LIGHT TRANSMISSION THROUGH GREENHOUSE COVERS

Abstract

The selection of the greenhouse covering material is essential for the success of the protected horticultural production, and among the properties of a material (optical, chemical, physical, etc…) the most important for the growers are the optical properties, as they define the amount and quality of solar radiation entering the greenhouse and thus the amount of photosynthetic active radiation (PAR) that will be intercepted by the crop, determining the production of dry biomass. However, there is no such a material that can be considered optimum, in terms of optical properties. Therefore, the grower has to choose a suitable covering material, with the optimum optical properties for its specific growing conditions. The most important covering materials used in horticulture in terms of covered surface in the world are flexible plastic films, glass and different types of screens. The industry is continuously incorporating new optical properties to these materials that may be of interest for the growers depending on their specific growing conditions. In the case of glass recent developments tested in The Netherlands include anti-reflection coatings, to increase the global radiation transmission at low sun radiation incident angles; new glasses have also been tested with different levels of light diffusion with none of minimum loss of global transmission, with very promising results in terms of light use efficiency by the crops, and thus, interesting yield increases. Other types of filters being tested are near infrared filters (absorption or reflection) both in glass and in plastic films, with results that suggest that reflection is preferable and that few materials are available that do not affect PAR transmission too. UV filters have provided very remarkable results in limiting the infestation by pests in plastic films. In the case of screens, a large variability of materials are available (in terms of porosity, colour of the threads, photoselective filters, etc.), and recently have been subject of different studies to understand better the combination of direct transmission of light through the pores and the interaction with the threads of the incoming radiation.