Chapter 7

Light and Lighting Control in Greenhouses

Supplemental Greenhouse Lighting

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

Most crops benefit from supplemental lighting, but the technology is only profitable when increased growth and quality of the crop generates enough revenue to offset the investment costs in lighting equipment and other associated operational costs. The floricultural crops lit most often are plugs and cut flowers, because in these cases increased growth rate also corresponds to greater economic yield. Adding supplemental light during the plug stage is especially important in the northern United States and Canada because a majority of plugs and liners are produced late in the winter and in early spring, when the natural DLI is low.

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 (See Figure 7.4) 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 lights are more efficient than incandescent bulbs, but like incandescent bulbs, the light they produce a relatively large amount of far-red light (See Figure 7.5). 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. 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 Lamps

Fluorescent lamps (See Figure 7.6) 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

T5 Full Spectrum Lamps

T5 full spectrum lamps 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 (See Figure 7.7). The letter “T” denotes the tubular shape of the lamp and the number 5 indicates its diameter in eighths of an inch. T5 lamps are slim, only 5/8” of an inch in diameter, which makes T5 fluorescent tubes more efficient than standard fluorescent tubes.

Compact Fluorescent Lamps

Compact fluorescent lamps (CFLs) are replacing incandescent bulbs in some applications. CFLs offer full-spectrum lighting and are ideal in greenhouses that are starting seeds (See Figure 7.8). 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. CFLs are usually available with magnetic or electronic ballasts.

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 halides (MH) lights (See Figure 7.9) 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. A common assertion is that typical MH lamps do not supply enough red light for optimal late-stage growth of photoperiodic plants.

High Pressure Sodium (HPS). High-pressure sodium (HPS) lamps are the most commonly used HID fixtures in commercial greenhouses (See Figure 7.10). 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. HPS lamps may require supplementation with fluorescent, metal halide, or other light sources high in blue light. While good-quality plants can be grown under HPS lamps, 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 (See Figure 7.11). LEDs 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. Thus, energy is saved using narrow-band LEDs for specific plant responses by not providing extraneous colors of broad-band light that otherwise would be an inefficient energy burden. 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 F° (32 C°), whereas HID lamps generally operate around 600 degrees F (315°C). This 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.

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.

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