Search

Cameras that take pictures at millimetre wavelengths are spawning a host of new security applications, including screening people and vehicles for hidden objects as well as seeing in poor weather conditions.

Computer-generated holography technique is a powerful technology suitable for a wide range of display types, including 2D, stereoscopic, autostereoscopic, volumetric, and true 3D imaging. Computer-generated holography is an emerging technology, made possible by increasingly powerful computers, that avoids the interferometric recording step in conventional hologram formation. Instead, a computer calculates a holographic fringe pattern that it then uses to set the optical properties of a spatial light modulator, such as a liquid crystal microdisplay. The SLM then diffracts the readout light wave, in a manner similar to a standard hologram, to yield the desired optical wavefront. Although CGH-based display systems can be built today, their high cost makes them impractical for many applications.

This paper discusses the design of an improved passive millimeter wave imaging system intended to be used for military forward operating base security in degraded visual environments. A simple end-to-end model of such an imager is described, including a simple scene model based on transformations applied to visible and infrared imagery, optical aberrations, focal plane sampling, scan conversion, receiver performance and image processing algorithms. The use of such a model as a design tool is discussed, especially with regard to optimizing scan conversion and image processing algorithms. The expected performance of the latest imager design is predicted.

Image processing techniques can be used to improve the cost-effectiveness of future interferometric Passive MilliMetre Wave (PMMW) imagers. The implementation of such techniques will allow for a reduction in the number of collecting elements whilst ensuring adequate image fidelity is maintained. Various techniques have been developed by the radio astronomy community to enhance the imaging capability of sparse interferometric arrays. The most prominent are Multi- Frequency Synthesis (MFS) and non-linear deconvolution algorithms, such as the Maximum Entropy Method (MEM) and variations of the CLEAN algorithm. This investigation focuses on the implementation of these methods in the defacto standard for radio astronomy image processing, the Common Astronomy Software Applications (CASA) package, building upon the discussion presented in Taylor et al., SPIE 8362-0F. We describe the image conversion process into a CASA suitable format, followed by a series of simulations that exploit the highlighted deconvolution and MFS algorithms assuming far-field imagery. The primary target application used for this investigation is an outdoor security scanner for soft-sided Heavy Goods Vehicles. A quantitative analysis of the effectiveness of the aforementioned image processing techniques is presented, with thoughts on the potential cost-savings such an approach could yield. Consideration is also given to how the implementation of these techniques in CASA might be adapted to operate in a near-field target environment. This may enable a much wider usability by the imaging community outside of radio astronomy and thus would be directly relevant to portal screening security systems in the microwave and millimetre wave bands. © 2014 SPIE.

This paper discusses the design of an improved passive millimeter wave imaging system intended to be used for base security in degraded visual environments. The discussion starts with the selection of the optimum frequency band. The trade-offs between requirements on detection, recognition and identification ranges and optical aperture are discussed with reference to the Johnson Criteria. It is shown that these requirements also affect image sampling, receiver numbers and noise temperature, frame rate, field of view, focusing requirements and mechanisms, and tolerance budgets. The effect of image quality degradation is evaluated and a single testable metric is derived that best describes the effects of degradation on meeting the requirements. The discussion is extended to tolerance budgeting constraints if significant degradation is to be avoided, including surface roughness, receiver position errors and scan conversion errors. Although the reflective twist-polarization imager design proposed is potentially relatively low cost and high performance, there is a significant problem with obscuration of the beam by the receiver array. Methods of modeling this accurately and thus designing for best performance are given.

This PhD programme is contributing to the development of Passive Millimetre-Wave Imagers (PMMWI) using the principles of interferometric aperture synthesis and digital signal processing. The principal applications are security screening, all-weather flight aids and earth observation. To enhance the cost-effectiveness of PMMWI systems the number of collecting elements must be minimised whilst maintaining adequate image fidelity. A wide range of techniques have been developed by the radio astronomy community for improving the fidelity of sparse interferometric array imagery. This paper brings to the attention of readers these techniques and discusses how they may be applied to imaging using software packages publicly available from the radio astronomy community. The intention of future work is to adapt these algorithms to process experimental data from a range of realistic simulations and real-world targets. © 2012 SPIE.

This paper will give an overview of some recent developments in electroholography for applications in interactive 3D visualisation. Arguably the ultimate technology for this task, it is the only approach having the potential to deliver full depth cue, 3D images, having resolutions beyond that which can be perceived by the human eye. Despite significant advances by many researchers, the high pixel counts required by the computer generated hologram (CGH) patterns in these systems remain daunting - in practice, systems able to calculate and display reconfigurable CGH having pixel counts of more than one billion may be required for 300 mm width, 3D images. Advances described include novel Fourier mode variants of diffraction specific algorithms and parallel binarisation techniques for design of the CGH patterns; computer architectures for effective implementation of these algorithms for interactive CGH calculation; the latest developments in the Active Tiling spatial light modulator technology and novel replay optics arrangements including folded mirror geometries, viewer tracking alternatives and new horizontal parallax configurations. Throughout, the emphasis is optimisation towards implementation as an interactive electroholography system having practical utility. Some recent results from demonstrations of aspects of the technology will be shown. These include monochrome and colour, static and dynamic, horizontal parallax only (HPO) and full parallax, 3D images, generated from true CGH systems with up to 24 billion pixels.

