Plants and Light
Plants use sunlight to produce sugars through photosynthesis, but they do not use the entire spectrum of light. Different plants have receptors for different wavelengths, and the wavelengths encourage different types of growth. Early in the growing season, sunlight is bluer, and this encourages overall plant growth. In the fall, redder sunlight encourages flowering and fruiting in many species. Different sorts of grow lamps are designed to emit different wavelengths, and each type has its strengths and weaknesses.
Incandescent grow lamps work like classic incandescent light bulbs; electricity runs through a thin wire in the middle of the bulb, heating it up until it glows white hot. Incandescent grow lamp bulbs can be very inefficient, turning more energy into heat than into light. They also have a much shorter lifespan than other lights and have to be changed often.
Fluorescent, Sodium and Metal Halide Lamps
Fluorescent, metal halide and sodium lamps all work by sending electricity through gasses. Molecules in the gas absorb electrons, then reemit the energy as photons of light. The type of gas used determines the spectrum of light. Metal halide lamps give off a lot of blue and violet light, which creates strong roots and leafy, compact plants. Sodium lights are the opposite; they give off mostly red and yellow light, which stimulates fruiting and budding. Fluorescent lights give off a balanced, broad-spectrum light, so they are good for all stages of growth.
LEDs, or light-emitting diodes, are the newest option in plant-growth lighting. Although LEDs have been used for displays in devices like alarm clocks for decades, they have only recently been made bright enough to use for grow lights. They work by pushing electrons through a barrier between two semiconductors. When the electrons jump through this barrier, they fall into a lower energy state, releasing some of their energy as light. Grow LED systems usually consist of a grid of blue and red LEDs instead of broad-spectrum lights. They produce a lot of light with very little energy, and last for a very long time. The main draw back is that LEDs are not good for display; the red and blue light is hard to see by and won't show the colors of a plant well.