Strong Lines Emitted from Positive Column of Narrow Bore T2 Low-Pressure Ar-Hg Discharges

Abstract

The electric field and absolute radiance of 13 strong lines in the positive column of narrow bore T2 (outer diameter 7 mm) low-pressure Ar-Hg discharges were measured experimentally, which includes 11 Hg lines ranging from 185 to 579 nm and 2 Ar lines of 811, 842 nm. The discharges were operated with different argon filling pressure ranging from 2 to 10 Torr (corresponding to 266 to 1333 Pa), for discharge currents 20-200 mA and cold spot temperature 20-80 °C (0.16-11.8 Pa Hg vapour pressure). The Koedam factors of important emission lines were also measured for various discharge parameters, in order to convert radiance to exitance, whereafter the radiant power of all the lines except 185 nm, could be calculated and their radiant efficiency could be compared as well. Considering the absorption of 185 nm radiation in air, the ratio of the radiance at 185 nm to that at 254 nm was measured instead of its Koedam factor for current 80-200 mA and cold spot temperature 20-60 °C. Therefore, 185 nm radiant power was derived indirectly from that of 254 nm in corresponding discharge conditions. According to our measured results, the argon pressure for the maximum production of 254 nm radiation is around 5 Torr. It is showed that the optimum cold spot temperature for 254 nm radiant efficiency is higher than 50 °C, which is consistent with the temperature dependence on the tube diameter. With increasing discharge current and cold spot temperature, 185 nm radiant power has the similar tendency to that of 254 nm, while the fraction of electrical power converted to 185 nm radiation increases slightly with these parameters. Generally, the ratio of radiant power at 185 nm to that at 254 nm is higher than 0.2. For evaluating the energy balance of the positive column as well as the luminous efficacy of the fluorescent lamp product, the radiant powers of other strong lines also has significantly effect though they are considerably smaller than that of 254 nm and 185 nm. Besides, it must be taken in consideration that mercury depletion on the axis of positive column is serious for T2 narrow tube discharge especially at low Hg vapour pressure and high current.