Evaluation of the candidacy of the Π-mechanisms of stiles for color-matching fundamentals

Linear combinations of small field color matching functions that yield best fits (minimum RMS deviations on a logarithmic scale) to the spectral sensitivities of Stiles's Π-mechanisms were found by a computer minimization technique. Good fits to Π₃ (RMS = 0.039 log units, λ ≦ 533 nm) and to Π₄ (RMS = 0.042 log units) and a surprisingly excellent fit to Π₅ (RMS = 0.024 log units) were found. The same fitting technique was applied also to a large set of pigment spectra obtained from the Dartnall A₁, nomogram and from Wald, Brown and Smith's iodopsin spectrum by varying (i) λ_max, (ii) optical density (iii) lens and macular pigment corrections. None of these spectra were found to be better fit than Π₃, Π₄ and Π₅. A formal analysis reveals that the apparently protanomaloas W.D.W. unit coordinates induced by Π₅, Π₄, Π₃ could result from small deviations of these spectral sensitivities from the "true" underlying sensitivities.