Introduction
Horticultural lighting has existed for many years as a supplement to, or replacement for sunlight as the catalyst for plant growth. While many light sources have been used, professional, large-scale operations tend toward high intensity discharge (HID) lamps for most supplemental and replacement lighting for plant growth. The success of HID lighting has been such that there are specific chemistries of high pressure sodium and metal halide lamps designed with grow lighting in mind. Since the commercialization of LEDs in lighting, there have been attempts at making LED plant growth lighting using the direct colors and relative adjustability of LED spectrums to try and optimize the light to the plant’s absorption. Significant steps in the efficiency of LED systems have increased the viability of LEDs as an alternative light source to HID lamps for output, and the widespread adoption of LEDs in general lighting has been driving improvements in drivers and cost reductions.
Horticultural industries may enjoy some potential benefits from LED characteristics, such as the ability to develop a spectral power that is tuned to the specific crop grown and adjusted over the plants’ lifecycle and the potential to have greater influence over the quality of the finished product by adjusting the light. Coupled with the ability to direct the LEDs and the long lifespan of the products, the potential for greenhouses is significant.
However, these benefits can only be realized with an understanding of the fixture’s performance, as not every LED fixture can provide the equivalent light output and intensity of the incumbent technologies, and plant lighting has special testing considerations.
This article will explore the differences between the measurement of lighting for vision and what is needed for plants.
Differences Between Horticultural and Traditional Light
Most people are familiar with the concept of light output and intensity, but what we are familiar with as humans is largely from the perspective of photopic vision. This is based on the average human eye response (known as the photopic curve) with metrics such as lumens and lux weighted according to this curve. The photometric curve peaks in the yellow-green region and drops off towards both the blue and red ends of the spectrum.; however, this response curve is not applicable to plants, as they do not have eyes.
Rather than reference the photopic response curve, horticultural lighting references the photosynthetic response region (typically referred to as Photosynthetically Active Radiation, or PAR) as a more appropriate reference. Also referenced by some is the McCree Curve, which was developed based on CO2 assimilation per mole of photons between 400nm and 700nm wavelengths. This widely referenced curve has peaks in the blue and red range.
This curve, however, is an average curve, and the many different plant species will vary in their absorption. Additionally, while photosynthesis is certainly an important aspect of horticultural lighting, it is not the only process in plants that relies on a spectrum of light. Additional responses from photoreceptors can impact plant growth characteristics, and some of these have reactions beyond the photosynthetically active range of wavelengths, into the UV and far-red regions. As plants have different responses and additional mechanisms which influence plant growth, using a single curve and weighting the light according to a curve does not make sense.
Evolution of Horticultural Lighting
Artificial lighting has been used to supplement the growth of plants since electric lighting was first developed. Early studies focused on carbon-arc lamps, but incandescent lamps filled the role over time. While these products were effective, they were less efficient and heavily weighted in the red and far red spectrum, which caused elongated stem growth. When discharge lamps came into commercial use, they were used to different effect, and the industry generally settled around high pressure sodium and metal halide lamps for supplemental lighting. Fluorescent lamps have also been used in some situations for a wider spectrum of light due to their use of phosphors, which can tailor the spectrum more towards plant growth.
The problem with the incumbent technology is that it is relatively static. The spectrum cannot be changed as plant growth needs change without actively changing out a lamp, which introduces additional labor. High pressure sodium lamps are cost effective, efficient, and long lasting, but provide a limited spectrum for growth, mainly in the orange and red region. Metal halide lamps, which can provide strong blues (a spectrum that tends to encourage leaf growth), experience rapid lumen depreciation due to darkening of the arc tube and contain very high pressures which often require the use of a lensed fixture. There have been attempts to combine the two into a single lamp, but these lamps end up having a lumen maintenance imbalance, with the blues of the metal halide depreciating more quickly than the high pressure sodium components. Finally, these sources become inefficient as they require reflectors and can deteriorate over time.
