Wageningen UR Greenhouse Horticulture has developed a new device for measuring the optical properties of greenhouse covers, the so-called Transvision. This device can determine the transmission of diffuse light and can deal with thick materials, which was impossible with the traditional measuring ball.
The measuring and mathematical method needed to be amended to be able to measure the light transmission of new greenhouse cover materials. This is why Wageningen UR Greenhouse Horticulture is now introducing a new technique for measuring light transmission of materials.
Scientist Gert-Jan Swinkels explains: “The device we have developed, the so-called Transvision, is unique. Until now, light transmission by materials could only be measured for perpendicular light. Only a few millimetres of thin materials could be measured. For greenhouse horticulture, however, measurement of the transmission of diffuse light is much more important and in case of multi-layer materials it is important that the transmission through thicker layers can also be determined. Our device can do this following an adaptation of the design of the Ulbricht ball. This ball is already for some hundred years being used for comparing perpendicular light.”
The new ball measures the amount of diffuse light transmitted through the greenhouse cover under a clouded sky. This is an important criterion, especially in the Netherlands, where global radiation in winter mainly consists of diffuse light. But there is more, the measuring ball can also determine the effect of, e.g., a coating that makes the light more diffuse by dispersion. The device can also measure relatively thick materials. This is, e.g., interesting for measurement of synthetic sheets consisting of several layers.
NEN norm revised
A large number of new materials with properties unfamiliar to traditional greenhouse glass have been placed on the market in recent years. Such materials have a higher isolation value, transmit more light, or increase light dispersion through a microstructure or pigment. The greenhouse glass norm NEN 2675 has for a long time been used to determine light transmission. This norm is based on determination of light transmission under perpendicular light incidence on clear materials. To enable a fair comparison between new materials, Wageningen UR and TNO have in 2009 developed a new measuring protocol and it has been decided that NEN 2675 needs to be revised. The Transvision will be used to support manufacturers in developing new materials, for benchmarking of materials, and for collecting measuring data for energy and light models.