Diffuse Light Field Research Articles

Key functional role of the optical properties of coral skeletons in coral ecology and evolution.

Multiple scattering of light on coral skeleton enhances light absorption efficiency of coral symbionts and plays a key role in the regulation of their internal diffuse light field. To understand the dependence of this enhancement on skeleton meso- and macrostructure, we analysed the scattering abilities of naked coral skeletons for 74 Indo-Pacific species. Sensitive morphotypes to thermal and light stress, flat-extraplanate and branching corals, showed the most efficient structures, while massive-robust species were less efficient. The lowest light-enhancing scattering abilities were found for the most primitive colonial growth form: phaceloid. Accordingly, the development of highly efficient light-collecting structures versus the selection of less efficient but more robust holobionts to cope with light stress may constitute a trade-off in the evolution of modern symbiotic scleractinian corals, characterizing two successful adaptive solutions. The coincidence of the most important structural modifications with epitheca decline supports the importance of the enhancement of light transmission across coral skeleton in modern scleractinian diversification, and the central role of these symbioses in the design and optimization of coral skeleton. Furthermore, the same ability that lies at the heart of the success of symbiotic corals as coral-reef-builders can also explain the 'Achilles's heel' of these symbioses in a warming ocean.

Nonlinear atomic spectroscopy inside a random porous medium

We have studied the pump-probe spectroscopy of rubidium vapor confined to the micrometric interstices of a random porous glass. Due to the propagation in the highly scattering medium, the light fields are randomized inside the sample with significant consequences for the atomic spectra. A two-frequency modulation technique was used to isolate the atomic response proportional to the product of the intensities of the pump and probe fields. Unusual line shapes were observed which include relatively narrow structures that are present in spite of the Doppler broadening due to the atomic velocity distribution. A simple theoretical modeling of the light-atom interaction that assumes statistical isotropy of the diffuse light field and mutual temporal incoherence of the pump and probe fields is presented. Using a single adjustable parameter to account for the atomic confinement, the model successfully describes the diversity of the observed spectral line shapes.

Modeling the aerosol effects on the light field below a tubular-pipe: A case of clear sky conditions

The question we have postulated is well founded since the air pollution is now recognized as one of the most actual problems of human life in many regions, especially in highly industrialized and densely polluted urbanized areas. Aerosols scatter the light depending on their sizes, shapes, and compositions, so the scattering pattern of a particulate matter appears fairly greatest modulator of the ground reaching diffuse light. Undoubtedly the aerosol optics represents an essential portion of information in modeling the illuminance in the interior spaces. However, the effects of both the diffuse light and the direct sunbeams are difficult to separate experimentally. We analyze the optical behavior of diffuse light field numerically using theoretical models of (a) light scattering in the atmospheric environment (based on mSOS – modified Successive Orders of Scattering) and (b) light propagation through the cylindrical pipe (HOLIGILM). The form of aerosol scattering function is controlled by asymmetry parameter (ASY) which is recognized to be one of the most important factors influencing the luminous intensity solid (LIS) – i.e., the angular distribution of luminous energy at the light tube base. Along with solar zenith angle and aerosol optical depth (AOD), the ASY plays a crucial role in transforming the daylight to the illuminance patterns on a horizontal plane situated below the light guide. The bulk single scattering albedo of aerosol particles – i.e., the overall scattering efficiency of aerosols, has only less impact on LIS.

APC: A new code for Atmospheric Polarization Computations

A new polarized radiative transfer code Atmospheric Polarization Computations (APC) is described. The code is based on separation of the diffuse light field into anisotropic and smooth (regular) parts. The anisotropic part is computed analytically. The smooth regular part is computed numerically using the discrete ordinates method. Vertical stratification of the atmosphere, common types of bidirectional surface reflection and scattering by spherical particles or spheroids are included. A particular consideration is given to computation of the bidirectional polarization distribution function (BPDF) of the waved ocean surface.

On Link Misalignment for Underwater Wireless Optical Communications

In wireless optical communications, laser sources have narrow divergence angles and therefore require precisely aligned links, which may increase system complexity and is hard to implement in practice. In the underwater environment, however, the diffuse light field formed at the receiver side due to the scattering property of seawater may reduce the requirement of link alignment. We investigate the performance of link misalignment for underwater wireless optical communications (UWOC) and present a closed-form expression of relationship of transmit power, link range and receiver offset distance. Numerical results suggest that, regardless of water types, an appropriate receiver offset distance is permitted with specific transmit power and link range for reliable communications.

