Holographic projection system

Abstract

A holographic projection system is provided. The system includes a detector device arranged to detect light received by the first detection region. The system is arranged to perform an optical alignment process that includes selecting a first pixel in an array of pixels. The optical alignment process includes forming a first holographic reconstruction of a first picture, the first picture including a main control region, the main control region including a selected first pixel. The optical alignment process also includes moving the first holographic reconstruction such that the first holographic reconstruction moves between the plurality of positions. The optical alignment process also includes determining a parameter value of the light received by the first detection region in each of the plurality of positions and comparing each parameter value to a threshold condition. If a threshold condition is not satisfied, the holographic projection system is arranged to repeat the optical alignment process by: selecting a second pixel of the array of pixels adjacent to the first pixel; and forming a second holographic reconstruction of a second picture, the second picture comprising a main control region comprising second pixels.

Description

Technical Field

[0001] The present disclosure relates to a holographic projection system arranged to perform an optical alignment process and a method for performing the optical alignment process. More specifically, the present disclosure relates to performing an optical alignment process for compensating for tilt changes of a display device of a holographic projection system. Even more specifically, the present disclosure relates to performing an optical alignment process that can be performed during the normal operating time of the system without adversely affecting the viewing experience. Some embodiments relate to a holographic projector, a picture generation unit, or a head-up display. Background Art

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

[0003] Computer generated holography can numerically simulate interference processes. Computer generated holograms can be calculated by techniques based on mathematical transformations such as Fresnel or Fourier transforms. These types of holograms may be referred to as Fresnel / Fourier transform holograms or simply Fresnel / Fourier holograms. Fourier holograms may be considered as Fourier domain / plane representations of objects or frequency domain / plane representations of objects. Computer generated holograms may also be calculated, for example, by coherent ray tracing or point cloud techniques.

[0004] The computer generated hologram may be encoded on a spatial light modulator arranged to modulate the amplitude and / or phase of the incident light. For example, the light modulation may be achieved using electrically addressable liquid crystals, optically addressable liquid crystals or micromirrors.

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

[0006] Using the system described herein, a holographic projector can be provided. Such projectors have been used, for example, in head-up displays "HUDs". Summary of the invention

[0007] Aspects of the disclosure are defined in the accompanying independent claims.

[0008] In summary, a holographic projection system is provided for forming a holographic reconstruction of a picture. The holographic projection system is arranged to perform an optical alignment process. The optical alignment process is suitable for repeated execution. The optical alignment process is suitable for execution during the (normal) operating time of the holographic projection system (without adversely affecting the viewing experience). The holographic projection system according to the present disclosure can be arranged to perform an optical alignment process for compensating for the misalignment of the holographic reconstruction at the sub-pixel level. That is, the optical alignment process performed by the holographic projection system can compensate for the misalignment of the formed holographic reconstruction by translating the holographic reconstruction by a distance less than the pixel pitch of the holographic reconstruction (i.e., the distance between the corresponding centers of adjacent pixels of the holographic reconstruction).

[0009] The holographic projection system may be arranged to spatially modulate light according to a hologram to form a holographic wavefront. The holographic projection system may be arranged so that the holographic wavefront forms a holographic reconstruction. For example, the holographic projection system may include a display device, such as a spatial light modulator (e.g., a liquid crystal on silicon spatial light modulator). The display device may be arranged to display a hologram (or a sequence of holograms). The light incident thereon (e.g., light received by the display device from a light source, such as a coherent light source, such as a laser) may be spatially modulated according to the corresponding displayed hologram to form a holographic wavefront. The holographic projection system according to the present disclosure may be arranged to compensate for the misalignment of the holographic reconstruction formed by the system caused by the misalignment of the display device (e.g., the tilt of the display device). For example, the display device may be substantially planar (and / or include a substantially planar surface for receiving light). The tilt of the display device may cause the normal of the substantially planar display device / surface to change. This may cause the holographic reconstruction formed by the (spatial modulated) light incident on the display device to move / become misaligned. The optical alignment process according to the present disclosure may compensate for this tilt of the display device.

[0010] There are many different factors that the holographic projection system can be arranged to compensate. If there is a predictable relationship between a particular factor and the required amount of compensation, the holographic projection system can be arranged to use a compensation model to compensate for the factor. For example, one factor can be temperature. Changes in temperature may cause optical misalignment of components of the holographic projection system. The amount of optical misalignment can change in a predictable (e.g., linear) manner as the temperature changes. Therefore, this optical misalignment can be corrected by programming the holographic projection system with a compensation model that relates factors (e.g., temperature) to the required misalignment / compensation amount. For example, the factor (such as temperature) is measured and input into the compensation model to output a compensation amount that can be applied to the target image (to be holographically reconstructed). In this arrangement, there is no need to directly measure the optical misalignment to correct it. Instead, the required compensation is inferred from the measurement of the corresponding factor (e.g., temperature). This is generally advantageous because measuring, for example, temperature may be more direct than quantifying optical misalignment by direct measurement of misalignment.

[0011] After thorough simulations and experiments, the inventors found that the relationship between environmental factors (such as temperature) of the holographic projection system and the tilt angle of the display device is unpredictable. In other words, the inventors have found that it is impractical to quantify or predict the tilt for given environmental (such as thermal) conditions. Therefore, the inventors have not been able to design a reliable compensation model for the tilt of the display device (such as taking temperature as input). The inventors therefore recognize that an optical alignment process that compensates for the tilt of the display device by directly quantifying the optical alignment may be necessary.

[0012] A holographic projection system for performing an optical alignment process for directly determining optical misalignment has been previously proposed in British patent GB2559112B published on August 1, 2018. GB2559112B discloses a holographic projection system comprising a spatial light modulator arranged to receive light from a light source and output spatially modulated light according to a computer-generated hologram of a picture presented on the spatial light modulator to form an image on a light receiving surface of the system. The image (which is a holographic reconstruction) includes a main image area containing user information and a secondary image area that is not intended to be observable by the user. The system also includes a detector arranged to detect the optical power of light traveling to or from the secondary image area of ​​the image and received by the detector. The system also includes a holographic controller arranged to perform an optical alignment process, which includes changing the position of the image on the screen and detecting the optical power of the received light at multiple positions of the image. In some examples, a computer generated hologram (displayed on a display device) includes components (e.g., grating functions) arranged to perform a holographic reconstruction / translation of an image, and wherein the position of the image is changed during an optical alignment process by changing the grating function, such as by changing the periodicity of the grating function. In such examples, the optical alignment process may include determining (the periodicity of) the grating function that produces a maximum detected optical power (also referred to herein as an optical power peak). In this way, an appropriate grating function (e.g., one that produces an optical power peak) may be used to quantify / compensate for optical misalignment.

[0013] The inventors have found that the optical alignment process of GB2559112B provides a reliable method for determining the optical alignment position at least initially (e.g., in the startup procedure of the holographic projection system). However, the position of the optical alignment / holographic reconstruction of the system may change over time. For example, this may be due to changes in environmental conditions such as temperature. For example, the tilt of the display device may change during the use of the system (e.g., running time). Therefore, it is necessary to repeat the optical alignment process during the operation of the holographic projection system to compensate for the change in tilt. The inventors have found that if the optical misalignment of the system has changed significantly relative to the previous / initial alignment (e.g., the secondary image area has been translated by a distance greater than or equal to the reconstructed pixel spacing, optionally, greater than or equal to a plurality of pixel spacings), then when performed during the running time of the holographic projection system, the optical alignment process of GB2559112B may adversely affect the viewing experience. This will be explained in more detail below.

[0014] The holographic reconstruction formed by the holographic projection system is typically pixelated. The target image (holographically reconstructed by the holographic projection system) may also be pixelated (i.e., the target image may include a pixel array and the holographic reconstruction may include a corresponding pixel array or image points). The target image (and the holographic reconstruction) includes a secondary image area that may be referred to as a control area. The optical alignment process of GB2559112B effectively includes running a scan by scanning (translating) the holographic reconstruction relative to the detector, for example using a grating function. The holographic reconstruction moves on the playback plane of the system. The scanning distance required to detect the optical power peak depends on the relative distance between the first or initial position of the control area of ​​the holographic reconstruction on the playback plane and the detector. The distance can be measured from the initial or first position of the control area of ​​the holographic reconstruction on the playback plane to the detector. The greater the relative distance, the greater the scanning distance required to detect the optical power peak. When the scan is performed (e.g., by applying a grating during the optical alignment process), the entire holographic reconstructed image is translated on the playback plane. This means that both the primary and secondary image (or control) areas of the holographic reconstruction move / translate on the playback plane. The inventors have recognized that this means that as part of the optical alignment process, the image content of the primary image area (which is intended to be visible at the eye box / observation window of the system during normal operation) is moved. The inventors have recognized that as the scanning distance becomes larger, the movement of the holographic reconstruction may be noticeable, which may adversely affect the viewing experience. For example, the holographic reconstruction may need to move a distance equal to a number of pixels or more (i.e., a distance greater than the pixel pitch of the holographic reconstruction, such as a distance equal to a multiple of the pixel pitch) before a peak in optical power is detected. Such a distance may be noticeable to a user of the system and may adversely affect the viewing experience. For example, the (primary image area) of the holographic reconstruction may appear to move or even jump when being scanned / translated, which may be distracting or uncomfortable. Therefore, the inventors have recognized that, for example, as the scanning distance increases, the optical alignment method may become unsuitable for use during normal operation of the holographic projection system.

[0015] The inventors have discovered that the amount of shift or translation of a pixel of a holographic reconstruction caused by a change in misalignment of the display device (e.g., a change in tilt) can be assumed to be substantially equal for all pixels of the holographic reconstruction. In other words, it can be assumed that a global shift or translation is applied to each pixel of the holographic reconstruction. Therefore, the inventors have recognized that by using a control region at any pixel position of the holographic reconstruction, the shift or translation caused by a change in misalignment of the display device can be quantified / determined / compensated. This is because the amount of misalignment / compensation required for all pixels is the same.

[0016] The inventors have also discovered that, in realistic environments, the proportion of the shift or translation of pixels of the holographic reconstruction caused by changes in the misalignment (e.g., tilt) of the display device is relatively small compared to other factors, such as scaling of the holographic reconstruction caused by changes in the wavelength of the light forming the holographic reconstruction (e.g., caused in turn by changes in temperature). Therefore, the inventors have recognized that the main reason why the scanning distance becomes relatively large as the holographic reconstruction shifts over time / because of changes in conditions is due to these "other" factors (e.g., wavelength changes), rather than because of changes in the misalignment of the display device (which is the optical misalignment that the present optical alignment process is intended to address). Therefore, the inventors have determined that the main reason why the scanning distance needs to be increased is due to translation caused by factors other than the misalignment / tilt of the display device (i.e., factors other than the factors to be determined, i.e., changes in the tilt of the display device).

[0017] Thus, the inventors have developed an optical alignment method comprising performing a first scan with a first holographic reconstruction of a first picture including a (primary) control region containing a first pixel within a limited scan range, and repeating the process by performing a second scan with a second holographic reconstruction of a second picture if a threshold condition is not met within the limited scan range. Like the first picture, the second image includes a (primary) control region. However, the (primary) control region of the second picture is reselected. In particular, the (primary) control region of the second picture is reselected to include a second pixel adjacent to the first pixel. The concept is that, because the threshold was not met in the first scan, it is known that the (primary) control region of the first picture was too far from the detector (at an initial or first position) to be detected during the first scan (over the limited scan area). It can be assumed that the second (adjacent) pixel is now closer to the detector than the first pixel (e.g., because a changing environmental factor / condition changes the position and distribution of the pixels of the holographic reconstruction). It can also be assumed that this repositioning is mainly caused by factors other than the tilt of the display device (in particular, scaling caused by changes in the wavelength of light incident on the display device). By reselecting the (main) control area of ​​the second picture, the initial or first position of the holographic reconstruction of the (main) control area of ​​the second picture is formed closer to the detector (and therefore within the limited scanning range of the second or repeated scan). Therefore, the optical power peak should be detected in the second scan. The inventors have recognized that because the position change of each pixel caused by the change of the misalignment / tilt of the display device is constant, it is not important to reselect the control area. The reselected control area (with a different pixel position on the playback plane) is as suitable as the initially selected control area for determining the change of misalignment caused by the display device (e.g., the tilt of the display device). In some embodiments, the holographic projection system can be arranged to introduce an offset that compensates for the reselection of the control area. For example, if the control area is reselected to include a second pixel adjacent to the first pixel in the first direction, a corresponding offset can be introduced in the second direction, which is equal to the distance between the first and second pixels. In some embodiments, the holographic projection system may include two control areas (e.g., first and second control areas). In such an embodiment, the optical alignment method may include determining the average of the shift required to align the first control area with the first detection area and the shift required to align the second control area with the second detection area. In such an embodiment, when the control area is reselected, it may not be necessary to introduce an offset. The average shift / average translation can automatically compensate for this. Similarly, the average shift / average translation can advantageously automatically compensate for scaling effects (e.g. caused by wavelength variations of the light forming the holographic reconstruction). Unlike the translation caused by the tilt of the display device (which is global), scaling effects may affect different pixels differently (e.g., possibly in opposite directions at different selected pixels).By averaging the measurements of the different shifts of the corresponding control regions of the first and second pictures, the differential shift caused by scaling is cancelled, leaving a global shift.

