Abstract
An instrument is described which makes possible the accurate measurement of the ratio of small changes in wavelength of a spectral line to the line width. Measurement of this ratio is the basis of a new method of measuring electron temperatures in plasmas. The instrument is based on a Fizeau fringe interferometer. Photoelectric detection is used, and, because the instrument has no scanning parts, a time resolution of 10-6 sec or better is easily obtained. The conditions for the use of the instrument and the corrections for instrumental effects are discussed.