Hologram Computation

Abstract

To provide methods and devices for breaking the convention of independently calibrating each single color channel.SOLUTION: A method of projecting a first image and a second image using one multi-wavelength hologram. The first image is different to the second image. The multi-wavelength hologram is arranged for illumination with light of a first wavelength to project the first image. The multi-wavelength hologram is further arranged for illumination with light of a shorter second wavelength to project the second image.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present disclosure relates to image projection. More specifically, the present disclosure relates to a method for projecting at least first and second different images using a single hologram. Some embodiments relate to providing a single multi-wavelength hologram representing two or more individual holograms, such as a single multi-color hologram representing multiple individually colored holograms. Some embodiments relate to a method for computing a multi-wavelength hologram. Some embodiments relate to a head-up display. [Background technology]

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

[0003] Computer-generated holography can numerically simulate interference processes. Computer-generated holograms can be calculated by techniques based on mathematical transforms such as the Fresnel transform or the Fourier transform. These types of holograms are sometimes called Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. Fourier holograms can be thought of as a Fourier domain / planar representation of an object or a frequency domain / planar representation of an object. Computer-generated holograms can also be calculated by, for example, coherent ray tracing or point cloud techniques.

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

[0005] A spatial light modulator typically contains a plurality of individually addressable pixels, which may also be called cells or elements. The light modulation scheme may be binary, multi-level, or continuous. Alternatively, the device may be continuous (i.e., not composed of pixels), and thus the light modulation may be continuous throughout the device. A spatial light modulator may be reflective, meaning that modulated light is output in reflection. A spatial light modulator may also be transmissive, meaning that modulated light is output in transmission.

[0006] Holographic projectors can be provided using the systems described herein, and such projectors find application in, for example, head-up displays (HUDs) and head-mounted displays (HMDs), including near-eye devices.

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

[0008] In summary, provided herein are methods and systems that allow a single hologram to simultaneously (or "simultaneously") represent multiple holograms, thereby enabling illumination of that single hologram to project multiple images displayed on a single display device. For example, the single hologram may be a multi-wavelength hologram that simultaneously represents two or more single-wavelength holograms. For example, the single hologram may be a multi-color hologram that simultaneously represents each of three individual holograms, such as red (R), green (G), and blue (B) component holograms of a target image.

[0009] Conventionally, monochromatic holograms are displayed, illuminated separately from one another with light of each wavelength / color, and the corresponding display device is calibrated separately to provide maximum holographic resolution for each color. The methods and systems disclosed herein defy convention by using a common calibration of the display device to display multi-wavelength (i.e., multi-image) holograms, effectively representing two or more different wavelength / color holograms simultaneously (or "simultaneously"). This necessitates the use of a non-ideal grayscale resolution corresponding to the number of available optical modulations / grayscale levels to represent at least one of the holograms, due to the well-known relationships and interactions between the display device's drive voltage, cell gap, birefringence, and wavelength. For clarity, in this context, "less than ideal grayscale resolution" is caused by a non-optimized calibration of the display system's cells for one or more wavelengths (i.e., for one or more colors of light) and / or the use of a non-optimized voltage operating range to provide a predetermined range of optical modulations / grayscale levels (e.g., phase delays) for one or more wavelengths (i.e., for one or more colors of light). Thus, if a display system has less than ideal grayscale resolution for a particular wavelength / color, this typically means that it is calibrated to use fewer discrete grayscales to provide different magnitudes of light modulation within the entire required modulation range (minimum to maximum angle values, such as 0 to 2π for phase modulation, or minimum to maximum amplitude values, 0 to 1 for amplitude modulation) than would be possible if the system were configured only for the hologram and illumination light of that particular wavelength / color. However, the inventors have surprisingly found that this loss of grayscale resolution for some of the holograms does not significantly affect the accuracy and resolution of the images formed from the illumination of multi-wavelength (i.e., multi-image) holograms calculated as described herein.

[0010] The inventors have observed and exploited the effect of a feature known as "phase wrapping," which allows multiple different drive voltages for a display cell to be used to provide the same optical modulation value for a particular wavelength of light. Disclosed herein are methods and apparatus that use the phenomenon of phase wrapping and, in particular, break the convention of independently calibrating each single color channel. In some embodiments, schemes are disclosed in which multiple different voltages can be used to provide all available modulation values ​​(e.g., across the entire modulation range from 0 to 2π or 0 to 1) for one or more colors, including the color with the longest wavelength (e.g., red light). This means that the display device is "overdriven" relative to its conventional operating range, at least for shorter wavelengths, and potentially for all wavelengths of illumination light. This approach allows for the determination of a single hologram that reconstructs multiple distinct images, i.e., multiple monochromatic components of a full-color image. In some embodiments, the same hologram reconstructs the red, green, and blue components of a full-color image. In particular, the red, green, and blue components of a full-color image are not identical. The technique of determining one hologram for all three colors represents a breakthrough in the field of holographic projection, enabling the use of a single display device instead of multiple ones without any of the traditional drawbacks associated with frame-sequential color display methods. This is because the hologram does not need to be changed or updated between subframes. In some embodiments, the same hologram can be simultaneously illuminated with multiple different colors, forming corresponding multiple different monochromatic images. In some embodiments, the described method is rapidly applied to multiple different combinations of multiple individual holograms to provide and illuminate multiple respective multi-wavelength holograms very rapidly, for example, at video image rates.

[0011] Aspects of the present disclosure are defined in the accompanying independent claims.

[0012] According to one aspect, there is provided a method of projecting a first image and a second image using a single multi-wavelength hologram, wherein the first image is different from the second image, and the multi-wavelength hologram is positioned for illumination with light of a first wavelength to project the first image, and the multi-wavelength hologram is further positioned for illumination with light of a second, shorter wavelength to project the second image.

[0013] According to one aspect, a projector is provided, the projector configured to project a first image and a second image using a multi-wavelength hologram, the projector comprising a display device for displaying the multi-wavelength hologram, the first image being different from the second image, the multi-wavelength hologram configured for illumination with light of a first wavelength to project the first image, and the multi-wavelength hologram further configured for illumination with light of a second, shorter wavelength to project the second image.

[0014] The display device may display the multi-wavelength hologram in its display area. For example, the display area may include an array of pixels. Thus, the same display area (e.g., array of pixels) is used to display a single hologram representing both the first and second images. The method and / or projector may further be configured to project a third image using the same multi-wavelength hologram, the third image being different from each of the first and second images, and the multi-wavelength hologram being configured for illumination with light of a third shortest wavelength to project the third image. For example, the first, second, and third images may include red, green, and blue images, respectively, which may include the red, green, and blue components of the target image, respectively. The image content of each of the red, green, and blue images may differ at least in part from one another. The first and second images (and optionally also the third image) may be projected onto a common reconstruction plane, and they may at least partially spatially overlap each other on the reconstruction plane.

[0015] A display device included in the projector may include a plurality of pixels, each configurable to provide phase modulation values ​​ranging from 0 to 2π at a first wavelength within a corresponding first operating range of voltage drive levels. The display device may be configured to provide phase modulation to the multi-wavelength hologram using a predetermined maximum number of discrete phase modulation levels. For example, phase modulation over 128 discrete phase levels may be provided. The projector may further include a display driver configured to distribute the discrete phase modulation levels over a voltage range equal to or greater than the first operating range of voltage drive levels.

[0016] Each pixel of the phase modulator may also be configurable to provide a phase modulation value in the range of 0 to 2π at a second wavelength within a corresponding second operating range of voltage drive levels, and the projector may be configured to drive one or more of the pixels to a voltage that exceeds a maximum voltage within the second operating range of voltage drive levels.

[0017] Because the second wavelength is shorter than the first wavelength, the display device may be configured to deliver phase modulation values ​​ranging from 0 to 2π over a smaller voltage range for the second wavelength than for the longer first wavelength. The projector may be configured to deliver phase modulation for the second hologram using discrete phase levels and their respective voltage levels defined for the first hologram. Because the second wavelength is shorter, the voltage gaps between the discrete voltage levels defined for the first hologram represent larger phase modulation gaps for the second hologram than for the first hologram. Therefore, the resolution of the second hologram is lower than the resolution of the first hologram when represented using common discrete voltage levels. Furthermore, higher voltage levels defined for the first hologram represent phase modulation greater than 2π for the second hologram.

[0018] The projector may include or operate in conjunction with any suitable light source configured to illuminate the multi-wavelength hologram with light at a first wavelength to form a first image and light at a second wavelength to form a second image. For example, first and second laser diodes may be provided. For example, a light source configurable to separately deliver light at a plurality of different respective wavelengths may be provided.

[0019] The projector, or a light source operating in conjunction with the projector, may be configured to illuminate the multi-wavelength hologram with light at the first wavelength and light at the second wavelength substantially simultaneously, such that the first and second images (and optionally also the third image) may be formed substantially simultaneously.

[0020] A multi-wavelength hologram may include representations of a first hologram including a first set of hologram pixel values ​​corresponding to a first image and a second hologram including a second set of hologram pixel values ​​corresponding to a second image. Thus, a single multi-wavelength hologram simultaneously represents both the first and second holograms (i.e., "simultaneously" or "contemporaneously"). For example, a multi-wavelength hologram may include a "combined" or "composite" representation of the first and second holograms. For example, it may include an averaged or aggregated hologram formed from the first and second holograms.

[0021] Each pixel of the multi-wavelength hologram may include a composite hologram pixel value determined from corresponding first and second hologram pixel values ​​of the first and second holograms, respectively. Each composite hologram pixel value may include an average value determined from corresponding first and second hologram pixel values ​​of the first and second holograms, respectively. A respective weighting may be applied to at least one of the first and second hologram pixel values ​​to determine the composite hologram pixel value. Thus, for one or more pixels, the composite hologram pixel value may be closer to the corresponding pixel value from the first hologram than the second hologram, or vice versa. Any suitable method may be used to determine whether and how much weighting to apply.

[0022] The projector may further include a processor configured to obtain, for a selected pixel of the display device, at least a first pixel drive level of the first hologram and at least a second pixel drive level of the second hologram, and determine a multi-wavelength pixel drive level for that pixel of the display device based on the first and second pixel drive levels, where the multi-wavelength pixel drive level may be closer to the first pixel drive level than to the second pixel drive level, or vice versa, depending on the weighting of the respective hologram values ​​for the given pixel.

[0023] The multi-wavelength pixel drive level can be determined based on a best fit between the first pixel drive level of the first hologram and the second pixel drive level of the second hologram, which may include approximations for the first pixel drive level and the second pixel drive level simultaneously.

[0024] The processor may be configured to obtain, for selected pixels of the display device, a plurality of second pixel drive levels of the second hologram, each of the plurality of second pixel drive levels corresponding to the same optical modulation level of the second hologram, and determine a multi-wavelength pixel drive level based on the first pixel drive level and a selected one of the plurality of second pixel drive levels. This is possible because the voltage level of the first hologram corresponds to overdriving the second hologram beyond a phase modulation value of 2π and because of phase wrapping, which the inventors identified as a phenomenon in which phase modulation repeats with a period of n(2π). As a result of these factors, for at least some pixels of the second hologram, there is a first available voltage level corresponding to a required phase modulation value of "θ" and a second available voltage level corresponding to a phase modulation of "θ+2π," which has the same modulation effect as the first available voltage level. Therefore, for that pixel (or pixels), options are available as to which of these voltage levels are used to represent the second hologram and to determine the corresponding multi-wavelength pixel drive level.

