Light Transport Phenomena Research Articles

Methodology for the fast direct estimation of spectral radiative transport properties in microalgae photobioreactors

Light distribution within photobioreactors is critical for the photosynthetic activity of microalgae in biotechnological processes. The radiative transport properties allow determining the radiant energy distribution, which is highly useful when designing, optimizing, and scaling photobioreactors. As radiation transport is strongly affected by process-dependent phenomena, simple, fast, and accurate in-situ estimation methodologies are of paramount importance. This paper presents a methodology for estimating microalgae cultures’ spectral radiative transport properties (i.e., the extinction coefficient, single-scattering albedo, and scattering phase function), considering measurements of incoming, transmitted, and dispersed energies by the microalgae culture within a modular spherical system. The experimental and numerical methods presented allow us to quickly and independently estimate the spectral radiative transport properties in a single setup. The methodology shows acceptable accuracy when estimating the spectral radiative transport properties of Chlorella vulgaris (400–700 nm). Thus, the proposed methodology has the potential for in-process monitoring and real-time analysis of light transport phenomena in photobioreactors.

Real‐time Deep Radiance Reconstruction from Imperfect Caches

AbstractReal‐time global illumination is a highly desirable yet challenging task in computer graphics. Existing works well solving this problem are mostly based on some kind of precomputed data (caches), while the final results depend significantly on the quality of the caches. In this paper, we propose a learning‐based pipeline that can reproduce a wide range of complex light transport phenomena, including high‐frequency glossy interreflection, at any viewpoint in real time (> 90 frames per‐second), using information from imperfect caches stored at the barycentre of every triangle in a 3D scene. These caches are generated at a precomputation stage by a physically‐based offline renderer at a low sampling rate (e.g., 32 samples per‐pixel) and a low image resolution (e.g., 64×16). At runtime, a deep radiance reconstruction method based on a dedicated neural network is then involved to reconstruct a high‐quality radiance map of full global illumination at any viewpoint from these imperfect caches, without introducing noise and aliasing artifacts. To further improve the reconstruction accuracy, a new feature fusion strategy is designed in the network to better exploit useful contents from cheap G‐buffers generated at runtime. The proposed framework ensures high‐quality rendering of images for moderate‐sized scenes with full global illumination effects, at the cost of reasonable precomputation time. We demonstrate the effectiveness and efficiency of the proposed pipeline by comparing it with alternative strategies, including real‐time path tracing and precomputed radiance transfer.

Interactive VPL-based global illumination on the GPU using fuzzy clustering

Physically-based synthesis of high quality imagery, including global illumination light transport phenomena, results in a significant workload, which makes interactive rendering a very challenging task. We propose a VPL-based ray tracing approach that runs entirely in the GPU and achieves interactive frame rates while handling global illumination light transport phenomena. This approach is based on clustering both shading points and VPLs and computing visibility only among clusters’ representatives. A new massively parallel K-means clustering algorithm, enables efficient execution in the GPU. Rendering artifacts, that could result from the piecewise constant approximation of the VPLs/shading points visibility function introduced by the clustering, are smoothed away by resorting to an innovative approach based on fuzzy clustering and weighted interpolation of the visibility function. The effectiveness of the proposed approach is experimentally verified for a collection of scenes, with frame rates larger than 3 fps and up to 25 fps being demonstrated.

Path-space differentiable rendering of participating media

Physics-based differentiable rendering---which focuses on estimating derivatives of radiometric detector responses with respect to arbitrary scene parameters---has a diverse array of applications from solving analysis-by-synthesis problems to training machine-learning pipelines incorporating forward-rendering processes. Unfortunately, existing general-purpose differentiable rendering techniques lack either the generality to handle volumetric light transport or the flexibility to devise Monte Carlo estimators capable of handling complex geometries and light transport effects. In this paper, we bridge this gap by showing how generalized path integrals can be differentiated with respect to arbitrary scene parameters. Specifically, we establish the mathematical formulation of generalized differential path integrals that capture both interfacial and volumetric light transport. Our formulation allows the development of advanced differentiable rendering algorithms capable of efficiently handling challenging geometric discontinuities and light transport phenomena such as volumetric caustics. We validate our method by comparing our derivative estimates to those generated using the finite differences. Further, to demonstrate the effectiveness of our technique, we compare both differentiable rendering and inverse rendering performance with state-of-the-art methods.

Visualization of Directional Beaming of Weakly Localized Raman from a Random Network of Silicon Nanowires.

Disordered optical media are an emerging class of materials that can strongly scatter light. These materials are useful to investigate light transport phenomena and for applications in imaging, sensing and energy storage. While coherent light can be generated using such materials, its directional emission is typically hampered by their strong scattering nature. Here, the authors directly image Rayleigh scattering, photoluminescence and weakly localized Raman light from a random network of silicon nanowires via real‐space microscopy and Fourier imaging. Direct imaging enables us to gain insight on the light transport mechanisms in the random material, to visualize its weak localization length and to demonstrate out‐of‐plane beaming of the scattered coherent Raman light. The direct visualization of coherent light beaming in such random networks of silicon nanowires offers novel opportunities for fundamental studies of light propagation in disordered media. It also opens venues for the development of next generation optical devices based on disordered structures, such as sensors, light sources, and optical switches.

