There are four basic building blocks on which plant life is based: Light, Water , Nutrition, and Climate.
The most common factor that limits plant growth is the light source. Gardening outdoors, this obviously is not a problem; Mother Nature has seen to proper light balance and intensity for healthy plant growth. The responsibility for proper indoor lighting falls on the gardener. If your plants are not furnished enough light of the correct spectrum, they often will be mere shadows of what they could have been, if they grow at all. When you can't rely on Mother Nature to handle the lighting for you, the next best thing is a High-Intensity Discharge (HID) Metal Halide light system.
It is hard to compare HID lights with fluorescent tubes or incandescent light bulbs. Although they each create light from electricity, that's where the similarity ends. Fluorescent tubes emit a gentle, low temperature light in a very low wattage. Excellent for the first two weeks of most any plant's life, fluorescent lights simply do not provide the intensity of light required for most vegetables, flowers and ornamentals.
Incandescent lights ('regular' light bulbs) are even worse for horticulture because they are very expensive to operate, put off as much heat as light, and do not offer the spectrums of light required for healthy plant growth. Even when incandescent light bulbs are altered with interior coatings to change their spectrum (like the "grow light" bulbs you see in the grocery store), they still do not come close to providing the kind of light a plant needs for robust, active growth. The only thing that will really grow and prosper under an incandescent grow bulb is your electric bill! Listed below are some common types of hydroponic grow lights.
What we call light is only a small sliver of the electromagnetic (EM) spectrum. Gamma rays occupy the short end of the EM spectrum with a wavelength of 10-5 nm, at the other end are radio waves with a wavelength of 1012nm. The part of the spectrum that can be seen by the human eye is called visible light and occupies the part of the spectrum between 380 and 750 nm. All life, beginning with photosynthetic organisms (plants) depends on this part of the spectrum for its energy. We see the different wavelengths of visible light as different colors.
Chlorophyll is the plant pigment responsible for photosynthesis in plants, but it does not absorb all the wavelengths of visible light equally. The most important form of chlorophyll, chlorophyll a, does not absorb light in the green part of the spectrum. Light not absorbed is reflected, this is why plants are green. The absorption of light by chlorophyll A is at its maximum at two points in the visible spectrum, at 430 and 662 nm. The rate of photosynthesis peaks at two points in the spectrum which correspond approximately to the peaks in absorption. There are some lesser forms of chlorophyll which use other parts of the visible spectrum, so requirements differ slightly from plant to plant, and depending on the stage a plant is at in its life cycle.
The trick in artificially lighting your indoor hydroponics system is to provide your plants with the right parts of the visible spectrum, at the right times, and in the right measure. In general, you should plan on lighting your hydroponics system for about 14-16 hrs. per day, with a corresponding 8-10 hrs. of dark. The dark periods are important to plants, just as sleep time is needed by animals. Lighting your plants uses a great deal of energy, and it can produce a lot of heat. The heat byproduct of lighting can be either a blessing or a curse, but may well need to be managed in one way or another.
There are three main types of lighting systems for indoor growing. These are High Intensity Discharge (HID), fluorescent, and the new kid on the block LED. HID is further broken down into High Pressure Sodium (HPS), and Metal Halide (MH) bulbs. To complicate matters just a bit more, some lighting systems use “conversion bulbs”, which allow you to switch back and forth between MH and HPS bulbs at different stages of growth. Each system has its advantages and draw backs. If you are unable to utilize natural lighting, choosing an artificial lighting system will be one of your most difficult yet most important decisions in planning your hydroponics operation.