Why do metal halide lamps require a ballast?

Metal halide (MH) ballasts are required to start the lamp, regulate the lamp starting and lamp operating currents, and provide appropriate sustaining supply voltage.

Starting the lamp: In MH lamps, ballasts provide the starting voltage and ignition pulses (pulse-start lamps) necessary to ignite the lamp. Probe-start MH ballasts, however, can take as long as 10 to 20 minutes to restrike (re-start) a lamp. Pulse-start MH ballasts can restrike the lamp within 2 to 8 minutes of an interruption in current, because they provide high-voltage pulses to start these lamps.

Regulating lamp current and power: The ballast regulates the lamp operating current flowing through the lamp after the lamp has been started. The ballast is set to deliver relatively stable power to the lamp while regulating the lamp current despite typical line voltage fluctuations. This maximizes lamp life and ensures other performance characteristics such as color and light output.

Providing appropriate sustaining voltage: MH ballasts must maintain suitable voltage and current wave shape to the lamp. MH lamp voltage typically increases over time, and the ballast must continue to provide sufficient voltage to the lamp as it ages. In addition, the American National Standards Institute (ANSI) and/or the lamp manufacturers specify a suitable current wave shape to the lamp to achieve good lumen maintenance. Lamp current crest factor (CCF) is defined by ANSI as the ratio of the peak value of lamp current to the root-mean-square value of the current. A low CCF (between 1.4 and 1.6) contributes to good lamp lumen maintenance and longer life.