Differentiable refraction-tracing for mesh reconstruction of transparent objects

Capturing the 3D geometry of transparent objects is a challenging task, ill-suited for general-purpose scanning and reconstruction techniques, since these cannot handle specular light transport phenomena. Existing state-of-the-art methods, designed specifically for this task, either involve a complex setup to reconstruct complete refractive ray paths, or leverage a data-driven approach based on synthetic training data. In either case, the reconstructed 3D models suffer from over-smoothing and loss of fine detail. This paper introduces a novel, high precision, 3D acquisition and reconstruction method for solid transparent objects. Using a static background with a coded pattern, we establish a mapping between the camera view rays and locations on the background. Differentiable tracing of refractive ray paths is then used to directly optimize a 3D mesh approximation of the object, while simultaneously ensuring silhouette consistency and smoothness. Extensive experiments and comparisons demonstrate the superior accuracy of our method.

Path-space differentiable rendering

Physics-based differentiable rendering, the estimation of derivatives of radiometric measures with respect to arbitrary scene parameters, has a diverse array of applications from solving analysis-by-synthesis problems to training machine learning pipelines incorporating forward rendering processes. Unfortunately, general-purpose differentiable rendering remains challenging due to the lack of efficient estimators as well as the need to identify and handle complex discontinuities such as visibility boundaries. In this paper, we show how path integrals can be differentiated with respect to arbitrary differentiable changes of a scene. We provide a detailed theoretical analysis of this process and establish new differentiable rendering formulations based on the resulting differential path integrals. Our path-space differentiable rendering formulation allows the design of new Monte Carlo estimators that offer significantly better efficiency than state-of-the-art methods in handling complex geometric discontinuities and light transport phenomena such as caustics. We validate our method by comparing our derivative estimates to those generated using the finite-difference method. To demonstrate the effectiveness of our technique, we compare inverse-rendering performance with a few state-of-the-art differentiable rendering methods.

A differential theory of radiative transfer

Physics-based differentiable rendering is the task of estimating the derivatives of radiometric measures with respect to scene parameters. The ability to compute these derivatives is necessary for enabling gradient-based optimization in a diverse array of applications: from solving analysis-by-synthesis problems to training machine learning pipelines incorporating forward rendering processes. Unfortunately, physics-based differentiable rendering remains challenging, due to the complex and typically nonlinear relation between pixel intensities and scene parameters. We introduce a differential theory of radiative transfer, which shows how individual components of the radiative transfer equation (RTE) can be differentiated with respect to arbitrary differentiable changes of a scene. Our theory encompasses the same generality as the standard RTE, allowing differentiation while accurately handling a large range of light transport phenomena such as volumetric absorption and scattering, anisotropic phase functions, and heterogeneity. To numerically estimate the derivatives given by our theory, we introduce an unbiased Monte Carlo estimator supporting arbitrary surface and volumetric configurations. Our technique differentiates path contributions symbolically and uses additional boundary integrals to capture geometric discontinuities such as visibility changes. We validate our method by comparing our derivative estimations to those generated using the finite-difference method. Furthermore, we use a few synthetic examples inspired by real-world applications in inverse rendering, non-line-of-sight (NLOS) and biomedical imaging, and design, to demonstrate the practical usefulness of our technique.

Path‐space Motion Estimation and Decomposition for Robust Animation Filtering

AbstractRenderings of animation sequences with physics‐based Monte Carlo light transport simulations are exceedingly costly to generate frame‐by‐frame, yet much of this computation is highly redundant due to the strong coherence in space, time and among samples. A promising approach pursued in prior work entails subsampling the sequence in space, time, and number of samples, followed by image‐based spatio‐temporal upsampling and denoising. These methods can provide significant performance gains, though major issues remain: firstly, in a multiple scattering simulation, the final pixel color is the composite of many different light transport phenomena, and this conflicting information causes artifacts in image‐based methods. Secondly, motion vectors are needed to establish correspondence between the pixels in different frames, but it is unclear how to obtain them for most kinds of light paths (e.g. an object seen through a curved glass panel).To reduce these ambiguities, we propose a general decomposition framework, where the final pixel color is separated into components corresponding to disjoint subsets of the space of light paths. Each component is accompanied by motion vectors and other auxiliary features such as reflectance and surface normals. The motion vectors of specular paths are computed using a temporal extension of manifold exploration and the remaining components use a specialized variant of optical flow. Our experiments show that this decomposition leads to significant improvements in three image‐based applications: denoising, spatial upsampling, and temporal interpolation.

Interplay of disorder andPTsymmetry in one-dimensional optical lattices

We study a one-dimensional binary optical lattice in the presence of diagonal disorder and alternating gain and loss, and examine the light transport phenomena for localized and extended input beams. In the pure $\mathcal{PT}$-symmetric case, we derive an exact expression for the behavior of light localization in terms of typical parameters of the system. Within the $\mathcal{PT}$-symmetric region light localization becomes constant as a function of the strength of the gain and loss parameter, but outside the $\mathcal{PT}$-symmetric window, light localization increases as the gain and loss parameter increases. When disorder is added, we observe that the presence of gain and loss inhibits (favors) the transport for localized (extended) excitations.

