Measuring facility for (shortwave) radiation transmission of condensation covered greenhouse claddings

To investigate the effect of water drops on the Photosynthetically Active Radiation (PAR) transmittance of greenhouse cladding materials, an experimental unit was built. A light-tight double climate room (hot box/cold box), equipped with a temperature and humidity control system, permits the controlled production of condensation on any cladding material. By means of a parallel light source inside the cold box and an integrating sphere in the hot box, it is possible to measure the PAR transmittance for several cladding materials, for the several phases of the condensation and evaporation process, and for several orientations of the material under investigation: vertical or inclined. Preliminary results for experiments on vertical polyethylene (PE) and glass covered with condensation without runoff resulted in a decrease of the PAR transmittance at normal incidence by about 22 and 3%, respectively. When compared to the transmittance curve for dry PE, the one for wet PE was found to be less steep for incidence: angles up to 60 degrees and to agree best with the average of simulated curves for cap shaped drops with contact angles between 40 and 90 degrees.