[0025] The processor may be further configured to obtain, for a selected pixel of the display device (or a selected pixel of the multi-wavelength hologram), a plurality of first pixel drive levels of the first hologram, each of the plurality of first pixel drive levels corresponding to the same light modulation level of the first hologram, and to determine the multi-wavelength pixel drive level based on a selected one of the plurality of first pixel drive levels and a selected one of the plurality of second pixel drive levels. In other words, the projector may be configured to distribute available voltage levels of the first hologram over a phase modulation range, for example, from 0 to greater than 2π, such that there are multiple possible voltage levels for delivering at least a portion of the phase modulation values ​​required for the first hologram (and the second hologram).

[0026] The step of determining the multi-wavelength pixel drive level can include identifying a best-matched pair of pixel drive levels, the pair including one from a plurality of first pixel drive levels and one from a plurality of second pixel drive levels, and weighting can be applied to influence whether both pixel drive levels should be equally matched or whether one should be preferred over the other.

[0027] According to one aspect, there is provided a method for determining a multi-wavelength hologram, the multi-wavelength hologram being configured to project a first image and a second image, the first image being different from the second image, when displayed on a pixelated display device and illuminated with light of a first wavelength to project the first image and light of a second, shorter wavelength to project the second image, the method including the steps of: acquiring a first hologram including a first set of hologram pixel values ​​corresponding to the first image; acquiring a second hologram including a second set of hologram pixel values ​​corresponding to the second image; determining a first operating range of voltage drive levels, wherein each pixel of the display device, when driven within the first operating range, is configurable to provide light modulation values ​​within the full range of light modulation values ​​at the first wavelength; determining a maximum number of discrete light modulation levels for the display device and distributing those discrete light modulation levels across a voltage range equal to or greater than the first operating range of voltage drive levels; and separately displaying each of the first hologram and the second hologram. using the distributed discrete light modulation levels to represent a first set of pixel drive levels for the first hologram and outputting a first set of corresponding pixel drive levels for the first hologram and a second set of pixel drive levels for the second hologram; for each pixel of the multi-wavelength hologram, selecting a first drive level from the first set of pixel drive levels to represent the corresponding pixel of the first hologram and a second drive level from the second set of pixel drive levels to represent the corresponding pixel of the second hologram, and outputting a multi-wavelength drive level for that pixel based on the selected first and second drive levels; and using the multi-wavelength drive level output for each pixel to form the multi-wavelength hologram.

[0028] The optical modulation values ​​may include phase modulation values, amplitude modulation values, or a combination thereof. As will be appreciated by those skilled in the art, the full range of phase modulation values ​​at the first wavelength may be defined from 0 to 2π, while the full range of amplitude modulation values ​​may be defined from 0 to 1. The plurality of discrete modulation values ​​may be referred to as gradations.

[0029] The multi-wavelength hologram may be further configured to project a third image when displayed on a pixelated display device and illuminated with light of a third shortest wavelength, the first, second, and third images all being distinct from one another. The method may further include obtaining a third hologram including a third set of hologram pixel values ​​corresponding to the third image, separately representing the third hologram using distributed discrete light modulation levels, and outputting corresponding pixel drive levels of the third set of the third hologram. The method may further include selecting, for each pixel of the multi-wavelength hologram, a third drive level from the third set of pixel drive levels to represent the corresponding pixel of the third hologram, and outputting a multi-wavelength drive level for that pixel based on the selected first, second, and third drive levels. The method may further include using the multi-wavelength drive level output for each pixel to form the multi-wavelength hologram.

[0030] The selected first drive level and the selected second drive level (and optionally the third drive level) may be closer in magnitude to each other. They may be closer in magnitude to each other than any other possible pair of drive levels including a first drive level from the first set of pixel drive levels and a second drive level from the second set of pixel drive levels. In embodiments in which three drive levels are combined, the selected drive levels may be closer in magnitude to each other than other possible sets of first, second, and third drive levels for a given pixel of the output multi-wavelength hologram.

[0031] The method may further include determining an average drive level from the first drive level and the second drive level (optionally the third drive level), the average drive level being output as the multi-wavelength drive level for the pixel. At least one of the first drive level and the second drive level (optionally the third drive level) may be weighted to obtain the average drive level.

[0032] The method may further include displaying the multi-wavelength hologram on a display device, and optionally further including illuminating the display device with light of a first wavelength and light of a second wavelength to project the first and second images (and optionally also illuminating the display device with light of a third wavelength to project a third image). A hologram engine or projector or other suitable optical system may be configured to carry out the methods of the above aspects.

[0033] According to one aspect, there is provided a diffractive structure formed by the method of the above aspect.

[0034] According to one aspect, a display system is configured to substantially simultaneously display a first image and a second image using a multi-wavelength hologram, the display system comprising: a light modulator including a plurality of pixels configured to display the multi-wavelength hologram, each pixel of the light modulator being a liquid crystal cell configured to provide light modulation (e.g., phase modulation, amplitude modulation, or a combination thereof), each liquid crystal cell having a cell gap; and a processor configured to determine the multi-wavelength hologram from a first hologram of the first image and a second hologram of the second image, the light modulator being configured such that at least some light modulation values ​​of the second hologram can each be provided by a corresponding plurality of different pixel drive levels for the respective pixels, and determining the multi-wavelength hologram includes selecting a pixel drive level from the corresponding plurality of different pixel drive levels for each of the at least some phase values ​​of the second hologram.

[0035] A light modulator may be arranged for illumination, and the amount of light modulation provided by a pixel may be determined by the pixel drive level and the wavelength of the illumination light, with longer wavelengths of illumination light typically requiring greater pixel drive levels to achieve a particular light modulation value than do shorter wavelengths of illumination light.

[0036] Selecting a pixel drive level for each pixel of the second hologram can be based on a best match with the pixel drive level of the corresponding pixel of the first hologram. There can be two or more possible pixel drive levels to select for at least some of the pixels of the second hologram.

[0037] The first hologram may be configured for illumination with light of a first wavelength, and the second hologram may be configured for illumination with light of a second, shorter wavelength.

[0038] The pixel values ​​of the first and second holograms may be within the full optical modulation range (e.g., 0 to 2π for phase modulation, or 0 to 1 for amplitude modulation). However, due to the phenomenon of phase wrapping, at least for the second hologram, it may be possible to drive the optical modulator to deliver the required modulation effect using optical modulation values ​​that exceed the maximum level of the full range (e.g., 2π for phase modulation, 1 for amplitude modulation).

[0039] Each pixel value of the multi-wavelength hologram may be determined from a corresponding pixel value of the first hologram and a corresponding pixel value of the second hologram, and the light modulators may be arranged such that for any pair of pixel values ​​of the first hologram and the second hologram, multiple combinations of corresponding pixel drive levels are possible. For example, two or more pixel drive levels may be suitable for delivering the required light modulation to at least one corresponding pixel of the first hologram or the second hologram. The processor may be configured to identify an optimal combination of pixel drive levels for determining the pixel drive level for each of the pixels of the multi-wavelength hologram.

[0040] The display system may be further configured to display a third image substantially simultaneously with the first and second images using a multi-wavelength hologram, which may include a red-green-blue (RGB) hologram.

[0041] According to one embodiment, the diffractive structure is configured to project a first image and a second image, the first image being different from the second image, and the diffractive structure is configured for illumination with light of a first wavelength to project the first image and for illumination with light of a second, shorter wavelength to project the second image.

[0042] The diffractive structure may be further arranged to also project a third image, the third image being different from each of the first and second images, and the diffractive structure being arranged for illumination with light of a third shortest wavelength to project the third image.

[0043] The diffractive structures may be arranged for display on a pixelated display device. The diffractive structures may be configured to present three single-wavelength diffractive structures, each corresponding to three individual images, using a common set of voltage levels on the pixelated display device, with each voltage level corresponding to a different respective light modulation level (e.g., a phase modulation level, an amplitude modulation level, or a combination thereof) for each of the three single-wavelength diffractive structures. Thus, the diffractive structures may be configured to simultaneously present each single-wavelength diffractive structure at a different respective resolution. Thus, the diffractive structures may be configured to present different respective ranges of light modulation values ​​for each individual single-wavelength diffractive structure. For phase modulation, the range may be at least 0-2π for the first (longest wavelength) diffractive structure and may exceed 0-2π for the other individual single-wavelength diffractive structures (configured for illumination with shorter and shortest wavelength light, respectively). However, it has been found that the diffractive structures, when illuminated, can nevertheless result in high-quality image projection (i.e., holographic reconstruction) for all of the first, second, and third images.

[0044] The diffractive structure may include a multi-wavelength hologram. It may include a kinoform.

[0045] According to one embodiment, an optical system is configured to project a first image, a second image, and a third image using a multi-wavelength hologram, each of the first, second, and third images being different, the multi-wavelength hologram being configured for illumination with light of a first wavelength to project the first image, further configured for illumination with light of a second shorter wavelength to project the second image, and further configured for illumination with light of a third shortest wavelength to project the third image.

[0046] The first, second, and third wavelengths of light may comprise red, green, and blue light, respectively. The first, second, and third images can be combined at the reconstruction plane to provide a multicolor image.

[0047] According to one aspect, there is provided a voltage selection unit for driving a pixelated display device to display a multi-wavelength diffractive structure, the multi-wavelength diffractive structure being configured to represent a first diffractive structure and a second different diffractive structure, the voltage selection unit determining a first plurality of discrete voltage levels capable of driving the display device, each level of the first plurality of discrete voltage levels being configured to correspond to a respective discrete light modulation value of the first diffractive structure over a full range of its light modulation values ​​(e.g., 0-2π in the case of phase modulation), and a voltage selection unit for determining a first plurality of discrete voltage levels capable of driving the display device to display a multi-wavelength diffractive structure, the multi-wavelength diffractive structure being configured to represent a first diffractive structure and a second different diffractive structure, the voltage selection unit determining a first plurality of discrete voltage levels capable of driving the display device, each level of the first plurality of discrete voltage levels being configured to correspond to a respective discrete light modulation value of the first diffractive structure over a full range of its light modulation values ​​(e.g., 0-2π in the case of phase modulation), and a voltage selection unit for determining ... and configured to determine a correspondence between discrete phase modulation values ​​over a range of light modulation values ​​(e.g., over 0 to 2π for phase modulation), and to determine, using the first plurality of discrete voltage levels, a first set of pixel drive values ​​for representing the first diffractive structure on the display device and a second set of pixel drive values ​​for representing the second diffractive structure on the display device; and configured to select, for each pixel of the display device, an optimized pixel drive value that represents each of the pixel drive value for the first diffractive structure and the pixel drive value for the second diffractive structure.

[0048] The multi-wavelength diffractive structure may be further configured to represent a third, different diffractive structure, and the voltage selection unit may also be configured to determine a correspondence between each level of the first plurality of discrete voltage levels and a respective discrete light modulation value of the third diffractive structure within a range exceeding the full range of its light modulation values ​​(e.g., from 0 to over 2π in the case of phase modulation).The display device is further configured to determine a third set of pixel drive values ​​for representing the third diffractive structure on the display device using the first plurality of discrete voltage levels, and to select, for each pixel of the display device, an optimized pixel drive value representing the pixel drive value for the first diffractive structure, the pixel drive value for the second diffractive structure, and the pixel drive value for the third diffractive structure.

[0049] For at least one pixel of at least one of the diffractive structures, there may be two or more possible voltage levels corresponding to a required light modulation value, and the voltage selection unit may therefore be configured to identify, for each such pixel of the display device, a best-fit voltage level representing one possible voltage level for each of the first, second and third diffractive structures.

