CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.K. Patent Application 1912158.1, filed Aug. 23, 2019, which is hereby incorporated herein by reference in its entirety.

FIELD

The present disclosure relates to a projector. More specifically, the present disclosure relates to a holographic projector and a holographic projection system. The present disclosure further relates to a method of holographically projecting a source image and a method of holographically projecting video images. Some embodiments relate to a head-up display and a head-mounted display.

BACKGROUND AND INTRODUCTION

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 holographic recording, or “hologram”, comprising interference fringes. The hologram may be reconstructed by illumination with suitable light to form a two-dimensional or three-dimensional holographic reconstruction, or replay image, representative of the original object.

Computer-generated holography may numerically simulate the interference process. A computer-generated hologram, “CGH”, may be calculated by a technique based on a mathematical transformation such as a Fresnel or Fourier transform. These types of holograms may be referred to as Fresnel or Fourier holograms. A Fourier hologram may be considered a Fourier domain representation of the object or a frequency domain representation of the object. A CGH may also be calculated by coherent ray tracing or a point cloud technique, for example.

A CGH may be encoded on a spatial light modulator, “SLM”, arranged to modulate the amplitude and/or phase of incident light. Light modulation may be achieved using electrically-addressable liquid crystals, optically-addressable liquid crystals or micro-mirrors, for example.

The SLM may comprise a plurality of individually-addressable pixels which may also be referred to as cells or elements. The light modulation scheme may be binary, multilevel or continuous. Alternatively, the device may be continuous (i.e. is not comprised of pixels) and light modulation may therefore be continuous across the device. The SLM may be reflective meaning that modulated light is output from the SLM in reflection. The SLM may equally be transmissive meaning that modulated light is output from the SLM is transmission.

A holographic projector for imaging may be provided using the described technology. Such projectors have found application in head-up displays, “HUD”, and head-mounted displays, “HMD”, including near-eye devices, for example.

A holographic projector projects an image onto a replay field on a replay plane. When using the described technology, the projected image is formed from a hologram displayed on pixels of the SLM, herein referred to as “SLM pixels”. Thus, the SLM pixels display pixels of the hologram, herein referred to as “hologram pixels”. The projected image is formed of “image pixels” which are also referred to herein as “image spots”. The image pixels have a finite size and adjacent image pixels in the replay field can interfere or blur together. This is referred to herein as pixel crosstalk. The problem of pixel crosstalk leads to reduced image quality.

Furthermore, a hologram engine takes time to determine a hologram for display from a source image. For example, the hologram may be a Fourier hologram calculated using at least one Fourier transform. The time taken to calculate the hologram can therefore limit the rate at which holograms can be written to the SLM and thus the rate at which a sequence of source images can be projected as a video stream, herein called the “frame rate”. Thus, it can be difficult to project images at acceptable video frame rates.

There is disclosed herein an improved holographic projection system and method.

SUMMARY

Aspects of the present disclosure are defined in the appended independent claims.

There is disclosed herein a holographic projector comprising an image processing engine, a hologram engine, a display engine and a light source. The image processing engine is arranged to receive a source image for projection and generate a plurality of secondary images from the source image. The source image comprises pixels. Each secondary image comprises fewer pixels than the source image. A first secondary image has more pixels than a second secondary image. The hologram engine is arranged to determine, such as calculate, a hologram corresponding to each secondary image to form a plurality of holograms. Thus, a first hologram corresponding to the first secondary image has more pixels than a second hologram corresponding to the second secondary image. The display engine is arranged to display each hologram in turn on the display device. The light source is arranged to Illuminate each hologram during display to form a holographic reconstruction corresponding to each secondary image on a replay plane.

The inventors have disclosed herein an approach in which a plurality of a secondary images of different resolution are derived from the same source image. A hologram is determined and displayed for each secondary image. A corresponding plurality of holographic reconstructions are therefore formed one after the other on the replay plane. The holographic reconstructions are formed within the integration time of the human eye such that a viewer cannot tell that the projected image that they see is formed from multiple holographic reconstructions formed one after the other. The projected image therefore appears to be a faithful and complete reconstruction of the source image.

This approach provides three significant technical contributions to the field. Firstly, the quality of the projected image is improved. Secondly, the speed at which the projected image can be updated (i.e., the frame rate) is increased. Thirdly, a more flexible display device is provided. These three technical contributions are explained respectively in the following three paragraphs.

First, the approach disclosed herein enables pixel crosstalk to be managed by displaying different image pixels at different times. More specifically, different groups of image spots are displayed at different times. For example, a first holographic reconstruction formed at a first time (e.g. corresponding to the first secondary image) may comprise a first group of image pixels (e.g., every other image pixel) of an image frame and a second holographic reconstruction at a second time (e.g. corresponding to the second secondary image) may fill in the gaps of the image frame by displaying a second group comprising the remaining image pixels. Since image pixels of the first and second groups (e.g. adjacent pixel groups) are not displayed at the same time, interpixel interference and pixel crosstalk is reduced. The inventors have therefore disclosed a technique of interlacing (in time) a plurality of holographic reconstructions to improve image quality by managing pixel crosstalk.

