Section 4: Supplemental Lighting in Greenhouses
Before selecting a light source for greenhouse (or growth chamber/germination chamber) lighting, numerous factors should be considered. Among these are the total energy emitted by the source, efficiency (% of electrical energy converted to light energy), wavelengths emitted (especially in the 400 to 700 nm wavelengths), cost, life expectancy (of bulbs and fixtures), and the fixtures required (including ballasts). The following is a discussion related to general types of light sources (i.e. types of bulbs), their properties and how they may be used in greenhouses, germination chambers or growth chambers.
Types of Lamps Used in Greenhouses and Other Controlled Environments:
Incandescent lamps (tungsten-filament) - These lamps are generally not used for supplemental lighting in greenhouses for photosynthetic purposes. A large portion of the radiation given off by these lamps is in the form of infrared (heat). Because of this, their efficiency rating is only 7%. Lamps range from 40 to 500 watts. Life span ranges from 750 to 1000 hours. In order to produce enough light for effective photosynthetic lighting, a large number of these lights would be required. This would require a large number of fixtures and would result in large amounts of heat being produced. Further, most of the visible radiation that these lamps produce is in the red and far-red wavelengths that cause plants to become tall and to have weak stems. However, because low light levels are required for photoperiodic lighting, incandescent lamps are suitable for photoperiod control (creating artificially long days) and are commonly used for this purpose in greenhouses.
Incandescent lights are often used in growth chambers. This is because most light in growth chambers is provided by cool white fluorescent bulbs (see below) which are high in blue light but relatively low in red light. A few incandescent bulbs are added to provide additional red light and thus broaden the light spectrum experienced by the plants in the growth chamber (provide light quality closer to natural sun light). Incandescent lamps are not used in germination chambers due to their high heat load.
Tungsten-halogen lamps - These lamps combine tungsten filament with iodine vapor. This allows for the output to remain constant throughout the life of the lamp. These lamps are available in up to 1,500 watts and have a have a 2000-hour lifespan. Again, these lamps are typically only suitable for photoperiodic lighting in greenhouses or in growth chambers.
Fluorescent lamps - These lamps are most commonly used in growth chambers and seed germination chambers. They are rarely used to produce crops in greenhouses. As with incandescent lamps, a large number of lamps would be required to produce enough light to benefit the crop. Fluorescent lamps require ballasts that provide adequate voltage to start operation and limits continuing current to the lamp. Ballasts can be heavy and thus increase the dead load on a structure. Additionally, ballasts generate significant amounts of heat. The required fixtures (ballasts and reflectors) cost money, require additional wiring and block natural sunlight. Fluorescent lamps are more efficient than incandescent lamps (20% efficiency) and provide their light over a broader spectrum (more in the blue region) than incandescent lamps. Both cool white and warm white lamps are used, but cool white lamps are most common.
- Cool white lamps have a lifespan of 12,000 hours, and come in lengths of 4 ft and 8 ft, and 75-watt bulbs are most commonly used for horticultural purposes. Warm white lamps are similar to cool white lamps, but warm white lamps emit more light in the red region of the spectrum and less in the blue region than cool white lamps.
- High output (HO) and very high output (VHO) fluorescent lamps are also available. Whereas cool white lamps generate 91 watts of light energy, HO lamps generate 126 watts and VHO lamps generate 225 watts.
- There are other types of fluorescent lamps designed for plant production. These are used in hobby situations since the same limitations apply as to other fluorescent lamps. These "plant lamps" or "grow lamps" usually have slightly broader emission spectra than standard cool white incandescent lamps. These lamps generally generate 46 watts of light energy and have a life span of 10,000 to 12,000 hours and cost more than traditional cool white lamps.
Light emitting diodes (LED) are devices that are much more like computer chips than actual light bulbs. They are small solid state semiconductor devices that emit light and the wavelength (or color of light) that is emitted depends upon the type of semiconductor material and the impurities or additives (i.e. phosphor coating is added to make blue) used to make the LED. Light emitting diodes are highly efficient and emit light in a very narrow range (i.e. as a single color). They can be designed to emit in specific colors such as blue and red, and they can be mixed to provide a broader spectrum of light. Advantages of LED are a very high efficiency, very low heat output, and a long life span. LED lights are being increasingly used in controlled enviroment agriculture systems. They are most commonly used to provide narrow wavelengths for specific purposes. They are also increasingly being used as lighting sources in germination chambers, growth chambers and factory farms (hydroponic shipping containers or boxes.
High Intensity Discharge (HID) Lamps - These are the most commonly used lamps for supplemental photosynthetic lighting in greenhouses. They are referred to as high intensity discharge lamps because they have a much higher quantum flux density than incandescent or fluorescent lamps (they give off more photons). As with fluorescent lamps, these lamps require ballasts that can be heavy and generate significant amounts of heat. Reflectors are used to direct the light generated downward and to improve uniformity of light distribution. These reflectors are manufactured in many different shapes. Numerous types of bulbs are available for use in HID lamps:
- High-pressure mercury bulbs have an emission spectrum similar to fluorescent lamps but with a higher concentration of their radiation being emitted in the red region. Light energy is produced by these lamps using a two-step process. First, the filament gives off UV light. This UV light excites a phosphor powder inside of the tube. This powder fluoresces and gives off visible light. Because of this two-step process, these lamps have an efficiency rating of only 13% and have a lifespan of about 10,000 hours. These lamps are more commonly used along roadways and in parking complexes. They may be purchased in wattages of up to 1000W.
