Lighting: The design phase

Due to the growing trend in consuming locally produced food and the necessity to address the negative impacts of climate change, controlled environment agriculture (CEA) is becoming increasingly popular. In addition to traditional greenhouses with transparent glazing, the number of “indoor” crop production facilities is rapidly expanding. These indoor facilities rely 100% on electric light (sole-source) to provide for their crops, unlike in greenhouses, where typically more than 70% of the light is provided by the sun. In an effort to boost production, greenhouses can also use supplemental lighting to increase the amount of light the plants receive.

As far as horticultural lighting fixtures, there are many options available in terms of technology, form factor, and spectral and intensity output, but not every fixture may be suitable for a particular application. When deciding on a lighting design for a traditional greenhouse or an indoor farm, there are several things to consider.

Technology

While not the only two electric light sources used for plant production, high-pressure sodium (HPS) and light emitting diode (LED) fixtures are most commonly used and will be discussed in this article. HPS technology has been around longer, and while it is less versatile or efficient, it is still the dominant technology for supplemental lighting applications. However, the advantages of LED technology make it an attractive alternative, especially considering the potential for substantial energy savings. So far, one of the main hurdles for broader adoption of LED fixtures has been their relatively high purchase price. Note that we’re talking about LED fixtures that were specifically designed for horticultural applications and that come with reasonable manufacturers’ warranties.

HPS fixtures require current-regulating ballasts that are designed to operate bulbs of around either 400, 600 or 1,000 Watt. This ballast consumes an additional approximately 10% of the bulb wattage but is necessary to prevent premature bulb failure. Traditionally, HPS bulbs were designed with a mogul base, but are also available in a double-ended version (usually for the higher bulb wattages). These double-ended bulbs require a different fixture design but have a somewhat higher conversion efficiency. The conversion efficiency (approximately 30% for HPS fixtures when comparing the electric energy consumed with the useful light energy produced) is termed the efficacy and for horticultural applications is expressed with the units µmol/J (pronounced micromole per Joule).

LED fixtures require drivers that deliver the correct electric characteristics to the LEDs. These fixtures typically have a number of LEDs mounted on a heat sink that maintains the desired operating temperature. LEDs with different spectral outputs (i.e., colors) can be combined to design a fixed or tunable spectral output. Some fixtures also have dimming capabilities, allowing growers to adjust the light intensity as needed. In addition to the benefits of being able to select an optimal spectrum and to easily remove heat from the fixture, many LED fixtures designed for horticultural applications have efficacies (up to 3.5 µmol/J) that well exceed the efficacy of the most efficient HPS fixture (maxing out at around 2.0 µmol/J). As a result, less electricity is needed to produce the same amount of light. For additional efficacy values for other light sources used for horticultural applications, see Table 1.

Heat management

Light is just one aspect of the growing environment that needs to be managed in controlled environments, with heat being another key parameter. In greenhouse production, heat produced from the supplemental lighting system is often useful as low-light periods often correspond with low-temperature periods (night and winter lighting). However, in a sole-source lighting situation, electric lighting must be applied throughout the year and any excess heat will need to be removed depending on how well the building is insulated and the outdoor climate conditions.

In terms of different lighting technologies and heat production, there are distinct differences between HPS and LED fixtures, that should be considered. The light output of most fixtures is temperature dependent, so managing their thermal environments is vital to ensuring uniform output and that fixtures perform for their expected lifetimes.

In addition to producing light with a unique and fixed spectrum, HPS fixtures produce a substantial amount of radiant heat. This radiant heat increases the crop temperature as long as the plant tissue has a direct line-of-sight with the fixture. However, radiant heat can result in uneven heating (especially in taller crops with a canopy with multiple leaf layers), and heating with electricity is expensive.

While higher wattage HPS fixtures are the most efficient, their bright output makes it more difficult to distribute the light evenly over a crop canopy. Because of this, and their radiant energy output, HPS fixtures are often installed some distance away from the top of the canopy, and this can result in light loss when some of the light produced is not directed to the crop canopy.

The heat sinks on LED fixtures are either actively (with a fan) or passively cooled and they convert the generated heat into mostly convective heat (the conversion efficiency is approximately 30-40%). Unlike radiant heat, convective heat is easier to deal with: We can simply move warm air when it is causing plant stress. This also means that LED fixtures can often be installed closer to the crop canopy, resulting in less light loss.

Indoor crop production on vertically stacked shelves will especially benefit from this characteristic of LED fixtures. LED fixtures can also be purchased that use water cooling to get rid of the excess heat. Water cooling makes it much easier to remove excess heat and may also create an opportunity to use this “waste.”

