The positive column of low pressure DC glow discharges in Xe/CI, and Kr/C1, gas mixtures has been investigated with respect to its UV radiation power and radiative efficiency for a wide range of parameters (total gas pressure: 1 -30 mbar, current 10-30 mA, partial pressure of CI,: 1 -33%). Also the radial distribution of the particle number densities of XeCl* ( B ) and KrCI* (B) has been determined by absolutely calibrated emission spectroscopy. Optimum UV (190-350 nm) output and efficiency has been found in gas mixtures with a relative CI2 partial pressure of 2-3% and total gas pressure of 12-18 mbar. At these parameters, the UV radiation power per column length is greater than 40 Watt/m with a radiative efficiency of 15-18%. These discharges could be used for UV induced photochemical processes.