Dennis Dunn

Passive millimetre-wave imaging architectures

This paper discusses various passive millimetre-wave imaging systems. It includes sources of radiation, atmospheric transmission and a brief summary of their various applications. This is followed by a review of methods for detecting millimetre-wave radiation. From a cost analysis it is shown that scanned systems are at present far more cost effective than focal plane array of receivers for high-performance systems. Then various types of imaging system available are considered with greater emphasis on recent developments and their methods of beam forming. It is concluded that at present, and for the foreseeable future, optical beam forming and beam steering are the most cost-effective solutions. Some general remarks are included as to how receivers are matched to their collection apertures, followed by a section on optical beam-forming components. The recent development of a lightweight, low-cost, high-performance reflective lens is included. It is then shown how this may be combined with mechanical scanning systems to form high-performance passive millimetre-wave imaging systems.

Review of imaging architecture

This paper reviews various passive mm-wave imaging systems. It includes sources of radiation, atmospheric transmission and a brief summary of their various applications. This is followed by a review of methods for detecting mm-wave radiation. From a cost analysis it is shown that scanned systems are at present far more cost effective than focal plane array of receivers for high performance systems. There is a review of the various types of imaging system available with greater emphasis being placed on recent developments and their methods of beam forming. It is concluded that at present, and for the foreseeable future, optical beam forming and beam steering are the most cost effective. Some general remarks are included as to how receivers are matched to their collection apertures, followed by a section on optical beam forming components. The recent development of a lightweight, low cost, high performance reflective lens is included. It is then shown how this may be combined with mechanical scanning systems to form high performance passive mm-wave imaging systems.

Review of super-resolution techniques for passive millimeter-wave imaging

This paper reviews the mathematical image processing methods available for super-resolving passive millimeter wave (PMMW) images. PMMW imaging has a number of advantages over infra- red (IR) and visible imaging in being able to operate under adverse weather conditions making it useful for all weather surveillance. The main disadvantage, however, is the size of aperture required to obtain usable spatial resolution. A typical aperture size would be 1 m diameter for a system operating at 94GHz. This aperture may be reduced if super- resolution techniques are employed. To achieve super- resolution non-linear methods of restoration are required in order to generate missing high frequency information. For thee to be genuine high frequencies it is necessary to restore the image subject to constraints. These constraints should apply directly to the scene content rather than to properties of any noise also present. The merits of the available super-resolution techniques are discussed with reference to sharpening noisy PMMW images. Any increase in sharpness of an image frequently results in an increase in the noise present. This can detract from the ability of a human observer to recognize an object in the scene. This problem is discussed with reference to a recent model of human perception.

Design and development of a high-performance passive millimeter-wave imager for aeronautical applications

This paper describes a high performance opto-mechanically scanned mm-wave imager intended to monitor the ground movement of aircraft in adverse weather conditions. It employs two counter-rotating mirrors that are tilted about their axes of rotation. They simulate the linear scan of a single high speed, large aperture flapping mirror. When used with a linear receiver array they can produce a two-dimensional scan of the scene at TV rates. In the present application they were used with a single receiver and a large flapping mirror to produce a two-dimensional scan of the scene of ±10° vertically and 60deg; horizontally. One of the rotating mirrors had a concave surface and acted as the focusing element in the imager. The two mirrors were driven from a single servo motor using timing belts and toothed pulleys. The flapping mirror was slaved to the motion of the rotating discs using an electronic cam. The single channel 94GHz receiver consisted of an InP LNA followed by a down-converter and a detector. The video output passed to an A/D converter and was displayed on a conventional PC. This system has virtually 100% transmission and can be used at any waveband.

