing the duration of the first driving event 650a, the hologram is written to the pixels of the display device more quickly, and the inventors have found that the liquid crystal reaches a stable state more quickly within the frame interval. This results in faster and more stable hologram display. Furthermore, each driving event corresponds to a subframe of the frame displaying the hologram. By including the first driving events 650a, 660a with shorter durations, a greater number of subframes can be displayed during the frame interval of the hologram display. Many advantages relate to increasing the number of subframes per frame. In particular, the inventors have discovered that image quality can be improved by displaying holograms differently in different subframes, for example by changing the tiling scheme used to display the holograms as described below.

[0108] Therefore, increasing the total number of subframes per frame allows for the display of a larger number of different subframes and subframe formats during frame intervals, thereby improving image quality. Furthermore, since the pixel ramp period / addressing period is faster during m-bit subframes, illumination events 600 can start earlier in the frame interval and can have a longer duration.

[0109] As mentioned above Figure 6AThe reduction in the duration of the driving events reduces the maximum number of clock pulses used for pixel driving. According to the invention, a reduced / shallower bit representation of the pixel voltage value can be written during the first and second driving events 650a, 660a. This, in itself, results in a reduction in resolution / image quality. However, according to this disclosure, further driving events are provided, and for these further driving events, the bit depth increases. Therefore, the duration of the driving events is variable during the frame interval. Thus, in the illustrated embodiment, the duration of subsequent driving events is increased to allow for a larger maximum number of clock pulses used for pixel driving. Therefore, a higher / deeper bit representation of the pixel voltage value can be written during the third and fourth driving events 670a, 680a. Therefore, high-resolution pixel values ​​are written to the display device for at least a portion of the frame interval, so that the resolution of the holographic reconstruction is not compromised.

[0110] The example scheme for driving a liquid crystal display device to display a hologram according to the embodiments can be implemented in various ways, such as Figure 6C Example schemes are provided. In one embodiment, the driving scheme can be implemented using driving circuitry having two or more modes or operations. Specifically, the driving circuitry may have at least a first low-bit depth mode for driving pixels with a reduced / shallower bit representation of their pixel values ​​(e.g., the least significant bit removed), and a second high-bit depth mode for driving pixels with a higher bit representation of their pixel values ​​(e.g., calculated by a holographic calculation algorithm). The driving circuitry may be integrated within the display device (e.g., LCOS) or may be external (e.g., part of the display device's combined controller / driver).

[0111] In low bit-depth mode, the driving circuitry can operate by providing a relatively high drive current to achieve (maximum) pixel voltage within a shorter addressing cycle. The shorter addressing cycle corresponds to the maximum number of clock pulses required to drive a pixel using a pixel value with a reduced number of bits (e.g., m bits). Therefore, in low bit-depth mode, row time is low because all pixels in a row are written within a shorter common addressing cycle. Consequently, driving events (including the sum of row times for all pixel rows) are faster. Low bit-depth mode can be used for... Figure 6C The example scheme includes first and second drive events 650a and 660a. In high bit-depth mode, the drive circuit can operate by providing a relatively low drive current because the (maximum) pixel voltage is obtained over a longer addressing cycle. A longer addressing cycle corresponds to the maximum number of clock pulses required to drive a pixel with a higher bit-depth (i.e., n-bit) pixel value. Therefore, in high bit-depth mode, charging to the (maximum) pixel voltage takes longer, meaning a relatively long (common) addressing cycle / line time, and thus a slower drive event. High bit-depth mode can be used for... Figure 6CThe example scheme includes the third and fourth driving events 670a and 680a.

[0112] Therefore, in the above embodiment, the operating mode of the driving circuit determines (1) the driving current (which determines how fast the pixels charge) and (2) the row time or (common) addressing period (i.e., the number of clock pulses counted before moving to the next row of pixels). The switching between modes is defined by a driving scheme that may be the same for all frames. In this case, the switching between modes is periodic and predictable, allowing the driving circuit itself to control the switching between modes simply by counting clock pulses. Alternatively, the operating mode can be indicated in a subframe (e.g., by an indicator such as a flag) that includes the pixel values ​​to be written during the corresponding driving event. In this case, the switching can be performed by the driving circuit when the next subframe has a different mode indicator than the previous subframe.

