Distribution Of Absorption Coefficient Research Articles

Monte Carlo ray-tracing simulation for radiative heat transfer in turbulent fluctuating media under the optically thin fluctuation approximation

The Monte Carlo ray-tracing method (MCRT) based on the concept of radiation distribution factor is extended to solve radiative heat transfer problem in turbulent fluctuating media under the optically thin fluctuation approximation. A one-dimensional non-scattering turbulent fluctuating media is considered, in which the mean temperature and absorption coefficient distribution are assumed and the shape of probability density function is given. The distribution of the time-averaged volume radiation heat source is solved by MCRT and direct integration method. It is shown that the results of MCRT based on the concept of radiation distribution factor agree with these of integration solution very well, but results of MCRT based on the concept of radiative transfer coefficient do not agree with these of integration solution. The solution of time-averaged radiative transfer equation by the concept of radiative transfer coefficient should be treated with caution.

Algorithmic techniques for quantitative Compton tomography

Compton tomography is a non-destructive technique that allows one to reconstruct the spatial distribution of the electronic density of the sample analyzed. This information is complementary to the absorption coefficient distribution at a given energy, which can be obtained by a more conventional transmission tomography. However, the reconstruction algorithms used in Compton tomography are different and more complex than those used in transmission tomography. This is mainly due to the absorption of the X-ray photons along the path from the source to the interaction point and from the interaction point to the detector. Only a few of the reconstruction algorithms reported in the literature take absorption corrections into account, and the problems of a quantitative reconstruction have seldom been discussed. In this paper a new kind of reconstruction algorithm is described and the first results are reported and discussed.

Tomographic studies of nontronite particles in aqueous media

X-ray microscopy allows the direct visualization of soil colloids in water. In combination with tomographic techniques it is possible to visualize the three dimensional distribution of the linear absorption coefficient of soil colloids and to investigate the surface amount and spatial arrangement of the particles. First experiments of Nontronite particles from the XM-1 are presented.

Modeling of gaseous radiant exchange with the smooth (reordered) band model

The smooth band model (also called the reordered band model) shows promise of providing the foundation for a radiant analysis method for enclosures containing participating gases. Existing models for the smooth absorption coefficient distribution, however, are not totally satisfactory. This paper presents a new model for the smooth absorption coefficient that addresses the problems with the current models. The new model is exercised on some benchmark calculations of total gas emissivity to test its accuracy. Also, a demonstration problem involving a spherical enclosure with reflective walls is solved to illustrate the utility of the smooth band model, as well as the ease of calculation.

Iteration algorithm to correct absorption in PET

One of the difficulties arising in PET is attenuation of emission projection data. The most difficult situation arises in the cases in which no information as to the distribution of the absorption coefficient is available. It is this case that is considered in the present paper. A new approach to the problem is proposed. Its main idea consists in that the measurement data, as soon as they all belong to one object, are bound to be self-consistent. The whole set of projection data is considered as an invariant function of the operator PR/sup -1,/ where R/sup -1/ is an algorithm solving the emission tomography problem and P is the projecting operator. An iteration algorithm that permits reconstruction of emission projection data is developed. Numerical simulations are carried out.

Analysis and optimization of a diffuse photon optical tomography of turbid media

In a numerical study, we investigate a diffuse-photon computed tomography of a turbid medium. Using a perturbation approach, we relate through a matrix K a bulk heterogeneous distribution of the optical absorption coefficient &mgr;(a) that characterizes the heterogeneity in an otherwise homogeneous turbid medium to the diffuse photon flux that emerges from its surface. By studying the condition number (N(C)) of the matrix K as a function of illumination-detection schemes and choices of reconstruction grids, we explore strategies that optimize the fidelity and spatial resolution of the computed tomography.

Two-Dimensional Multi-Lens Array with Circular Aperture Spherical Lens for Flat-Top Irradiation of Inertial Confinement Fusion Target

A two-dimensional multi-lens array 350 mm in diameter with spherical element lenses has been tested to improve the irradiation uniformity of a high power glass laser for inertial confinement fusion (ICF). A circular shape for the element lens aperture is chosen to eliminate azimuthally asymmetric intensity distribution. An approximate flat-top intensity distribution is obtained with a lens array of 37 lenses. Beam patterns of a coherent laser and partially coherent light (PCL) with the random phase plate (RPP) and the multi-lens array (MLA) are measured in a GEKKO XII glass laser system. Spherical mode structure of the irradiation nonuniformity of 5.6% with MLA is estimated with cos3 θ distribution of absorption coefficient in the target plasma.

