PAR TRANSMITTANCE OF DRY AND WET GLASS

Abstract

A single, a low emissivity, and a double glass plate were investigated with respect to their transmittances in the photosynthetically active radiation (PAR) range. Directional-hemispherical transmittances of these glass plates were determined under laboratory conditions in the dry and the wet (i.e., condensate covered) states. The transmittance curve of the low emissivity glass plate lay below the one of the single glass plate and above the one of the double glass plate. The presence of a condensate layer caused a similar transmittance reduction for single and double glass. This reduction reached a maximum of 13 to 15% at incidence angles of 50-65 degrees. The transmittance reductions determined for the wet low emissivity glass were smaller and restricted to at most 11% at an incidence angle of 70 degrees. Assuming uniformly diffuse radiation, condensation was found to lower the transmittance by 8% for single and double glass and 5% for low emissivity glass.