The discharge process is initiated by high
voltage
ignition which ionizes the pressurized gap between the electrodes. The high
voltage
current is transmitted for approximately one second, and then is reduced to the lamp's stable operating level. Immediately following ignition, the
electric arc
that initiates the conversion of the materials to a gaseous light emitting state is established. If a
lamp
is started cold, pulses of only a few thousands volts are needed. For hot starting, considerably higher
voltage
pulses are required, i.e. 20,000 to 70,000 volts depending on the type of
lamp
and its wattage. After two to three minutes of operation the
lamp
reaches its maximum output level. During this time the
luminous flux
gradually increases until it reaches its nominal values, while current and
color temperature
which start at high levels gradually decrease to their nominal values.
Ignition time and leveling off a discharge lamps.
During the ignition period, the filter substances (mercury, halide and rare
earth
metals) vaporize. The
lamp
is connected to a
ballast
which regulates current and voltage, and acts as a current limiter during start up and warm up and as a
voltage
regulator after the
electric arc
has been established. As the
lamp
works, the electrodes are gradually eroded by the
electric arc
and the space between them increases. The larger the space between the electrode, the less electric current is consumed, and as compensation the
voltage
increases. In the past few years electronic ballasts have been developed which enable the
lamp
to operate on a square-wave current. This produces virtually flicker free light and increases the
luminous efficacy
of the lamp. Lamp life is also increased because the lamps are run at a constant output.
