colour blindness
Worldwide, 8% of men and 0.5% of women have a colour vision deficiency
People with colour vision deficiency find it difficult to identify and distinguish between certain colours. People may experience red-green, blue-yellow, or total colour blindness, although red-green deficiencies are by far the most common – affecting around 1 in 12 men and 1 in 200 women. In contrast, blue-yellow colour blindness or total colour blindness are relatively uncommon – possibly 1 in 30-50,000 people may experience these types of colour blindness.
People who are red-green colour blind generally have difficulty distinguishing between shades of red, yellow, and green. About half of people with red-green colour blindness will have a mild anomalous deficiency, while the other half have moderate or severe anomalous conditions. In contrast, people who are blue-yellow colour blind will have problems distinguishing blues, green and yellows. People who experience total colour blindness may see the world in shades of grey.
A brief overview about colour blindness
Normal Vision:
More about normal vision:
The typical human retina contains rod cells, which are active in low light, and cone cells, which are active in normal daylight. Normally, there are three kinds of cone cells (referred to as blue, green and red cones), each containing a different pigment, which are activated when the pigments absorb light at defined wavelengths within the visible range.
The sensitivity of normal colour vision actually depends on the overlap between the absorption ranges of the three systems: different colours are recognized when the different types of cones are stimulated to different degrees. People with normal vision, trichromats, can distinguish three primary colours
Colour blindness
Normal vision, ‘trichromacy’, depends on three types of cone cells, as shown above.
Colour blindness results from a deficiency in the cone cells, which may take various forms, depending on how the cones are compromised. In some instances, only two types of cone cells are functional, which results in people seeing some mixture of just two primary colours (‘dichromacy‘). In other people, three types of cone cells may be functioning, but one of the cones is altered in its spectral sensitivity (‘anomalous trichromacy‘).
The type of colour blindness that results from cone deficiencies depends on which type of cones are affected (i.e. red, green or blue).
Dichromacy
One type of cone cells may be compromised, which results in people seeing with some mixture of just two primary colours.
The type of colour-blindness that results from dichromacy depends on which cones cells are affected. Protanopes lack red cones, deuteranopes lack green cones, and tritanopes lack blue cones. Dichromats usually know they have a colour vision problem, and it can affect their daily lives.
Anomalous trichromacy
This is a common type of inherited colour vision deficiency, occurring when one of the three cone pigments is altered in its spectral sensitivity. This results in people with anomalous trichromacy making colour matches that differ from the normal.
Again, the types of colour-blindness that arise due to anomalous trichromacy depends on which cones are affected. Protanomaly results from anomalous red cones, deuteranomaly from anomalous green cones, and tritanomaly from anomalous blue cones. From a practical standpoint, many protanomalous and deuteranomalous people have very little difficulty carrying out tasks that require normal colour vision.
Monochromacy
Monochromacy occurs in two primary forms; either cone monochromacy or rod monochromacy (achromatopsia).
With rod monochromacy, the retina contains no cone cells, so that in addition to the absence of colour discrimination, vision in lights of normal intensity is difficult. This is extremely rare (1/30,000) and the most severe form of colour blindness. People with rod monochromacy see the world in black, white, and grey. This is because the brain needs to compare the signals from different types of cones in order to see colour.
In the case of cone monochromacy, which results from having both rods and cones but only a single type of cone, the person can have good pattern vision at normal daylight levels but will not be able to distinguish hues. Cone monochromacy is even rarer than rod monochromacy (1/100,000).
Types of colour blindness that result from deficiencies in cone cells:
Red-Green colour Blind
Those affected have difficulty with discriminating red and green hues due to the absence or mutation of the red or green retinal photoreceptors. This is the most common form of colour blindness, and can be divided into four types:
- Protanopia: unable to perceive any ‘red’ light – caused by the complete lack of the red cones for long-wavelength sensitive retinal cones.
- Deuteranopia: unable to perceive any ‘green’ light – caused by the complete lack of the green cones for medium-wavelength sensitive retinal cones. A deuteranope suffers the same hue discrimination problems as protanopes, but without the abnormal dimming.
- Protanomaly: reduced sensitivity to red light – caused by having a mutated form of the long-wavelength (red) pigment, whose peak sensitivity is at a shorter wavelength than in the normal retina. This is a mild colour vision defect in which an altered spectral sensitivity of red retinal receptors (closer to green receptor response) results in poor red–green hue discrimination.
- Deuteranomaly: caused by having a mutated form of the medium-wavelength (green) pigment – results in a reduction in sensitivity to the green area of the spectrum. This is by far the most common type of colour vision deficiency, mildly affecting red–green hue discrimination.
People with deuteranomaly may be better at distinguishing shades of khaki than people with normal vision and may be at an advantage when looking for predators, food, or camouflaged objects hidden among foliage.
blue-yellow colour blind
Those with blue-yellow colour blindness have difficulty discerning between bluish and greenish hues, as well as yellowish and reddish hues. Blue-yellow colour blindness is rarer than red-green colour blindness, and can be divided into two types:
- Tritanopia: unable to perceive any ‘blue’ light – caused by the complete lack of the short-wavelength (blue) retinal receptors – only the red and green cone pigments are present. This is a very rare colour vision disturbance in which only the red and the green cone pigments are present, with a total absence of blue retinal receptors.
- Tritanomaly: extremely rare – reduced sensitivity to blue light – caused by the mutation of short-wavelength (blue) sensitive cones. This is the rarest form of anomalous trichromacy colour blindness, where the short-wavelength pigment is shifted towards the green area of the spectrum.
completely colour blind
Monochromacy is extremely rare.
People with complete colour blindness can see no colour at all and their world consists of different shades of grey ranging from black to white, rather like only seeing the world on an old black and white television set.
People who are red-green or blue-yellow colour blind will see colours very differently to people with normal vision. For example:
Colour Blindness and Teaching
The differences in colour perception between people with normal vision and those who are colour blind could have a significant impact on access to learning if the colours used in slides or images etc. have been chosen without due consideration. It is therefore important to ensure equitable access to materials by choosing colour schemes carefully. To find out more about what works (and what doesn’t!) when creating teaching materials, click on the button below.
