Eric Jakeman

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.

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.

Particle shape determination from polarization fluctuations of scattered radiation

When the number of scattering centers is small and/or varying, non-Gaussian fluctuations of coherently scattered radiation can yield vestigial information about the character of the individuals. The probability density functions of fluctuations in polarized light scattered from ensembles of small spheroidal particles are calculated. The particle dynamics are governed by Brownian motion, so their orientation is random relative to the incident polarized radiation, and the number of illuminated scattering centers can fluctuate according to Poisson statistics. It is demonstrated that the probability density functions of the scattered radiation may be used to discriminate the particle shape unambiguously, provided that the mean number of them is sufficiently small.

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.

Structure of polarization fluctuations and their relation to particle shape

Abstract The results of calculations and experiment are presented for second-order fluctuations of the polarization state of light scattered by ensembles of small non-spherical particles. Analytical results for spheroids in the small particle (Rayleigh) limit are extended by numerical modelling using the T-matrix or extended boundary condition technique and validated with a laboratory experiment. Both Gaussian and non-Gaussian scattering regimes are discussed, and the possibility of using measurements of these fluctuations to characterize the shape of both small (Rayleigh) and larger particles is considered.

Heterodyne detection of enhanced backscatter.

Heterodyne detection has been used to measure the enhanced backscattering from a standard target consisting of a plane mirror positioned behind a moving ground-glass disk (assumed to act as a phase screen). A tilt of the plane mirror in combination with spectral filtering of the detector output allows isolation of the double-scattered component of the light signal. When the illuminating laser and the local oscillator beams are correctly mode matched, the intensity of this signal displays the factor-of-2 increase that denotes full coherent enhancement. This full enhancement is preserved for a wide range of mirror to phase-screen spacing, in contrast with earlier observations in which direct-detection techniques were used.

Perturbation on the perfect lens: the near-perfect lens

We present the full wave solution for a dipole source above a slab of left-handed medium of refractive index n = −1, but with and μ differing from those ideal values that create the perfect lens through taking = −(1+δ)−1 and μ = −(1+δ), where δ, and hence and μ, are real. Any modifications in resolution are therefore not due to loss effects within the lens. Finite solutions for the form of the fields throughout all space are obtained using the method of Hertz potentials, thereby regularizing the singular perfect lens solution. This regularization will facilitate subsequent perturbation analyses. Using an appropriately defined criterion, we examine the sensitivity of the lens resolution to the material imperfection δ. It is found that the resolution converges logarithmically with δ upon that of the perfect lens. Comparison of theoretical results with experimental data suggests that even this slow convergence is the best that can be achieved.

Lidar frequency modulation vibrometry in the presence of speckle.

We report laboratory target vibration measurements that use an easily aligned and adjusted fiber-based 1.5-microm heterodyne lidar. The targets are simple spherically curved retroreflectors with well-controlled vibration frequencies and amplitudes. A rotating ground-glass screen creates Gaussian speckle. We wish to understand the modulated and fast-fading lidar returns seen from real target. We frequency demodulated the recorded laboratory data by phase differencing to provide estimates of dphi/dt, where phi is the phase of the received carrier-plus-noise phasor. Experimental results for signal strength and signal-to-noise ratio, for specific target modulation parameters, agree well with our recently developed dphi/dt correlation-function theory.