Diffuse Reflection Model Research Articles

Frequency analysis of the non-principal-axis rotation of uniaxial space debris in circular orbit subjected to gravity-gradient torque

The non-principal-axis rotational motion of uniaxial space debris can be decomposed into periodic motions associated with two frequencies: the polhode frequency of the space debris rotating around the symmetry axis, and the tumbling frequency of the symmetry axis rotating around the angular momentum. To determine from optical measurements the rotational motion of upper rocket stages in circular orbits subjected to gravity-gradient torque, the evolutions of these two frequencies need to be analyzed. Taking into account only the long-term changes in the long-period variables, the differential equations of the non-principal axis rotational motion of the uniaxial space debris are averaged and reduced, from which the evolutions of the polhode and tumbling frequencies are then obtained analytically. The theoretical results are verified by numerical simulations of the diffuse reflection model. The frequencies in the variation of the reflected light intensity in the simulation are analyzed using the frequency map analysis (FMA) method. Errors of these results are found to be less than 1%. Based on the theoretical expressions, the rotational state of the uniaxial space debris can be estimated in the simulation without any prior information except the orbital parameters. A series of state variables are estimated, including the ratio of the moments of inertia about the transverse axis and the symmetry axis, the instantaneous rotation velocity, the orientation of the angular momentum, and the precession cone of the symmetry axis.

Performance Analysis of Indoor Diffuse VLC MIMO Channels Using Angular Diversity Detectors

We consider specular and diffuse reflection models for indoor visible light communications using a mobile receiver with angular diversity detectors in multiple input multiple output (MIMO) channels. We aim to improve the MIMO throughput compared to vertically oriented detectors by exploiting multipath reflections from different surfaces in the room. We then evaluate data throughput across multiple locations in the small room by using repetition coding, spatial multiplexing, and spatial modulation approaches. In spatial modulation, we also propose a novel approach called adaptive spatial modulation. This makes use of channel matrix rank information to decide which TX/RX setup to be used, and is developed to cope with rank deficient channels. In a scenario, where the receiver is moving, channel gains are weak in some locations due to the lack of line of sight (LOS) propagation between transmitters and receivers. This effect is mitigated by employing adaptive modulation and coding together with per antenna rate control. We then compare the throughput for LOS only channels against LOS with specular or diffuse reflection conditions, for both vertical and angular oriented receivers. The results show that exploiting specular and diffuse reflections provide significant improvements in link performance.

Implementation of the swept source optical coherence tomography (SSOCT) system with a statistical method to analyze optical properties of turbid media

We present a spectroscopic OCT (sOCT) system to extract spectroscopic properties of turbid liquid media using principle component analysis (PCA) and swept source optical coherence tomography (SSOCT). Extracted results are well comparable with the standard photo spectrometer technique. This system has potential for real time measurement of blood oxygen saturation and monitoring of relative concentration for other metabolic compounds inside human body. Liquid media was prepared with ethanol (10% v/v), citric acid (5% v/v) and water. A linear mathematical model for depth resolved diffuse reflectance was build from sOCT signal. Then PCA was applied on resultant model to extract absorption coefficients of different constituents of the media. Extracted information is well comparable with the standard photo spectrometer technique. This system has potential to monitor relative concentration change of metabolic compounds inside any system.

Influence of earth's reflective radiation on space target for space based imaging

