Greenhouse Lighting
Greenhouse Lighting System
Supplemental lighting can extend the hours in a day, compensate for the light-limiting effects of overcast weather, and increase the amount of available light energy. Supplemental lighting can be used to increase the available light energy either across the full visible spectrum or within specific spectral ranges. Supplemental lighting is only feasible if light is the only factor limiting productivity. If other variables (e.g., carbon dioxide concentration, water, nutrient status) are insufficient, then supplemental lighting will not achieve the desired effect. When installing supplemental lighting systems in greenhouses, several factors should be considered. First, the (average) amount of solar radiation for the location should be investigated. This will give an idea of the range of solar radiation conditions at the site. Second, as discussed before, the type of greenhouse structure and cover will have an impact on the transmission of sunlight. Third, the type of crop (or crops) grown in the greenhouse will point to the plant requirements (such as light intensity, duration, or light integral), and the available space in the greenhouse to hang lamps (less space is available for taller crops in lower greenhouses resulting in loss of light uniformity). Next, the plant requirements should be compared to the available amounts of sunlight to calculate the necessary amounts of supplemental lighting. It is usually not economical to install lighting systems that provide high light intensities in greenhouses because of the large number of lamps required.
Benefits of Supplmental Lighting
When the solar DLI is low, there are many compelling reasons to install and operate supplemental lighting in greenhouses. Although investment costs in lighting equipment and other associated operational costs can be high, supplemental lighting typically shortens production time and improves quality parameters such as rooting, stem thickness, compactness, branch number, flower count, and flower size. There can also be “hidden” benefits, such as a decrease in sensitivity to some environmental conditions when grown under high light. Under a high DLI (at least 10 mol∙m–2∙d–1), plant responses to low temperature, photoperiod and light quality often decrease. However, with supplemental lighting during the dark and short days of winter there is greater tolerance to non-optimal conditions when plants are grown.
Lamp Types for Greenhouses
Several lamp technologies, each of which produces light of different spectral composition and overall light intensity, are commercially available for greenhouse applications. As the element that produces light, lamp technology is the keystone to overall lighting system capabilities and potential. All other types of lighting equipment merely exist to provide auxiliary support to the lamp, either by enhancing or distributing its light output or by regulating its power draw and controlling its intensity.
Incandescent Bulbs
Incandescent bulbs are not used for supplemental light purposes because of excessive heat, poor light quality for growth, and low efficiency (conversion of electricity to usable light is about 7 percent, the rest is lost as heat). They are, however, useful for phytochrome-dependent photoperiod control since they are relatively inexpensive to install and operate, they can be cycled on and off frequently, and they produce large amounts of red and infrared radiation.
Halogen Incandescent Bulbs
Halogen incandescent bulbs are more efficient than incandescent bulbs, but like incandescent bulbs, the light they produce a relatively large amount of far-red light. In contrast, fluorescent bulbs produce light that spreads out over a wider area, allowing a single bulb to uniformly illuminate a larger space and be more efficient than halogen bulbs.
Fluorescent Lamps
Fluorescent lights are most often used in growth chambers (rooms) or in small seed germination setups. The lower lamp temperatures and moderate light intensities allow growers to position fluorescent lights closer to plant foliage, making fluorescents a good candidate for germination benches. Fluorescent lighting is rarely used in commercial greenhouses for supplemental lighting because of its low-intensity output and the considerable shading the fixtures and reflectors create.
Compact Fluorescent Lamps
Compact fluorescent lights (CFLs) are replacing incandescent bulbs in some applications. Compact fluorescent lights offer full-spectrum lighting and are ideal in greenhouses that are starting seeds. Because the full-spectrum light provided by CFLs is like sunlight, it allows young seeds exposure to the light source that will ensure they develop properly. Cool white fluorescent light is a good choice for young seedling plants. Compact fluorescent lights are usually available with magnetic or electronic ballasts.
Full Spectrum T5 Fluorescent Lamps
Full spectrum T5 fluorescent lights are more compact and efficient than older forms of fluorescent lighting, which allows them to be used for all plants rather than just for seedlings (Figure 8.7). The letter T denotes the tubular shape of the lamp and the number five indicates its diameter in eighths of an inch. Full spectrum T5 fluorescent lamps are slim, only 0.625 inches in diameter, which makes T5 fluorescent tubes more efficient than standard fluorescent tubes.
High-Intensity Discharge Lamps
The most efficient lamps used for supplemental lighting in greenhouses are the so-called high intensity discharge (HID) lamps. Two such lamps are the metal halide (MH) and the high-pressure sodium (HPS) lamps. Today, they are the primary choice for indoor plant growth. The light intensities and efficiencies obtained by high intensity discharge lamps are higher than either incandescent or fluorescent lamps. These lamps have a high light output, (and produce a lot of heat) so they should be placed about two feet or more above the top of the plants.
Metal Halide (MH). Metal halide (MH) lamps are used for their blue light, although they appear bright-white to the human eye. Metal halides lights are commonly used during vegetative plant growth but are less popular than HPS lights for flowering, fruiting, or full-life cycle lighting. If MH lights are used in the flowering stage, they are often of a higher rated power, such as 1,000 W, or are enhanced to provide more red-light output. A common assertion is that typical MH lights do not supply enough red light for optimal late-stage growth of photoperiodic plants.
