Alan H. Lettington

Applications of diamond-like carbon thin films

Abstract Diamond-like carbon (DLC) does not have a unique composition but consists of a mixture of amorphous and crystalline phases. It is otherwise known as amorphous hydrogenated carbon. This paper reviews the preparation and properties of DLC films and their use in infrared optical, mechanical, electronic and biomedical applications.

Compact real-time (video rate) passive millimeter-wave imager

This paper describes a novel real time mechanically scanned passive millimeter wave imager. This imager produces a field of view of 40 degree(s) X 20 degree(s) with diffraction limited performance and a 25 Hz frame update rate. It is relatively inexpensive because the scene is imaged using 32 direct detection receivers with a frequency of operation from 28 - 33 GHz. The compact antenna uses polarization techniques to fold the beam and is constructed from readily available low cost materials.

Mechanically scanned real-time passive millimeter-wave imaging at 94 GHz

It is well known that millimetre wave systems can penetrate poor weather and battlefield obscurants far better than infrared or visible systems. Thermal imaging in this band offers the opportunity for passive surveillance and navigation, allowing military operations in poor weather. We have previously reported a novel real time mechanically scanned passive millimetre wave imager operating at 35GHz and in this paper a 94GHz variant will be described. This 94GHz imager has a field-of-view of 60° x 30° and has diffraction limited performance over the central two thirds of this field-of-view. It is relatively inexpensive because the scene is imaged using a linear array of direct detection receivers and compact folded optics. The receiver array has been constructed using indium phosphide monolithic microwave integrated circuits (MMICs) allowing high gain and low noise figure to be achieved. The compact optics consist of a polarisation sensitive mirror and a Faraday rotator. readily The mirror is constructed from expanded polystyrene, supporting a printed copper grid etched onto a PTFE/glass fibre substrate. These materials are low cost and readily available. The Faraday rotator is made from a commercial grade plasto-ferrite sandwiched between antireflection coatings. The optics produce a conical scan pattern and image processing is used to generate a raster scan pattern and to perform gain and offset corrections.

Quantitative analysis of remote gas temperatures and concentrations from their infrared emission spectra

Techniques for obtaining quantitative values of the temperatures and concentrations of remote hot gaseous effluents from their measured passive emission spectra have been examined in laboratory experiments and on field trials. These emission spectra were obtained using an adapted FTIR spectrometer with 0.25 cm-1 spectral resolution. The CO2 and H2O vapour content in the plume from a 55 m smoke stack and the temperature of these gases were obtained by comparing the measured emission spectra with those modelled using the HITRAN atmospheric transmission database. The spatial distributions of CO2, CO and unburnt CH4 in a laboratory methane flame were reconstructed tomographically using a matrix inversion technique.

Scene-based techniques for nonuniformity correction of infrared focal plane arrays

Images employing focal plane array detectors suffer from aliasing and fixed pattern noise generated by nonuniformities in the response of the detector elements. Aliasing which is caused by undersampling may be removed by microscan which extends the spatial bandwidth of the images by a factor of two. The presence of fixed pattern noise which is caused by nonuniformities in the detector responses is more difficult to eliminate. It severely reduces the performance of imagery and its removal is essential. This paper compares the performance of representative methods for reducing the effect of fixed pattern noise through simulated results. Preliminary results are presented for an alternative method currently under development.

Image Restoration Using a Lorentzian Probability Model

Abstract The distribution of edge values for an image of a general scene often has a sharp peak with a long tail. This property, which can be well described by a Lorentzian probability function, has been used to develop an efficient nonlinear image restoration algorithm for reducing the various artifacts that often arise in the restored images. The algorithm starts with a Wiener filter solution which is used to model the edge image by the Lorentzian function so that the likelihood of the image can be estimated. A nonlinear correction term is then introduced which increases this image likelihood under the mean square error criterion. This process ensures that the resulting image retains its sharpness while reducing the noise and ringing artifacts. An iterative procedure has been developed to implement this method. Computer simulated results show that the algorithm is robust in reducing artifacts and easily implemented. The algorithm also possesses a superresolution capability due to the highly nonlinear pr...

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.

Measurement of droplet velocity, size and refractive index using the pulse displacement technique

There is a current requirement to detect and quantify the presence of gas bubbles in oil exploration. For this reason a novel method has been developed to measure simultaneously the velocity, size and refractive index of large optically transparent bubbles and droplets. The method is based on the time displacement of refracted and reflected beams scattered from the moving particles. Experimental results obtained from water droplets are included.

Nonuniformity correction and restoration of passive millimeter-wave images

We discuss methods of improving the quality and resolution of passive millimeter-wave images, particularly those obtained using the Defense Evaluation and Research Agency (DERA) MITRE imager and the more recent MERIT imager. This later real time imager consists of some novel optics followed by a conical scanner in the form of a disk rotating about an axis through its center and tilted with respect to its normal. A horizontal array of receivers is scanned such that each receiver performs a conical scan pattern in the scene. The resulting image, which has a 40x20 deg field of view, consists of a series of circles whose centers are uniformly displaced horizontally. Each receiver is calibrated initially using a two point correction but then drifts in time and a scene based correction is applied. Following preprocessing the images are superresolved using nonlinear restoration techniques. Two such techniques are described. The first is an error energy reduction algorithm that uses background subtraction and the second assumes a Lorentzian distribution of gradients within an image. Both are able to superresolve without significantly enhancing the noise. These various processes are described and images presented.

High-performance passive millimeter-wave imaging

We discuss the current status of passive millimeter-wave radiometry as a thermal imaging technique. The major problems are poor spatial resolution and slow response time. Techniques for overcoming these difficulties are identified, including the use of aperture synthesis, multichannel receivers, correlation, and inverse transform techniques. A comparison is made with infrared imaging.