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Different Artificial lights used in polyhouses

 

Various methods of Supplemental Lighting in Greenhouses: Before selecting a light source for greenhouse lighting, numerous factors should be considered. Among these are the

1. Total energy emitted by the lamp,

2. Efficiency (% of electrical energy converted to light energy),

3. wavelengths emitted (especially in the 400 to 700 nm wavelengths),

4. Cost, life expectancy (of bulbs and fixtures), and the fixtures required (including ballasts).

Supplemental lighting during daylight hours to enhance photosynthesis is proved to be highly effective. Such lighting is more profitable in High Density Plantings, such as rooting and seedling beds and the production of young plants. Many types of lamps have been used in the greenhouse. Basically, they fall into 3 groups

1. 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 relatively low light levels are required for photoperiodic lighting, incandescent plants are suitable and commonly used for this purpose.

2. Fluorescent lamps: - These lamps are most commonly used in growth chambers and seed germination rooms. 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. These fixtures 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.

3. High Intensity Discharge (HID) Lamps: - Now a day, the HID lamps are preferred types for the final stages of the crop growth in the greenhouse. These are the most commonly used lamps for supplemental lighting in greenhouses. As with fluorescent lamps, these lamps require ballasts that can be very heavy and generate significant amounts of heat. Reflectors are used to direct the light generated downward and to improve uniformity of light distribution. Numerous types off bulbs are available for use in HID lamps such as high-pressure mercury, metal halide, low pressure sodium and high-pressure mercury. The most commonly used HID system at present utilizes high pressure sodium lamps.

a. High-pressure mercury bulbs: have emission spectrum similar to fluorescent lamps but with a greater 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 radiation excites a phosphor powder in the tube. This powder fluoresces and gives off visible light. Because of this two-step process, these lamps have an efficiency of only 13% and have a lifespan of about 10,000 hours.

b. 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 is 20%. These bulbs have a lifespan of 8,000 to 15,000 hours.

c. Low-pressure sodium bulbs: have an efficiency of 27% and have a lifespan of 18,000 hours. However, these bulbs have very narrow emission spectrums. This narrow emission spectrum can cause adverse effects on crop development.

d. High-pressure sodium lamps: are the most common type of bulb used for supplemental lighting in greenhouses. They have a broader emission spectrum than low-pressure sodium bulbs and are cheaper than mercury bulbs. These bulbs have efficiency of 25% and a lifespan of 24,000 hours.

               Supplemental lighting is used for most crops but is particularly popular with Chrysanthemum and geranium stock plants, rose and plug seedlings. Light intensities of 3.2to 6.5 Klux at plant height are generally used for seedlings and ornamental plants, with 4.3 Klux being the most common level. Intensities of 6.5 to 10.8 Klux are used for vegetable crops.

RAKESH KUMAR PATTNAIK

Asst. Prof. Horticulture

 

 

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