[0050] For each pixel for which there are two or more possible combinations of voltage levels to represent each of the first, second, and third diffraction structures, the voltage selection unit is configured to determine all possible pairs of voltage levels, each pair including a possible voltage level for one diffraction structure and a corresponding possible voltage level for one of the other diffraction structures, determine the difference in magnitude of the two voltage levels in each possible pair, and identify an optimized combination of the three possible pairs for that pixel that represents the difference in magnitude of the voltage levels between each diffraction structure and each of the others, wherein the total difference in magnitude of the pairs in the optimized combination is minimized.

[0051] The first, second, and third diffractive structures may include red, green, and blue holograms, respectively, and thus the pairs may include red-green (RG) pairs, green-blue (GB) pairs, and blue-red (BR) pairs.

[0052] A bias may be applied to the magnitude difference in voltage levels between the two diffractive structures of at least one of the three pairs in the optimized combination. For example, it may be determined that minimizing the voltage gap of the RG pair is more important than minimizing the voltage gaps of the GB pair and the BR pair, or vice versa. The bias size and selection may be determined based on any suitable factors, such as image type, display type, cell gap, etc. The voltage selection unit may be further configured to output voltage levels representing optimized combinations of the three possible pairs for each pixel, the output voltage levels comprising optimized pixel drive values ​​for the respective pixels.

[0053] The above aspects allow a single multi-wavelength diffractive structure to "simultaneously" or "simultaneously" represent at least two individual (single-wavelength) diffractive structures and, when properly illuminated, clearly and accurately project two (or more) corresponding images. The multi-wavelength diffractive structure can be said to be a single optimized representation (e.g., approximation) of at least two single-wavelength diffractive structures. This is done in a computationally intelligent and efficient manner not previously considered in conventional holography. Because of the aspects and embodiments disclosed herein, the potential for economic savings, improved compactness, and enhanced practical applications of multi-wavelength holography are significant.

[0054] The term "hologram" refers to a recording containing amplitude or phase information about an object, or some combination thereof. The term "holographic reconstruction" refers to an optical reconstruction of an object formed by illuminating a hologram. Because the holographic reconstruction is a real image and spatially separated from the hologram, the system disclosed herein is described as a "holographic projector." The term "reconstruction field" refers to the 2D region in which the holographic reconstruction is formed and perfectly focused. When a hologram is displayed on a spatial light modulator containing pixels, the reconstruction field is repeated in the form of multiple diffraction orders, each of which is a replica of the zeroth reconstruction field. The zeroth reconstruction field is the brightest reconstruction field and therefore generally corresponds to the preferred or first-order reconstruction field. Unless otherwise specified, the term "reconstruction field" should be interpreted as referring to the zeroth reconstruction field. The term "reconstruction plane" refers to a plane in space that contains all reconstruction fields. The terms "image," "reconstructed image," and "image region" refer to the region of the reconstruction field illuminated by the light of the holographic reconstruction. In some embodiments, an "image" may include discrete spots that may be referred to as "image spots" or, for convenience only, "image pixels."

[0055] The terms "encoding," "writing," or "addressing" are used to describe the process of providing a plurality of pixels of an SLM with respective control values ​​that respectively determine the modulation level of each pixel. The pixels of the SLM can be said to be configured to "display" a light modulation distribution in response to receiving the control values. The SLM can therefore be said to "display" a hologram, and a hologram can be thought of as an array of light modulation values ​​or levels.

[0056] It has been shown that holographic reconstructions of acceptable quality can be formed from "holograms" that contain only phase information related to the Fourier transform of the original object. Such holographic recordings are sometimes called phase-only holograms. While the embodiments relate to phase-only holograms, the present disclosure is equally applicable to amplitude-only holography.

[0057] The present disclosure is also equally applicable to forming a holographic reconstruction using amplitude and phase information associated with the Fourier transform of the original object. In some embodiments, this is achieved by complex modulation using a so-called fully complex hologram, which contains both amplitude and phase information about the original object. Such holograms are sometimes referred to as fully complex holograms because the value (grayscale) assigned to each pixel of the hologram has an amplitude and a phase component. The value (grayscale) assigned to each pixel may be represented as a complex number having both an amplitude and a phase component. In some embodiments, a fully complex computer-generated hologram is calculated.

[0058] "Phase delay" can be shorthand for a phase value, phase component, phase information, or simply the phase of a computer-generated hologram or a pixel of a spatial light modulator. That is, any phase value described is actually a number (e.g., in the range of 0 to 2π) representing the amount of phase delay provided by that pixel. For example, a spatial light modulator pixel described as having a phase value of π / 2 delays the phase of received light by π / 2 radians. In some embodiments, each pixel of a spatial light modulator is operable at one of multiple possible modulation values ​​(e.g., phase delay values). The term "grayscale" may be used to refer to multiple available modulation levels. For example, the term "grayscale" may be conveniently used to refer to multiple available phase levels in a phase-only modulator, even if different phase levels do not provide different grayscales. The term "grayscale" may also be conveniently used to refer to multiple available complex modulation levels in a complex modulator.

[0059] Thus, a hologram comprises an array of gray scales, i.e., an array of optical modulation values, such as an array of phase delay values ​​or complex modulation values. A hologram is also considered a diffraction pattern because it is a pattern displayed on a spatial light modulator and causes diffraction when illuminated with light having a wavelength comparable to, but generally less than, the pixel pitch of the spatial light modulator. Reference is made herein to combining holograms with other diffraction patterns, such as diffraction patterns that function as lenses or gratings. For example, a diffraction pattern that functions as a grating may be combined with a hologram to translate the reconstruction field on the reconstruction plane, or a diffraction pattern that functions as a lens may be combined with a hologram to focus the holographic reconstruction on the reconstruction plane in the near field.

[0060] In the following detailed description, different embodiments and groups of embodiments may be disclosed separately, but 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, i.e., all possible combinations and permutations of features disclosed in this disclosure are contemplated.

[0061] Particular embodiments will now be described, by way of example only, with reference to the following figures: [Brief explanation of the drawings]

[0062] [Figure 1] FIG. 1 is a schematic diagram showing a reflective SLM generating a holographic reconstruction on a screen. [Figure 2A] FIG. 1 illustrates the first iteration of an exemplary Gerchberg-Saxton type algorithm. [Figure 2B] FIG. 1 illustrates second and subsequent iterations of an exemplary Gerchberg-Saxton type algorithm. [Figure 2C] FIG. 10 illustrates alternative second and subsequent iterations of an exemplary Gerchberg-Saxton type algorithm. [Figure 3] FIG. 1 is a schematic diagram of a reflective LCOS SLM. [Figure 4] FIG. 10 is a diagram showing the phase delay dependence on voltage for light of three wavelengths. [Figure 5] FIG. 1 shows a column representing three monochromatic holograms divided into conventional full-resolution grayscales. [Figure 6] FIG. 10 shows a column representing three monochromatic holograms divided into gray levels according to an embodiment. [Figure 7] 7A-7C illustrate possible phase values ​​for pixels of each of the three monochromatic holograms of FIG. 6, according to an embodiment. [Figure 8] 1 shows a conventional multicolor holographic reconstruction of a target image; and [Figure 9]FIG. 9 illustrates a multi-color holographic reconstruction of the target image of FIG. 8, according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0063] The same reference numbers are used throughout the drawings to refer to the same or similar parts. The present invention is not limited to the embodiments described below, but extends to the full scope of the appended claims, i.e., the present invention may be embodied in different forms and should not be construed as being limited to the embodiments set forth for illustrative purposes.

[0064] Singular terms may include plurals unless specifically stated otherwise.

[0065] A structure described as being formed on top / bottom of or above / below another structure should be interpreted to include when the structures contact each other and also when a third structure is disposed between them.

[0066] In describing temporal relationships, for example, when the temporal order of events is described as "after," "succeeding," "next," "before," etc., the present disclosure should be construed to include consecutive and non-sequential events unless otherwise stated. For example, unless expressions such as "just," "immediately," or "directly" are used, the description should be construed to include non-sequential cases.

[0067] In this specification, terms such as "first" and "second" may be used to describe various elements, but these elements are not limited by these terms. These terms are used only to distinguish one element from another. For example, a first element could be referred to as a second element, and similarly, a second element could be referred to as a first element, without departing from the scope of the appended claims.

[0068] Features of different embodiments may be partially or wholly combined or combined with one another and may interoperate with one another in various ways. Some embodiments may be implemented independently of one another or may be implemented together in a co-dependent manner.

[0069] In this disclosure, the term "substantially" when applied to a structural unit of a device may be interpreted as the technical characteristics of the structural unit being produced within the technical tolerances of the method used to produce it.

[0070] optical configuration FIG. 1 shows an embodiment in which a computer-generated hologram is encoded on a single spatial light modulator. The computer-generated hologram is the Fourier transform of the object for reconstruction. Therefore, the hologram can be said to be 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 on a replay field, e.g., a receiving surface such as a screen or diffuser.

[0071] A light source 110, e.g., a laser or laser diode, is positioned to illuminate the SLM 140 through a collimating lens 111. The collimating lens directs a substantially planar wavefront of light into the SLM. In FIG. 1, the wavefront direction is not normal (e.g., 2 or 3 degrees away from true orthogonal to the plane of the transparent layer). However, in other embodiments, a substantially planar wavefront is provided at normal incidence, and a beam splitter device is used to separate the input and output optical paths. In the embodiment shown in FIG. 1, light from the light source is positioned to reflect off the mirror-inverted rear surface of the SLM and interact with the light modulating layer to form an exit wavefront 112. The exit wavefront 112 is applied to an optical system including a Fourier transform lens 120 having its focus at a screen 125. More specifically, the Fourier transform lens 120 receives the beam of modulated light from the SLM 140 and performs a frequency-space transformation to generate a holographic reconstruction on the screen 125.

[0072] In particular, in this type of holography, each pixel of the hologram contributes to the overall reconstruction: there is no one-to-one correlation between a specific point on the reconstructed field (or image pixel) and a specific light-modulating element (or hologram pixel). In other words, the modulated light leaving the light-modulating layer is distributed across the reconstructed field.

[0073] In these embodiments, the position of the holographic reconstruction in space is determined by the refractive power (focusing power) of the Fourier transform lens. In the embodiment shown in FIG. 1, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens, and the Fourier transform is performed optically. Any lens can act as a Fourier transform lens, but the performance of the lens limits the accuracy of the Fourier transform it performs. Those skilled in the art understand how to perform an optical Fourier transform using lenses.

[0074] Hologram Calculation In some embodiments, the computer-generated hologram is a Fourier transform hologram, or simply a Fourier hologram or Fourier-based hologram, in which an image is reconstructed in the far field by exploiting the Fourier transform properties of a positive lens. A Fourier hologram is calculated by Fourier transforming a desired light field in the reconstruction plane to the lens plane. A computer-generated Fourier hologram can be calculated using the Fourier transform.

[0075] Fourier transform holograms can be calculated using algorithms such as the Gerchberg-Saxton algorithm. Furthermore, the Gerchberg-Saxton algorithm can be used to calculate Fourier domain holograms (i.e., Fourier transform holograms) from amplitude-only information in the spatial domain (such as a photograph). Phase information about an object is effectively "retrieved" from amplitude-only information in the spatial domain. In some embodiments, computer-generated holograms are calculated from amplitude-only information using the Gerchberg-Saxton algorithm or variations thereof.

