Toward a more accurate and extensible colorimetry. Part Ii. Discussion

Technical advances in the forty years since the Stiles-Burch color-matching work-among them the bright split-field visual colorimeter coupled by quartz light pipe to a spectroradiometer-allow measurement of an accurate absolute spectral power distribution (SPD) of every viewed light. Furthermore, the ability to measure accurately the power-content of each spectral constituent of a viewed light has clarified both colorimetric procedures and signficance, for example, of color-matching functions (CMFs) and tristimulus values. Three disparate sets of three spectral primaries were used throughout the work. This has not been done before, and its use elucidated much of the difficulty (as described by Judd, Stiles, MacAdam, Wyszecki, and others) inherent in traditional colorimetry. Errors in computed chromaticities of sets of 28 visually-matching lights depend strongly on their spectral content, and are particularly large in the presence of high spectral content in the violet, deep red, and near 500 nm or 580 nm. Peaks of visual efficiency, and minima in chromaticity errors, were found with power in three intermediate spectral regions near 450 nm, 530 nm, and 610 nm. Computed luminance (1964 CIE Standard Observer) and power-content also depend strongly on spectral content of the visually-matching white lights. That this is true of luminance is generally unknown. Perceived brightness of single spectral lights in isolation, and of mixtures, was examined at length: As a rule, removing one spectral component from a mixture increases perceived brightness, thus invalidating both luminance and R + G + B as correlates of brightness. Visual tests of the Grassmann proportionality and additivity assumptions, with highly metameric mixtures, tended to confirm them. Nevertheless, numerous tests-substituting pairs of actual visually-matching lights from one primary-set for pairs from another, which is how “transformation of primaries” is (mathematically) carried out-led to the inescapable conclusion that the normal human visual system does not honor such transformation. At the same time, numerous indications supported a particular set of primaries as an invariant of the visual system, making transformation needless as well as invalid. Chromaticity errors are not the fault of the particular maximum-saturation CMFs utilized; the observer's own CMFs do no better, whatever the primary-set. Soundness of traditional construction of the chromaticity diagram is shown to depend on the primary-set, breakdown occurring for some otherwise legitimate primary-sets.