Unambiguous evidence for the additive effect in chromatic adaptation.

The perceived color of a light can be greatly affected by superimposing it upon a chromatic adapting field. In 1976, Walraven investigated the effect of chromatic adaptation on color perception and concluded it may be characterized completely by a gain change which operates only on the incremental stimulus (i.e. the superimposed test field). He proposed that the quanta from the background are somehow discounted and only the incremental light further processed to give the resulting color percept. Later experiments (Shevell, 1978) disconfirmed Walraven’s proposal. The data revealed that the background is not discounted completely with a red adapting field above about 50 td. Instead, the results were consistent with a “two-process” theory of chromatic adaptation in which the adapting field both causes a gain change (as proposed by Walrann) and also contributes directly to the color signal (this contribution is called the “additive effect” since it affects the color signal in the test area by a fixed amount rather than a fixed proportion). Recently Waltaven (1979) suggested my results do not conflict with his “discounting the background” hypothesis. He makes two points, both of which are necessary for his argument against the two-process theory of chromatic adaptation. First, he claims the fixation instructions given to the observers are responsible for my failure to co&m his hypothesis. Second, in another experiment (where the test field was briefly flashed) Walraven concludes there is no evidence for the additive effect. As shown below, Walraven is incorrect on each point.