The reduction of the tube radius is an effective technique for enhancing power concentration. The increased wall losses are compensated by larger field strengths, which counteract the decrease in radiation efficiency with rising current density. The causal relationships are investigated for tube radii of 18-4 mm a t constant discharge current 1 = 200 mA and constant current density j = 1.2 mA/mm2 and for parameter variation of the field strength only. The measured efficiencies of the Hg-resonance lines 254 and 185 nm and the electron energy distribution functions agree well with the calculated values. Moreover, the results of the model calculations provide information on the elementary processes involved, such as elastic losses, collisions of second kind, stepwise processes and Coulomb interaction. The results show that Hg-rare gas discharge with tube radii R = 5-10 mm are no less suitable for light production than those with radii between 13 and 18 mm.