P C Y Chang

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.

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.

Imaging and multiple scattering through media containing optically active particles

Abstract This paper examines the behaviour of polarized light scattered by a medium containing small chiral spheroidal particles. We show that for single scattering the observed phenomena of optical activity may be interpreted in terms of an averaged Mueller matrix and describe how the degree of polarization is affected by such a medium. The polarization properties of multiply scattered light by chiral particles are considered through the use of Monte Carlo simulations. It is shown that the effects of chirality under multiple scattering can be interpreted as an order-preserving influence in a disordered system and that this influence can, in principle, be exploited for the purposes of imaging.

Polarization properties of light multiply scattered by non-spherical Rayleigh particles

Abstract Multiple scattering of incoherent polarized light propagating through a random medium comprised of spheroidal Rayleigh particles is studied using Monte Carlo simulations. Two approaches are taken for the implementation of the simulation: the first uses individual realizations of particle orientation and the second, an accelerated method, averages over the particle orientation. These different methods produce results that are indistinguishable within statistical errors. The depolarization of light is examined in both transmission and backscatter for media comprised of spheroids of different polarizability ratios. In media containing spheroidal particles the depolarization is greater than that for spherical particles. Media containing prolate spheroids are more depolarizing than media comprising oblate particles of the same polarizability ratio. The extra depolarization due to asphericity is much less pronounced in the multiple scattering regime than for single scattering.

Properties of a Polarized Light-Beam Multiply Scattered by a Rayleigh Medium

Multiple scattering of an incoherent beam of polarized light propagating through a random medium is studied using Monte-Carlo simulations. The medium comprises a slab of monodispersive spherical Rayleigh particles. Spatial, angular and path-length distributions of the scattered light are examined in both transmission and backscatter. The change of polarization state is discussed as a function of propagation distance for two initial states. Contrasting linear with circularly polarized light shows the former to be better preserved in all the results obtained. Backscattered light is dominated by single scattering events occurring from the medium closest to the exit plane.

Polarization Fluctuation Spectroscopy

Polarization fluctuation spectroscopy is a dynamic light scattering technique that extends photon correlation spectroscopy to account for particles changing the polarization state of an incident beam of coherent light. This extension enables the shape in addition to the size of the particles to be sensed. The technique requires the particles to be sufficiently numerous for the scattered light to be in the Gaussian scattering regime, but sufficiently sparse for multiple-scattering effects to be neglected. The temporal cross-correlation function of intensities scattered into separate polarization states depends on the polarization state of the input beam, the scattering angle, and the relative refractive index, size and shape of the particles. Measurement of the temporal cross-correlation function enables independent determination of the size and aspect ratio of mono-disperse samples. The theory underpinning the method is developed from first principles and the processing and experimental techniques are discussed, together with the accuracy of measurement that can be achieved. The viability of the technique is demonstrated experimentally.