Image analysis – Applications
Patent
1993-10-01
1998-09-22
Razavi, Michael T.
Image analysis
Applications
382288, 348172, G06T 760
Patent
active
058126902
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a centroiding method for use with photon counting detectors.
BACKGROUND OF THE INVENTION
Over the past few years a number of photon counting detectors that employ charge-coupled devices (CCDs) for event capture have been developed. These detectors all use interpolative centroiding, at some level, to enhance the resolution of the system, event centres being found with an accuracy as high as 1/8th of a CCD pixel in each of the x and y axes, i.e. to 1/64th of a pixel. This process is referred to as the allocation of an event to a channel. Thus, each pixel may be divided into as many as 8 channels, or special subdivisions along each axis. It should be understood that although the invention is described below in relation to dividing each pixel into 8 channels along each axis, it is equally applicable if the number of channels is more or less than 8.
Recent publications (Fordham, J. L. A. and Book, R. N. 1988, Robinson, L. B., ed., "Instrumentation for Ground-Based optical Astronomy, Present and Future (Springer-Verlag, Now York), p.582; and Dick, J., Jenkins, C. and Ziabicki, J. 1989, PASP, 101, 684) discuss errors associated with the centroiding algorithms and the effect on system fixed pattern noise. In these papers it is assumed that the event profile is a Gaussian, showing the effect of variations in, for example, pulse width, noise, and DC bias on centroiding accuracy. However, when analyzed it is found that events do not have a Gaussian profile and so the approach adopted in those papers is less effective than it would otherwise be.
Photon counting detectors employing CCD's can be split into two categories, namely where there is lens coupling between the intensifier and the CCD; and where there is fibre optic coupling between the intensifier and CCD.
For the lens coupled system, distortions are introduced onto the event profile by the lens, where the distortion is dependent upon the radial position of the event with respect to the optical centre. These distortions are due to aberrations within the lens. For this type of detector, where there is no defined pulse shape, a single accurate centroiding algorithm, whether hardware or software generated, cannot be employed. This means that, from the algorithm, there always will be centroiding errors which cause fixed pattern noise similar to that discussed in the above referenced papers. The only way to overcome this is to include a smoothing function within the centroiding circuit.
For the fibre optically coupled systems, if very localized shears in the coupler are discounted, then the CCD will produce an accurate image of each event generated on the intensifier output phosphor. However, fibre optic coupling is only normally employed on micro-channel plate (MCP) intensifiers, and these give rise to an additional form of distortion due to the bias angle of the channel plates with respect to the output phosphor. This bias angle, typically of 8 degrees, introduces an asymmetrical profile to each event which is constant over the whole active area and can thus be fitted, in principle, by a single algorithm. A typical event is shown in FIG. 1a of the accompanying drawings and a profile of a slice through the centre of this event is shown in FIG. 1b.
The event is noticeably asymmetrical, the direction of the asymmetry being dependent upon the orientation of the CCD with respect to the channel plate. Fitting an error free algorithm to this profile such that each event is accurately centered to 1/8th of a CCD pixel or better would be very complex in hardware and in software would be very time consuming, limiting the dynamic range by introducing a maximum count rate that can be processed. It is an object of the present invention to provide a centroiding technique that overcomes such problems and provides accurate centroiding with low fixed pattern noise.
Before describing the present invention a description will be given of the centroiding algorithm conventionally used to allocate an event to a channel.
A typical five
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Chang Jon
Razavi Michael T.
University College London
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