Modifications of discrete ordinate method for computations with high scattering anisotropy: Comparative analysis

A numerical accuracy analysis of the radiative transfer equation (RTE) solution based on separation of the diffuse light field into anisotropic and smooth parts is presented. The analysis uses three different algorithms based on the discrete ordinate method (DOM). Two methods, DOMAS and DOM2+, that do not use the truncation of the phase function, are compared against the TMS-method. DOMAS and DOM2+ use the Small-Angle Modification of RTE and the single scattering term, respectively, as anisotropic parts. The TMS method uses the Delta-M method for truncation of the phase function along with the single scattering correction. For reference, a standard discrete ordinate method, DOM, is also included in analysis. The obtained results for cases with high scattering anisotropy show that at low number of streams (16, 32) only DOMAS provides an accurate solution in the aureole area. Outside aureole, the convergence and accuracy of DOMAS, and TMS is found to be approximately similar: DOMAS was found more accurate in cases with coarse aerosol and liquid water cloud models, except low optical depth, while the TMS showed better results in case of ice cloud.

Pigmentation or transparency? Camouflage tactics in deep‐sea cephalopods

Pigmentation or transparency? Camouflage tactics in deep‐sea cephalopods

Isolating the singularities of a brightness field in a turbid medium on the basis of small-angle solutions of transfer theory

The paper suggests a new improved approach to calculating the anisotropic part of a diffuse light field in a turbid medium. We discuss some specific applications related to specific problems of radiative transfer theory, which demonstrate the accuracy and flexibility of this approach in practice.

Analysis of the radiative transfer equation with highly assymetric phase function

This paper considers a scalar radiative transfer problem with high scattering anisotropy. Two computational methods are presented based on decomposition of the diffuse light field into a regular and anisotropic part. The first algorithm (DOMAS) singles out the anisotropic radiance in the forward scattering peak using the Small-Angle Modification of RTE. The second algorithm (DOM2+) separates the single scattering radiance as an anisotropic part, which largely defines the fine detail of the total radiance in the backscattering directions. In both cases, the anisotropic part is represented analytically. With anisotropy subtraction, the regular part of the signal, which requires a numerical solution, is essentially smoothed as a function of angles. Further, the transport equation is obtained for the regular part that contains an additional source function from the anisotropic part of the signal. This equation is solved with the discrete ordinates method. A conducted numerical analysis of this work showed that algorithm DOMAS has a strong advantage as compared to the standard discrete ordinates method for simulation of the radiance transmission, and DOM2+ is the best of the three for the reflection computations. Both algorithms offer at least a factor of three acceleration of convergence of the azimuthal series for highly anisotropic phase functions.

Breakdown of the isotropy of diffuse radiation as a consequence of its diffraction at multidimensional regular structures

This article is devoted to the results of an experimental test of the theoretical assumption that the basic axiomatic postulate of statistical physics according to which it is equally probable for a closed system to reside in any of the microstates accessible to it may be invalid for nonergodic cases. In the course of photometric experiments for the purpose of recording the predicted loss of isotropy by a diffuse light field when it came into contact with a two-dimensional phase-type diffraction grating, a significant deviation from Lambert's law was detected when the diffuse photon gas was scattered by the grating surface. This caused angular anisotropy of the radiation fluxes to appear in the initially homogeneous light field. These results provide a basis for revising the determination of the most probable macrostate of a closed system.

Suppression of undesired diffraction orders of binary phase holograms

A method to remove undesired diffraction orders of computer-generated binary phase holograms is demonstrated. Normally, the reconstruction of binary Fourier holograms, made from just two phase levels, results in an undesired inverted image from the minus first diffraction order, which is superposed with the desired one. This can be avoided by reconstructing the hologram with a diffuse light field with a pseudorandom, but known, phase distribution, which is taken into account for the hologram computation. As a consequence, only the desired image is reconstructed, whereas all residual light is dispersed, propagating as a diffuse background wave. The method may be advantageous to employ ferroelectric spatial light modulators as holographic display devices, which can display only binary phase holograms, but which have the advantage of fast switching rates.

Tutorial on diffuse light transport

A tutorial introduction to diffuse light transport is presented. The basic analytic equations of time-resolved, steady-state and modulated light transport are introduced. The perturbation method for handling slight heterogeneities in optical properties is outlined. The treatment of boundary conditions such as an air/tissue surface is described. Finite mesh-based numerical methods are introduced to calculate the diffuse light field in complex tissues with arbitrary boundaries. Applications in tissue spectroscopy and imaging illustrate these theoretical and computational tools.

