Total and regular transmittances of dry and condensate covered transparent materials

I.V. Pollet, J.G. Pieters, and R. Verschoore, Ghent University, Belgium

Keywords

transmission, forward scattering, diffusion, photosynthetically active radiation (PAR), condensation, greenhouse, solar energy, optical transmission properties

Abstract

By means of a laboratory measuring unit, the total and regular normal transmittances of four different transparent materials were investigated, namely a non-coated single glass plate (SG); a low-density polyethylene film (LDPE); an anti-drop condensation polyethylene film (ADCPE); and a diffusive polyethylene film (DPE). The transmittances were measured in the dry state and when covered with condensate.

In the dry state, the total transmittances were highest and almost similar (89 %) for those products (SG, LDPE, ADCPE) whose transmittance curves showed no or a small spectral dependence. The total transmittance of DPE, however, varied between 67 and 84 % in the visible radiation range.

In the dry state, the SG plate did not scatter the transmitted radiation, in contrast to the plastic films which scattered at least 50 % of the transmitted radiation. The total normal transmittance of two materials, namely SG and ADCPE, was not affected by the presence of condensate due to their relatively high wettability, whereas the total transmittance of the plastic films with no anti-drop agents, namely LDPE and DPE, was obviously decreased (up to 25 %) by the condensate.

With the exception of ADCPE, the forward scattering of the claddings was broadened by the presence of condensate. The most diffusive material in the dry state, DPE, remained the most diffusive one when covered with condensate. More than 90 % of the radiation transmitted by wet DPE was diffuse. Transmittance differences between materials were due to the different structure and composition of the materials and to the variation in shape and size of the condensate drops.