Color centers in alkali halide crystals possess a peculiar optical cycle, which delivers an emission very often with quantum efficiency near 1, a few of them having the qualities required by active media. Among these the FA center was the first one to lase, and it is the most stable, efficient and easy to be produced. However up to now, as far as its basic properties are concerned, it remains a rather elusive point defect.Indeed recent results brought up again for discussion the old classification between FA(I) and FA(II) centers, the last ones only being optical active centers. It has ben found that both species can coexist in the same host crystal, their relative number being a function of temperature. This phenomenon opens up new perspectives for studying in detail relaxation, thermal excitation, and energy transfer processes at a microscopic level after optical excitation.