[0018] The inventors have found that by reselecting the control area as described above (and performing a scan for each image / control area), the scan range can be significantly reduced. For example, in some examples, the scan range can be less than the pixel pitch of the holographic reconstruction. The inventors have recognized that such a small scan range may not be perceived by a user of the holographic projection system. Therefore, the jittery appearance of the holographic reconstruction when performing the method can be advantageously reduced or minimized. The inventors have also found that an optical alignment process can be used to achieve sub-pixel alignment accuracy.

[0019] In a first aspect, a holographic projection system is provided. The holographic projection system is arranged to (is suitable for) forming a holographic reconstruction of a picture including an array of pixels or image points at a playback plane. Because the picture is pixelated, the holographic reconstruction can also be pixelated. The system includes a detector device, which is arranged to detect light received by a first detection area of ​​the detector device. The holographic projection system is arranged to perform an optical alignment process. In some embodiments, the holographic projection system includes a controller (e.g., a holographic controller) arranged to perform an optical alignment process.

[0020] The optical alignment process includes the step of selecting a first pixel in the pixel array.

[0021] The optical alignment process also includes a step of forming a first holographic reconstruction of the first picture. The first holographic reconstruction can be formed at the playback plane. The first picture includes a main control area including a selected first pixel. One or more pixels (including the first pixel) of the main control area of ​​the first picture can be referred to as being in an "on" state or turned on. An area adjacent to and optionally surrounding the main control area may include pixels that are in an "off" state or turned off. In other words, the main control area can be a picture area that includes content and can be adjacent to (and optionally surrounded by) an area that does not include content. The main control area can be composed of the selected first pixels. In other words, the main control area can be a single picture control area. The holographic projection system can be arranged so that the first holographic reconstruction of the first picture is formed at a first position. It can initially be formed at this first position. At the first position, it can be expected that the main control area is aligned with the first detection area. For example, a calibration process may have been performed previously. The previously performed calibration process may have been used to align the main control area with the first detection area. For example, an initial grating function may have been determined during the calibration process, which (at the time of the calibration process) correctly aligns the first control area with the first detection area (in the first dimension of the playback plane). The initial diffraction grating may be stored in a memory of the holographic projection system. If nothing has changed since the calibration process, the first holographic reconstruction (e.g., at the first position) may be substantially aligned with the first detection area using the initial diffraction grating. However, if the optical alignment of the holographic projection system has changed (e.g., because the tilt angle of the display device of the system has changed), the first holographic reconstruction (at the first position) may not be substantially aligned with the first detection area.

[0022] The optical alignment process also includes moving the first holographic reconstruction relative to the first detection area. In other words, the first holographic reconstruction can be translated or scanned. This step of the process can be referred to as performing a first scan. In the first scan, the first holographic reconstruction is moved / translated / scanned in a first dimension of the playback plane. The first holographic reconstruction moves between multiple positions. The distance between the two farthest (or most extreme) positions of the multiple positions (in the first dimension of the playback plane) can be referred to as a scan range (i.e., the range over which the first holographic reconstruction is moved / scanned). It can be said that the first scan has a first scanning range.

[0023] The optical alignment process also includes determining a parameter value of the light received by the first detection area in each of a plurality of positions. The parameter value may be based on a signal received from the first detection area. For example, the first detector may be arranged to continuously monitor the parameter value as the first reconstruction is moved / throughout the first scan. The parameter may be related to the optical power detected / received by the first detection area. The optical alignment process also includes comparing each correspondingly determined parameter value with a threshold condition. The threshold condition may be related to an expected value of the parameter value in an alignment state between the main control area and the first detection area (e.g., where the first detection area receives a maximum amount of light). For example, if the parameter is optical power, the threshold condition may be an optical power value that is equal to or exceeds a predetermined value of optical power, which may indicate that the first detection area is substantially aligned with the first detection area. If the threshold condition is not met at any or each of the plurality of positions in the first holographic reconstruction, this may indicate that the main control area was not aligned with the first detection area during the first scan. This may indicate that the holographic projection system causes the main control area to be misaligned so that it is (initially) formed in an area outside the scan area of ​​the first scan on the playback plane (i.e., when the main control area is moved / scanned, it does not coincide / align with the first detection area).

[0024] If the threshold condition is not met at multiple positions of the first holographic reconstruction (for example, if the threshold condition is not met at any one or each of the multiple positions), the optical alignment process also includes the following steps. In particular, the holographic projection system is arranged to repeat the optical alignment process. The repeated optical alignment process includes: a) selecting a second pixel adjacent to the first pixel in the pixel array; and b) forming a second holographic reconstruction of the second picture (at the playback plane). The second picture includes a main control area (which may be different from the main control area of ​​the first picture). Here, the main control area of ​​the first picture may be referred to as the first main control area, and the main control area of ​​the second picture may be referred to as the second main control area. The second main control area (of the second picture) includes the selected second pixel. The second picture may include "on" and "off" pixels that are different from the first picture. For example, a pixel of the second picture corresponding to a first pixel of the first picture may be turned off in the second picture.

[0025] The first and second main control areas may have substantially the same form (e.g., the same shape). The position of a first pixel of the first main control area in the first main control area may correspond to the position of a second pixel in the second main control area. For example, if the first pixel is substantially located at the center of the first main control area, the second pixel may be substantially located at the center of the second main control area. In this case, it can be said that the main control area is centered on the corresponding selected (first or second) pixel. Therefore, when the optical alignment process is repeated, by selecting different (second) pixels, the main control area can be effectively shifted or moved on the playback plane. It can be said that the pixel position of the second main control area is shifted relative to the pixel position of the first main control area. In other words, the position of the main control area can be changed without changing the form (e.g., shape) of the main control area. In some embodiments, the first and second main control areas are both composed of a single pixel (and therefore only composed of the first or second selected pixels, respectively).

[0026] The advantages of performing a first scan using a first holographic reconstruction of a first picture including a (first) main control area and then forming a second holographic reconstruction of a second picture including a (second) main control area (which is shifted in pixel position relative to the first main control area) have been described above. In particular, it allows a relatively small scan range to be selected (e.g. for the first scan), and if a threshold condition is not met, another scan is performed using the shifted (initial position) main control area. In this way, the viewing experience of a user of the holographic projection system is not adversely affected by the holographic reconstruction moving around (e.g. the phenomenon of the holographic reconstruction "jumping around" can be substantially minimized).

[0027] As used herein, a second pixel being "adjacent to" a first pixel means that the first pixel and the second pixel are close to each other. In some examples, the first pixel may be no more than five pixels away from the second pixel (i.e., there may be no more than five pixels between the first pixel and the second pixel). In some examples, the first pixel may be no more than one, two, three, or four pixels away from the second pixel. In some examples, the first pixel may be immediately adjacent to the second pixel. The first and second pixels may be adjacent in a direction substantially along the first dimension. In other words, in the first dimension, the second pixel of the second holographic reconstruction may be closer to the first detection area than the first pixel of the first holographic reconstruction.

[0028] In some embodiments, the detector arrangement is further arranged to detect light received by the second detection area. In other words, the detector arrangement may include a first detection area and a second detection area different from the first detection area. The first and second detection areas may be spatially separated. The detector arrangement may include a photodiode. The photodiode may be a quadrant photodiode, wherein each active area of ​​the quadrant photodiode may be used to detect light, thereby improving the accuracy of the light detection and, therefore, the accuracy of the optical alignment process.

[0029] In some embodiments, the optical alignment process further comprises selecting a third pixel in the pixel array. Optionally, the third pixel is selected while the first pixel in the pixel array is selected.

[0030] In some embodiments, the first picture also includes a secondary control area containing a selected third pixel. The secondary control area of ​​the first picture can be a first secondary control area of ​​reference. The (first) main control area can be spatially separated from the (first) secondary control area. The holographic projection system can be arranged so that light from the main control area and the secondary control area is not receivable at the observation system / eye box of the holographic projection system. In other words, the user of the holographic projection system (at the observation window / eye box) may not be able to observe the main and secondary control areas.

[0031] In some embodiments, the first picture includes a picture content area. The picture content area may be spatially separated from the (first) main control area and the (first) secondary control area (if present). The light of the holographic reconstruction forming the image content area (in the holographic reconstruction of the first picture) may be received at the observation window / eye box of the holographic projection system. In other words, the picture content area may include content intended to be observable by the user. In some embodiments, the second picture includes a picture content area. The picture content area of ​​the second picture may take a form corresponding to the picture content area of ​​the first picture. In other words, the picture content areas of the first and second pictures may have substantially the same shape and may appear in substantially the same position. The picture content area of ​​the second picture may be referred to as the second picture content area. The picture content of the first picture content area may be the same or different from the picture content of the second picture content area.

[0032] In some embodiments, the optical alignment process further comprises detecting a parameter of light received by the second detection area at a plurality of locations of the first holographic reconstruction. In some embodiments, the optical alignment process further comprises comparing the detected parameter to a threshold condition. The threshold condition for the parameter of light received by the second detection area may be the same as the threshold condition for the parameter of light received by the first detection area. Thus, the first scan may comprise simultaneously detecting the parameter of light received by the first and second detection areas.

[0033] In some embodiments, the distance from the first pixel to the center of the pixel array in the first dimension may be equal (in magnitude) to the distance from the third pixel to the center of the pixel array (and similarly in the second dimension).

[0034] In some embodiments, the holographic projection system is arranged to determine the first alignment position of the holographic reconstruction of the first main control area based on a threshold condition satisfied by the detection parameters of the light received by the first detection area. In some embodiments, the holographic projection system is arranged to determine the second alignment position of the holographic reconstruction of the first secondary control area based on a threshold condition satisfied by the detection parameters of the light received by the second detection area. However, as described above, during the first scan, the threshold condition may not be satisfied. For the first or second detection area, the threshold condition may not be satisfied. Therefore, the threshold condition may only be satisfied during the second scan (or even further scans, such as the third or fourth scan).

[0035] In some embodiments, the holographic projection system is arranged to determine an average associated with the first and second alignment positions. As described above, such an average can allow scaling effects (caused by wavelength variations) to be offset. The average can be a positional average of the first and second alignment positions, or an average of a grating function used to achieve the first and second alignment positions. If the first and third pixels are not at the same distance from the center of the picture, the average can be a weighted average. The weight of each of the first and third pixels can be determined based on the distance of each of the first and third pixels from the center of the picture / replay plane. This may be because the scaling of the pixels of the holographic reconstruction (caused by changes in the wavelength of light used to form the holographic reconstruction) is spatially varying, and may be spatially varying, so that the farther each pixel is from the center of the picture / replay plane, the greater the displacement of the corresponding pixel.

[0036] In some embodiments, the repeated optical alignment process includes selecting a fourth pixel in the pixel array that is adjacent to the second pixel; and wherein the second picture further includes a second secondary control area including the selected fourth pixel.

[0037] In some embodiments, the optical alignment process further includes detecting parameters of light received by the second detection region at multiple locations of the second holographic reconstruction and comparing the detected parameters to a threshold condition.

