# Patent

## Title
2D/3D holographic display system

## Abstract

A display system (300) comprising an optical system and a processing system. The optical system comprising a spatial light modulator (380), a light source, a Fourier transform lens, a viewing system (320, 330) and a processing system. The spatial light modulator is arranged to display holographic data in the Fourier domain, illuminated by the light source. The Fourier transform lens is arranged to produce a 2D holographic reconstruction in the spatial domain (310) corresponding to the holographic data. The viewing system is arranged to produce a virtual image (350) of the 2D holographic reconstruction. The processing system is arranged to combine the Fourier domain data representative of a 2D image with Fourier domain data representative of a phase only lens to produce first holographic data, and provide the first holographic data to the optical system to produce a virtual image.

## Background
This application is the U.S. National Stage of International Application No. PCT/GB2011/051328, filed Jul. 14, 2011, which designates the U.S., published in English, and claims priority under 35 U.S.C. §§119 or 365(c) to Great Britain Application No. 1011829.7, filed Jul. 14, 2010.
The present invention relates a display system and a method of displaying images. Embodiments relate to virtual image display systems and methods, and some embodiments relate to head-up display systems.
BACKGROUND
Light scattered from an object contains both amplitude and phase information. This amplitude and phase information can be captured on, for example, a photosensitive plate by well known interference techniques to form a hologram comprising interference fringes. The hologram may be reconstructed to form an image, or holographic reconstruction, representative of the original object by illuminating the hologram with suitable light.
Computer-generated holography may numerically simulate the interference process using Fourier techniques.
It has been proposed to use holographic techniques in a two-dimensional image projector.
Referring to FIG. 1, there is shown a light source 100 which applies light via a Fourier lens (120) onto a spatial light modulator (140) in this case as a generally planar wavefront. The spatial light modulator is reflective and consists of an array of a large number of phase-modulating elements. Light is reflected by the spatial light modulator and consists of two parts, a first specularly reflected portion (known as the zero order) and a second portion that has been modulated by the phase-modulating elements to form a wavefront of spatially varying phase. Due to the reflection by the spatial light modulator all of the light is reflected generally back towards the light source (100) where it impinges on a mirror with aperture (160) disposed at 45° to the axis of the system. All of the image part of the light is reflected by the mirror towards a screen (180) that is generally parallel to the axis of the system. Due to the action of the Fourier lens (120), the light that impinges on the screen (180) forms a real image that is a reconstruction of an image from which the information applied to the phase modulating elements was derived.
Embodiments relate to an improved 2D real-time projector for forming virtual images of holographic reconstructions and providing adaptive positional control of the virtual image in space, and allow for spatial filtering of the reconstruction.
SUMMARY OF THE INVENTION
Aspects of the invention are defined in the appended independent claims.
In summary, a spatial light modulator (SLM) forms an array of phase-modulating elements that collectively represent a phase-only Fourier transform of a desired image which can be reconstructed by correctly illuminating the SLM, to form a projector. The phase-only distribution may be referred to as a hologram. The image may be described as the holographic reconstruction. The elements of the SLM may be referred to as pixels.
The holographic reconstruction is imaged by an optical viewing system to form a virtual image. The inventor has recognised that by providing variable lensing data to the hologram, the position of the virtual image relative to a viewer can be changed. This can provide a “depth” to the display system and allow virtual images to be presented at different distances from the viewer to provide a pseudo 3D system in real-time. In particular, the inventor has recognised that by forming an intermediate reconstruction, spatial filtering may be performed to remove higher diffracted orders produced by the hologram. This gives rise to an improved viewing system particular for real-time applications such as head-up displays.



BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention will now be described to the accompanying drawings in which:
FIG. 1 shows the basic principle of a conventional holographic image display;
FIG. 2 shows a schematic drawing of an example of a reflective SLM;
FIG. 3 shows a schematic drawing of a display;
FIG. 4 shows the effect of varying lensing information at the position of the virtual image.
FIG. 5 shows a schematic drawing of a LCOS SLM device;



In the figures like reference numerals referred to like parts.
DETAILED DESCRIPTION OF THE DRAWINGS
It is found that the phase information alone is sufficient to generate a hologram which can give rise to a holographic reconstruction of acceptable quality. That is, the amplitude information in the hologram can be discarded. This can reduce the power of the required laser light sources but has other advantages too. Fourier-based computer generated holographic techniques have therefore been developed using only the phase information.
The image reconstructed by a hologram is given by the Fourier transform of the hologram. The hologram is therefore a phase-only pattern representative of the Fourier transform of the object whereas the reconstructed image (or holographic reconstruction) may contain both amplitude and phase information.
Gerchberg-Saxton is one example of an iterative algorithm for calculating a phase only hologram from input image data comprising only amplitude information. The algorithm starts from a random phase pattern and couples this with amplitude data to form complex data. A discrete Fourier transform is performed on the complex data and the resultant dataset is the Fourier components, which are made up of magnitude and phase. The magnitude information is set to a uniform value, and the phase is quantised, to match the phase values available. An inverse discrete Fourier transform is then performed. The result is another complex dataset, where the magnitude information is overwritten by the target image and the process is repeated. The Gerchberg-Saxton algorithm therefore iteratively applies spatial and spectral constraints while repeatedly transferring a data set (amplitude and phase), between the spatial domain and the Fourier (spectral) domain.
The Gerchberg-Saxton algorithm and derivatives thereof are often much faster than other “non-Fourier transform” algorithms such as direct binary search algorithms. Modified algorithms based on Gerchberg-Saxton have been developed—see, for example, co-pending published PCT application WO 2007/131650 incorporated herein by reference.
These improved techniques are able to calculate holograms at a sufficient speed that 2D video projection is realised. Embodiments described herein relate to 2D video projection using a computer-generated hologram calculated using such a modified Gerchberg-Saxton algorithm
Holographically generated 2D video images are known to possess significant advantages over their conventionally projected counterparts, especially in terms of definition and efficiency. However, the computational and hardware complexity of the current hologram generation algorithms preclude their use in real-time applications. Recently these problems have been solved—see, for example, published PCT application WO 2005/059881 incorporated herein by reference.
To display the phase only holographic data, a phase modulating device is required. Since these devices do not modulate the amplitude, they are optically transparent, in general. Therefore no light is lost to absorption, for example. This has the major advantage that all of the reconstruction light is used in the creation of the holographic reconstruction. This translates to a more energy efficient display system.
The phase modulating device may be pixellated and each pixel will act as a diffractive element. The diffraction pattern from each pixel will give rise to a complex interference pattern at a screen referred to as a replay field. Due to this complex relationship, each pixel on the hologram contributes to multiple parts of the reconstructed image.
An example phase modulating device is a spatial light modulator (SLM). Typically a SLM has a field of addressable phase-modulating elements. In some SLMs the phase-modulating elements are a linear or one-dimensional array of elements; in others a two dimensional array are provided. For simplicity many SLMs have a regular 2-D array of like, generally square, phase-modulating elements; it is however not necessary for the phase-modulating elements to be alike in size or shape.
FIG. 2 shows an example of using a reflective SLM, such as a LCOS, to produce a holographic reconstruction at a replay field location, in accordance with the present disclosure.
A light source (210), for example a laser or laser diode, is disposed to illuminate the SLM (240) via a collimating lens (211). The collimating lens causes a generally planar wavefront of light to become incident on the SLM. The direction of the wavefront is slightly off-normal (i.e. two or three degrees away from being truly orthogonal to the plane of the transparent layer). The arrangement is such that light from the light source is reflected off a mirrored rear surface of the SLM and interacts with a phase-modulating layer to form an exiting wavefront (212). The exiting wavefront (212) is applied to optics including a Fourier transform lens (220), having its focus at a screen (225).
The Fourier transform lens receives light from the SLM and performs a frequency-space transformation to produce a holographic reconstruction at the screen (225) in the spatial domain.
In this process, the light from the light source is generally evenly distributed across the SLM (240), and across the phase modulating layer. Light exiting the phase-modulating layer may be distributed across the screen. There is no correspondence between a specific image region of the screen and any one phase-modulating element.
Referring to FIG. 3, there is shown an embodiment in accordance with the present disclosure using the SLM based system described above. FIG. 3 shows a head-up display (300) having an SLM based system (305) for providing a real image of a holographic reconstruction (310). The holographic reconstruction is formed at a so-called replay field. The spatial position of the replay field may be varied in accordance with embodiments described herein.
The display consists of an optical combiner (320) and a lens (330) disposed between the holographic reconstruction (310) and the combiner (320). The arrangement is such that a viewer (340) looking towards the combiner (320) will see a virtual image (350) of the holographic reconstruction (310) at a distance d from the viewer and behind the combiner (320). Such a system can be used for example in a head-up display or head-mounted display.
The optical system (335) may consist of a lens having a focal length f, and located at distance ed from the viewer. The holographic reconstruction (310) is at a real distance od behind the lens. If the holographic reconstruction (310) is disposed in the focal plane of the lens (330) then the viewer (340) will perceive the image (350) to be at infinity.
However if the holographic reconstruction (310) is closer to the lens (330) then the focal length of the lens (330) the image (350) will no longer be at infinity.
Provided the holographic reconstruction is closer to the lens (330) than the focal length of the lens then the image (350) can be arranged to appear closer than at infinity, and appear at a virtual distance vd. The calculations are as follows:




