Light emanating from ac arcs operating on 60-Hz power exhibits a 120-Hz modulation of intensity. Although time-averaged spectra of such arcs are usually reported, the instantaneous light spectrum and its dependence on the oscillating plasma temperature is a more valuable source of spectroscopic information. An experimental technique is described for scanning the spectrum of an ac discharge at any fixed phase in the arc operating cycle. Spectra were recorded at phases corresponding to maximum and minimum light output for a pure Hg arc, a Hg-Na, Sc, Th metal-iodide arc, a Hg high-pressure Na arc, and a Hg-SnI2 arc. Intensity ratios Imax/Imin of spectral lines generally increased with increasing upper-state energy. Many of the dominant lines also exhibited sizable changes of line profile with variations of phase. In the Hg-Na, Sc, Th metal-iodide arc, the intensity variations of optically thin lines were used to obtain estimates of upper-state energies and relative gA values of some Th lines for which these quantities were previously unknown.
P. L. PATTERSON, "Modulated Spectra of ac Arc Lamps," J. Opt. Soc. Am. 62, 627-633 (1972)