[0038] In some embodiments, the first pixel is adjacent to the third pixel in a first direction of the first dimension, and the second pixel is adjacent to the fourth pixel in a second direction of the first dimension. In some embodiments, the first direction is opposite to the second direction. This may be the case when the first pixel is located on the opposite side of the center of the image / playback field of the holographic projection system from the second pixel.

[0039] In some embodiments, the optical alignment process further includes receiving (or selecting or forming) a first picture based on the selected first pixel and then forming a first holographic reconstruction.

[0040] In some embodiments, the repeated optical alignment process also includes moving (translating / scanning) the second holographic reconstruction relative to the first detection area in the replayed first dimension. This may be referred to herein as performing a second scan. The first and second scans may include, for example, sequentially moving the first and second holographic reconstructions in the first and second directions of the first dimension. Thus, the first and second scans may be centered at the initial position of the corresponding holographic reconstructions, and may then move in the first direction and then return along the first dimension in the second direction within the corresponding scan range.

[0041] In some embodiments, the repeated optical alignment process also includes detecting parameters of the light received by the first detection area at multiple positions of the second holographic reconstruction, and comparing the detected parameters with threshold conditions. In some embodiments, the holographic projection system is arranged to determine the alignment position of the holographic reconstruction of the first main control area based on the threshold condition satisfied by the detection parameters of the light received by the first detection area. This may occur in the first scan or the second scan. The holographic projection system can be arranged to determine the alignment position based on the position of the corresponding holographic reconstruction (the position in the multiple positions of the corresponding scan) that produces the maximum detection parameter (e.g., optical power). In other words, when the corresponding holographic reconstruction is moved / scanned, the alignment position can be based on the detected optical power peak.

[0042] In some embodiments, the holographic projection system is arranged to move the first holographic reconstruction in the first dimension by a total distance less than or equal to the (expected) pixel spacing of the first holographic reconstruction. In other words, the scanning range can be less than or equal to the expected pixel spacing of the first holographic reconstruction. Therefore, if the scan is centered on the first position of the first holographic reconstruction, the scan may include moving the first holographic reconstruction in the first direction (from the first position) by no more than half a pixel spacing, and in the second direction (from the first position) by no more than half a pixel spacing. In some embodiments, the holographic projection system is also arranged to move the second holographic reconstruction in the first dimension by a total distance less than or equal to the (expected) pixel spacing of the second holographic reconstruction. In some embodiments, the holographic projection system can be arranged so that the first and / or second scans have a scanning range of less than 1 mm, optionally less than 0.5 mm. The scanning range (measured in millimeters) can refer to the scanning range at the playback plane (the detection area adjacent to the detector). Such a scanning range can advantageously not be perceived by the user of the holographic projection system (or at least will not adversely affect the viewing experience).

[0043] In some embodiments, the holographic projection system includes a display device arranged to display a diffraction pattern of a hologram including a corresponding picture, and spatially modulate light incident thereon according to the diffraction pattern to form a holographic wavefront. The hologram may be a computer-generated hologram of the corresponding picture.

[0044] In some embodiments, the holographic projection system is arranged so that during the step of forming the first holographic reconstruction of the first picture, a diffraction pattern of the first hologram of the first picture is displayed on the display device. The holographic projection system can also be arranged so that the diffraction pattern also includes a grating function (or a phase ramp function), and wherein the holographic projection system is arranged to change the grating function of the diffraction pattern to move the first holographic reconstruction (on the playback plane). The holographic projection system (e.g., a holographic controller) can be arranged to change the grating function of the diffraction pattern to move the first holographic reconstruction between multiple positions. There may be a grating function associated with each of the multiple positions. The holographic projection system can be arranged to determine the grating function that produces the maximum detection parameter (e.g., optical power).

[0045] A grating function (or phase ramp function) can be calculated to provide a displacement range with high precision (e.g., sub-pixel precision). The displacement can be a linear displacement in a first dimension (when performing a scan in the first dimension) or a linear displacement in a second dimension (when performing a scan in the second dimension). The diffraction pattern displayed on the display device may include a grating function / phase ramp combined with a corresponding hologram by addition or superposition. Two perpendicular grating functions can be added to the hologram component separately and modified separately to fine-tune the position of the light encoded by the corresponding hologram. Throughout this disclosure, "grating functions" (or "phase ramp functions") are referenced only by the example of functions that provide linear translation of a holographic image or replay field. That is, an array of light modulation values, when added to a hologram, linearly displaces the replay field with a defined amplitude and direction. The displacement can be measured in pixels, millimeters, or degrees. The phase ramp can also be referred to as a phase wedge. The phase values ​​of the phase ramp can be wrapped (e.g., modulo 2π). The wrapped phase ramp can be considered a phase grating. However, the present disclosure is not limited to "grating functions", the terms "software grating" and "blazed grating" may be used as examples of beam steering functions, such as a winding modulation ramp. A grating function or phase ramp may be characterized by its phase gradient.

[0046] The above-described optical alignment process is used to align the holographic reconstruction in a first dimension. For example, when scanning the corresponding first or second holographic reconstruction in the first dimension, the grating function is determined based on the position where the parameter (e.g., optical power) is maximum. In some embodiments, the holographic reconstruction is arranged to perform an optical alignment process in a second dimension. For example, the optical alignment process in the second dimension may include selecting a pixel in the pixel array. The pixel may be a first pixel or may be a (different) fifth pixel. The optical alignment process may further include forming a third holographic reconstruction at the playback plane of the third picture. The picture may include a (third) main control area containing the (first or fifth) pixel. The optical alignment process may further include moving the third holographic reconstruction relative to the first detection area in the second dimension of the playback plane so that the third holographic reconstruction moves between multiple positions. In other words, the holographic projection system may be arranged to perform a third scan. Unlike the first and second scans, the third scan may be in the second dimension (different from the first scan). The movement of the third holographic reconstruction can be achieved by a holographic projection system arranged to display a diffraction structure including different grating functions (a computer-generated hologram in combination with the third picture). The optical alignment process may further include determining a parameter value of light received by the first detection area in each of a plurality of positions. The optical alignment process may further include comparing each corresponding determined parameter value with a threshold condition. If the threshold condition is not met at a plurality of positions (e.g., any or each position) (of the third holographic reconstruction), the holographic projection system may be arranged to repeat the optical alignment process by selecting a sixth pixel adjacent to the third pixel in the pixel array; and forming a fourth holographic reconstruction (at the playback plane) of the second picture, the second picture including a fourth main control area (different from the third main control area) containing the sixth pixel.

[0047] In some embodiments, the holographic projection system further comprises a mask comprising an aperture. The first detection area may be arranged to detect light received through the aperture of the mask. The mask may be upstream of the light receiving surface, for example, directly in front of the light receiving surface. In some embodiments, the width of the aperture in the first dimension may be smaller than the size (e.g., width) of the first pixel in the first dimension. In other words, the aperture may be described as having a sub-pixel width. The presence of such a mask may improve the accuracy of the optical alignment process. In particular, a mask with a sub-pixel width may achieve sub-pixel accuracy when detecting the alignment position of the corresponding holographic reconstruction. In some embodiments, the aperture in the mask comprises an elongated slit extending in a second dimension different from the first dimension. The elongated slit may be suitable for a first scan, which may be performed in a first dimension as described above. The second dimension (wherein the elongated slit is elongated) may be perpendicular to the first dimension.

[0048] In some embodiments, the elongated slit of the aperture is a first elongated slit. In some embodiments, the aperture includes a first elongated slit and a second elongated slit. The first and second elongated slits may overlap each other so that the aperture forms a cross. The second elongated slit may extend / elongate in the first dimension. Thus, the second elongated slit may be suitable for scanning performed in a second direction.

[0049] In a second aspect, a holographic projection system is provided for forming a (pixelated) holographic reconstruction of a picture including an array of pixels (or image points) at a playback plane. The system includes a detector device arranged to detect light received by a first detection area and a second detection area. In other words, the detector device includes a first and a second detection area. The holographic projection system is arranged to perform an optical alignment process. The optical alignment process includes selecting a first pixel in the pixel array and a third pixel in the pixel array. The optical alignment process also includes forming a first holographic reconstruction of a first picture (at the playback plane). The first picture includes a main control area including the selected first pixel and a secondary control area including the selected second pixel. The optical alignment process also includes moving (translating / scanning) the first holographic reconstruction relative to the first and second detection areas in a first dimension of the playback plane so that the first holographic reconstruction moves between multiple positions. The optical alignment process also includes determining a parameter value of the light received by the first detection area and a corresponding parameter value of the light received by the second detection area in each of the multiple positions (based on the signal received from the first detection area), and comparing each corresponding determined parameter value with a threshold condition. If the threshold condition is not met at multiple positions (e.g., any or each) (of the first holographic reconstruction), the holographic projection system is arranged to repeat the optical alignment process by selecting a third pixel in the pixel array that is adjacent to the first pixel, and selecting a fourth pixel in the pixel array that is adjacent to the second pixel. The optical alignment process also includes forming a second holographic reconstruction (at the playback plane) of a second picture, the second picture including a first control region including the third pixel and a second control region including the fourth pixel.

[0050] In a third aspect, there is provided an optical alignment method, for example for a holographic projection system arranged to form a holographic reconstruction of a picture comprising an array of pixels. The holographic projection system may be a holographic projection system as defined in the first or second aspect. The method includes: selecting a first pixel in a pixel array; forming a first holographic reconstruction of a first image (at a playback plane), the first image including a first main control area, the first main control area including the selected first pixel; moving (translating / scanning) the first holographic reconstruction relative to the first detection area in a first dimension of the playback plane so that the first holographic reconstruction moves between multiple positions; and determining a parameter value of light received by the first detection area of ​​a detector of the holographic projection system in each of the multiple positions (based on a signal received from the first detection area), and comparing each corresponding determined parameter value with a threshold condition, if the threshold condition is not met at multiple positions (for example, any or each) (of the first holographic reconstruction), the method includes repeating the optical alignment process by: selecting a second pixel in the pixel array adjacent to the first pixel; and forming a second holographic reconstruction of a second image (at the playback plane), the second image including a second main control area (different from the first main control area) containing the second pixel.

[0051] Features or advantages described in relation to one aspect may also apply to other aspects. In particular, features or advantages described in relation to the first aspect may apply to the second or third aspect.

[0052] In the present disclosure, the term "replica" is used only to reflect that the spatially modulated light is split so that the composite light field is directed along multiple different optical paths. The word "replica" is used to refer to each occurrence or instance of the composite light field after a replication event - such as a partial reflection-transmission of a pupil expander. Each replica propagates along a different optical path. Some embodiments of the present disclosure relate to the propagation of light encoded with a hologram rather than an image - that is, light that is spatially modulated with a hologram of an image rather than the image itself. It can therefore be said that multiple replicas of the hologram are formed. Those skilled in the art of holography will understand that the composite light field associated with the propagation of light encoded with a hologram will vary with the propagation distance. The term "replica" as used herein is independent of the propagation distance, so two branches or paths of light associated with a replication event are still referred to as "replicas" of each other, even if the branches have different lengths, so that the composite light field evolves differently along each path. That is, according to the present disclosure, even if two composite light fields are associated with different propagation distances, they are still considered "replicas" - as long as they are generated from the same replication event or series of replication events.

[0053] According to the present disclosure, a "diffraction light field" or "diffracted light field" is a light field formed by diffraction. A diffraction light field can be formed by illuminating a corresponding diffraction pattern. According to the present disclosure, an example of a diffraction pattern is a hologram, and an example of a diffraction light field is a holographic light field or a holographically reconstructed light field that forms an image. The holographic light field forms a (holographic) reconstruction of the image on a playback plane. The holographic light field propagated from the hologram to the playback plane can be said to include light encoded with the hologram or light in the holographic domain. The diffraction light field is characterized by a diffraction angle determined by the minimum characteristic size of the diffraction structure and the wavelength of light (of the diffraction light field). According to the present disclosure, it can also be said that the "diffraction light field" is a reconstructed light field formed on a plane spatially separated from the corresponding diffraction structure. An optical system for propagating a diffraction light field from a diffraction structure to an observer is disclosed herein. The diffraction light field can form an image.

