Are your plants receiving the right amount of light?

21-09-2020    12:32   |    Greenhouse Grower

Of all the factors in a greenhouse that growers can control, including light, humidity, temperature, water, and carbon dioxide levels, light typically is the least controlled of all of them.

Keeping grower needs in mind, Marc van Iersel of the University of Georgia and Erico Mattos of Cornell University developed a lighting system that monitors how much sunlight is inside the greenhouse and controls the amount of supplemental light that is provided to achieve the daily light integral.
Photo by Marc van Iersel

“Researchers of ornamental and vegetable crops have developed guidelines for a wide range of different crops regarding how much light they should receive each day,” says University of Georgia Horticulture Professor Marc van Iersel. “These light recommendations are referred to as the daily light integral (DLI), which is the total amount of light integrated over an entire day. There aren’t DLI recommendations for all crops. However, growers can estimate the DLI for those crops for which specific recommendations aren’t available.”

Van Iersel says one issue growers face when trying to deliver the optimum DLI for their crops is that most lighting systems are not designed to assist growers with achieving these DLIs.

“The simplest lighting control systems are timers, and they obviously provide crude control,” he says. “More commonly, lighting systems are controlled based on the amount of sunlight. In many cases, that sunlight is measured outside the greenhouse.”

Accurate Light Measurement Comes Down to the Sensors

When determining light levels, challenges can arise for growers relating to the accuracy of the sensors they use to measure light.

“In many cases, these light sensors haven’t been recalibrated since they were initially installed,” van Iersel says. “As a result, growers are not going to achieve very good light control. Even if the light sensors are accurate, the control algorithms that turn supplemental lights on and off based on a simple light threshold are still not going to deliver the same DLI from day to day.

“Considering how many greenhouse growers operate, most of their crops are presold for a specific delivery day. It is critical for growers that they can meet their production schedules. A few days of overcast weather can delay crops and create havoc in the growers’ overall schedules.”

With light sensors located outside the greenhouse, van Iersel says growers may be able to estimate the amount of light delivered to inside the greenhouse if they know the light transmission of the glazing material.

“This light transmission changes over time,” van Iersel says. “Growers don’t measure the changes in transmission of the glazing material. Over time, the light transmission information becomes less reliable. Typically, the sensors measure the energy in the light rather than measuring the photosynthetic photon flux. It is the photosynthetic photon flux that drives photosynthesis and growth.

“For growers to be able to have confidence that they can ship their crops on time, having consistent control of the DLI is really important. Typical lighting control systems don’t offer that capability. On a day-to-day basis, light levels can differ very drastically. Growers need to have an intelligent light control system that takes into account the changing light levels and respond accordingly.”

The lighting control system connects to light fixtures via a low-voltage wire from the lights to a wiring panel and the controller.
Photo courtesy of Candidus

Precision Light Delivery: How It Works

To help growers accurately measure and deliver the optimum light needed for controlled-environment crops, van Iersel along with Dr. Erico Mattos of Cornell University developed a unique light control system.

The system measures photosynthetic photon flux, which is more relevant to the crops, and uses an algorithm to monitor how much sunlight is received in a greenhouse. It then integrates that over the day. In real time, the system calculates how much supplemental light should be provided.

In the case of dimmable LED lights, the system controller sends a signal to the lights to dim them or make them brighter to provide exactly the amount of light that is needed to reach the DLI by the end of the day. Non-dimmable LEDs and high-pressure sodium (HPS) lights are turned on and off throughout the day based on the amount of light received by the plants and the number of hours available to provide supplemental lighting.

In other words, the light control system monitors how much sunlight is inside the greenhouse and controls the amount of supplemental light that is provided. This ensures growers aren’t providing more light than their crops need.

At the same time, crops receive the optimum amount of light, assuming that growers have installed a lighting system that can deliver the necessary light.

“Lighting systems should have enough capacity to assure that desired DLIs can be reached most days, but not necessarily all days,” van Iersel says. “This is one thing we are seeing in some greenhouses. If the sunlight DLI during the winter months is 1 to 2 moles, providing another 15 to 6 moles from supplemental lights to reach a crop’s optimum DLI may be cost prohibitive.”

The light control system Iersel and Mattos developed is now marketed under the name Candidus, and is designed from a crop perspective. In addition to providing a consistent DLI day to day, the controller can also be used to control flowering of photoperiod-sensitive crops.

“Growers can do day length extension and night interruption,” van Iersel says. “And dimmable lights can be operated at a fraction of their full power. Growers can turn on their LED lights at 10% percent of full power, which should be enough light for night interruption or daylength extension. This is controlled through a simple, user-friendly interface.”

The Candidus light control system sends growers a monthly report that summarizes the percentage of days the daily light integral has been achieved and the energy and cost savings.
Photo courtesy of Candidus

The lighting control system is also tied to a local area network that growers can tie into with their computers using Wifi. Growers set the desired DLI and photoperiod to determine when the lights turn on and shut off. If they want to do night interruption or daylength extension, they specify the on and off times and the percentage of full power. Those are the only settings growers need to worry about. The interface shows what the lights are doing in real time, so growers can monitor the performance of the system very easily.”

The system is also entirely scalable, allowing for control of different lighting zones. The basic system has two lighting zones. If growers want to do more lighting zones, it’s a simple matter of installing additional hardware to cover more zones.

Source: Greenhouse Growers
Photo created by freepik



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