The dichotomy between the logarithmic and power-law models of lightness perception is revisited. Both laws (and no other) are permitted by the luminance scale-invariance of the equality of lightness differences. The logarithmic law, which emerges out of integration of Weber's law for just-noticeable differences (JNDs), is the only one of these that is compatible with the observed background-lightness-invariance of the lowest JND and also with the additivity of JNDs. The power law, although defensible as a fit to empirical data, is not compatible with JND additivity. Both laws must be compromised in form to fit other mathematical necessities (e.g., continuity and domain), hence the remaining question is: can JNDs be summed to total lightness. (c) 2012 Wiley Periodicals, Inc. Col Res Appl, 39, 99-101, 2014