How do color defectives see the world?

The effects of a color deficiency on visual performance in the real world depends on the type and severity of the defect. A person with slight deuteranomaly may have some difficulty differentiating between certain colors, particularly when they are pale or desaturated, but the condition is unlikely to be a significant handicap. In contrast, dichromats have significant problems differentiatiating between a wide range of colors and any tasks which involve choosing between these colors will be difficult or impossible to carry out.

Protans and deutans (both anomalous trichromats and dichromats) share a common difficulty with the red-orange-yellow-green range of hues, a characteristic which makes it difficult to tell when tomatoes are ripe!

Deutans and protans will also tend to confuse various other colors. The easiest way of representing the combinations of colors which are confused by color defectives is with reference to the CIE chromaticity diagram described earlier. Pitt (1944) investigated the colors which dichromats confuse and found them to lie along straight lines when plotted on a chromaticity diagram. The lines radiate from different points according to the type of defect. These lines are called isochromatic confusion loci and can be viewed from the View menu of the computer program. Dichromats will tend to confuse colors over the whole length of the confusion loci, whereas anomalous trichromats will tend to confuse colors over a more limited range.

The line which passes through the white point and intersects the spectral locus, indicates the wavelength which will appear 'white' or neutral to the color defective individual. The neutral point occurs at about 490nm for protanopes, 500nm for deuteranopes and 570nm for tritanopes. The protanope's neutral locus also passes through saturated reds as well as blue-green indicating that these colors could not be differentiated from a white/grey. The deuteranope will not be able to differentiate green-blues and purples from white/grey.

Protans (dichromats and anomalous trichromats) are generally very insensitive to red light, i.e. red light looks very dim. This is because of the absence or altered sensitivity of long-wave sensitive cones. This can be a significant problem in terms of the detection of red signals, warning lights, rear lights on cars etc. While there is no statistical evidence that protans have more road traffic accidents than color normals, there is some evidence that protanopes are more likely to be involved in rear-end shunts and accidents occurring as a result of a failure to notice red rear lights and warning lights. For this reason, protan defects are generally considered to be more serious than deutan defects.