Since the spectrum of the resonance radiation from an atomic vapor consists only of those spectral lines that are absorbed by the vapor, resonance lamps can serve as monochromators for isolating these lines. The atomic vapor is produced by cathodic sputtering in a rare gas discharge or, in the case of elements with high vapor pressure at low temperatures, by indirect electrical heating of the appropriate metal. Alternatively, the resonance lines of an element can be isolated by allowing the spectrum of the element emitted by a dc operated atomic spectral lamp to pass through a pulsating cloud of atomic vapor of the same element produced by sputtering in a modulated discharge. The resonance lines are selectively modulated and produce a signal at the output of an ac detection system. Other lines in the spectrum produce no output signal since they are not absorbed by passage through the atomic vapor and are therefore not modulated. Atomic absorption spectrophotometers incorporating these two techniques for the isolation of resonance lines have been constructed and their performance studied.
J. V. Sullivan and A. Walsh, "The Isolation and Detection of Atomic Resonance Lines," Appl. Opt. 7, 1271-1280 (1968)