Contributions of the opponent mechanisms to brightness and nonlinear models.

Additivity of heterochromatic brightness matching was investigated between unique-green and red (660 nm), and between unique-blue and unique-yellow with 2-deg bipartite field composed of a 100 td white reference field and a bichromatic mixture field. Ten subjects participated in the measurements and the results showed wide varieties among subjects. Previous models for brightness perception based on the contributions of red-green and yellow-blue opponent channels can explain some properties of the results, but they fail to explain other aspects. To overcome the difficulties, a new model is proposed for the brightness perception at photopic levels.