The relative emission efficiency of F-3(+) and F-2 centers as a function of their concentration has been investigated in LiF crystals colored at room temperature and 260degreesC with 3 MeV electrons. From a careful study of their absorption and emission spectra, we discovered that the emission efficiency h of both F-3(+) and F-2 defects is constant up to similar to10(16) centers/cm(3). When increasing the concentration up to 10(18) centers/cm(3), h decreases slightly for the F-2 centers and considerably for the F-3(+) ones, a difference which is bigger at room temperature with respect to liquid nitrogen temperature. A thermally activated process is taking place, where the F-2 center and other aggregated defects play an important role. (C) 2003 The Electrochemical Society.