What is the difference between High Pressure Sodium (HPS) and Metal Halide (MH)?
High Pressure Sodium (HPS) lamps yield yellow lighting and burn around 2200 Kelvin (K). HPS lights are used for the second (or bloom) phase of the growth and are not recommended for the first (or vegetative) phase. Plants grown in their vegetative phase with HPS lamps will usually grow slightly more quickly and with longer internodal spacing resulting in a “leggy” plant that is less stable. It is for this reason that San Diego Hydroponics recommends that you use Metal Halide (MH) lights for the vegetative phase.
Metal Halide (MH) lamps are used primarily in the vegetative phase of plant growth and can yield a wide range of light color, anywhere from 3,000 K to over 20,000 K. Most MH bulbs will range from 6000 K to 10,000 K.
MH lamps are very popular among reef aquarists who need a high intensity light source for their live coral. HPS lamps are the preferred supplemental greenhouse lights due to their high efficiency and the fact that plants grown in greenhouses get all the blue light they need naturally.