In absorption measurement techniques a spectral source is desired that has a resonance spectral line of high intensity. This spectral line must have a profile as narrow as possible without self-absorption. Different solutions of the problem in which a hollow cathode is used are given in the literature.
Z. van Gelder, "New High-Intensity Spectral Source with a Narrow Line Profile," Appl. Spectrosc. 22, 581-582 (1968)