PAR transmittances of dry and condensate covered glass and plastic greenhouse cladding

In this study, the angular transmittances over the photosynthetically active radiation (PAR) range of two types of glass (single glass (SG) and low emissivity glass (LEG)) and three types of plastic films (low density polyethylene (LDPE), anti-drop condensation polyethylene (ADCPE) and photoselective diffusive polyethylene (DPE)) were determined experimentally in the laboratory. The measurements were performed on dry as well as on condensate covered materials, since condensate frequently occurs on a greenhouse cladding material. Based on the angular measurements, the transmittance of diffuse PAR incident on a 25° inclined slab was calculated. For the dry state, it was found that the PAR transmittance decreased with increasing photoselectivity of the materials. When covered with condensate, the PAR transmittance of the glass plates was only reduced at incidence angles higher than 15° by at most 20% on a relative scale, while the transmittance of non-anti-drop condensation films was especially reduced at small angles of incidence by at most 25%. Due to the addition of anti-drop condensation agents to a plastic film, the PAR transmittance of the film was not affected by the condensate and, therefore, showed the highest PAR transmittance in the wet state. For the photoselective cladding materials, the spectral dependency of the transmittance was only slightly changed by the condensate.