How does a colour blind person see traffic lights?

Defects in colour vision or dyschromatopsia affect a significant percentage of the world’s population. Approximately 1 in 13 men and 1 in 230 women suffer from different degrees of impaired colour vision. This means that 8% of men and 0.5% of women in the world do not perceive colours properly, which amounts to approximately 656 dyschromatopsics, popularly known as colour blind.

How does colour vision work?

The ocular system, through the cornea, lens and internal fluids, projects an image onto the retina. It acts as a sensor that detects light and transmits a nerve impulse to the brain, where the image is processed and vision is generated.

The human retina has three different colour receptors called cones. They are colloquially known as “red”, “green” and “blue” cones due to the range of the colour spectrum they are related to, the hues of light for which they are more sensitive. The red cones correspond to long wavelengths, the green ones to medium waves, and the blue ones to short waves. Therefore, if we have a defect or absence of the red cones, we will have a specific type of colour vision defect, different from the ones corresponding to a defect or absence of the green or blue cones.

How many types of dyschromatopsia are there?

Since there are three different types of cones (red, green and blue), the defects are classified according to the set of altered cones. Therefore, dyschromatopsics are deutan (altered “green”), protan (altered “red”) or tritan (altered “blue”). Furthermore, colour vision can be impaired for two reasons: the set of cones may not work properly or it may not exist at all. Consequently, the colour vision defect will be classified as -anomaly if they do not work efficiently, or -anopia when they do not exist. In summary, these are the colour vision defects that exist:

• Protanomaly or protanopia.
• Deuteranomaly or deuteranopia.
• Tritanomaly or tritanopy.

How does a dyschromatopsic see traffic lights?

Everyone who meets a dyschromatopsic asks him how he sees traffic lights. The dyschromatopsic’s answer is clear: “I see it normally: green, red and yellow”. How can that be if he does not see colours well?

Dyschromatopsia affects a specific region of the chromatic space, i.e. the difficulty to perceive colours is shown in the ability to differentiate among similar tones of similar colours. Therefore, a dyschromatopsic will never have difficulties distinguishing a red light from a green light. However, he will have some difficulties finding a brown paint between brown-green ones; he will have difficulties distinguishing some trees from others in a forest; and he will also find it difficult to interpret scales classified by similar colour tones. The problems of colour vision thus lie in the ability to interpret as different “similar or close” hues in the colour spectrum, not in differentiating extreme colours such as yellow, red, green, or blue.

Is it possible not to see any colour?

Indeed, there are people who suffer from monochromacy, which is defined as total colour blindness. None of the three existing cones in the retina work correctly and the person is unable to distinguish colours. It is in these cases of severe colour vision alteration where enormous difficulties will arise, making it almost impossible to differentiate the colours in traffic lights.