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While natural light is essential for plant growth in hydroponics, it may well be replaced by artificial lighting. The use of grow lights, as they're called, efficiently extends daylight for the plants (or substitutes for daylight altogether). Because plants require a certain amount of intensity of light and draw from the full light spectrum for various stages of growth, standard incandescent lights don't work well as grow lights. (Consider that you need to emulate sunlight as much as possible.) Among the ideal lighting systems for hydroponic plants are the High Intensity Discharge (HID) bulbs ñ two of which are listed below. Aside from those two, there are still several kinds of bulbs that are ideal for hydroponics:

Metal Halide Bulbs (MH)
Arguably the most popular HID-type bulb, metal halide bulbs produce the closest emulation of summer sunlight available, and yield the spectral colors plants thrive on most most particularly the blues, which are ideal for vegetative growth. MH bulbs need to be replaced before they actually burn out (they tend to die out gradually) as they cannot produce sufficient lumens anymore to help out the plants. Good thing that they're long-lasting, having an average life of 10,000 hours, or a year.

High Pressure Sodium Bulbs (HPS)
The other HID-type bulb, the high pressure sodium bulb, is most appropriate for supplementary lighting, used together with natural sunlight. HPS bulbs emphasize the orange-red band of the spectrum, which is good for flowering plants. Compared to metal halide, HPS bulbs as grow lights are longer-lasting (on the average 18,000 hours) and are less costly. However, because they are deficient in blue light, HPS bulbs are not usually recommended as a replacement for natural light, nor as an alternative to metal halide. Instead, they'd do well in a greenhouse environment.

Fluorescent Bulbs
Where in earlier days fluorescent light bulbs weren't intense enough, at present fluorescents are made that have enough lumens to supply light for hydroponics. Notwithstanding if the bulb is high-output or low-output, it can do this job perfectly. Fluorescents emit much smaller amounts of heat than HID bulbs, which means they can be placed much closer to the plants and when they are placed closer (but not too close), they emit enough of the spectral colors to encourage growth.

LED Grow Lights
The newest version of grow light, the advantage of LED bulbs is that they are comparable to the output of a high pressure sodium bulb while being extremely energy efficient, and emitting almost no heat. On the flip side, these bulbs promote slower growth rate and are painful to your wallet. In fact, it can be argued that the same growth results can be achieved with fluorescent grow lights, which are far less costly. Thus, people are still doubtful if LED bulbs are the future of grow lights, even if they're definitely the newest thing to artificial lighting.