Groups of iron lines with relative intensities independent of excitation are proposed to serve as intensity standards for rapid and reliable plate calibration. One such group between 3140 and 3240A has been tested under a great variety of conditions and found to be satisfactory for this purpose. Lines of Fe I selected at random in general have their relative intensities changed much more by self-reversal than by changes in excitation. A general quantitative study of self-reversal was made which allows an estimate of the degree of self-reversal of any line under given conditions. Weak or moderately strong lines coming down to all but the lowest levels are free from self-reversal under the conditions at which arcs or sparks are usually operated.
G. H. DIEKE and H. M. CROSSWHITE, "The Use of Iron Lines as Intensity Standards," J. Opt. Soc. Am. 33, 425-432 (1943)