  od
  =
  
    1
    
      (
      
        
          1
          f
        
        -
        
          1
          
            -
            vd
          
        
      
      )
    
  




The above-mentioned replay field location may be varied by varying the lensing characteristic of phase only lensing data applied to the spatial light modulator (380). Thus for a first lensing characteristic the position of the real image (310) can be relatively close to the lens (330) and for a second value of lensing data the real image 310 is relatively more distant from the lens (330). This means that the image (350) created by the virtual image display can be varied in apparent depth.
In summary the information that is applied to the phase modulating elements of the SLM (380) consists of two parts, a first part that comprises the information representative of the final image and a second part which has the effect of providing a negative lensing and adjustment characteristic. By varying this latter part it is possible to cause the position of the holographic reconstruction and therefore virtual image (350) to be varied.
By using a sufficiently fast spatial light modulator, an appropriate computational algorithm and by writing data appropriately to the spatial light modulator it is possible to image different sub-frames of data at different apparent depths. The SLM must be sufficiently fast to allow information to be electrically written and optically read-out multiple times in a standard video frame. If the sub-frames are displayed sufficiently quickly, they may appear to a human viewer to be present simultaneously.
For example a general image can appear to be 2.5 meters from the viewer (340) but a part of the image—for example an image of especial importance to the viewer—can, by providing its imaging data in a different sub-frame and by changing the lensing data for that sub-frame—cause that image to appear in front of the general image plane. This is shown schematically in FIG. 4 which shows four different subframe image positions, denoted 501, 502, 503 and 504.
The arrangement of FIG. 3 should be distinguished from configurations in which the viewer is positioned at the real image (310). Such configurations may be referred to as “direct view”. In such cases, the viewer's eyes as the Fourier lens.
In summary the present disclosure relates to a virtual image display in which a holographic reconstruction (310) is first formed as a real image in space. The real image (310) forms the “object” for lens (330) which produces a virtual image (350) of the real image (310). The virtual image (350) may be seen by the viewer by looking through the optical combiner (320) as shown in FIG. 3.
By modifying the lensing data applied to the spatial light modulator (380), the position of the real image (310) can be changed. Accordingly, the position of the virtual image (350) can also be changed.
In contrast, when the viewer is position at the real image (310) the viewer functions as the Fourier lens and so sees all diffracted orders of the reconstruction field. That is, the viewer would see multiple replicas of the primary reconstruction—in order words, multiple reconstructions. The presence of multiple orders may lead to confusion particularly in a head-up display, for example.
Additionally the quality of the reconstructed hologram is also affect by the so-called zero order problem which is a consequence of the diffractive nature of the reconstruction.
Such zero-order light can be regarded as “noise” and includes for example specularly reflected light, and other light that is unrefracted by the patterns on the spatial light modulator.
This “noise” is generally focussed at the focal point of the Fourier lens, leading to a bright spot at the centre of a reconstructed hologram. In a direct view application the zero order would be a substantial distraction when looking at the virtual image.
Advantageously, by imaging the intermediate reconstruction it is possible to filter out the zero order and the higher diffracted orders of the reconstruction field at the intermediate reconstruction. This may be achieve, for example, by positioning a spatial filter at the real image (310) to provide a physical aperture through which only preferred orders such as the primary order can pass.
Conventionally, the zero order light is simply blocked out however this would clearly mean replacing the bright spot with a dark spot.
However as the hologram contains three dimensional information, it is possible to displace the reconstruction into a different plane in space—see, for example, published PCT application WO 2007/131649 incorporated herein by reference.
The application of the present invention includes head up displays and head mounted displays, inter alia. The invention allows for full colour holograms with different information at different distances or depths from the viewer, full 3D with a very limited volume by stacking multiple sub-frames, a large number of different images at different distances, perspective tracking of objects and enhance reality, for example a near-eye augmented-reality system with the ability to overlay different information at different depths.
