Measuring light transmissivity: transmission and reflection of dry and wet materials
When light strikes a transparent material, three things can happen. The light can be transmitted, it can be reflected, or it can be absorbed. Transmission, reflection and absorption together always account for 100% of the incoming light. WUR LightLab uses a variety of equipment to identify these characteristics in glass, films, plastic sheets, screens, nets and seasonal coatings.
This device consists of an integrating sphere, a light source comparable to sunlight and different optical components, the device is designed by us in order to measure hemispherical transmission based on the double-beam method.
The material is placed on the top of the sphere, and a light source is directed at the material as well as at an opening next to the material to act as a control. A sensor (spectrophotometer) in the sphere determine how much light is transmitted by the material compared to the control. The entire spectrum of UV, PAR and NIR solar radiation can be assessed. All angles of incidence are measured, simulating the movement of the sun in relation to the greenhouse cover.
The Transvision can measure a wide range of materials: ordinary glass, diffused glass, glass with coatings such as anti-reflection and low emission, double glazing, films, plastic sheets, screens, nets and even fluorescent materials. The equipment can easily handle complex materials such as those involving structures or lenses. Measurements of PAR transmission through glass and screens are done in accordance with NEN2675:2018 +C1.
Customers receive a table showing the transmission levels at various angles of incidence and a calculation of the hemispherical transmission.
Perkin Elmer Lambda 950TM (UL270)
The Perkin Elmer Lambda 950TM (Perkin Elmer Inc.) is a commercially available device which consists of an integrating sphere to measure the transmission and reflection of semi-transparent materials. Supplementing it with the UL270 Integrating Sphere Accessory (OMT Solutions B.V.) enables us to measure perpendicular transmission and reflection at an 8° angle. This can be done for every part of the entire solar radiation spectrum from 300 to 2000 nm, meaning UV, PAR and NIR.
Customers receive a measurement of the transmission in accordance with NEN2675 for the entire solar spectrum or parts of it.
The optical characteristics of materials are typically assessed in dry conditions. But a horticultural greenhouse is rarely dry. Moisture can condense on a material in different ways – for example in water drops (hydrophobic) or as a water film (hydrophilic) – and this in turn affects light transmission.
De Transvision Wet measures the effect of condensation on a material. The device measures the hemispherical reflection of the greenhouse cover material both under dry conditions and when condensation is present, for several consecutive hours. The material stands at an angle for this measurement, as it would in a greenhouse. High levels of moisture are generated, just like in a greenhouse containing a crop. The hemispherical reflection measured in both dry and wet conditions then enables us to express the change in hemispherical transmission as a percentage.