[0113] Therefore, a method for driving a liquid crystal display device to display a hologram is provided. The grayscale value of each pixel of the hologram is received. A pixel voltage is determined based on the grayscale value of each pixel of the hologram. During at least one first driving event, pixels in the pixel array of the liquid crystal display device are driven according to a first representation of the pixel voltage. After at least one first driving event, during at least one second driving event, pixels in the pixel array of the display device are driven according to a second representation of the pixel voltage. The first representation is an n-bit representation, the second representation is an m-bit representation, and n... <m。

[0114] A method for driving a liquid crystal display device to display a hologram is also provided. The grayscale value of each pixel of the hologram is received. A pixel voltage is determined based on the grayscale value of each pixel of the hologram. Pixels in the pixel array of the liquid crystal display device are driven by at least one first driving event having a first duration. During at least one second driving event having a second duration, the pixels in the pixel array of the display device are driven according to a second representation of the pixel voltage. The at least one second driving event follows at least one first driving event. The second duration is greater than the first duration.

[0115] exist Figure 6C The illustrated embodiment proposes a method comprising a sequence of four display events with two pairs of “DC balance” driven events. In other embodiments, sequences comprising any other even number of DC balance display events may be used, wherein the first pair of display events has a shorter duration than at least one subsequent pair of display events. Furthermore, as described herein, embodiments comprising sequences of display events comprising an odd number of display events with variable durations are possible and contemplated, with or without compensation for DC balance.

[0116] block diagram

[0117] Figure 7 A holographic projection system 700 according to an embodiment is illustrated. Specifically, the system includes a liquid crystal display device 740 and a controller 710, the controller 710 being arranged to drive the display device 740 to display a hologram using a driving scheme according to the present disclosure. The liquid crystal display device 740 may include a spatial light modulator, such as an LCOS SLM, as described herein.

[0118] Typically, controller 710 includes a hologram engine 720 and a display engine 730. Hologram engine 720 is arranged to determine a hologram of an input image and provide the hologram to display engine 730. For example, hologram engine 720 can use the algorithms described herein to compute a computer-generated hologram. In the illustrated system 700, hologram engine 720 receives an image from image source 750 (such as a camera) and computes a corresponding hologram. In some embodiments, image source 750 can provide a sequence of images for display at a video frame rate, and hologram engine 720 can compute a corresponding sequence of holograms, which are then sequentially provided to display engine 730.

[0119] According to this disclosure, display engine 730 receives a hologram from hologram engine 720 and generates drive signals for driving display device 740 to display the hologram. Therefore, display engine 730 can be considered as operating as a display driver. Display engine 730 includes a first stage 760 and a second stage 770. In the first stage 760, display engine 730 determines pixel voltages based on the grayscale levels (e.g., phase modulation levels) of the received hologram. Specifically, first stage 760 determines pixel voltages corresponding to grayscale levels based on a predetermined calibration scheme 765 of display device 740. Calibration scheme 765 may include a lookup table stored in controller 710 that provides a mapping between grayscale levels and pixel voltages, or may include a predetermined formula for calculating pixel voltages from grayscale levels based on previous calibrations of system 700. First stage 760 determines pixel voltages with high precision, i.e., high bit depth. Second stage 770 generates drive signals for multiple display events (subframes) on display device 740 according to drive scheme 775. Driving scheme 775 can be stored in controller 710 and determined according to design requirements. According to this disclosure, driving scheme 775 includes at least one driving event (subframe) based on a first representation of pixel voltage, followed by at least one second driving event (subframe) based on a second representation of pixel voltage, wherein the first representation has a lower bit depth than the second representation. It can be said that at least one driving event (subframe) of driving scheme 775 has a first duration, and at least one second driving event (subframe) of driving scheme 775 has a second duration, wherein the first duration is shorter than the second duration. Therefore, the second stage 770 of display engine 730 can determine the first representation Rep1 and the second representation Rep2 of pixel voltage received from the first stage 760, and determine the subframe according to driving scheme 775. Specifically, the second stage 770 can generate a driving signal for a sequence of driving events including at least one driving event (subframe) based on the first representation Rep1 of pixel voltage, followed by at least one second driving event (subframe) based on the second representation Rep2 of pixel voltage. In some implementations, the sequence of driving events may include a first driving event consisting of one or more pairs of “DC balancing” events, followed by a second driving event consisting of one or more pairs of “DC balancing” events, as described herein. As those skilled in the art will understand, each subframe displayed on the display device 740 may be determined by the display engine 730 using conventional techniques as required by design, including tiling techniques for optimizing image quality as described below.