In embodiments, the spatial light modulator is a Liquid Crystal over silicon (LCOS) device. The image quality is, of course, affected by the number of pixels and the number of possible phase levels per pixel.
LCOS devices are a hybrid of traditional transmissive liquid crystal display devices, where the front substrate is glass coated with Indium Tin Oxide to act as a common electrical conductor. The lower substrate is created using a silicon semiconductor process with an additional final aluminium evaporative process being used to create a mirrored surface, these mirrors then act as the pixel counter electrode.
Compared with conventional glass substrates these devices have the advantage that the signal lines, gate lines and transistors are below the mirrored surface, which results in much higher fill factors (typically greater than 90%) and higher resolutions.
LCOS devices are now available with pixels between 4.5 μm and 12 μm, this size is determined by the mode of operation and therefore amount of circuitry that is required at each pixel.
The structure of an LCOS device is shown in FIG. 5.
A LCOS device is formed using a single crystal silicon substrate (402). It has a 2D array of square planar aluminium electrodes (401), spaced apart by a gap (401a), arranged on the upper surface of the substrate. Each of the electrodes (401) can be addressed via circuitry (402a) buried in the substrate (402). Each of the electrodes forms a respective planar mirror. An alignment layer (403) is disposed on the array of electrodes, and a liquid crystal layer (404) is disposed on the alignment layer (403). A second alignment layer (405) is disposed on the liquid crystal layer (404) and a planar transparent layer (406), e.g. of glass, is disposed on the second alignment layer (405). A single transparent electrode (407) e.g. of ITO is disposed between the transparent layer (406) and the second alignment layer (405).
Each of the square electrodes (401) defines, together with the overlying region of the transparent electrode (407) and the intervening liquid crystal material, a controllable phase-modulating element (408), often referred to as a pixel. The effective pixel area, or fill factor, is the percentage of the total pixel which is optically active, taking into account the space between pixels (401a). By control of the voltage applied to each electrode (401) with respect to the transparent electrode (407), the properties of the liquid crystal material of the respective phase modulating element may be varied, thereby to provide a variable delay to light incident thereon. The effect is to provide phase-only modulation to the wavefront, i.e. no amplitude effect occurs.
A major advantage of using a reflective LCOS spatial light modulator is that the liquid crystal layer is half the thickness that it would be if a transmissive device were used. This greatly improves the switching speed of the liquid crystal (a key point for projection of moving video images). A LCOS device is also uniquely capable of displaying large arrays of phase only elements in a small aperture. Small elements (typically approximately 10 microns) result in a practical diffraction angle (a few degrees) so that the optical system does not require a very long optical path.
It is easier to adequately illuminate the small aperture (a few square centimeters) of a LCOS SLM than it would be for the aperture of a larger liquid crystal device. LCOS SLMs also have a large aperture ratio, there is very little dead space between the pixels (as the circuitry to drive them is buried under the mirrors). This is an important issue to lowering the optical noise in the replay field.
The above device typically operates within a temperature range of 10° C. to around 50° C., with the optimum device operating temperature being around 40° C. to 50° C.
As a LCOS device has the control electronics embedded in the silicon backplane, the Fill factor of the pixels is higher, leading to less unscattered light leaving the device.
Using a silicon backplane has the advantage that the pixels are optically flat, which is important for a phase modulating device.
A colour 2D holographic reconstruction can be produced and there are two main methods of achieving this. One of these methods is known as “frame-sequential colour” (FSC). In an FSC system, three lasers are used (red, green and blue) and each laser is fired in succession at the SLM to produce each frame of the video. The colours are cycled (red, green, blue, red, green, blue, etc.) at a fast enough rate such that a human viewer sees a polychromatic image from a combination of the three lasers. Each hologram is therefore colour specific. For example, in a video at 25 frames per second, the first frame would be produced by firing the red laser for 1/75th of a second, then the green laser would be fired for 1/175th of a second, and finally the blue laser would be fired for 1/75th of a second. The next frame would then be produced, starting with the red laser, and so on.
An alternative method, that will be referred to as “spatially separated colours” (SSC) involves all three lasers being fired at the same time, but taking different optical paths, e.g. each using a different SLM, and then combining to form the colour image.