The space-based surveillance, which would mainly use the space-based visible, has great value for civil and military applications currently and for a fairly long future period. In space-based surveillance, the visible and near-infrared radiation characteristics of the space target are influenced by its attitude variation. This influence is especially prominent in space-based imaging. In some ways, solar radiation cannot arrive at the surface of the space target, or the arriving radiation is not uniformly distributed because of the space target's strong reflection at a particular position. In order to solve these problems, the visible and near-infrared illumination characteristics of the space target surface are analyzed. Moreover, a notion that earth's reflective radiation can be used as illumination for space target imaging is given, and an accurate modeling method is proposed. Firstly, based on diffuse reflectance model, a method of mathematically calculating the illumination at space target's position from earth's reflective radiation is established. And a formula for calculating illumination is derived. Secondly, the coordinates of sun and space target at any time can be obtained by the Satellite tool kit software, in which the complicated multiplying matrix and coordinate transformation algorithm introduced in some references are avoided. Thirdly, the method of estimating earth's reflective radiation region at arbitrary moment is introduced. The grid division method is generated and the uniform sampling is used in each small area. Fourthly, the position of a surface cell is transformed from the sphere reference frame into the J2000.0 inertial frame. The earth's reflective radiation can be calculated through numerical integration. Finally, the illumination from earth's reflective radiation to a sun synchronous orbit satellite in an imaging mission based on space is calculated by the given parameters. The results show that the earth's reflective radiation is luminous enough for space target imaging when the satellite passes through arctic. When the satellite moves on the orbit, we can obtain more detailed information about target satellites' bottom then the ground simulation imaging. The on-orbit imaging results demonstrate the validity of the modeling method, which could support the foundation of our space-based surveillance system theoretically and technically and could be used as a reference of space-based orbit measurement and determination in deep space exploration.

Influence of the phase function in generalized diffuse reflectance models: review of current formalisms and novel observations.

Diffuse reflectance spectroscopy, which has been demonstrated as a noninvasive diagnostic technique, relies on quantitative models for extracting optical property values from turbid media, such as biological tissues. We review and compare reflectance models that have been published, and we test similar models over a much wider range of measurement parameters than previously published, with specific focus on the effects of the scattering phase function and the source-detector distance. It has previously been shown that the dependence of a forward reflectance model on the scattering phase function can be described more accurately using a variable, γ, which is a more predictive variable for reflectance than the traditional anisotropy factor, g. We show that variations in the reflectance model due to the phase function are strongly dependent on the source-detector separation, and we identify a dimensionless scattering distance at which reflectance is insensitive to the phase function. Further, we evaluate how variations in the phase function and source-detector separation affect the accuracy of inverse property extraction. By simultaneously fitting two or more reflectance spectra, measured at different source-detector separations, we also demonstrate that an estimate of γ can be extracted, in addition to the reduced scattering and absorption coefficients.

A Physically‐Based BSDF for Modeling the Appearance of Paper

AbstractWe present a novel appearance model for paper. Based on our appearance measurements for matte and glossy paper, we find that paper exhibits a combination of subsurface scattering, specular reflection, retroreflection, and surface sheen. Classic microfacet and simple diffuse reflection models cannot simulate the double‐sided appearance of a thin layer. Our novel BSDF model matches our measurements for paper and accounts for both reflection and transmission properties. At the core of the BSDF model is a method for converting a multi‐layer subsurface scattering model (BSSRDF) into a BSDF, which allows us to retain physically‐based absorption and scattering parameters obtained from the measurements. We also introduce a method for computing the amount of light available for subsurface scattering due to transmission through a rough dielectric surface. Our final model accounts for multiple scattering, single scattering, and surface reflection and is capable of rendering paper with varying levels of roughness and glossiness on both sides.

Design on Diffuse Optical Instrument Based on Computer Simulation Technology

Using new science and technology, art design embodies the times characteristics and the rich connotation based on visual communication. With the progress of science & technology and crossing with other areas, this field has gradually formed a new one, which has association and interaction with other visual media. Based on the times characteristics of the art design, we analyze the current mainstream of 3D design, and analyze the features of space rendering integration. On the basis of this, we further explore the translation transform and rotation transformation principle of 3D design, and construct diffuse reflectance model based on design flow of 3D image. We use computer to verify the performance of the teaching practice and education stability, it has strong operability.

