Metal-halide lamp

Operation

A Metal Halide lamp is a gas discharge lamp, like a mercury vapor lamp, with metal halide salts added to improve luminous efficacy and CRI. The arc tube emits harmful UV light, but this light is blocked by the outer envelope. They are more efficient than an incandescent bulb. Metal halide lamps can be probe start or pulse start.

Starting methods

Pulse start

Pulse start metal halide (PSMH) lamps start the lamp with a high voltage pulse, doing so by an igniter, or an electronic ballast. This is generally more efficient than probe start and allows for faster warm up times

Probe start

Probe start metal halide lamps came first, and start in a way similar to a mercury vapor lamp. There is a ‘starting electrode’ run in series with a resistor, next to the main electrode to form a small arc which ignites the large arc. This means the lamp is slower to start, and less efficient than PSMH.

Ballast

All gas-discharge lamps require a ballast to function. These can either be magnetic or electronic ballasts.

Magnetic

Magnetic ballasts make use of an inductor to limit current flow, which is often paired with an igniter in a PSMH lamp. These ballasts waste some energy and may cause a poor power factor. A capacitor is often connected across the input terminals to correct power factor. Magnetic ballasts cause the lamp to pulse at 60 hz.

Electronic

Electronic ballasts first rectify the incoming AC power to DC power then, this DC power is converted to high frequency AC, at 20 - 100 KHz. This reduces the pulsing of the lamp.