Diffuse screens.

Scattering sunlight so no leaf is left in the shade

Besides diffuse glass or a roof coating, diffuse light can also be created with screens. Although these screens are used first and foremost to distribute incoming light evenly, they double as sun protection and energy saving, and they come in a range of shading percentages that can be matched to the light needs of each crop. Over the past decade diffuse screens have become steadily more popular with growers of many different crops and climates.

The defining feature of every diffuse screen is the white strips woven or knitted into the fabric, which scatter the light passing through. Fabrics are available with an open structure, which combined with ridge ventilation gives optimal cooling and sun protection, or with a closed structure, which reduces cooling in the evening, lowers the risk of condensation forming on the crop and saves energy at night. The choice between them is a climate and crop decision as much as a light one.

The agronomic value comes from how diffuse light behaves in the canopy. With direct sun, distribution is far from ideal: the greenhouse structure, equipment and the leaves themselves cast shadows, so some plants receive too much light and suffer stress or burn, while others sit in shade and grow poorly. High-intensity light on a single leaf is also less efficient for photosynthesis than the same light spread evenly. Diffuse light solves both problems at once — horizontally it evens out the bright and dark patches across the crop, and vertically it penetrates deeper into tall crops like tomatoes, peppers and cucumbers, where under direct light the top leaves capture more than they can use while the lower leaves are starved. The result is more uniform, healthier growth and better overall photosynthesis.

Diffusion is measured in two ways, and the distinction matters when comparing products. The traditional Haze Factor states the percentage of perpendicular incoming light that is deflected by more than 2.5 degrees — a haze factor of 60% means 60% of direct light has been redirected. The problem is that sunlight almost never enters the greenhouse perpendicularly, so Wageningen University developed the F-Scatter (or HortiScatter) method to give an accurate diffusion value at realistic angles of 10 degrees or more. Both methods are integrated into the NEN 2675:2018 standard, allowing growers to compare glass, screen fabrics and horticultural coatings on a common basis. We help select the right diffuse fabric and diffusion value for your crop and radiation profile — get in touch to discuss it.