Metal halide (MH) is a type of high-intensity discharge (HID) lamp. Metal halide lamps produce light though an electric arc and a mixture of gaseous mercury and metal halides.
Metal halide lamps emit a bluish-white light that encourages plants to enter a vegetative growth stage. When used properly, plants will grow thick and stocky foliage.
Metal halides are part of the high-intensity discharge (HID) lighting family, which includes high-pressure sodium (HPS) and ceramic metal halide (CMH).
Metal halide grow lights are popular for indoor gardening. They are a cheaper alternative to other types of indoor grow lighting while still producing a high quality and high output light. These lights are often employed by gardeners when they want to keep their plants in a short and bushy state.
Metal halide lamps improve the color rendition and efficiency of light, with a luminous efficiency of 75 – 100 lumens per watt, and have a lamp life of 6,000 – 15,000 hours.
One downside to metal halide lights is that they do not emit in the red/orange portion of the light spectrum, which is necessary for plants to produce fruit. However, special metal halide lamps with an enhanced red-light spectrum have been developed.
MH lights also increase the heat in the growroom or greenhouse, so some form of exhaust system must be used to cool down the room and vent out the heat. They must also be positioned away from the plant’s canopy so they do not inadvertently burn the foliage.