[0076] The Gerchberg Saxton algorithm calculates the intensity cross section I of a light beam at planes A and B. A (x, y) and I B (x, y) are known, and I A (x, y) and I B Consider the situation when (x, y) are related by a single Fourier transform: with a given intensity cross section, the phase distribution Ψ in planes A and B A (x, y) and Ψ B An approximation to (x, y) is found for each. The Gerchberg-Saxton algorithm finds a solution to this problem by following an iterative process. More specifically, the Gerchberg-Saxton algorithm finds the solution to I A (x, y) and I BA data set (amplitude and phase) representing (x, y) is repeatedly transferred between the spatial and Fourier (spectral or frequency) domains, while iteratively applying spatial and spectral constraints. A corresponding computer-generated hologram in the spectral domain is obtained through at least one iteration of the algorithm. The algorithm is configured to converge and produce a hologram representing the input image. The hologram may be an amplitude-only hologram, a phase-only hologram, or a fully complex hologram.

[0077] In some embodiments, phase-only holograms are calculated using an algorithm based on the Gerchberg-Saxton algorithm, such as those described in British Patent Nos. 2,498,170 or 2,501,112, the entire contents of which are incorporated herein by reference. However, the embodiments disclosed herein describe calculating phase-only holograms by way of example only. In these embodiments, the Gerchberg-Saxton algorithm retrieves phase information Ψ[x,y] of the Fourier transform of a data set that yields known amplitude information T[u,v], which represents a target image (e.g., a photograph). Because magnitude and phase are inherently combined in the Fourier transform, the transformed magnitude and phase contain useful information about the accuracy of the calculated data set. Therefore, the algorithm can be used iteratively with feedback on both the amplitude and phase information. However, in these embodiments, only the phase information Ψ[u,v] is used as a hologram to form a holographic representation of the target image at the image plane. A hologram is a data set (e.g., a 2D array) of phase values.

[0078] In another embodiment, an algorithm based on the Gerchberg-Saxton algorithm is used to calculate a full complex hologram. A full complex hologram is a hologram that has a magnitude component and a phase component. A hologram is a data set (e.g., a 2D array) that includes an array of complex data values, each of which includes a magnitude component and a phase component.

[0079] In some embodiments, the algorithm processes complex data and the Fourier transform is a complex Fourier transform. The complex data can be thought of as including (i) a real component and an imaginary component, or (ii) a magnitude component and a phase component. In some embodiments, the two components of the complex data are treated differently at various stages of the algorithm.

[0080] FIG. 2A illustrates the first iteration of an algorithm according to some embodiments for computing phase-only holograms. The input to the algorithm is an input image 210, which includes a two-dimensional array of pixels or data values, each of which is a magnitude or amplitude value. That is, each pixel or data value in the input image 210 does not have a phase component. Therefore, the input image 210 can be thought of as a magnitude-only, amplitude-only, or intensity-only distribution. An example of such an input image 210 is a photograph or a frame of a video, which includes a time series of frames. The first iteration of the algorithm begins with a data formation step 202A, which involves assigning a random phase value to each pixel of the input image using a random phase distribution (or random phase seed) 230 to form a starting complex data set, where each data element of the set includes both magnitude and phase. The starting complex data set can be said to represent the input image in the spatial domain.

[0081] First processing block 250 receives a starting complex data set and performs a complex Fourier transform to form a Fourier transform complex data set. Second processing block 253 receives the Fourier transform complex data set and outputs hologram 280A. In some embodiments, hologram 280A is a phase-only hologram. In these embodiments, second processing block 253 quantizes each phase value and sets each amplitude value to units to form hologram 280A. Each phase value is quantized according to the phase levels that can be represented on a pixel of a spatial light modulator used to "display" the phase-only 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 phase-only Fourier hologram representing an input image. In other embodiments, hologram 280A is a full complex hologram including an array of complex data values ​​(each including an amplitude component and a phase component) derived from the received Fourier transform complex data set. In some embodiments, second processing block 253 constrains each complex data value to one of a plurality of allowable complex modulation levels to form hologram 280A. The constraining step may include setting each complex data value to the closest allowable complex modulation level in the complex plane. Hologram 280A may be said to represent the input image in the spectral domain, Fourier domain, or frequency domain. In some embodiments, the algorithm stops at this point.

[0082] However, in other embodiments, the algorithm continues as represented by the dotted arrow in Figure 2A. In other words, the steps following the dotted arrow in Figure 2A are optional (i.e., not required for all embodiments).

[0083] A third processing block 256 receives the modified complex data set from the second processing block 253 and performs an inverse Fourier transform to form an inverse Fourier transformed complex data set. The inverse Fourier transformed complex data set is said to represent the input image in the spatial domain.

[0084] The fourth processing block 259 receives the inverse Fourier transformed complex data set and extracts a magnitude value distribution 211A and a phase value distribution 213A. Optionally, the fourth processing block 259 evaluates the magnitude value distribution 211A. Specifically, the fourth processing block 259 may compare the magnitude value distribution 211A of the inverse Fourier transformed complex data set with the input image 210, which is itself a magnitude value distribution. If the difference between the magnitude value distribution 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 magnitude value distribution 211A and the input image 210 is sufficiently small, the fourth processing block 259 can determine that the hologram 280A represents the input image 210 sufficiently accurately. In some embodiments, the phase value distribution 213A of the inverse Fourier transformed complex data set is ignored for comparison purposes. It will be understood that any number of different methods for comparing magnitude value distribution 211A to input image 210 may be used, and the present disclosure is not limited to any particular method. In some embodiments, a mean squared difference is calculated, and if the mean squared difference is less than a threshold, hologram 280A is deemed acceptable. If fourth processing block 259 determines that hologram 280A is unacceptable, further iterations of the algorithm may be performed. However, this comparison step is not required, and in other embodiments, the number of iterations of the algorithm performed is predetermined, pre-set, or user-defined.

[0085] 2B represents the second iteration of the algorithm and any further iterations of the algorithm. The phase value distribution 213A of the previous iteration is fed back through the processing blocks of the algorithm. The magnitude value distribution 211A is rejected in favor of the magnitude value distribution of the input image 210. In the first iteration, the data formation step 202A combined the magnitude value distribution of the input image 210 with the random phase distribution 230 to form the first complex data set. However, in the second and subsequent iterations, the data formation step 202B includes forming a complex data set by combining (i) the phase value distribution 213A from the previous iteration of the algorithm and (ii) the magnitude value distribution of the input image 210.

[0086] The complex data set formed by data formation step 202B of FIG. 2B is then processed in the same manner as described with reference to FIG. 2A to form second iteration hologram 280B. Therefore, the description of the process will not be repeated here. The algorithm can stop when second iteration hologram 280B is calculated. However, any number of further iterations of the algorithm may be performed. It will be understood that third processing block 256 is only required if fourth processing block 259 or further iterations are required. The output hologram 280B generally gets better with each iteration. However, in practice, a point is usually reached where no measurable improvement is observed or the positive benefits of performing further iterations are offset by the negative effects of additional processing time. Therefore, the algorithm is described as iterative and convergent.

[0087] 2C depicts an alternative embodiment for the second or subsequent iteration. The distribution of phase values ​​213A of the previous iteration is fed back through the processing blocks of the algorithm. The distribution of magnitude values ​​211A is rejected in favor of an alternative distribution of magnitude values. In this alternative embodiment, the alternative distribution of magnitude values ​​is derived from the distribution of magnitude values ​​211A of the previous iteration. Specifically, processing block 258 subtracts the distribution of magnitude values ​​of the input image 210 from the distribution of magnitude values ​​211A of the previous iteration, scales the difference by a gain factor α, and subtracts the scaled difference from the input image 210. This is mathematically represented by the following equation, where the subscripts and numbers indicate the iteration number:

number

number

[0088] The gain factor α may be fixed or variable. In some embodiments, the gain factor α is determined based on the size and rate of the incoming target image data. In some embodiments, the gain factor α depends on the number of iterations. In some embodiments, the gain factor α is simply a function of the number of iterations.

[0089] The embodiment of Figure 2C is in all other respects identical to the embodiment of Figures 2A and 2B. A phase-only hologram Ψ(u,v) can be said to contain a phase distribution in the frequency domain or Fourier domain.

[0090] In some embodiments, the Fourier transform is performed using a spatial light modulator. Specifically, the hologram data is combined with second data that provides optical power. That is, the data written to the spatial light modulator includes hologram data representing the object and lens data representing the lens. When displayed on the spatial light modulator and illuminated with light, the lens data emulates a physical lens, i.e., brings light to a focal point in the same manner as a corresponding physical optical system. Thus, the lens data provides optical or focusing power. In these embodiments, the physical Fourier transform lens 120 in FIG. 1 may be omitted. Methods for calculating the data representing a lens are known. The data representing a lens is sometimes referred to as a software lens. For example, a phase-only lens may be formed by calculating the phase delay caused at each point in the lens due to its refractive index and spatially varying optical path length. 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. An amplitude-only lens may be formed by a Fresnel zone plate. In the field of computer-generated holography, methods are also known for combining data representing lenses with holograms so that the Fourier transform of the hologram can be performed without the need for a physical Fourier lens. In some embodiments, lens data is combined with the hologram by simple addition, such as simple vector addition. In some embodiments, physical lenses are used in conjunction with software lenses to perform the Fourier transform. Alternatively, in other embodiments, the Fourier transform lens is omitted entirely, so that the holographic reconstruction occurs in the far field. In further embodiments, the hologram can be combined in the same way with grating data, i.e., data configured to perform the function of a grating, such as image steering. Again, how to calculate such data is known in the art. For example, a phase-only grating can be formed by modeling the phase delay caused by each point on the surface of a blazed grating. An amplitude-only grating can simply be superimposed with an amplitude-only hologram to provide angular steering of the holographic reconstruction.The second data providing lens and / or steering may be referred to as a light processing function or a light processing pattern to distinguish it from the holographic data, which may be referred to as an image forming function or an image forming pattern.

[0091] In some embodiments, the Fourier transform is performed jointly by a physical Fourier transform lens and a software lens, i.e., some of the optical power contributing to the Fourier transform is provided by a software lens, and the remaining optical power contributing to the Fourier transform is provided by a physical or optical system.

[0092] In some embodiments, a real-time engine is provided that is configured to receive image data and use an algorithm to calculate a hologram in real time. In some embodiments, the image data is a video that includes a sequence of image frames. In other embodiments, the hologram is pre-calculated, stored in computer memory, and recalled as needed for display on the SLM. That is, in some embodiments, a repository of pre-defined holograms is provided.

[0093] The embodiments relate, by way of example only, to Fourier holography and Gerchberg-Saxton type algorithms. The present disclosure is equally applicable to Fresnel holography and Fresnel holograms, which can be calculated by similar methods. The present disclosure is also applicable to holograms calculated by other techniques, such as those based on point cloud methods.

[0094] Light Modulation Spatial light modulators can be used to display diffraction patterns, including computer-generated holograms. If the hologram is a phase-only hologram, a spatial light modulator that modulates phase is required. If the hologram is a fully complex hologram, spatial light modulators that modulate phase and amplitude may be used, or one spatial light modulator that modulates phase and a second spatial light modulator that modulates amplitude may be used.

[0095] In some embodiments, the light modulation elements (i.e., pixels) of the spatial light modulator are cells containing liquid crystals. That is, in some embodiments, the spatial light modulator is a liquid crystal device in which the optically active component is a liquid crystal. Each liquid crystal cell is configured to selectively provide a plurality of light modulation levels. That is, each liquid crystal cell is configured at any one time to operate at one light modulation level selected from a plurality of possible light modulation levels. Each liquid crystal cell is dynamically reconfigurable to a light modulation level different from the plurality of light modulation levels. In some embodiments, the spatial light modulator is a reflective liquid crystal on silicon (LCOS) spatial light modulator, although the disclosure is not limited to this type of spatial light modulator.

