Monte Carlo Ray Tracing Algorithm Research Articles

Effective emissivities of isothermal blackbody cavities calculated by the Monte Carlo method using the three-component bidirectional reflectance distribution function model

This paper proposes a three-component bidirectional reflectance distribution function (3C BRDF) model consisting of diffuse, quasi-specular, and glossy components for calculation of effective emissivities of blackbody cavities and then investigates the properties of the new reflection model. The particle swarm optimization method is applied for fitting a 3C BRDF model to measured BRDFs. The model is incorporated into the Monte Carlo ray-tracing algorithm for isothermal cavities. Finally, the paper compares the results obtained using the 3C model and the conventional specular-diffuse model of reflection.

A monostatic illumination function with surface reflections from one-dimensional rough surfaces

When solving scattering or emissivity problems for rough surfaces, the shadowing effect is often taken into account. Furthermore, for rough surfaces with large root mean square slope, surface reflections of the incidence or emission ray should not be neglected, especially at large observation angles. In this paper, a model of the monostatic statistical illumination function for one-dimensional rough surfaces with single surface reflection is developed, which is based on the Smith illumination function. A Monte Carlo ray-tracing algorithm is used to evaluate the accuracy of the present model. It is shown that, when neglecting the correlation between heights and slopes of the surface, the present model agrees quite well with the Monte Carlo result. Moreover, the result is improved if the correlation between heights and slopes is taken into account. For practical purposes, an empirical factor is introduced to improve the performance of the uncorrelated first-order illumination function to avoid computing the correlated one, which takes a long computation time. Besides, the first-order illumination function is significant at large observation angles, which could be promising to overcome problems in models of surface infrared emissivity where underestimation occurs compared with experimental measurements.

Modeling the role of microstructural parameters in radiative heat transfer through disordered fibrous media

Understanding the influence of microstructural parameters on the rate of heat transfer through a disordered fibrous medium is important for the design and development of heat insulation materials. In this work, by generating virtual 3-D geometries that resemble the internal microstructure of fibrous insulation materials, we simulated the influence of diameter, orientation, and emissivity of the fibers, as well as the media’s porosity and thickness on the radiative heat transmittance. Our simulations are based on a Monte Carlo ray tracing algorithm that we have developed for studying radiative heat flow in 3-D disordered media. The media were assumed to be made up of cylindrical opaque fibers with specular surface. The advantage of our modeling approach is that it does not require any empirical input values, and can directly be used to isolate and study the role of individual microstructural parameters of the media. The major limitation of the model is that it is accurate as long as the fibers can be considered large relative to the wavelength of the incoming rays. Our results indicate that heat flux through a fibrous medium decreases by increasing the packing fraction of the fibers when the thickness and fiber diameter are kept constant. Increasing the fibers’ absorptivity (or emissivity) was observed to decrease the radiation transmittance through the media. Our simulations also revealed that for constant porosity and thickness, the heat flux transmitted across the medium can be reduced by using finer fibers. The steady state temperature profiles across the thicknesses of media with different properties were obtained and found to be independent of the fibers’ emissivity.

Spatially multiplexed multi-input-multi-output optical imaging system in a turbid, turbulent atmosphere

Active optical imaging is preferred over radio frequency counterparts due to its higher resolution, faster area search rate, and relatively easier learning and interpretation of the image by a human observer. However, in imaging through atmosphere, one should consider dispersive effects of multiple scatterings and turbulence-induced wave perturbations, which give rise to intensity fluctuations and wavefront distortions. All these phenomena broaden and distort the spatial impulse response known as the point spread function (PSF). In this paper, a spatially multiplexed multi-input-multi-output imaging system design, inspired by multispot diffuse indoor communications configuration first introduced by Yun and Kavehrad [IEEE International Conference Selected Topics in Wireless Communications (IEEE, 1992), pp 262-265], is presented. At the transmitter, a computer-generated holographic beam splitter is used to generate arrays of beamlets, providing a faster area search rate and a uniformly distributed illumination over the entire target area. Then, at the receiver, an array of photodetectors is used to collect the reflected rays. While a Monte Carlo ray-tracing algorithm developed at Pennsylvania State University, Center for Information and Communications Research (CICTR), is used to model imaging in multiple-scattering turbid media, phase screens are employed to simulate turbulence-induced wavefront distortions. Hence, a comprehensive framework is exploited that takes into account possible sources of degradation. Using this framework, system performance is analyzed under different meteorological conditions. Restoration techniques such as adaptive-optics corrections, blind deconvolution, and time gating are used to improve the contrast and enhance the sharpness and resolution of the images.

