McCART is a numerical procedure to solve the radiative transfer equation for light propagation through the atmosphere especially developed to study the effect of the atmosphere on the response of hyperspectral sensors for remote sensing of the earth's surface. McCART is based on a single Monte Carlo simulation run for a reference layered plane non-absorbing atmosphere and a plane ground with uniform reflectance. The spectral response of the sensor for a given distribution of ground reflectance and for a specific profile of scattering and absorption properties of the atmosphere is obtained in a short time from the results of the Monte Carlo simulation, making use of scaling relationships and of symmetry properties. The response includes the effects of adjacent pixels. The results can be used to establish the limits of applicability of approximate algorithms for the processing and analysis of hyperspectral images. The algorithm can be also used to develop procedures of atmospheric compensation.
The paper addresses the problem of extending conventional Monte Carlo procedures to cases of multiple scattering in media with suspensions of non-spherical or chiral particles. Extinction coefficients of the media depend on polarization of radiation. Along the propagation path polarization of radiation changes, unless the field is polarized according to one of two particular modes. The relationship between these modes and the elements of the amplitude scattering matrix for the type of particle is shown by means of a simple formalism, tested with reference to simple shapes and orientation of the particles. Some possibilities for extending Monte Carlo procedures are suggested. A case of small chiral spheres is considered.
A mathematical model and a Monte Carlo algorithm were developed to simulate radiation transfer in dispersive media that is optically anisotropic with respect to zenith angle of the light beam. The algorithm was created with the purpose to simulate radiation transfer processes in the atmosphere with optically anisotropic clouds (for instance, cirrus clouds). A numerical experiment was performed for a cloud with ice crystals of hexagonal cylinder shape. We compared results for scattering media with particles stochastically oriented in a horizontal plane and in space. It was shown that orientation of particles could strongly affect the albedo of clouds and the shape of halos.
The paper addresses the problem of extending conventional Monte Carlo procedures to cases of multiple scattering in media with suspensions of non-spherical or chiral particles. Extinction coefficients of the media depend on polarization of radiation. Along the propagation path polarization of radiation changes, unless the field is polarized according to one of two particular modes. The relationship between these modes and the elements of the amplitude scattering matrix for the type of particle is shown by means of a simple formalism, tested with reference to simple shapes and orientation of the particles. Some possibilities for extending Monte Carlo procedures are suggested. A case of small chiral spheres is considered.
This note describes a possible Monte Carlo procedure to deal with propagatiaon of optical radiation in a medium containing non-spherical particles, without the limit of isotropic orientation of their axes.
It is based on the choice of two polarization states which does not change during the propagation in the turbid medium. An approximate procedure is considered for permitting the use of only two polarization states.
Multiple-scattering effects sometime bias the ground-based lidar measurements, in particular for density aerosol and cirrus cloud. Both analytical and Montecarlo methods are very useful tools to study this influence. However, for analytical solution, it needs to make some hypotheses and the Montecarlo simulation is only a forward method. In this paper, an itinerative method is introduced based on Montecarlo simulation. Both extinction and backscattering coefficients, obtained by Raman lidar, are corrected for the multiple-scattering influence. For the typical cirrus cloud, the error of the multiple-scattering influence on extinction can be as large as 100%. However, it is negligible of the influence on backscattering coefficient. Therefore, the lidar ratio is also sensitive to the multiple-scattering effect.
This work overviews recent advances that have been made in an analytical theory of elastic and Raman lidar returns with multiple scattering and polarization from clouds and seawaters and outlines newly developed software for computer simulation of airborne oceanic lidar performance.
KEYWORDS: Scattering, Vegetation, Backscatter, Data modeling, Chemical elements, Particles, Monte Carlo methods, Radar, Multiple scattering, Systems modeling
This paper describes an application of the Monte Carlo method to the evaluation of backscattering response to microwave sounding of vegetation. After a brief introductory discussion on the different approaches commonly employed to the numerical simulation of scattering from vegetation, we describe our model based on representing the vegetation medium as a collection of elementary scatterers of simple shapes, and dealing directly with electromagnetic field interaction with these elements. Plant structures are built assembling the single elements by the Lindenmayer systems fractal technique. We presents some examples of computations on models of different kinds of vegetation showing the potential of modeling in understanding scattering behavior. A brief discussion on the issue of second order scattering effects is also included.