[0120] As described above, the driving circuit (not shown) associated with the display device can receive driving signals from the second stage 770 of the display engine 730 and drive the pixels of the display device with the corresponding pixel value of each driving event in the sequence. The driving circuit can switch between the operation modes of the driving events in the sequence according to the driving scheme 775, for example by counting clock pulses based on indicators associated with subframes / driving events or other means.

[0121] Additional features

[0122] In some embodiments, the light source is a laser, such as a laser diode. In some embodiments, the light-receiving surface is a diffuser surface or screen, such as a diffuser. The holographic projection system disclosed herein can be used to provide an improved head-up display (HUD) or head-mounted display. In some embodiments, a vehicle is provided that includes a holographic projection system mounted in the vehicle to provide a HUD. The vehicle can be a motor vehicle, such as a car, truck, van, delivery truck, motorcycle, train, airplane, boat, or ship.

[0123] The quality of holographic reconstruction can be affected by the so-called zero-order problem, which is a result of the diffraction properties of pixelated spatial light modulators. This zero-order light can be considered "noise" and includes, for example, specular reflections and other unwanted light from SLMs.

[0124] In examples of Fourier holography, this "noise" is concentrated at the focal point of the Fourier lens, resulting in a bright spot at the center of the holographic reconstruction. Zero-order light can be simply blocked out; however, this means replacing the bright spot with a dark spot. Some embodiments include angle-selective filters to remove only collimated rays of the zeroth order. Embodiments also include methods for managing the zeroth order as described in European Patent 2,030,072, which is incorporated herein by reference in its entirety.

[0125] In some embodiments, the size of the hologram (the number of pixels in each direction) is equal to the size of the spatial light modulator, such that the hologram fills the spatial light modulator. That is, the hologram uses all the pixels of the spatial light modulator. In other embodiments, the hologram is smaller than the spatial light modulator. More specifically, the number of pixels in the hologram is less than the number of light modulation pixels available on the spatial light modulator. In some of these other embodiments, a portion of the hologram (i.e., a consecutive subset of the pixels of the hologram) is repeated in unused pixels. This technique may be referred to as "tiling," where the surface area of ​​the spatial light modulator is divided into multiple "tiles," each representing at least one subset of the hologram. Thus, the size of each tile is smaller than the size of the spatial light modulator. In some embodiments, the "tiling" technique is implemented to improve image quality. Specifically, some embodiments implement the tiling technique to minimize the size of the image pixels while maximizing the amount of signal content entering the holographic reconstruction. In some embodiments, the holographic pattern written into the spatial light modulator includes at least one complete tile (i.e., the complete hologram) and at least a small portion of the tile (i.e., a consecutive subset of the pixels of the hologram).

[0126] In this embodiment, only the primary playback field is utilized, and the system includes physical blocks, such as baffles, arranged to restrict the propagation of higher-level playback fields through the system.

[0127] In some embodiments, the holographic reconstruction is in color. In some embodiments, a method known as Spatially Separated Color (SSC) is used to provide color holographic reconstruction. In other embodiments, a method known as Frame Order Color (FSC) is used.

[0128] The SSC method uses three spatially separated arrays of light-modulated pixels for three monochrome holograms. Therefore, as described herein, a driving event involves driving all (addressable) pixels of one of the three pixel arrays to display a particular monochrome hologram. In the SSC method, three driving events can be executed simultaneously to drive the corresponding arrays in the three pixel arrays, so that the three monochrome holograms can be displayed substantially simultaneously. An advantage of the SSC method is that the images can be very bright because all three holographic reconstructions can be formed simultaneously. However, if three spatially separated arrays of light-modulated pixels are provided on a common SLM due to space constraints, the quality of each monochrome image will be suboptimal because only a subset of the available light-modulated pixels is used for each color. Therefore, relatively low-resolution color images are provided.

[0129] The FSC method can use all pixels of a common spatial light modulator (SLM) to sequentially display three monochrome holograms. The monochrome reconstruction loop (e.g., red, green, blue, red, green, blue, etc.) is fast enough that a human viewer perceives a multicolor image from the integration of the three monochrome images. An advantage of FSC is that the entire SLM can be used for each color. This means the quality of the resulting three color images is optimal because all pixels of the SLM are used for each color image. However, a disadvantage of the FSC method is that the brightness of the synthesized color image is about three times lower than that of the SSC method because each monochrome illumination event only occurs for one-third of the frame time. This deficiency can be addressed by overdriving the laser or using a more powerful laser, but this requires more power, leading to higher costs and increased system size.

