An artificial light source is necessary for areas where natural light is not enough to support optimal plant growth. For instance, during summers, container crops can be moved indoors and supplied with the light they need by artificial sources. This means that adverse seasonal effects on plants yields are controlled . It is also used by farmers to extend the natural sunlight by disrupting the night. This would enable the plant to continue its photosynthesis process, hence, results in better yields (Nick, 2009). There are about four primary sources of artificial light in greenhouses. Each of this sources has advantages and disadvantages, as well as different specific applications. Although light is necessary for plant growth, strength, and good yields, too much of it negatively affect the plant. Scientists have thus come up with ways of calculating the optimal amount of light required by the plant.

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Artificial Sources of Light
This type of lamps is cheap and easy to install compared to other types of lighting. They are also easily operated without many skills. They have the advantage of having high fatigue resistance to frequent repeated switching them off and on (Apex Publishers, 2018). The main disadvantage of an incandescent type of lighting is the fact that they give less amount of light for sufficient greenhouses application. They, however, radiate a lot of infrared. One primary application of incandescent lamps is providing lighting to greenhouse doorways. They are also used to give a specific type of light known as ‘spot grow’ that has a range of 75 – 150-watt ability. This type of light is used to provide light for a single plant.

High-Intensity Discharge (HID)
These lamps have high light output with a low amount of shading. They are of two types; one is the high-pressure sodium ( HPS) which emits light in the yellow or orange spectrum wavelength, and the other is the metal halide ( MH) and emits light in the blue region thus appears bluish . HID produces light of a wavelength ranging from the red to the blue spectrum (visible light) thus very beneficial to the plant. HPS is preferred by farmers as light in the yellow spectrum, provides conditions that initiate flowering or budding in plants. This supports better yields. MH is applied in areas where they act as the primary sources of light, like when there is no sunlight. Light in the blue spectrum emitted by Metal halide lamp has long wavelength but minimum frequency scientifically . This characteristic promotes plant growth. The presence of conversion lamps having ballasts and fixtures has made it possible to use MH bulbs for excellent plant growth and HPS bulbs in flowering stages .

Although High-Intensity Discharge lamps are the most preferred type of lighting in greenhouses, they have two main disadvantage. The first is they take up a lot of space owing to the heavy ballast they require separate from the fixtures. This also degrades the aesthetic value of the greenhouse. The second significant problem associated with HID is the fact that they radiate heat which necessitates that they are moved further from the crop especially in the warm season (Nick, 2009). However, this act reduces the intensity of light reaching the plant and so affecting efficiency. Never the less, High-intensity discharge bulbs are more expensive to use than other types of lighting.

Traditional fluorescent fixtures had very little lumen output. In modern time, however, the introduction of T5 lights has made the application of fluorescent lamps in greenhouse significant. T5 gives three times lumen power compared to traditional bulbs. The advantage of using fluorescent bulbs is that it radiates very little infrared thus can be placed close to the plant without overheating them. This property is the principle behind the application of these type of bulbs in germination shelves. There are two disadvantages of using fluorescent bulbs; one is that the fixture used with fluorescent bulbs produce a lot of shading and the other is that, even with the introduction of T5, the intensity of light given off by fluorescent bulb can still not be compared to that produced by HID lamps (Andersson,1997).

Light Emitting Diode (LED)
LED technology form the fundamental principle behind most modern grow light. LED lights are of small size, and weight, thus, can be mounted and configured more efficiently. They produce heat out of the back but not with the light and can thus be placed close to the plant just as in the case of fluorescents lamps. They function under the principle of maximising the blue and the red light while minimising orange and the yellow light so as only to produce light that is needed for photosynthesis. LED has better energy economy than HID. It emits light that appears dimmer than that from fluorescent source. It has a better lifespan than all the other traditional light sources .

Problems with Using Extra Lighting
A plant needs light to provide the energy required for photosynthesis. However, the very nature of the light can slower the photosynthesis process. The visible and the ultraviolet light, for example, have very adverse effects on photosynthesis process . This made worse when light supply exceeds the optimal range required by the plant. Every plant has an optimal range of light it requires. Too much light triggers degradation of essential plant proteins known as D1 – polypeptide (Andersson,1997). This happens as are a result of photosystem II reaction involving oxidising agents and species of toxic oxygen. The reaction is less under normal conditions but accelerates with the increase in light intensity. The photosystem is inversely proportional to photosynthesis thus when it increases, photosynthesis reduces.

Too much light increases the rate of transpiration. Transpiration is a pant transport mechanism whereby water is lost by the plant to the atmosphere through the stomata. Stomata are triggered to open by light and are more sensitive to light in the blue region (Plant & Soil Sciences e-Library, 2018). Also, high temperature caused by heat as a result of infrared light causes more full opening of the stomata. When the light especially infrared is too much, more water is lost to the atmosphere. This led to more water uptake by the plant by capillary living the soil dry (Plant & Soil Sciences e-Library, 2018). For this reasons, it is essential to select the most appropriate source of artificial light and not to exceed the optimum level of light required by the plant.

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