In several Lidar techniques molecular backscattering from atmospheric gases is considered, e.g. Raman lidar technique, and elastic molecular scattering by HSR1 technique are employed. When larger particles are also present in the sounded zone, Mie scattering contributions are superposed to the Lidar molecular signal. Because the efficiency of the Mie scattering is very high with respect to the molecular one, the effect of Mie scattering can be very strong for typical tropospheric clouds or aerosol structures, when multiple scattering contribution becomes important. In this case the incident, or the molecular scattered radiation, can be multiply scattered from the aerosols. There is no significant multiple molecular scattering, but larger particle scattering processes occur in addition to one molecular scattering. A Monte Carlo code developed for studying this effect is described by means of a flow diagram showing the details of the procedure. The code allows the consideration of depolarization for the molecular return due to the intervening effect of Mie scattering. Some results in some cases of realistic models of atmospheric structures are presented, showing increase of the molecular returns by factors even of the order of a few units. Comparisons with published data by other authors will be shown.
Since multiple scattering (`MS') makes it more difficult to interpret lidar returns in terms of characteristics and parameters of a sounded medium, since 1987 there have been a series of workshops aimed at comparing research dealing with MS and lidar technique.
The purpose of this paper is to present and summarize the activity of the Florence group, relevant to the effect of multiple scattering in the lidar technique of sounding clouds or fog.
The atmosphere is the optical medium between the imaging system and the observed object. The effect of this optical turbulent, absorbing and scattering element on the quality of an image is difficult to predict. Since the beginning of the sixties there were several attempts to build a model that will describe the degrading effect of the atmosphere on spatial resolution of imaging systems. In the field of atmospheric turbulence there is a common agreement on its relative contribution to the degradation of the spatial resolution of an image. On the other hand in the field of atmospheric scattering there is a disagreement on its degrading effects and an international scientific discussion has been developed in the past five years in this scientific field. A model, that was suggested several years ago by Sadot and Kopeika, claimed that the effect of the atmospheric scattering on the spatial resolution of imaging systems is a function of the properties of the imaging system, apart from the inherent properties of the atmosphere. The results of their model were in contrast with the results of the work of other scientists and therefore, caused a scientific debate. The purpose is to propose an alternative theoretical model which describes the effects of atmospheric scattering by focusing on the point spread function of the imaging system.
In this paper a physical model that describes the relationship between the optical properties of the atmosphere and the characteristics of an imaging system is suggested. The model describes how different components of the light reaching the imaging system, after passing through the atmosphere, are detected by it. The model includes the effects of the final size of the detector elements of the imaging system and the dynamic range and the final field of view limits of the imager. It is found that for common imaging systems (with resolution of 8 bit or 12 bit) working in general atmosphere conditions (VIS >= 5 km), the processes of atmospheric scattering and absorption hardly contribute to spatial blurring of the recorded images. A field experiment was carried out in order to verify the predictions of the suggested model. The measurements were performed using a scanning point radiometer, while a local meteorological station and a visibility meter measured the properties of the atmosphere. Theoretical predictions, which were accomplished by using a Monte-Carlo simulation of atmospheric scattering effects, are compared with the experimental data acquired in the field tests. A good agreement was obtained between the measured data and the theoretical predictions.
The effect of multiple scattering on received power and its polarization state is examined by considering clouds made of spherical water droplets and non-spherical Chebyshev particles. A Monte Carlo code was used and its capability of dealing with homogeneous and stratified clouds is shown by a series of examples.
A numerical code is used to examine the features of the effect of atmospheric turbidity on the modulation transfer function of an optical system operating on ground, on an airplane or a satellite. Models of size distributions and optical properties of particulate suspended in the atmosphere are considered. The relevant scattering phase functions are calculated by Mie theory and are later used by a code using both Monte Carlo and geometrical optics procedures to evaluate the contribution of atmospheric turbidity to the augmentation of the point spread function. Comparison of ours with other researchers procedures is shown. The effect of atmospheric turbidity is evaluated as due to the presence of scatterers (the secondary sources) whose defocused images are distributed on the plane of the image of the primary source. The positions of the scatterers are determined by a Monte Carlo procedure, while the contribution of each secondary source to the irradiance on the image plane is evaluated by means of geometrical optics. Cases of different aerosols types, geometry aspects of viewing through the atmosphere and atmospheric absorption effects on the MTF are shown.