[0054] The term "hologram" is used to refer to a record containing amplitude information or phase information or some combination thereof about an object. The term "holographic reconstruction" is used to refer to the optical reconstruction of an object formed by illuminating a hologram. The system is described as a "holographic projector" because the holographic reconstruction is a real image and is spatially separated from the hologram. The term "replay field" is used to refer to the 2D area within which the holographic reconstruction is formed and is fully focused. If the hologram is displayed on a spatial light modulator comprising pixels, the replay field will be repeated in the form of multiple diffraction orders, each of which is a replica of the zero-order replay field. The zero-order replay field usually corresponds to the preferred or main replay field because it is the brightest replay field. Unless otherwise explicitly stated, the term "replay field" should be considered to refer to the zero-order replay field. The term "replay plane" is used to refer to a plane in space that contains all replay fields. The terms "image", "replay image" and "image area" refer to the area of ​​the replay field illuminated by the light of the holographic reconstruction. In some embodiments, an "image" may include discrete points, which may be referred to as "image points," or, for convenience only, "image pixels."

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

[0056] It has been found 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 may be referred to as phase-only holograms. The embodiments relate to phase-only holograms, but the 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 related to the Fourier transform of the original object. In some embodiments, this is achieved by using complex modulation of a so-called fully complex hologram that contains amplitude and phase information related to the original object. Because the value (gray level) assigned to each pixel of the hologram has an amplitude and a phase component, such a hologram may be referred to as a fully complex hologram. The value (gray level) assigned to each pixel can be represented as a complex number with an amplitude and a phase component. In some embodiments, a fully complex computer-generated hologram is calculated.

[0058] Reference may be made to the phase value, phase component, phase information, or simply phase, of a pixel of a computer-generated hologram or spatial light modulator as shorthand for "phase delay". 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 pixel of a spatial light modulator described as having a phase value of π / 2 will delay the phase of the received light by π / 2 radians. In some embodiments, each pixel of the spatial light modulator may be operated at one of a plurality of possible modulation values ​​(e.g., phase delay values). The term "grayscale" may be used to refer to a plurality of available modulation levels. For example, the term "grayscale" may be used for convenience to refer to a plurality of available phase levels in a phase modulator alone, even if the different phase levels do not provide different shades of gray. For convenience, the term "grayscale" may also be used to refer to a plurality of available complex modulation levels in a complex modulator.

[0059] Thus, a hologram comprises an array of gray levels, i.e. an array of light modulation values, such as phase delay values ​​or an array of complex modulation values. A hologram is also considered to be a diffraction pattern, since it is a pattern that causes diffraction when displayed on a spatial light modulator and illuminated with light of a wavelength relative to (usually less than) the pixel pitch of the spatial light modulator. Reference is made herein to combining a hologram with other diffraction patterns, such as a diffraction pattern used as a lens or a grating. For example, a diffraction pattern used as a grating may be combined with a hologram to shift the replay field on the replay plane, or a diffraction pattern used as a lens may be combined with a hologram to focus the holographic reconstruction on the replay plane in the near field.

[0060] Although different embodiments and embodiment groups may be disclosed separately in the following detailed description, any feature of any embodiment or embodiment group may be combined with any other feature or combination of features of any embodiment or embodiment group. That is, all possible combinations and permutations of the features disclosed in this disclosure are contemplated. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Specific embodiments are described by way of example only with reference to the following drawings:

[0062] Figure 1is a schematic diagram showing a reflective SLM producing a holographic reconstruction on a screen;

[0063] Figure 2A A first iteration of an example Gerchberg-Saxton type algorithm is shown;

[0064] Figure 2B The second and subsequent iterations of an example Gerchberg-Saxton type algorithm are shown;

[0065] Figure 2C An alternative second and subsequent iteration of an example Gerchberg-Saxton type algorithm is shown;

[0066] Figure 3 is a schematic diagram of a reflective LCOS SLM;

[0067] Figure 4 An example HUD in a vehicle is shown;

[0068] Figure 5 is a schematic diagram of a holographic reconstruction formed by a spatial light modulator showing a diffraction pattern;

[0069] Figure 6 is a schematic diagram showing a translation of a holographic reconstruction formed by a spatial light modulator, the translation being caused by a tilt variation of the spatial light modulator;

[0070] Figure 7 yes Figure 6 Schematic diagram of pixels of holographic reconstruction;

[0071] Figure 8 is a schematic diagram showing scaling of a holographic reconstruction formed by a spatial light modulator, the scaling being caused by a change in the wavelength of light incident on the spatial light modulator;

[0072] Fig. 9 yes Figure 8 Schematic diagram of pixels of holographic reconstruction;

[0073] Figures 10A-10F Shows Figure 6 and 8 Movement of the image area of ​​the arrangement shown;

[0074] Fig.11 An arrangement is shown in which a holographic reconstruction includes primary and secondary control regions and first and second masks;

[0075] Fig. 12A The steps of the optical alignment process are shown, including moving the holographic reconstruction so that the master control region moves relative to the first mask;

[0076] Fig. 12B It shows that when the holographic reconstruction is Fig. 12A The optical power measured when moving in the manner shown;

[0077] Fig.13A The steps of the optical alignment process are shown, including moving the holographic reconstruction so that the secondary control region moves relative to the second mask;

[0078] Fig. 13B It shows that when the holographic reconstruction is Fig.13A The optical power measured when moving in the manner shown;

[0079] Fig.14 Shown based on Fig. 12B and 13B The average misalignment of the primary and secondary control areas is determined by using the optical power peak detected in the image;

[0080] Fig.15A shows the reselection of pixels forming the main control area;

[0081] Fig. 15B shows the reselection of pixels forming the secondary control area;

[0082] Fig.16A showing pixels of a holographic reconstruction forming a first picture of a first master control area;

[0083] Fig. 16B Pixels of the holographic reconstruction of the second picture forming the second master control area are shown, wherein the pixels of the holographic reconstruction of the second picture have been reselected, such as Fig.15A as shown; and

[0084] Fig.17 is a flow chart of a method of performing an optical alignment process according to the present disclosure.

[0085] The same reference numbers will be used throughout the drawings to refer to the same or like parts. DETAILED DESCRIPTION

[0086] The present invention is not limited to the embodiments described below, but extends to the full scope of the appended claims. That is, the present invention can be implemented in different forms and should not be construed as limited to the described embodiments, which are set forth for illustrative purposes.

[0087] A structure described as being formed on / under another structure or above / below another structure should be construed to include the case where the structures are in contact with each other and, further, also include the case where a third structure is provided therebetween.

[0088] When describing a temporal relationship, for example, when the temporal order of events is described as "after," "subsequently," "next," "before," etc., the present disclosure should be considered to include both sequential and non-sequential events unless otherwise specified. For example, unless terms such as "just," "immediately," or "directly" are used, the description should be considered to include non-sequential situations.

[0089] Although the terms "first", "second", etc. may be used herein to describe various elements, these elements are not limited by these terms. These terms are only used to distinguish between various elements. For example, a first element may be referred to as a second element, and similarly, a second element may be referred to as a first element without departing from the scope of the appended claims.

[0090] The features of different embodiments may be coupled or combined with each other in part or in whole, and may interoperate with each other in different ways. Some embodiments may be performed independently of each other, or may be performed together in a mutually dependent relationship.

[0091] Optical Configuration

[0092] Figure 1 An embodiment is shown in which a computer generated hologram is encoded on a single spatial light modulator. The computer generated hologram is a Fourier transform of the object used for reconstruction. Therefore, it can be said that the hologram is a Fourier domain or frequency domain or spectral domain representation of the object. In this embodiment, the spatial light modulator is a reflective liquid crystal on silicon "LCOS" device. The hologram is encoded on the spatial light modulator and a holographic reconstruction is formed at a playback field, such as a light receiving surface such as a screen or diffuser.

[0093] A light source 110, such as a laser or laser diode, is arranged to illuminate the SLM 140 via a collimating lens 111. The collimating lens causes a substantially planar wavefront of light to be incident on the SLM. Figure 1 In some embodiments, the direction of the wavefront is off-normal (e.g., two or three degrees from a plane that is truly normal to the transparent layer). However, in other embodiments, a substantially planar wavefront is provided at normal incidence and a beam splitter arrangement is used to separate the input and output optical paths. Figure 1 In the embodiment shown, the arrangement is such that light from the light source reflects from the mirrored rear surface of the SLM and interacts with the light modulating layer to form an exit wavefront 112. The exit wavefront 112 is applied to an optical device including a Fourier transform lens 120, the focus of which is located at a screen 125. More specifically, the Fourier transform lens 120 receives the modulated light beam from the SLM 140 and performs a frequency-space transform to produce a holographic reconstruction at the screen 125.

[0094] It is important to note that in this type of hologram, every pixel of the hologram contributes to the entire reconstruction. There is no one-to-one correlation between a specific point on the replay field (or image pixel) and a specific light modulation element (or hologram pixel). In other words, the modulated light leaving the light modulation layer is distributed over the entire replay field.

[0095] In these embodiments, the position of the holographic reconstruction in space is determined by the power (focus) of the Fourier transform lens. Figure 1 In the illustrated embodiment, the Fourier transform lens is a physical lens. That is, the Fourier transform lens is an optical Fourier transform lens and performs a Fourier transform optically. Any lens can act as a Fourier transform lens, but the performance of the lens will limit the accuracy of the Fourier transform it performs. The skilled person understands how to use a lens to perform an optical Fourier transform.

[0096] Hologram computing

[0097] In some embodiments, the computer generated hologram is a Fourier transform hologram, or simply a Fourier hologram or a Fourier based hologram, where the image is reconstructed in the far field by utilizing the Fourier transform properties of a positive lens. The Fourier hologram is calculated by Fourier transforming the desired light field in the replay plane back to the lens plane. The computer generated Fourier hologram can be calculated using the Fourier transform.

[0098] The Fourier transform hologram may be calculated using an algorithm such as the Gerchberg-Saxton algorithm. In addition, the Gerchberg-Saxton algorithm may be used to calculate a hologram in the Fourier domain (i.e., a Fourier transform hologram) from amplitude-only information in the spatial domain (e.g., a photograph). Phase information about the object is effectively "retrieved" from amplitude-only information in the spatial domain. In some embodiments, a computer-generated hologram is calculated from amplitude-only information using the Gerchberg-Saxton algorithm or a variation thereof.

[0099] The Gerchberg-Saxton algorithm considers that when the intensity cross section I of the beam in planes A and B is known, A (x,y) and I B (x,y) and I A (x,y) and I B When (x,y) is related by a single Fourier transform. For a given intensity cross section, the phase distribution approximation Ψ in planes A and B is obtained separately A (x,y) and Ψ B(x, y). The Gerchberg-Saxton algorithm finds a solution to this problem by following an iterative process. More specifically, the Gerchberg-Saxton algorithm iteratively applies spatial and spectral constraints while repeatedly transferring representations of I between the spatial domain and the Fourier (spectral or frequency) domain. A (x,y) and I B A data set (amplitude and phase) of (x,y). A corresponding computer generated hologram in the spectral domain is obtained by at least one iteration of the algorithm. The algorithm is convergent and arranged to produce a hologram representing the input image. The hologram can be an amplitude-only hologram, a phase-only hologram, or a full complex hologram.

[0100] In some embodiments, a phase-only hologram is calculated using an algorithm based on the Gerchberg-Saxton algorithm, such as the algorithm 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 a phase-only hologram by way of example only. In these embodiments, the Gerchberg-Saxton algorithm retrieves the phase information Ψ[u,v] of the Fourier transform of a data set, which produces known amplitude information T[x,y], where the amplitude information T[x,y] represents a target image (e.g., a photograph). Since amplitude and phase are essentially combined in the Fourier transform, the transformed amplitude and phase contain useful information about the accuracy of the calculated data set. Therefore, the algorithm can be used iteratively with feedback of 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. The hologram is a data set (e.g., a 2D array) of phase values.

[0101] In other embodiments, an algorithm based on the Gerchberg-Saxton algorithm is used to calculate a full complex hologram. A full complex hologram is a hologram having an amplitude component and a phase component. A hologram is a data set (e.g., a 2D array) comprising an array of complex data values, where each complex data value comprises an amplitude component and a phase component.

[0102] In some embodiments, the algorithm processes complex data and the Fourier transform is a complex Fourier transform. The complex data can be considered to include (i) a real component and an imaginary component, or (ii) an amplitude component and a phase component. In some embodiments, the two components of the complex data are processed differently at various stages of the algorithm.

