Many people that want to grow marijuana do not know what lighting equipment they need. For anyone that can afford it, LED (light emitting diode) grow lights are the way to go. For many, LED grow lights are too large an investment. HID (High Intensity Discharge) grow lights are purchased instead and are perfectly capable of producing top quality product.
There are two types of HID grow lights used for marijuana, Metal Halide and High Pressure Sodium (MH and HPS). There are two primary differences between MH and HPS grow lights: color (spectrum) and brightness.
A color spectrum is the amount of each color of light emitted by a light source. The color spectrum of metal halide lamps is fairly close to that of the sun, while HPS is closer to that of a candle. To get an idea of the difference in color of the two lights, have a look outside at night; the orange glow from street lights is from HPS lamps (sometimes older street lights are mercury vapor). Most landscape lighting and stadium lighting are MH, and give off a bluish white light. Topbulb has excellent spectrum examples, near the bottom of the page.
The different spectrums of light emitted by the different bulbs encourage different types of growth in marijuana.
The blue-white light of MH bulbs encourages growth of large leaves packed densely together on the stem, with little overall branching. A plant grown using only MH light will be colorful, leafy, and pine tree shaped with slightly airy buds.
HPS grow lights increase the plants tendency to branch out, which increases the number of bud sites and the amount of empty space between them. A plant grown using only HPS will be very bushy with lots of space between the leaves and dense buds.
Using MH light until after the first week of flowering gives plants a lot of leaves, which in turn provide more energy to the plant, without compromising harvest weights.
The brightness of the lamps is the main reason most growers use HPS lights for flowering over MH. HPS lamps are 40%-60% brighter than MH lamps of equal wattage.
For example, a 400 watt metal halide puts out 32,000 initial lumens (80 lumens/watt) and a 400 watt HPS puts out 50,000 initial lumens (125 lumens/watt). The most efficient HID grow light available is the 600 watt HPS, which puts out 90,000 initial lumens (150 lumens/watt). For reference, a 100 watt incandescent light bulb puts out only 1,750 initial lumens (17.5 lumens/watt).
During vegetative (non-flowering) growth, marijuana requires only 1/4 the light it will need during flowering, which makes MH grow lights more useful for young plants. Many growers prefer to use MH bulbs for the plants before flowering is induced, and HPS for the remainder. Many others have one room with a metal halide grow light for vegetative plants and a separate, larger grow room with an HPS light for flowering plants.