When saving energy and long life are important in your lighting design, High Intensity Discharge lighting (HID) is the best option for saving energy and long life. The technology involved in HID lighting is similar to fluorescent lighting in this manner: an arc is established between two electrodes in a gas-filled tube, which causes a special vapor to produce the light you see. HID lights do not need a coating of phosphors in order to produce the light like fluorescent light bulbs do.
There are three main types of HID lamps: mercury vapor, sodium, and metal halide. This article deals with mercury vapor bulbs. GE, Philips, Sylvania, Westinghouse, and other brands all carry this type of bulb in various wattages.
The main reason to use mercury vapor light bulbs is energy efficiency. The lumen or light output from one of these bulbs is equivalent to the output from a normal incandescent bulb three times the wattage. This will save the end user a lot of money over the life of the bulb in energy. It will also require installation of fewer light fixtures. The average life of this type of bulb is typically about 24,000 or more hours. Compare this to a normal incandescent bulb, which has an average life of about 1,000 hours.
The most significant drawback to mercury vapor lighting is the color of the light it emits (color rendering index or CRI) and how items look when viewed underneath the light. Imagine taking your brand new cherry red Mustang convertible to your local store. If you were to park it under mercury vapor lighting, the cherry red would most likely look a drab brown! When color quality is important, you should consider installing metal halide bulbs. In addition, the initial cost of fixtures (which contain specialized ballasts) and the bulbs is much higher than normal incandescent lighting. Keep in mind the energy saved over the life of the bulb and the need for fewer fixtures will more than make up for this, however.
All HID bulbs, inlcluding Sylvania mercury vapor, are designed to be matched with the appropriate ballast. This means that the bulbs are not interchangeable with one another. A sodium cannot be swapped with a mercury vapor, a 100 watt mercury cannot be swapped with a 250 watt mercury, etc.
Some of the most common varieties of sodium bulbs are as follows: 75/DX, 75DX, 100/DX, 100DX, 175/DX, 175DX, 250/DX, 250DX, 400/DX, AND 400DX. Of course, there are other far less common wattages, shapes, and sizes available.