Several kinds of lights are used to grow plants indoors--two of these, metal halide and high pressure sodium, are also called HID (high-intensity discharge) lights. LED (light- emitting diode) lights are greatly improved over its original versions, and relatively inexpensive. Recent advances have made the fluorescent lights more attractive for growers. Incandescent lights are not recommended for growing plants.
The best light to grow plants under without sunlight is a metal halide lamp that produces an abundance of light in the blue spectrum--light that promotes plant growth.
A metal halide tube will give you 125 lumens of light. A lumen is a 1-foot candle falling on 1 square foot. One foot-candle is the measurement of how much light you get when standing 1 foot away from the light source.
High-pressure sodium bulbs emit an orange-red glow that triggers hormones in plants. This increases budding and flowers. High pressure sodium lights are often used in greenhouses to supplement sunlight. They produce up to 140 lumens and last twice as long as a metal halide.
LED (light-emitting diode) lights are bright, cheap and emit only wavelengths of light that are needed for the best absorption of chlorophyll. Whether or not LED lights are better for indoor growing than HID lights is a subject of controversy, since claims of efficiency by advocates are based on anecdotal evidence and not scientific studies.
When you look at fluorescent tubes pay attention to the “T” numbers. They refer to the diameter of tubes in eighths of an inch. Early models are T12. Newer tubes are narrower, T8 or even T5. They are more efficient, cheaper and last longer.
CFL (compact fluorescent lights, which replaced long tubes) and T5 lights that emit a full spectrum of lights are the fluorescent lights of choice. Fluorescent lights formerly gave you 39 lumens for each watt. A CFL or T5 will yield 75 to 90 lumens. Fluorescent lights less heat than metal halide lights so they can be placed closer to the plants.
Color temperature is measured in Kelvins (K). Temperatures of 2,700 to 3,000 K are yellow-red “warm” colors. Temperatures of 5,000 K are green-blue “cool” colors. You want lots of blue light for growing plants, especially seedlings and clones. Fluorescent lights give lots of light in the blue zone.
Standard T12 full spectrum tubes are good for starting plants and growing seedlings. They’re good for herbs, African violets and other plants that don’t need much light. T5 lights are also good for budding and flowering.
You can grow plants using an incandescent light, the standard light bulb lit by a heated filament, but it is a poor choice. They’re cheap to buy, but they use a lot of electricity, don’t last long, and their light isn’t good for plants.