Guide to UV Measurement
The Mercury Vapor Arc Lamp
A well-known and widely used type of lamp is the medium-pressure mercury vapor arc lamp. In this type of lamp, clear fused quartz tubing is used to enclose the plasma that produces the ultraviolet energy. The fused quartz tubing has a wall thickness of about one millimeter and an outer diameter of 20 to 25 millimeters. The ends of the quartz tubing are sealed around the electrode ribbons to provide a vacuum-tight enclosure. |
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 Lamps |
When power is applied to the electrodes, the voltage between them rises until ionization of the starting gas occurs. As the lamp warms and the mercury vaporizes, the plasma changes from that of the starting gas to a constricted mercury arc.
UV (~30%), visible light (~25%), infrared radiation (~25%) and convected heat (~20%) are emitted from the mercury arc lamp. This can vary depending on the fill pressure and additives in the bulb with the mercury. The bulb will typically last from 1000 to 2000 hours before degradation at the electrodes and deposits on the interior wall of the bulb reduce its output to the point where it is replaced. Typical bulb diameters range from 18 to 30 mm.
In printing applications and other continuous-feed processes it is desirable or necessary to turn the lamp on and off. It is usual for an arc lamp to have a mechanical shutter incorporated in it, as the bulb requires a few minutes to stabilize at power each time it is started.