A Research on the Imaging Strategy and Imaging Simulation of Toutatis in the Chang’e-2 Flyby Mission

For the Chang’e-2 extended mission of asteroid exploration, the illumination conditions for imaging the asteroid Toutatis are calculated in this paper according to the orbital parameters of both the Chang’e-2 detector and the asteroid, as well as the incident angles of sunlight. On this basis, it is suggested to take photographs after flyby, and the orientation of the camera's optical axis in the coordinate system deflned by Earth's mean equator and equinox at J2000.0 is proposed to be (118.02°, 22.03°). Based on the shape model of Toutatis determined by the foreign radar data, the orientation of the asteroid in the inertial space is calculated at the rendezvous time. Using the Oren-Nayar diffuse-reflection model and the relative positions among the sun, the asteroid, and the detector, together with the cameras orientation, the imaging simulations are performed on the starry sky background respectively at the distances of 300km, 500km, and 1000km from the asteroid after flyby. The results of simulations are verified further by the optical images of Toutatis obtained in the mission.

Aerodynamic Analysis Based on Challenging Minisatellite Payload Satellite Lift-to-Drag Measurements

A lift-to-drag ratio analysis is performed based on accelerometer measurements made by the Challenging Minisatellite Payload satellite to investigate the nature of gas-surface interactions taking place in orbit. The results described are for the November 2002 satellite yaw maneuver. At this time, the satellite flew at a mean altitude of 407 km. Two types of gas–surface interaction are considered in the analysis. The first is a cosine (fully diffuse) reflection model with incomplete energy accommodation. The second is a blended cosine reflection model having full accommodation combined with a fraction of quasi-specular reflection having an accommodation coefficient dependent on the surface material. It is found that the fully diffuse model best reproduces the satellite data during the yaw maneuver, and that the accommodation coefficient is approximately 0.89 during this time.

Phase-function corrected diffusion model for diffuse reflectance of a pencil beam obliquely incident on a semi-infinite turbid medium

Oblique incidence reflectometry (OIR) is an established technique for the estimation of tissue optical properties. However, a sensing footprint of a few transport mean-free paths is often needed when diffusion-regime-based algorithms are used. Smaller-footprint probes require improved light-propagation models and inversion schemes for diffuse reflectance close to the point of entry but might enable micro-endoscopic form factors for clinical assessments of cancers and precancers. The phase-function corrected diffusion theory presented by Vitkin et al. [Nat. Commun. 2, 587 (2011)] to the case of pencil beams obliquely incident on a semi-infinite turbid medium is extended. The model requires minimal computational resources and offers improved accuracy over more traditional diffusion-theory approximations models when validated against Monte Carlo simulations. The computationally efficient nature of the models lends itself to rapid fitting procedures for inverse problems. The analytical model is used in a nonlinear fitting algorithm to demonstrate the recovery of optical properties using a measurement footprint that is significantly smaller than needed in previous diffusion-regime OIR methods.

Diffuse reflection diameter and radius for convex-quadrilateralizable polygons

In this paper we study the diffuse reflection diameter and diffuse reflection radius problems for convex-quadrilateralizable polygons. In the usual model of diffuse reflection, a light ray incident at a point on the reflecting surface is reflected from that point in all possible inward directions. A ray reflected from a polygonal edge may graze that reflecting edge but an incident ray cannot graze the reflecting edge. The diffuse reflection diameter of a simple polygon P is the minimum number of diffuse reflections that may be needed in the worst case to illuminate any target point t from any point light source s inside P. We show that the diameter is upper bounded by 3n−104 in our usual model of diffuse reflection for convex-quadrilateralizable polygons. To the best of our knowledge, this is the first upper bound on diffuse reflection diameter within a fraction of n for such a class of polygons. We also show that the diffuse reflection radius of a convex-quadrilateralizable simple polygon with n vertices is at most 3n−108 under the usual model of diffuse reflection. In other words, there exists a point inside such a polygon from which 3n−108usual diffuse reflections are always sufficient to illuminate the entire polygon. In order to establish these bounds for the usual model, we first show that the diameter and radius are n−42 and ⌊n−44⌋ respectively, for the same class of polygons for a relaxed model of diffuse reflections; in the relaxed model an incident ray is permitted to graze a reflecting edge before turning and reflecting off the same edge at any interior point on that edge. We also show that the worst-case diameter and radius lower bounds of n−42 and ⌊n−44⌋ respectively, are sometimes attained in the usual model, as well as in the relaxed model of diffuse reflection.

