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.
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.
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.
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.
When a beam of light is incident on a translucent sample, a significant fraction of the light is scattered at high angles. Some of this scattered light may be trapped inside the substrate through multiple reflections and total internal reflection, similar to light coupling into an optical fiber. The trapping depends on factors such as the surface roughness of the external surfaces and/or the size and distribution of scattering particles inside the sample. The scattered light may thus escape out of the sample at a shifted position relative to the incident beam. This leads to port losses in an integrating sphere. The detected signal from the light entering the sphere then underestimates the hemispherical transmittance. In this paper the signal versus lateral position has been measured in an attempt to estimate the error and to find an extrapolation procedure for the correct transmittance value. The lateral measurements were carried out by moving a detector behind the sample, a procedure carried out at several angles of incidence. Different illumination methods have also been studied both theoretically and experimentally to further investigate what effect light trapping can have when characterising scattering samples.
Surface reflections from optical transmission components are in many cases unwanted and cumbersome. Thin film coating is the conventional technique used for anti-reflection treatment of optical components. In recent years subwavelength gratings have been studied as a replacement for thin films. Subwavelength gratings are microstructures that can be formed on one or both sides of a substrate. Typically an optical component needs to be AR-coated on
both sides. We have fabricated injection moulded subwavelength gratings superimposed upon a blazed grating structure in polycarbonate. The gratings are initially formed by electron-beam lithography and subsequently replicated using the same process which is used to manufacture standard plastic compact discs (CDs).
There are several problems when trying to characterize a component such as a blazed transmittance grating. First of all there is the spread of internal reflections. Light that is reflected inside the substrate is shifted in lateral position due to the angle of the grating. We have thoroughly investigated the effects of decrease in grating efficiency due to internal reflections and also tried to minimize these effects by appropriately treating both sides of the plastic CD.
An optical measurement system composed of an optical microscope (Olympus BX60) and an optical multichannel analyzer (EG&G OMA 1460) has been assembled and tested. The optical microscope allows the user to make measurements on a small and well defined area of the sample. The light source, a 100 W halogen lamp, and the diode array detector, result in high sensitivity in the wavelength region of 450-750 nm. The spectral resolution of the instrument is listed as 0.59 nm/channel. The full width at half maximum (FWHM) of the strongest peaks in calibration measurements on a mercury lamp is 5 channels corresponding to 3 nm. Quasifractal clusters of gold particles have been produced with electron beam lithography. The clusters consists of different numbers of particles, giving a cluster size variation from 1.6 (mu) m to 50 (mu) m. The individual gold particles are 50 nm in diameter each. The measurement system has been used to measure both absolute transmittance and the relative transmittance using the uncoated substrate as a reference.
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