Karl Sewalt MSc
- product innovation
- product development
- industrial design engineering
The light transmittance of clear glass greenhouse roofs can be determined with an existing test protocol based on reference values. One simple measurement provides perpendicular incident light and diffuse light transmittance (sunlight scattered by clouds). However, this method is not suitable for new, light-diffusing glasses and plastics. TNO has therefore developed new measurement protocols so growers can make an informed choice of material.
The less light a greenhouse covering or greenhouse roof blocks, the better. Growers therefore want to be able make an informed choice from various material options. Indeed, choosing a different type of glass could yield extra points for a Green Label Greenhouse Certificate. When a grower intends to build a new greenhouse, it is therefore important to know what the light transmittance of the greenhouse roof will be. Until recently, the amount of light transmission was measured according to a protocol based on perpendicular incident light. With this protocol, the transmittance for clear glass can be determined by making a perpendicular measurement and the diffuse light transmittance (sunlight scattered by clouds) can then be calculated on the basis of a standard curve. The current generation of greenhouse roof materials, however, is often made of plastic or coated glass, for which this measurement protocol is unsuitable.
This prompted TNO and Wageningen UR Greenhouse Horticulture to develop reliable new measurement methods for determining the diffuse transmittance values of these materials. The first method is a hemispherical measurement, which involves illuminating a concave reflector that reflects the light. This method is fast, as just two measurements are needed, yet the arrangement is complex and large and the measurement only yields the diffuse transmittance value.
The second method, the angle-dependent measurement, takes more time because several measurements are required. Light is shone straight through the material at various angles, and the hemispherical transmittance is then derived from the weighted average. The arrangement is more compact than that for the hemispherical measurement and provides additional information, such as the transmittance values from various angles. This information may lead to changes in the design.