The hemispherical transmittance properties of a variety of coated and uncoated plastics (teflon, tedlar, acrylic) and fiberglass composites (e.g., greenhouse coverings) were measured as a function of wavelength, polarization and incident angle. In addition the normal hemispherical reflectance for each sample was measured. The normal incidence data were solar averaged over an air mass two solar spectrum for the following cases: (1) an average over the solar energy spectrum as reported separately by Moon and Thekaekara to obtain a solar transmittance which is applicable to a solar thermal collector; (2) an average over the solar photon spectrum to obtain a solar transmittance which is applicable to a solar photon converter; and (3) an average over the solar photon spectrum which has been limited to the active wavelength region of several common solar cells. Solar energy transmittance values for the materials studied ranged from 0.95 to 0.77. The solar energy averages calculated using Thekaekara's data were from 0 to 3.5% lower than the averages calculated from Moon's data. Depending upon the material, the difference between the solar energy average and the solar photon average limited to the active range of a solar cell ranged from 1% to as much as 11%. The angular dependence of the solar transmittance was found to agree with calculations using a multiple beam transmittance model for materials that have very little scattering in the transmitted beam. However, for materials with appreciable scattering in the transmitted beam, differences between calculated and measured angular transmittance values as large as 0.04 transmittance units were found using this model. © 1979.
