Dichromats have two instead of three cone types in the retina (Birch, 2001).
The method is based on pseudoachromatic stimuli identification, that is, those stimuli seen as chromatic (greenish or reddish) by common observers and as grey by dichromats.
uv] (chromatic angle) values correspond to pseudoachromatic stimuli for real or simulated dichromats.
At the time when the survey was carried out, most dichromats were aware of their defect and 71% became aware of it because they experienced difficulties with colour-related tasks.
In total 49% of dichromats and 18% of anomalous trichromats who participated in the Steward and Cole survey (1) reported difficulties with distinguishing the colours of traffic light signals.
In the Steward and Cole survey,' 43% of dichromats and 29% of anomalous trichromats reported that having a colour vision defect affected their choice of career and almost 25% of the respondents said that the defect precluded them from an occupation.
A classification of the defect may be possible at the same time, but the anomaloscope will be needed to differentiate between anomalous trichromats and dichromats
Therefore, some anomalous trichromats may have colour discrimination which is nearly as good as that of normals, whereas others may resemble dichromats
Other signs, such as reduced visual acuity (VA) or visual field defects, may accompany acquired deficiencies but other aspects of visual function are not altered in congenital dichromats
and anomalous trichromats.