The minimally distinct border (MDB) method for comparing two fields of differing chromaticity was first reported by Boynton and Kaiser in 1968, and has been the subject of further experimental and theoretical investigation since that time. The evidence suggests that the MDB criterion is achieved when the two fields being compared produce equal effects upon the achromatic channels of the photopic visual system. The comparisons prove to be transitive and Abney’s law is strictly obeyed; spectral sensitivity evaluated by the MDB is very similar to that obtained by flicker photometry. An advantage of the MDB over flicker photometry is that the strength of the border at the MDB setting can be evaluated. This is done either by border matches with an achromatic comparison field, or with direct subjective estimates. At the MDB setting, the more saturated of two fields appears brighter than the other one. This implies that the chromatic channels of the visual system contribute to brightness as well as to chromaticness. The MDB criterion provides a new method for saturation scaling: Chromatic stimuli are juxtaposed in turn with white, and the strength of the border at the MDB setting is taken in each case as an index of saturation. More generally, any two fields can be compared by this method, as we have done with a matrix of 16 spectral stimuli plus white, employed in all possible combinations. Because the results can be accurately displayed in a two-dimensional euclidean space, the MDB method also shows potential as a new basis for color scaling.
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