Neural mechanism of the retina may perform the principal role of luminosity transformation. In this paper, non-linearity of the Munsell value function was discussed on the physiological basis. Amplitude response function of the retinal neurons is approximated as a sigmoidal one V=V_max•X/(X+S). This function may more essentially express the visual process, because it could be shown that this function includes parts well fitting to the well known psychological response functions, Weber-Fechner and Steven's law.
Introducing an idea of neural adaptation to this function, the Munsell value function can be more precisely approximated. The conclusive equations were expressed as V=V_max•X/{X+S(X)} and S(X)=X₀{1+5.55•X/(X+1.8•X₀)}, where X₀ was background light intensity or refrectance, and X was test light one. This equation means that the adaptation level S(X) varied with two components, groval intensity (X₀) and local one (X).