High-resolution optical diffraction microscopy

In an optical diffraction microscopy experiment, one measures the phase and amplitude of the field diffracted by the sample and uses an inversion algorithm to reconstruct its map of permittivity. We show that with an iterative procedure accounting for multiple scattering, it is possible to visualize details smaller than lambda/4 with relatively few illumination and observation angles. The roles of incident evanescent waves and noise are also investigated.     

Calculated effects of backscattering on skin dosimetry for nuclear fuel fragments

The size of hot particles contained in nuclear fallout rangesfrom 10 nm to 20 µm for the worldwide weapons fallout.Hot particles from nuclear power reactors can be significantlybigger (100 µm to several millimetres). Electron backscatteringfrom such particles is a prominent secondary effect in betadosimetry for radiological protection purposes, such as skindosimetry. In this study, the effect of electron backscatteringdue to hot particles contamination on skin dose is investigated.These include parameters such as detector area, source radius,source energy, scattering material and source density. The Monte-CarloNeutron Particle code (MCNP4C) was used to calculate the depthdose distribution for 10 different beta sources and variousmaterials. The backscattering dose factors (BSDF) were thencalculated. A significant dependence is shown for the BSDF magnitudeupon detector area, source radius and scatterers. It is clearlyshown that the BSDF increases with increasing detector area.For high Z scatterers, the BSDF can reach as high as 40 and100% for sources with radii 0.1 and 0.0001 cm, respectively.The variation of BSDF with source radius, source energy andsource density is discussed.     

Phase function effects on oceanic light fields

Numerical simulations show that underwater radiances, irradiances, and reflectances are sensitive to the shape of the scattering phase function at intermediate and large scattering angles, although the exact shape of the phase function in the backscatter directions (for a given backscatter fraction) is not critical if errors of the order of 10% are acceptable. We present an algorithm for generating depth- and wavelength-dependent Fournier-Forand phase functions having any desired backscatter fraction. Modeling of a comprehensive data set of measured inherent optical properties and radiometric variables shows that use of phase functions with the correct backscatter fraction and overall shape is crucial to achieve model-data closure.     

水色遥感的精确Rayleigh散射算法研究

偏振问题是卫星水色遥感需要解决的一个重要问题.通过对带偏振的辐射传输方程进行求解得到精确的Rayleigh散射计算结果,便可以对水色遥感器进行偏振修正.本研究将矢量辐射传输方程进行傅立叶展开,得到与方位角独立的矢量辐射传输方程,利用逐次散射法对其进行求解.本文给出了HY-1卫星上水色遥感器COCTS在不同观测角度和不同风速情况下的精确Rayleigh散射结果,并通过比较得到了在不同观测条件下Rayleigh散射结果之间的差异.     

Modeling low-coherence enhanced backscattering using Monte Carlo simulation

Constructive interference between coherent waves traveling time-reversed paths in a random medium gives rise to the enhancement of light scattering observed in directions close to backscattering. This phenomenon is known as enhanced backscattering (EBS). According to diffusion theory, the angular width of an EBS cone is proportional to the ratio of the wavelength of light lambda to the transport mean-free-path length l(s)* of a random medium. In biological media a large l(s)* approximately 0.5-2 mm > lambda results in an extremely small (approximately 0.001 degrees ) angular width of the EBS cone, making the experimental observation of such narrow peaks difficult. Recently, the feasibility of observing EBS under low spatial coherence illumination (spatial coherence length Lsc < l(s)*) was demonstrated. Low spatial coherence behaves as a spatial filter rejecting longer path lengths and thus resulting in an increase of more than 100 times in the angular width of low coherence EBS (LEBS) cones. However, a conventional diffusion approximation-based model of EBS has not been able to explain such a dramatic increase in LEBS width. We present a photon random walk model of LEBS by using Monte Carlo simulation to elucidate the mechanism accounting for the unprecedented broadening of the LEBS peaks. Typically, the exit angles of the scattered photons are not considered in modeling EBS in the diffusion regime. We show that small exit angles are highly sensitive to low-order scattering, which is crucial for accurate modeling of LEBS. Our results show that the predictions of the model are in excellent agreement with the experimental data.     

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.     

Development of a real-time stereo TEM

We have developed a real-time stereo transmission electron microscope (TEM) with tilting illumination. This system allows us to observe three-dimensional (3-D) images directly with a video-rate of 1/30 s. The system comprises two electrostatic deflectors. The first, included in the illumination system, is able to illuminate a specimen at two oblique stereoscopic angles (left and right of the optical axis) up to ±2.3 °. The second deflector, in the imaging system, is used to correct the image separation caused by defocusing of the tilted illumination. These deflectors are operated in synchronization with an NTSC video signal output from a CCD camera to record left projections on odd fields and right projections on even fields. The time series of stereo pairs is transferred to a 3-D display that enables viewing of the 3-D images without special glasses. Real-time observation of ZnO particles demonstrated that 3-D images were clear even while moving the specimen. We applied this system to observing dislocations in an Al thin film.     