An advantage of the frame-sequential colour (FSC) method is that the whole SLM is used for each colour. This means that the quality of the three colour images produced will not be compromised because all pixels on the SLM are used for each of the colour images. However, a disadvantage of the FSC method is that the overall image produced will not be as bright as a corresponding image produced by the SSC method by a factor of about 3, because each laser is only used for a third of the time. This drawback could potentially be addressed by overdriving the lasers, or by using more powerful lasers, but this would require more power to be used, would involve higher costs and would make the system less compact.
An advantage of the SSC (spatially separated colours) method is that the image is brighter due to all three lasers being fired at the same time. However, if due to space limitations it is required to use only one SLM, the surface area of the SLM can be divided into three equal parts, acting in effect as three separate SLMs. The drawback of this is that the quality of each single-colour image is decreased, due to the decrease of SLM surface area available for each monochromatic image. The quality of the polychromatic image is therefore decreased accordingly. The decrease of SLM surface area available means that fewer pixels on the SLM can be used, thus reducing the quality of the image. The quality of the image is reduced because its resolution is reduced.
Embodiments implement the technique of “tiling”, in which the surface area of the SLM is further divided up into a number of tiles, each of which is set in a phase distribution similar or identical to that of the original tile. Each tile is therefore of a smaller surface area than if the whole allocated area of the SLM were used as one large phase pattern. The smaller the number of frequency component in the tile, the further apart the reconstructed pixels are separated when the image is produced. The image is created within the zeroth diffraction order, and it is preferred that the first and subsequent orders are displaced far enough so as not to overlap with the image and may be blocked by way of a spatial filter.
As mentioned above, the image produced by this method (whether with tiling or without) comprises spots that form image pixels. The higher the number of tiles used, the smaller these spots become. If one takes the example of a Fourier transform of an infinite sine wave, a single frequency is produced. This is the optimum output. In practice, if just one tile is used, this corresponds to an input of a single phase of a sine wave, with a zero values extending in the positive and negative directions from the end nodes of the sine wave to infinity. Instead of a single frequency being produced from its Fourier transform, the principle frequency component is produced with a series of adjacent frequency components on either side of it. The use of tiling reduces the magnitude of these adjacent frequency components and as a direct result of this, less interference (constructive or destructive) occurs between adjacent image pixels, thereby improving the image quality.
Preferably, each tile is a whole tile, although it is possible to use fractions of a tile.
There is provided a method of displaying images comprising varying lensing data on a spatial light modulator while varying imaging data applied to the spatial light modulator, whereby images of objects may be formed at different depths with regard to an image plane.
This image plane may be used in a virtual imaging system.
The step of varying data may be carried out in such a way that the plural images formed at different depths appear to the human eye to be simultaneously present.
There is provided a method of displaying, the method comprising applying data for forming an image to a SLM, illuminating the SLM, applying the resultant light to an optical system for forming a virtual image, wherein the data applied to the SLM includes first data and second data, the first data related to the content of the image and the second data determined to provide at least a lensing function by the SLM, the method further comprising varying the second data in such a way that plural images formed at different depths appear to the human eye to be simultaneously present.
The method may comprise varying the first data whereby the plural images differ from one another.
There is provided a display comprising an SLM, circuitry for operating the SLM, an illumination device for illuminating the SLM and an optical system adapted to form a virtual image reconstructed from data on the SLM, wherein the circuitry is adapted to apply data for forming an image to the SLM, the data applied including first data and second data, the first data related to the content of the image and the second data determined to provide a lensing function by the SLM, and the circuitry adapted to vary the second data in such a way that plural images formed at different depths appear to the human eye to be simultaneously present.
The optical system may comprise a Fourier lens
The display may form a head-up display.
The invention is not restricted to the described embodiments but extends to the full scope of the appended claims.

