We have investigated the luminescence of CaF₂ thin films doped with very low concentrations of Sm²⁺ ions using scanning confocal optical microscopy at low temperatures. The film morphology was studied independently by atomic force microscopy. The Sm²⁺ ions are homogeneously distributed in the films and show photobleaching. Unexpectedly, on the film surface strongly luminescent small topographic features are observed that are found to contain Sm³⁺ by spectral analysis. The formation of Sm³⁺ is probably due to the presence of oxygen during film growth. In the lowest doped films on-off blinking behavior of isolated luminescent spots provides strong evidence for the first observation of single ions in a crystal.