By employing three homologous spectrum lines, two in one wave-length range, and a third in the second wave-length range, it is possible to construct the two characteristic curves for heterochromous photometry. Spectra containing the three lines are recorded, at several levels of density, on the photographic emulsion to be calibrated. This provides data for a homochromous and a heterochromous preliminary curve. Calibration curves are constructed in the usual manner from these two preliminary curves.
MILTON GREEN, "Three-Line Method of Obtaining Characteristic Curves for Heterochromous Photometry," J. Opt. Soc. Am. 40, 842-842 (1950)