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Monte Carlo integration is an established method in simulating light transport. However, due to its stochastic nature, it requires copious amounts of samples to eliminate the estimation error, and variance reduction techniques such as importance sampling are used to improve the convergence speed as a remedy. Path guiding is a class of adaptive importance sampling methods devised specifically for light transport simulation, which demonstrates significant improvements over classical sampling techniques. Yet most of the previous path guiding methods only account for the radiance term, omitting the bidirectional scattering distribution function (BSDF) term. This results in suboptimal sample quality for non-diffuse light transport. This paper presents a path guiding method which guides paths according to the full product of the BSDF and radiance terms in light transport equation. Extending a spatio-directional tree based path guiding method, we use a semi-adaptive grid-quadtree hybrid subdividing the directional domain in a spatial subdomain to learn and represent the radiance field. With the help of BSDF lookup tables used to accelerate BSDF evaluations, this grid-quadtree allows us to efficiently construct approximate product distributions and guide paths according to the product of incident radiance and cosine-weighted BSDF at each path vertex. The resulting method is relatively simple to implement and enables more robust sampling on scenes with many glossy surfaces.

Aiming to explore its performance as energy efficient system, in this study, the window is subdivided according to functions, in which the upper part would act as a daylight optimization unit. Two variant positions of lasercut panel, known as ‘louvre-style’ and ‘venetian-style’ are placed in the upper window unit, in order to take advantage of their angle selective redirecting capabilities. Their effectiveness to provide a satisfactory visual comfort is evaluated by assessing their potential for solar control and daylight redirection indoors. The first results indicate that the use of lasercut panel as a sole system, its insufficient to provide an adequate visual environment, with a reduced risk of glare for the occupants. Aiming to improve its performance, horizontal blinds are added as supplementary element to the interior-upper part of the window, blended in an arrangement called ‘secondary system’. As a result of a comprehensive evaluation, in which, the upper and middle window units are considered, two lasercut panel styles are identified as those providing sufficient daylight provision indoors, and a reduced risk of discomfort glare. The properties of the fenestration systems tested here, were described using their corresponding BSDF data, while the evaluation was performed using the RADIANCE matrix multiplication methods.

The cheapest method for an instrument to perform radiometric monitoring in orbit is to compare its radiometric response from a scene to the known radiance of that same scene. This is known as vicarious calibration. The known radiance of this scene comes mostly from other space instruments. The limiting factors of this vicarious calibration approach arise from differences in the acquisition time and illumination/viewing geometry between the two measurements. Earth scenes may change over time, which limits vicarious calibration to quasi stable scenes. The level of stability of these scenes limits the level of accuracy that can be achieved. Likewise, the bi-directional scattering distribution function (BSDF) of the observed scene is likely to cause differences in observed radiance if the illumination and/or viewing geometry changes. If the observation of the scene is at the same time, stability of the scene is no longer an issue, and if the observation is at the same geometry then BSDF effects will cancel, and direct comparison is possible. This is rarely the case unless the instrument is on the same satellite. In this paper we present the design and measurement concept of such a small, on-board calibration instrument; the Absolute Radiometric Reference Instrument (ARRI). We believe this concept will revolutionize the approach to in-orbit absolute reflectance calibration.

The objects with microroughness surfaces, heterogeneous media are widely used in modern optical devices, for instance in various light guiding devices, car dashboards and other illuminating systems as elements of scenes aimed to generate images with photorealistic quality. Surface or volume scattering of these objects can have very complex shape with pronounced picks, far from ideal specular or ideal diffuse ones and typically described with Bi-Directional Scattering Functions (BSDF). If model reconstructing BSDF measuring device contains inaccurateness, it can result in errors of parameters reconstruction for scattering objects and serious damage for entire optical simulation for scattering objects. In simulation model it is very preferable simulate optical parts maximally close to real measuring device. Another problem is calculation speed of model aimed for BSDF simulation. BSDF measurement devices have very high angular and spatial resolutions that results in a very ineffective from viewpoint of simulation. Moreover, BSDF functions is multidimensional, so many calculations are required to fulfill. These problems impose careful choosing of simulation model, its reasonable simplifications, selection of proper ray tracing engine. In the paper the virtual prototyping of different industrial measurement devices is considered. These devices are quite different and thus require different approaches in computer models design. The results of virtual prototyping for several samples with complex scattering properties are presented as in numerical shape of angular intensity distribution as well in qualitative as photorealistic images. All virtual prototyping simulated results are compared with measurements to prove reliability and reasonability of built models.

International audience; A statistical multi-lobe approach was recently introduced in order to efficiently handle layered materials rendering as an alternative to expensive general-purpose approaches. However, this approach poorly supports scattering volumes as the method does not account for back-scattering and resorts to single scattering approximations. In this paper, we address these limitations with an efficient solution based upon a transfer matrix approach which leverages the properties of the Henyey-Greenstein phase function. Under this formalism, each scattering component of the stack is described through a lightweight matrix, layering operations are reduced to simple matrix products and the statistics of each BSDF lobe accounting for multiple scattering effects are obtained through matrix operators. Based on this representation, we leverage the versatility of the transfer matrix approach to efficiently handle forward and backward scattering which occurs in arbitrary layered materials. The resulting model enables the reproduction of a wide range of layered structures embedding scattering volumes of arbitrary depth, in constant computation time and with low variance.

