An interpretation of the bril scale of subjective brightness.

Abstract

The bril scale, obtained from extensive experimental data by Hanes, represents an attempt to establish a subjective scale of brightness by the determination of particular luminances which produce specific ratios of brightness. This scale, like other scales obtained from fractionation data, necessarily assumes a one-to-one correspondence between intensity of stimulus and magnitude of response—an assumption in conflict with known facts regarding adaptation.It is shown that the reformulation of the Fechner law proposed several years ago, aided by results obtained from rating-scale techniques, can predict quantitatively all of the data used for the construction of the bril scale. Since this theoretical treatment takes explicit account of adaptation, it follows that each of the ratios determined by fractionation techniques has meaning by itself, but that the combination of these ratios into a scale of wide range is not justified. The fallacy involved in such combination is the assumption that a given stimulus is psychologically equivalent to itself when it is presented under different conditions, and, therefore, under different states of adaptation.