Monte Carlo simulation of solar radiation in maize canopies and its visualisation

The spatial distribution of solar radiation casts important influences on eco‐physiological functions of plant canopies. A simulation model of the three‐dimensional (3D) effect of direct and indirect solar radiation in real maize canopies is developed from measured 3D canopy structure meshes. The model includes two procedures: parallel direct light pass and diffusive indirect radiation pass. The former one is based on the Monte Carlo ray tracing algorithm. After enough ray casting, the radiation is obtained. Then, the diffusive indirect radiation pass uses a modified Monte Carlo radiosity model to evaluate the diffusion distribution. Results indicate that simulated sun fleck ratio is significantly consistent with the measured dataset.

Laboratory measurements of spectral reflection from ice clouds of various habits

Spectral light from 550 to 650 nm reflected from the surface of an ice cloud produced in a temperature-controlled column is measured at seven different angles between 16.7 degrees and 29.9 degrees . Cloud optical depth (tau) is determined from the extinction of a 670 nm laser and is corrected for forward scattering using a Monte Carlo ray-tracing algorithm. Reflection measurements are compared to expectations from a plane-parallel radiative transfer model with input parameters based on the measured tau and a phase function for the observed ice crystal types. The plane-parallel radiative transfer model can be used to interpret the measured reflection for tau less than about 0.4 for this particular experiment, ideal for providing a validation data set to assist with the development of satellite bidirectional remote sensing.

Validity of Hybrid Models for the Bidirectional Reflectance of Coated Rough Surfaces

Modeling of the bidirectional reflectance of thin-film coatings on rough surfaces has been an important and challenging problem. In the past, there have been studies that use thin-film optics to take account of the coating-induced interference effects and the geometric optics approximation to model the surface roughness. In an effort to delineate the validity regime of the hybrid method, the predicted bidirectional reflectance is compared with that obtained from the rigorous electromagnetic-wave solution for one-dimensional rough surfaces, considering the effect of coating thickness and roughness parameters. Two hybrid models are implemented using the Monte Carlo ray-tracing algorithm: the surface generation method and microfacet slope method. The former relies on statistically generated surfaces according to the root-mean-square roughness and the autocorrelation length, whereas the latter uses the microfacet whose orientation is stochastically determined, when a ray hits the surface, according to the slope distribution without creating deterministic rough surfaces a priori. The validity regimes of the hybrid models are established for silicon dioxide films on silicon substrates. The advantages and disadvantages of each hybrid model are discussed. Some useful guidelines are provided for the simulation of optical scattering from coated rough surfaces.

Coherent backscattering effect for non-zero elements of Mueller matrix of discrete media at different illumination–observation geometries

The coherent backscattering effects in media composed of spherical particles with x=5 and m=1.5+0.5i and medium volume density ρ=0.05 are studied using a Monte-Carlo ray-tracing algorithm. All non-zero Mueller matrix elements are calculated as a function of phase angle α for different illumination–observation geometries both with and without the constructive interference of reciprocal rays. Four orders of scattering are taken into consideration. If the scattering plane includes the surface normal, only Mueller matrix elements S11, S12, S21, S22, S33, S34, S43 and S44 are non-zero. We find pronounced differences between cases when the coherent multiple scattering is and is not included with the incoherent multiple scattering for phase curves of S11 and the ratios S21/S11, S33/S11, and S44/S11. An anti-spike effect related to the coherent backscattering is found in S44/S11. When the angle of incidence is not perpendicular to the surface, the elements S12 are not zero, even at α = 0. For oblique-incident illumination, there may be sharp asymmetries in the phase-angle dependence of elements S11 and the ratio S21/S11 relative to α = 0; whereas, the ratios S22/S11, S33/S11, S34/S11, and S44/S11 all appear symmetric.

Concentrator and lens models for calculating the impulse response on IR‐wireless indoor channels using a ray‐tracing algorithm

AbstractThis paper presents concentrator and lens models for the detector and emitter, respectively, upon which a Monte Carlo ray‐tracing algorithm allows the evaluation of impulse response on infrared (IR) wireless indoor channels. We also present computer simulation results that show the effects of concentrator FOV and reception direction on the impulse response, received optical power, and channel rms delay spread, which allow us to propose structures for angle‐diversity receivers. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 36: 262–267, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10738

Error estimation of the impulse response on diffuse wireless infrared indoor channels using a Monte Carlo ray-tracing algorithm

The paper describes a method to determine the error in two Monte Carlo based ray-tracing algorithms used to compute the impulse response on infrared (IR) wireless indoor channels. The equations for calculation of this error are given and several results concerning the error estimation are reported, which verify the reliability of the equations.