SOLAR-ENERGY TRANSMITTANCE OF TRANSLUCENT SAMPLES - A COMPARISON BETWEEN LARGE AND SMALL INTEGRATING SPHERE MEASUREMENTS

Optical transmittance and reflectance of a translucent plastic PTFE hi m have been measured over the solar wavelength range using different i ntegrating spheres. The same sample has been measured with small and l arge spheres and the total solar transmittance has been obtained from both broad band measurements and from integration of spectral data. Th e fact that the sum of reflectance and transmittance often exceeds 100 % shows that all types of spheres tend to overestimate the transmittan ce of this highly scattering sample. This error can be attributed to t he sphere geometry in combination with the light scattering properties of the sample, and unless proper correction of recorded data is carri ed out the error may be as large as 5-10%. Some specific errors are pr esented and an approximate correction procedure is suggested. These re sults show that there is a need for a transmittance standard which can be used to calibrate integrating spheres. Such a standard with neglig ible thickness would be especially useful for measurements with large, broadband integrating spheres, but would also be helpful for the corr ect handling of data from small spectral instruments. (C) 1998 Elsevie r Science B.V. All rights reserved.