Data-driven models replicate the irregular Bidirectional Scattering Distribution Functions (BSDFs) of optically Complex Fenestration Systems in daylight simulation. RADIANCE employs the ten...

Microfacet distributions are considered nowadays as a reference for physically plausible BSDF representations. Many authors have focused on their physical and mathematical correctness, while introducing means to enlarge the range of possible appearances. This article is dedicated to Normal Distribution Functions (NDFs) and the influence of their shape on the rendered material aspect. We provide a complete framework for studying the impact of NDFs on the observed Bidirectional Scattering Distribution Functions (BSDFs). To explore very general NDFs, manually controlled by the user, and including anisotropic materials, we propose to use a piecewise continuous representation. It is derived with its associated Smith shadowing-masking function and importance sampling formulations for ensuring efficient global illumination computations. A new procedure is also proposed in this article for generating an explicit geometric micro-surface, used to evaluate the validity of analytic models and multiple scattering effects. The results are produced with a computer-generated process using path tracing. They show that this generation procedure is suitable with any NDF model, independently from its shape complexity.

The pattern and permeability characteristics that compose three-dimensional (3D) textiles are important as a textile shading component to the architecture, engineering and construction (AEC) industry. Moreover, due to their complex geometry, 3D fabrics require a detailed modeling for correct evaluation of performance. This paper aims to present and investigate the optical properties of six 3D fabrics proposed as an alternative to traditional roller blind shading systems, which could guarantee high daylight availability and visual comfort. The light transmission distribution for the different samples has been characterized by carrying out angular visual transmittance measurements, using integrating sphere optical benches, and virtual modeling using Radiance's genBSDF tool. The Bidirectional Scattering Distribution Function (BSDF) calculated with Radiance shows a good fit with the optical angular measurements, which were used as a benchmark and to calibrate models. The performance of the 3D textiles, in terms of illuminance, daylight availability, and glare, was assessed considering two of the set of 3D textiles as alternative materials for a movable internal roller shade system in an open office. The innovative system was compared with a traditional single layer fabric roller blind under different shading control strategies, 3D fabrics demonstrate the potential of increasing Daylight Autonomy while providing better uniformity and better connection with the outdoors due to a higher openness factor. The shading effect created by the 3D geometry of the textile can increase the openness factor while restricting glare.

Straylight requirements for optical instruments become ever more stringent. This applies not only to optical instruments in space, but also extends to optical ground support equipment (OGSE) and any other instrument that is sensitive to straylight. One of the means to control the straylight within instruments and OGSE’s is the application of black coatings, e.g. on optical mounts, lens barrels and baffles. There are different types of black coatings on the market. The choice of a suitable black coating does not only depend on its optical properties, but also on the compatibility with the substrate material and its geometry. For many coatings, the manufacturers provide only a hemispherical reflectivity value, which is not very suitable for straylight predictions, as it does not include the angle dependent scattering behavior. This information is provided by the Bi-directional Scatter Distribution Function (BSDF). The optics and opto-electronics laboratories (OOEL) at ESTEC receive many requests to characterize black coatings by measuring the BSDF. The OOEL has created a database for black coatings to collect the BSDF data in support to space industry to facilitate the selection of the most suitable black coating and make more accurate predictions of straylight at instrument level. The database, called ‘Black Coatings BSDF Database’, will be presented here to showcase its functionalities. We will present some examples of BSDF data to demonstrate the added value over hemispherical reflectivity. The BSDF data contained within the Black Coatings BSDF Database is structured according to coating type, measured wavelength and, if that information was available, according to specific information on the coating and/or substrate. So far, all BSDF measurements data has been measured at the OOEL using the two currently available CASI and Albatross TT scatterometers. Additional data will be added in the future. The European space industry is invited to contribute to the Black Coatings BSDF Database by either providing a sample to be characterized or contribute with already measured BSDF data. The Black Coatings BSDF database will be a database created for and by the European space industry. Access to the database is free and can be requested by contacting the OOEL via: ooel@esa.int.

A Bidirectional Scattering Distribution Function (BSDF) describes how light from each incident direction is scattered (reflected and transmitted) by a simple or composite surface, such as a window shade. Compact, tabular BSDFs may be derived via interpolation, discretization and/or compression from goniophotometer measurements. These data-driven BSDFs can represent any measurable distribution to the limits of their tabulated resolution, making them more general than parametric or analytical BSDFs, which are restricted to a particular class of materials. However, tabulated BSDFs present a trade-off between higher sampling loads versus lower directional accuracy during simulation. Low-resolution BSDFs (e.g., Klems basis) may be adequate for calculating solar heat gains but fall short when applied to daylight glare predictions. The tensor-tree representation moderates this trade-off using a variable-resolution basis, providing detail where needed at an acceptable cost. Independently, a peak extraction algorithm isolates direct transmission from any tabular BSDF, enabling highresolution beam radiation and glare analysis through transmitting systems with a “vision” component. Our data-driven BSDF methods were validated with a pilot study of a fabric shade installed in an outdoor, full-scale office testbed. Comparisons between measurement and simulation were made for vertical illuminance, specular and near-specular transmission, and daylight glare probability. Models based on high resolution BSDF measurements yielded superior results when accounting for anisotropy compared to isotropic models. Models with higher resolution produced more accurate source luminance data than low-resolution models. Further validation work is needed to better characterize generality of observed trends from this pilot study., Building and Environment, 196, ISSN:0360-1323

