8 Lighting System Design Principles

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A good lighting system will result in both higher yields and lower energy consumption.

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March 11, 2019

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A good lighting system will result in both higher yields and lower energy consumption. But what’s good for one business may not be good for another—and a plethora of options can make selecting the right lighting system for your cannabis operation confusing.

For example, the right lighting design depends on factors such as the desired light intensity level, the layout of a cultivation environment, the available height and the light distribution of the luminaires. What's more, almost all horticulture lighting companies will provide light plans—but not all know or follow good design principles.

So, if you plan to rely on the recommendations of a lighting company, or if you want to start from scratch yourself, it’s valuable to familiarize yourself with some basic lighting principles. Here are eight principles to get you started.

1. Determine your target-level light intensity.

The optimal target level will depend on the growing strategy, plant stage, cultivar and electricity prices. Our research, as well as research conducted by the industry, suggests that level may be more than twice the light provided by a double-ended, 1,000W HPS fixture. So, if you’re growing in a greenhouse in the U.S. or Canada, odds are that the sun isn’t providing enough light to grow cannabis optimally most of the year. You should determine what the lighting deficit is in January, the darkest month. You’ll need to install enough lights to compensate for that deficit—unless you are willing to sacrifice yields during periods of the year with lower natural light levels.

2. Position lights to maintain even intensity across the canopy.

Place lights at heights and in locations relative to one another so that you provide, on average, your target light intensity to the canopy while minimizing variation in that light intensity. To clarify, imagine each of your lights is a showerhead that sprays water (i.e., light) onto your plants. When sprays (i.e., beams of light) from multiple showerheads cross each other, more water hits those spots. Your goal is to place those showerheads such that each plant receives close to your target level. Of course, some plants will get hit by more water, but you can place the showerheads such that those differences are minimized.

This fan is reducing light intensity by about 15 percent in the canopy region that is covered by the fan’s shadow.
photo provided by James Eaves

3. Buy a quantum sensor.

You can only evaluate a lighting system’s design and improve upon it if you can measure the amount of light the system delivers to your canopy. Quantum sensors allow you to measure the number of photons that fall on a particular location each second and that are within the wavelength range that plants use for photosynthesis. The unit of measurement of light intensity is called photosynthetic photon flux density (PPFD). Lux, lumens and foot candles are misleading metrics when dealing with plants because they are measures based on what a human eye sees, which is not the same spectrum range that plants use for photosynthesis.

4. The shape of your growing space matters.

For instance, plant lighting research from Cornell University has shown that if your growing space is shaped like a rectangle, staggering your lights is more likely to improve the distribution of light—but that's not the case for square-shaped grow spaces, which, research has shown, are more likely to benefit from lining lights up in rows.

5. Don’t underestimate the cost of obstructions.

Think carefully about the placement of fans, crossbeams, ventilation socks and other potential lighting obstructions. Their shadows, which are almost unperceivable to the eye, can reduce light intensity by nearly 10 percent at specific locations. A general rule of thumb is that a 1-percent decrease in intensity equates to a 1-percent decrease in yields.

6. Light-intensity variation is often lower toward the center of a room and increases as you move toward the perimeter.

One way to increase uniformity at the edges of your grow space is to reduce the distance between lights—or increase their intensity—around the perimeter. In fact, light distribution and energy efficiency can be improved by placing lower-wattage lights toward the center of the room and higher-wattage lights toward the perimeter.

7. Understand that the plants will do some work for you.

Inevitably, when designing a room or imagining improvements—at some point, additional improvements to light distribution will become very expensive. But plants will always do some of the work for you. For example, plants will change the morphology and angle of individual leaves to use available light more efficiently, but it is difficult to notice that plants are doing that. So when you are considering more complex changes to your room design in order to improve light distribution (e.g., making lights movable), ask yourself if that investment will add enough improvement above what the plants will do themselves to make the investment worth it.

8. Understand the Inverse Square Law.

You can increase light-intensity uniformity by increasing the distance between the plants and the lights—but because of the Inverse Square Law, small changes in distance will have a big impact on average light intensity. Think about the showerhead example. As you increase the distance between the showerhead and a bucket on the floor, you decrease the amount of water that falls inside the bucket and increase what lands outside of it. The photons produced by your light behave in a similar way, to a degree that depends on the shape of the light’s lens. As you increase the distance between the lights and the plants, you increase the amount of light that spreads out and hits things like aisles and walls. So, there is the ability to increase PPFD and energy savings by moving lights closer to plants. One of the primary benefits of light-emitting diode (LED) fixtures is that you can move them closer to plants because an LED’s light beam spreads less and it produces far less radiant heat. At the same time, as you move the lights closer to the plants, you increase variability of light intensity across the room. Imagine moving all the showerheads 2 inches from the floor. Some spots would receive a lot of water while others would receive none. Finding the optimal point in that trade-off is perhaps the most central element of an optimal lighting design.

James Eaves, Ph.D., is an agricultural economist and professor at Université Laval. He is also the Head of Innovation at GreenSeal Cannabis Company, which uses advanced vertical farming methods to grow cannabis in Stratford, Ontario.