Operation
When a mercury vapor lamp is first turned on, it will produce a dark blue glow because only a small amount of the mercury is ionized and the gas pressure in the arc tube is very low, so much of the light is produced in the ultraviolet mercury bands. As the main arc strikes and the gas heats up and increases in pressure, the light shifts into the visible range and the high gas pressure causes the mercury emission bands to broaden somewhat, producing a light that appears more nearly white to the human eye, although it is still not a continuous spectrum. Even at full intensity, the light from a mercury vapor lamp with no phosphors is distinctly bluish in color. The pressure in the quartz arc-tube rises to approximately one atmosphere once the bulb has reached its working temperature. If the discharge should be interrupted (e.g. by interruption of the electric supply), it is not possible for the lamp to restrike until the bulb cools enough for the pressure to fall considerably. The reason for a prolonged period of time before the lamp restrikes is because mercury vapor ballasts along with other HID lamp ballasts send relatively low voltage to the lamp upon start up, but as pressure increases inside the arc-tube, higher voltage is required to keep the lamp lit so the ballast sends higher voltage to the lamp. Once the ballast is shut off and turned on again, it starts over at a low voltage but if the lamp is still hot, then high pressure inside the arc-tube prevents the lamp from striking an arc and turning on.
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