[0103] Figure 2AA first iteration of an algorithm for calculating a phase-only hologram according to some embodiments is shown. The input to the algorithm is an input image 210 comprising a 2D array of pixels or data values, each of which is an amplitude or magnitude value. That is, each pixel or data value of the input image 210 does not have a phase component. Therefore, the input image 210 can be viewed as an amplitude-only or amplitude-only or intensity-only distribution. An example of such an input image 210 is a frame of a photograph or a video comprising a time sequence of frames. The first iteration of the algorithm begins with a data formation step 202A, which includes 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, wherein each data element of the data set comprises an amplitude and a phase. It can be said that the starting complex data set represents the input image in the spatial domain.

[0104] The first processing block 250 receives the starting complex data set and performs a complex Fourier transform to form a Fourier transformed complex data set. The second processing block 253 receives the Fourier transformed complex data set and outputs a hologram 280A. In some embodiments, the hologram 280A is a phase-only hologram. In these embodiments, the second processing block 253 quantizes each phase value and sets each amplitude value to 1 to form the hologram 280A. Each phase value is quantized according to the phase level that can be represented on the pixel of the spatial light modulator that will be used to "display" the 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. The hologram 280A is a phase-only Fourier hologram representing the input image. In other embodiments, the hologram 280A is a full complex hologram that includes an array of complex data values ​​(each including an amplitude component and a phase component) derived from the received Fourier transformed complex data set. In some embodiments, the second processing block 253 constrains each complex data value to one of a plurality of allowable complex modulation levels to form the hologram 280A. The constraining step may include setting each complex data value to the closest allowable complex modulation level in the complex plane. It can be said that the hologram 280A represents the input image in the spectral or Fourier or frequency domain. In some embodiments, the algorithm stops at this point.

[0105] However, in other embodiments, the algorithm continues as Figure 2A In other words, following Figure 2A The steps indicated by dashed arrows are optional (ie, not essential to all embodiments).

[0106] The 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 can be said to represent the input image in the spatial domain.

[0107] The fourth processing block 259 receives the inverse Fourier transformed complex data set and extracts the distribution of amplitude values ​​211A and the distribution of phase values ​​213A. Optionally, the fourth processing block 259 evaluates the distribution of amplitude values ​​211A. Specifically, the fourth processing block 259 can compare the distribution of amplitude values ​​211A of the inverse Fourier transformed complex data set with the input image 510, which itself is of course a distribution of amplitude values. If the difference between the distribution of amplitude values ​​211A and the input image 210 is small enough, the fourth processing block 259 can determine that the hologram 280A is acceptable. That is, if the difference between the distribution of amplitude values ​​211A and the input image 210 is small enough, the fourth processing block 259 can determine that the hologram 280A is a sufficiently accurate representation of the input image 210. In some embodiments, for comparison purposes, the distribution of phase values ​​213A of the inverse Fourier transformed complex data set is ignored. It will be appreciated that any number of different methods may be employed to compare the distribution of amplitude values ​​211A and the input image 210, and the present disclosure is not limited to any particular method. In some embodiments, a mean square error is calculated, and if the mean square error is less than a threshold, the hologram 280A is deemed acceptable. If the fourth processing block 259 determines that the 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 or preset or user defined.

[0108] Figure 2B 202B. The second iteration of the algorithm and any further iterations of the algorithm are represented. The distribution of phase values ​​213A of the previous iteration is fed back through the processing blocks of the algorithm. The distribution of amplitude values ​​211A is rejected in favor of the distribution of amplitude values ​​of the input image 210. In the first iteration, the data forming step 202A forms a first complex data set by combining the distribution of amplitude values ​​of the input image 210 with the random phase distribution 230. However, in the second and subsequent iterations, the data forming step 202B includes forming a complex data set by combining (i) the distribution of phase values ​​213A from the previous iteration of the algorithm with (ii) the distribution of amplitude values ​​of the input image 210.

[0109] Then, refer to Figure 2A The same method as described is handled by Figure 2BThe complex data set formed by the data forming step 202B is used to form a second iterative hologram 280B. Therefore, the description of the process is not repeated here. When the second iterative hologram 280B has been calculated, the algorithm can stop. However, any number of further iterations of the algorithm can be performed. It will be understood that the third processing block 256 is only required when the fourth processing block 259 is required or further iterations are required. The output hologram 280B generally gets better with each iteration. However, in practice, a point is often reached where no measurable improvement can be observed, or the positive benefits of performing further iterations are offset by the negative effects of the additional processing time. Therefore, the algorithm is described as iterative and convergent.

[0110] Figure 2C 213A for a previous iteration. The distribution of phase values ​​213A for a previous iteration is fed back through a processing block of the algorithm. The distribution of amplitude values ​​211A is rejected in favor of an alternative distribution of amplitude values. In this alternative embodiment, the alternative distribution of amplitude values ​​is derived from the distribution of amplitude values ​​211 for a previous iteration. Specifically, processing block 258 subtracts the distribution of amplitude values ​​for input image 210 from the distribution of amplitude values ​​211 for a previous iteration, scales the difference by a gain factor α, and subtracts the scaled difference from input image 210. This is mathematically expressed by the following equation, where the subscript text and number represent the number of iterations:

[0111] R n+1 [x,y]=F'{exp(iψ n [u,v])}

[0112] Ψ n [u,v]=∠F{η·exp(i∠R n [x,y])}

[0113] η=T[x,y]-α(|R n [x,y]|-T[x,y])

[0114] in:

[0115] F' is the inverse Fourier transform;

[0116] F is the forward Fourier transform;

[0117] R[x,y] is the complex data set output by the third processing block 256;

[0118] T[x,y] is the input or target image;

[0119] ∠ is the phase component;

[0120] Ψ is the phase-only hologram 280B;

[0121] η is the new distribution of amplitude values ​​211B; and

[0122] α is the gain factor.

[0123] 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 input target picture data. In some embodiments, the gain factor α depends on the number of iterations. In some embodiments, the gain factor α is only a function of the number of iterations.

[0124] In all other respects, Figure 2C Examples and Figure 2A and Figure 2B It can be said that only the phase hologram Ψ(u,v) comprises the phase distribution in the frequency or Fourier domain.

[0125] In some embodiments, a spatial light modulator is used to perform a Fourier transform. Specifically, the hologram data is combined with second data that provides optical power. That is, the data written to the spatial light modulation 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 simulates a physical lens - that is, it focuses light in the same way as a corresponding physical optical element. Therefore, the lens data provides optical power or focusing power. In these embodiments, the lens data can be omitted. Figure 1 A physical Fourier transform lens 120 of FIG. 120. It is known in the art of computer generated holography how to calculate data representing a lens. The data representing a lens may be referred to as a software lens. For example, a phase-only lens may be formed by calculating the phase delay caused by each point of the lens due to its refractive index and the 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. It is also known in the art of computer generated holography how to combine data representing a lens with a hologram so that a Fourier transform of the hologram may be performed without the need for a physical Fourier lens. In some embodiments, the lensing data is combined with the hologram by a simple addition, such as a simple vector addition. In some embodiments, a physical lens is used in combination with a software lens to perform a Fourier transform. Alternatively, in other embodiments, the Fourier transform lens is omitted entirely, so that holographic reconstruction occurs in the far field. In further embodiments, the hologram may be combined in the same manner with grating data - i.e., data arranged to perform a grating function such as beam steering. Again, it is known in the art of computer generated holography how to calculate such data. 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.

[0126] In some embodiments, the Fourier transform is performed jointly by a physical Fourier transform lens and a software lens. That is, some of the optical power that contributes to the Fourier transform is provided by the software lens, while the remaining optical power that contributes to the Fourier transform is provided by one or more physical optical devices.

[0127] In some embodiments, a real-time engine is provided that is arranged to receive image data and calculate a hologram in real time using an algorithm. In some embodiments, the image data is a video comprising a sequence of image frames. In other embodiments, the hologram is pre-calculated, stored in a computer memory and called up as needed for display on the SLM. That is, in some embodiments, a repository of predetermined holograms is provided.

[0128] The embodiments relate to Fourier holography and Gerchberg-Saxton type algorithms by way of example only. The present disclosure is equally applicable to Fresnel holography and holograms calculated by other techniques, such as those based on point cloud methods.

[0129] Light Modulation

[0130] A spatial light modulator may be used to display a diffraction pattern including a computer generated hologram. If the hologram is a phase-only hologram, a spatial light modulator that modulates the phase is required. If the hologram is a full complex hologram, a spatial light modulator that modulates both phase and amplitude may be used, or a first spatial light modulator that modulates the phase and a second spatial light modulator that modulates the amplitude may be used.

[0131] In some embodiments, the light modulation elements (i.e., pixels) of the spatial light modulator are cells comprising 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 multiple light modulation levels. That is, each liquid crystal cell is configured to operate at a light modulation level selected from a plurality of possible light modulation levels at any time. Each liquid crystal cell can be dynamically reconfigured 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, but the present disclosure is not limited to this type of spatial light modulator.

[0132] LCOS devices provide a dense array of light modulating elements or pixels within a small aperture (e.g., a few centimeters wide). Pixels are typically about 10 microns or smaller, which results in a diffraction angle of a few degrees, meaning that the optical system can be compact. It is much easier to fully illuminate the small aperture of an LCOS SLM than the large apertures of other liquid crystal devices. LCOS devices are typically reflective, which means that the circuitry that drives the LCOS SLM pixels can be buried under the reflective surface. This results in a higher aperture ratio. In other words, the pixels are densely packed, meaning there is almost no dead space between pixels. This is advantageous because it reduces optical noise in the playback field. LCOS SLMs use a silicon backplane, which has the advantage that the pixels are optically flat. This is particularly important for phase modulation devices.

[0133] The following is just an example, Figure 3 A suitable LCOS SLM is described below. An LCOS device is formed using a single crystal silicon substrate 302. It has a 2D array of square planar aluminum electrodes 301, separated by gaps 301a, arranged on the upper surface of the substrate. Each electrode 301 can be addressed by a circuit 302a buried in the substrate 302. Each electrode forms a respective plane mirror. An orientation layer 303 is disposed on the electrode array, and a liquid crystal layer 304 is disposed on the orientation layer 303. A second orientation layer 305 is disposed on a planar transparent layer 306, for example made of glass. A single transparent electrode 307, for example made of ITO, is disposed between the transparent layer 306 and the second orientation layer 305.

[0134] Each square electrode 301 together with the covered area of ​​the transparent electrode 307 and the intervening liquid crystal material defines a controllable phase modulation element 308, commonly 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 the pixels 301a. By controlling the voltage applied to each electrode 301 relative to the transparent electrode 307, the properties of the liquid crystal material of the respective phase modulation element can be varied to provide a variable retardation to light incident thereon. The effect is to provide only phase modulation to the wavefront, i.e. no amplitude effects occur.

[0135] The described LCOS SLM outputs spatially modulated light in a reflective manner. A reflective LCOS SLM has the advantage that the signal lines, grating lines and transistors are located below the mirror, which results in a high fill factor (typically greater than 90%) and high resolution. Another advantage of using a reflective LCOS spatial light modulator is that the thickness of the liquid crystal layer can be half the thickness required when using a transmissive device. This greatly increases the switching speed of the liquid crystal (a key advantage for projecting moving video images). However, the teachings of the present disclosure can also be implemented using a transmissive LCOS SLM.

[0136] Head-up display

[0137] In some embodiments, a holographic projection system is provided as part of a heads-up display (or "HUD"). Figure 4 A HUD in a vehicle such as a car is shown. Figure 4 4 shows a windshield 430 and a hood (or engine cover) 435 of a vehicle. The HUD includes a picture generation unit "PGU" 410 and an optical system 420. The PGU 410 and the optical system 420 may be collectively referred to as a holographic projector.

[0138] PGU410 includes a light source, a light receiving surface, and a processor (or computer) arranged to computer-control the image content of a picture. PGU410 is arranged to generate a picture or a series of pictures on the light receiving surface. The light receiving surface can be a screen or a diffuser. In some embodiments, the light receiving surface is plastic (i.e., made of plastic). The light receiving surface is disposed on the primary playback plane. That is, the holographic playback plane of the image is first formed.

