Light transmission through condensation on glass and polyethylene

Laboratory measurements of the light transmittance through dry and condensation covered glass and polyethylene were made. Theoretically condensation on glass is expected to behave as a film, while that on polyethylene as drops. The experimental measurements showed that for both surfaces the behaviour was between these two limiting situations. To allow for a more accurate description of the influence of condensation on the radiometric properties of cladding materials, a model for the simulation of light transmission through drops with any geometry is presented. By means of this model, it is demonstrated that a model of cap-shaped drops with contact angles between 40 degrees and 70 degrees for glass and between 40 degrees and 90 degrees for polyethylene describes better the behaviour of condensation without run-off. Comparing the results for a series of cap-shaped drops, drop size is shown to be unimportant in assessing light transmittances. The contact angle of the drop, however, is demonstrated to be a very important parameter. Furthermore, it is shown that diffusion by condensation drops takes place in a narrow solid angle around the direction of the light rays entering the drop and that it is almost independent of the contact angle of the drops. (C) 1997 Elsevier Science B.V.