Secondly, the inventors have disclosed herein an approach which is suitable for real-time (i.e. video rate) processing. Specifically, the holograms can be determined and displayed within the frame time of video. This technical contribution is achieved because each secondary image has fewer pixels than the source image. Although more holograms are required for each source image, each individual hologram can be determined much quicker.

For example, it is quicker to calculate two holograms comprising x pixels using a Fourier transform method than it is to calculate one hologram comprising 2x pixels; it is even quicker to calculate one hologram having x pixels and another hologram having fewer than x pixels. The inventors have therefore disclosed a technique to increase the speed of calculating holograms corresponding to a source image to enable holographic projection at acceptable video frame rates.

Thirdly, the approach disclosed herein provides multiple degrees of design freedom which are highly valuable in a real-world projector. For example, the image processing engine may dynamically change the scheme used to derive the secondary images from the source image, including the size/resolution of different secondary images, based on external factors. Likewise, the display engine may dynamically change the tiling scheme used to display a hologram based on external factors. “Tiling” involves writing a hologram to an SLM that has fewer hologram pixels that the number of pixels of the SLM, and repeating part of the hologram (that is, a continuous subset of the hologram pixels) in the unused pixels of the SLM. The more tiles used to display a hologram, the better the uniformity of the image pixels. The new approach therefore enables each of the holograms to be tiled differently, for example by displaying the holograms using different numbers of tiles. In some real-world applications, it may be better to display some image content of the source image with maximum quality but, optionally, only a low refresh rate is needed and display other image content at lower quality but, optionally, with a higher refresh rate. The inventors have therefore disclosed a technique that can be flexibly adapted to a wide variety of real-world scenarios.

These and other advantages of the new approach disclosed herein will be further appreciated from the following detailed description.

The term “hologram” is used to refer to the recording which contains amplitude information or phase information, or some combination thereof, about the object. The term “holographic reconstruction” is used to refer to the optical reconstruction of the object which is formed by illuminating the hologram. The term “replay plane” is used herein to refer to the plane in space where the holographic reconstruction is fully formed. The term “replay field” is used herein to refer to the sub-area of the replay plane which can receive spatially-modulated light from the spatial light modulator. The terms “image”, “replay image” and “image region” refer to areas of the replay field illuminated by light forming the holographic reconstruction. In embodiments, the “image” may comprise discrete spots which may be referred to as “image pixels”.

The terms “encoding”, “writing” or “addressing” are used to describe the process of providing the plurality of pixels of the SLM with a plurality of control values which respectively determine the modulation level of each pixel. It may 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 may be said to “display” a hologram.

It has been found that a holographic reconstruction of acceptable quality can be formed from a “hologram” containing only phase information related to the original object. Such a holographic recording may be referred to as a phase-only hologram. Embodiments relate to a phase-only hologram but the present disclosure is equally applicable to amplitude-only holography.

The present disclosure is also equally applicable to forming a holographic reconstruction using amplitude and phase information related to the original object. In some embodiments, this is achieved by complex modulation using a so-called fully complex hologram which contains both amplitude and phase information related to the original object. Such a hologram may be referred to as a fully-complex hologram because the value (grey level) assigned to each pixel of the hologram has an amplitude and phase component. The value (grey level) assigned to each pixel may be represented as a complex number having both amplitude and phase components. In some embodiments, a fully-complex computer-generated hologram is calculated.

Reference may be made to the phase value, phase component, phase information or, simply, phase of pixels of the computer-generated hologram or the spatial light modulator as shorthand for “phase-delay”. That is, any phase value described is, in fact, a number (e.g. in the range 0 to 2π) which represents the amount of phase retardation provided by that pixel. For example, a pixel of the spatial light modulator described as having a phase value of π/2 will change the phase of received light by π/2 radians. In some embodiments, each pixel of the spatial light modulator is operable in one of a plurality of possible modulation values (e.g. phase delay values). The term “grey level” may be used to refer to the plurality of available modulation levels. For example, the term “grey level” may be used for convenience to refer to the plurality of available phase levels in a phase-only modulator even though different phase levels do not provide different shades of grey. The term “grey level” may also be used for convenience to refer to the plurality of available complex modulation levels in a complex modulator.

Although different examples and embodiments may be disclosed separately in the detailed description which follows, any feature of any example or embodiment may be combined with any other feature or combination of features of any example or embodiment. That is, all possible combinations and permutations of features disclosed in the present disclosure are envisaged.