[0139] The optical system 420 includes an input port, an output port, a first mirror 421, and a second mirror 422. The first mirror 421 and the second mirror 422 are arranged to guide light from the input port of the optical system to the output port of the optical system. More specifically, the second mirror 442 is arranged to receive light of a picture from the PGU 410, and the first mirror 421 is arranged to receive light of a picture from the second mirror 422. The first mirror 421 is also arranged to reflect the received picture light to the output port. Therefore, the optical path from the input port to the output port includes a first optical path 423 (or a first optical path component) from the input to the second mirror 422 and a second optical path 424 (or a second optical path component) from the second mirror 422 to the first mirror 421. Of course, there is a third optical path (or optical path component) from the first mirror to the output port, but in Figure 4 Due to the shape of the light path, Figure 4 The optical configuration shown may be referred to as a "z-fold" configuration.

[0140] The HUD is configured and positioned within the vehicle so that the light of the picture from the output port of the optical system 420 is incident on the windshield 430 and is at least partially reflected by the windshield 430 to the user 440 of the HUD. Thus, in some embodiments, the optical system is arranged to form a virtual image of each picture in the windshield by reflecting the spatially modulated light from the windshield. The user 440 of the HUD (e.g., the driver of the car) sees a virtual image 450 of the picture in the windshield 430. Thus, in an embodiment, the optical system is arranged to form a virtual image of each picture on the windshield of the vehicle. The virtual image 450 is formed at a distance below the hood 435 of the car. For example, the virtual image may be 1 to 2.5 meters away from the user 440. The output port of the optical system 420 is aligned with a hole in the dashboard of the car so that the light of the picture is directed to the user 440 by the optical system 420 and the windshield 430. In this configuration, the windshield 430 serves as an optical combiner. In some embodiments, the optical system is arranged to form a virtual image of each picture on an additional optical combiner included in the system. The windshield 430 or an additional optical combiner (if included) combines the light from the real world scene with the light of the picture. It can therefore be understood that the HUD can provide an augmented reality including a virtual image of the photo. For example, the augmented reality information may include navigation information or information related to the speed of the motor vehicle. In some embodiments, the light forming the picture is output by being incident on the windshield at a Brewster angle (also called a polarization angle) or within a 5-degree Brewster angle (e.g., within a 2-degree Brewster angle).

[0141] In some embodiments, the first mirror and the second mirror are arranged to fold the optical path from input to output so as to increase the optical path length without unduly increasing the physical size of the HUD.

[0142] The picture formed on the light receiving surface of the PGU410 may be only a few centimeters wide and a few centimeters high. The light receiving surface of the PGU410 can be the display plane of the alignment method. The first mirror 421 and the second mirror 422 provide magnification together or individually. That is, the first mirror and / or the second mirror can have optical power (i.e., diopter or focusing power). The user 440 therefore sees an enlarged virtual image 450 of the picture formed by the PGU. The first mirror 421 and the second mirror 422 can also correct optical distortions, such as those caused by the windshield 430, which typically has a complex curved shape. The folded optical path in the mirror and the optical power together allow the virtual image of the picture to be appropriately magnified.

[0143] The PGU 410 of the present disclosure includes a holographic projector and a light receiving surface such as a screen or a diffuser.

[0144] Optical misalignment

[0145] Figure 4 The HUD is susceptible to various optical misalignments. This will refer to Figure 5 to 1 0 for a more detailed explanation. The HUD includes a processor. The PGU 410 includes an SLM 603 connected or coupled to the processor of the HUD. The processor causes the SLM 603 to display a diffraction pattern including a computer-generated hologram (a hologram of a picture). The SLM 603 receives incident light having a wavelength from a light source and spatially modulates the incident light according to the computer-generated hologram to form an image within a main playback field at a playback plane 607. In this example, the light source is a laser and the incident light is a collimated beam of coherent light.

[0146] The image is a holographic reconstruction of a computer-generated hologram and is formed by diffraction. The image includes a main image area containing user information (e.g., a holographic reconstruction of the original image or object represented by the computer-generated hologram displayed on the SLM 603) and main and secondary control areas. These main and secondary control areas are located outside the main image area, in the secondary image area of ​​the playback field. The first and second control image areas include diffracted light formed by the displayed hologram.

[0147] exist Figure 5 In the example of FIG. 6 , the light in the first and second control regions can be considered as “control points”, i.e., focal points of diffracted light outside the main image region, for control of the projection system. An image including the main image region and the first and second control image regions is formed in a main playback field 619 at the playback plane. The playback field 619 has an angular range 623, centered on the optical axis 605 of the spatially modulated light output from the SLM 603. In other words, the size of the playback field 619 that forms the image depends on the angle.

[0148] The system also includes a detector device. The detector device is arranged to detect light traveling to or from the main control area and light traveling to or from the secondary image area (assuming that the main control area and the secondary control area are properly aligned with the detector device). The detector device described here includes two independent detectors (e.g., a first detection area and a second detection area). The two detectors are camera devices, each detector monitoring the amount of light received from one control point (i.e., one control area). For example, the two detectors determine the optical power of the received light. Although any other suitable detector device capable of determining the amount of received light from each control point may be adopted in the corresponding embodiment.

[0149] The position of the primary and secondary control regions depends on many factors. The main factors discussed here are: physical misalignment of SLM603 (particularly tilt of SM603); and b) variation in the wavelength of the incident light. The optical alignment process described here is arranged to determine a). The optical alignment process described here is arranged to determine a) while ignoring other factors that affect the position of the primary and secondary control regions (e.g., ignoring b at the same time).

[0150] Figure 6 The effect of physical misalignment of the SLM (e.g., a change in the tilt of the SLM) is shown. This misalignment results in a shift in the playback field. For example, as described above, the incident light is spatially modulated by the computer-generated hologram displayed on the SLM703 to form an image at the playback plane 709. The image at the playback plane 709 includes a main image area and main and secondary control image areas. In this arrangement, the light in the main image area and the secondary image area can again be considered as "control points", i.e., light focal points located outside the main image area. The playback field 719 has an angular range 723 and is centered here on the optical axis 722 of the spatially modulated light output from the SLM703. In other words, the size of the playback field 719 at the playback plane 709 is determined by the angle 723.

[0151] Mechanical translation may occur if the SLM 703 is subjected to vibration or changes in environmental conditions such as temperature. Such translation results in optical misalignment within the system and causes a shift in the playback field at the playback plane 709. In particular, the playback field containing the primary and secondary control areas may no longer be centered on the optical axis 722, but may be tilted or translated relative to the optical axis 722 to form a playback field 717 having an angular range 721. Angle 721 is equal to angle 723 - in other words, the image at the playback plane 709 has the same size before and after the translation, it is just located in a different position in space because the playback field 717 is shifted relative to the playback field 719. The entire playback field has been translated, so the relative position of the primary and secondary control areas has not changed (i.e., the spacing between the two areas has not increased or decreased). The relative position of the primary and secondary control areas relative to their corresponding first and second detection areas has changed. The relative position changes of the primary and secondary control areas are the same.

[0152] The computer generated hologram displayed on the SLM 703 is a hologram of a picture. The picture is pixelated (and in this example formed by an array of pixels). Therefore, the holographic reconstruction formed at the replay fields 717, 719 is also pixelated. In other words, Figure 6 Each of the main image area and the main and sub control areas includes pixels. Figure 7 The effect of the tilt variation of the SLM 703 on the pixel position of the holographic reconstruction is shown. Figure 7A first pixel array 802 shown as solid black circles is shown. The first pixel array 802 corresponds to the pixels of the holographic reconstruction formed at the replay field 719. Figure 7 A second pixel array 804 is shown as a hollow black circle. The second pixel array 804 corresponds to the pixels of the holographic reconstruction formed at the replay field 717. Because the replay field 717 is translated relative to the replay field 719 (as a result of the change in the tilt of the SLM 703), the second pixel array 804 is translated relative to the first pixel array 802. In fact, each pixel 802 in the first array is translated to form a corresponding pixel 804 in the second array. This translation is caused by Figure 7 Because the entire playback field is translated across the playback field as a result of the tilt change, each pixel 802 in the first array is translated by the same amount to form a corresponding pixel 804 of the second array. Figure 7 All arrows 806 in are of equal length. In other words, all pixels globally experience the same translation.

[0153] The effect of the change in the wavelength of the incident light is as follows: Figure 8 919 depends on the wavelength and the spacing of the pixels of the SLM 903. The diffraction of the light forming the image containing the primary and secondary control areas is diffracted according to the diffraction pattern displayed on the SLM - which can be just a computer generated hologram, or a combination of a hologram and a second diffraction pattern (e.g. a grating function component) having a defined diffraction spacing.

[0154] For a given SLM 903 and a given diffraction pattern including a computer generated hologram, a change in wavelength results in a change in the diffraction angle, and thus in a change in the position of the primary and secondary control regions - that is, when the wavelength of the incident light changes (for example due to temperature fluctuations in the laser cavity of the laser), the position of the control regions also changes. Therefore, the angular extent of the replay field containing the image including the first and second control image regions changes due to the change in wavelength. For example, if the wavelength of the incident light increases, the diffraction angle also increases, resulting in a larger replay field 917 whose angular extent 921 is greater than the angle 923. In other words, a change in wavelength results in a change in the size of the replay field. This change in image size can be defined as follows, where L is the distance to the replay plane 909 of the optical system and θ is the diffraction angle: Image size = 2L tan(θ).

[0155] Fig. 9 The effect of varying wavelength on the pixel position of the holographic reconstruction is shown. Fig. 9 A first pixel array 1002 shown as solid black circles is shown. The first pixel array 1002 corresponds to the pixels of the holographic reconstruction formed at the replay field 919. Fig. 9A second pixel array 1004 is shown as an open black circle. The second pixel array 1004 corresponds to the pixels of the holographic reconstruction formed at the replay field 917. Because the replay field 917 is scaled relative to the replay field 919 (as a result of the wavelength change), the second pixel array 1004 is scaled relative to the first pixel array 1002. This means that each pixel 1002 in the first array is translated by a different amount to form a corresponding pixel 1004 in the second array. This translation is given by Fig. 9 1006 in FIG. 1006. Each arrow 1006 has a separate length and / or direction. In other words, the translation experienced by each pixel is unique at each individual position.

[0156] Figure 7 and Fig. 9 A 6×6 pixel array is shown in each. It should be clear that this is exemplary only. A playback field may include any number of pixels, and typically includes many more pixels than a 6×6 array. For example, a playback field may include a 2000×1000 pixel array.

[0157] Figures 10A to 10F It is further shown how different factors affect the position of the primary control region and the secondary control region respectively relative to the first or detection region. Fig. 10A and 10B , the projection system includes a detector arrangement. The detector arrangement includes a first detector area 1101 and a second detector area 1103, which are located in the image area (i.e., the image area located in the playback plane 709 / 909) to detect light in the image control area. Here, the first detector area 1101 corresponds to the first CCD and the second detector area 1103 corresponds to the second CCD, but any other suitable detector arrangement can be used. When the system is in optical alignment and the wavelength of the incident light from the light source is equal to the expected wavelength, the control point of the main control area (the first control point) is located at a position 1105 in the first detector area 1101, and the control point of the secondary control area (the second control point) is located at a position 1107 in the second detector area 1103.

[0158] As reference Fig. 10C and 10D Said and Figure 6As shown, when the system is subject to mechanical misalignment, the positions of the control points are equally translated, causing the image (e.g., the image at the playback plane 709) to be translated. For example, there may be a tilt of the SLM 703, resulting in an optical misalignment to the left, where the position of the first control point is shifted from position 1105 to position 1109, and the position of the second control point is shifted from position 1107 to position 1111. Alternatively, there may be an optical misalignment to the right, where the position of the first control point is shifted from position 1105 to position 1113, and the position of the second control point is shifted from position 1107 to position 1115. In both cases, the image is translated in space.

