Laboratory measurements of PAR transmittance of wet and dry greenhouse cladding materials

A laboratory set-up which consisted of two climate rooms (cold box/hot box) was used to assess the effects of condensation on the PAR transmittance of a vertical greenhouse cladding material under controlled climate conditions for several incidence angles. It was observed that the absolute transmittance difference between dry and wet polyethylene decreased gradually from 23% to about 0% for increasing incidence angles from 0 degrees to 75 degrees. Results for radiation incident in the horizontal and the vertical planes agreed within 3% over all incidence angles. The diffuse radiation transmittance for a wet polyethylene Blm was 15% lower than the one for a dry polyethylene film. (C) 1999 Elsevier Science B.V. All rights reserved.