Optomechanical scanners for passive millimeter-wave imaging

This paper describes the design and development of a novel reflective lens for use at millimeter and microwave frequencies. This lens uses twist-polarization techniques and effectively simulates a refractive lens of index 3 in a cheap, light-weight structure consisting of two wire grid polarizers and a Faraday rotator. This paper also discusses how a high-sped rotary scanning mechanism can be used to produce a linear scan pattern again using twist-polarization techniques. This linear scanner employs a wire grid polarizer in the form of a roof prism, a Faraday rotator or quarter wave plate and a rotating disc inclined with respect to its axis of rotation. Through multiple reflections the original conical scan is cancelled in one direction while it is doubled in the other direction to form a linear scan pattern.

A new opto-mechanical scanner for millimeter and sub-millimeter wave imaging

This paper describes a new opto-mechanical scanner that is able to meet the established requirements for mm-wave imaging in remote sensing and security applications. The ideal system would employ a 2-D array of electronically scanned receivers, but at present their cost is prohibitively high. Fortunately low cost, high performance, opto-mechanically scanned imagers are able to meet the current requirements. They can establish the market and pave the way for lower-cost receiver developments, necessary for electronic scanning. An opto-mechanical scanner should be able to scan a 2-D image of the scene in real-time, with a linear raster scan pattern. It should have high optical efficiency so that an imager can achieve the required thermal sensitivity with the minimum number of receivers. It should be compact to fit inside a small space envelope. The size of the imager should be dominated by the size of the collection aperture and not by any relay optics. In mm-wave imaging this size is controlled by the required spatial resolution and the space available. It is also desirable that the scanner employs the minimum number of frequency-selective optical components. This ensures that it can easily operate at any wavelength, and be active or passive. The new scanning arrangement meets these requirements and is being developed into a high performance, low-cost, compact prototype system that hopefully will meet the present and future needs for mm-wave and terahertz imaging.

Use of constraints in the superresolution of passive millimeter-wave images

This paper discusses the use of constraints when super-resolving passive millimeter wave (PMMW) images. A PMMW imager has good all-weather imaging capability but requires a large collection aperture to obtain adequate spatial resolution due to the diffraction limit and the long wavelengths involved. A typical aperture size for a system operating at 94GHz would be 1m in diameter. This size may be reduced if image restoration techniques are employed. A factor of two in recognition range may be achieved using a linear technique such as a Wiener filter; while a factor of four is available using non-linear techniques. These non-linear restoration methods generate the missing high frequency information above the pass band in band limited images. For this bandwidth extension to generate genuine high frequencies, it is necessary to restore the image subject to constraints. These constraints should be applied directly to the scene content rather than to any noise that might also be present. The merits of the available super-resolution techniques are discussed with particular reference to the Lorentzian method. Attempts are made to explain why the distribution of gradients within an image is Lorentzian by assuming that an image has randomly distributed gradients of random size. Any increase in sharpness of an image frequently results in an increase in the noise present. The effect of noise and image sharpness on the ability of a human observer to recognise an object in the scene is discussed with reference to a recent model of human perception.

A Chebyshev differentiation method

We investigate Chebyshev collocation methods for the evaluation of derivatives, show that the inclusion of an interval arithmetic version of the cosine transform makes it possible to eliminate the severe errors normally associated with such methods, and demonstrate a numerical procedure which finally achieves the goal of obtaining accurate derivatives.

Techniques for millimetre-wave imaging

This paper reviews the formation of an image with coherent and incoherent radiation. It discusses the various mm-wave methods for electronic beam-forming and beam-steering such as phased array, leaky-wave antennas, up-conversion, tapped delay lines and digital beam-forming techniques. These methods are related in the paper to their optical analogues of beam-forming and steering by a lens and the measurement of the aperture function in the case of holography. It concludes that digital techniques will be used in the future when the cost of receivers is reduced but that at present opto-mechanical techniques are more cost effective. A high efficiency, compact opto-mechanical system is described. This is able to operate at any wavelength and be active or passive. Typical 94GHz images are presented.

Calculation of continuum wave functions

Abstract A numerical method is presented for the calculation of single-particle normalized continuum wavefunctions which is particularly suited to the case where the wavefunctions are required for small radii and low energies. Three methods are investigated for integrating the equations and three methods for determining the normalization.