Metal halide grow lights create fantastic light under which your plants will thrive. These grow lights are a HID, or high-intensity discharge, light. They are small grow lamps that produce a lot of light, and yet are more efficient to run than either incandescent or fluorescent light bulbs. Indoor gardeners and hydroponics enthusiasts get great results with their plants when they use metal halide lights.

Metal halide lamps are made up of several different parts. There are tungsten electrodes which are connected to a quartz arc tube, where the light is produced. Inside the arc tube is where you will find mercury vapor, different metals and noble gas, all of which play a part in creating the light produced by MH grow lights. The arc tube is covered by a glass bulb. There is a metal base, and a connection to the power source. Some metal halide grow lights use an arc tube made of alumina or aluminum oxide, as well. This type of grow light requires the use of a ballast to control the flow of the current through the arc tube in order for the grow lamp to function properly.

Each different type of grow light has a particular color-rendering index and correlated color temperature. This is true of the metal halide grow lights. Some of these lamps have an 80 color-rendering index, on a scale of zero to 100. This index ranks lights to show how well or poorly they reproduce colors of objects being lit by the lamp.

100 is the best rating, and zero the worst. Therefore, an 80 is reflective of a very good quality white light. In terms of correlated color temperature, MH grow lights can range from 3,000 K to 20,000 K. 3,000 is in the yellow range of the spectrum, while 20,000 is in the blue range. (K refers to the Kelvin temperature scale.) In terms of comparison, daylight registers in at 6,500 K, whereas a television screen is at 9,300 K and moonlight is at 4,100 K. If you are an indoor gardener, you are trying to recreate daylight in order for your plants to thrive, so you will want a bulb as close to daylight's 6,500 K as possible.

Light produced by a grow lamp can fall at different spots on the electromagnetic spectrum. Some lights lean more toward the blue end of spectrum, while others land on the orange and red end. This matters to the indoor gardener, because baby plants need light in the blue spectrum in order to grow well. If you want a mature plant to produce fruit or flowers, then you will need lights in the red and orange spectrum.