Metal halide bulbs, high-pressure sodium bulbs and mercury lamps make up the family of high-intensity-discharge (HID) lighting. A ballast serves as a lamp starter and regulates power to the light. The ballast assembly consists of a transformer, capacitor and lamp starter sized for the specific type of lamp.
Examine the bulb for signs it has burned out. A mercury HID bulb exhibits a blackening at the end of the arc tube. High-pressure-sodium lamps and metal-halide lamps do not exhibit obvious symptoms when power is off.
Check the specifications on the ballast. An incorrect lamp-and-ballast combination will exhibit the same symptoms as a bad bulb. If the bulb is rated at 100 volts and 150 watts, the ballast characteristics must match.
For high-pressure sodium lamps, use a known good bulb to first ensure the ballast and starter are operable. Once verified, insert the suspect bulb and test operation.