After a survey of various definitions of the photometric characteristic curve of photographic emulsions, this paper reintroduces a long forgotten photometric parameter, the opacitance ω = opacity - 1 = 10D - 1 and describes its application to a characteristic function I = Aωn(n ≃ ½), Which is linear in log-log co-ordinates over the whole range of densities of practical interest in photographic photometry. In a special case n = 1 and J = I/A = ω.
Gerard de Vaucouleurs, "Linearization of Characteristic Curves in Photographic Photometry," Appl. Opt. 7, 1513-1518 (1968)