AN EXPERIMENTAL INVESTIGATION OF THICK-SAMPLE EFFECTS IN THE MEASUREMENT OF DIRECTIONAL-HEMISPHERICAL TRANSMITTANCE OF ADVANCED GLAZING MATERIALS

Samples that are thick with respect to the measuring apparatus can pos e problems in the measurement of directional-hemispherical transmittan ce. Due to the lateral displacement of light that these samples induce , traditional small-beam techniques are often no longer applicable. Br oad-area irradiation offers a solution to these problems, but, in prac tice, imperfections in the extent and the uniformity of the irradiatio n can create errors. This paper reports measurement of the ''detector- integrated inscattering function'', which describes the lateral displa cement of light, for a number of thick samples over a range of inciden ce angles. This function is used to determine the minimum area that mu st be irradiated for an accurate broad-area irradiation transmittance measurement. The errors caused by transverse non-uniformities in the i rradiance distributions produced by a state-of-the-art broad-area irra diation spectrophotometer are also reported. Errors caused by transver se irradiation non-uniformities are estimated, and transmittance measu rements made with small-area and broad-area irradiation are compared. It is shown that in most cases small-area irradiation transmittance me asurements underestimate the transmittance of thick samples due to out scattering, and that broad-area irradiation measurements using state-o f-the-art irradiation optics can contain large errors of up to 20%, du e to irradiance non-uniformities, particularly at large angles of inci dence. Copyright (C) 1997 Elsevier Science Ltd.