The display of 3D images containing all the depth cues required by the human vision system can be achieved using a reconfigurable Computer Generated Hologram (CGH) with high pixel count. Giga-pixel scale displays or spatial light modulators are required in order to form directly viewable 3-D images of 0.5m in size. A new Spatial Light Modulator (SLM) solution, Active Tiling (AT)1, has been developed by the authors to replay giga-pixel scale CGHs at video refresh rates. This has overcome a key bottleneck preventing commercial development of electro-holography to date. At the heart of an AT system is a set of replication optics which produces multiple images of an electrically addressed Spatial Light Modulator (SLM) on an optically addressed spatial light modulator (OASLM). Solutions employed within electronic holography will be discussed. A holographic 3D display system using a 4 channel Active Tiling modulator with a new replay optics system has demonstrated directly viewable 3-D images and animations from 100 Mega-pixel CGH data. This provided viewing of both horizontal parallax only (HPO) and full parallax 3-D images up to 140mm in size.

Reconfigurable computer generated holography - sometimes known as electroholography - is the only technique capable of using computer held data to generate interactive, high quality, 3D images containing all the depth cues used by the human visual system. Practical applications of electroholography require computer generated hologram (CGH) patterns of between 10/sup 9/ to 10/sup 10/ pixels to be calculated and displayed at interactive rates. These pixel counts are necessary in order to generate images which are both big enough (upwards of 300 mm width) and possess a large enough field of view (FOV) to permit simultaneous, multiuser viewing. Advances in several key areas may now make such high performance electroholographic systems practical in the near future. We describe CGH design algorithms, computer architectures and CGH display engines, including the Active Tiling/spl trade/ system.

Giga-pixel scale displays or spatial light modulators are required in order to form directly viewable 3-D images of 0.5m in size using the principles of computer generated holography (CGH). This has been a key bottleneck preventing commercial development of electro-holography. Active Tiling is a modular spatial light modulator system developed by the authors to provide a route to replay images from giga-pixel scale CGHs. This paper will present the latest development of a multi-channel Active Tiling unit and results from this system for the first time. A holographic 3D display system using a 4 channel Active Tiling modulator with a new replay optics system has demonstrated directly viewable 3-D images and animations from 100 Mega-pixel CGH data. This provided viewing of both horizontal parallax only (HPO) and full parallax 3-D images up to 140mm in size. 25 Mega-pixels of CGH data is written by each channel onto a liquid crystal optically addressed spatial light modulator at high resolution. The modular design of Active Tiling permits CGH data to be written seamlessly across multiple channels which can be updated at rates up to 30 Hz. A Fourier Transform optical replay system was developed and integrated with the 4-channel Active Tiling system to form the CGH images.

Holographic techniques offer a route to the generation of 3D images having all the depth cues used by the human vision system. A new electro-optic modulator system has been developed by the authors to replay dynamic holographic images. This Active Tiling (AT) system offers a route to replay giga-pixel computer generated holographic (CGH) images with video refresh rates. A key component of the AT system is an Optically Addressed Spatial Light Modulator (OASLM), onto which segments of the large pixel count CGH are loaded or written sequentially before the whole CGH frame is read out simultaneously. The OASLM device structure used consists of an amorphous silicon photosensor layer combined with surface stabilised ferroelectric liquid crystal (SSFLC) light modulation layer. A number of experiments have been conducted to determine the performance and suitability of this device for replaying a CGH. These experiments include electro-optic switching to determine the operating window and diffraction efficiency (DE) measurements to determine spatial resolution performance. A detailed description of the experimental apparatus and method used for measuring DE is presented, and results show the OASLM to be capable of diffracting light from fringe patterns with spatial periods as low as 3 micrometers (333 lp/mm). Examples of CGH replay of 3D images from the OASLM when operating within the AT system are also presented.

Continuing advances in both computing and modulator techniques and technologies increase the likelihood of electro-holography displays becoming practical in the next five years or so. These displays aim to allow high quality, interactive, 3D images to be generated from compte held dat. Until now, large pixel counts have precluded any systems of practical utility. This paper will describe recent progress towards meeting the challenges of implementing such displays. Despite more than exponential increases in computer performance, interactive hologram calculation remains an issue. A significant part of the cost of any electro-holography product will be associated with the computational requirements. These are strongly influenced by the choice of computer generated hologram (CGH) type, the algorithm used to calculate the CGH and the computer architecture chosen for implementation. The leading optics will be discussed and some experimental results presented indicating performance, cost and image quality tradeoffs. Eventual choice will depend on the specifications of the required system. Another traditional bottleneck has been the optical modulator employed. As one of the leading candidates for practical implementation, the current and projected performance of the DERA Active Tiling system will be explored, and the latest experimental results presented. These will include the first published, full parallax, true CGH, 3D image replays from an Active Tiling channel.

