Interferometric wavelengths, suitable for use as secondary standards of class “B”, are given for 68 thorium lines in the range 2650 to 3400 A, as emitted by a thorium-iodide electrodeless discharge tube without carrier gas. The rms deviation from the mean is 3 × 10-4 A. The method of preparation of a reliable and very intense thorium electrodeless tube is described.
A. DAVISON, A. GIACCHETTI, and R. W. STANLEY, "Interferometric Wavelengths of Thorium Lines between 2650 A and 3400 A," J. Opt. Soc. Am. 52, 447-451 (1962)