Autonomous driving enable more use of in-vehicle interior lighting effects using advanced interior materials for illumination. Targeting premium visual ergonomics requires additional know how of today’s and future materials regarding light scattering. Within the presented work, we improved a goniophotometer towards a 2 standard deviation of < 0.08 %. This is in the range of a suitable limit so we successfully measured light scattering in terms of BSDF and/or BTDF of wood, transparent plastic (including 3D printed), and leather. These materials scatter light differently, so the results enable the base for optical simulation tools to design special visual effects.

This white paper summarizes the current state of the art in the field of measurement and simulation characterization of daylighting systems by bidirectional scattering distribution functions (BSDFs) and provides recommendations broken down by classes of systems and use cases.

Monte Carlo light transport simulations often lack robustness in scenes containing specular or near-specular materials. Widely used uni- and bidirectional sampling strategies tend to find light paths involving such materials with insufficient probability, producing unusable images that are contaminated by significant variance., This article addresses the problem of sampling a light path connecting two given scene points via a single specular reflection or refraction, extending the range of scenes that can be robustly handled by unbiased path sampling techniques. Our technique enables efficient rendering of challenging transport phenomena caused by such paths, such as underwater caustics or caustics involving glossy metallic objects., We derive analytic expressions that predict the total radiance due to a single reflective or refractive triangle with a microfacet BSDF and we show that this reduces to the well known Lambert boundary integral for irradiance. We subsequently show how this can be leveraged to efficiently sample connections on meshes comprised of vast numbers of triangles. Our derivation builds on the theory of off-center microfacets and involves integrals in the space of surface slopes., Our approach straightforwardly applies to the related problem of rendering glints with high-resolution normal maps describing specular microstructure. Our formulation alleviates problems raised by singularities in filtering integrals and enables a generalization of previous work to perfectly specular materials. We also extend previous work to the case of GGX distributions and introduce new techniques to improve accuracy and performance.

Future cars with (semi-) autonomous driving enable more use of interior lighting with special visual effects. This deals as well with interior materials – like plastics, wood, metals - which are illuminated or backlighted by RGB LEDs. We performed 3D BRDF measurements from a luminance imager and compare them to traditional goniophotometers. Perception test with 37 subjects for transflective materials show that there is a narrow width of backlight intensity for “pleasant” (not bothering) intensity. Such fundamental optical examinations enable special visual effects for future vehicles.

Real-world materials are often layered: metallic paints, biological tissues, and many more. Variation in the interface and volumetric scattering properties of the layers leads to a rich diversity of material appearances from anisotropic highlights to complex textures and relief patterns. However, simulating light-layer interactions is a challenging problem. Past analytical or numerical solutions either introduce several approximations and limitations, or rely on expensive operations on discretized BSDFs, preventing the ability to freely vary the layer properties spatially. We introduce a new unbiased layered BSDF model based on Monte Carlo simulation, whose only assumption is the layer assumption itself. Our novel position-free path formulation is fundamentally more powerful at constructing light transport paths than generic light transport algorithms applied to the special case of flat layers, since it is based on a product of solid angle instead of area measures, so does not contain the high-variance geometry terms needed in the standard formulation. We introduce two techniques for sampling the position-free path integral, a forward path tracer with next-event estimation and a full bidirectional estimator. We show a number of examples, featuring multiple layers with surface and volumetric scattering, surface and phase function anisotropy, and spatial variation in all parameters.

Retro-reflective coatings applied to blinds of reduced geometric complexity promise to provide view to the outside while effectively controlling solar gains and glare. To characterize the reflection characteristics of such coatings over the entire solar spectrum, a novel extension to a scanning gonio-photometer is developed. The extended instrument is tested and applied to measure a coating's Bidirectional Reflection Distribution Function including the region of the retro-reflected peak. The measured data-sets are compiled into a data-driven reflection model for the daylight simulation software Radiance. This model is applied to illustrate the coating's effect in a comparison to purely diffuse and specular surface finishes on geometrically identical, flat blinds. Daylight supply, the probability of glare, and solar gains are assessed for an exemplary, South-oriented office under sunny sky conditions. The results indicate the potential of the coating to effectively shade direct sun-light even if applied on blinds with minimalistic geometries. The modeling technique is shown to be a general means to replicate the irregular optical properties of the coating, which cannot be represented by the standard models in daylight simulation software., + ID der Publikation: hslu_49059 + Art des Beitrages: Wissenschaftliche Medien + Sprache: Englisch + Letzte Aktualisierung: 2018-02-09 16:47:24

