Traditional metal halide light bulbs use probe start technology, which uses three electrodes – a starter electrode and two operating electrodes – in the arc tube to start the light bulb. When the light bulb first turns on, a discharge is created across a small gap between the starter probe electrode and the nearby operating electrode. Electrons then jump across the arc tube to the other operating electrode to help start the light bulb. Once the light bulb is started, a bi-metal switch removes the starter probe electrode from the circuit. Probe start metal halides experience lumen depreciation over the life of the light bulb due to tungsten sputtering off of the electrodes and blackening the arc tube wall.
Metal halide light bulbs cannot immediately begin producing their full lumen output at start-up because the temperature and pressure in the inner arc tube require time to reach full operating levels. Starting the initial arc sometimes takes a few seconds, and the warm-up period can be as long as five minutes, depending upon light bulb type. During this time, the light bulb exhibits different colors as the various metal halides vaporize in the arc chamber.
If power is interrupted, even briefly, the light bulb’s arc will extinguish, and the high pressure that exists in the hot arc tube will prevent re-striking the arc. The light bulb needs time to cool down before the arc can be re-started. In a typical probe start metal halide, this could be anywhere from 10 to 20 minutes. The amount of time it takes for the HID light bulb to return to its full lumen output after a momentary power interruption is known as the hot restart time or hot restrike. This is a concern in some lighting applications where prolonged lighting interruption could create manufacturing shut-down or a safety issue. Pulse start metal halide light bulbs, which use a ballast with exceptionally high operating voltage, have instant restrike capabilities to restart a hot light bulb.
Pulse start metal halide light bulbs do not have the starter probe electrode. Instead they have a high-voltage igniter that works with the ballast to start the light bulb using a series of high-voltage pulses (typically 3 to 5 kilovolts). The high pulses of voltage allow the light bulb to warm-up and reach full lumen output faster than probe start light bulbs and reduce hot restrike time; a pulse start metal halide light bulb needs only 5-7 minutes to restrike.
Without the probe electrode, the amount of seal area at the end of the arc tube is reduced, which results in reduced heat loss. Furthermore, using an igniter with a light bulb reduces the tungsten sputtering by heating up the electrodes faster during start-up; hence, there is less lumen depreciation caused by tungsten blackening the bulb. Moreover, because the tungsten remains on the electrodes longer, the life of the pulse start light bulb is longer than the probe start metal halide light bulb.
Another difference between traditional metal halide bulbs and pulse start metal halide bulbs is their degree of color shift. Probe start light bulbs have a color shift of plus or minus 300K, whereas pulse start light bulbs have improved color rendering and a more controlled CCT. The color shift for pulse start light bulbs is plus or minus 100K, which is still a noticeable difference but better than probe start light bulbs.
Pulse start technology was developed to increase bulb life and to have both the energy efficacies of high-pressure sodium light bulbs and the desirable color characteristics of metal halide bulbs. The benefits of pulse start light bulbs include superior efficacy; more rapid, reliable starting – even in cold temperatures; longer life; and increased lumen maintenance.