Three-dimensional emission tomography of optically thick plasma for the known absorption

The three-dimensional emission tomography of a plasma partially absorbing intrinsic emission is studied. The distribution of local absorption coefficients is assumed to be known. To determine the distribution of local emission coefficients under the given conditions, the use of algebraic algorithms inverting the projection matrix constructed taking into account absorption is proposed. NDAT and NDFBP algorithms are developed, which are based on the expansion of the product of the operator of energy transport in an absorbing medium and the operator of the solution of the problem of emission tomography of a transparent plasma in a Neumann series. A numerical simulation is made, which shows that the emission field can be reconstructed with a reasonable accuracy for an optical thickness as high as 6–8.

Passive acoustic thermotomography—Correlation and noncorrelation variants

Passive thermoacoustic tomography is a method for reconstructing temperature, 2D and 3D distributions inside objects, using the intensity of ultrasonic thermal acoustic radiation emitted from within the object. This radiation can be measured by piezotransducers (PT) both in noncorrelation and correlation mode. For noncorrelation mode, a priori information about ultrasound absorption coefficient distribution is needed for reconstruction. In this case, the methods for the reconstruction of temperature distributions were analyzed by computer simulation. The standard error of reconstruction did not exceed 0.25 K (for about 300 measurements and fluctuation error of every measurement, 0.1 K). The physical experiments confirmed the predictions of the theory. In correlation mode, two spatial distributions—of the temperature and of the absorption coefficient—could be potentially reconstructed. The spatial correlation function of sound pressure from a source of thermal acoustic radiation was measured experimentally by a two-probe acoustic interferometer. The acoustic blackbody made from a narrow strip of a material with a large absorption coefficient served as the source of radiation. The spatial correlation function was computed; this function turned to be in good agreement with experimental data. [Work supported by Russian Basic Research Foundation Grant No. 98-01-00100.]

A comparison study of linear reconstruction techniques for diffuse optical tomographic imaging of absorption coefficient

We compare, through simulations, the performance of four linear algorithms for diffuse optical tomographic reconstruction of the three-dimensional distribution of absorption coefficient within a highly scattering medium using the diffuse photon density wave approximation. The simulation geometry consisted of a coplanar array of sources and detectors at the boundary of a half-space medium. The forward solution matrix is both underdetermined, because we estimate many more absorption coefficient voxels than we have measurements, and ill-conditioned, due to the ill-posedness of the inverse problem. We compare two algebraic techniques, ART and SIRT, and two subspace techniques, the truncated SVD and CG algorithms. We compare three-dimensional reconstructions with two-dimensional reconstructions which assume all inhomogeneities are confined to a known horizontal slab, and we consider two `object-based' error metrics in addition to mean square reconstruction error. We include a comparison using simulated data generated using a different FDFD method with the same inversion algorithms to indicate how our conclusions are affected in a somewhat more realistic scenario. Our results show that the subspace techniques are superior to the algebraic techniques in localization of inhomogeneities and estimation of their amplitude, that two-dimensional reconstructions are sensitive to underestimation of the object depth, and that an error measure based on a location parameter can be a useful complement to mean squared error.

Influence of Two-Dimensional Gas Phase Radiation on Downward Flame Spread

This work investigates how radiation heat transfer influences downward flame spread by presenting a gas phase radiation model, described by a two dimensional P-1 approximation method, to incorporate with the combustion model of Duh and Chen (1991). The parametric study is based on the variation of gravity, which changes the Damkohler number (Da) and radiation to conduction parameter (I/N∞) simultaneously. Comparing the results with the previous studies of Duh and Chen (199l) and Chen and Cheng (1994). which only considered the radiation effect in cross stream direction, the role of stream-wise radiation was identified. The stream-wise radiation contributes to reinforce the forward heat transfer rate subsequently increasing the flame spread rate. However, this model also provides more directional radiation loss than that of Chen and Cheng (I994) and, in doing so, draws more energy out from the flame to further reduce its strength. The results indicates that the effect of heat loss is greater than that of enhancing the upstream heat transfer since the flame spread rate in the present model is always lower than the one predicted by Duh and Chen (1991), Finally, a contour of the Planck mean absorption coefficient distribution is illustrated to demonstrate the effectiveness of gas radiation distribution. It reveals that the strongest radiation occurs near the pyrolyzing surface and the other significant one is in the plume region. A comparisons with available experimental data also given to evaluate the ability of predict in the present combustion model.

