Light Energy and Brightness Sensation

Abstract

I AM encouraged to note that Hunt finds my data on brightness magnitude¹ in broad agreement with his deductions, from a consideration of the neural responses, about the basic sensation–stimulus relations for the eye; but I would not wish to read too much into this agreement. My communication contained only a brief report of these recent brightness-estimate experiments; but I referred to earlier studies made by a method which involved mental estimates of equal, supra-threshold contrasts, and to the broad general agreement but detailed disagreement between these earlier studies^2,3 and the later work. A more detailed exposition is given in another paper⁴. The differences between the sensation–stimulus relations obtained by one method and by another may be important, both theoretically and also in the practice of lighting engineering. They may arise from different experimental conditions; but I cannot avoid the suspicion that there are real differences between the stimulus–contrast relation and the stimulus–brightness relation. Our training leads us to expect that brightness magnitude can be deduced from contrast magnitude, and vice versa, and thence that we can construct a scale of brightness by adding contrasts, either minimal, as Fechner did, or supra-threshold, as I did in my earlier work, just as we can construct a foot-rule by adding inch-intervals one to the other. To a limited extent we can, and this is why these early brightness scales have been so useful in practice. Sooner or later, the employment of these scales leads us into discrepancies, because ‘contrast’ does not always seem to be the same thing as ‘difference of brightness’. Perhaps there is no reason why it should be.