John G. Walker

Blind deconvolution by means of the Richardson-Lucy algorithm

A blind deconvolution algorithm based on the Richardson–Lucy deconvolution algorithm is presented. Its performance in the presence of noise is found to be superior to that of other blind deconvolution algorithms. Results are presented and compared with results obtained from implementation of a Weiner filter blind deconvolution algorithm. The algorithm is developed further to incorporate functional forms of the point-spread function with unknown parameters. In the presence of noise the point-spread function can be evaluated with 1.0% error, and the object can be reconstructed with a quality near that of the deconvolution process with a known point-spread function.

Improving visibility depth in passive underwater imaging by use of polarization

Results are presented that demonstrate the effectiveness of using polarization discrimination to improve visibility when imaging in a scattering medium. The study is motivated by the desire to improve visibility depth in turbid environments, such as the sea. Most previous research in this area has concentrated on the active illumination of objects with polarized light. We consider passive or ambient illumination, such as that deriving from sunlight or a cloudy sky. The basis for the improvements in visibility observed is that single scattering by small particles introduces a significant amount of polarization into light at scattering angles near 90 degrees: This light can then be distinguished from light scattered by an object that remains almost completely unpolarized. Results were obtained from a Monte Carlo simulation and from a small-scale experiment in which an object was immersed in a cell filled with polystyrene latex spheres suspended in water. In both cases, the results showed an improvement in contrast and visibility depth for obscuration that was due to Rayleigh particles, but less improvement was obtained for larger scatterers.

Visibility depth improvement in active polarization imaging in scattering media.

A simple image-subtraction technique for further enhancement of the visibility depth in polarized imaging of surfaces immersed in scattering media is proposed and assessed. The technique is based on active illumination with circular or linear polarization states and image detection in the original and the opposite, or orthogonal, states. Contrast enhancement is achieved by subtraction of a fraction of the image recorded in the original state from that recorded in the opposite state. Results demonstrating the effectiveness of this method, obtained with Monte Carlo techniques, show that the visibility depth can be increased by as much as a mean free path. The results obtained are compared with those obtained by use of two alternative methods.

Analysis of the spatial distribution of polarized light backscattered from layered scattering media

The scattering of polarized light from a two layer scattering medium is investigated using Monte Carlo simulations. First order and normalized second order moments are used to analyze the spatial properties of the emerging light in different polarization states. Linearly and circularly polarized illumination is used to probe different depths. Absorption and layer thickness are varied and it is demonstrated that the determination of these values is aided by the inclusion of polarization information. The lateral and depth localization of light by polarization subtraction is also quantified. Potential applications of these techniques are burn depth and melanoma thickness measurements.

Polarization discrimination for active imaging in scattering media

The results of a Monte Carlo computer simulation of active-illumination imaging of surfaces immersed in a scattering media are presented. The simulation is based on scattering by spherical or spheroidal Rayleigh particles and rigorously accounts for the polarization effects of the scattering process. Illumination with linear or circular polarization states and detection in the original and orthogonal polarization states are investigated. The object surfaces are modelled as diffuse scatterers which either preserve or randomize the polarization of the reflected light. The simulations clearly indicate that, in some situations, polarization discrimination can be effective in extending the depth of visibility. The effectiveness of this approach depends largely on the polarization properties of the surface to be imaged rather than the properties of the intervening scattering medium or the imaging geometry.

Importance of shadowing and multiple reflections in emission polarization

Abstract Polarization characteristics of thermal radiation emitted from surfaces are investigated within the geometrical optics approximation. Analytical results are presented for photons emitted without subsequent reflection from surfaces having sawtooth corrugations with different slope distributions. Analytical results are used to validate a Monte Carlo simulation designed to determine and quantify the effects of multiple reflection of emitted photons from surface structures and, in addition, to treat two-dimensional surfaces. Results are shown that illustrate the dependence of the degree of polarization on the relative orientation of the viewing angle with respect to the corrugations. Simulations of emission from structured and random two-dimensional surfaces show that, whilst the total emission can saturate, the degree of polarization decreases with increasing roughness of the surface morphology. The prospect for manipulating surfaces to have specific polarization signatures is discussed.

Non-scanning confocal fluorescence microscopy using speckle illumination

An arrangement in which a diffuser may be utilised to provide full field speckle illumination in a fluorescence microscope is described. The design requires that the conventional image of the speckle-illuminated object and a separate image of the speckle pattern are recorded or processed in real time. An alternative design could utilise a light valve. An analysis of the performance of the proposed microscope is given which shows that with adequate averaging the microscope response is equivalent to that of a scanning confocal arrangement. A computer simulation is used to demonstrate the enhanced lateral resolution and depth discrimination and also to assess the amount of averaging required for high fidelity imaging.

Computer simulation of a method for object reconstruction from stellar speckle interferometry data

A method for reconstructing a diffraction-limited image from data consisting of many short-exposure turbulence degraded images is described. The results of a computer simulation of the method are presented. The method should prove useful for obtaining high-angular resolution images from large earth bound telescopes. The simulation indicates that the method may be applicable to astronomical objects as faint as approximately 11th magnitude.

Speckle-illuminated fluorescence confocal microscopy, using a digital micro-mirror device

An implementation of a speckle-illuminated fluorescence confocal microscope using a digital micro-mirror device (DMD) is described. The DMD not only projects a sequence of imaged binary speckle patterns onto the specimen at a very high frame rate but also operates as a spatial light modulator to perform real-time optical data processing. Frame averaging is accomplished by CCD charge accumulation during a single exposure. The recorded time-averaged image is a confocal image plus an unwanted non-confocal image which can be removed by recording a separate image. Experimental results with image acquisition within a fraction of a second are shown. Images of a thin biological sample are also shown to demonstrate practical application of the technique.

The influence of particle size in active polarization imaging in scattering media

Simulations of active-illumination imaging of surfaces immersed in a scattering media utilizing Monte Carlo techniques are presented. The observing medium comprises finite-sized spherical particles and the simulations rigorously account for polarization effects of the scattering process. Illumination with linear or circular polarization states and detection in the original and orthogonal polarization states are investigated. The object surfaces are modelled as diffuse scatterers which randomize the polarization state of the reflected light. Polarization discrimination can be effective in extending the depth of visibility through scattering particles with diameters as large as the wavelength of the light.