Sunlight has a wavelength range of 300 - 2500 nanometres on the Earth's surface. This range includes ultraviolet (UV), photosynthetically active radiation (PAR), and near-infrared (NIR). Each of these wavelengths has its own effect on the greenhouse climate or the crop. For example, PAR light influences photosynthesis: more PAR leads to higher production. Infrared mainly ensures that the air and the greenhouse are heated. The Greenhouse Horticulture and Flower Bulbs Business Unit of Wageningen University & Research is investigating whether there are materials with optical properties that can ensure the correct light balance in the greenhouse.
UV usually makes up about 5 to 6% of sunlight; PAR and near-infrared account for 45 to 50% each. The exact ratio between the different types of light depends on many factors, such as location, season, time of day, and possible cloud cover. The desired ratio also differs per location (climate), per season and even per crop. When it is warm, it is wise to block near-infrared as much as possible, for example, with a screen or with a seasonal coating on the greenhouse roof. On cold days with less sunlight, anti-reflective coatings reduce the reflection of light away from the greenhouse cover, thus increasing the overall entry of light inside. UV light provides an extra boost to color or flavor in some crops (e.g., basil) but may cause damage in other crops.
Some materials can help achieve the desired balance of the light wavelengths. WUR is investigating which materials can be used for this, either in the greenhouse cover or in a screen or coating. One example is 'fluorescent' materials, which are made of molecules or atoms that can absorb light of a particular energy (having a certain wavelength) and briefly get 'excited' to a higher energy level - as they stabilize back to their original state, they emit lower-energy (or longer-wavelength) light. In the greenhouse, such material might then absorb high-energy UV light and emit lower-energy red light, increasing the total PAR light as a result. However, they also tend to lose part of this 'extra' PAR radiation by emitting some of it out of the greenhouse or by re-absorbing it, so they need to be improved to achieve the best outcomes in the greenhouse. Another example is materials that cause light to spread so that the diffuse radiation penetrates deeper into the crop.