Measuring and representing light reflection and transmission accurately are core to high fidelity visual simulation of materials. In this paper, we analyze state-of-the-art Bidirectional measurements and models. We show that the most of the state-ofthe-art BSDF models do not suggest a general solution for any surface class, from glasses to metals, isotropic to anisotropic materials, and daylight redirecting films. Furthermore, it’s shown that an accurate and dense BSDF acquisition is not a trivial task at especially some specific measurement angles, such as normal incidence and grazing angles. In this paper, we address the problem of finding a general solution for efficient BSDF measurement and representation. We also outline the main issues that do not allow the effective use of current BSDF representations. Finally, we suggest open research issues that need to be investigated in BSDF literature, Işık yansıması ve iletiminin doğru bir şekilde ölçümlenip sunulması malzemelerin yüksek doğrulukta görsel simülasyonu için çok önemlidir. Bu makalede, literatürdeki en son İki Yönlü Saçılım Dağılım Fonksiyonu (BSDF) ölçümlemeleri ve modelleri incelenmektedir. Literatürdeki en son BSDF modellerinin camlardan metallere, izotropikden anizotropiğe ve ışığı yeniden yönlendiren filmlere kadar bir çok yüzey tipi için genel bir çözüm önermedikleri gösterilmiştir. Bunun dışında, özellikle normal geliş yönü ve süpürme açıları gibi spesifik ölçümleme açılarında doğru ve yoğun BSDF ölçümlemesinin kolay olmadığı gösterilmiştir. Bu makalede, etkin BSDF ölçümlemesi ve sunumu için genel bir çözüm bulma problemine yönelinmiştir. Ayrıca, mevcut BSDF sunumlarının etkin olarak kullanımına müsaade etmeyen ana konular belirtilmiştir. Son olarak, BSDF literatüründe incelenmesi gereken açık araştırma konuları önerilmiştir

Realistic Rendering of thin transparent layers bounded by rough surfaces involves substantial expense of computation time to account for multiple internal reflections. Resorting to Monte Carlo rendering for such material is usually impractical since recursive importance sampling is inevitable. To reduce the burden of sampling for simulating subsurface scattering and hence improve rendering performance, we adapt the microfacet model to the material with a single thin layer by introducing the extended normal distribution function (ENDF), a new representation of this model, to express visually perceived roughness due to multiple bounces of reflections and refractions. With such a representation, both surface reflection and subsurface scattering can be treated in the same microfacet framework, and the sampling process can be reduced to only once for each bounce of scattering. We derive analytical expressions of the ENDF for several cases using joint spherical warping. We also show how to choose proper shadowing-masking and Fresnel terms to make the proposed bidirectional scattering distribution function (BSDF) model energy-conserving. Experiments demonstrate that our model can be easily incorporated into a Monte Carlo path tracer with little extra computational and storage overhead, enabling some real-time applications.

Bidirectional scattering distribution function (BSDF) represents the scattered light distribution in forward and backward directions. BSDF is an important function in precise lighting design, because surface scattering is difficult to determine and including it in simulations. However, to measure a whole field BSDF is time consuming, as it can take as long as a day. In this paper, we propose and demonstrate a new way to measure BSDF. We utilize a screen that is illuminated by the scattered light, and a camera that captures the image on the screen. A complex calibration between the grey level of the camera and intensity is performed to make sure the measurement is valid. Through continuously imaging the screen for various scattered light distributions, an image fusion is performed to present the final BSDF. We call this instrument as screen image synthesis (SIS) BSDF meter. In this paper, two generations of SIS BSDF meter was developed, and is shown in details.

Recently, physically based rendering (PBR) [1] has become popular with respect to visual realism in the computer graphics field. PBR model is more of a ray tracing calculation than a legacy Lambert or Phong model. This technique has become the industry standard for movies and video game developments. This paper refers to a simple model for subsurface light transport in translucent materials with respect to the PBR system. We attempt to simulate a realistic subsurface scattering by choosing to mimic a cake-surface texture, which has a sophisticated translucent material, and nest-like structure that can demonstrate the characteristics of the bidirectional scattering distribution function (BSDF). We used the Arnold engine with 3ds max software for Monte Carlo ray-tracing calculation in CPU. We compared the difference between the photo-scanning map and the procedural shader, which were fed into the displacement channel to achieve a random organic texture of a cake-surface. This approach showed the advantages and disadvantages of these different situations in the PBR workflow. With the knowledge of the examining mathematical model of PBR, we describe that the PBR theory and the practical aspect to validate this procedural shader, can provide more advantages in a variety of material mimic. Finally, the workflow of the experiment can help 3D artists design realistic looking surfaces as effortlessly as possible based on physically accurate formulas.

In the field of earth-observation, on-board calibration is often necessary to guarantee the radiometric accuracy of space instruments. A typical method is to use large diffusers in front of the instrument, illuminated with a reference source like the sun [1]. Hence, it is necessary to characterize the scattering properties of the diffuser with excellent accuracy. Given the large size and weight of the diffusers to characterize, CSL have developed a bench which uses a robot arm to manipulate the sample. For the most stringent applications, the typical accuracy of robotic arms is not good enough to measure the BSDF with a satisfactory accuracy. A method have been developed which uses laser tracker measurements of the sample during a calibration phase and compensate for the robot errors. This paper describes the principle of the method and the results obtained. We also present the model of how the orientation error of the sample affects the BSDF relative error.

