RADIATION TRANSPORT AND IMAGE TRANSMISSION THROUGH AEROGELS

Optical and thermal properties of aerogels play an important role in their application as transparent insulation, daylighting systems and superinsulated windows. Directional-directional transmission, t(dd), as well as the directional-hemispherical transmission, t(dh), are presented for 20 mm thick monolithic and granular SiO2 aerogel. For optimized monolithic aerogels, averaged transmission values of t(dh,sol) = (91 +/- 1/4)% and t(dd,vis) = (84 +/- 2)% were obtained in the solar and visible spectral region. The granular aerogels show a visual transmission of t(dh,vis) = (58 +/- 4)% for waterglass as a precursor and of t(dh,vis) = (78 +/- 4)% for tetramethoxysilane. To characterize the visual image transmission the modulation transfer function (MTF) was measured for 20 mm thick monolithic layers. In order to explain the measured data a Monte Carlo based simulation model was developed. Results of this analysis indicate that the MTF is mainly affected by large inhomogeneities (> 1 mu m) i.e., predominantly be surface scattering.