[0096] LCOS devices offer a dense array of light-modulating elements, or pixels, within a small aperture (e.g., a few centimeters wide). The pixels are typically about 10 microns or smaller, resulting in a diffraction angle of only a few degrees, allowing for compact optics. Properly illuminating the small aperture of an LCOS SLM is easier than the larger apertures of other liquid crystal devices. LCOS devices are typically reflective, meaning that the circuitry driving the LCOS SLM's pixels can be embedded beneath the reflective surface, resulting in a high aperture ratio. In other words, the pixels are densely packed, meaning there is little dead space between them. This is advantageous because it reduces optical noise in the playback field. LCOS SLMs use silicon backplanes, which have the advantage that the pixels are optically flat. This is particularly important for phase-modulating devices.

[0097] A suitable LCOS SLM is described below, by way of example only, with reference to FIG. 3. The LCOS device is formed using a single-crystal silicon substrate 302. It has a two-dimensional array of square planar aluminum electrodes 301, spaced apart by gaps 301a, disposed on the upper surface of the substrate. Each of the electrodes 301 can be addressed via circuitry 302a embedded in the substrate 302. Each electrode forms a respective 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 of glass. A single transparent electrode 307, for example of ITO, is disposed between the transparent layer 306 and the second alignment layer 305.

[0098] Each square electrode 301, together with the area overlying the transparent electrode 307 and the intervening liquid crystal material, defines a controllable phase-modulating element 308, often referred to as a pixel. The effective pixel area, or fill factor, is the percentage of the total pixel that is optically active, taking into account the space between pixels 301a. By controlling the voltage applied to each electrode 301 relative to the transparent electrode 307, the properties of the liquid crystal material in the respective phase-modulating element can be changed, thereby providing a variable retardation to light incident thereon. The effect is to provide phase-only modulation of the wavefront; i.e., no amplitude effects occur.

[0099] The described LCOS SLM outputs spatially modulated light in reflection. Reflective LCOS SLMs have the advantage that the signal lines, gate lines, and transistors are located below the mirror surface, 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 liquid crystal layer can be half the thickness that would be required using a transmissive device. This significantly improves the switching speed of the liquid crystal (an important advantage for projecting moving images). However, the teachings of this disclosure can be implemented using a transmissive LCOS SLM as well.

[0100] Multiwavelength holographic reconstruction In many practical applications, it is desirable to provide multi-wavelength (i.e., multi-color) holographic reconstructions ("images"). This traditionally requires providing a separate hologram for each color, which must be displayed separately. Each individual hologram must be illuminated separately by light of its corresponding color / wavelength. There are two well-known conventional approaches to multi-color holography: the first is known as spatially-separated colors (SSC), and the second is known as frame-sequential color (FSC).

[0101] The SSC method uses three spatially separated arrays of light-modulating pixels for three respective monochromatic (usually red / green / blue (RGB)) holograms. In SSC, all three holographic reconstructions are formed simultaneously and can be combined (e.g., superimposed) in a common plane to form a resulting multicolor image (i.e., a multicolor holographic reconstruction), which can be very bright. The three spatially separated arrays of light-modulating pixels can be provided spatially separated from one another on a common spatial light modulator (SLM), for example, when spatial and / or financial constraints prevent the provision of multiple SLMs. However, because only a subset of the available light-modulating pixels on the SLM is used for each color, the quality of each monochromatic image in such an arrangement is suboptimal. Therefore, a relatively low-resolution color image is provided. Alternatively, three separate SLMs can be used for SSC, one for each color. Each SLM displays a different respective color hologram, each of which is individually illuminated to output a respective optical channel of spatially modulated light. In such a configuration, beam-combining optics (e.g., X-cube mirrors and dichromic mirrors) are required to combine the three optical channels to allow for the formation of a multicolor image. While this provides a high-quality image, providing multiple SLMs has the advantage that the other required optics are expensive and also significantly impact the size of the resulting optical system. In many situations, such as but not limited to vehicle head-up displays (HUDs), where space is limited and real estate is at a premium, compactness is generally highly desirable. Furthermore, providing multiple SLMs is economically costly.

[0102] The FSC method can display three monochromatic holograms sequentially (i.e., one after the other) using all pixels of a single common spatial light modulator. The monochromatic reconstructions are cycled quickly enough that a human observer can perceive a multicolor image from the integration of the three monochromatic images (e.g., red, green, blue, red, green, blue, etc.). An advantage of FSC is that the entire SLM is used for each color. This means that the quality of the three color images generated is optimal because all pixels of the SLM are used for each color image. However, a disadvantage of the FSC method is that each monochromatic illumination event can only occur for one-third of the frame time, resulting in a composite color image that is approximately three times lower in brightness than the SSC method. This disadvantage can potentially be addressed by overdriving the laser or using a more powerful laser, but this requires more power, resulting in higher costs and larger system size. Furthermore, FSC has the disadvantage of fewer subframes available for tile shifting and other techniques traditionally applied to improve image quality.

[0103] The inventors have devised an improved method and system for delivering high-quality multicolor (i.e., multi-wavelength) holography in a compact, efficient, and cost-effective manner, which has not been possible before.

[0104] The present inventors have devised a method and system for providing diffractive structures (which may be referred to as "kinoforms" or "holograms") that can simultaneously deliver multiple images when displayed and illuminated on a single display device, such as a spatial light modulator (SLM), such as a liquid crystal on silicon (LCOS) SLM. For example, when displayed and illuminated with light of corresponding wavelengths, two or more different color images can be simultaneously (or at least substantially simultaneously) delivered. The image content of each color may at least partially physically overlap with the image content of each of the other colors, or may be physically separated from each other on the reproduction plane. The image content of each color may be different from the image content of each of the other colors, or individually colored images may contain common or overlapping image content that combine / superimpose on the reproduction plane to form a single multicolor image of that image content.

[0105] It is known that high-quality holographic projection typically requires a display device including pixels capable of providing a phase difference of up to 2π. The present disclosure relates to the display of diffractive structures in high-quality display devices. For example, a display may be made using a high-resolution (high pixel density) reflective liquid crystal display and fast-switching pixels for holographic projection at video rates using phase holograms. However, other types of holograms, such as complex holograms (containing both phase and amplitude components), are also contemplated within the present disclosure. According to at least some embodiments of the present disclosure, a high-resolution display may be defined as one having a pixel pitch of 5 μm or less, e.g., less than 2 μm. However, this numerical example should not be considered limiting of the present disclosure.

[0106] The total retardation (ie, phase delay) Φ in the reflective cell satisfies the following equation: Φ=4πdΔn / λ (1)

[0107] where d is the cell gap (thickness), Δn is the birefringence of the liquid crystal, and λ is the wavelength of light. The product dΔn is known as the path difference. Therefore, both the type of liquid crystal and the cell thickness affect the configurable retardation that the cell applies to incident light. According to one embodiment, the method and system can be implemented using planar-aligned nematic cells containing liquid crystals with positive dielectric anisotropy, as this configuration has been found to be effective for phase holography. The response time of such cells is related to the square of the intercellular gap. However, the present disclosure is not limited to such cells.

[0108] The effective birefringence Δn exhibited by liquid crystals is known to be voltage-dependent. Therefore, the actual retardation applied by a given LC cell depends not only on the wavelength of the illuminating light but also on the voltage applied to the cell at any given time. Furthermore, the voltage range required for the LC cell to deliver the full range of phase retardation (i.e., a phase retardation ranging from 0 to 2π) depends on the wavelength of the illuminating light. The longer the wavelength, the greater the voltage range required to achieve the full range of phase retardation. Thus, each cell (or "pixel") in a high-quality display device can be controlled to deliver a specific phase retardation (i.e., a selected retardation within the range of available retardations) at any given time, depending on the voltage applied to that cell and the wavelength of the illuminating light. Thus, when the cell is illuminated with light of different respective wavelengths, the cell typically applies a different respective phase retardation to each wavelength of light at any given voltage. Conventionally, when multicolor holography is required, this results in the display of separate diffractive structures, each tuned for illumination with light of a different respective wavelength, and appropriate voltages can be applied separately to the cells displaying each individual diffractive structure to meet the phase delay requirements for each color image. However, the present inventors have devised a method, as described in more detail below, that avoids the need to display multiple different diffractive structures for multicolor holography.

[0109] When a diffractive structure, such as a hologram or kinoform, is computed, it contains multiple individual pixels, each of which can be displayed in a respective pixel of a display device. During computation, each hologram pixel is quantized into a number of allowable "grayscales," where grayscale is a quantization of the magnitude of the phase delay that the hologram pixel imparts when displayed and properly illuminated. The number of available grayscales determines the tonal resolution, i.e., it is a measure of the number of different possible discrete modulation levels each hologram pixel can impart, ranging, for example, from 0 to 2π. This traditionally affects the precision of the hologram, i.e., the tonal resolution, and therefore the quality or accuracy (i.e., fidelity) of the corresponding image reconstruction.

[0110] When a hologram is displayed on a display device, the voltage range over which the full (2π) range of phase retardation can be delivered by the display device's cells for a particular wavelength determines the voltage increments required by the cells between adjacent hologram gray levels ("GL"). For example, if each cell requires a voltage range of 0V to 5V to achieve a maximum phase retardation of 2π for red light, and if the displayed scheme contains 128 evenly spaced gray levels, each gray level must be approximately 40mV apart. Thus, for example, achieving the 12th gray level, GL12, for red light requires an applied voltage of 480mV. The display system can be calibrated accordingly.

[0111] On the other hand, the same cell (with the same thickness and birefringence) may require only a voltage range of 0V to 2V to achieve a phase delay range of 0 to 2π for a different wavelength of light, such as green light. If those cells are calibrated for a "red hologram" (i.e., a hologram configured to be illuminated by red light) with a 40mV gap between adjacent discrete gray levels, when the cell displays a "green hologram" configured to be illuminated by green light, the entire range of phase delay is delivered by the first 50 gray levels. As a result, the difference in phase delay that can be imparted by the available gray levels of the green hologram is reduced compared to the red hologram. In other words, for the same cell voltage calibration, the "gray level resolution" (or precision) of the green hologram will be smaller than that of the red hologram.

[0112] Thus, the conventional approach is to use different calibrations (i.e., different respective cells (e.g., different cell gaps), or use the same cells but calibrated differently at different respective times) to display holograms illuminated by different respective wavelengths. Thus, the practice is to ensure that the grayscale resolution is individually optimized / maximized for each (i.e., all) respective color hologram. Thus, in the above example, the cells could be calibrated to separate adjacent grayscales in the green hologram by 15.6 mV, resulting in the full 128 grayscales (not just 50) being available for displaying the green hologram.

[0113] The inventors defy convention in determining that accurate multicolor images can be achieved by providing non-ideal tonal resolution for one or more color holograms. Furthermore, they have determined that using imperfect tonal resolution for one or more colors means that the same (single) hologram displayed on a display device can be illuminated with light of different colors simultaneously (or at least in very rapid succession) to form multiple different monochromatic images, all of which can be of acceptably high quality. This has not been possible with conventional holography. Furthermore, it defies significant preconceptions and conventional expectations in the field of holography.

