Screening for mild anomalous trichromacy using the Ishihara Plates test

The Ishihara Plates test is one of the most established and widely used means of identifying color vision deficiencies. However, literature examining the effectiveness of the Ishihara Plates test has identified weaknesses, particularly when screening for milder anomalous trichromacy. We constructed a model of the chromatic signals expected to contribute to false negative readings by calculating, for particular anomalous trichromatic observers, the differences in chromaticity between the ground and pseudoisochromatic portions of plates. Predicted signals from 5 plates were compared for 7 editions of the Ishihara Plates test, for 6 observers with 3 severities of anomalous trichromacy, under 7 illuminants. We found significant effects of variation in all of these factors other than edition on the predicted color signals available to read the plates. The impact of edition was tested behaviorally with 35 CVD observers and 26 normal trichromats, which corroborated the minimal effect of edition predicted by the model. We found a significant negative relationship between predicted color signals for anomalous trichromats and behavioral false negative plate readings (? = -.46, p = .005 for deuteranomals, ? = -.42, p = .01 for protanomals), suggesting that residual observer-specific color signals in portions of plates designed to be isochromatic may be contributing to false negative readings, and validating our modelling approach.