Optical properties of glazing materials at normal incidence
Measurements of spectral transmittance T and reflectance R at normal incidence continue to be the most common and accurate source of energy performance data for glazing materials. Prediction of these radiometric properties from more fundamental materials data is often confounded by the complexity and uncertainty of coating structures. Angle-dependent radiometric properties of coated glazing will probably be predicted from normal-incidence data rather than being measured at many angles. The general error level demonstrated in round-robin tests is on the order 1-2 percent; it is often necessary to achieve better levels of performance. Based on results obtained following the round-robin tests, it is expected that accuracy of better than 0.5 percent can be generally achieved. A new type of absolute standard reference is described and tested with promising results.