High intensity discharge (HID) lights are the most commonly used grow light since they give off a greater amount of lumens (visible light power) per watt of incoming electricity to the light. HID lights are commonly preferred over fluorescent and incandescent lamps because their power will is displayed in their lumen output more than their heat output. An HID light can put out as much as five (5) times the lumens per watt as an incandescent lamp can produce.
There are two types of HID grow light bulbs used by indoor gardeners. There is the metal halide lamps and the high pressure sodium lamps.
Metal Halide (MH)
Metal halide lights are powerful and efficient light sources for growing. They emit more of a blue spectrum which is ideal for the vegetative stage of growing. The metal halide lights are designed to produce the same kind of blue and indigo light spectra that your plants would get from the sun in the spring time when most plants are beginning to sprout. This type of blue light will produce more compact stems, tighter internode spacing and a faster growth rate. The MH lights are best for the early stages of plant growth, when the stem production and leaf production are so important to the overall quality of plant you produce. Metal Halide lamps also produce enough red and yellow light spectra to produce decent flowering or fruiting, however, for maximum flowering production you will want to switch to an HPS light.
High Pressure Sodium (HPS)
High pressure sodium lights, as a ‘flowering’ and ‘fruiting’ light is in a class of its own and can only be out performed by the sun. HPS lights produce strong red and yellow light spectra. This end of the light spectrum generates longer stems, bigger blooms and more bountiful crop. The high pressure sodium lights produce brighter and more efficient lumens per watt than a metal halide. The greater your efficiency and production of lumens per watt can affect the size and quality of the yield of your plants.
Because of the fact that metal halide lamps require metal halide ballasts and high pressure sodium lamps require high pressure sodium ballasts, many growers prefer to use a digital ballast which has the ability to run either type of bulb.