Growers are increasingly learning that steering the greenhouse climate, which includes the related factors of light levels, temperature and humidity, can yield big gains in terms of plant quality and productivity as well as economic efficiency in heating and cooling expenses. Researchers, meanwhile, have been demonstrating that diffuse light plays an important role in achieving those goals. Materials have been developed to diffuse direct solar radiation, and use of diffuse light begins with an understanding of light and its behavior.
Solar radiation passes through the top of the atmosphere essentially unchanged by its passage through space. As it moves, it interacts with gas molecules, water droplets and aerosols and is gradually absorbed into the atmosphere. Radiant energy is converted into internal energy. It ceases to exist as radiant energy and heats the atmosphere directly.
Very little of this absorption happens in visible wavelengths. Rather, ozone absorbs most of the ultraviolet rays. Carbon dioxide, water vapor and water droplets absorb near infrared radiation.
Likewise, sunlight passes through the atmosphere without being altered. Light arrives at the Earth’s surface directly from the sun. That’s what makes the disk of the sun visible, as well as creating patterns of strong light and shadows.
With diffuse radiation, some light reaches the surface after being scattered. It appears to originate from the location of the last scattering event. You can see it illuminating the sky outside of the direct solar beam. Because there is no atmosphere on the moon, in contrast, the sky around it is black except for the direct light source.
The amounts of direct and diffuse light reaching the surface vary depending on atmospheric conditions.
A clear, dry atmosphere will produce a large amount of direct light but little diffuse light. You’ll see a high contrast between dark shadows and brightly illuminated surfaces.
On the other hand, a sky with a great deal of cloud cover results in only diffuse light and no direct light. Light comes from all directions of the sky, so the only shadows are directly underneath things. The disk of the sun is not visible. It’s this light that’s been shown to be most beneficial to greenhouse crops.
Advantages Of Diffuse Light
Simply put, diffuse light is light scattered by particles, which can be found in clouds or in whitewash, various types of glazing or shades. Most plants can benefit from diffuse light. Diffuse light penetrates deeper for greater photosynthesis by activating more of the canopy (less shading by upper leaves). In addition, diffuse light helps encourage better growth due to a better distribution of light (more evenly spread rather than with hot spots and shady spots).
In studies on vegetables and pot plants, researchers at the Netherlands’ Wageningen UR Greenhouse Horticulture showed that diffuse light can help provide benefits such as improved crop yield, higher leaf count, lower crop temperature and shorter crop time.
However, researchers also found that in the “darker” portion of winter (such as November), diffuse light’s benefits did not make up for the loss of light transmission when using a greenhouse film. Thus, they concluded, there is a need for greenhouse coverings that scatter the light but do not reduce it.
We will see a larger benefit from diffuse light in areas with more direct light. For example, Holland is already rather cloudy due to its geography, so the effect of diffusing light will not be as great as it will be in sunnier areas where heat stress can be created in the greenhouse environment by high levels of direct solar radiation.
Haze and light transmission can be altered. The challenge is converting direct light to diffuse light without decreasing light transmission to the crops.
Research in Holland on cucumbers in 2008 found that fruit production in kilos was 4.3 percent higher and the number of cucumbers was 7.8 percent more with diffuse light compared to a clear covering, even while their diffuse covering provided 4 percent less light. This is due to diffuse light’s ability to better penetrate the plant canopy to stimulate photosynthesis. With today’s advanced light-diffusing curtains, production would be even higher. Researchers estimated then that with just 4 percent more light, production in kilos would have been 7.8 percent more with 11 percent more fruit.
Results also depend on whether growers are using fixed or moveable solutions. With fixed solutions such as glazing and whitewash, light transmission is lost when conditions are too dark. With a permanent glazing, this can happen during any dull period in the year. Growers are typically losing light in the winter, for example.
Determining The Best Way To Diffuse Light
With whitewash (semi-permanent), light can be lost any time after it is applied but before it is removed. This is because whitewash is typically applied in the spring when greenhouse temperatures begin to rise to uncomfortable levels. Growers must guesstimate when to apply. However, often a late winter storm or a cloudy or rainy period rolls in, and the grower regrets having applied this labor-intensive, semi-permanent shading.
The ideal answer is a moveable solution to provide diffusion only when needed.Plastic films that diffuse light evenly help improve production, manage water stress control and provide greater photosynthesis. They also help prevent conditions like leaf burn during high-light days, and if installed properly, can help change the light’s angle as well. But a fixed roof glazing is also a permanent solution that reduces light transmission during all periods, including dim months and hours when light may be desired.
As our understanding of diffuse light and its properties deepens, we are able to tailor products to harness its benefits. For example, Svensson designed a product with proprietary white strips to reflect sunlight plus retain heat in the greenhouse when there is no sun or the outdoor temperature drops.
Testing at greenhouses in different parts of the world showed that the screens, designed in both open and closed structure designs integrating Svensson’s white strips, offered high-grade light diffusion, which allows for greater light delivery into the canopy. The results: excellent cooling, improved plant quality and even shortened production time.
The best action is to diffuse light only when the sun is intense with an on-demand, movable solution that protects, cool and brings light only when you want to — when your plants need it. Diffuse light offers many benefits for greenhouse crops; the challenge is finding the answer that enables you to best harness it for your business.