The use of a spectroscopic method for analysing an rf‐sputtering glow discharge is described. Both pure CaO disks and ones doped with Sm and Tm were used as targets, and the purpose of the sputtering was to prepare luminescence films. The sputtering was carried out in atmospheres of Ar, O2, N2, and Ne. The intensity of the strongest peak of calcium was measured in these discharges as a function of the distance from the target and of the rf voltage used. It was observed that impurities, in this case H and N2, were revealed, and a regular calcium flux from the target to the substrate could be well controlled by this method.