Research Presented at Conferences
http://hdl.handle.net/1813/2655
2016-04-30T15:12:37ZReflections and the focusing effect from an ideal three-dimensional rough surface
http://hdl.handle.net/1813/2667
Reflections and the focusing effect from an ideal three-dimensional rough surface
Clavano, Wilhelmina R.; Philpot, William D.
An analytical expression for higher-order reflectances from a shallow-water homogeneous ocean bottom modeled as an egg-carton surface is presented. Roughness of this ideal surface is expressed as the amplitude-to-length ratio of its basic sinusoidal function. Any real surface that can be approximated by an egg-carton function will effectively have a comparable roughness metric. Incidence and reflection directions are considered in full azimuthal variation. The detector is located just below the water surface so that only in-water reflections are considered and there are no air-water transmission effects. Furthermore, this setup allows for an understanding of reflections that occur in media with any index of refraction or absorption coefficient. Fixing the detector footprint but adjusting its field-of-view enables the observation of the same bottom surface area as the depth varies while keeping the roughness and the number of waveforms viewed constant.
First-order reflectance decreases as the roughness increases, as was shown in the two-dimensional case. This is true as the roughness varies, regardless of the bottom reference level chosen. Focusing effects are expected from (but are not limited to) second-order reflectance and are due to parts of the bottom whose angles maximize both incoming light and the reflections toward the detector. Along a plane about the vertical axis, the roughness ratio for a fixed-length waveform that returns the highest reflectance can be found. In three dimensions, this phenomenon is complicated by reflections from all hemispherical directions. Shadowing and obscuration behave similarly as in the two-dimensional case although shadowed areas will have an increased potential to reflect light from other directions (than the plane defined by the source incidence and the vertical directions). This is expected to cause higher order reflections to increase as the roughness increases.
2006-03-04T15:33:50ZThe off-specular peak and polarisation effects of an undulating underwater suface
http://hdl.handle.net/1813/2666
The off-specular peak and polarisation effects of an undulating underwater suface
Clavano, Wilhelmina R.; Philpot, William D.
Periodic undulations are used to describe underwater bottom roughness. An expression of the bi-directional reflectance distribution function (BRDF) is given that is dependent on the given roughness metric. Highlights include an off-specular peak and polarisation effects. For an undulating underwater surface, we have shown through geometric optics that reflectance from a rough diffuse surface increases as the viewing direction approaches the backward direction even in the absence of shadowing and/or self-shading (Clavano & Philpot (2003), see also Cox & Munk (1956)). The effects of shadowing and self-shading are equivalent to applying a geometrical attenuation factor to specular reflectance, which is similar to an analysis of morphological effects using triangular waves by Zaneveld & Boss (2003). We show that a reflectance peak displaced away from the specular direction occurs at large angles of incidence (relative to the global normal) as the surface gets rougher (part of work in Clavano & Philpot (2004)). Similar results have been shown for oil films on ocean surfaces using Monte Carlo methods by Otremba & Piskozub (2004) and Otremba (2004). As a general result, an expression of the full bi-directional reflectance distribution function (BRDF) is given. While geometrical effects play a significant role in the reflectance distribution, we consider polarisation effects (as in Mullamaa (1962, 1964)) to gain more insight into real-world reflectances and compare with empirical distributions described by Cox & Munk (1956).
http://www.hydrooptics.spb.ru/onw2005/index.php
2006-03-03T18:45:20ZBackscattering anisotropy near $180^{\circ}$: an indication of particle size and shape
http://hdl.handle.net/1813/2656
Backscattering anisotropy near $180^{\circ}$: an indication of particle size and shape
Clavano, Wilhelmina R.; Boss, Emmanuel; Agrawal, Yogesh C.
By modelling the single scattering of particles in the exact backward direction ($180^{\circ}$) and $5^{\circ}$ around, the field of view of an instrument measuring backscattering is simulated. Calculations of the scattering Mueller matrix $M_{ij}$ using a development of the extended boundary condition method [1] are made for spheroidal particles with sizes ($D$ in $\mu$), shapes (defined by spheroidal aspect ratio $\frac{s}{t}$) and refractive indices similar to ($m = 1.05 + 0.01i$) marine particles found in the natural environment.
Results show that information about size and shape can be gathered from the intensity patterns of the backscattering for particles within the anomalous diffraction region. Comparison between the polarised scattering intensity patterns ($I_{\parallel}$ and $I_{\perp}$) produced by these non-spheres and their volume-equivalent spheres provides insight into the information available from backscattering polarimetry on the effects of size and shape in light scattering by differently shaped particles.
2006-03-01T18:45:55Z