[0114] Broadly, the inventors have devised a method and system that can provide one (i.e., a single) hologram that, when displayed and illuminated using a single display device, forms multiple images. The hologram can fill all pixels of the display device. Each (i.e., all) pixel of the hologram can contribute to each of the multiple images. For example, a single display device can be used to provide a single hologram that, when properly illuminated, can (substantially) simultaneously form red (R), green (G), and blue (B) images. This is possible even if the image content of each of the R, G, and B target images is different. This is possible even if the image content of each of the R, G, and B target images physically overlaps each other or is spatially distinct from each other on the reconstruction plane.

[0115] The disclosed method broadly involves computing multiple separate individual holograms and identifying an optimization that can adequately represent each of those holograms simultaneously by displaying the single hologram at a common (i.e., the same) pixel of a single display device. For example, this may involve computing a separate hologram of the target image for each of multiple wavelengths, e.g., computing separate red (R), green (G), and blue (B) holograms, as is well known in traditional multicolor holography. However, in a departure from conventional practice, the disclosed method considers one or more voltages to which each cell of the display device can be driven to achieve the required phase delay for the corresponding hologram pixel of each of the multiple individual holograms. Then, by applying intelligently selected biases and using computationally accurate optimization methods, the individual holograms are combined into a single optimized hologram, and the display device can be calibrated and driven accordingly. Key to the success of this approach is the inclusion of "phase wrapping" to provide appropriate selection options.

[0116] Thus, in effect, the method disclosed herein identifies an optimized hologram that simultaneously sufficiently well represents each of multiple individual holograms. Thus, the method eliminates the need to separately display each individual hologram, since each hologram is adequately represented by an optimized hologram and can be illuminated by its respective light source using a single display device and a single (common) calibration for the display device. Furthermore, the optimized holograms (or common calibration) simultaneously representing each hologram can be displayed across the entire display device, thereby enabling good image quality and low noise for each image. This is surprising and provides significant space and economic savings compared to conventional holographic techniques. It also significantly reduces the complexity of multicolor projectors. In some embodiments, only one display device (and associated optics and electronics) is required, rather than three.

[0117] The disclosed method takes advantage of so-called "phase wrapping" (or "phase repetition"). The inventors observed that phase wrapping occurs when an LC cell is driven to a voltage greater than the minimum voltage required to provide a 2π phase retardation for any given wavelength. In conventional holography, each cell in a display device is driven only to the minimum voltage required to achieve that cell's specific phase retardation for a given wavelength of illumination. However, the inventors recognized that, due to phase wrapping, increasing the voltage applied to an LC cell beyond the minimum voltage required to provide a 2π phase retardation for any given wavelength results in the phase retardation provided by the cell increasing beyond 2π for that wavelength. Furthermore, they recognized that the retardation imparted by the cell is, in fact, repeatable with a 2π repeat period. This means that the retardation imparted by the cell between 0 and 2π is repeated between 2π and 4π. Therefore, the phase retardation at "θ" is the same as the phase retardation imparted at "θ+m2π," where "m" is any non-zero integer. For example, a phase delay of π has the same effect as a phase delay of 3π, and so on.

[0118] Thus, the inventors have recognized that there are multiple different phase retardation values ​​(i.e., one value between 0 and 2π, another value between 2π and 4π, etc.) that have the same retardation effect on light of a given wavelength, i.e., multiple different phase retardation values ​​that spatially modulate light of that wavelength in the same way. Correspondingly, the inventors have recognized that there are multiple different voltages that can drive an LC cell of a particular type and thickness to provide a desired phase retardation for light of a given wavelength. With this in mind, the inventors have deviated from convention and further recognized that, at least in some circumstances, it is possible to simultaneously identify a common voltage for each of multiple different holograms illuminated by different respective wavelengths of light that can be selected to provide at least an acceptable approximation of the respective desired phase retardation. Thus, when a common voltage is identified for each pixel, the result can be a set of voltage pixel drive values ​​that corresponds to a single optimized hologram that simultaneously represents each of the multiple different holograms. Thus, a single optimized hologram can be used instead of multiple individual monochromatic holograms. This has not been possible until now.

[0119] The inventors have recognized that, typically, to identify such a common voltage for each pixel of an optimized hologram, the display system must be configured to be driven over a voltage range that provides multiple voltage options for achieving the required phase delay for at least some, and possibly all, of the different wavelengths (corresponding to the different holograms to be represented). For example, if the display system is configured to provide a single hologram for representing multiple monochromatic holograms of different colors, each configured for illumination with light of different wavelengths, consideration must be given to what happens to the shorter wavelength light when the display device cell is driven with the relatively high voltage required to achieve the full range of phase delay for the longer wavelength hologram. For example, as will be further appreciated from the detailed example below, if individual red (R), green (G), and blue (B) holograms are replaced with a single optimized hologram, the display system may need to be driven at least within a voltage range that provides a phase delay range of 0 to 2π for red light, which has the longest wavelength of the three colors. Such a range may provide, for example, two voltage level options for each phase delay value of the green hologram and two or three voltage level options for each phase delay value of the blue hologram. At least in some cases, it may be desirable to drive the display system to still higher voltage values ​​to allow multiple voltage level options for at least some of the phase retardation values ​​of the red hologram.

[0120] Thus, the inventors have determined that it is possible to use a single common encoding (or "configuration") of the cells of a display device to effectively display and illuminate multiple different holograms simultaneously via a single optimized hologram. The encoding involves applying a "best fit" or "optimized" voltage to each cell of the display device to accurately represent each of multiple individual wavelength holograms simultaneously. The voltage applied to at least some of the cells may be higher than the minimum voltage required to achieve a full 2π phase delay for the wavelength of light corresponding to at least one of the holograms. Thus, the applied voltage may be higher than conventionally expected or required to represent one or more of the holograms on the display device.

[0121] In practice, when displaying an optimized hologram, the display device's cells are calibrated to provide the same predetermined voltage gap between adjacent gray levels for each of the multiple holograms represented by the optimized (multi-wavelength) hologram. While this may require using a lower-than-ideal resolution for at least one hologram (e.g., a hologram illuminated with relatively short wavelength light, such as blue (B) light) to utilize fewer gray levels to subdivide the respective 2π phase delay range, the inventors have found that the resulting image is still of acceptably high quality. Therefore, on balance, the methods and systems disclosed herein are highly beneficial because they enable a single display device to be used for the simultaneous display of multiple holograms. For example, this allows a single display device to simultaneously display a group of red / green / blue (RGB) holograms without the typical sacrifices associated with conventional FSC technology. This has significant advantages in terms of the compactness and efficiency of the resulting optical system as well as its economic cost-effectiveness.

[0122] The methods and systems disclosed herein may be further understood in connection with the accompanying drawings.

[0123] FIG. 4 shows the voltage dependence of the phase retardation (in the range of 0 to 2π) that can be provided by an exemplary display device (in this case, an LCOS SLM) for each of red, green, and blue (RGB) holograms when displayed on an LCOS display and illuminated with light of the corresponding color. As can be seen from the top line 410 of FIG. 4, the LCOS must be driven between 0 V and 5.0 V to provide the full range of phase retardation for red light. As can be seen from the middle line 420 of FIG. 4, the LCOS only needs to be driven between 0 V and 2.0 V to provide the full range of phase retardation for green light. As can be seen from the bottom line 430 of FIG. 4, the LCOS only needs to be driven between 0 V and 0.7 V to provide the full range of phase retardation for blue light.

[0124] FIG. 5 shows that there are 128 possible hologram gray levels ranging from GL0 to GL127, subdividing the 0-2π phase delay range along the x-axis. While this disclosure is not limited to the use of 128 gray levels, this is common for display devices using 7-bit drive schemes. FIG. 5 also shows three columns of histograms for each of the red 510, green 520, and blue 530 wavelengths of light, uniformly subdividing the 0-2π phase delay range into 128 discrete gray levels. As FIG. 4 shows, if each color is represented with the same resolution using all 128 gray levels, a different respective voltage is conventionally required to achieve a maximum phase delay of 2π for each color in the full 0-2π phase delay range, such that the voltage gap between adjacent gray levels is wavelength dependent. Therefore, conventionally, a display device is arranged for separate calibration of the holograms of each respective color (or multiple display devices are provided, each calibrated to a single respective color) so that the holograms of each color can be displayed using all 128 gray levels. Thus, red light in this example has a gap of about 40mV between adjacent grey levels, while green light has a gap of about 15.6mV between adjacent grey levels, and blue light has a gap of about 5.5mV between adjacent grey levels.

[0125] Contrary to convention, the inventors have discovered that it is possible to provide a single common calibration of the cells of an LC device to effectively display multiple holograms simultaneously, each hologram configured for illumination with a different respective wavelength of light. This is illustrated in Figure 6, which includes three columns each representing the deliverable phase delay over a common voltage range by a single display device for three holograms (RGB) represented by a single optimized hologram according to the present disclosure. In this example, the display device is similar to the exemplary display device of Figure 4, and the voltage range shown is 0 to 5.0 V.

[0126] As can be seen in Figure 6, we have effectively extended the range of phase angles for holograms constructed for illumination with each of the two shorter wavelengths (green 620 and blue 630) to provide a sufficient voltage range (0-5.0 V) for the full 0-2π phase delay range deliverable for a third hologram constructed for illumination with a longer wavelength (red 610). The 0-5.0 V voltage range is subdivided into 128 gray levels (GL0-GL127) for each of the three holograms.

[0127] For green light 620, a voltage range of 0 to 5.0 V allows for a maximum phase delay of greater than 3π, and for blue light 630, a voltage range of 0 to 5.0 V allows for a maximum phase delay of greater than 6π. However, due to phase wrapping, the phase delay at a phase angle of "θ" becomes the same as the phase delay imparted at "θ + m2π" (where "m" is any non-zero integer). Therefore, this expansion of the phase angle range does not actually increase the number of discrete phase delays that can be imparted by the green and blue holograms. On the contrary, this means that the full 0 to 2π range of each of the green and blue wavelengths is compressed into smaller respective voltage ranges and correspondingly subdivided into fewer than 128 discrete gray levels. In this example, as can be seen in Figure 6, the full 0 to 2π range of green light is compressed to 73 gray levels (GL0 to GL72), and the full 0 to 2π range of blue light is compressed to approximately 36 gray levels (GL0 to GL35). Conversely, the red hologram has a phase delay range of 0 to 2π over a voltage range of 0 to 5.0 V, thereby using all 128 gray levels to distribute the possible phase delays that the red hologram can impart. As a result, the resolution of each of the green and blue holograms is reduced compared to that of the red hologram, and the resolution of the blue hologram is reduced compared to that of the green hologram. This would traditionally be expected to adversely affect the corresponding green and blue images when those holograms are displayed and illuminated on a display device. However, the inventors have confirmed that this effect is insignificant with the optimized multi-wavelength holograms disclosed herein.

[0128] FIG. 7 illustrates an example of how a multi-wavelength hologram can be determined based on three individual RGB holograms using the exemplary display device of FIGS. 4-6 and the common RGB cell calibration and expanded phase angles (i.e., phase wrapping) of the G and B holograms described above in connection with FIG. 6 . This example should be considered merely illustrative, and the colors and phase delay values ​​used in this example are not intended to limit the present disclosure. In FIG. 7 , each of the three columns relates to the same single cell of the display device and indicates the phase delay (and therefore the corresponding required voltage) required for each pixel of the three holograms displayed in that single cell when each color hologram is displayed individually. The phase delay required for red hologram 710 is indicated by first dashed line 711 and is shown to be just below π, which corresponds to approximately the gray level GL62 of the red hologram. The phase delay required for green hologram 720 is indicated by second dashed line 721 and is also shown to be just below π, which corresponds to approximately the gray level GL33 of the green hologram. The required phase delay for blue hologram 730 is shown by third dashed line 731 and is also shown to be less than π, which corresponds to approximately gray level GL14 for the blue hologram. Thus, even though the magnitude of the required phase delay is similar for this pixel in each of the three holograms, the gray levels corresponding to each phase delay will be (significantly) different for each wavelength in this example, due to the common calibration and wavelength dependence of the phase delay deliverable by the LC cell.

