When red and green stimuli are alternated at some appropriate frequency, we perceive flicker. The flicker can be increased or decreased by changing the ratio of the two radiances; some particular ratio gives minimum flicker. When an adapting field of a certain color is applied, the ratio must be changed in order to obtain the minimum flicker, owing to selective chromatic adaptation. A red adapting field, for example, causes greater sensitivity loss for red compared to other spectral ranges. Thus the radiance ratio should be altered so as to increase the radiance of red stimulus over the green one. Factors H <sub>R</sub> and H<sub>G</sub> are introduced which specify the ratio change caused by the red and green adapting fields, respectively. A large H<sub>R</sub> is especially characteristic of normal color vision, and virtually zero H <sub>R</sub> is obtained for color defectives. Thus H<sub>R</sub>, in combination with H<sub>O</sub>, which specifies the radiance ratio with no adaptation, provides a new method of discriminating color defectives from normals and distinguishing types of defects. The present investigation shows that the discrimination among the three groups is very clear and that no misclassification occurs. However, dichromats are not distinguished from anomalous subjects; further investigation is needed concerning that point.
MITSUO IKEDA and MITSUO URAKUBO, "Flicker HTRF as Test of Color Vision," J. Opt. Soc. Am. 58, 27-29 (1968)