Fluorescent or incandescent lighting can be used to supplement natural daylight, or to replace it entirely. Where artificial light is the only source of illumination for your plants, it is best given for 12-16 hours each day, at any time during the 24 hours.
Economically speaking, it is cheaper to use fluorescent light than incandescent: for the same output in watts the latter uses more power. In addition, the heat radiated from incandescent bulbs can be a problem, and plants will be scorched if they are closer than about 4 feet from the incandescent bulbs for any length of time. Thus, for wholly artificial lighted plantings, fluorescent tubes generally are best: though for supplemental lighting in small areas an incandescent floodlights or spotlight is fine.
For general groups of plants the optimum light at average plant height is 300-400 foot candles: however, most foliage plants will grow at 100-150 foot candles(bright to average room light), and they will survive for a year or more with much less light- even 50 foot candles.
Foot candles indicate the intensity of light at any given distance from its source. Thus, 30-400 foot candles can be supplied by four 40-watt fluorescent tubes, with reflector, at a height of 2 feet above the plants. For an intensity of 100-150 foot candles, use either two 40 watt tubes at 2 feet or one 40 watt tube only 1 foot above the plants. Incandescent light sources can also provide this intensity with little danger scorching the plants if there is some air movement: one 300-watt bulb with reflector supplies about 100 foot candles of light at a distance of 4 feet.
Although plants may not make new growth with less than 100 foot candles of light, they can remain attractive for long periods. Interior landscapes, and office and reception area displays are examples in which rapid plant growth may not be needed nor desired in the displays. Several combinations of light source and distance form the plants will provide sufficient light for these plants:
INCANDESCENT bulb with reflector:
75-watt at 2 feet* supplies about 60 foot candles
100-watt at 2 feet* supplies about 80 foot candles
150-watt at 3 feet* supplies about 60 foot candles
300-watt at 6 feet* supplies about 50 foot candles
*For short periods only or scorching will result.
FLUORESCENT 40-watt tub(s) with reflector:
One tube at 2 feet supplies about 75 foot candles.
Two tubes at 3 feet supplies about 80 foot candles.
Four tubes at 5-6 feet (ceiling to desk) supply about 40 foot candles.
The quality of light from cool white fluorescent tubes is adequate for indoor foliage plant life and growth, though it may be too harsh for our eyes. where no natural daylight is available a combination of warm white and cool white tubes will provide a well balanced and eye-pleasing light: a one-to-one ratio is suggested. Another way to warm up the cool white light for our eyes is to add one watt of incandescent light to every two wats fo fluorescent
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