The Centre Spatial of Liege (CSL) is involved from more than 10 years in the BTDF/BRRDF metrology of large Lambertian diffusers used for on-board calibration on space instrumentation. In this context, a dedicated automatized BSDF calibration facility has been developed, suitable for calibration of all types of diffusers (from industry to space applications). Accurate calibration of such systems induces constraining requirements on the calibration bench: manipulate large diffusers and mechanisms in class ISO5 environment; incidence beam divergence close to the Sun divergence; large spectral range coverage; measurement of very low diffused signals (highly stable, with low noise, straylight free set-up), high accurate measurement of attenuator and a high knowledge of all calibration properties for BSDF modelling. Based on previous heritage, recent improvements have been implemented in order to correct some defaults like mechanical stabilities (complete new design), specimen alignment (new iterative procedure), absolute error (super stable light bulb, improved filters wheel repeatability, optics to limit diffraction effects in UV, incidence beam non-uniformity compensation …). This paper presents the design of the improved bench, highlights critical parameters for BTDF/BRDF calibration and relates main improvements to reach todays performances. The current bench performances are illustrated by calibration campaigns results performed at CSL for Sentinel 2, Sentinel 3 and Sentinel 4 calibration assemblies.

Swiss Federal Office of Energy, International Conference on Climate Resilient Cities - Energy Efficiency and Renewables in the Digital Era 2019, CISBAT 2019 -- 4 September 2019 through 6 September 2019 -- -- 155202, Data-driven modelling provides a general means to represent optically complex fenestration in daylight simulation by its Bidirectional Scattering Distribution Function (BSDF). Radiance employs the tensor tree as a compact data structure to store the BSDF at high directional resolution. The application of such models under sunny sky conditions is, however, computationally demanding, since the density of stochastic backward samples must match the BSDF resolution. The bidirectional Photon Map is proposed to rapidly forward-sample the BSDF, starting from the known sun direction. Its exemplary application shows a potential speed-up of 98% when compared to backward ray-tracing. © Published under licence by IOP Publishing Ltd.

An early phase design exercise for shading a South facade of an office building in Bucharest is presented here. The problem to solve is deciding in a simple and quick way (not using the complicated BSDF approach), based strictly on energy-efficiency considerations, between two options in principle: the first, exterior screens, is much cheaper and the send is unmovable horizontal aluminium slats. The tool used to produce the necessary result quantities by building energy simulation is COMFEN 4.1. The conclusion is positive: if aesthetic reasons are ignored, in Bucharest and very likely many other Romanian cities having a quite similar climate, screens can be at least equally effective in saving energy by South facade shading. As they allow a flexible shading strategy (removing them during some months of the heating season), the energy-efficiency realized by having them on throughout the year can be increased further.

Microfacet-based material appearance models are commonly considered as a physical plausible representation of matter–light interaction. With such models, the microgeometry of a surface element is defined by a statistical distribution of microfacets. The mathematical formulation ensures physical plausibility, such as energy conservation and reciprocity. Many authors have addressed microfacet bidirectional scattering distribution function (BSDF) representations, with various normal distribution functions (NDFs) and their relationship with shadowing and masking, or the effects due to multiple light scattering on the microgeometry. However, an extensive study on how an actual microgeometry drives material appearance still is missing. This question is a key issue for inverse design and manufacturing. This paper contributes to filling this gap by proposing a complete pipeline composed of a microgeometry generation process and numerical lighting simulation. From any input NDF, our method generates a controlled and structured microgeometry, integrated within numerical light scattering simulation. Reflected light is gathered using a virtual goniophotometer. From a given set of parameters, we use our pipeline to study the impact of microgeometry structures on material light scattering in the case of rough surfaces. The obtained results are discussed and compared with already existing approaches when they exist in the pipeline.

© 2018 Elsevier B.V. It has long been established that shading windows with overhangs, fins, and other types of non-coplanar systems (NCS) is one of the most effective ways of controlling solar heat gains in buildings because they intercept solar radiation prior to entry into the building. Designers however often specify non-opaque materials (e.g., louvers, fritted glass, expanded metal mesh) for these systems in order to admit daylight, reduce lighting energy use, and improve indoor environmental quality. Most simulation tools rely on geometric calculations and radiosity methods to model the solar heat gain impacts of NCS and cannot model optically-complex materials or geometries. For daylighting analysis, optically-complex NCS can be modeled using matrix algebraic methods, although time-efficient parametric analysis has not yet been implemented. Determining the best design and/or material for static or operable NCS that minimize cooling, heating, and lighting energy use and peak demand requires an iterative process. This study describes and validates a matrix algebraic method that enables parametric energy analysis of NCS. Such capabilities would be useful not only for design but also for development of prescriptive energy-efficiency standards, rating and labeling systems for commercial products, development of design guidelines, and development of more optimal NCS technologies. A facade or “F” matrix, which maps the transfer of flux from the NCS to the surface of the window, is introduced and its use is explained. A field study was conducted in a full-scale outdoor testbed to measure the daylight performance of an operable drop-arm awning. Simulated data were compared to measured data in order to validate the models. Results demonstrated model accuracy: simulated workplane illuminance was within 11–13%, surface luminance was within 16–18%, and the daylight glare probability was within 6–9% of measured results. Methods used to achieve accurate results are discussed. Results of the validation of daylighting performance are applicable to solar heat gain performance. Since exterior shading can also significantly reduce peak demand, these models enable stakeholders to more accurately assess HVAC and lighting impacts in support of grid management and resiliency goals.