[0129] In this example, because phase wrapping is used for the green 720 and blue 730 holograms, the green 720 and blue 730 columns each show multiple possible voltages and corresponding gray levels that can impart the desired phase delay for that respective color. Thus, the green 720 column shows a fourth dashed line 721' at GL 104, where the desired phase delay for the green hologram is imparted. The blue column 730 shows a fifth dashed line 731' at approximately GL 53 and a sixth dashed line 731'' at approximately GL 124, where the desired phase delay for the blue hologram is imparted. Thus, in this example, within the voltage range of 0-5.0 V for the selected display device, there is one possible choice of gray level to represent the required red phase delay, two possible choices of gray level to represent the required green phase delay, and three possible choices of gray level to represent the required blue phase delay. The possible options indicated by the dashed lines on the three columns of Figure 7 are sometimes referred to as "ideal" hologram values ​​for each color. However, the inventors have found that it is possible to use hologram values ​​that differ from the ideal values ​​for at least one color (often different from the ideal values ​​for all three colors) and still represent each hologram sufficiently accurately.

[0130] The inventors have recognized that this provision of multiple options for imparting desired / required phase delays to one or more holograms of multiple colors (i.e., in the case of holograms configured to be illuminated by one or more different wavelengths of light) provides a range for finding a phase delay that is at least an acceptable approximation of the phase delay required for two or more different holograms simultaneously. In other words, the use of a common cell calibration to display each of multiple holograms of different colors and using an increased voltage range to exploit the effect of phase wrapping, thereby delivering a corresponding expanded range of phase angles for at least one of those holograms, makes it possible to determine a single "optimized" or "best-fit" hologram that can simultaneously represent (and therefore replace) multiple individual holograms. This is remarkable and has not previously been possible using conventional holographic techniques.

[0131] Although not shown in the example of FIG. 6 (and to which the present disclosure is not limited), the inventors have determined that in some embodiments, it may be appropriate to extend the drive voltage range of a display device beyond the typical voltage range required to achieve a phase delay of 0 to 2π for the longest wavelength hologram (i.e., the red hologram in the example above). This provides two or more options for delivering at least some of the phase delays required for each individual hologram (i.e., each time) to provide a single optimized hologram, thereby improving the likelihood of finding an acceptable compromise between them. Furthermore, the cell type (e.g., birefringence of the LC) and / or cell gap / thickness can be considered and specifically selected to help achieve an optimized combination of the desired phase delays for the selected image or set of images to be rendered.

[0132] In the example of FIG. 7 , a line 750 is drawn across each of the three columns 710, 720, and 730 at approximately GL 46. This line 750 represents the “best fit” or “optimized” hologram value (i.e., optimized phase delay value) for each of the three holograms 710, 720, and 730 simultaneously on a single pixel of a display device, with corresponding “optimized” pixel voltage drive levels determined according to the methods disclosed herein. One embodiment of a method for determining the optimized hologram value is described in detail below. However, broadly, the graphical illustration of FIG. 7 shows that the optimized hologram value is an average value based on three hologram values ​​that are relatively close to each other (one value representing the phase delay required for each of the three respective monochromatic holograms). In this example, these values ​​are the red hologram value 711 (GL 62), the first green hologram value 721 (GL 33), and the second blue hologram value 731′ (GL 53). The optimized hologram values ​​in this example are weighted rather than simply being "averaged" values ​​of the three monochromatic hologram values. The detailed description of the embodiments below provides more information regarding possible approaches to such weighting. However, in at least some circumstances, it is possible to derive the optimized hologram values ​​without weighting and / or using a weighting scheme different from that detailed below.

[0133] The optimized hologram value 750 derived for this pixel may be displayed in a cell of a display device to represent the corresponding pixel of all three (RGB) holograms simultaneously. The process of determining the optimized hologram value may be performed for all pixels of the three holograms, or the entire set of three holograms may be represented by a single optimized hologram that can be displayed and illuminated with light of each of the three colors. Furthermore, this process may be repeated for multiple holograms in rapid succession, for example, to represent successive images in a video-rate sequence of images.

[0134] According to one embodiment, the method for determining an optimized hologram disclosed herein is as follows.

[0135] 1. Separating the target image into individual wavelength-specific channels, e.g., R, G, B. This is known from conventional holography and can be done in any suitable way. 2. Using any suitable hologram calculation technique, calculate the individual holograms for the channels, e.g., R, G, B holograms. This provides the respective phase delay values ​​for each pixel of each individual color hologram. 3. Expand the pixel voltage range for at least the shorter wavelengths (i.e., green and blue) to provide at least the voltage range necessary to achieve a phase delay range of 0 to 2π for the longest wavelength (red). The pixel voltage range for all wavelengths can be expanded. The decision regarding the extent of pixel voltage expansion can be based on the display's look-up table (LUT) for the entire wavelength range being utilized (i.e., spanning the wavelengths of the three colors), which relates voltage values ​​to phase shifts of the diffracted waves. For each pixel, the required phase delay to be imparted for each color (within the range of 0 to 2π) can be identified, and any available iterations of that phase delay can be identified with corresponding distances / phase differences above 2π and above the next nearest integer multiple of 2π, thereby identifying all possible "ideal" R, G, and B hologram values ​​for each pixel. 4. For each pixel, complex phase mixing is performed individually by calculating all possible R, G, and B combinations for all addressable voltages to determine the combination with the smallest distance between the "ideal" R, G, and B phase values. This can be done by examining possible voltage pairs, each pair having a voltage level representing one color and a voltage level representing the respective other color—so, in this example, we have pairs RG, GB, and BR. A "selection bias" (Sb) can then be applied to increase or decrease the weight of any RG, GB, and / or BR distance. In other words, it is possible to separately / independently weight or bias the difference between the two gray levels in each pair (RG, GB, BR). An example of this is shown in Equation (2) below. Regardless of the exact details of how complex phase mixing is performed, this step identifies three hologram values ​​(one for each color) that are relatively close to each other and should be combined to provide an optimized hologram value for that respective pixel. 5. For each pixel, the ideal R, G, and B pixel values ​​identified in step 4 are combined to provide an average value. At this stage, an "averaging bias" (Ab) can be applied to favor certain wavelengths over others. An example of this is shown in equation (3) below. The average value is used to assign a common R, G, and B pixel value containing a gray scale (e.g., GL0-GL127) that can be used as a single optimized hologram value to represent the corresponding pixel in all three holograms simultaneously. 6. Once the above steps are performed pixel by pixel, an optimized multi-wavelength hologram is output.

[0136] The weight / bias values ​​used in step 4 and / or step 5 above may vary, for example, based on the type of image and / or the type or thickness of the LC cell, and / or based on any other suitable factors. The weight / bias values ​​may be predetermined and / or calculated based on one or more instantaneous conditions.

[0137] An example of one way to select which "ideal" hologram values ​​to use for each color in step 4 is given in equation (2) below. |(RG)*RGb|+|(GB)*GBb|+|(BR)*BRb| (2)

[0138] where, for example, "R" = pixel value / second of the "ideal" hologram value (one) of the corresponding color (which can be expressed as a grayscale, GL0 to GL127, as in the example of Figure 7), and, for example, "RGb" = red-green bias (i.e., a coefficient representing the importance / priority / importance placed on the proximity of red and green values).

[0139] Once the R / G / B value combinations have been selected in step 4, one example of how to calculate the average hologram value in step 5 is given by equation (3) below.

number

[0140] For example, Rb = red bias, indicating a preference for a particular wavelength(s) over others, and a bias value of 1 means no bias for that respective wavelength.

[0141] In the above embodiment, the LC cell is configured to modulate the phase of light. Therefore, the light modulation value corresponds to the phase value of a phase-based hologram. In other embodiments, the LC cell may be configured to modulate the amplitude of light or both the amplitude and phase of light. Those skilled in the art will understand that the same principles can be applied to determine the combined (e.g., average) light modulation value (e.g., gray level with corresponding drive voltage), particularly when the light modulation value corresponds to the amplitude value of an amplitude-only hologram or the amplitude and phase values ​​of a fully complex hologram. This is due to the fact that the LC cell can provide substantially the same level of amplitude / amplitude and phase modulation of light in response to different drive voltages, similar to the phenomenon of "phase wrapping" described above. For example, British Patent No. 2,576,552 describes the complex modulation behavior of an LC cell. In particular, for a single wavelength of light, the characteristic curves of amplitude and phase in the complex plane as a function of voltage follow a spiral path. Thus, two or more discrete optical modulation levels (grayscales with corresponding drive voltages) on each adjacent overlapping / concentric spiral portion of the spiral path can be utilized to provide substantially identical amplitude and phase modulation. Similar effects can be observed with respect to the amplitude modulation behavior of LC cells. Therefore, as described herein, by utilizing these effects, it is possible to compute a single multi-wavelength hologram representing two or more amplitude, phase, or complex holograms of different wavelengths / colors.

[0142] We have found that multi-wavelength holograms calculated as described herein can provide high-quality approximations to multiple individual color holograms, as shown in Figures 8 and 9 herein.

[0143] Figure 8 shows the results obtained using the standard Gerchberg-Saxton (GS) algorithm, which involves two known "optimization" techniques, to compute individual R, G, and B holograms of a target image, view and illuminate each hologram separately, and combine the corresponding holographic reconstructions (i.e., (reconstructed) images) at the reconstruction plane to form a multicolor image. The top-left image 801 is an RGB-reconstructed multicolor image from an optimized GS hologram. The top-right image 802 is an individual red reconstruction. The bottom-left image 803 is an individual green reconstruction, and the bottom-right image 804 is an individual blue reconstruction.

[0144] FIG. 9 shows corresponding results obtained using a multi-wavelength hologram according to the present disclosure. The same optimized GS hologram of FIG. 8 was subjected to additional processing, including "selection bias" (Sb) and "average bias" (Ab), according to steps 3-6 above, to form an optimized multi-wavelength hologram (or "multi-wavelength kinoform," (MWK)) according to the present disclosure. The top-left image 901 is an RGB reconstructed polychromatic image from the optimized multi-wavelength hologram. The top-right image 902 is an individual red reconstruction using the red values ​​selected in step 4 of the disclosed method. The bottom-left image 903 is an individual green reconstruction using the green values ​​selected in step 4 of the disclosed method, and the bottom-right image 904 is an individual blue reconstruction using the blue values ​​selected in step 4 of the disclosed method.

[0145] The "quality" of a holographic reconstruction can be quantified in two ways: contrast and mean square error (MSE). Contrast is measured by designating squares of 25x25 pixels over white (255, 255, 255) and black (0, 0, 0) areas and calculating the contrast value by measuring the average grayscale value of each before subtraction. The higher the contrast, the more accurate the reconstructed image is to the original target image. Mean square error (MSE) is measured by taking the pixel-by-pixel intensity difference between the original and reconstructed images. The lower the MSE, the more accurate the reconstructed image is to the original. In our case, we used the original / source image as a control image to compare the reconstruction against. Table 1: [Table 1]

[0146] Table 1 above shows the contrast and MSE results for the images in Figures 8 and 9 herein. There is little difference in the MSE values, and the contrast values ​​are very similar. Thus, the quality of the images produced by the multi-wavelength holograms disclosed herein is comparable to that of images produced by conventional holography, but without the sacrifices, compromises, and difficulties associated with those conventional techniques, such as system bulk, financial cost, and control complexity. Thus, the methods and systems disclosed herein represent a significant improvement over conventional holography, offering the potential for multicolor holography to be more compact, efficient, and cost-effective than conventional holography, making it more accessible and applicable to a much wider range of applications.

