Next to the irradiation level, the extent to which the transmitted radiation is diffused, gains interest in the field of protected plant production and other kinds of solar collector systems. By means of a laboratory measuring unit, the diffusive behaviors of four transmitting materials, namely single glass (SG), low density polyethylene (LDPE), anti-drop condensation polyethylene (ADCPE) and diffusive polyethylene, were investigated in the dry state and when covered with condensate. Dry SG hardly diffused radiation and behaved as an optically smooth material. Due to their surface roughness and bulk heterogeneities, the LDPE film and the ADCPE film diffused about 50% of the transmitted radiation, while the diffusive polyethylene film diffused more than 80% of the transmitted radiation in the dry state. When covered with condensate, the diffuse fractions of the transmitted radiation of all four materials, except the ADCPE, were obviously enhanced. Wet SG and wet LDPE showed a similar diffusion pattern. Slightly more than 80% of the transmitted radiation was diffused by both materials. The diffusive polyethylene film remained the most diffusive transmitting material in the wet state. More than 90% of the transmitted radiation was diffused. About one quarter of the transmitted radiation was diffused at angles greater than 45degrees. None of the materials investigated in the dry as well as in the wet state approached a Lambertian diffusion pattern. (C) 2004 Elsevier B.V. All rights reserved.