Measurements of excited-state populations in processing plasmas can be useful because those populations often are indicators of, or participants in, chemical reactions. An optical self-absorption technique has been used to measure the relative densities of species in long-lived excited states in high-density plasma reactors. The technique is advantageous because it is simple and inexpensive compared to many laboratory diagnostic techniques, and thus it has potential for industrial manufacturing applications. The technique is useful when absorption strength and wavelength are in acceptable ranges. This paper describes the technique, compares its performance to a more sophisticated laser-absorption technique, and presents self-absorption data from a laboratory reactor and from a 300 mm production-prototype reactor.