Including of specular component in a BTDF or BRDF assessment based on digital imaging

Bi-directional Transmission (or Reflection) Distribution Functions, commonly named BTDFs (and BRDFs), are essential quantities to describe any complex fenestration system in details. They are defined as the ratio of the luminance diffused from a surface element in a given direction (after transmission or reflection), and the illuminance incident on the sample. However, these functions are capable of describing the regular (specular) as well as the diffuse components of emerging light, and their mutual knowledge is necessary to assess a glazing or shading system’s optical performances properly. Although the analytical expression of a BT(R)DF differs whether it is related to regular (specular) or diffuse light, a simultaneous assessment of the two components can be achieved under certain conditions, presented in this paper. They are thereafter analyzed for the particular data acquisition procedure developed for a novel type of bi-directional photogoniometer, based on digital imaging

Solar Energy and Building Physics Laboratory - Activity Report 2009

The Solar Energy and Building Physics Laboratory (LESO-PB) works at the forefront of research and technological development in renewable energy, building science and urban physics. It is part of the Civil Engineering Institute (IIC) of the School of Architecture, Civil and Environmental Engineering (ENAC) of the Swiss Federal Institute of Technology (EPFL) in Lausanne, Switzerland. Placed under the responsibility of Prof. Dr Jean-Louis Scartezzini and four group leaders, the laboratory counts about 50 scientists, engineers and technicians. This report presents the teaching, research and dissemination activities for 2009

Comparison between ray-tracing simulations and bi-directional transmission measurements on prismatic glazing

Evaluation of solar heat gain and daylight distribution through complex window and shading systems requires the determination of the bi-directional transmission distribution function (BTDF). Measurement of BTDF can be time-consuming, and inaccuracies are likely because of physical constraints and experimental adjustments. A general calculation methodology, based on more easily measurable component properties, would be preferable and would allow much more flexibility. In this paper, measurements and calculations are compared for the specific case of prismatic daylight-redirecting panels. Measurements were performed in a photogoniometer equipped with a digital-imaging detection system. A virtual copy of the photogoniometer was then constructed with commercial ray-tracing software. For the first time, an attempt is made to validate detailed bi-directional properties for a complex system by comparing an extensive set of experimental BTDF data with ray-tracing calculations. The results generally agree under a range of input and output angles to a degree adequate for evaluation of glazing systems. An analysis is presented to show that the simultaneously measured diffuse and direct components of light transmitted by the panel are properly represented. Calculations were also performed using a more realistic model of the source and ideal model of the detector. Deviations from the photogoniometer model were small and the results were similar in form. Despite the lack of an absolute measurement standard, the good agreement in results promotes confidence in both the photogoniometer and in the calculation method

Inclusion of the specular component in the assessment of bidirectional distribution functions based on digital imaging

To describe complex fenestration systems such as novel solar blinds, new glazing or coating materials, daylight and sunlight-redirecting devices, a detailed description of their optical properties is needed, given by their Bidirectional Transmission (or Reflection) Distribution Functions (commonly named BTDFs and BRDFs). These functions are angle-dependent at both the incidence and the emission levels, and are defined as the ratio of the luminance of a surface element in a given direction (after diffuse transmission or reflection) to the illuminance on the sample. However, these functions are capable of describing the specular as well as the diffuse components of emerging light, and their mutual knowledge is necessary to properly assess a glazing or shading system’s daylighting performances and benefit from their potential as energy-efficient and users’ comfort strategies. Although the analytical expression of a BT(R)DF differs whether it is related to specular or diffuse light, a simultaneous assessment of the two components can be achieved under certain conditions. These conditions are analyzed for the particular data acquisition procedure developed for a novel type of bidirectional goniophotometer, based on digital imaging

Lighting simulation for External Venetian blinds based on BTDF and HDR sky luminance monitoring

The precise daylighting simulation in buildings can potentially contribute to designers’ and occupants’ smart utilization of it. The traditional method of employing sky models was indicated with noticeable errors in transient lighting simulation for complex fenestration systems (CFS), due to the mismatch between the real sky and standard sky models. This paper evaluates the performance of a calibrated embedded photometric device based on sky luminance monitoring, capable of real-time on-board lighting computing. Daylighting experiments for external Venetian blinds (EVB) were conducted in a lighting test module to demonstrate its accuracy in the simulation of horizontal work-plane illuminance. The BTDF of the EVB was generated for two different tilt angles of slats respectively. With partly cloudy skies, the photometric device was validated with improved accuracy in simulating the work-plane illuminance distribution compared with a common practice employing the Perez all-weather sky model

Mixed-Dimensionality Approach for Advanced Ray Tracing of Lamellar Structures for Daylighting and Thermal Control

The appropriate choice of the type of glazing and glazed area in a façade depends on many factors. They include amongst other criteria: location, orientation, climatic condition, energetic efficiency, usage of the building, required user comfort, and the architectural concept. All requirements cannot be fulfilled at all times and priorities have to be set to find a compromise between occupant comfort, design objective, cost and energetic efficiency. An innovative glazing system combining daylighting, glare protection, seasonal thermal control and clear view was developed [1] and patented by the authors. This design was developed using a novel ray tracing approach to obtain a strongly angular dependent transmission with specific angular distribution. Taking advantage of the changing elevation of the sun between seasons, a seasonal variation is created by a strongly angular dependent transmittance. In this paper we present the mixed dimensionality approach used to achieve a very fast and accurate ray tracing of any lamellar structure that has a two dimensional profile. The originality of the presented Monte Carlo algorithm is the separation of intersection and interaction. Intersections are computed using only the two dimensions of the profile thereby increasing significantly computational speed. Interactions are computed using vector calculus in three dimensions and provide accurate results with very little computational load. With such optimizations, the user interface could be designed to give an instantaneous idea of the light path in the modelled system. The model also calculates an accurate bidirectional transmittance distribution function that is used in a Radiance simulation to obtain a rendering of the daylighting distribution in an office space. Hereby we can compare the daylighting performances of the novel design based on optical microstructures with those of other CFSs. Finally the combination of simulated angular dependent transmittance and Meteonorm data provides an estimate of transmitted energy over the year and proves the efficiency of the presented optical microstructures for dynamic thermal control. The proposed working principles of redirection and angular dependent transmittance are thereby demonstrated. The software provides all the mentioned results in the user interface where the performances of different designs can also be compared, making the optimization process of a profile with a defined objective very intuitive