can be screwed into a standard incandescent light socket supplied with the proper voltage. Metal halide A very closely related lamp design called the metal halide lamp uses various compounds in an amalgam with the mercury. Sodium iodide and scandium iodide are commonly in use. These lamps can produce much better quality light without resorting to phosphors. If they use a starting electrode, there is always a thermal shorting switch to eliminate any electrical potential between the main electrode and the starting electrode once the lamp is lit. (This electrical potential in the presence of the halides can cause the failure of the glass/metal seal). More modern metal halide systems do not use a separate starting electrode; instead, the lamp is started using high voltage pulses as with high-pressure sodium vapor lamps. Self-ballasted lamps Self-ballasted (SB) lamps are mercury vapor lamps with a tungsten filament inside connected in series with the arc tube that functions as an electrical ballast. This is the only kind of mercury vapor lamp that can be connected directly to the mains without an external ballast. These lamps have only the same or slightly higher efficiency than incandescent lamps of similar size, but have a longer life. They give light immediately on startup, but usually need a few minutes to restrike if power has been interrupted. Because of the light emitted by the filament, they have slightly better color rendering properties than mercury vapor lamps. Self-ballasted lamps are typically more expensive than a standard mercury vapor lamp. 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.