A hologram can produce all the 3D depth cues that the human visual system uses to interpret and perceive real 3D objects. As such it is arguably the ultimate display technology. Computer generated holography, in which a computer calculates a hologram that is then displayed using a highly complex modulator, combines the ultimate qualities of a traditional hologram with the dynamic capabilities of a computer display producing a true 3D real image floating in space. This technology is set to emerge over the next decade, potentially revolutionizing application areas such as virtual prototyping (CAD-CAM, CAID etc.), tactical information displays, data visualization and simulation. In this paper we focus on the computational challenges of this technology. We consider different classes of computational algorithms from true computer-generated holograms (CGH) to holographic stereograms. Each has different characteristics in terms of image qualities, computational resources required, total CGH information content, and system performance. Possible trade- offs will be discussed including reducing the parallax. The software and hardware architectures used to implement the CGH algorithms have many possible forms. Different schemes, from high performance computing architectures to graphics based cluster architectures will be discussed and compared. Assessment will be made of current and future trends looking forward to a practical dynamic CGH based 3D display.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper discusses certain developments in our understanding of the writing of large-area CGH arrays of the absorption type. The calibration of grey-scale structures is considered, using different forms of the linearization process. It is also pointed out that the feature shape and packing fraction play a dominant role in the achievement of high dynamic range of the holographic recording.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Advances in computing and optical modulation techniques now make it possible to anticipate the generation of near real- time, reconfigurable, high quality, three-dimensional images using holographic methods. Computer generated holography (CGH) is the only technique which holds promise of producing synthetic images having the full range of visual depth cues. These realistic images will be viewable by several users simultaneously, without the need for headtracking or special glasses. Such a data visualization tool will be key to speeding up the manufacture of new commercial and military equipment by negating the need for the production of physical 3D models in the design phase. DERA Malvern has been involved in designing and testing fixed CGH in order to understand the connection between the complexity of the CGH, the algorithms used to design them, the processes employed in their implementation and the quality of the images produced. This poster describes results from CGH containing up to 108 pixels. The methods used to evaluate the reconstructed images are discussed and quantitative measures of image fidelity made. An understanding of the effect of the various system parameters upon final image quality enables a study of the possible system trade-offs to be carried out. Such an understanding of CGH production and resulting image quality is key to effective implementation of a reconfigurable CGH system currently under development at DERA.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

As a support to the advances in theoretical understanding and computational methods, we describe a new laser plotter technique that enables, in principle, an unlimited size of pixel array to be plotted efficiently with a rigorous estimate of duration of the plot run time. Developments in laser plotter design are presented that allow the formation of pixellated holographic structures of high precision (c. 1 - 10 micron pixel dia.) with an accompanying high pixel count (e.g. at least up to, and beyond, 104 per side within a square array). The case of absorption holograms offers an easy route to a good quality result. We can then exploit the many tricks of amplitude holography borrowed from lithographic and holographic experience using ultra-fine grain silver halide materials. The problem of exposure quantization and linearization is addressed in a pragmatic fashion. The central issue of why such holograms can tolerate intrinsic diffraction artifacts within each pixel is considered along with the exposure level quantization -- it is difficult to print individual pixels within which the optical density is clinically uniform. We cannot over-estimate the reliability difficulties that can arise in a system designed to print massive arrays of pixels in a serial fashion. The electronic testing involved has to be associated with error-free repeatability and high accompanying switching speeds. This may look easy but it is the major issue that distinguishes serially printed digital holography from the simple one-step parallel process of forming the ordinary hologram.

Over many years, the subject of computer generation of holograms has been visited in various guises. Historically, the obvious restrictions imposed by computational power and computer generated hologram (CGH) fabrication techniques have placed limits on what can be taken seriously in terms of image complexity. Modern advances in computational hardware and electro-optic systems now permit both the calculation and the manufacture of CGH's of complex 3D objects which fill a significant volume of space. New methods permit the recording to be made within a reasonable timescale. In addition to advancing fixed CGH generation techniques, the motivation for the work reported here includes assessment of design algorithms, modulation strategies and image quality metrics. These results are of relevance for a novel electroholography system, currently under development at DERA Malvern. This paper describes a complete process of data generation, computation, data manipulation and recording leading to practical techniques for the creation of large area CGH's. As a support to the advances in theoretical understanding and computational methods, we describe (in Part II) a new laser plotter technique that enables, in principle, an unlimited size of pixel array to be plotted efficiently with a rigorous estimate of duration of the plot run time. The results reported here are limited to 2048 X 2048 pixels. In this example, the novel switching techniques employed on the laser plotter permit the pixel array to be printed in approximately 1 hour. However, paths towards easily raising the pixel count and its associated printing rate are presented for both the computational engine and laser plotting processes.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.