The commercialization of LEDs, specifically since LED lights became available in high power packages, has generated interest in the horticultural community due to their long life and multiple color options and this interest has increased as the technology has become more efficient. Using a combination of direct color and/or phosphor-converted white LEDs in a single fixture has the capability of tuning the spectrum to one that is optimal for the specific species of plant being grown. This rapid improvement in LEDs has many considering the switch from other technologies, but the difficulty is determining which fixture is best.
Metrics
The sheer number of metrics that can be associated with visible light can get confusing, and with horticultural lighting this is no different. To reiterate an earlier point, lumens (and lux, candela, etc.) are all adjusted for the human eye response and, as such, are not an appropriate reference and should not be considered when evaluating a fixture for horticultural applications. The primary metrics for horticultural use are focused on the quantities of photons produced (typically measured in micromoles, or 6.022×1017 photons per micromole) as these are what get absorbed by the plant. Though others can be explored, the most common metrics are listed below:
Photosynthetic Photon Flux (PPF) – This metric is the unweighted photon flow in micromoles per second, but not necessarily in any one direction. This would be somewhat analogous to lumens, as it is an overall quantity.
Photosynthetic Photon Flux Density (PPFD) – A density based metric that focuses on how many moles hit one square meter per second. This would be similar to a lux, where it is the quantity hitting a specific area.
Daily Light Integral (DLI) – This metric involves the number of moles of photons that hit a surface over a 24 hour period.
Additional metrics and definitions are being worked on by organizations such as the American Society of Agricultural and Biological Engineers (ASABE), as are the test methods which are used to acquire these measurements. One such potential metric expands the wavelength range beyond photosynthetically active radiation and could include the ranges in UV, far-red and infrared to which plants are also sensitive.
Measurement Considerations
Part of the challenge of measuring the light output of horticultural luminaires has been the lack of available metrics and test methods. This lacking does not necessarily prevent measurement, but it does make a true comparison between two different products tested in different laboratories difficult.
The standard test methods for photometrically oriented luminaires have been used by some, but these methods typically have conditions that are not necessarily applicable to horticultural fixtures. For example, many horticultural luminaires will be exposed to higher temperatures and humidity levels, and the fixture’s performance at 25°C may not be representative of the fixture’s performance when installed in appropriate environments. As the methods can vary, the test report provided may not be able to provide the appropriate amount of information on the light coming out of fixtures for horticultural use.
The ASABE is making efforts to improve these deficiencies by using existing methodologies as much as possible and taking lessons in measurement already learned from photometric measurement. Where applicable, the intent is to reference setup, conditions, and methods in IES test methods such as LM-79 and others; however, these methods cannot be referenced directly as many are focused on photometric references, and the test conditions are not necessarily appropriate as noted above.
Application Considerations
One does not simply install any fixture in a greenhouse and call it a day. Lighting for horticultural applications will have different considerations for operating conditions than most other applications. The specific conditions may vary by application but they all focus around the following conditions and ingredients that help plants grow:
Irrigation and Humidity
The most obvious and potentially damaging environmental condition that horticultural lighting is exposed to is water. Any type of lighting will have this concern, as electricity and water require some consideration, but this condition is typically magnified in a horticultural environment.
Efficient lighting solutions such as HID or LED have some manner of ballast or driver to convert the incoming AC line voltage into something useful for the light source as well as a method of directing the generated light. For HID, this could be a magnetic or electronic ballast for starting the lamps and maintaining the proper electrical conditions and reflectors for distributing the intense light over an area. For LEDs, this driver can be simple or complex and may have a reflector or a diffuser. Regardless of source type, degradation in either the electrical controls or optical controls will reduce the light output (to zero if the light becomes inoperable) and effectively reduce the yield and/or quality of the product.
Depending on the location of the fixture and the irrigation method in the facility, different levels of protection may be necessary. For drip irrigation or hydroponics, for example, the concern may be focused on keeping the high humidity from damaging internal components rather than guarding against direct spray. For other applications, rain like spray or even more forceful sprays may be a concern with the fixture, either from the irrigation system or from errant sprays due to manual watering.
