Integrating Sphere Tests Efficiency

Dr. Flavio Fontana

As building developers struggle to incorporate energy conservation into their designs, controlling heat and light fluxes through windows is crucial. As the "intelligent window" becomes more complex, so does the objective assessment of its overall optical properties. Straightforward spectrophotometric measurement of absorption becomes difficult.

The researchers' facility for measuring the optical properties of windows incorporates a large integrating sphere. Courtesy of Labsphere Inc.

"The instruments exploited for similar tasks work usually on small samples and are almost useless when the spectral properties of multiple glazings with overall thickness up to 100 mm have to be specified," said M. Bianchi of VitroTherm, an Italian manufacturer of glazing materials.

The Transparent Materials Laboratory at Italy's Agency for Energy, New Technologies and Environment -- traditionally involved in research on intelligent building concepts -- has developed a method to provide the spectral data needed for designers to select the best windows for their applications. The system, which can be configured to perform transmittance, reflectance and absorptance measurements for visible and infrared light (up to 2500 nm), can accommodate samples as thick as 5 cm and as large as 100 × 100 cm.

The key component of the apparatus deployed by Augusto Maccari and other researchers is the optimized use of a 1-m-diameter integrating sphere from Labsphere Inc. of North Sutton, N.H., which eliminates the problems related to the acceptance angle limitations typical of spectrophotometers. The light transmitted or reflected by the sample is not routed directly to the spectrum analyzer, but is distributed evenly over the internal surface of the detector. The input slit of the spectrophotometer is not illuminated by the input beam, but instead collects the light diffused by a well-defined area of the sphere. The sphere's highly efficient diffusive coating allows full exploitation of its integrating nature, making measurement insensitive to such variations as the aiming of the beam impinging on the sample and local inhomogeneities of the sample.

The system allows engineers to spectrally characterize whole samples, including windows with multiple glazings, even with spectrally selective coatings; photochromic windows; and glazings with shading structures -- with benefits in terms of precision and direct use of data for thermal budget analysis.