Chromaticity co-ordinates of Ishihara plates reveal that hidden digit plates can be read by S-cones

The Ishihara pseudoisochromatic plates constitute one of the most commonly used screening tools for red-green colour vision deficiencies. Even though hidden digit plates are supposed to be read only by those who are colour vision defective, studies report that some normal trichromats can indeed read these plates. By measuring the chromaticity co-ordinates of the dots used in Ishihara plates, the purpose of this study was to clarify the mechanism that enables normal trichromats and colour vision defectives to read the plates, particularly hidden digit plates.

Spectrophotometric measurements were made for a 24-plate version of the Ishihara pseudoisochromatic plates and chromaticity co-ordinates of the dots were expressed in the MacLeod-Boynton diagram.

As theoretically expected, reading of Ishihara plates by normal trichromats was mediated by the dot chromaticity differences along the L/(L + M) axis. On the other hand, reading by colour vision defective observers was made possible mainly by the dot chromaticity differences along the S/(L + M) axis. This would also explain why some normal trichromats can read hidden digit plates, the plates that are supposed to be read only by colour vision defective observers.

Normal trichromats read Ishihara plates using their chromatic discrimination ability along the L/(L + M) axis. Red-green colour vision defective observers rely on S-cones in reading the plates. Some normal trichromats can read the hidden digit plates because they can extract S-cone differences efficiently despite the distraction from the L/(L + M) axis.