[0130] The example describes illuminating an SLM with visible light, but those skilled in the art will understand that light sources and SLMs can also be used to guide infrared or ultraviolet light, as disclosed herein. For example, those skilled in the art will know techniques for converting infrared and ultraviolet light into visible light to provide information to a user. For example, this disclosure extends to the use of phosphors and / or quantum dot technologies for this purpose.

[0131] Some embodiments describe 2D holographic reconstruction by way of example only. In other embodiments, the holographic reconstruction is 3D holographic reconstruction. That is, in some embodiments, each computer-generated hologram forms a 3D holographic reconstruction.

[0132] The methods and processes described herein can be embodied on a computer-readable medium. The term "computer-readable medium" includes media arranged for temporary or permanent storage of data, such as random access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" should also be considered to include any medium or combination of media capable of storing instructions for machine execution, such that when the instructions are executed by one or more processors, the machine performs, wholly or partially, any or all of the methods described herein.

[0133] The term "computer-readable medium" also covers cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible and non-transitory data repositories (e.g., data volumes) in the example forms of solid-state storage chips, optical discs, disks, or any suitable combinations thereof. In some example embodiments, instructions for execution may be transmitted by a carrier medium. Examples of such carrier media include transient media (e.g., propagation signals for transmitting instructions).

[0134] It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope of the appended claims. This disclosure covers all modifications and variations within the scope of the appended claims and their equivalents.

Claims

1. A method for driving a liquid crystal display device to display a hologram, the method comprising: Receive the grayscale value of each pixel in the hologram; The pixel voltage is determined based on the grayscale value of each pixel in the hologram; The pixels in the pixel array of the liquid crystal display device are driven according to a first representation of the pixel voltage during at least one first driving event, and the pixels in the pixel array of the display device are driven according to a second representation of the pixel voltage during at least one second driving event following at least one first driving event, wherein the first representation is an 1 / 2 bit representation, the second representation is an 1 / 2 bit representation, and 1 / 2 < 1 / 2, and wherein at least one first driving event is shorter in duration than at least one second driving event.

2. The method of claim 1, further comprising illuminating the pixels of the display device after at least one first driving event to form a holographic reconstruction corresponding to the hologram on the playback plane.

3. The method as described in claim 2, wherein, Illumination of the pixels of the display device begins at a predetermined time after the first drive event in at least one first drive event.

4. The method of claim 1, wherein, The at least one first driving event includes at least one pair of complementary first driving events, wherein each pair of first driving events is DC balanced.

5. The method of claim 1, wherein, The at least one second driving event includes at least one pair of complementary second driving events, wherein each pair of second driving events is DC balanced.

6. The method of claim 1, further comprising deriving the first representation from the second representation.

7. The method of claim 6, further comprising removing one or more least significant bits of the second representation to derive the first representation.

8. A liquid crystal display device arranged to display a hologram, the display device comprising: The display driver is arranged to receive the grayscale value of each pixel of the hologram and determine the pixel voltage based on the grayscale value of each pixel of the hologram. The display driver is arranged to drive pixels in the pixel array of the liquid crystal display device to a determined pixel voltage according to a driving scheme including at least one first driving event and at least one second driving event following the at least one first driving event, wherein each driving event writes the pixel voltage to all pixels in the pixel array of the display device, wherein each first driving event is shorter in duration than each second driving event, and wherein the display driver is further arranged to drive the pixels of the display device according to a first representation of the pixel voltage during at least one first driving event, and to drive the pixels of the display device according to a second representation of the pixel voltage during at least one second driving event, wherein the first representation is an 1 / 2 bit representation, the second representation is an 1 / 2 bit representation, and 1 / 2 < 1 / 2.

9. The display device as claimed in claim 8, wherein, The display driver is also arranged to derive a first representation of the pixel voltage from a second representation of the pixel voltage.

10. The display device as claimed in claim 9, wherein, The display driver is arranged to derive a first representation of the pixel voltage by removing one or more least significant bits of the second representation.

11. The display device of claim 8, further comprising: A hologram engine, which is arranged to calculate the grayscale value of each pixel in a hologram.

12. The display device of claim 8, comprising a pixelated spatial light modulator "SLM".

13. The display device as claimed in claim 12, wherein, The pixelated spatial light modulator is an LCOS SLM.

14. A head-up display or head-mounted display, comprising the display device as claimed in any one of claims 8 to 13.

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