[0159] In contrast, as referenced Fig.10E and 10F Said and Figure 8 As shown, when the wavelength of the incident light changes, the control points move together or further apart, so that the image (e.g., the image at the playback plane 909) is scaled according to the above equation. For example, when the wavelength increases, the position of the first control point shifts from position 1105 to position 1117, the position of the second control point shifts from position 1107 to position 1119, and the image is enlarged, as shown in FIG. Figure 8 As shown. Alternatively, when the wavelength decreases, the position of the first control point shifts from position 1105 to position 1121, the position of the second control point shifts from position 1107 to position 1123, and the image is reduced. In other words, the radial distances of the first and second control image regions relative to the center of the SLM both change. This change in radial distance can be used to calculate the fractional change in wavelength of the light by considering the relative positions of the two control image regions, or by determining the fractional change in radial distance of just one point.

[0160] In this way, based on the relative positions of the control points or lights of the first and second control image areas, the detector arrangement can distinguish between the reference Fig. 10C and 10D Devices described and references Fig.10E and 10F This is advantageous because scaling or translation of an image displayed to a user needs to be compensated differently, as described in more detail below. In this way, the present projection system is able to apply appropriate compensation to restore the distorted image to the original intended image.

[0161] The detector arrangement is arranged to output a first signal representative of the position of the first control image area based on detection of light in the first control image area and to output a second signal representative of the position of the second control image area based on detection of light in the second control image area.

[0162] Optical Alignment Process

[0163] Typically, a PGU / HUD may experience a combination of the two types of misalignment described above (caused by SLM tilt and wavelength variations). Fig.11 Features of a PGU of a HUD according to the present invention are shown.The HUD according to the present disclosure is arranged to determine optical misalignments caused by tilt of the SLM and to distinguish those "tilt" misalignments from misalignments caused by wavelength variations.

[0164] Previously (for example, in Figures 10A-10F The main and sub-control areas are displayed as square bright areas on the playback plane. Fig.11 In the example of FIG. 1 , the primary and secondary control regions 1202 and 1204 are composed of a single pixel (the primary control region 1202 is composed of a first pixel, and the secondary control region 1204 is composed of a second pixel). The PGU also includes a first mask 1206 and a second mask 1212, the first mask 1206 includes a cross-shaped aperture 1208 located between the primary control region 1202 and the first detection region 1210, and the second mask 1212 includes a cross-shaped aperture 1214 located between the secondary control regions 1216. The width of each of the cross-shaped apertures 1208 and 1214 is smaller than the width of a pixel (i.e., smaller than the width of the primary control region 1202 and the secondary control region 1204). As will be explained in more detail below, the inventors have recognized that this arrangement enables very precise (at the sub-pixel level) identification of the alignment state of the primary and secondary control areas 1202, 1204 (because when the primary and secondary control areas 1202, 1204 are precisely aligned with the first mask 1206 or the second mask 1212, respectively, the light forming the primary and secondary control areas 1202, 1204 may only be received at the corresponding first or second detection areas 1205, 1207).

[0165] The HUD is arranged to perform the following process to determine corrections / compensations that may be used to correct the tilt of the SLM (and changes in the tilt of the SLM over time).

[0166] As described above, the PGU includes a processor. The processor is configured to cause the SLM of the PGU to display a diffraction pattern. The diffraction pattern includes a computer-generated hologram (it is a hologram of a picture). The diffraction pattern also includes one or more diffraction gratings. Technicians will be familiar with the concept of using diffraction gratings with desired grating spacing to achieve translation of the reproduced plane image to position the image as needed (holographic reconstruction). The diffraction pattern may include a first diffraction grating arranged to translate the image in a first dimension of the playback plane. The diffraction pattern may include a second diffraction grating arranged to translate the image in a second dimension of the playback plane (in this example, the second dimension is perpendicular to the first dimension).

[0167] Initially, for example, when the PGU is started, the processor is configured to cause the SLM of the PGU to display a first diffraction pattern including a first computer-generated hologram and first and second diffraction gratings. In this example, the first and second diffraction gratings are stored in the memory of the PGU. The first and second diffraction gratings stored in the memory of the PGU are predetermined gratings that have been found in a previous calibration to correctly align the image (holographic reconstruction), thereby compensating for the tilt of the SLM during the previous calibration. Therefore, in this example, it is expected that the first and second diffraction gratings will accurately align the main control area 1202 with the aperture of the first mask 1206, and accurately align the secondary control area 1204 with the aperture of the second mask 1212 (under the conditions of the previous calibration). However, this is not required.

[0168] Generally speaking, the conditions when the PGU is started up are different from the conditions when the previous calibration was run. For example, the tilt of the SLM may have changed and / or the wavelength of the light may have changed. Therefore, it is necessary to run the optical alignment process to compensate for these changes. Fig. 12A , 12B , 13A and 13B show the optical alignment in the first dimension (parallel to the x-direction).

[0169] Fig. 12A and 13A A close-up of the cross-shaped apertures 1208, 1214 of the first and second masks 1206, 1212, respectively, is shown. The alignment process involves shifting the holographic reconstruction (image) in a first dimension by displaying a series of different diffraction gratings on the SLM. This may be referred to as performing a first scan. The diffraction grating causes the entire image to shift (so that a global shift is applied to all pixels of the image). Thus, the first pixel of the primary control region 1202 is shifted by the same amount as the second pixel of the secondary control region 1204.

[0170] Fig. 12A 1302. The movement of the main control area 1202 relative to the first mask 1206 is shown in . The initial (misaligned) position of the main control area 1202 is represented by the shaded circle 1302. During the first scan, the main control area 1202 moves between a plurality of different positions along the first dimension (each position having a different diffraction grating associated with it). When the main control area 1202 moves between the plurality of positions, the processor is arranged to receive signals from the first detection area. These signals represent parameters detected by the first detection area receiving light. In this example, the parameter is optical power. Therefore, based on the signal, the processor is arranged to determine the optical power associated with each position of the main control area. In other words, the processor is arranged to determine the optical power associated with each (first) diffraction grating. Fig. 12B The position of the main control area on the x-axis and the optical power curve on the y-axis are shown. Fig. 12BIt shows how there is an optical power peak 1304 at a specific location of the main control region, and how the optical power is at a low level in other situations. Based on the location of the power peak 1304, the processor determines the alignment position of the main control region 1202 relative to the first mask 1206. In particular, Fig. 12A , the "aligned" position of the main control region 1202 is represented by a circle 1306 including striped shading. Because the aperture of the first mask 1206 is narrow, the intensity peak is also narrow. This allows the alignment position of the main control region 1202 to be accurately determined.

[0171] Fig. 13B The movement of the secondary control area 1204 relative to the first mask 1212 is shown. The initial (misaligned) position of the secondary control area 1204 is represented by the shaded circle 1312. During the first scan, the secondary control area 1204 moves between a plurality of different positions along the first dimension (each position having a different diffraction grating associated therewith). When the secondary control area 1204 moves between the plurality of positions, the processor is arranged to receive signals from the second detection area. These signals represent parameters detected by the second detection area receiving light. In this example, the parameter is optical power. Therefore, based on the signal, the processor is arranged to determine the optical power associated with each position of the second control area. In other words, the processor is arranged to determine the optical power associated with each (first) diffraction grating detected by the second detection area. Fig. 13B A graph showing the position of the secondary control region on the x-axis and the optical power on the y-axis is shown. Fig. 13B It shows how there is a power peak 1314 at a specific location of the primary control region, and how the optical power is otherwise at a low level. Based on the location of the power peak 1314, the processor determines the alignment position of the secondary control region 1204 relative to the second mask 1212. In particular, Fig. 13B , the "aligned" position of the secondary control region 1204 is represented by a circle 1316 including striped shading. Because the aperture of the second mask 1212 is narrow, the intensity peak is also narrow. This allows the alignment position of the secondary control region 1204 to be accurately determined.

[0172] If all pixels of the holographic reconstruction experience the same misalignment relative to a previous calibration (e.g. because the misalignment is simply due to the tilt of the SLM), then it can be expected that the same diffraction grating will cause the primary control region 1202 and the secondary control region 1204 to be aligned with the corresponding detection regions. Figures 12A to 13BAs shown, the alignment position of the main control region 1202 is different from the alignment position of the secondary control region 1204. In particular, the translation or displacement of the holographic reconstruction required to align the main control region 1202 is smaller in the first dimension (from the initial position) than that required to align the secondary control region. This is represented by arrows 1410 and 1412. Arrow 1410 (associated with the main control region) is shorter than arrow 1412 (associated with the secondary control region). The reason for this is that the misalignment of each pixel is not only due to SLM tilt, but also due to the change in wavelength (relative to the previous calibration). As shown Fig. 9 As shown, changes in wavelength cause each pixel to move by a different amount. For example, pixels on opposite sides of the center of the playback field may shift in opposite directions. Fig.13A In this case, the translation of the main control region 1202 in the first dimension caused by the wavelength change is opposite to the translation caused by the tilt change of the SLM. This causes arrow 1410 to be shortened. Fig. 13B In this case, the translation of the secondary control region 1202 in the first dimension caused by the wavelength change is in the same direction (parallel to the first dimension) as the translation caused by the tilt change of the SLM. This causes the arrow 1412 to become longer.

[0173] The inventors have realised that by averaging the position, misalignment caused by SLM tilt can be separated from misalignment caused by wavelength variations. The average aligned position therefore gives an indication of the compensation needed to compensate for tilt (independent of the compensation needed for wavelength variations). This is explained by Fig.14 1502. In some examples, the processor is arranged to determine a diffraction grating that will cause the desired translation in the first dimension indicated by the dashed line 1502. Wavelength variations may be compensated in a separate procedure, not disclosed herein.

[0174] The above process has been described for alignment in a first dimension. However, it will be clear to those skilled in the art that the process may be repeated for alignment in a second dimension (to compensate for misalignment in the second dimension caused by tilting of the SLM).

[0175] Pixel re-centering

[0176] The inventors have found that in real-world scenarios, it is necessary for the above optical alignment process to be performed during the runtime of the PGU. For example, during the use of the PGU, the tilt of the SLM may change due to environmental changes and / or due to vibrations or other mechanical effects acting on the SLM. The optical alignment process needs to be rerun repeatedly during runtime to check for these changes. However, the inventors have also found that the above method may adversely affect the viewing experience because the main and / or sub-control areas may drift / shift relatively far away from the corresponding masks. This is because the scanning range required to detect the relative distance to the main and / or sub-control areas increases as the distance of the control area from the corresponding detection area increases. Because the diffraction grating used in the scan causes the entire playback field to be shifted, the image content observable by the user will also be shifted by the diffraction grating. The inventors have recognized that during runtime, if a relatively large (e.g., multi-pixel length) scan is used, the user may perceive the displacement of the image content. For example, the picture content may jump around.

[0177] If a relatively small scan range is used during the optical alignment process, any interference with the viewing experience (e.g., during run time) can be minimized. For example, the inventors have discovered that interference can be minimized using a scan range equivalent to a single pixel spacing (e.g., 0.5 pixel spacing in a first direction of the first dimension from an initial position, and 0.5 pixel spacing in a second direction of the first dimension from the initial position). Of course, if the shift / misalignment of the control region is greater than 0.5 pixel spacing (such that the control region is greater than 0.5 pixel spacing from the first or second detection region), then such a scan range may not be large enough to detect the first and / or second control region. The present disclosure provides a technical advance in repositioning / reselecting the first and second control regions and in which another (relatively shorter) scan can be performed using the repositioned / reselected first and second control regions. This is in Fig.15A and 15B Shown in.

[0178] Fig.15A Reselection of the first control area is shown. Fig.15A Focus on the first mask 1206, but for simplicity, Fig.15A Only a single branch 1602 of the aperture 1208 of the mask 1206 is shown. The scanning range relative to the first mask 1206 is indicated by a double arrow 1604. Fig.16A The location of the first control region of the first holographic reconstruction of the first scan is shown as an unshaded dashed circle 1606. Fig.15AIt can be clearly seen that the unshaded dashed circle 1606 is outside the scan range 1604. Therefore, during the first scan, no power peak will be detected. Therefore, a second scan is performed using a second holographic reconstruction, in which the first control region has been re-centered / repositioned closer to the aperture 1602. The first control region of the second holographic reconstruction is represented by Fig.15A The shaded circle 1608 in FIG.