Environment maps have been used for decades in production path-tracers to recreate ambient lighting conditions captured from real world scenes. Stochastic sampling of the radiance integral can be very challenging however, as both the BSDF and the environment can have strong peaks that are not aligned with each other. Multiple importance sampling (MIS) between the environment and the BSDF is a common way to reduce variance by re-weighting each estimator, but can still result in wasted samples. Product importance sampling is an effective way to reduce the variance by drawing samples using a probability distribution built from the product of the BSDF and the environment map. To our knowledge, the most practical product sampling technique [Clarberg and Akenine-Moller 2008] is still relatively costly for production rendering because it approximates the BSDF by a sparse quad-tree built on the fly from a few hundred BSDF samples. Due to the high complexity of the multi-lobed models used in film rendering, this cost can be prohibitive.

Window systems play a key role in establishing both the thermal and luminous environments within buildings, as well as the consequent energy required to maintain these for the comfort of their occupants. Various strategies have been employed to improve the thermal and optical performance of window systems. Some of these approaches result in products with relatively complex structures. Thus, it becomes difficult to characterise their optical and thermal properties for use in building performance prediction. This review discusses the experimental and numerical methods used to predict the thermal and optical behaviour of complex window systems. Following a discussion of thermal characterisation methods available in the literature that include experimental test methods, theoretical calculation methods and Computational Fluid Dynamic methods, sophisticated optical methods, such as use of Bidirectional Scatter Distribution Functions (BSDF) to optically characterise complex window systems, are introduced. The application of BSDF allows advanced daylight assessment metrics along with daylight evaluation tools to be used to realise dynamic annual prediction of the luminous environment. Finally, this paper reviews methods that permit the prediction of the combined thermal, daylight and energy behaviour of buildings that make use of complex window systems.

A physically accurate description of the optical properties of surfaces is the one of the most important requirements in optical simulation for both imaging and non-imaging optics. Uncertainty in the specification of the optical properties might influence the simulation image or the spatial distribution of radiation in optical system. One of the ways of describing the optical properties is using the Bidirectional Scattering Distribution Function (BSDF). As a rule, BSDF is measured by goniospectrophotometers, but sometimes it is not possible to perform such measurements. In some cases, the measurement should be done inside the material, but it is impossible to measure BSDF of the boundary there. One of the possible solutions is to measure the microrelief heights distribution by profile measurement machine or atomic force microscope and assign measured data for given model. But, not every optical design software solution has the ability to specify microrelief directly, while majority of them just have the ability to specify BSDF. In this article, authors show methods of BSDFs generation from measurements of the real microrelief in the form of spatial distribution of heights.

We present a computational workflow using annual daylight simulation and image-based postprocessing to visualise cumulative annual irradiance and glare duration on the built environment due to reflections from building integrated photovoltaics. The annual daylight simulation considers reflections from measured BSDFs of proposed PV materials. The postprocessing includes a spatio-temporal feature detection to identify sustained glare duration based on recommended thresholds. We demonstrate this workflow with a case-study of a proposed PV roof retrofit for a church in Lucerne, Switzerland., + ID der Publikation: hslu_55683 + Art des Beitrages: Präsentation Konferenzpapier/Tagungsbeitrag + Sprache: Englisch + Letzte Aktualisierung: 2018-10-03 14:55:50

© 2017 Elsevier Ltd Simulation tools that enable annual energy performance analysis of optically-complex fenestration systems have been widely adopted by the building industry for use in building design, code development, and the development of rating and certification programs for commercially-available shading and daylighting products. The tools rely on a three-phase matrix operation to compute solar heat gains, using as input low-resolution bidirectional scattering distribution function (BSDF) data (10–15° angular resolution; BSDF data define the angle-dependent behavior of light-scattering materials and systems). Measurement standards and product libraries for BSDF data are undergoing development to support solar heat gain calculations. Simulation of other metrics such as discomfort glare, annual solar exposure, and potentially thermal discomfort, however, require algorithms and BSDF input data that more accurately model the spatial distribution of transmitted and reflected irradiance or illuminance from the sun (0.5° resolution). This study describes such algorithms and input data, then validates the tools (i.e., an interpolation tool for measured BSDF data and the five-phase method) through comparisons with ray-tracing simulations and field monitored data from a full-scale testbed. Simulations of daylight-redirecting films, a micro-louvered screen, and venetian blinds using variable resolution, tensor tree BSDF input data derived from interpolated scanning goniophotometer measurements were shown to agree with field monitored data to within 20% for greater than 75% of the measurement period for illuminance-based performance parameters. The three-phase method delivered significantly less accurate results. We discuss the ramifications of these findings on industry and provide recommendations to increase end user awareness of the current limitations of existing software tools and BSDF product libraries.

International audience; We derive a novel framework for the efficient analysis and computation of light transport within layered materials. Our derivation consists in two steps. First, we decompose light transport into a set of atomic operators that act on its directional statistics. Specifically, our operators consist of reflection, refraction, scattering, and absorption, whose combinations are sufficient to describe the statistics of light scattering multiple times within layered structures. We show that the first three directional moments (energy, mean and variance) already provide an accurate summary. Second, we extend the adding-doubling method to support arbitrary combinations of such operators efficiently. During shading, we map the directional moments to BSDF lobes. We validate that the resulting BSDF closely matches the ground truth in a lightweight and efficient form. Unlike previous methods we support an arbitraty number of textured layers, and demonstrate a practical and accurate rendering of layered materials with both an offline and real-time implementation that are free from per-material precomputation.