High Pressure Sodium (HPS). High-pressure sodium (HPS) lights are the most commonly used HID fixtures in commercial greenhouses (Figure 8.8). High-pressure sodium lights produce light mainly in the yellow and red end of the light spectrum, which makes these lighting systems a great fit for late-phase (flowering and fruiting) plant growth. High-pressure sodium lights may require supplementation with fluorescent, metal halide, or other light sources high in blue light. While good-quality plants can be grown under HPS lights, the lack of blue light can lead to plant etiolation.
Light-Emitting Diodes
Light-emitting diodes (LEDs) represent a promising technology for the greenhouse industry that has technical advantages over traditional lighting sources but are only recently being tested for greenhouse applications (Figure 8.9). Light-emitting diodes can be manufactured to emit photon colors that match the absorbance peaks of important plant pigments, such as the red and far-red-absorbing forms of phytochrome, or the red and blue peaks of leaf photosynthetic action spectra.
Benefits. One of the most important features of LEDs for greenhouse application is that the generation of light in LEDs does not produce heat in the beam of light, and LEDs are cool to the touch. Heat is generated at the fixture level, where it can be more easily dissipated. The standard operating temperature of LED arrays are approximately 90 degrees F (32°C), whereas HID lamps generally operate around 600 degrees F (315°C). HIDs can present a working hazard and results in excess heat that needs to be removed from an environment. This is especially true in the summer months.
Light Fixture Efficacy of LEDs. Light fixture efficacy refers to the photon flux of a fixture divided by the energy consumed to emit that quantity of light. Light fixture efficacy is quantified by the number of photons with wavelengths from 400 to 700 nm (the photosynthetically active radiation waveband) emitted by a lighting device divided by the input power to emit that light. This term is referred to as the photosynthetic photon efficacy (PPE) and is in the unit of micromoles of photons per joule (µmol∙J-¹). There is a large variation in photon flux among LED products, which means the fixture number, installation height and spacing and light uniformity can vary dramatically. Fixtures with the highest efficacies usually emit mostly red light, since red LEDs are the most efficient.
Full Spectrum Versus Narrow Spectrum LEDs. Growers generally have two options when it comes to horticultural LED light spectrums: (1) full spectrum and (2) narrow-band spectrum. Full spectrum LED lights include a broad band of the light spectrum (more similar to the sun) renders a “white” light (there are no true white wavelengths). The narrow band spectrum LED lights are often referred to as red/blue spectrum lights because the wavelengths they emit are within a narrow band of light.
Controlling Greenhouse Lightings
Supplemental lighting of crops during the day is conducted most economically by only lighting when ambient daytime light levels are so low that productivity is significantly reduced (e.g., during the winter, in the early morning or early evening, and during cloudy days). The operation of lighting can be controlled by a timer or a greenhouse climate control system. If controlled by a timer, there are two basic options: lighting from one hour before sunset until some time during the night, such as midnight or 2 am, or lighting from four to five hours after sunset until one hour after sunrise. Both options prodvide a period of darkness because some crops do not tolerate continuous ligh. A disadvantage with this approach is that timers do not automatically turn on during overcast days.
Determining Greenhouse Lighting Requirements
When installing supplemental lighting systems in greenhouses, several factors should be considered. First, the (average) amount of solar radiation for the location should be investigated. This will give an idea of the range of solar radiation conditions at the site. Second, as discussed before, the type of greenhouse structure and cover will have an impact on the transmission of sunlight. Third, the type of crop (or crops) grown in the greenhouse will point to the plant requirements (such as light intensity, duration, or light integral), and the available space in the greenhouse to hang lamps (less space is available for taller crops in lower greenhouses resulting in loss of light uniformity).
Calculating the Number of Lamps for Supplementing Lighting
To calculate number of lamps for a greenhouse, we need fixture specific information. The lamp efficiency (µmol J-1 or µmol W-1 s-1) defined as the number of light photons (µmol) in the range of PAR coming out of the fixture every second (s) per every watt of electrical power (W). Manufactures report these values on the specification sheet and they generally range between 0.84 to 2.3 µmol W-1 s-1.
Selecting a Light Fixture
here are many factors to consider when determining what type of lighting fixture to install to deliver the desired light intensity. Much of the decision is based on economics, although depending on the application, the spectrum of light can also play a major role. As mentioned earlier, the two major lamp types appropriate for high-intensity lighting applications are high-pressure sodium (HPS) and light-emitting diode (LED). Metal halide fixtures are another option, but they are not commonly used because of their lower energy efficiency. While the light spectrum of HPS lamps is relatively fixed, there is a very wide range of LED products on the market that emit various spectra. There are three major horticultural lighting applications: (1) low-intensity greenhouse lighting to create long days, (2) moderate-intensity supplemental greenhouse lighting to increase growth, and (3) high-intensity sole-source lighting for indoor use. To increase the DLI in greenhouses purchase light fixtures intended for supplemental greenhouse lighting. Lamps designed for photoperiodic lighting are not effective at increasing growth.
DLC Approved
It is important to ensure that fixtures have been designed to tolerate growing conditions (for example, high humidity) and meet benchmark standards specific to horticulture. To help accomplish this, growers are advised to consider light fixtures that are on the Horticultural Lighting Qualified Product list by the DesignLights Consortium (DLC).
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