Analytical solution of the poiseuille flow problem using the ellipsoidal-statistical model of the Boltzmann kinetic equation

An analytical solution (in the form of a Neumann series) of the problem of rarefied gas flow in a plane channel with infinite walls in the presence of a pressure gradient (Poiseuille flow) parallel to them is constructed within the framework of the kinetic approach in an isothermal approximation. The ellipsoidal-statistical model of the Boltzmann kinetic equation and the diffuse reflection model are used as the basic equation and the boundary condition, respectively. Using the resulting distribution function, the mass and heat flux densities in the direction of the pressure gradient per unit channel length in the y′ direction are calculated, and profiles of the gas mass velocity and heat flux in the channel are constructed. The results obtained for the continuum and free-molecular flow models are analyzed and compared with similar results obtained by numerical methods.

Influence of the Prandtl and Knudsen numbers on heat-transfer process in the problem of planar Poiseuille flow

The analytic solution (in the form of the Neumann series) has been derived for the problem of computing the heat flux in a planar channel in the presence of a pressure gradient parallel with the walls (in the problem of planar Poiseuille flow) within the framework of the kinetic approach for arbitrary values of the Prandtl number. The ellipsoidal-statistical model of the Boltzmann kinetic equation is used as the governing equation, and the model of diffuse reflection is used as the boundary condition. The conducted numerical analysis of final expressions obtained in the present work showed a substantial dependence of the heat flux on the value of the Prandtl number of gas for channels whose thickness is comparable with the mean free path of gas molecules.

Scattering and absorptive characteristics of a cenosphere

The scattering and absorptive characteristics of the cenosphere to monochromatic light were studied by Monte Carlo ray tracing method. The reflection, refraction and absorption model of the cenosphere were built by geometrical optical approximation under specular and diffuse reflection assumptions. Validation of the model was made by comparing absorptive data from ray tracing model and that from Mie theory, and results show that when κ < 0.01 and χ > 80, the difference between these two models is no more than 5%. The influences of the size parameter χ and optical constants m on the radiation characteristics were studied and discussed, and the scattering phase function of the cenosphere under both reflection models were calculated and plotted. The radiation characteristics of the cenosphere are related to the reflection model at the interface. Under specular reflection model, the scattering of the cenosphere is preferentially forward, while the proportion of backward scattering under diffuse reflection model is much larger. Under specular reflection, the scattering albedo rises monotonically with the increase of n. And as the particle becomes larger, the peak of the internal volumetric absorption gradually moves from inner part of the medium to outer layer. Under diffuse reflection model, the scattering albedo changes non-monotonically with the variation of n and the distribution of the volumetric internal absorption is non-monotonic with the minimum value locating in the middle part of the medium.

Modeling the diffuse reflectance due to a narrow beam incident on a turbid medium

We present a model for the diffuse reflectance when a continuous beam is incident normally on a half space composed of a uniform scattering and absorbing medium. This model is the result of an asymptotic analysis of the radiative transport equation for strong scattering, weak absorption, and a narrow beam width. Through comparison with the diffuse reflectance computed using the numerical solution of the radiative transport equation, we show that this diffuse reflectance model gives results that are accurate for small source--detector separation distances.

Modeling diffuse reflectance from homogeneous semi-infinite turbid media for biological tissue applications: a Monte Carlo study

Diffuse reflectance spectroscopy is one of the simplest and widely used techniques for the non-invasive study of biological tissues but no exact analytical solution exists for the problem of diffuse reflectance from turbid media such as biological tissues. In this work, a general treatment of the problem of diffuse reflectance from a homogeneous semi-infinite turbid medium is presented using Monte Carlo simulations. Based on the results of the Monte Carlo method, simple semi-empirical analytical solutions are developed valid for a wide range of collection geometries corresponding to various optical detector diameters. This approach may be useful for the quick and accurate modeling of diffuse reflectance from tissues.

