Mercury-vapor lamp

A Mercury-vapor lamp is a gas release lamp which uses mercury in an excited state to produce light. The arc discharge is generally restricted to a little fused quartz tube mounted within a larger borosilicate glass bulb. The outer bulb may be clear or coated with a phosphor; in either case, the outer bulb provides thermal insulation, defense from ultraviolet radiation, and a convenient mounting for the fused quartz arc tube.

Mercury vapor lamps (and their relatives) are often used because they are comparatively efficient while still offering better color rendition than either pressure sodium vapor lamps. They also offer a very long lifetime.

Operation
When the lamp is first turned on, it will make a dark blue glow because only a small amount of the mercury is ionized and the gas force 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 able to be seen range and the high gas pressure causes the mercury emission bands to broaden somewhat, producing a light that appears more-white to the person 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.

Color considerations
To correct the bluish tinge, many mercury vapor lamps coat the within of the outer bulb with a phosphor that converts some portion of the ultraviolet emissions into red light. This helps to fill in the or else very-deficient red end of the electromagnetic spectrum. These lamps are generally called "color corrected" lamps. Most new mercury vapor lamps have this coating. One of the original complaints against mercury lights was they tended to make people look like "bloodless corpses" because of the lack of light from the red end of the spectrum.

Light pollution considerations
For placements where light pollution is of prime significance (for example, an observatory parking lot), low pressure sodium is preferred. As it emits light on only one wavelength, it is the easiest to filter out. Mercury vapor lamps without any phosphor are second best; they create only a few distinct mercury lines that need to be filtered out.

Ultraviolet hazards
All mercury vapor lamps (including metal halide lamps) must have a feature (or be installed in a fixture that contains a feature) that prevents ultraviolet radiation from escaping. Usually, the borosilicate glass outer bulb of the lamp performs this function but special care must be taken if the lamp is installed in a position where this outer envelope can become damaged. There have been documented cases of lamps being broken in gymnasiums and sun burns and eye inflammation have resulted.

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