[0147] While the example images in Figures 8 and 9 have red, green, and blue components spatially separated from one another with different respective image content, this example is purely illustrative and should not be considered limiting. The methods disclosed herein can be performed on any target image, including those in which the individual color reconstructions have common or overlapping image content and / or in which the individual color reconstructions partially or completely spatially overlap on the reconstruction plane. Furthermore, the methods disclosed herein can be applied to any type of individual wavelength hologram, including those configured to provide multiple images on different respective reconstruction planes when properly displayed and illuminated. Thus, a multi-wavelength hologram calculated as described herein may be a multiplexed image.

[0148] While the above example combines three holograms, this should not be considered limiting. The disclosed methods apply to combining any number of holograms (i.e., combining two or more holograms). Furthermore, while the above example includes R, G, and B holograms, the disclosed methods may also be applied to holograms configured for illumination with any selected individual wavelength of light.

[0149] Additional Features 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, such as a diffuser, or a screen. The holographic projection systems of the present disclosure can be used to provide improved head-up displays (HUDs) or head-mounted displays. In some embodiments, a vehicle is provided that includes a holographic projection system installed in the vehicle to provide a HUD. The vehicle may be a motor vehicle, such as a car, truck, van, lorry, motorcycle, train, airplane, boat, or ship.

[0150] The methods and processes described herein can be implemented on a computer-readable medium. The term “computer-readable medium” includes media configured to temporarily or permanently store 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 interpreted to include any medium, or combination of media, that can store instructions for execution by a machine, such that, when executed by one or more processors, the instructions cause the machine to perform, in whole or in part, any one or more of the methodologies described herein. In examples, the instructions may be executed by a processor (e.g., a hologram engine) for performing hologram calculations or by a processor (e.g., a voltage selection unit or a display driver) for driving a display device. The term “processor” may include a microcontroller, an FPGA, an ASIC, or any other type of hardware component suitable for the image processing described herein.

[0151] The term "computer-readable medium" also encompasses cloud-based storage systems. The term "computer-readable medium" includes, but is not limited to, one or more tangible, non-transitory data repositories (e.g., data volumes) in the exemplary form of a solid-state memory chip, an optical disk, a magnetic disk, or any suitable combination thereof. In some exemplary embodiments, instructions for execution may be communicated by a carrier medium. Such carrier media include, for example, transient media (e.g., a propagating signal communicating the instructions).

[0152] 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 includes all modifications and variations that come within the scope of the appended claims and their equivalents. [Explanation of symbols]

[0153] 110 Light source 111 Collimating Lens 112 Exit wavefront 120 Fourier transform lens 125 screens 140 SLM 202A Data formation step 202B Data formation step 210 input images 211A Distribution of magnitude values 211B New distribution of magnitude values 213A Distribution of Phase Values 230 Random Phase Distribution 250 First Processing Block 253 Second Processing Block 256 Third Processing Block 258 processing blocks 259 Fourth Processing Block 280A Hologram 280B Second repeat hologram 301 Planar aluminum electrode 301a pixel 302 Single crystal silicon substrate 302a circuit 303 Orientation layer 304 Liquid Crystal Layer 305 Second alignment layer 306 Planar transparent layer 307 Single transparent electrode 308 Controllable Phase Modulation Element 410 top line 420 Chuo Line 430 Bottom Line 510 Red 520 Green 530 blue 610 Red Light 620 Green Light 630 blue light 710 Red Hologram 711 First dashed line 720 Green Hologram 721 Second dashed line 721' Fourth dashed line 730 Blue Hologram 731 Third dashed line 731' Fifth dashed line 731'' 6th dashed line 750 straight line 801 Top left image 802 Top right image 803 Bottom left image 804 Bottom right image 901 Top left image 902 Top right image 903 Bottom left image 904 Bottom right image

Claims

1. 1. A projector configured to project a first image and a second image using a single multi-wavelength hologram, comprising: the projector includes a display device for displaying the multi-wavelength hologram; the first image is different from the second image; the multi-wavelength hologram is configured to be illuminated by light at a first wavelength to project the first image; the multi-wavelength hologram is further configured to be illuminated by light of a second wavelength having a wavelength shorter than the first wavelength to project the second image; the display device is configured to provide phase modulation to the multi-wavelength hologram using a predetermined maximum number of discrete phase modulation levels; the display device comprises a plurality of pixels, each pixel capable of providing phase modulation values ​​in the range of 0 to 2π corresponding to the discrete phase modulation levels at the first wavelength with a corresponding voltage drive level within a first operating range; the projector further comprising a display driver configured to distribute the discrete phase modulation levels across voltage drive levels within an extended voltage range, the extended voltage range being a voltage range that exceeds the first and second operating ranges; voltage drive levels within the expanded voltage range correspond to the discrete phase modulation levels with corresponding voltage drive levels within the first operating range and correspond to discrete phase modulation levels with corresponding voltage drive levels within the second operating range; each said pixel of said display device is capable of providing phase modulation values ​​in the range of 0 to 2π corresponding to said discrete phase modulation levels at said second wavelength with a corresponding voltage drive level within a second operating range; the projector is configured to illuminate the multi-wavelength hologram with light at the first wavelength to form the first image and to illuminate the multi-wavelength hologram with light at the second wavelength to form the second image. Projector.

2. the first image and the second image are projected onto a common reconstruction plane; 2. The projector according to claim 1.

3. the multi-wavelength hologram includes a first hologram representation with a first set of hologram pixel values ​​corresponding to the first image and a second hologram representation with a second set of hologram pixel values ​​corresponding to the second image; the first set of hologram pixel values ​​being pixel values ​​consisting of each of maximum discrete phase modulation levels for light of the first wavelength; The second set of hologram pixel values ​​are pixel values ​​that are each comprised of a maximum discrete phase modulation level for light of the second wavelength.

2. The projector according to claim 1.

4. each pixel of the multi-wavelength hologram has a composite hologram pixel value determined from corresponding first and second hologram pixel values ​​of the first and second holograms, respectively; 4. The projector according to claim 3.

5. For a pixel of the display device, obtaining at least a first pixel drive level for the first hologram and obtaining at least a second pixel drive level for the second hologram; the first pixel drive level is a drive voltage level within the first operating range; the second pixel drive level is a drive voltage level within the second operating range; and a processor configured to determine a multi-wavelength pixel drive level for the pixel of the display device based on the first pixel drive level and the second pixel drive level.

5. The projector according to claim 4.

6. the processor is configured to obtain, for the pixel of the display device, a plurality of second pixel drive levels of the second hologram, each of the plurality of second pixel drive levels corresponding to a same discrete phase modulation level of the second hologram, and to determine the multi-wavelength pixel drive level based on the first pixel drive level and a selected one of the plurality of second pixel drive levels.

6. The projector according to claim 5.

7. the processor is further configured to: obtain, for the pixel of the display device, a plurality of first pixel drive levels of the first hologram, each of the plurality of first pixel drive levels corresponding to a same discrete phase modulation level of the first hologram; and determine the multi-wavelength pixel drive level based on a selected one of the plurality of first pixel drive levels and a selected one of the plurality of second pixel drive levels.

7. The projector according to claim 6.

8. 1. A projector configured to project a first image, a second image, and a third image using a single multi-wavelength hologram, comprising: each of the first image, the second image, and the third image is different, and the multi-wavelength hologram is configured for illumination with light of a first wavelength to project the first image, is further configured for illumination with light of a second wavelength having a wavelength shorter than the first wavelength to project the second image, and is further configured for illumination with light of a third wavelength having a wavelength shorter than the first wavelength and the second wavelength to project the third image.

2. The projector according to claim 1.

9. 1. A method for displaying a first image and a second image using a single multi-wavelength hologram, comprising: The method includes determining a multi-wavelength hologram; the multi-wavelength hologram is displayed on a display device, the display device being included in a projector; configured to project a first image and a second image when illuminated with light of a first wavelength to project a first image and with light of a second wavelength having a wavelength shorter than the first wavelength to project a second image, the first image being different from the second image; the display device is configured to provide phase modulation to the multi-wavelength hologram using a predetermined maximum number of discrete phase modulation levels; the display device comprises a plurality of pixels, each pixel configured to provide a phase modulation value in the range of 0 to 2π at the first wavelength corresponding to the discrete phase modulation levels with a corresponding voltage drive level within a first operating range; the projector further comprising a display driver configured to distribute the discrete phase modulation levels across voltage drive levels within an extended voltage range, the extended voltage range being a voltage range that exceeds the first and second operating ranges; voltage drive levels within the expanded voltage range correspond to the discrete phase modulation levels with corresponding voltage drive levels within the first operating range and correspond to discrete phase modulation levels with corresponding voltage drive levels within the second operating range; each said pixel of said display device is capable of providing phase modulation values ​​in the range of 0 to 2π corresponding to said discrete phase modulation levels at said second wavelength with a corresponding voltage drive level within a second operating range; the projector is configured to illuminate the multi-wavelength hologram with light at the first wavelength to form the first image and to illuminate the multi-wavelength hologram with light at the second wavelength to form the second image; The method comprises: i) acquiring a first hologram comprising a first set of hologram pixel values ​​corresponding to the first image; ii) acquiring a second hologram comprising a second set of hologram pixel values ​​corresponding to the second image; iii) determining a first operating range of drive voltage levels, each pixel of the display device being configurable to provide a light modulation value within a range of light modulation values ​​provideable by the first wavelength when driven within the first operating range; iv) determining a maximum number of discrete light modulation levels for said display device and distributing those discrete light modulation levels over a voltage range equal to or greater than said first operating range of voltage drive levels; v) using the distributed discrete light modulation levels to separately represent each of the first hologram and the second hologram, and outputting a first set of pixel drive levels for the first hologram and a second set of pixel drive levels for the second hologram; vi) for each pixel of a multi-wavelength hologram, selecting a first drive level from the first set of pixel drive levels to represent a corresponding pixel of the first hologram, and selecting a second drive level from the second set of pixel drive levels to represent a corresponding pixel of the second hologram, and outputting a multi-wavelength drive level for that pixel based on the selected first drive level and the selected second drive level; vii) using said multi-wavelength drive level output pixel by pixel to form said multi-wavelength hologram; Contains method.

10. the optical modulation values ​​include phase modulation values, and the full range of the phase modulation values ​​at the first wavelength is from 0 to 2π; 10. The method of claim 9.

11. each pixel of the multi-wavelength hologram has a composite hologram pixel value determined from corresponding first and second hologram pixel values ​​of the first and second holograms, respectively; The display device includes: For a pixel of the display device, obtaining at least a first pixel drive level for the first hologram and obtaining at least a second pixel drive level for the second hologram; the first pixel drive level is a drive voltage level within the first operating range; the second pixel drive level is a drive voltage level within the second operating range; The method of claim 10 , further comprising a processor configured to determine a multi-wavelength pixel drive level for the pixel of the display device based on the first pixel drive level and the second pixel drive level.

12. A processor configured to perform the method of claim 9; or A computer readable medium containing instructions that, when executed by a processor, perform the method of claim 9.

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