Di. Milburn et Kgt. Hollands, AN EXPERIMENTAL INVESTIGATION OF THICK-SAMPLE EFFECTS IN THE MEASUREMENT OF DIRECTIONAL-HEMISPHERICAL TRANSMITTANCE OF ADVANCED GLAZING MATERIALS, Solar energy, 57(4), 1996, pp. 261-275
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, traditional small-beam techniques are often no longer applicable.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.