An investigation of the characteristics of multiply-scattered lidar returns from homogeneous layres of nonspherical Chebyshev particles is presented. A Monte Carlo procedure has been employed to simulate lidar measurements in a ground-based configuration. Total detected power and depolarization of the return signal have been calculated for a variety of particle sizes and deformations, as well as for different fields of view of the instrument and optical thicknesses of the medium. As far as depolarization is concerned, particles characterized by a high backscattering depolarization ratio have shown a peculiar behavior in multiple scattering. Results have been checked, for double scattering, employing an analytical formula previously developed.
KEYWORDS: Scattering, Clouds, LIDAR, Polarization, Modulation transfer functions, Monte Carlo methods, Multiple scattering, Receivers, Mass attenuation coefficient, Beam splitters
The features of a code for calculation of lidar returns from clouds are recalled. Some laboratory measurements on models showed the validity of the code with regard to polarization of received power. Reference is also made to a code for calculations of turbidity effects on optical systems.
The absorption and the reduced scattering coefficients of living tissues were obtained from measurements of time-resolved reflectance at near-IR wavelengths. The inversion procedure was based on the temporal spread function for a semi-infinite medium given by the diffusion approximation. Measurements showed significant variations between the optical parameters measured on different organs and different volunteers. The single scattering properties (extinction coefficient and scattering function at small forward scattering angles) of bovine and swine brain were obtained from transmissometric measurements on thin slices of tissues. Large differences between the optical properites of white and grey matter were observed, whereas minor differences were found between bovine and swine samples.
KEYWORDS: Clouds, LIDAR, Scattering, Monte Carlo methods, Multiple scattering, Atmospheric propagation, Modulation transfer functions, Remote sensing, Mass attenuation coefficient, Receivers
A Monte Carlo procedure to deal with propagation of radiation in a turbid medium presenting some simple kinds of inhomogeneity is briefly outlined. Cases of lidar returns from inhomogeneous clouds are shown.
Numerical and experimental results of a study carried out to investigate the capabilities of the time gating technique to enhance images of biological tissues are reported. In particular we have investigated the confocal scanning configuration. The experimental results were obtained by using a picosecond laser and a streak camera system. The numerical results were obtained by using Monte Carlo simulations. Both elementary and semianalytic Monte Carlo methods were used. The numerical results showed that by using the time gating technique the contrast can be significantly increased when a short gating time is used. The measured time resolved spread functions were in general good agreement with numerical results. However, the experimental results showed that with our experimental setup it is difficult to measure with sufficient accuracy the small energy received during a short gating time when the slab of tissue is larger than 1 cm.
By using both picosecond time-resolved spectroscopy and Monte Carlo simulations, we studied photon migration in a cylindrical phantom. Fan beam style source-receiver geometry was considered. The influence of inhomogeneities inside the phantom on impulse response of short incident light was studied considering a black body located at various positions inside the scattering medium. Applicability of time gating technique to the fan beam geometry for enhancement of image resolution was also examined.
A numerical procedure is applied to studying the effect of some geometrical parameters on point spread function and modulation transfer function of an optical system operating in a turbid medium. The possibilities offered by an inverse scheme of calculation are also examined.
KEYWORDS: Scattering, Time resolved spectroscopy, Tissues, Ischemia, Monte Carlo methods, Absorption, Light scattering, Near infrared, Spectroscopy, Imaging spectroscopy
Human forearm oxygenation and scattering changes during ischemia were investigated using picosecond near infrared laser spectroscopy. Path lengths were calculated for different geometries. At 760 nm, a plateau phaser was reached after 4 min occlusion. No changes were observed during the occlusion at 800 nm, the hemoglobin isosbetic wavelength. Time-resolved Monte Carlo simulations were performed also to mimic the propagation of a light pulse in a forearm phantom containing different quantity and size of polystyrene spheres as scattering media. The influence of both the scattering and absorption effects was studied.
A Monte Carlo based method of calculating the effect of a turbid medium on the MTF of an optical system is presented. Results are compared with experimental data.
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