Jacob C. Jonsson

Simulating the Daylight Performance of Complex Fenestration Systems Using Bidirectional Scattering Distribution Functions within Radiance

Abstract We describe two methods which rely on bidirectional scattering distribution functions (BSDFs) to model the daylighting performance of complex fenestration systems (CFS), enabling greater flexibility and accuracy in evaluating arbitrary assemblies of glazing, shading, and other optically-complex coplanar window systems. Two tools within Radiance enable a) efficient annual performance evaluations of CFS, and b) accurate renderings of CFS despite the loss of spatial resolution associated with low-resolution BSDF datasets for inhomogeneous systems. Validation, accuracy, and limitations of the methods are discussed.

Experimental validation of bidirectional reflection and transmission distribution measurements of specular and scattering materials

The development of advanced materials for facades aims to achieve higher energy efficiency of buildings. Successful application of these materials depends on the availability of reliable characterization data. While data derived from integrated measurements of transmission and reflection is widely available, it does not allow to characterize the angular dependence of the performance of such materials. The Bidirectional Reflection-Transmission Distribution (BRTD) can be measured by commercially available Gonio-Photometers and, complimenting integrated transmittance and reflectance, allows the assessment of facade materials and thus supports both their development and application. Validation of the obtained data is crucial to back these measurements. Integration of validation procedures into the operation of a characterization laboratory allowing a well-defined approach to quality control is presented for a range of typical material and sample types: * consistency checks of measurement data * cross-checking of integrated material properties derived from BRTD data with integrating sphere measurements * round-robin comparison between laboratories using comparable devices The results of of these first measurements are discussed. Potential to further improve the availability of reliable angular resolved characterization data for the building sector is identified.

Light-scattering properties of a woven shade-screen material used for daylighting and solar heat-gain control

Shade-screens are widely used in commercial buildings as a way to limit the amount of direct sunlight that can disturb people in the building. The shade screens also reduce the solar heat-gain through glazing the system. Modern energy and daylighting analysis software such as EnergyPlus and Radiance require complete scattering properties of the scattering materials in the system. In this paper a shade screen used in the LBNL daylighting testbed is characterized using a photogoniometer and a normal angle of incidence integrating sphere. The data is used to create a complete bi-directional scattering distribution function (BSDF) that can be used in simulation programs. The resulting BSDF is compared to a model BSDFs, both directly and by calculating the solar heat-gain coefficient for a dual pane system using Window 6.

Method for more accurate transmittance measurements of low-angle scattering samples using an integrating sphere with an entry port beam diffuser

For most integrating sphere measurements, the difference in light distribution between a specular reference beam and a diffused sample beam can result in significant errors. The problem becomes especially pronounced in integrating spheres that include a port for reflectance or diffuse transmittance measurements. The port is included in many standard spectrophotometers to facilitate a multipurpose instrument, however, absorption around the port edge can result in a detected signal that is too low. The absorption effect is especially apparent for low-angle scattering samples, because a significant portion of the light is scattered directly onto that edge. In this paper, a method for more accurate transmittance measurements of low-angle light-scattering samples is presented. The method uses a standard integrating sphere spectrophotometer, and the problem with increased absorption around the port edge is addressed by introducing a diffuser between the sample and the integrating sphere during both reference and sample scan. This reduces the discrepancy between the two scans and spreads the scattered light over a greater portion of the sphere wall. The problem with multiple reflections between the sample and diffuser is successfully addressed using a correction factor. The method is tested for two patterned glass samples with low-angle scattering and in both cases the transmittance accuracy is significantly improved.

Inter-laboratory comparison using integrating sphere spectrophotometers to measure reflectance and transmittance ofspecular, diffuse, and light-redirecting glazing products

An inter-laboratory comparison (ILC) between glazing manufacturers that submit data to the International Glazing Database (IGDB) is carried out every four years. This time a large number of independent laboratories were included in addition to the IGBD submitters, in total over 50 boxes of samples were sent out in parallel. Each box contained 5 specular samples, consisting of clear float glass, low-e coated glass, laminates, and an applied film on clear glass. New for the IGDB submitters were 5 diffuse samples, 2 fritted glass samples, a diffuse laminate, a light-redirecting daylighting film, and a shade fabric with an inhomogeneous pattern. The samples were characterized by each participant in the solar optical range, 300 nm - 2500 nm, as well as the thermal infrared from 5µm–25µm. Spectral data was inspected for anomalies such as systematic absorption and non-continuous steps due to instrument design and operation. Spectral averaged data was calculated and used to compare the results from the different laboratories. Such comparisons indicated that use of a diffuse reference for specular measurements marginally increased the measured result. For diffuse products the effects of sphere geometry and design influenced the results to a significant degree.

An Edge-Heating Device for Optical Measurement of Thermochromic Glazing Materials and Recommended Test Procedure

Thermochromic materials have optical properties that vary with temperature. To simulate energy performance of such materials, it is important to have spectral data in the solar range, 300–2500 nm, for each temperature that the material will have in the simulation. This paper describes a temperature control strategy that allows for measurement of reflectance and transmittance at a fixed temperature using a commercial spectrophotometer. A specimen frame is used to clamp heating strips to the surface at the edge of the sample that is being tested. Multiple thermocouples are used to monitor the temperature gradient over the sample as the center is cooler than the edge. Verification using an infrared (IR) camera and time-resolved transmittance measurements show that the center sample temperature is stable and how long it takes to achieve equilibrium. An interpolation method is described and verified to reduce the number of states that need to be measured. A recommended test procedure is described and used on two different materials.

Light-loss when measuring transmittance of thick scattering samples with an integrating sphere

Light scattering materials have several uses in solar energy applications, ranging from a purely aesthetic function as a cover glass to a way of increasing the path-length of photons inside a semiconductor. Knowing the transmittance of such elements is of essence to properly model, simulate, and design a solar energy system. The traditional method for obtaining the transmittance is to use a spectrophotometer fitted with an integrating sphere detector. However, it is well-known that most commercial integrating spheres underestimate the true transmittance of thick scattering samples. This study investigates a method to obtain quantitative values of the losses associated with measuring a scattering sample. The International Commission on Glass (ICG TC-10) is conducting an inter-laboratory comparison (ILC) on scattering samples to improve the methodology for characterizing such samples. A fritted glass sample similar to one in the ILC was used as an example. One side of a clear glass sample has a highly scattering layer. The bi-directional transmittance distribution function (BTDF) for the sample was obtained using a goniophotometer and then used as scattering function in a ray-tracing simulation. The ray-tracer was configured to report the amount of light exiting all six surfaces of the sample as well as through various ports defined by the integrating sphere geometry. The sample was then measured with a commercial integrating sphere in several different configurations, verifying the accuracy of the model.