The human visual system is able to adapt its sensitivity so as to compensate, to a considerable extent, for changes in the level and the color of the illumination. The means whereby this adaptation is provided include: changes in the diameter of the iris; changes between cone and rod vision; bleaching of the visual pigments in the receptors; adjustments to the amplification of the electronic signals produced by the receptors; and cortical effects. The extent of the compensation is not complete, so that the color appearance of objects is only approximately constant. Because original scenes and images often involve various levels and colors of illumination, the effects of this incomplete adaptation can have important implications for producing satisfactory pictures. The recent availability of an agreed Color Appearance Model (CIECAM97s) with its included Chromatic Adaptation Transform (CAT97), and a derived Color Inconstancy Index (CON97), now make it possible to allow for these effects quantitatively.