Chromatic Difference Steps of Moderate Size Measured along Theoretically Critical Axes

Abstract A new method of measuring chromatic discrimination steps is described where observers must indicate in which direction a change has occurred, as well as the fact of its occurrence. This procedure seems to stabilize the subject's criterion of difference and yields step sizes more akin to those of Wright's “dashes” than MacAdam's ellipses. Tests have been made along two critical dimensions of variation: (1) the tritan axis, where discriminations depend initially only on variations in blue‐cone (B) excitation and (2) the red‐green axis, where discriminations depend on the substitution of red‐cone (R) excitation for that of green cones (G), or vice versa. Discriminations dependent on blue cones are affected by the level of B‐cone excitation but are independent of R/G. After individual differences are taken into account by a simple scaling factor, an equation of the form ΔB/ B + B o = K accounts well for the data. Blue‐cone discrimination is found to be independent of the ratio of red‐to‐green‐cone excitation, with an optimal Weber fraction of about 18%. Discriminations dependent on the exchange of red‐and green‐cone excitation yield an optimal Weber fraction of about 2%. This ratio is increased, though not greatly, by an imbalance of the red‐green ratio relative to a neutral condition, and by increasing the level of blue‐cone excitation. At 120 td, the ratio between a MacAdam step and ours is approximately constant at about 13:1.