The most practical method of generating germicidal radiant energy is by passage of an electric discharge through a rare gas (usually argon) at low pressures (on the order of 130 to 400 pascals, or 1 to 3 torr) containing mercury vapor enclosed in a special glass tube with no fluorescent coating that transmits short-wavelength UV. Hot-cathode germicidal lamps are identical in shape, electrical connection, operating power, and life to standard fluorescent lamps, both linear and compact types. Maintaining the transmission of the lamp over life is more difficult than for standard fluorescent lamps.
Cold-cathode germicidal lamps are also available in various sizes, usually for shorter, smaller diameter lamps. Their operating characteristics are similar to those of hot-cathode lamps, but their starting mechanisms are different
Approximately 45% of the input power from such a device is emitted at a mercury-discharge wavelength of 253.7 nm, in the middle of the UV-C band. The second major emission line is at 184.9 nm, but this emission is normally absorbed by the glass, since—if emitted through the glass, as it is with pure quartz—it would create ozone at levels far above the safety limit. Other mercury lines in the UV-B and UV-A regions are present at much lower emitted-power levels and not considered important in germicidal action; unfortunately, they can add to the safety concern.