The Importance of Light Diffusion on Your Canopy

The horticultural industry is always looking for new ways to optimize yields in a sustainable manner, with a focus on lighting crops correctly and efficiently. When we discuss lighting for crops, we tend to define optimal lighting with three measurements: light intensity (or quantity), light duration (or photoperiod) and light quality (typically spectrum).

With the onslaught of so many LED supplemental lighting companies coming into the industry there has been many studies conducted on light quality from light spectrum. What is the right spectrum of light for your crop and at what stage of growth?

I am not going into detail on this part of light quality, but want to point out that light quality should not only be thought of as light spectrum. The amount of diffusion of light photons should be considered just as important as spectrum when discussing light quality. From sunlight to supplemental lighting, let’s take a look at light diffusion.

What Is Light Diffusion?

Simply put, light diffusion is the scattering of light. It is photons bouncing around in all directions. In its most simple form, think about standing under a large outdoor shelter when the sun is highest in the sky. If there was no diffusion of light at all, it would be very bright outside the shelter but totally dark under the shade of it. The reason we can still see well in the shade is the natural diffusion of sunlight by our atmosphere.

Photons from the sun get scattered as they enter through our atmosphere and bounce around in the air enabling us to see when not directly under the sun. This natural diffusion of sunlight is also why the sky looks blue. This is called “Rayleigh Scattering” (named after the nineteenth-century British physicist Lord Rayleigh) which states that the shorter wavelength of blue light that is higher energy gets scattered by air particles in our atmosphere that are smaller than the wavelengths of light themselves.

The high energy, shorter wavelength, of blue gets scattered much more than the longer wavelength photons in the visible spectrum. Conversely, when we see clouds in the sky that look white, this is because the water droplets in the sky are larger than the wavelengths of light so they capture and scatter all of the colors of the light spectrum, making them appear white. This type of natural light diffusion by clouds is referred to as “Mie Scattering” (named after German physicist Gustav Mie).

Now that we understand light diffusion, why is it so important for light quality in horticulture?

Light Quality

Plants have evolved for millions of years and are very efficient in utilizing the sun’s rays for photosynthesis. As man began cultivating plants for food, there was a long part of our history that we only farmed and cultivated plants that grew naturally in our local environment. As of today, as our population grows there is a large demand to sustainably grow our food closer to our population centers.

This has obviously been largely accomplished by the greenhouse sector and the growing indoor farming sector. As my example above stated, sunlight is naturally diffused in our atmosphere. Plants have learned to utilize the direct sunlight with their large canopy leaves as well as utilizing diffused light (or indirect light) in their leaves on lower branches.

If there was no light diffusion in nature, plants would only grow large canopy top leaves. Plants are very efficient at utilizing light for photosynthesis at all levels of the plant. Now that we are moving crops into greenhouses and indoors, there are more and more studies being done to show the importance of ensuring there is good diffusion in these facilities to optimize light quality.

Horizontal Distribution

Recent studies have shown that uniform horizontal distribution of light caused by diffusion can increase overall photosynthesis and production. A study in 2015 by the Chinese Academy of Agricultural Sciences stated: “Diffuse light improves spatial light distribution in the crop canopy, thereby stimulating crop photosynthesis; the more uniform horizontal light distribution within the canopy plays the most important role for this effect. Diffuse light also lessens the variation of the temporal light distribution at any specific point in the canopy.” This study utilized glass greenhouse glazing with high light diffusion that does not restrict light transmission.

Another more recent study on light diffusion on cucumbers by Wageningen University stated: “Diffuse light has a positive influence on the production of cucumbers, especially during the summer. This positive effect can be explained by a change in light penetration into the crop and by an increased photosynthesis capacity, so that a crop like cucumber can utilize diffuse light better than direct light. In addition, diffuse roof material resulted in a lower crop temperature, especially higher in the crop which likely leads to a more optimal conditions for photosynthesis.”

This study also found that when a greenhouse covering with high light diffusion properties was used, this helped reduce the temperature in the upper canopy while maintaining a warmer temperature lower in the canopy. Not only were the plants able to photosynthesize better in the lower canopy but the more uniform temperature also aided in overall plant health.

Does Supplemental Lighting Diffuse Light?

The horticultural supplemental lighting industry has known this importance of light diffusion for years. Have you ever noticed how HID lighting manufacturers all have reflectors that are “dimpled”? If you look at your shadow under an HID light with a dimpled reflector, you will see that your shadow looks blurry on the edges and is not very dark. This is because those dimples on that highly reflective material in the reflector are scattering photons around and diffusing light. Conversely, if you look at your shadow under an LED light which does not utilize a reflector you will see a darker shadow with defined edges because LEDs are a much more directional light.

While LEDs can reach high light levels with less watts, they simply do not diffuse light or spread it uniformly on the canopy which can cause “hot spots” and uneven growth from some of the higher output greenhouse specific LEDs. Some LED companies even tout that their directional LEDs are optimal because HID reflectors “waste light by bouncing photons all over instead of concentrating them toward the canopy” but as we just learned above, this is actually optimal for better photosynthesis – especially if you are utilizing lights for your sole light source in a room with reflective surfaces on the walls.

LED Controversy

You can spend time scouring the web and see images from LED grow light companies with their lights above a canopy of cannabis. When you take a closer look and compare to that of HID and HPS, you will instantly notice how dark the plants look underneath the top canopy due to shadowing.

However, to be clear I am by no means saying HID or HPS is the better light choice. I’ve covered this topic back in my days with one of the leading horticultural lighting companies in the world and I believe both technologies have their place in the industry. Some growers have definitely proven better yields and flavor profiles because of the possible spectral advantages of LED. I believe the LED manufacturers will soon combine the known light quality advantages of spectrum while also utilizing light diffusion for the perfect combination for optimum performance.

Some LED manufacturers are already embracing this truth as stated by Osram in the following HortiDaily article: “Current LED fixtures often have a lambertian primary lens and no secondary optic, or a simple glass cover, which lead to hotspots with a high concentration of light directly under the luminaire. The result is areas with different intensities of illumination, causing inconsistent yields.” LED companies like Osram are looking for and developing ways to diffuse light better for this reason.

At UbiQD we know that quantum dots have great light diffusing properties. The large digital display market knows this as well and currently utilize quantum dots on LEDs to enhance color, sharpen images, etc with QD-LEDs for TV screens. At UbiGro we bring this technology to greenhouses by not only altering the sun’s spectrum for optimal photosynthesis, but to also diffuse these photons in a greenhouse.

Recently, we have begun a study with only 50% coverage of our UbiGro retrofit film in a greenhouse. The initial thought when looking at the picture above is that maybe some of this crop will be shaded from the UbiGro film, or that some areas of the canopy will be utilizing the optimal spectrum from UbiGro while other areas are receiving natural sunlight but this is not the case. When PAR and spectral data are recorded from various spots in the canopy, we find that the light levels are uniform and the spectrum is consistent.

Even at 50% coverage of the UbiGro film, it is diffusing the sun’s spectrum so that the enhanced spectrum is the same all across the canopy as is the PAR light level. We know that combining this optimal sun spectrum with the scattering of these photons are both contributing to increased photosynthesis, and therefore, yields for the grower.

For more information about the capabilities of the UbiGro film or questions about this post, reach out to Eric Moody directly at [email protected].

The Importance of Light Diffusion on Your Canopy

Eric Moody

Eric has more than 8 years of experience in horticulture lighting industry, building relationships with greenhouse growers of all sizes and crops on optimal lighting for their growing operation, and most recently managed a North American sales team for PL Light Systems.