Measuring the light transmittance of greenhouse covering materials has become a standard procedure for the horticultural industry. This is done standard on dry materials. However, transmittance changes (strongly) when water condenses on the inside of the material. Condensation on clear glass lowers the transmittance by a few percent while condensation on diffuse increased transmittance with a few percent.
For example, transmittance of clear non-diffusing class usually decreases because the condensed droplets tend to reflect the light instead of transmit it, especially if the glass has a hydrophobic coating.
With the introduction of diffuse glass and practical experiments that have been done with these materials it has become clear that the advantage lies not only in a better light distribution but also in an increased light transmission for wet conditions. In contrast to clear glass, not droplets are formed on diffuse glass because the condensate forms a layer that has an anti-reflecting effect which increases the transmission.
Because a greenhouse is wet on the inside during the period of the year when sunlight is limiting, it is important to be able to measure transmission with a standardized protocol. Because the current light transmission protocols are based on dry measurements, a reliable and robust measurement protocol for greenhouse covers with condensate is being developed which will become available for horticultural industry. The protocol is based on real condensation which occurs with the right combination of temperature and humidity and a material which is put at an inclination equal to a Venlo greenhouse cover.
The preliminary results show that condensation on clear glass lowers the transmittance by a few percent while condensation on diffuse increased transmittance with a few percent and that the measurement is reproducible.