- High-pressure metal-halide bulbs cost more than high-pressure mercury bulbs and have a shorter life span than high-pressure mercury bulbs. However, they have a broader emission spectrum than mercury bulbs. Their efficiency rating is 20%, and they have a lifespan of 8,000 to 15,000 hours. They may be purchased in wattages of up to 2000W.
- Low-pressure sodium bulbs have an efficiency rating of 27% and have a lifespan of 18,000 hours. However, these bulbs have a very narrow emission spectrum (nearly all light is emitted around 590 nm). This narrow emission spectrum can cause adverse effects on crop development. These bulbs are available in wattages of 35, 55, 135 and 180W.
- High-pressure sodium (HPS) lamps are the most common type of bulb used for HID lighting in greenhouses. They have a broader emission spectrum than low-pressure sodium bulbs and are lower in cost than high pressure mercury bulbs (approximately $150 -$250 per unit or $2.25 - $3.00/ft2 to purchase and install). These bulbs have efficiency ratings of 25% and a lifespan of 24,000 hours. They are available in wattages of 250, 400 and 1000W. The 400 watt and 1000 watt bulbs are most commonly used in greenhouses.
Using Supplemental Lighting in Greenhouses
As discussed above, supplemental lighting in greenhouses may be conducted for two purposes. The first being to alter the photoperiod experienced by the plants (photoperiodic lighting) and the second being to increase to amount of light available for photosynthesis (HID lighting).
Controlling Photoperiod in Greenhouses
The duration that light is perceived by the plant (photoperiod) is usually controlled in order to time flowering or to maintain plants in a vegetative condition. During long-day photoperiods (i.e. late spring and summer), black out cloth (a 100% light excluding cotton polypropylene fabric) may be pulled to artificially shorten the length of the photoperiod if required. Black out cloth may also need to be pulled over the crop during naturally short days in order to block out light pollution coming from external sources.
During short-day photoperiods, supplemental photoperiodic lighting may be used to increase photoperiod (specifically to shorten the length of the night experienced by the plants and create a long day). Photoperiodic lighting requires only very low levels of light (typically 10 µmol/m2/s are recommended), but does require that the light source emit in the red region of the spectral distribution. Incandescent lamps work well for this purpose because they emit enough photons (have a high enough light level), in the red region and they are low in cost and simple to install. If already available and installed in the greenhouse, high pressure sodium or metal halide HID lamps may also be used for photoperiodic lighting. However, they will produce more light than required and will cost more to operate than incandescent lamps.
Compact fluorescent bulbs have become increasingly common in home and commercial applications. Researchers have tested these types of bulbs as an alternative to incandescent lamps for creating long-days. For some plant species, it has been found that compact fluorescent lamps are not as effective at creating the long-day effect as incandescent lamps. It has been speculated that this may be due to the differences in the light quality emitted by the two light sources. Mixing incandescent lamps with compact fluorescent lamps at a 1:1 ratio was effective for photoperiod control.
When manipulating photoperiod through lighting, the lights may be turned on for 4 - 8 hours at sunset or before sunrise (extending the day). However, more commonly, the lights are turned on during the middle of the night (night interruption). This places a light period in the middle of the night, thus creating two short night periods rather than having a single long night. Night interruption has been shown to be the most effective method of creating a long-day photoperiod. For this method, lights are usually turned on at 10:00PM and turned off at 2:00AM. Cyclic lighting may also be used to interrupt the night period. There are several potential lighting cycles that can be used, but a common method is to turn the lights on for 5 - 10 minutes per hour during the night. Finally, if HID lighting is being used on the crop, and long days need to be maintained, the lights can simply be turned on for 18 hours as usual. This period is sufficient to create long days for all greenhouse crops.
High Intensity Discharge Lighting in Greenhouses
Supplemental HID lighting is specifically used to increase the light level (number of photons) available for photosynthesis. Because of the costs associated with HID lighting, it is not economical to use HID lights with all crops even if a positive growth response can be achieved. The increased plant quality, increased production or reduced cropping time resulting from the increased light levels must compensate for the increased cost associated with HID lighting. Therefore, HID lighting is most commonly used for higher value crops such as roses, vegetative stock plants, ornamental and vegetable plugs or seedlings, and some greenhouse-grown vegetable crops. In some cases, HID lighting might be used on containerized crops if ambient light levels are very low and limiting.
It is best to work with the manufacturer to decide on the best placement of fixtures to achieve desired light levels with maximum efficiency. In some cases, 1000 watt lamps are used. However, tall greenhouses are required with these lamps to insure uniform light distribution. The reflector used with the light source is important. The goal is to achieve the most uniform distribution of light that is possible while using as few lamps as possible. HID lamps using the older style circular reflectors are generally spaced apart at 1.5 times the distance between the lamp and the plant material. New types of reflectors allow spacing to be up to 4.5 times the distance between the lamps and the plants.
As discussed above, greenhouse managers may look to maintain an optimal quantum flux density (light level) throughout the day. However, another useful way to control light level is to measure the daily light integral (DLI) and make sure that the crop receives its required daily light integral. On days with more natural light, little supplemental HID lighting might be required. On other days with less natural light, more supplemental HID lighting may be required. The required daily light integral may be met by increasing the quantum flux density or by increasing the length of time the crops receive light. However, the quantum flux density must be higher than the light compensation point or no net photosynthesis occurs. The best way to manage light quantity is to provide light at a quantum flux density in the optimal range for a period of time required to provide the optimal daily light integral. When conducting supplemental HID lighting, light should be provided for no longer than 16 - 18 hours per day. Providing supplemental light for 24 hours per day can be detrimental to plants.
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