The paper presents a new spatio-temporal learning-based descriptor called binarised statistical dynamic features (BSDF) for representation and classification of dynamic texture. The BSDF descriptor operates by applying three-dimensional spatio-temporal filters on local voxels of an image sequence where the filters are learned via an independent component analysis, maximising independence over spatial and temporal domains concurrently. The BSDF representation is formed by binarising filter responses which are then converted into codewords and summarised using histograms. A robust representation of the BSDF descriptor is finally obtained via a sparse representation approach yielding very discriminative features for classification. The effects of different hyper-parameters on performance including the number of filters, the number of scales, temporal depth, number of samples drawn are also investigated. The proposed approach is evaluated on the most commonly used dynamic texture databases and shown to perform very well compared to the existing methods.

We develop a way to find volumetric scattering model parameter values based on measured BSDF characteristics. Example models often use Mie or Gegenbauer particles. The accuracy and flexibility of this approach are illustrated.

We 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.

Complex fenestration systems (CFS) need to be evaluated to quantify how well they contribute to avoid glare and manage the solar heat gains through building facades. It is possible to use solar bidirectional properties of CFS in some building energy simulations tools in order to evaluate more accurately their impact on lighting, cooling and heating energy consumption. genBSDF is a Radiance's program whose capabilities of assessing the bidirectional solar properties of CFS are evaluated in this paper. This evaluation was carried out for 16 cases of a venetian blind varying the solar properties and slope angle of the slats. Results of directional and bidirectional solar properties obtained by genBSDF with radiosity assumptions were compared with the results of the radiosity-based WINDOW algorithm. Results show an excellent agreement between the two approaches which demonstrates that genBSDF is a very precise and robust tool for assessing the solar bidirectional properties of CFS.

Estimation of intrinsic images still remains a challenging task due to weaknesses of ground-truth datasets, which either are too small or present non-realistic issues. On the other hand, end-to-end deep learning architectures start to achieve interesting results that we believe could be improved if important physical hints were not ignored. In this work, we present a twofold framework: (a) a flexible generation of images overcoming some classical dataset problems such as larger size jointly with coherent lighting appearance; and (b) a flexible architecture tying physical properties through intrinsic losses. Our proposal is versatile, presents low computation time, and achieves state-of-the-art results.

Visual comfort assessments employing luminance-based metrics rely on efficient CBDM techniques for image synthesis. Data-driven BSDF models allow to isolate internal light paths in optically CFS from CBDM. Bidirectional photon mapping is proposed for the efficient sampling of such models in the calculation of the direct solar component in CBDM. The method allows accurate image synthesis for visual comfort assessments with only two calculation steps, achieving comparable accuracy as the established but complex 5PM. The validity of the approach is confirmed by comparison with backward ray-tracing. Its exemplary application to compare two CFS in terms of glare control demonstrates the importance to achieve reconcilability of conflicting targets such as view and glare control in daylighting.

Fenestration attachments are anticipated to produce significant reductions in building energy use because they can be deployed quickly at low-cost. New software tools enable users to assess the building energy impacts of optically complex fenestration systems (CFS) such as shades, Venetian blinds, or daylighting systems. However, such tools require users to provide bi-directional scattering distribution function (BSDF) data that describe the solar-optical performance of the CFS. A free, open-source Radiance tool genBSDF enables users to generate BSDF data for arbitrary CFS. Prior to genBSDF, BSDF data for arbitrary fenestration systems could only be produced using either expensive software or with expensive equipment. genBSDF outputs CFS data in the Window 6 XML file format and so can be used with CFS-enabled software tools to model multi-layered window systems composed of glazing and shading layers. We explain the basis and use of the genBSDF tool and validate the tool by comparing results for four different cases to BSDF data produced via alternate methods. This validation demonstrates that BSDFs created with genBSDF are comparable to BSDFs generated analytically using TracePro and by measurement with a scanning goniophotometer. This tool is expected to support accelerated adoption of fenestration attachments and daylighting technologies.

Rapid development of computing and visualisation systems has resulted in an unprecedented capability to display, in real time, realistic computer-generated worlds. Advanced techniques, including three-dimensional (3D) projection, supplemented by multi-channel surround sound, create immersive environments whose applications range from entertainment to military to scientific. One of the most advanced virtual reality systems are CAVE-type systems, in which the user is surrounded by projection screens. Knowledge of the screen material scattering properties, which depend on projection geometry and wavelength, is mandatory for proper design of these systems. In this paper this problem is addressed by introducing a scattering distribution function, creating a dedicated measurement setup and investigating the properties of selected materials used for rear projection screens. Based on the obtained results it can be concluded that the choice of the screen material has substantial impact on the performance of the system

Physically accurate lighting simulation requires precise account of the optical properties (BSDF) which are usually measured using gonio-spectrophotometer. In this paper, the authors analyzed the accuracy of BSDF shape measured for later use of measurements in special software for photorealistic visualization and virtual prototyping. Visual and numerical analysis were done. In the first case we look at the sample image rendered under specified lighting conditions, replacing its properties on measurement results and visually estimate the similarity (or difference). In the second case we compare the results of simulation of spatial or angular radiance distribution with results of corresponding radiometric measurements.