## Claims

1. A display system for displaying a virtual image of a first two-dimensional (2D) image to a viewer, the display system comprising:
a processing system configured to combine data representative of a Fourier transform of the first 2D image with Fourier-domain data representative of a first phase only lens to produce first holographic data;
a phase modulating spatial light modulator arranged to display the first holographic data;
a light source arranged to illuminate the spatial light modulator to form spatially modulated light in accordance with the first holographic data;
a Fourier transform lens arranged to Fourier transform the spatially modulated light and produce a first 2D holographic reconstruction image of the first 2D image, the first 2D holographic reconstruction image being a real image in space spatially remote from the viewer; and
a viewing system arranged to display a first virtual image of the real 2D holographic reconstruction image to the viewer.

2. The display system of claim 1, wherein the processing system is further arranged to combine data representative of a Fourier transform of a second 2D image with Fourier-domain data representative of a second phase only lens to produce second holographic data, and provide the second holographic data to the phase modulating spatial light modulator to produce a second 2D holographic reconstruction image;
wherein the first and second 2D holographic reconstruction images are spatially displaced relative to each other to form replay fields at different locations.

3. The display system of claim 2, wherein the spatial light modulator comprises an array of pixelated diffractive elements.

4. The display system of claim 3, wherein the first virtual image and a second virtual image of the second 2D holographic reconstruction image appear to a viewer to be simultaneously present.

5. The display system of claim 4, wherein the spatial light modulator is a reflective liquid crystal on silicon spatial light modulator.

6. The display system of claim 5, wherein the optical power of the first phase only lens is user-controlled.

7. The display system of claim 6, wherein the first phase only lens has a first optical power and the second phase only lens has a second optical power different to the first phase only lens.

8. The display system of claim 7, wherein the display comprises a heads-up display.

9. The display system of claim 8, wherein the replay fields are spatially remote from the viewer.

10. The display system of claim 9, further comprising a spatial filter configured to selectively block at least one diffraction order of the first 2D holographic reconstruction image.

11. The display system of claim 10, wherein the first and second virtual images are sequential frames of a 2D video.

12. The display system of claim 11, wherein the pixelated array includes pixels having respective diameters less than 15 μm.

13. The display system of claim 10, wherein the spatial filter is further configured to selectively block a zeroth diffracted order of the first 2D holographic reconstruction image.

14. A method for displaying a virtual image to a viewer, the method comprising:
combining, with a processing system, first lensing data with holographic image data representative of a first two-dimensional (2D) image to produce first holographic data;
writing the first holographic data to a phase-modulating spatial light modulator;
illuminating the spatial light modulator to phase modulate light in accordance with the first holographic data;
performing a Fourier transform of the spatially modulated light using a Fourier transform lens to produce, as a real image in space spatially remote from the viewer, a first 2D holographic reconstruction image of the first 2D image; and
displaying a first virtual image of the first 2D holographic reconstruction image using an optical viewing system.

15. The method of claim 14, further comprising spatially filtering the resultant light from the spatial light modulator to selectively block at least one diffraction order of the first 2D holographic reconstruction image.

16. The method of claim 14, further comprising:
combining, with the processing system, second lensing data with second holographic image data representative of a second 2D image to produce second holographic data;
writing the second holographic data to the phase-modulating spatial light modulator;
illuminating the spatial light modulator to phase modulate light in accordance with the second holographic data;
performing a Fourier transform of the spatially modulated light using the Fourier transform lens to produce, as a real image in space spatially remote from the viewer, a second 2D holographic reconstruction image of the second 2D image; and
displaying a second virtual image of the second 2D holographic reconstruction image using the optical viewing system, wherein the first and second virtual images are spatially displaced relative to each other to form replay fields at different locations.