What is Colour Vision Deficiency (CVD)?

CVD is a defect in ‘colour perception’ by the eyes. This condition is more common in men compared to women. About 8% of males (1 in 12 males) and 0.4 % of females (1 in 200 females) are affected by various types of colour vision deficiencies (CVD). The red-green colour defect is very common, where the person confuses with the spectrum of red and green colours. CVD can be inherited (the defective gene is passed on by the parents) or acquired (as a result of other diseases or an injury that damages the parts of eye).

People with colour vision deficiency (CVD) can only see a limited number of colours. This means mistakes can be made in colour identification and some colours are confused. People with significant colour defect confuse pale or dark colours. The range of colour confusions increases in low level of lighting and if the coloured areas are small.


What causes CVD?

Colour perception by our eye is similar to a television. A television contains three electron guns (red, green and blue) through which an image with varied colours is created. Similarly, in the retina (light sensitive tissue) of our eyes, there are two types of cells - rods and cones, of which cones are responsible for colour vision. There are three types of cones (L-cone, M -cone and S-cone), each containing a specific pigment called opsin which is sensitive to a particular part of the light spectrum.

A defect (mutation or rearrangement of the gene) in any one or two of the gene producing these cones will result in the production an abnormal cone causing CVD in a person.

This mutation passes from one generation to another( i.e. a genetic condition which is inherited from parents to their children) known as ‘congenital’ colour vision deficiency. Most commonly the inheritance takes place from mother to her child. “In some cases, if a single pigment is missing, the eye might have problem seeing the specific colour (Colour Vision Deficiency). But if all the pigments are missing in the cones, the eye will not see any colour at all. This severe condition is known as ‘total colour blindness’ and it is very rare to occur.”

Trichromacy

Normal human colour vision uses all three types of light cones correctly and is known as trichromacy. People with normal colour vision are known as trichromats.

Types of Colour Vision Deficiencies

Normal human colour vision uses all three types of light cones correctly and is known as trichromacy. People with normal colour vision are known as trichromats

Colour vision deficiencies


1. Anomalous trichromacy

People with ‘faulty’ trichromatic vision will have mild to severe form of colour vision deficiencies and are known as anomalous trichromats. In people with this condition all of their three cone types are used to perceive colours but one out of the three cones perceives colours slightly out of alignment, so that there are three different types of effect produced depending upon which cone type is ‘faulty’.

The different anomalous conditions are as follows
Protanomaly (Red weak) - which is a reduced sensitivity to red light.


NORMAL

PROTANOMALY

Deuteranomaly (Green weak) - which is a reduced sensitivity to green light and is the most common form of colour vision deficiency.


NORMAL

DEUTERANOMALY

Tritanomaly (Blue weak)- which is a reduced sensitivity to blue light and is extremely rare.


NORMAL

TRITANOMALY

2. Dichromacy

People with dichromatic colour vision have only two types of cones which are able to perceive colour (i.e. they have two cones which functions normally and one cone which doesn’t function/absent ). For such individuals a specific section of the light spectrum can’t be perceived.

The different types of dichromic colour vision deficiency are as follows

People suffering from ‘protanopia’ are unable to perceive the spectrum of ‘red’ light. This people are most severely handicapped.

Those with ‘deuteranopia’ are unable to perceive the spectrum of ‘green’ light. People with deuteranopia are also referred as ‘deutranopes’

Those with ‘tritanopia’ are unable to perceive the spectrum of ‘blue’ light. People with tritanopia are also referred as ‘tritanopes’

Colour’s confused by ‘Dichromat’ people

Protanopia
Protanopes are more likely to
confuse
  • Black with many shades of red
  • Dark brown with dark green, dark orange and dark red
  • Some blues with some reds, purples and dark pinks
  • Mid-greens with some oranges
Deuteranopes
Deuteranopes are more likely to
confuse
  • Mid-reds with mid-greens
  • Blue-greens with grey and mid-pinks
  • Bright greens with yellows
  • Pale pinks with light grey
  • Mid-reds with mid-brown
  • Light blues with pale violet
Tritanopes
The most common colour confusions for tritanopes are
  • Light blues with greys
  • Dark purples with black
  • Mid-greens with blues
  • Oranges with reds

3.Total colour blindness


NORMAL

TOTAL COLOUR BLINDNESS


People with monochromatic vision(mono – one; Chroma - colour) 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.

This is the condition which should be called as colour blindness and as we know it is very rare to occur.These rare types are mostly associated with some form of disorders in eyes.

There are two types:
1. Rod Monochromacy, 2. Cone Monochromacy

clourvision.org.in as such provides information focusing only on ‘colour vision deficiency’

4. Acquired colour vision defect

Causes that increase the risk of acquired colour vision deficiency include: aging, accidental injury in brain/eye, diabetic retinopathy, glaucoma, cataract, age related macular degeneration, multiple sclerosis, liver disorders, Alzheimer's disease, leukaemia as well as intake of some drugs that treat psychological problems, high blood pressure, heart disease, chronic illness, chemical exposure/chemical deposition in body and so on.

Difference between congenital and acquired colour vision deficiency

CONGENITAL COLOUR VISION DEFECT ACQUIRED COLOUR VISION DEFECT

Present at birth

Onset after birth

Type and severity of the defect the same throughout life

Type and severity of the defect fluctuates

Both eyes are equally affected

One eye is affected more, other is affected less

Visual acuity(i.e.vision) is unaffected

Visual acuity(i.e.vision) is often reduced and visual field defects frequently occur

Higher incidence in males

Equal incidence in males and females



History of colour vision deficiency


JOHN DALTON
The first scientific paper about colour blindness was written by John Dalton in 1793 entitled “Extraordinary facts relating to the vision of colours“. Dalton himself was red-green colour vision deficient and as a scientist he took interest in this topic. He claimed, that a coloured liquid inside the eyeball(vitreous part of the eye) is the source for a different colour perception. This was proved wrong only after his death, when his eyes were examined and no such liquid was found.

After that Thomas Young and Hermann von Helmholtz were the first who described the trichromatic colour vision. And once a theory for human colour vision was ready, the basics of colour vision deficiency weren’t far away.

Based upon the early contributions made by John Dalton we at colourvision.org.in have named our effective CVD screening tool as ‘Daltons’ . We are extremely proud and amazed looking at this researcher of late 17th century making a remarkable outbreak in search of the knowledge about colour vision deficiency. His interest in colour vision during those days still inspires many researchers to work on projects regards to colour vision deficiency.
Know More

John Dalton’s Eyes-Museum of science and industry (MOSI), Manchester, England(UK)

Are there any treatments available?

Colour deficiency cannot be corrected by wearing coloured spectacles or contact lenses or by any other means. Colour filters can sometimes provide assistance in some of the tasks by enhancing the brightness differences between colours that are seen as identical by the colour defective.

There are strategies available that will help the affected person to function better in a colour oriented environment. Various clinical trials are being conducted for treating complete colour vision defect (total colour blindness) through gene therapy, but currently, there is no cure for colour vision deficiency.
Strategies for CVD – click here!


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