Bidirectional Scattering Distribution Function (BSDF): A Systematized Bibliography

In conjunction with the development of a bidirectional scattering metrology project, a large number of papers pertaining to the theory and measurement of bidirectional scattering from optical surfaces were collected and categorized. This collection includes papers that deal with various aspects of the bidirectional scattering distribution function (BSDF), its measurement, interpretation, use, and implications. Each paper is classified in one or more subject categories on the basis of its technical content. The subject categories are included just to serve as a key to the most salient characteristics of each paper cited. Because of the interest in this field, this bibliography is being published as a service to the public.     

Bidirectional reflectance distribution function of Spectralon white reflectance standard illuminated by incoherent unpolarized and plane-polarized light

A Lambert surface would appear equally bright from all observation directions regardless of the illumination direction. However, the reflection from a randomly scattering object generally has directional variation, which can be described in terms of the bidirectional reflectance distribution function (BRDF). We measured the BRDF of a Spectralon white reflectance standard for incoherent illumination at 405 and 680?nm with unpolarized and plane-polarized light from different directions of incidence. Our measurements show deviations of the BRDF for the Spectralon white reflectance standard from that of a Lambertian reflector that depend both on the angle of incidence and the polarization states of the incident light and detected light. The non-Lambertian reflection characteristics were found to increase more toward the direction of specular reflection as the angle of incidence gets larger.     

Scattering observations for tilted transparent fibers: evolution of airy caustics with cylinder tilt and the caustic merging transition

When a dielectric circular cylinder is obliquely illuminated, the scattering angle associated with the Airy caustics of the cylinder's primary rainbow depends on the tilt of the cylinder. We display records of the scattering pattern for a transparent poly(methyl methacrylate) fiber ranging from small values of tilt through values of tilt that are sufficiently large for the Airy caustics from both sides of the fiber to merge in a meridional plane containing the incident wave vector and the fiber's axis. The records are compared directly with the evolution of the caustic projected onto the observation plane, and certain qualitative features of the global evolution of the caustics are confirmed. Although the observations used laser illumination, they are relevant to anticipating the scattering by sunlit transparent tilted cylinders.     

Radiative transfer calculations in multilayer systems with smooth or rough interfaces

Abstract

A very general discrete-ordinate method is presented for radiative transfer calculations in plane-parallel systems with partially reflecting interfaces. It is the natural extension of existing treatments for atmospheres (like DISORT) to account for changes of refractive index (Fresnel interfaces). The system under study is made of several dielectric layers, each layer containing spherical particles which produce anisotropic scattering (Mie scatterers). Incident light can be either collimated at normal or oblique incidence, or diffuse. A reflecting substrate can be added to the system. A procedure can be used to calculate the scattered fluxes at arbitrary angles, from the fluxes firstly obtained at a fixed set of polar angles. An efficient matrix formalism allows one to consider various boundary conditions: smooth or rough (scattering) interfaces. Surface scattering is introduced through a combination of Kirchhoff's expressions (for the specular reflection attenuation) and a geometrical optics model (for the scattering lobe). Comparisons with previous models and examples of simulations are presented. The main limitation of the method seems to be the approximate account for the polarization in this scalar treatment.     

Surface waves and atomic force microscope probe-particle near-field coupling: discrete dipole approximation with surface interaction

Evanescent waves on a surface form due to the collective motion of charges within the medium. They do not carry any energy away from the surface and decay exponentially as a function of the distance. However, if there is any object within the evanescent field, electromagnetic energy within the medium is tunneled away and either absorbed or scattered. In this case, the absorption is localized, and potentially it can be used for selective diagnosis or nanopatterning applications. On the other hand, scattering of evanescent waves can be employed for characterization of nanoscale structures and particles on the surface. In this paper we present a numerical methodology to study the physics of such absorption and scattering mechanisms. We developed a MATLAB implementation of discrete dipole approximation with surface interaction (DDA-SI) in combination with evanescent wave illumination to investigate the near-field coupling between particles on the surface and a probe. This method can be used to explore the effects of a number of physical, geometrical, and material properties for problems involving nanostructures on or in the proximity of a substrate under arbitrary illumination.     

Optical properties (bidirectional reflectance distribution function) of shot fabric

To study the optical properties of materials, one needs a complete set of the angular distribution functions of surface scattering from the materials. Here we present a convenient method for collecting a large set of bidirectional reflectance distribution function (BRDF) samples in the hemispherical scattering space. Material samples are wrapped around a right-circular cylinder and irradiated by a parallel light source, and the scattered radiance is collected by a digital camera. We tilted the cylinder around its center to collect the BRDF samples outside the plane of incidence. This method can be used with materials that have isotropic and anisotropic scattering properties. We demonstrate this method in a detailed investigation of shot fabrics. The warps and the fillings of shot fabrics are dyed different colors so that the fabric appears to change color at different viewing angles. These color-changing characteristics are found to be related to the physical and geometrical structure of shot fabric. Our study reveals that the color-changing property of shot fabrics is due mainly to an occlusion effect.     

Optical sizing of aggregated combustion particles: computational development of a two-angle laser scattering technique

To simplify the experimental efforts involved in the nonintrusive optical characterization of typical combustion-generated particulates, an inverse analysis of scattering measurements at only two angles is developed and evaluated based on the exact scattering computations on simulated fractallike aggregates. Two optimum angles are suggested by combination of theoretical (sensitivity analysis) and practical (spatial resolution) considerations that have led to the interpretation of spherule diameter from the ratio of large-angle scattering and absorption coefficients and the aggregate gyration radius from the ratio of scattering dissymmetry. The inferred spherule and aggregate sizes are found to be generally in good agreement with the initially prescribed values for the range of physical and optical particulate properties considered here. These computational results under precise simulation conditions demonstrate the accuracy of the proposed particulate-sizing technique by avoiding the common experimental uncertainties that prevent a definitive performance assessment of any experimental method.     

Aperiodic Fourier modal method in contrast-field formulation for simulation of scattering from finite structures

This paper extends the area of application of the Fourier modal method (FMM) from periodic structures to aperiodic ones, in particular for plane-wave illumination at arbitrary angles. This is achieved by placing perfectly matched layers at the lateral sides of the computational domain and reformulating the governing equations in terms of a contrast field that does not contain the incoming field. As a result of the reformulation, the homogeneous system of second-order ordinary differential equations from the original FMM becomes non-homogeneous. Its solution is derived analytically and used in the established FMM framework. The technique is demonstrated on a simple problem of planar scattering of TE-polarized light by a single rectangular line.     

Resonant scattering of light from a glass/Ag/MgF2/air system with rough interfaces and supporting guided modes in attenuated total reflection

We report experimental results of the resonant scattering of light from a prism-glass/Ag/MgF2/air system with use of the attenuated total reflection technique for p and s polarized light. Two MgF2 film thicknesses were used. The system with the thinner dielectric layer supports two transverse magnetic (TM) and two transverse electric (TE) guided modes at a wavelength of 632.8 nm, and the system with the thicker dielectric layer supports three TM and three TE guided modes. In both cases we found dips in the specular reflection as a function of incident angle that is due to excitation of guided modes in the MgF2 film. The scattered light shows peaks at angles corresponding to the measured excitation of the guided modes. These peaks are due to single-order scattering and occur for any angle of the incident light. All features in the scattering response are enhanced in resonance conditions, and the efficiency of injecting light into the guide is reduced.     

Incoherent Fourier Transformation: A New Approach to Character Recognition

It is possible to determine any Fourier coefficient (spatial frequency) in a real two-dimensional distribution of illumination by allowing the distribution to throw the shadow of a suitably placed grid onto an observation plane, where the contrast of the shadow measures the modulus of the Fourier coefficient and its position measures the phase of the coefficient. A pilot model of a scheme for reading the Arabic numerals 0, 1, …, 9 using only three Fourier coefficients has been set up and run successfully. The possibilities of the method are assessed.     

Characterization of spatial and temporal variations in the optical properties of tissuelike media with diffuse reflectance imaging

We describe a method to characterize spatial or temporal changes in the optical properties of turbid media using diffuse reflectance images acquired under broad-beam illumination conditions. We performed experiments on liquid phantoms whose absorption (mu(a)) and reduced scattering (mu(s)') coefficients were representative of those of biological tissues in the near infrared. We found that the relative diffuse reflectance R depends on mu(a) and mu(s)' only through the ratio mu(a)/mu(s)' and that dependence can be well described with an analytical expression previously reported in the literature [S. L. Jacques, Kluwer Academic Dordrecht (1996)]. We have found that this expression for R deviates from experimental values by no more than 8% for various illumination and detection angles within the range 0 degrees-30 degrees. Therefore, this analytical expression for R holds with good approximation even if the investigated medium presents curved or irregular surfaces. Using this expression, it is possible to translate spatial or temporal changes in the relative diffuse reflectance from a turbid medium into quantitative estimates of the corresponding changes of (mu(a)/mu(s)')(1/2). In the case of media with optical properties similar to those of tissue in the near infrared, we found that the changes mu(a)/mu(s)' should occur over a volume approximately 2 mm deep and 4 mm x 4 mm wide to apply this expression.     

Bragg scattering from an obliquely illuminated photonic crystal fiber

Scattering at visible frequencies from a two-dimensional silica/air photonic crystal material in the form of a fine fiber reveals the hexagonal crystal structure of the material. Oblique illumination allows the observation of first-order Bragg conditions even for a crystal structure with a pitch several times the wavelength of light. These scattering measurements demonstrate the feasibility of a low-loss waveguide based on photonic bandgap effects.