A simplified wide band model of the cumulative distribution function for carbon dioxide

An efficient and accurate wide band absorption coefficient cumulative distribution function is used to model the 2.0, 2.7, 4.3, 9.4, 10.4 and 15.0 μm bands of carbon dioxide. Band parameters for these bands are determined, and a study of the accuracy of the formulation is performed over a wide range of pressures and temperatures. The model accurately describes the absorption coefficient distributions compared to existing line-by-line and narrow band results. The simplified formulation of the absorption coefficient provides accurate values for the wide band absorption, generally with differences below 10% when compared to benchmark results.

Tomographic Reconstruction of 2-D Absorption Coefficient Distributions from a Limited Set of Infrared Absorption Data

The results of infrared absorption experiments, performed on a steady, nonreacting flow of methane and argon to test a new inversion method for tomographic reconstruction of spatially nonuniform distributions, are presented. The absorption coefficient distributions to be reconstructed are expressed as a weighted sum of Karhunen-Loève eigenfunctions, formed by applying the Karhunen-Loève procedure to a training set containing a priori information regarding the distributions to be reconstructed. Reconstruction of asymmetric absorption coefficient distributions is accomplished by utilizing a set of laser absorption measurements obtained using an infrared helium neon laser. Issues pertinent to the practical application of the new inversion method, such as the construction of a training set using probe sampling, the effect of experimental error, and the precise modeling of laser absoroption paths, are discussed. Reconstruction of absorption coefficient distributions from only 42 laser absorption measurements typically achieve normalized errors of 10% or less over 80% of the domain.

エキシマレーザによるアブレーション加工の基礎的研究 ２ プローブレーザビームを用いたエキシマレーザ誘起プラズマの解析

We made time resolved measurements of laser-plasma interaction to study the effects of laser-induced plasma on laser processing during laser irradiation. A probe laser of 40 microns diameter was used for the measurements. The reduction of the probe laser intensity by laser-induced plasma was observed in the latter part of a laser pulse. The radial distributions of plasma absorption coefficients and removal depth were measured at various diameters of uniform intensity beams. A plasma absorption model where plasma expands vertically was developed and, using the model, the effect of laser-induced plasma on drilling speed was estimated.

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X-ray transmission imaging that creates image contrast from the distribution of the X-ray absorption coefficient is not sensitive to materials consisting of light elements such as hydrogen, carbon, nytrogen, and oxygen. On the other hand, the X-ray phase shift caused by the light elements is substantial enough to be detected even when absorption is almost zero. Hence, phase-contrast X-ray imaging is a promissing technique for observing the structure inside biological soft tissues without the need for staining and without serious radiation exposure. Using fringe scanning X-ray interferometry, the X-ray phase shift caused by an object was measured. Three-dimensional image reconstruction of cancerous tissues using the measured phase shifts was enabled under tomographic configuration phase-contrast X-ray computed tomography (CT) .

An investigation of the porous silicon optical absorption power law near the band edge

A theoretical investigation of the absorption coefficient of p-type doped porous silicon near the band edge is presented. We assume that the absorption coefficient is constructed by taking an average over a distribution (in terms of band gap) of absorption coefficients of individual crystallites. Exploiting physics fundamental to the crystallite optical absorption process, we derive the relation between the absorption coefficient and the averaged conduction density of states near the band edge for porous silicon. By postulating a specific form for the effective conduction density of states we find excellent agreement with recent optical absorption data for p-type doped porous silicon. We attempt to explain the basis for this postulate phenomenologically by suggesting a certain large-scale behaviour of the particle size distribution. The implication of further experimental verification will be discussed.

Antiferromagnetic spin correlation suppression and superconducting characteristics improvement in YBa2Cu3O6+δ films under light illumination (abstract)

Improvement of YBa2Cu3O6+δ film superconducting characteristics that are dependent on the charge carrier concentration is possible not only by means of oxygen doping, but also by light illumination. New results may be presented as follows: (i) The spectral distribution of the light absorption coefficient of Y-Ba-Cu-O films strongly depends on the index δ: there are relatively narrow bands in the spectrum at small δ and their intensity falls down as δ increases. The films with δ∼0.5–0.75 are metallized under light illumination: the resistivity at T<Tc decreases, the critical temperature Tc rises, the absorption band with an energy of E=2.2 eV vanishes gradually, and the two-magnon band in the Raman spectrum also vanishes, which indicates a decrease of the antiferromagnetic spin correlation. The above changes occur only if the photon energy is higher than that of the optical gap. The photoinduced changes are long lasting at T<180 K. (ii) The charge carrier density increase is not the only effect of light illumination. We have observed an increase in the critical current density Jc (measurements were carried out in the pulse regime) of YBa2Cu3O6.9 films after continuous red light illumination. Such photoinduced changes are also long lasting if T<Tc and a current pulse amplitude do not exceed Jc.

