Measurement and prediction of angle-dependent optical properties of coated glass products: results of an inter-laboratory comparison of spectral transmittance and reflectance

The European Union ADOPT project has been concerned with the accurate determination of the optical properties of coated glass products, describing reliable procedures for angle-dependent measurements and the development of predictive algorithms to enable accurate angle-dependent performance characterisation based upon near-normal values only. An essential element of the ADOPT project was the inter-laboratory comparison of transmittance and reflectance measurements for commercially available glazings. Thirteen European and three USA laboratories participated. The selected samples represent major categories of coated glass products used in buildings. Each participant received a set of samples and measured the angle-dependent spectral transmittance and reflectance under appropriate polarisation conditions for incident wavelengths within the solar spectral distribution. Calibrated mirrors were also provided for reference purposes. This paper presents the results of the inter-comparison which show that near-normal solar, visible and spectral transmittance and reflectance can be measured with an uncertainty of better than ±0.02. The inter-comparison illustrates the difficulties still experienced in making accurate oblique incidence measurements. The predictive algorithms, which require near-normal input data only, are capable of predicting the angle-dependent transmittance, reflectance and total solar energy transmittance for double and triple glazed windows to an accuracy which enables reliable calculation of the annual solar heat gain through windows.