Advanced optical ray tracing software, CFSpro, was developed for the study and optimization of complex fenestration systems (CFSs). Using an algorithm mixing 2D and 3D approaches, accurate computation of large numbers of rays in extruded geometries can be performed and visualized in real time. A thin film model was included to assess the spectral control provided by coatings. In this paper, the ray tracing model is described and validated. A novel glazing, engineered with this simulation tool, is presented. It combines the functions of daylight provision, glare protection, and seasonal thermal control while conserving a view to the outside at near normal incidence.

The low cost, simplicity, and aesthetic appearance of external and internal shading devices, make them commonly used for daylighting and glare-control applications. Shading devices, such as Venetia ...

The present paper describes the challenging diffuser design and verification activities of TNO under contract of a customer for an earth observation instrument with observation conditions that require feasible BRDF under large angles of incidence of up to 70° with respect to the surface normal. Not only the design though but also the verification of the diffuser performance under such angles including out-of-plane , i.e. angle theta detection of the scattered light, was an essential activity to be executed. In this paper we will summarize the R&D activities with respect to diffuser design and verification that were recently carried out at TNO and present its applicability to current and future earth observation missions with challenging observation conditions and thus challenging diffuser requirements under high illumination angles.

The "Bi-Directional Scattering Function" BSDF of a diffuser depends on several parameters, such as surface properties, observational conditions and further. This paper describes experimental activities to achieve a better understanding about the interaction between diffuser properties and performance with regards to its scattering behavior. For this purpose a set of 24 diffusers with defined surface properties have been manufactured and systematically been investigated in a dedicated radiometric calibration measurement facility. The experimental data are compared with existing theoretical models.

Over the last two decades there has been extensive research done to improve the design of Organic Light Emitting Diodes (OLEDs) so as to enhance light extraction efficiency, improve beam shaping, and allow color tuning through techniques such as the use of patterned substrates, photonic crystal (PCs) gratings, back reflectors, surface texture, and phosphor down-conversion. Computational simulation has been an important tool for examining these increasingly complex designs. It has provided insights for improving OLED performance as a result of its ability to explore limitations, predict solutions, and demonstrate theoretical results. Depending upon the focus of the design and scale of the problem, simulations are carried out using rigorous electromagnetic (EM) wave optics based techniques, such as finite-difference time-domain (FDTD) and rigorous coupled wave analysis (RCWA), or through ray optics based technique such as Monte Carlo ray-tracing. The former are typically used for modeling nanostructures on the OLED die, and the latter for modeling encapsulating structures, die placement, back-reflection, and phosphor down-conversion. This paper presents the use of a mixed-level simulation approach which unifies the use of EM wave-level and ray-level tools. This approach uses rigorous EM wave based tools to characterize the nanostructured die and generate both a Bidirectional Scattering Distribution function (BSDF) and a far-field angular intensity distribution. These characteristics are then incorporated into the ray-tracing simulator to obtain the overall performance. Such mixed-level approach allows for comprehensive modeling of the optical characteristic of OLEDs and can potentially lead to more accurate performance than that from individual modeling tools alone.

The "Bi-Directional Scattering Function" BSDF of a diffuser depends on several parameters, such as surface properties, observational conditions and further. This paper describes experimental activities to achieve a better understanding about the interaction between diffuser properties and performance with regards to its scattering behavior. For this purpose a set of 24 diffusers with defined surface properties have been manufactured and systematically been investigated in a dedicated radiometric calibration measurement facility. The experimental data are compared with existing theoretical models.

The bidirectional scatter distribution function (BSDF) characterizes the scattering properties of a material for any angle of illumination or viewing, and offers as such a complete description of the spatial optical characteristics of the surface. An accurate determination of the BSDF is important in many scientific domains, such as computer graphics, architectural and lighting design, and the field of material appearance characterization (e.g. the color and gloss properties). Many BSDF measuring instruments have been reported in the literature. The majority of these instruments are goniometric measurement devices, by use of which the BSDF is determined by scanning all incoming and outgoing light flux directions in sequence. For this, the sample, detector, and/or source perform relative individual movements. In result, the major restriction of this type of instruments constitutes the measurement time, which may run to the order of several hours depending on the accuracy (angular resolution) and the complexity (spectral coverage, absolute measurement capability, etc.) of the reported measurement data. This paper describes the results of a feasibility study, in which an alternative goniometric measurement system is designed, enabling to acquire the photometric BSDF in a full three-dimensional (3D) space, with a high mechanical angular resolution (0.1°) in a time efficient way (about 30 minutes). A near-field goniophotometer, originally intended to measure luminance intensity distributions and luminous fluxes of light sources and luminaires, was converted for this purpose. Besides a discussion of the design and the measurement procedure, test sample measurements are presented to illustrate the versatility of the device. ispartof: pages:1-8 ispartof: Measuring, Modeling, and Reproducing Material Appearance vol:9018 pages:1-8 ispartof: SPIE/IS&T Electronic Imaging location:San Francisco date:Jan - Jan 2014 status: published