Spectral sensitivities of the normal human visual system, color-matching functions and their principles, and how and why the two sets should coincide

Abstract

The objectives of this article are (1) to help reestablish in technical thinking the three spectral sensitivity curves, dating from Helmholtz, of the normal human visual system (particularly their peak wavelengths); (2) to remind the reader of the principles of the sets of three color-matching functions (CMFs) comprising a CIE Standard Observer, and to make these principles more easily understandable; (3) to show how the visual data comprising today's Standard Observers lead directly to the peak wavelengths of the spectral sensitivity curves; (4) to use modern color-matching data to restore essential details to CMFs damaged by manipulation over the years; (5) to suggest that coincidence of corrected CMFs and the actual spectral sensitivities of the normal human visual system (a feature long tacitly assumed by color scientists of the past) is close at hand; and (7) to point out that CMFs embody a wealth of significance concerning the nature of the spectral response of the normal human visual system, despite the fact that they do not work well as weighting functions in the practice of colorimetry. The color-matching data of the CIE 1964 10° Standard Observer are used to reproduce the visual matches upon which it is based, and to model the principles of CMFs in general. The CIE 1964 data are treated as if they had been collected directly from modern-day visual matching experiments, in which an accurate, high-resolution, absolute spectral power distribution (SPD) of every viewed light is measured, and power content of each component of the light determined. The experimental units and dimensions of the resulting three CMFs are established. The significance of CMF plots above and below the zero-power level, and of the spectral shapes of the CMFs, is shown. The positions in wavelength of the nominal maximum of the red, green, and blue CMF, for a wide range of wavelengths of real spectral primaries, are noted (following MacAdam) to be “amazingly similar.” Then the much-manipulated data of the CIE Standard Observers are left behind, and modern raw unmanipulated visual data are analyzed in the same manner. The results yield characteristics of CMFs that are more representative of the normal human visual system than are those of the CIE Standard Observers. The peak heights of the nominal maxima of the red, green, and blue CMF, for the same wide range of primaries, are importantly significant also. They serve to define, at least approximately, the forms of the three spectral sensitivities, assuming the traditional model of the visual system. © 1999 John Wiley & Sons, Inc. Col Res Appl, 24, 139–156, 1999