A simplified curve-shape indexing system for spectrophotometric absorption curves is described by means of which the tentative identification of an “unknown” (whose curve is already at hand) may ordinarily be made in 2 or 3 minutes even in the presence of small amounts of colored impurities. Quantitative solutions are not required.
WM. A. SHURCLIFF, "Curve Shape Index for Identification by Means of Spectrophotometric Curves," J. Opt. Soc. Am. 32, 160-162 (1942)