Light Requirements of Plants
The plants' specific needs determine which lighting is most appropriate for optimum growth; artificial light must mimic the natural light to which the plant is best adapted. The bigger the plant gets the more light it requires; if there is not enough light, a plant will not grow, regardless of other conditions.
For example, vegetables grow best in full sunlight, and to flourish indoors they need equally high light levels; thus fluorescent lights or MH-lights are best. Foliage plants (e.g., Philodendron) grow in full shade and can grow normally with much lower light levels, thus regular incandescents may suffice.
In addition, plants also require both dark and light ("photo"-) periods. Therefore, lights may be turned on or off at set times. The optimum photo/dark period ratio depends on the species and variety of plant, as some prefer long days and short nights and others prefer the opposite or intermediate "day lengths".
Illuminance, or luminous flux density, measured in lux is an important factor in indoor growing. Illuminance is the amount of light incident on a surface. One lux equals one lumen of light falling on an area of one square meter (lm/m2), which is approximately 0.093 foot-candle (lm/ft2). A brightly lit office would be illuminated at about 400 lux.
Lux are photometric units, in that different wavelengths of light are weighted by the eye's response to them. In professional farming, radiometric (watt/metre2 or microeinstein /second·meter2) or photosynthetically active radiation weighted (PAR watt) units are used instead.
Read more about this topic: Grow Light
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