Rapport GTB-1252Gert-Jan Swinkels, Silke Hemming, Vida Mohammadkhani, Jim van Ruijven Wageningen UR Greenhouse HorticultureProtocol for measuring light transmission of horticultural screens2AbstractThe light transmission is an important property for horticultural screens. For energy screens in particular, an accurate measurement of the light transmission is important because these screens are often used during the day, in the winter period when radiation is limited. For shading screens the shading factor is an important factor. To enable a fair comparison between horticultural screens Wageningen UR in collaboration with screen producers Ludvig Svensson, Novavert and Bonar, developed a new protocol for measuring the transmission of horticultural screens. The protocol is based on the earlier developed protocol for measuring the light transmission of greenhouse covering materials, which was developed by TNO and Wageningen UR. The protocol covers the measurement of the transmission of horticultural screens for photosynthetically active radiation (PAR) in terms of hemispherical transmission. The scope of the protocol is limited to transparent screens with hemisferical transmittance greater than 10% and does not include the measurement of blackout screens. The protocol is regarded as the standard by the parties involved. The transmission measured can serve as a basis for comparing horticultural screens and can be used in calculating the performance of greenhouses.© 2012 Wageningen, Foundation Stichting Dienst Landbouwkundig Onderzoek (DLO) research institute, Wageningen UR Greenhouse Horticulture (Wageningen UR Glastuinbouw).Wageningen UR Greenhouse HorticultureAddress : P.O. Box 644, 6700 AP Wageningen, the NetherlandsTel. : +31 317 - 48 60 01Fax : +31 317 - 41 80 94E-mail : greenhousehorticulturewur.nlInternet : www.greenhousehorticulture.wur.nl3Inhoudsopgave1 Introduction 52 Scope 73 Terminology 94 Equipment 114.1 Light source 114.2 Integrating sphere 114.3 Detector 125 Measurement principles 135.1 Single beam and double beam system 135.2 Critical dimensions 156 Protocols 176.1 Direct transmission 176.2 Hemispherical transmission 19Literature 21Appendix I Determination of the single beam substitution error 23451 IntroductionLight is the energy source for plant growth. Solar radiance is sustainable and free and therefore the most favourable source of light in greenhouse crop production. Plant growth and with it the economical production depend strongly on the (daily) light sum at plant level. Therefore knowing the light transmission of the greenhouse covers and screens is important when considering building a new greenhouse. For energy screens in particular, an accurate measurement of the light transmission is important because these screens are often used during the day, in the winter period when radiation is limited. For shading screens the light transmission (shading factor) is an important factor for plant growth.For greenhouse covering materials, a standard for measuring light transmission which is used traditionally is the Dutch norm NEN 2675 (NEN 2675, 1990). This norm describes a method for measuring the PAR transmission of standard float glass for normal incidence (parallel to the normal of the surface) and is primarily intended for product comparison. For the optical performance in greenhouses however, the hemispherical transmission is a more important factor than the perpendicular transmission, especially at Northern latitudes. Here diffuse light dominates the global radiation, up to 80% in winter. Moreover, since the last 5 years, a large variety of new covering materials with improved thermal insulation, light transmission and diffusing properties has been developed. For these materials, often with coatings, the relation between the normal and hemispherical transmission is not straightforward which makes the NEN 2675 inappropriate. To enable a fair comparison between greenhouse covering materials, TNO and Wageningen UR developed a new protocol for measuring the transmission of greenhouse covering materials (Ruigrok, 2008). Before the publication of this document, no standard protocol was available for measuring the light transmission of shading and energy screens used in greenhouse horticulture. Different screen manufacturers and research institutes used different methods, which made a comparison of light transmission values for growers and industrial partners difficult. For that reason the standard protocol for measuring the light transmittance of horticultural screens, described in this document, was developed in co-operation with relevant screen manufacturers.Because measuring the transmittance of covering materials has many similarities with horticultural screens the screen protocol is based on the protocol for measuring covering materials. Screens are often inhomogeneous materials composed from different materials such as plastic, aluminium. They are three-dimensional structures influencing light transmission paths through the material. Depending on the screen type the patterns is relatively large which requires relatively large samples for transmission measurements.The measurement protocol for horticultural screens has been developed in cooperation with: Bonar (bonartf.com) Ludvig Svensson (svenssonglobal.com) Novavert (novavert.com)672 ScopeThis protocol covers the measurement of the transmission of horticultural screens for photosynthetically active radiation (PAR) in terms of hemispherical transmission. The scope of the protocol is limited to transparent screens with hemisferical transmittance greater than 10% and does not include the measurement of blackout screens. The protocol is regarded as the standard by the parties involved. The transmission measured can serve as a basis for comparing horticultural screens and can be used in calculating the performance of greenhouses. This protocol does not address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this protocol to establish appropriate safety and health practices.893 TerminologyHemispherical light Light coming from a hemisphere over the observer or target and which is distributed equally over the hemisphere surface.The hemispherical transmission (Them) The total transmission for hemispherical light.Single beam substitution errorThe systematic, predictable, and non-random error inherent in single beam integrating spheres measuring reflectance or transmittanceLight intensityThe value for each wavelength (range) measured by the detectorAngle of incidence The angle of incidence (AOI) is the angle between a beam incident on a surface and the line perpendicular to the surface at the point of incidence, called the normal.10