The requirements on light sources for double resonance and level crossing spectroscopy are reviewed. A modern version of the Cario-Löchte-Holtgreven flow lamp is described and its performance for a number of elements is compared with that of sealed-off electrodeless discharge lamps and Schfiler-type hollow cathode sources. None of these sources is useful under all circumstances. The electrodeless lamp is particularly effective in the case of the group IIb intercombination lines. The high atomic density available in the flow lamp makes it useful for partially forbidden resonance lines for a number of chemical species including highly reactive metals, such as Ca. The hollow cathode lamp is well suited to the fully allowed resonance lines of a wide range of elements including highly refractory met. © 1965 Optical Society of America.