Form factor

Horticultural lighting fixtures come in a variety of form factors and shapes that may include a reflector to help focus or spread the light output. HPS and Metal Halide fixtures rely on ballasts to produce the high voltages necessary for them to start and such devices may or may not be attached to the bulb/reflector. LED fixtures have drivers that provide the correct voltage and currents to the individual LEDs that make up a fixture. Rather than a single bulb, a number of LEDs are typically placed on a circuit board which may also double as a heat sink. LEDs can also be arranged into a “corn cobb” to mimic the traditional lamp bulb design.

In a greenhouse the goal is to maximize the amount of solar radiation and therefore ideally overhead obstructions should be minimized. For this reason, fixtures tend to be fewer with more power, mounted higher above the crop, and rely on reflectors to provide a uniform light level across the crop canopy.

Indoor production does not have the challenge of maximizing the use of solar radiation, and thus light uniformity can be maximized through the use of many more lower-output fixtures. When using LED lighting, fixtures can be placed much closer to the crop, which allows for racks/shelves of growing systems to be placed on top of one another.

Installed capacity

In sole-source lighting, all the lighting needs of the crop are provided by the electric lighting system. The lighting system must be capable of meeting the target daily light integral. In a greenhouse, natural sunlight can provide the bulk or even more light than a crop needs, however meeting a crop’s needs is highly location, crop, weather and season dependent. In a greenhouse, it may be necessary to install a shading system to protect the crop from receiving too much sunlight. During darker times of the year (or day), the supplemental lighting system may need to be used to deliver the crop’s lighting needs. Even on the darkest days, some amount of natural light will usually be available (provided the greenhouse is located below the Arctic Circle!). Designing a lighting system to meet the needs of the crop will require information about historical solar radiation data for the greenhouse location, an estimate for the transmissivity of the greenhouse structure, as well as knowledge of the lighting needs of the target crop, and the light distribution pattern produced by the fixtures used. Careful consideration and planning is needed when deciding on the installed intensity of supplemental lighting in a greenhouse. Such calculations are somewhat easier in a sole-source lighting application.

Spectrum

One important difference between greenhouse and indoor cultivation of crops is that crops grown in greenhouses will receive a substantial amount of sunlight. The solar spectrum is quite different from the spectrum produced by either HPS or LED fixtures. For example, sunlight contains ultraviolet (UV) and far-red radiation that can benefit some plant species. And if those wavebands are not present in the electric light used for supplemental lighting, crops grown in greenhouses may still get those wavelengths from the sun. But that is not the case for crops grown in a fully enclosed environment. In that case, certain wavelength may need to be added to the light spectrum in order for the plants to develop as expected.

LED fixtures allow growers to experiment with different light recipes to grow their crops and make adjustments throughout the growing cycle. Given the multitude of crops grown and the variable output of the most sophisticated LED fixtures, it will take time and plenty of experimentation to discover the optimum supplemental lighting strategy for a given location, growing structure, weather conditions, crops grown, growing strategy and grower preferences before an optimal light recipe can be identified.

Control systems and sensors

A critical part of a lighting system is how it is controlled, and this is true for both sole-source and greenhouse applications. Lighting control can be as simple as turning the lights on and off at set times of the day, and for a sole-source situation, this may be a good strategy for providing the same amount of light each day. However, growers using sole-source lighting systems may want to “ramp” the output of their lights such that the full intensity is not applied to the crop all at once, and they can do so through the use of dimmable LEDs. Similarly, the spectral output can be adjusted according to the time of day, or crop needs.

Time clock control is commonly used in greenhouse production, however this type of control does not take into account the natural light the crop is receiving, and the amount of light a crop receives in a day can vary greatly. For this reason, more sophisticated controls can be used and these controls depend on light sensors to monitor how much light the crop is receiving. Based on the recorded intensity of the light, control systems can determine when to open or close shading systems, and turn lights on or off. Even more sophisticated control algorithms will use the light history to predict how much natural light the crop is likely to receive and then provide supplemental light to make up any deficit, without lighting more than necessary.

Light sensors do require maintenance to ensure they are reading accurately; however, they are also useful for monitoring the degradation of the light output over time. If light sensors are not a part of a control system, hand-held light meters can be used periodically to check that the output of a lighting system has not degraded too much and, if so, would be in need of replacement.

Tim Shelford is a research associate with a joint appointment in the School of Integrative Plant Sciences, Cornell University and the Department of Environmental Sciences at Rutgers University; tjs47@cornell.edu

A.J. Both is a professor and extension specialist in the Department of Environmental Sciences at Rutgers University; both@sebs.rutgers.edu

Author’s note: This article is part of a series on horticultural lighting by members of Project LAMP (Lighting Approaches to Maximize Profit). LAMP work is funded by USDA-NIFA-SCRI Award Number # 2018-51181-28365. More information is available at hortlamp.uga.edu.