|Title||Evaluation of diffusing properties of greenhouse covering materials|
|Author(s)||Hemming, S.; Swinkels, G.L.A.M.; Breugel, A.J. van; Mohammadkhani, V.|
|Source||Acta Horticulturae 1134 (2016). - ISSN 0567-7572 - p. 309 - 316.|
|Department(s)||WUR GTB Tuinbouw Technologie|
|Publication type||Refereed Article in a scientific journal|
|Keyword(s)||BTDF - Diffuse light - F-scatter - Glass - Haze - Light scattering - Plastic films|
Modern greenhouse covering and screen materials are able to transform direct sunlight into diffuse light. After entering the greenhouse the sunlight is scattered and penetrates the crop where it is absorbed and used for photosynthesis. Diffuse light has been shown to increase light interception of crops, increase crop photosynthesis and results in higher yields. Many different types of diffuse covering materials are available, and growers select materials based on crop, outside climate where the greenhouse is located and market situation. Earlier measurement methods to evaluate diffuse covering materials have been proposed (Hemming et al., 2008) and are mainly based on the measurement and evaluation of the hemispherical light transmittance, the average amount of light coming into the greenhouse (Nijskens et al., 1985; Papadakis et al., 2000) and the Haze (e.g., ASTM-D1003-07), the average amount of scattered light reaching the crop. A more detailed evaluation of the angular and spectral light transmittance of materials is needed (Hemming et al., 2014), including the spatial distribution of scattered light, or the way light is scattered by a material. In this paper we propose a new method of measuring the spatial distribution of scattered light by a bi-directional transmittance distribution function (BTDF) measurement. By comparing the BTDF values of a diffuse covering or screen material with the BTDF values of a Lambertian diffuser, we calculate one simple value for material characterisation, the F-scatter. F-scatter is a more representative value for the scattering properties of a material than Haze. The detailed material scatter properties can be used for further raytracing analysis of materials in greenhouses with a specific roof construction, at a specific location for any moment of the year. It can also be used for 3D functional crop models in the future.