[0179] Fig. 15B Reselection of the second control area is shown. Fig. 15B Focus on the second mask 1212, but for simplicity, Fig. 15B Only a single branch 1612 of the aperture of the mask 1212 is shown. The scanning range relative to the second mask 1212 is indicated by a double arrow 1614. Fig. 15B The location of the first control region of the first holographic reconstruction of the first scan is shown as an unshaded dashed circle 1616. Fig. 15B It can be clearly seen that the unshaded dashed circle 1616 is outside the scan range 1614. Therefore, during the first scan, no power peak will be detected. Therefore, a second scan is performed using a second holographic reconstruction, in which the second control region has been re-centered / repositioned closer to the aperture 1612. The first control region of the second holographic reconstruction is represented by Fig. 15B The shaded circle 1618 in FIG.

[0180] Fig.16A and 16B Corresponding portions of the first and second holographic reconstructions in combination with the aperture 1208 of the first mask 1206 are shown. Fig.16A 1702 is shown how a first control region for a first holographic reconstruction is formed at a first pixel position (and is composed of a first pixel 1702 represented by a black circle). This first pixel 1702 can be said to be "turned on". All other adjacent pixels are "turned off" (in Fig.16A The "off" pixel is represented by a cross 1704). Fig. 16B 1706 is now "on", while the first pixel 1702 (and the second pixel 1706 represented by the black circle) is now "on". Fig. 16B All other pixels shown) are "off".

[0181] For the avoidance of doubt, Fig.16A and 16B The crosses 1704 simply represent "off" pixels. These crosses are not visible in the actual holographic reconstruction.

[0182] Now combine Fig.17The flowchart describes the pixel re-centering process in more detail.

[0183] The image generation unit is arranged to form a holographic reconstruction of a picture (or a sequence of pictures) comprising a pixel array. Step 1802 of the process comprises selecting a first pixel in the pixel array and a second pixel in the pixel array. The selected first pixel in the pixel array is used to form a first control area for a first scan. The selected second pixel in the pixel array is used to form a second control area for the first scan.

[0184] Step 1804 of the process includes selecting an appropriate first picture including first and second control regions (composed of first and second selected pixels, respectively). Step 1804 of the process also includes forming a first holographic reconstruction of the first picture. Step 1804 includes determining a diffraction pattern by computer generating a first hologram of the first picture. Step 1804 may optionally further include superimposing or adding a diffraction grating to the first hologram so that the diffraction pattern includes a first component as the computer-generated first hologram and the diffraction grating. The diffraction grating may be a diffraction grating determined in a different (previous) calibration process to align the holographic reconstruction, or a diffraction grating determined in a previous run of the optical alignment process according to the present disclosure. The first holographic reconstruction includes primary and secondary control regions of the first and second holographic reconstructions.

[0185] Step 1806 of the process includes moving / scanning the first holographic reconstruction on the playback plane relative to the first and second detection regions such that the first holographic reconstruction moves between a plurality of positions. Fig. 12A and 13A Shown in.

[0186] Step 1808 of the process includes determining a parameter value of light received by the first and second detection areas in each of a plurality of locations in the first holographic reconstruction. The parameter in this example is optical power. The value of the optical power (at each location) is determined based on the signal received from each of the first and second detection areas at the processor of the HUD.

[0187] Step 1810 of the process includes comparing each corresponding determined optical power value (for each of a plurality of positions) to a threshold condition. The threshold condition in this example is whether the optical power value is equal to or exceeds a minimum value. The minimum value is a predetermined value stored in a memory of the HUD. The minimum value is a value representing the peak optical power as expected when the corresponding primary or secondary control area is in an aligned state with the first or second detection area. If both the first and second detection areas meet the threshold condition, step 1813 is performed and a diffraction grating that achieves the peak optical power is determined for each of the first and second control areas. If the threshold condition is not met for one or both of the first and second detection areas, the process proceeds to step 1812 and the optical alignment process is repeated for each control area (e.g., a primary and / or secondary control area) that was not detected in an aligned position during the first scan.

[0188] Step 1812 of the process includes repositioning the primary and / or secondary control regions. Repositioning the primary control region includes selecting a third pixel of the second picture, the third pixel being different from the first pixel and adjacent to the first pixel. In this example, the third pixel is adjacent to the first pixel in a first direction of the first dimension. Repositioning the secondary control region includes selecting a fourth pixel of the second picture, the fourth pixel being different from the second pixel and adjacent to it. In this example, the fourth pixel is adjacent to the second pixel in a second direction of the first dimension (opposite to the first direction).

[0189] Step 1814 includes selecting an appropriate second picture that includes the reselected primary and secondary control regions (composed of the third and fourth selected pixels, respectively). Step 1816 of the process includes forming a second holographic reconstruction of the second picture. Step 1816 includes determining a diffraction pattern by computer-generating a second hologram of the second picture. Step 1816 may optionally further include superimposing or adding a diffraction grating to the second hologram so that the diffraction pattern includes a first component that is a computer-generated second hologram and the diffraction grating. The diffraction grating may be a diffraction grating that has been determined in a previous calibration process to align the holographic reconstruction. The second holographic reconstruction includes the primary and secondary control regions of the first and second holographic reconstructions at the third and fourth pixel locations, respectively.

[0190] Step 1816 of the process includes moving / scanning the second holographic reconstruction in the playback plane relative to the first and second detection areas such that the first holographic reconstruction moves between a plurality of positions.

[0191] Step 1818 of the process includes determining a parameter value of light received by the first and second detection areas in each of a plurality of locations in the second holographic reconstruction. The parameter in this example is optical power. The value of the optical power (at each location) is determined based on the signal received from each of the first and second detection areas at the processor of the HUD.

[0192] Step 1820 of the process includes comparing each corresponding determined optical power value (for each of the plurality of locations of the second holographic reconstruction) to a threshold condition. If both the first and second detection regions satisfy the threshold condition, step 1822 is performed and a diffraction grating that achieves a peak optical power is determined for each of the first and second control regions of the second holographic reconstruction.

[0193] The final step in the process is step 1824, which is performed after step 1813 or step 1822 (once the diffraction grating associated with the peak optical power of the primary and secondary control regions is detected, which can be after the first scan or after the second scan, as described above).

[0194] Step 1824 includes determining a new raster function that is used to compensate for tilt misalignment in the first dimension. Step 1824 includes determining an average of the gradient functions required to achieve alignment of the primary and secondary control regions. Step 1824 also includes adding the average to the initial gradient function.

[0195] Additional Features

[0196] The methods and processes described herein may be embodied on a computer-readable medium. The term "computer-readable medium" includes media arranged to store data temporarily or permanently, such as random access memory (RAM), read-only memory (ROM), buffer memory, flash memory, and cache memory. The term "computer-readable medium" should also be considered to include any medium or combination of media capable of storing instructions for execution by a machine, so that when the instructions are executed by one or more processors, the machine performs, in whole or in part, any one or more of the methods described herein.

[0197] 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 and non-transitory data storage repositories (e.g., data volumes) in the example form of solid-state memory chips, optical disks, magnetic disks, or any suitable combination thereof. In some example embodiments, instructions for execution may be conveyed by a carrier medium. Examples of such carrier media include transient media (e.g., propagated signals conveying instructions).

[0198] 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. The present disclosure covers all modifications and variations within the scope of the appended claims and their equivalents.

Claims

1. A holographic projection system for forming a holographic reconstruction of a picture comprising an array of pixels at a replay plane, the system comprising a detector arrangement arranged to detect light received by a first detection area; Wherein the holographic projection system is arranged to perform an optical alignment process, the optical alignment process comprising: selecting a first pixel in the pixel array; forming a first holographic reconstruction of a first picture, the first picture comprising a primary control region including the selected first pixel; moving the first holographic reconstruction relative to the first detection region in a first dimension of the playback plane such that the first holographic reconstruction moves between the plurality of positions; and determining a parameter value of light received by the first detection region in each of the plurality of locations and comparing each respective determined parameter value to a threshold condition; Wherein, if the threshold condition is not met at multiple positions, the holographic projection system is arranged to repeat the optical alignment process by: selecting a second pixel adjacent to the first pixel in the pixel array; and forming a second holographic reconstruction of a second image, the second image including a main control area containing the second pixel.

2. The holographic projection system according to claim 1, wherein: The detector arrangement is further arranged to detect light received by the second detection area.

3. The holographic projection system according to claim 2, wherein: The optical alignment process further includes selecting a third pixel in the pixel array, and wherein the first picture further includes a secondary control area including the selected third pixel.

4. The holographic projection system according to claim 2 or 3, wherein: The optical alignment process also includes detecting parameters of light received by the second detection area at multiple locations of the first holographic reconstruction and comparing the detected parameters to a threshold condition.

5. The holographic projection system according to claim 4, wherein: The holographic projection system is arranged to determine a first alignment position of a holographic reconstruction of the first master control area based on a threshold condition satisfied by a detection parameter of light received by the first detection area; and a holographically reconstructed second alignment position of the first secondary control area based on a threshold condition satisfied by a detection parameter of light received by the second detection area.

6. The holographic projection system according to claim 5, wherein: The holographic projection system is arranged to determine an average of the first and second alignment positions.

7. The holographic projection system according to any one of claims 3 to 6, wherein: The repeated optical alignment process includes selecting a fourth pixel in the pixel array that is adjacent to the second pixel; and wherein the second picture also includes a secondary control area including the selected fourth pixel.

8. The holographic projection system according to any one of claims 3 to 7, wherein: The first pixel is adjacent to the third pixel in a first direction of the first dimension, and the second pixel is adjacent to the fourth pixel in a second direction of the first dimension opposite to the first direction.

9. A holographic projection system as claimed in any one of the preceding claims, wherein: The parameter of the light measured by the first detection region is related to the light power.

10. A holographic projection system as claimed in any one of the preceding claims, wherein: The holographic projection system is arranged to select the second pixel as a nearest neighbor of the first pixel in a direction of the first dimension.

11. A holographic projection system as claimed in any one of the preceding claims, wherein: The first and second main control areas of the respective first and second pictures are respectively constituted by the selected first or second pixels.

12. A holographic projection system as claimed in any one of the preceding claims, wherein: The holographic projection system is arranged to move the first holographic reconstruction a total distance in the first dimension, which total distance is less than or equal to a pixel pitch of the first holographic reconstruction.

13. A holographic projection system as claimed in any one of the preceding claims, wherein: The holographic projection system comprises a display device arranged to display a diffraction pattern comprising a picture hologram and to spatially modulate light incident thereon according to the diffraction pattern to form a holographic wavefront.

14. The holographic projection system according to claim 13, wherein: The holographic projection system is arranged such that during the step of forming a first holographic reconstruction of the first picture a diffraction pattern comprising a first hologram of the first picture is displayed on the display device.

15. The holographic projection system according to claim 14, wherein: The holographic projection system is arranged such that the diffraction pattern further comprises a grating function, and wherein the holographic projection system is arranged to vary the grating function of the diffraction pattern to shift the first holographic reconstruction.

16. The holographic projection system according to claim 15, wherein: The holographic projection system is arranged to vary a grating function of the diffraction pattern to move the first holographic reconstruction between the plurality of positions.

17. The holographic projection system according to claim 15 or 16, wherein: The holographic projection system is arranged to determine a grating function which yields a maximum detection parameter.

18. The holographic projection system of any one of the preceding claims, further comprising a mask having an aperture, wherein: The first detection region is arranged to detect light received through an aperture of the mask.

19. The holographic projection system according to claim 18, wherein: The width of the aperture in the first dimension is smaller than the size of the first pixel in the first dimension.

20. An optical alignment method for a holographic projection system arranged to form a holographic reconstruction of an image comprising an array of pixels, the method comprising: selecting a first pixel in the pixel array; forming a first holographic reconstruction of a first picture, the first picture comprising a first master control region including the selected first pixel; moving the first holographic reconstruction relative to the first detection region in a first dimension of the playback plane such that the first holographic reconstruction moves between a plurality of positions; as well as determining a parameter value of light received by a first detection region of a detector of the holographic projection system in each of a plurality of locations, and comparing each respective determined parameter value to a threshold condition; Wherein, if the threshold condition is not satisfied at the plurality of locations, the method includes repeating the optical alignment process by: selecting a second pixel in the pixel array that is adjacent to the first pixel; and forming a second holographic reconstruction of a second picture, the second picture including a second primary control area including second pixels.

Images