The radiation transmittances of a standard glass plate and an ordinary polyethylene (PE) film were measured during a complete condensation cycle (dry phase; condensation without run-off; condensation with run-off evaporation phase). The measurements were performed in a specially developed laboratory measuring unit, using perpendicularly polarized monochromatic radiation in the range of photosynthetically active radiation (PAR). Transmittances were determined for three angles of incidence: 0, 30, and 60°. In the dry phase, at normal incidence, the transmittance of the ordinary PE film (about 90% was almost similar to the one of the standard glass plate (about 89%). For increasing incidence angles, the transmittance of both dry cladding materials decreased. As soon as condensate was formed, a quick decrease of the transmittance was found for the PE film for the three angles of incidence (23% at normal incidence). This transmittance decrease adopted an equilibrium value after approximately 1 h. On the glass plate, however, the quick drop of the transmittance was only observed for higher incidence angles. At normal incidence, the effect of condensate on the transmittance of the glass plate was found to be negligible. In most cases, the occurrence of run-off caused a slight change of the transmittance of both cladding materials with respect to the phase of condensation without run-off. As soon as the evaporation of the condensate started, the transmittance returned gradually to its original value for the dry state. In general, it can be stated that the effects of condensation on the radiation transmittance were greater on the PE film than on the glass plate.
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