Measurement of hemispherical transmittance of structured materials like transparent insulation materials

The determination of the hemispherical-hemispherical (or diffuse-diffuse) solar transmittance Tdif of transparent insulation materials like honeycomb structures or of shading devices like venetian blinds is important to estimate the efficiency of passive solar components using these materials. Tdjf may be calculated from the measured angle-dependent hemispherical (or diffuse) transmittance 7cj(). Alternatively one may try to measure the quantity directly by using a large radiation source for isotropic irradiation of the sample. The measuring device in both cases is a large integrating sphere. A calculation of Td.f needs an underlying physical model for the radiation transmittance of the sample. This model might not be availab'e for complicated structures and is idealized for samples in practice. The arising problems for a honeycomb structure will be discussed in detail. The differences of up to 10 percent in Tdjf are not negligible. The main problem is a practical one, not a fundamental one. In order to save measurement time and costs, only two scans along the principal directions of the cell walls are selected, thus essentiallyvarying only the polar incidence angle. These two curves can be fitted with a transmittance model. Then for every arbitrary incidence angle the transmittance can be calculated using the theoretical model, i.e. also Tdjf may be determined. The selection of the right model, however, needs information about the azimuthal dependance. This in principle can be obtained by varying the azimuthal angle systematically in additional scans. Information about this dependance can be obtained also from a scanning radiometer, where both angles for incident and outgoing radiation can be chosen without restrictions. However, this is rather time-consuming and poses new problems as well. Therefore the use of isotropic irradiation of the sample certainly is the fastest way. This approach may be realized in different ways. Two of them are discussed in the article and possible problem areas are identified.