摘要:

A finite-element approach, with the aid of the perturbation technique, is used to determine the complex propagation characteristics of metal-clad optical fibers. Such structures of sub-micron dimension find an important application in near-field optical scanning microscopy, a technique which has been developing rapidly in recent years, providing high-resolution imaging in both the semiconductor industry and the biological sciences. The fundamental optical modal field profiles of these structures, suitable for optical scanning microscopy applications, for both TE and TM polarisations, are investigated

ISSN:1058-9759    

DOI:10.1080/10589759808953045

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Model based iterative reconstruction techniques (MOR) in Electrical Tomography have the potential to produce the most physically realistic reconstructed images. However the real-time performance of this method is presently constrained by the high computing requirements for the calculation of the “ forward problem” which utilises the finite element method. Most of the recent work towards a resolution of this computing problem has centered on the improvement of the computer hardware involved. However, our work has concentrated on improving the performance of the numerical algorithms. This paper demonstrates that a direct solution method for the finite element method can be utilised to determine the capacitance matrix of a multi-electrode sensor without recourse to intermediate potential field solutions. The paper discusses aspects of the finite element method which can influence the solution time and accuracy, and in particular the construction of an optimum mesh for particular multi-electrode problems. The mesh strategy and accuracy are compared with analytical models of the sensors constructed using conformal maps

ISSN:1058-9759    

DOI:10.1080/10589759808953046

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A gamma ray tomograph has been designed for the study of the modes of flow in dense phase pneumatic pipelines. The instrument has six americium-241 gamma ray sources and thirty miniature sodium iodide detectors arranged symmetrically around a pipe section. The instrumentation system comprises a multi-channel preamplifier, a CAMAC crate containing multi-channel discriminators, scalers and a GPIB crate controller, and an IBM-compatible personal computer. The detector gains are individually controlled by analogue voltage outputs provided by multi-channel DAC interface cards. The detector count-rates are used with a filtered linear back-projection algorithm to estimate the distribution of solid material over the pipe cross-section at a rate of ten images per second in real time. Preliminary laboratory tests have been carried out on a fluidised bed formed inside a permeameter tube and a full-scale test pipeline and instrument room have been developed for conveying tests.

ISSN:1058-9759    

DOI:10.1080/10589759808953047

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The rationale for using computer modelling as a technique for investigating systems in electrical impedance tomography is explained. The methodology of modelling is described along with the hazards concomitant with the modelling process. New software tools are currently being used to perform ac analysis over a range of excitation frequencies and the initial results from these analyses are compared with previous results obtained with static models. A new descriptor for the visibility of an object is proposed and the behaviour of this measure with anomaly radius, conductivity contrast and frequency is described

ISSN:1058-9759    

DOI:10.1080/10589759808953048

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Most tomographic analysis seeks to find the conductivity or another property of a domain, directly or by iteration, assuming nothing a priori about the distribution of internal features. Finite element (FE) analyses therefore require large numbers of variables, each corresponding to The conductivity etc. of one pixel or element. In 3D such analyses produce very large systems of equations

ISSN:1058-9759    

DOI:10.1080/10589759808953049

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Optical spectroscopy techniques are frequently used to non-invasively monitor various physiological functions such as tissue and blood oxygenation. It is desirable to develop methods to localise these measurements and quantify the parameters of interest, e.g. oxygen saturation. There is also a need for the localisation of abnormal tissues using non-harmful radiation, for example localisation and characterisation of breast tumours. This is possible in principle, provided the optical properties of abnormal tissues are sufficiently different. To establish a baseline reference, against which more advanced optical imaging systems and methods can be compared, a rotate-translate optical imaging system was developed. Image reconstruction was performed using a conventional filtered back projection scheme, originally developed for CT. With this system the limits of spatial and contrast resolution as a function of the absorption and reduced scattering coefficient were investigated. The limiting factor was found to be contrast resolution. Given the often small differences in the absorption and scattering properties of different tissues this makes such a system of limited use. A second generation imaging system, using a rotate-rotate type scanning system, has been constructed. Data obtained with this system utilises all the light emerging in the plane of measurement, and does not assume rectilinear propagation. The image reconstruction algorithm developed for this uses diffusion theory to model the light transport, and an iterative reconstruction algorithm. Results obtained with this model are presented. A further method to improve image quality is the use of a phase resolved technique, which uses amplitude modulation of the light source. An outline is given of the principle of this technique, and the system designed for the measurement is discussed

ISSN:1058-9759    

DOI:10.1080/10589759808953050

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor