Showing papers on "Light scattering published in 1994"
TL;DR: In this article, the authors used the discrete-dipole approximation (DDA) for scattering calculations, including the relationship between the DDA and other methods, including complex-conjugate gradient algorithms and fast-Fourier transform methods.
Abstract: The discrete-dipole approximation (DDA) for scattering calculations, including the relationship between the DDA and other methods, is reviewed. Computational considerations, i.e., the use of complex-conjugate gradient algorithms and fast-Fourier-transform methods, are discussed. We test the accuracy of the DDA by using the DDA to compute scattering and absorption by isolated, homogeneous spheres as well as by targets consisting of two contiguous spheres. It is shown that, for dielectric materials (|m| ≲ 2), the DDA permits calculations of scattering and absorption that are accurate to within a few percent.
3,283 citations
26 Oct 1994
TL;DR: In this paper, the scattering of pure water is recalculated using the Einstein-Smoluchowski equation and the input for this equation is evaluated and the temperature dependency is included.
Abstract: The optical properties of pure water are basic input data for many geophysical investigations such as remote sensing of surface water and underwater radiative transfer calculations. Knowledge of the spectral properties of components in surface water is required for accurate interpretation of measured reflection and attenuation spectra in terms of their concentrations. Also the sources and sizes of errors in the basic data must be known. Absorption measurements were done with a submersible absorption meter in the temperature range 2.5 till 40.5 degree(s)C. The scattering of pure water is recalculated using the Einstein-Smoluchowski equation. The input for this equation is evaluated and the temperature dependency is included. New values for the absorption coefficient are given based on these results and analysis of data from the literature. Absorption in the wavelength range 300 - 550 nm is lower than presently used values. In the wavelength range about 700 nm the spectrum has a different shape. A formulation of the effect of temperature on the absorption spectrum is given.
423 citations
TL;DR: A frequency-domain near-infrared tissue spectrometer capable of measuring the reduced scattering coefficient of tissue with enough precision to detect changes in glucose levels in the physiological and pathological range is designed and constructed.
Abstract: Tissue glucose levels affect the refractive index of the extracellular fluid. The difference in refractive index between the extracellular fluid and the cellular components plays a role in determining the reduced scattering coefficient (micro(s)') of tissue. Hence a physical correlation may exist between the reduced scattering coefficient and glucose concentration. We have designed and constructed a frequency-domain near-infrared tissue spectrometer capable of measuring the reduced scattering coefficient of tissue with enough precision to detect changes in glucose levels in the physiological and pathological range.
382 citations
TL;DR: The application of quasi-elastic light scattering for the determination of particle size distributions is reviewed in this article, where the basic principles and assumptions on which the method relies are presented on an introductory level.
302 citations
TL;DR: In this paper, it is demonstrated that calculating the T-matrix using extended precision instead of double precision floating-point variables is an effective approach for suppressing the numerical instability in computations for spheroids.
277 citations
TL;DR: This study highlights the efficiency and ease of use of the LED technique, which lends itself to in vivo spectroscopy of biological tissues, and derives an analytical expression for the absorption and scattering coefficients that is based on a two-distance measurement technique.
Abstract: The absorption and scattering coefficient of a macroscopically homogeneous strongly scattering medium (lipid emulsion) containing Methylene Blue is quantitatively measured in the spectral range from 620 to 700 nm. We conduct the measurements in the frequency domain by using a light-emitting diode (LED) whose intensity is modulated at a frequency of 60 MHz. We derive an analytical expression for the absorption and scattering coefficients that is based on a two-distance measurement technique. A comparison with other measurement protocols such as measurement at two modulation frequencies shows that the two-distance method gives a better determination of the scattering and absorption coefficients. This study highlights the efficiency and ease of use of the LED technique, which lends itself to in vivo spectroscopy of biological tissues.
255 citations
26 Oct 1994
TL;DR: The only reasonable correction method for an absorption meter without attenuation measurements or a spectrophotometer is the method in which the measured absorption at a wavelength in the near infrared is subtracted as discussed by the authors.
Abstract: In this paper we examine correction methods for the scattering error of reflecting tube absorption meters and spectrophotometers. We model the scattering error of reflecting tube absorption meters for different tube parameters and different inherent optical properties. We show that the only reasonable correction method for an absorption meter without attenuation measurements or a spectrophotometer is the method in which the measured absorption at a wavelength in the near infrared is subtracted. A better correction is obtained if attenuation is measured simultaneously and the absorption at the reference wavelength is multiplied by the ratio of the measured scattering at the measurement wavelength divided by the measured scattering coefficient at the reference wavelength. This is the proportional method. We showed that the important geometrical parameters of the reflecting tube can be obtained by a comparison of measurements and models of polystyrene beads. Finally, we examine the improvements that could be obtained if a direct scattering measurement were made simultaneously with the absorption and attenuation measurements.
251 citations
TL;DR: The effect of glucose on light transport in highly scattering, tissue-simulating phantoms is demonstrated both experimentally and theoretically by application of diffusion theory.
Abstract: The presence of glucose dissolved in an aqueous solution increases the refractive index of the solution and therefore has an influence on the scattering properties of any particles suspended within it. We present experimental data on the effect of glucose concentration on the scattering coefficient of a suspension of spherical polystyrene particles. The experimental results are in good agreement with Mie theory. The effect of glucose on light transport in highly scattering, tissue-simulating phantoms is demonstrated both experimentally and theoretically by application of diffusion theory. The possible application of this effect for noninvasive glucose monitoring of diabetic patients is discussed.
229 citations
228 citations
TL;DR: The main interest is in light scattering by polydisperse models of nonspherical particles because averaging over sizes provides more realistic modeling of natural ensembles of scattering particles and washes out the interference structure and ripple typical of monodisperse scattering patterns, thus enabling us to derive meaningful conclusions about the effects of particle nonsphericity on light scattering.
Abstract: We report the results of an extensive study of the scattering of light by size and size–shape distributions of randomly oriented prolate and oblate spheroids with the index of refraction 1.5 + 0.02i typical of some mineral terrestrial aerosols. The scattering calculations have been carried out with Waterman’s T-matrix approach, as developed recently by Mishchenko [J. Opt. Soc. Am. A 8, 871 (1991); Appl. Opt. 32, 4562 (1993)]. Our main interest is in light scattering by polydisperse models of nonspherical particles because averaging over sizes provides more realistic modeling of natural ensembles of scattering particles and washes out the interference structure and ripple typical of monodisperse scattering patterns, thus enabling us to derive meaningful conclusions about the effects of particle nonsphericity on light scattering. Following Hansen and Travis [Space Sci. Rev. 16, 527 (1974)], we show that scattering properties of most physically plausible size distributions of randomly oriented nonspherical particles depend primarily on the effective equivalent-sphere radius and effective variance of the distribution, the actual shape of the distribution having a minor influence. To minimize the computational burden, we have adopted a computationally convenient power law distribution of particle equivalent-sphere radii n(r) ∝ r−3, r1 ≤ r ≤ r2. The effective variance of the size distribution is fixed at 0.1, and the effective size parameter continuously varies from 0 to 15. We present results of computer calculations for 24 prolate and oblate spheroidal shapes with aspect ratios from 1.1 to 2.2. The elements of the scattering matrix for the whole range of size parameters and scattering angles are displayed in the form of contour plots. Computational results are compared with analogous calculations for surface-equivalent spheres, and the effects of particle shape on light scattering are discussed in detail.
221 citations
TL;DR: In this article, the generalized Lorenz-Mie theory describes electromagnetic scattering of an arbitrary light beam by a spherical particle and the computationally most expensive feature of the theory is the evaluation of the beam-shape coefficients, which give the decomposition of the incident light beam into partial waves.
Abstract: Generalized Lorenz–Mie theory describes electromagnetic scattering of an arbitrary light beam by a spherical particle. The computationally most expensive feature of the theory is the evaluation of the beam-shape coefficients, which give the decomposition of the incident light beam into partial waves. The so-called localized approximation to these coefficients for a focused Gaussian beam is an analytical function whose use greatly simplifies Gaussian-beam scattering calculations. A mathematical justification and physical interpretation of the localized approximation is presented for on-axis beams.
TL;DR: The new device appeared to be particularly sensitive to small aggregates such as those formed in platelet activation induced by low concentrations of agonists, and allows the aggregate size distribution and the extent of aggregation to be estimated.
TL;DR: In this paper, the static structure factor was used to calculate asymmetry parameters of the phase function for densely packed particles with real refractive indices 1.31 and 1.66, approximating water ice and soil particles, respectively.
Abstract: Spatial correlation among densely packed particles can substantially change their single-scattering properties, thus making questionable the applicability of the independent scattering approximation in calculations of light scattering by planetary regoliths. The same problem arises in geophysics in light scattering computations for snow, frosts, and bare soil. In this paper, we use a dense-medium light-scattering theory based on the introduction of the static structure factor to calculate asymmetry parameters of the phase function for densely packed particles with real refractive indices 1.31 and 1.66, approximating water ice and soil particles, respectively, and imaginary refractive indices 0, 0.01, and 0.3. For sparsely distributed, independently scattering grains, the calculated asymmetry parameters are always positive and always larger than those for densely packed particles. For densely packed grains, the asymmetry parameters may be negative but only for radius-to-wavelength ratios from about 0.1 to about 0.4. With decreasing particle size, the calculated asymmetry parameters tend to zero independently of the compaction state. In the geometrical optics regime, the asymmetry parameters for densely packed scatterers are positive and very close to those for independently scattering grains. These results may have important implications for remote sensing of the Earth and solid planetary surfaces. In particular, it is demonstrated that negative asymmetry parameters derived with some approximate multiple-scattering theories may be physically irrelevant and can be the result of using an inaccurate bidirectional reflection function combined with the ill-conditionally of the inverse scattering problem.
TL;DR: In this article, the authors employed accurate vector and scalar multiple-scattering calculations to perform a systematic study of the errors induced by the neglect of polarization in radiance calculations for a homogeneous, plane-parallel Rayleigh scattering atmosphere above a Lambertian surface.
Abstract: Although neglecting polarization and replacing the rigorous vector radiative transfer equation by its approximate scalar counterpart has no physical background, it is a widely used simplification when the incident light is unpolarized and only the intensity of the reflected light is to be computed. We employ accurate vector and scalar multiple-scattering calculations to perform a systematic study of the errors induced by the neglect of polarization in radiance calculations for a homogeneous, plane-parallel Rayleigh-scattering atmosphere (with and without depolarization) above a Lambertian surface. Specifically, we calculate percent errors in the reflected intensity for various directions of light incidence and reflection, optical thicknesses of the atmosphere, single-scattering albedos, depolarization factors, and surface albedos. The numerical data displayed can be used to decide whether or not the scalar approximation may be employed depending on the parameters of the problem. We show that the errors decrease with increasing depolarization factor and/or increasing surface albedo. For conservative or nearly conservative scattering and small surface albedos, the errors are maximum at optical thicknesses of about 1. The calculated errors may be too large for some practical applications, and, therefore, rigorous vector calculations should be employed whenever possible. However, if approximate scalar calculations are used, we recommend to avoid geometries involving phase angles equal or close to 0 deg and 90 deg, where the errors are especially significant. We propose a theoretical explanation of the large vector/scalar differences in the case of Rayleigh scattering. According to this explanation, the differences are caused by the particular structure of the Rayleigh scattering matrix and come from lower-order (except first-order) light scattering paths involving right scattering angles and right-angle rotations of the scattering plane.
TL;DR: The time-resolved optical Kerr effect of water at 298 K was investigated using pulse widths (full width at half-maximum) as short as 38 fs to characterize the dynamic response of water from tens of femtoseconds to several picosecond time scales.
Abstract: The time-resolved optical Kerr effect of water at 298 K was investigated using pulse widths (full width at half-maximum) as short as 38 fs to characterize the dynamic response of water from tens of femtoseconds to several picosecond time scales. A frequency analysis of the free induction decay of the excited Raman modes observed maxima at 40 and 165 cm[sup [minus]1], with a broad distribution of higher frequency components centered around 470 cm[sup [minus]1]. There is an initial rapid decay in the induced birefringence of less than 50 fs due to interference between these modes. These features are in good agreement with previous depolarized light scattering studies. Additional relaxation components of 500 [+-] 30 fs and 1.7 [+-] 0.3 ps are resolved in the Kerr results. In total, these observations depict an overall dynamic pathway for the relaxation of water. 40 refs., 6 figs.
Patent•
23 Dec 1994
TL;DR: In this article, a wedge shaped parallelized light flux element made of a light scattering guide of an emitting directionality has a specified ranged effective scattering irradiation parameter and correlation distance "a".
Abstract: A wedge shaped parallelized light flux element made of a light scattering guide of an emitting directionality has a specified ranged effective scattering irradiation parameter and correlation distance "a". A light incident into the parallelized light flux element from a fluorescent lamp arranged facing a light incident surface of the parallelized light flux element is emitted from a light output surface as a parallelized light flux G1 by composite factors including forward scattering of the inside, repeated reflection on wedge shaped two sides, critical angular condition and boundary face transmittance for the light output surface. A total amount of parallel light flux G1 emitted from the parallel light flux emitting section F is reflected on a two side prism surface of two prism reflection element PR thereby being converted into sectional area enlarged light fluxes G2 and G3. They are incident on a liquid crystal panel LP through a light diffusion plate DF, and utilized as a backlight. Reflective appearance of a light supply source for supplying a light to the light scattering guide is prevented by forming a reverse slope portion or the like on back surface of the light output surface in adjacent to the light incident surface of the parallelized light flux element.
TL;DR: In this article, the surface chemical parameters (dissociation constant of the acidic carboxylic groups, total density of ionizable sites and Stern capacitance) are determined from fits of a Stern layer model to the titration data.
Abstract: In this paper we report on the charging behavior of latex particles in aqueous suspensions. We use static light scattering and acid–base titrations as complementary techniques to observe both effective and bare particle charges. Acid–base titrations at various ionic strengths provide the pH dependent charging curves. The surface chemical parameters (dissociation constant of the acidic carboxylic groups, total density of ionizable sites and Stern capacitance) are determined from fits of a Stern layer model to the titration data. We find strong evidence that the dissociation of protons is the only specific adsorption process. Effective particle charges are determined by fits of integral equation calculations of the polydisperse static structure factor to the static light scattering data. A generalization of the Poisson–Boltzmann cell model including the dissociation of the acidic surface groups and the autodissociation of water is used to predict effective particle charges from the surface chemical parameters determined by the titration experiments. We find that the light scattering data are best described by a model where a small fraction of the ionizable surface sites are sulfate groups which are completely dissociated at moderate pH. These effective charges are comparable to the predictions by a basic cell model where charge regulation is absent.
TL;DR: In this article, the role of correlated isotropy in the study of microroughness in high-quality optical coatings is emphasized and compared, and the sensitivity of the investigation method to the stack parameters is studied.
Abstract: We emphasize the role of correlated isotropy in the study of microroughness in high-quality optical coatings. First, cross correlation between surfaces and cross coherence between scattering sources are discussed and compared. An isotropy degree of roughness is then introduced as a quantitative value to describe the angular disorder of a surface connected with the polar dependence of scattering. We show how the frequency variations of this isotropy degree allow one to solve the inverse problem and obtain a unique solution for the scattering parameters that describe structural irregularities of the stacks. Light scattering can also be used to detect an oblique growth of the materials in thin-film form. Finally, we study the sensitivity of the investigation method to the stack parameters.
TL;DR: The propagation of light through the suspension is modeled and a set of Monte Carlo simulations is performed to substantiate the inference that the relative variation of the backscattered flux is proportional to the gradients of deformation of the RBC's, and that such gradients must be known in order to apply a rheological model describing the non-Newtonian behavior of RBC membranes.
Abstract: The backscattered and transmitted diagrams of He–Ne laser light illuminating a concentrated suspension of red blood cells (RBC's) are investigated. The shapes of these diagrams are closely related to the state of the suspension (at rest or submitted to a simple shear flow) and to the parameters that govern the non-Newtonian behavior of the blood suspension (such as the viscosity of the suspending medium and the volume concentration of the cells). An asymmetry in the backscattering diagram, which is absent on transmitted diagrams, is observed when the suspension is in a simple shear flow. This asymmetry is related to the deformation and orientation of the RBC's. The propagation of light through the suspension is modeled and a set of Monte Carlo simulations is performed to substantiate the inference that the relative variation of the backscattered flux is proportional to the gradients of deformation of the RBC's, and that such gradients must be known in order to apply a rheological model describing the non-Newtonian behavior of RBC membranes.
TL;DR: The use of gel permeation chromatorgraphy (GPC) coupled with the multiangle laser light scattering (MALLS) detector for the determination of molecular weight and size distribution and for the identification of polymer branching is demonstrated on several examples of both organic and water-soluble polymers.
Abstract: The use of gel permeation chromatorgraphy (GPC) coupled with the multiangle laser light scattering (MALLS) detector for the determination of molecular weight and size distribution and for the identification of polymer branching is demonstrated on several examples of both organic and water souluble polymers The influence of the second virial coefficient and the way of the light scattering data evaluation on the results obtained is shown Measurements of the root mean square radius are presented for two series of polystyrene and polymethylmethacrylate standard, and the results are compared with the radii calculated from the Flory-Fox equation © 1994 John Wiley & Sons, Inc
TL;DR: In this paper, a three-component ionic microemulsion system consisting of AOT/water/decane shows an unusual phase behaviour in the vicinity of room temperature. But this phase behaviour can be understood in terms of a system of polydispersed spherical water droplets, each coated by a monolayer of AO, dispersed in a continuum of oil.
Abstract: The three-component ionic microemulsion system consisting of AOT/water/decane shows an unusual phase behaviour in the vicinity of room temperature. The phase diagram in the temperature-volume-fraction (of the dispersed phase) plant: exhibits a lower consolute critical point at about 40 degrees C and 10% volume fraction. A percolation line, starting from the vicinity of the critical point, cuts across the plane, extending to the high-volume-fraction side at progressively lower temperatures. This phase behaviour can be understood in terms of a system of polydispersed spherical water droplets, each coated by a monolayer of AOT, dispersed in a continuum of oil. These droplets interact with each other via a hard-core plus a short-range attractive interaction, the strength of which increases with temperature. We show that Baxter's sticky-sphere model can account for the phase behaviour, including the percolation line, quantitatively provided that the stickiness parameter is a suitable function of temperature. We use the structure factors measured by small-angle neutron scattering (SANS) below the critical temperature to determine this functional dependence. We also investigate the dynamics of droplets, below and approaching the critical and percolation points, by dynamic light scattering. The first cumulant and time evolution of the droplet density correlation function can be quantitatively calculated by assuming the existence of polydispersed fractal clusters formed by the microemulsion droplets due to attraction. The relaxation phenomena observed in an extensive set of measurements of electrical conductivity and permittivity close to percolation can also be interpreted through the same cluster-forming mechanism, which reproduces the most relevant features of the frequency-dependent complex dielectric constant of this system.
TL;DR: In this paper, a near-forward light scatter detection method was proposed to estimate the particle size and velocity in the presence of a dispersed phase in a two-phase turbulent flow.
TL;DR: In this article, the dynamic behavior of hygroscopic multicomponent aerosols under the influence of changing relative humidity in the atmosphere is investigated, where the phase transformation of the aerosol particle is monitored by laser light scattering, and the relative humidity at the transition point is determined by directly measuring the water vapor pressure in the cell.
Abstract: The dynamic behavior of hygroscopic multicomponent aerosols under the influence of changing relative humidity in the atmosphere is investigated. Laboratory measurements of the deliquescence humidity as a function of temperature between 5° and 35°C are carried out with single aerosol particles individually suspended in an electrodynamic cell. The single-particle levitation cell is placed in a vacuum chamber that can be evacuated and backfilled with water vapor. The phase transformation of the aerosol particle is monitored by laser light scattering, and the relative humidity at the transition point is determined by directly measuring the water vapor pressure in the cell. Results are obtained for aerosol particles composed of binary mixtures of NACl, KCl, NaNO3, Na2SO4, and (NH4)S2SO4, which are common constituents of ambient aerosols. The measured temperature and composition dependence of the deliquescence properties agrees well with a theoretical model based on thermodynamic considerations. A stud...
TL;DR: In this paper, the initial step of particle growth in a low pressure radio-frequency discharge has been studied in situ by laser induced particle explosive evaporation (LIPEE) in situ.
Abstract: The initial step of particulate growth in a dust forming low pressure radio‐frequency discharge has been studied in situ by laser induced particle explosive evaporation (LIPEE). With respect to the conventional light scattering, this method has been found much more efficient to observe small nanometer size particles, especially in the case of UV excimer laser radiation. Experimental results interpreted by a simple model of laser‐particle interaction show that the intensity of LIPEE continuum emission depends on the particle radius roughly as r4. This interaction is essentially different from Rayleigh scattering, as the latter varies as r6. A study of time evolution of powder formation by LIPEE emission reveals the initial formation of nanometer size crystallites and the coalescence process leading to larger scale particles. It could be demonstrated that the critical step of dust formation is the initial clustering process leading to nanometer scale crystallites.
TL;DR: In this paper, the authors used the geometric optics approximation to evaluate the scattering, absorption and polarization properties of hexagonal ice crystals whose sizes are much larger than the incident wavelength, and demonstrated that the radiative forcing at the top of the atmosphere involving most cirrus clouds is positive, implying that the IR greenhouse effect outweighs the solar albedo effect.
TL;DR: Computer simulations which test the accuracy of the diffusion theories used in the analysis of multiple light scattering data find that the predicted probability for transmission through a slab is accurate to 1% if the slab thickness is greater than about 5 transport mean free paths.
Abstract: We report computer simulations which test the accuracy of the diffusion theories used in the analysis of multiple light scattering data. Explicitly including scattering anisotropy and boundary reflections, we find that the predicted probability for transmission through a slab is accurate to 1% if the slab thickness is greater than about 5 transport mean free paths. For strictly isotropic scattering and no boundary reflections, the exact diffusion theory prediction is accurate to this level for all thicknesses. In addition, we predict how the angular distribution of transmitted photons is affected by boundary reflectivity, both with and without refraction. Simulations show that, to a similar extent, corrections to diffusion theory from a more general transport theory are not needed here, either. Our results suggest an experimental means of measuring the so-called extrapolation length ratio which characterizes boundary effects, and thus have important implications for the analysis of static transmission and diffusing-wave spectroscopy data.
Patent•
14 Jun 1994
TL;DR: In this article, a low coherence light beam is split into first and second light beams, and the frequency of the first beam is shifted to match the coherence length of the second beam.
Abstract: A low coherence light beam is split into first and second light beams, and the frequency of the first light beam is shifted. The first and second light beams are then combined with each other at a position at which the optical path difference between the two light beams is larger than the coherence length of the low coherence light beam. The combined light beam is guided to a position in the vicinity of a medium having light scattering properties and split into a third light beam, which travels reversely to the direction of travel of the combined light beam, and a fourth light beam, which is irradiated to the medium. A light beam scattered backwardly from a predetermined depth in the medium is caused to interfere with the third light beam, and the intensity of the resulting interference light beam is detected. Optical heterodyne detection of the intensity of the backward scattered light beam is carried out in accordance with the detected intensity of the interference light beam, and information representing the microstructure of the predetermined deed portion in the medium is thereby obtained.