A backward Monte Carlo study of the multiple scattering of a polarized laser beam

A backward Monte Carlo estimator is developed to describe the multiple scattering of a polarized, narrow light beam by a plane-parallel medium. The case of a right circularly polarized beam is analyzed in this paper. Results indicate that the diffuse light field is partially polarized even at significant optical radii from the incident light beam. The degree of polarization of the diffuse light field is dependent on the optical thickness of the medium and the size parameter of the scatterers.

A backward Monte Carlo estimator for the multiple scattering of a narrow light beam

A backward Monte Carlo estimator is derived for the multiple scattering of a narrow light beam of arbitrary spatial distribution incident on a plane parallel layer. A means of obtaining an error estimate which provides an upper bound on the actual error is suggested. The intuitively clear result that the higher order intensities asymptotically approach each other for all optical radii was observed. It is observed that at reasonably modest optical distances from the beam, consideration of only the first or second orders of scatter in the evaluation of the diffuse light field will lead to erroneous results.

Laser Manipulation of Atoms and Atom Optics

In this paper experiments on laser cooling, collimation and manipulation of a sodium atomic beam, such as the transverse collimation and decollimation of an atomic beam by a standing wave or a misaligned standing wave, longitudinal cooling of an atomic beam by a diffuse light field, sub-Doppler cooling in a blue detuned standing wave, are reported. The basic concept on atom optics is developed. An experiment on a method for the injection of atoms into an atomic cavity is also discussed.

Absorption and scattering estimates from irradiance measurements: Monte Carlo simulations

A method of estimating the absorption (a), scattering (b), and backscattering (bb) coefficients of a homogeneous ocean based on near-surface measurements of the beam attenuation coefficient (c), the irradiance reflectance (R), and the attenuation coefficient of downwelling irradiance (Kd) is proposed, and it is tested with Monte Carlo simulations of radiative transfer in the ocean-atmosphere system. The technique utilizes the variation of R with solar zenith angle to estimate the value R would have if the sun were at zenith and the atmosphere were removed. This value of R is then combined with Kd to derive bb and the ratio of scattering to beam attenuation, from which a and b are obtained. In addition, the volume scattering function β(α) for scattering angles 60° a 150° is also estimated. Such estimates of bb and β(α), which are based on the diffuse light field existing over volumes of the order of 10–1,000 m3, are important in assessing the applicability of measurements made with classical scattering instruments (sampling volume ∼ 1 cm3) to predicting radiative transfer in the upper layers of the ocean. Direct application of the method is limited to situations in which the mixed layer is sufficiently deep and in which inelastic processes, e.g. Raman scattering and fluorescence, make negligible contributions to Kd and R near the sea surface.

Avoiding Ship-Induced Light-Field Perturbation in the Determination of Oceanic Optical Properties

THE DETERMINATION of in-water spectral irra- diance and the subsequent estimation of ocean optical properties is essential for the meaningful understand- ing and predictive modeling of the marine photoenvironment; this in turn is fundamental to the understanding of both biological and physical oceano- graphic processes. Further, the accurate estimation of near-surface optical properties is essential for the surface validation of ocean color satellite sensors. Two primary sources of error in the determination of optical prop- erties at sea are the perturbation of the in-water radiant energy field due to the ship (Gordon, 1985; Smith and Baker, 1986) and the estimation of quantitative cor- rections for atmospheric variability in radiant energy incident on the sea-surface during in-water measure- ments (Smith and Baker, 1984). Here we briefly describe two new optical oceanographic instrument and de- ployment techniques designed to reduce or eliminate these sources of error. The first is an Optical Free Fall Instrument (OFFI) designed to profile the watercolunm away from potential perturbation effects of the ship: the second is an Optical Surface Floating Instrument (OSFI) designed to obtain continuous optical data just below the air-water interface. A cartoon illustrating deployment of these instruments is shown in Fig. 1. Shipboard observations of spectral irradiance and radiance as a function of depth in the water column are commonly obtained by lowering an instrument on a hydrowire on the sunny side of the ship. Typically the instrument is lowered relatively close to the ship, which results in significant perturbation of the mea- sured in-water light field. Using Monte Carlo simula- tions, Gordon (1985) has shown that the bias in deter- mining downwelling irradiance seldom exceeds 1-2% for clear skies with the sun within + 45 ° of the beam of the ship, while the error can exceed 10% if the sun is off the bow or stem. While generally small, this error under clear skies is a function of wavelength. The error for overcast skies with a diffuse light field will be increased by the fraction of the sky obscured by the ship, after accounting for refraction and the Fresnel