Scalable Realistic Rendering with Many‐Light Methods

AbstractRecent years have seen increasing attention and significant progress in many‐light rendering, a class of methods for efficient computation of global illumination. The many‐light formulation offers a unified mathematical framework for the problem reducing the full lighting transport simulation to the calculation of the direct illumination from many virtual light sources. These methods are unrivaled in their scalability: they are able to produce plausible images in a fraction of a second but also converge to the full solution over time. In this state‐of‐the‐art report, we give an easy‐to‐follow, introductory tutorial of the many‐light theory; provide a comprehensive, unified survey of the topic with a comparison of the main algorithms; discuss limitations regarding materials and light transport phenomena and present a vision to motivate and guide future research. We will cover both the fundamental concepts as well as improvements, extensions and applications of many‐light rendering.

Instant Caching for Interactive Global Illumination

Abstract The ability to interactively render dynamic scenes with global illumination is one of the main challenges in computer graphics. The improvement in performance of interactive ray tracing brought about by significant advances in hardware and careful exploitation of coherence has rendered the potential of interactive global illumination a reality. However, the simulation of complex light transport phenomena, such as diffuse interreflections, is still quite costly to compute in real time. In this paper we present a caching scheme, termed Instant Caching, based on a combination of irradiance caching and instant radiosity. By reutilising calculations from neighbouring computations this results in a speedup over previous instant radiosity‐based approaches. Additionally, temporal coherence is exploited by identifying which computations have been invalidated due to geometric transformations and updating only those paths. The exploitation of spatial and temporal coherence allows us to achieve superior frame rates for interactive global illumination within dynamic scenes, without any precomputation or quality loss when compared to previous methods; handling of lighting and material changes are also demonstrated.

Monte Carlo Modeling of the Light Transport in Polymer Light-Emitting Devices on Plastic Substrates

A Monte Carlo method for modeling the light transport phenomena in organic polymer light-emitting devices (PLEDs) has been reported previously (Badano and Kanicki, 2001). The advantage of this simulation method is its ability to model bulk absorption, thin-film coatings, and uneven or irregular surfaces by tracking the photon polarization in realistic device structures. We have applied this method to analyze the PLEDs spectral outputs and out-coupling efficiencies. We have established that the calculated out-coupling efficiencies are approximately the same (/spl eta//sub coup//spl sim/0.2) for the red and green PLEDs. Using a description of uneven surfaces with Fresnel analysis, we showed that the nonsmooth interfaces (as modeled by the algorithm for uneven surfaces) between light-emitting polymer and hole transporting layer increase the probabilities of out-coupling and wave-guiding of the internally generated light. In this paper, we use this method to calculate the angular distribution of the PLED light-emission. We found that the Monte Carlo simulated PLED light-emission angular distribution shows better agreement with the experimental data than previously used models relying on standard refraction theory at one interface.

Monte Carlo analysis of the spectral photon emission and extraction efficiency of organic light-emitting devices

We report on a Monte Carlo method for modeling light transport phenomena in multilayer organic polymer light-emitting devices on plastic flexible substrates. The method allows modeling of Cartesian geometrical structures describing the fate of photons through multiple scattering events determined by the wavelength-dependent material optical properties. We apply the method to analyze the wavelength distribution of emitted light spectra. We find that for all organic polymers considered, the light emission is slightly shifted toward the longer wavelengths, and that this shift is maximum for light emissions with peaks around 530 nm. The photon extraction efficiency is higher (0.430) for organic polymers emitting in the longer wavelengths, while the photon absorbed fraction is higher (0.676) for spectra with a maximum in the short wavelengths.

A novel compact Ge-BGO Compton-suppression spectrometer

A novel Compton-suppression spectrometer has been designed and its performance has been tested experimentally. The device consists of a single BGO crystal with a cylindrical well that holds a Ge crystal without interfering material between the crystals. Both crystals are cooled to an equilibrium temperature of 104 K which allows a proper performance of the Ge detector. The BGO crystal is read out by a single PM tube. Photodiodes have been applied to investigate the light yield and timing properties of BGO at low temperature. The light transport phenomena were well reproduced by computer simulations. The photopeak-to-total ratio amounts to 0.74 with an average suppression factor of 10.1 for a 137Cs source and 0.60 and 6.4, respectively, for a 60Co source. These results are well reproduced by Monte Carlo codes simulating the γ-ray interactions, also taking into account some charge-collection inefficiency.

Wavelength dependent light attenuation in organic scintillators

A dual beam photometer with a dye laser light source has been developed to study the strongly wavelength dependent light transport phenomena in organic scintillators. The device is capable of measuring spectral attenuation lengths between several centimetres and 100 m in the wavelength range of visible light. The accuracy of the measured intensity ratio was better than 3 × 10 −3. For mineral oil based scintillators the necessity of a wavelength dependent description of light transport in large detectors is demonstrated. A dramatic decrease in transparency caused by impurities and characteristic resonances is shown.