Self-calibrated, Multi-spectral Photometric Stereo for 3D Face Capture

This paper addresses the problem of obtaining 3d detailed reconstructions of human faces in real-time and with inexpensive hardware. We present an algorithm based on a monocular multi-spectral photometric-stereo setup. This system is known to capture high-detailed deforming 3d surfaces at high frame rates and without having to use any expensive hardware or synchronized light stage. However, the main challenge of such a setup is the calibration stage, which depends on the lights setup and how they interact with the specific material being captured, in this case, human faces. For this purpose we develop a self-calibration technique where the person being captured is asked to perform a rigid motion in front of the camera, maintaining a neutral expression. Rigidity constrains are then used to compute the head’s motion with a structure-from-motion algorithm. Once the motion is obtained, a multi-view stereo algorithm reconstructs a coarse 3d model of the face. This coarse model is then used to estimate the lighting parameters with a stratified approach: In the first step we use a RANSAC search to identify purely diffuse points on the face and to simultaneously estimate this diffuse reflectance model. In the second step we apply non-linear optimization to fit a non-Lambertian reflectance model to the outliers of the previous step. The calibration procedure is validated with synthetic and real data.

Analytic solution to the problem of the couette flow in a plane channel with infinitely large parallel walls

An analytic (in the form of a Neuman series) solution to the problem of the Couette flow in a plane channel with infinitely large parallel walls is constructed using the kinetic approach in the isothermal approximation. For the basic equation, the Bhatnagar-Gross-Krook (BGK) model of the kinetic Boltzmann equation is used, while the boundary condition is determined by the diffuse reflection model. The mass flux through half the channel thickness in the direction parallel to the channel walls as well as the nonzero component of the viscous stress tensor are calculated taking into account the constructed distribution function. The results are compared with analogous data obtained by numerical methods.

A simplified two layer model for light diffuse reflectance in thin skin fruits

A simple model of light diffuse reflectance in thin skin fruits is presented. The model is based in a semi-infinite two-layer geometry (skin and flesh) and introduces simplifications due to the fact that the first layer is very thin. The flesh is described in terms of the usual reduced scattering and absorption coefficients µs′ and µa but the skin is modeled by simple reflection (R), absorption (A) and transmission (T) coefficients. The problem of diffuse reflectance is thus reduced to the problem of determining the four constants µs′, µa, R and T. It is shown that the problem can be solved by using absolute reflectance and spatially resolved reflectance measurements simultaneously. The coefficients µs′ and µa are determined by the usual fit of the diffuse reflectance profile (photons re-emitted from the flesh far from the incidence point) to the diffusion approximation. The coefficients R and T are determined by a second fit involving the total reflectance profile (photons reemitted both from skin and flesh), the incident beam profile and the diffuse reflectance profile calculated according to the previously determined values of µs′ and µa. The anisotropy of the light re-emitted by the fruit can also be roughly compared with the Lambertian expected behavior. In order to test the model we have followed a population of 22 ‘Rocha’ pears along a period of two weeks. We have then performed the population averages along the time and checked the plausibility of the values obtained for µs′, µa, R and T according to the expected fruit physiological changes along ripening. The results show that all parameters are physically acceptable and evolve in time according to the expected fruit ripening physiology.

Effect of boundary conditions on the performances of gas-lubricated micro journal bearing

As significant components of micromechanics, gas-lubricated microbearings are more prevalent for their special advantages than other types. The fluid dynamics of the microbearing is different from their larger cousins due to the noncontinuum effect and surface-dominated effect, which may make the Navier-Stokes equations invalid. In this paper, by considering the accommodation coefficients on journal (α i) and that on bearing (α o) separately, the microbearings with different bearing numbers under the assumption of large L/D (length to diameter) are simulated using direct simulation Monte Carlo (DSMC) program incorporated with a Volume-CAD software. The diffuse reflection model and Cercignani-Lampis-Lord (CLL) model are applied to model the molecule-surface interaction. The flow field characteristics, as well as the performances of gas-lubricated journal bearings including load-carrying capacity, attitude angle and bearing drag are obtained. The results reveal that αi and αo have different effects to flow field characteristics and bearing performances. The bearing number has significantly impact on the bearing performances. The method developed in this paper would be very useful for designing and evaluating the gas-lubricated journal microbearing.