The main types of plant grow lights are metal halide, high pressure sodium, LED (Light Emitting Diode) and fluorescent lights. You can grow plants under incandescent lights, but they're inefficient, expensive to operate and are not usually included in lists of grow lights. Metal halide and high pressure sodium lamps are also called HID lights (High Intensity Discharge). Grow lights give off light at different intensities and parts of the light spectrum, both of which are important to the gardener considering indoor growing.
A metal halide lamp contains a ceramic or quartz arc tube, sometimes called a burner or discharger, inside a larger bulb. The arc tube contains argon, mercury and metal halide salts. The heated gas in the arc tube is vaporized, giving off light and ultraviolet radiation. The glass outer jacket or bulb helps keep the arc tube from oxidizing.
A metal halide tube will yield 125 lumens of light above 5,000 K---blue spectrum light that is a close approximation of sunlight. A lumen measures the quantity of light; it is the light produced by one foot-candle falling on one square foot. A foot-candle is how much light you get one foot away from the source.The rough standard is that the more light a plant gets, the better it will grow.
The "temperature" or quality of light, its place in the light spectrum, is measured in Kelvins (K). Temperatures of 2700 to 3000 K are "warm colors," yellow-red. Temperatures of 5000 K or higher are "cool" colors, green-blue. Seedlings, clones and growing plants need blue light. Flowering plants usually need red-orange light.
The drawback is that metal halide lights are expensive and they consume a lot of electricity.
High Pressure Sodium
In high pressure sodium lights, heated mercury vaporizes sodium. High pressure sodium bulbs emit an orange-red glow that triggers hormones in plants, making them good for budding and flowers. High-pressure sodium lights are often used for indoor hydroponic systems. They're also used in greenhouses to supplement sunlight. They produce up to 140 lumen. They last twice as long as metal halide.
LED stands for Light Emitting Diode. LEDs work much like an incandescent light bulb except that they don't have a filament. The lights you see in television sets, computers and DVD players are LED. They don't get hot and consume little energy. LEDs that are used for growing plants emit only wavelengths of light that plants need to absorb chlorophyll. These special LEDs give off a pink glow so they're not good if you want to enjoy the natural color of the plant.
An electrical charge in a fluorescent tube ignites argon gas that vaporizes a small amount of mercury, giving off light. Fluorescent tubes yield abundant light in the desirable blue zone of the spectrum. Fluorescent lights marketed as "full spectrum" give you the entire spectrum. The T number rating of a fluorescent tube indicates the diameter of the tube in eighths of an inch. Earlier, thicker fluorescent tubes were T12. More recent tubes on the market are cheaper and more efficient--T8 down to T5. They also last longer.
CFL (Compact Fluorescent Light) are narrow T5 lights that emit a full spectrum of light. Fluorescent lights formerly yielded 39 lumens for each watt. A modern CFL or T5 will give you 75 to 90 lumens of light. Fluorescent lights produce almost no heat, so you can place them closer to your plants.