For low-pressure Hg-Ar discharges line intensities of UV and visible radiation have been measured for tube diameters of 36, 22.8, and 9.5 mm and for different Hg densities and currents. A computer-controlled experiment was used for angularly resolved measurements. For 36-mm tube diameter the measured angular intensity distribution agrees well with published results. For tubes with 9.5-mm diameter the maximum emission of the 254- and 436 nm radiation is reached at a coldest spot temperature of about 45 °C for a lamp current of 200 mA. The ratio of visible to UV radiation becomes larger for smaller diameters which can be important for lamp applications. Increasing the lamp current from 200 to 600 mA leads to saturation effects in the radiation. A further increase to 1 A changes remarkably the measured angular intensity distribution. This indicates that for higher currents a broader radial radiation profile is present and/or an increased trapping especially of the visible radiation. The total radiated power of the discharge column and the intensity normal to the emitting surface are correlated by the Koedam factor β. The investigation shows that this factor is constant within certain parameter ranges but can change drastically, e.g., for tubes having a very small diameter of 2 mm.

The radiation efficiency of Hg-Ar low-pressure surface wave (SW) discharges has been measured in a broad range of Ar pressures (40 mTorr-12 Torr), Hg partial pressures ( approximately 1-100mTorr) and excitation frequencies (200-1000 MHz). Gas filling and tube geometry (26 mm outer diameter) were chosen to allow a comparison with the conventionally excited Hg-Ar low-pressure discharge used for general lighting purposes in the well known fluorescent lamp. The study focuses on the quantitative measurement of the two Hg UV resonance lines at 185 and 254 nm. The RF power input into the plasma column extending outside the surfatron coupling structure was determined quantitatively. This was done by measuring the power dissipated in the part of the plasma column hidden within the surfatron by a calorimetric method. The measurements of the Hg UV radiation efficiency are supplemented by radial scan experiments of a visible Hg line, and by the study of phenomena occurring at a higher power density such as saturation and standing wave formation.