Choosing an indoor light for plants requires a little more thought than simply grabbing the first light you see on a store shelf. There are different colors, intensities and bulbs to consider. Each of the selections bears a direct impact on the plants, and some plants need more of one item than another and do better with different types of lights. Your location has a lot to do with the type of light you need for your indoor plants, as well, so select wisely.
There are seven main colors in the light spectrum, but only red and blue are used well by plants. Grow plants come in both colors. If you have to choose a bulb of one particular color choose red. Plants use red more than blue. Full-spectrum lights use the theory that even though plants mainly use red and blue, they benefit to some degree from all colors in the spectrum. Full-spectrum lights emulate natural lighting. It is good for people and atmosphere, but much of the light is wasted on plants.
People often choose incandescent bulbs for their indoor plant lights because they are so common. Regular household incandescent bulbs are too hot for plant health, however, and although they produce red light, they have little blue at all. Florescent bulbs are cooler, more energy efficient and come in plant-specific spectrum colors for maximum benefits. You can even use common household florescent bulbs for your plant needs, but they have the opposite color problem from incandescent bulbs. If you use them you will get too much blue, and not enough red.
In order to make household bulbs work you need a one to three ratio of incandescent and florescent bulbs. It is important to hang the incandescent lights higher to avoid burning sensitive leaves. The best ordinary lights are florescent plant lights that have both red and blue spectrum light. LED bulbs are cooler yet, but they have drawbacks. They are more expensive, do not perform any better in other aspects than florescent, and the biggest drawback is that they make plants look funny when the lights are on. That's okay if you are just growing for commercial use, but if you want to use the lights to make houseplants look their best, LED bulbs won't help.
The trickiest part of setting up indoor plant lights is obtaining the right intensity. Different plants have different needs to begin with. Start by ascertaining what amount of light your particular plants need. They are rated by low, medium and high light. To provide the proper amount of light you must determine how many watts of light each light makes available per square foot. This figure is known as footcandles. A footcandle is the amount of light 1 foot away from the light source. That amount will depend on the distance the light is from the surface.
An easier way to figure the watts, or footcandles, is to buy a candle meter and hold it where your plant sits to read the amount of light in that spot. Low-light plants require between 50 and 250 footcandles. Medium-light plants need 500 to 1000 footcandles and high-light plants require up to 2000 footcandles of light for the best development.
There are three types of professional-level, high-intensity discharge lights that work well for growing plants indoors. They are high-pressure sodium and metal halide. Both are on the expensive side, but last between 10,000 and 18,000 hours. Metal halide is higher-density blue spectrum, and sodium is red. They are so strong they are used mostly for large, greenhouse, industrial settings or office spaces.