The density transformation according to Seidel, Dt=log[(d0/d)-1] results in a calibration curve that is nearly straight in the wavelength region normally used in spectrographic analysis.
The properties of the “transformed density curve” are studied in detail and it is shown that by choice of favorable conditions the remaining curvature is negligible. This offers the possibility to extend the density difference method to the (H and D) density range 0.1–2.
B. VERKERK, "A Possible Extension of the Density-Difference Method of Spectrographic Analysis," J. Opt. Soc. Am. 41, 1017-1022 (1951)