Diel variations in the optical properties of a marine diatom

Diel variations in the optical properties of the marine diatom Thalassiosira p̀seudonana were examined in a nutrient‐replete laboratory culture exposed to a day‐night cycle in natural irradiance which simulated the light regime in upper layers of the euphotic zone (~10 m). Measurements of the beam attenuation coefficient, absorption coefficient, and size distribution of cells in suspension were used to determine optical cross‐sections and refractive index of cells. The optical cross‐sections varied greatly over a diel cycle; the absorption cross‐section (Xa) increased by as much as 35% and the attenuation (σ̄c) and scattering (σ̄b) cross‐sections by 80% between minima near sunrise and maxima near sunset. These changes were associated with significant variations in refractive index and cell size. Diel changes in the imaginary part of the refractive index (n'), driven by variation in intracellular pigment concentration, accounted for 58– 72% of the difference between the daily maxima and minima in σ̄a. Variations in the real part of the refractive index (n) contributed substantially (up to 85%) to the daily range in σ̄c and σ̄b. Increases in cell size were important during the morning, increasing the attenuation cross‐section to a similar extent as the refractive index. Variability in n correlated strongly with changes in intracellular C concentration. The normalization of attenuation and scattering cross‐sections to cellular C content resulted in a threefold reduction in the magnitude of diel variations. If such patterns are general features of planktonic microorganisms in the natural environment, it would suggest that varying particle concentration need not be the only (or even major) factor responsible for diel variability in the bulk optical properties of seawater. As a consequence, estimating particle production rates from diel variability in the beam attenuation coefficient may not be justified in some oceanic situations; however, using this variability to estimate production in terms of particulate organic C may be appropriate.

A k distribution method considering centres and wings of atmospheric absorption lines

The k distribution method is applied to band overlap and pressure as well as temperature dependence of absorption coefficients. The overlap of spectral absorption coefficients of H2O, CO2, O3, N2O, and CH4 is considered by a multivariate frequency distribution of absorption coefficients for a given pressure and temperature. In a first approach the vertical inhomogeneity is taken into account by the separation into the absorption coefficient at a reference state, the temperature dependence of the line strengths, and the dependence of the line profiles on pressure and temperature. If at distinct spectral positions absorption coefficients are changing from line wing absorption in the lower atmosphere to line centerlike absorption in the stratosphere, this is considered by a rough approximation. The known far wing scaling approximation is included, modified as special case. Comparison of infrared radiation flux densities in a standard atmosphere from this k distribution method with those from a line‐by‐line model leads to a difference about −1.8 W/m2 for the upward radiation flux density at the top of the atmosphere and −0.25 W/m2 for the downward directed at the surface. Cooling rate differences remain smaller than |0.1| K/d for the entire troposphere, but reach an intolerable 5 K/d at 50‐km height. In a second approach an extended version of the k distribution model is obtained by separating the spectral absorption coefficients into central and wing contributions. A multivariate frequency distribution which considers both strongly increases accuracy to less than 0.2% relative error for flux densities at all heights. And for cooling rates, a relative error of at most ±15% is found in the upper stratosphere.

Light transport in tissue

The propagation of light in tissue may be calculated by exact transport theory, or the approximate diffusion theory, provided the optical properties are known at the source wavelength. Optical properties for the exact methods are the absorption coefficient, scattering coefficient, and angular distribution of scattering. Appropriate properties for diffusion theory are the diffusion length and diffusion coefficient (corrected for anisotropic scattering). Computer programs and analytical solutions (for some simple geometries) exist, but the optical properties have to be determined experimentally and are not well defined as yet. The radiant energy fluence rate and the diffuse transmittance and reflectance have been measured in several tissues and in a few geometries, but there are gaps in the data as a function of wavelength. Calculations and measurements reveal that very large errors can result if the optical properties (for example, the diffusion length) are inaccurate, if anisotropic scattering is neglected, or if the finite size of the irradiating light beam is not taken into account. Furthermore, the radiant energy fluence and transmittance are perturbed by local regions of lesser or greater absorption, although recovery of the fluence and transmittance occurs beyond some three diffusion lengths.