Metal Halide (MH) and High Pressure Sodium (HPS) are two types of High Intensity Discharge (HID) lights used in hydroponic gardening. MH grow lights are generally used during a plant's growing cycle - i.e., when it is not blooming or bearing fruit. HPS lights are used during a plant's fruiting/blooming phase. There are two sub-types of HPS lighting: one is enhanced for the blue spectrum, which is better for fruiting, and the other is red spectrum enhanced, which specific benefits flowering. In addition, there are mercury-vapor lamps, which are similar to modern streetlights, and fluorescent grow lights.

All indoor grow lights make use of what is known as a ballast system. With MH and HPS lights, these are remote, meaning that an external box is required in order to house an electronic pre-heating system, which in turn runs the lamp itself.

Historically, all types of HID grow lights have a useful life of approximately 1 - 1-1/2 years. Although they will usually operate beyond this time, their luminescence is compromised - meaning that they will produce substantially less light, yet continue to consume the same amount of electricity.

With the advent of LED grow lights, are HPS and MH lights obsolete?

LED stands for "Light-Emitting Diode." LED lights have actually been around for a long time - some people who built models of spacecraft from popular science fiction may recall the old "grain-o-wheat" bulbs used to illuminate them. LED lights have also been used in calculators, digital displays and movement sensors (your new cordless optical mouse probably has a motion-sensing LED device).

Unlike most traditional types of glow lights - each of which have its own specific application - LED grow lights can be tuned to specific wavelengths according to the needs of the particular plant. LED grow lights are also much more efficient, using only a fraction of the energy required even by fluorescent lights while providing much greater luminescence.

The best part of LED grow lights is their durability. Unlike MH or HPS lights which usually must be replaced after 18 months, LED lamps are built to function for up to 100,000 hours - nearly twelve years with normal usage averaging 18 hours per day.

Another feature of LED grow lights that make them superior to other types is the substantially lower amount of hear produced. Unlike other types of grow lights, LEDs are cool burning. This means that less heat is produced to affect the plant, nor will an elaborate ventilation system be required in order to carry away excess heat.

The drawback of LED grow lights is their relative expense; they require a greater initial investment than most other types of grow lights. However, because of their efficiency and durability, they quickly pay for themselves in terms of long-term energy savings.