Ingress protection is a necessary rating for most of these fixtures and should be considered when designing or selecting lighting options. Additional considerations such as ballast potting, conformal coatings, and gasketing for humidity protection may also be beneficial.
Temperature
Plants grow better when it is warm. That is fairly common knowledge, but the temperatures at which plants are grown – temperatures that can ranges as high as 30-40C (86-104F) – are often higher than the temperatures at which many luminaires are tested. While HID products are relatively insensitive to higher heat, their ballasts must still stay below their maximum temperature in order to achieve the expected lifetime. LED products typically lose intensity and efficiency as their temperature increases, so testing in an environment that is 10-15C (18-27F) below where it will be operated could result in overstating the amount of light that will be available to the plants.
For indoor applications, the proper ventilation of excess heat is also a significant consideration. Warmth is good for plants, but excess temperatures can be a problem as well. While lighting has become much more efficient over the years, there is no perfectly efficient lighting system. Any electrical power that is not converted into light and absorbed by the plants has to be converted into something (see the first law of thermodynamics) and that something is heat. In an indoor, warehouse-type growing situation where active ventilation is required, the heat generated by lights can become problematic and costly to remove. Using the least amount of electrical power for the amount of light needed by the plants is a wise strategy for managing energy consumption and the building costs used to control the air temperature.
Chemical Exposure
Plants need nutrients to grow properly and, when in an environment that has some exposure to the outdoors, pest control may be a necessity. The specific chemicals vary by crop, and unfortunately there is not a standardized set of chemical tests to consider when designing or selecting horticultural lighting fixtures.
Spectral Power Distribution
Selecting the best spectral power distribution is not a simple matter. When light affects the growth characteristics of a plant, it is known as phototropism. At its most basic, phototropism can cause a plant to grow towards or away from a light source, but additional growth changes are possible. The same is also true with spectrum. As noted earlier, a spectrum heavier in reds tends to make plants grow longer stems, and a spectrum heavier in blues encourages leaf growth. While these colors may seem obvious given the McCree Curve, other studies have shown that supplementing these colors with green light (up to a certain point) will enhance growth for some species, further supporting the need for multiple spectrums of light for optimal plant growth.
It stands to reason that to get the best out of a crop of plants, there may be a specific light formula that is appropriate, and that light formula may change over time depending on what the desired growth of the plant is and what stage of the plant lifecycle it is in. With HID lamps, changing the spectrum is possible with certain ballasts by swapping out a metal halide lamp for a high pressure sodium lamp, or vice versa. This is limited, however, to the spectrums that can be made with these technologies. With fluorescent lamps, switching to different phosphor combination lamps can achieve this as well, but with similar limitations to the number of available options. Multi-source LEDs have the potential to offer a wider range of tenability by using multiple direct color LEDs (potentially including UV and IR) and/or including some phosphor-converted white LEDs for a wider spectrum of light. The ‘right’ recipe of light for a plant will depend on the specific species, and this information can be closely guarded as a competitive advantage.
Controllability
The ability to alter the lighting according to changes in weather can be a factor for plants grown in greenhouses with supplemental lighting. Plants have their own desired daily light integrals, and it may be beneficial or desirable to not only supplement after sundown, but to allow for ad-hoc supplementation when the weather has been cloudy and less solar radiation is hitting the plants. HID sources can be dimmed to a degree, but they not as efficient when dimmed and have a reduced lifetime when they are turned on or off frequently. Alternatively, LED sources are typically dimmable with the proper driver and can be tied into a sensor similar to those used for indoor daylight harvesting applications, allowing for the proper light level to be maintained in the greenhouse without excess energy use.
Summary
Horticultural lighting is not new, but the options to choose from are wider than ever, and the interest in growing a wide variety of crops indoors has put additional focus on lighting options. Intended use, environmental conditions, targeted crops, plant lifecycles and desired flexibility can all be considerations when selecting and designing horticultural lighting fixtures for plant growth. These considerations are particularly important for fixtures that are intended for long term use, as research may lead to more light recipes and insight into how lighting impacts the quality and quantity of plants that are grown with artificial light.
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