Light and Lighting Control in Greenhouses
Supplemental Greenhouse Lighting
In many regions the decision to install supplemental lighting is driven by a predictable lack of sunlight due to inclement weather patterns or in regions located between 40 to 80 degrees latitude where in the winter months there is insufficient sunlight levels for successful greenhouse growth. For example, parts of Washington, Oregon, and southwestern British Columbia, average just 2 to 3 hours of sunshine per day during the winter months, and because of the relatively low sun angle, the overall intensity can be as low as 5 percent of summer levels. 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 should not be confused with photoperiodic lighting which is used to modify the day length to control growth and development. Supplemental lighting involves higher light intensities (300 to 600 fc at plant height) whereas photoperiodic lighting involves much lower light intensities (7 to 10 fc). Comparisons between several light supplementation sources are offered below (Table 6.3).
Lamp Technology Options 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. Some of the lighting technologies include, incandescent bulbs, halogen incandescent bulbs, fluorescent lamps, compact fluorescent lamps, high-intensity discharge lamps, and light-emitting diodes.
Over time, incandescent bulbs (See Figure 6.4) will be phased out due to increasingly stringent national efficiency performance standards for lamps sold in the US. 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
The halogen incandescent bulb is a more energy-efficient modification of the traditional incandescent bulb and will continue to be available. Halogen bulbs are filled with halogen gas—usually bromine or compounds of bromine—in contrast to the vacuum or a low-pressure argon/nitrogen mixture in conventional incandescent bulbs.
Fluorescent or “cool-white” lamps (See Figure 6.5) deliver mostly blue-band light and thus is used to encourage leafy plant growth. Blue-dominant lights are also used to start seedlings where light is needed for germination. 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 producing mature flowering or fruiting plants because of its low-intensity output and the considerable shading the fixtures and reflectors create. Florescent lamps are more light efficient than incandescent bulbs and they have a much longer life span.
Compact Fluorescent Lamps (CFLs)
Compact fluorescent lamps (CFLs) (See Figure 6.6) are replacing incandescent bulbs in some applications. CFLs offer full-spectrum lighting and are ideal in greenhouses that are starting seeds. Because the full-spectrum light provided by CFLs is similar to 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. CFLs are usually available with magnetic or electronic ballasts.
High-intensity Discharge (HID) 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 halides (MH) lights (See Figure 6.7) are commonly used during vegetative plant growth but are less popular than HPS lamps for flowering, fruiting, or full-lifecycle lighting. If MH lamps 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.
High Pressure Sodium (HPS). High pressure sodium (HPS) lamps 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, and can also deliver greater light intensities than lamps used exclusively for vegetative growth.
Light-Emitting Diodes (LEDs)
Light-emitting diodes (LEDs) (See Figure 6.8) represent a promising technology for the greenhouse industry that has technical advantages over traditional lighting sources, but are only recently being tested for horticultural applications. LEDs are solid-state light-emitting devices (See Figure 6.8), and as such, are much more robust and longer-lived than traditional light sources with fragile filaments, electrodes, or gas-filled, pressurized lamp enclosures. LEDs can be designed to emit broad-band (white) light or narrow-spectrum (colored) wavebands specific for desired plant responses. One of the most important features of LEDs for horticultural 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. Reflectors are not necessary since the LED housing guides the light photons in one direction.
Controlling Greenhouse Lighting Systems
The operation of lighting can be controlled by a timer or a greenhouse climate control system. A major downside of using timers is that the lights are not automatically turned on during overcast days. A better way to control lighting requires an environmental control computer with a light sensor connected to it. The light sensor is typically mounted on a weather station. The climate control system tracks the amount of light received since sunrise.
Determining Greenhouse Lighting Requirements
Whenever the amount of light provided by sunlight is less than the target daily light integral (DLI) for the crop, natural light levels are adjusted with supplemental lights. For plants that have specific day-length requirements for flower initiation, it may not be possible to simply extend the number of hours the lights are run to increase the DLI to more optimal levels. In this case, the only option is to increase light intensity by installing more lamps if the DLI needs to be increased to maximize growth. Supplemental light levels need to be only a fraction of complete light levels, while photoperiodic light levels can be even lower. In order to appreciably increase the DLI, a supplemental light intensity (typically 400 to 600 foot-candles or 50 to 75 µmol?m−2?s−1) from high-intensity discharge (HID) lamps, such as high-pressure sodium (HPS), metal halide (MH) lamps or light-emitting diodes (LEDs) are used.
Carbon Dioxide and Supplemental Light
For photosynthesis, plants need both light (PAR) and carbon dioxide. Both need to be available in sufficient quantities for either one not to become the limiting factor (if there is enough light but not enough carbon dioxide, carbon dioxide becomes the limiting factor and vice versa). Therefore, when using supplemental lighting to increase plant production, it is important to maintain sufficiently high carbon dioxide concentrations inside the greenhouse.
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