Colour vision is the ability to detect various wavelengths of light waves and to distinguish between these different wavelengths and their corresponding colours. An individual with normal colour vision is known as trichromats. This is because the normal human eye can appreciate three primary colours (red, green and blue) due to presence of three different types of cones, i.e., red sensitive, green sensitive and blue sensitive.
What is colour blindness?
In colour blindness, mechanism to appreciate one or more primary colours is either defective (anomalous) or absent (anopia). It may be congenital or acquired.
A. Congenital colour blindness
- It is a hereditary condition affecting males more (3-4%) than females (0.4%).
- It may be of the following types:
Dyschromatopsia, literally means colour confusion due to deficiency of mechanism to perceive colours.
It can be classified into:
- Anomalous trichromacy
- Blue cone monochromatism
a. Anomalous trichromatic colour vision
Here, the mechanism to appreciate all the three primary colours is present but is defective for one or two of them. It may be of following types:
- Protanomalous: It refers to defective red colour appreciation.
- Deuteranomalous. It means defective green colour appreciation.
- Tritanomalous It implies defective blue colour appreciation.
b.Dichromatic colour vision
In this conditon faculty to perceive one of the three primary colours is completely absent.
Such individuals are called dichromates and may have one of the following types of defects:
- Protanopia, i.e., complete red colour defect.
- Deuteranopia, i.e., complete defect for green colour.
- Tritanopia, ie, absence of blue colour appreciation.
- Red-green deficiency (protanomalous, protanopia, deuteranomalous and deuteranopia) is more common.
- Such a defect is a source of danger in certain occupations such as drivers, sailors and traffic police.
- Blue deficiency (tritanomalous and tritanopia) is comparatively rare.
c. Blue cone monochromatism (BCM)
It is a condition of complete absence of red and green cone function. It is characterized by:
- Poor day vision but normal night vision,
- Colour vision severely limited, only differences in blue hues are detectable,
- Reduced visual acuity (6/24 to 6/6).
- Small amplitude nystagmus, and
It is an extremely rare condition presenting as rod monochromatism.
Rod monochromatism may be complete or incomplete. It is inherited as an autosomal recessive trait affecting both sexes equally. It is characterized by:
- Total colour blindness,
- Day blindness (visual acuity is about 6/60),
- Nystagmus, and
- Fundus is usually normal.
B. Acquired colour blindness
It may follow damage to macula or optic nerve. Usually, it is associated with a central scotoma or decreased visual acuity.
- Blue-yellow impairment is seen in retinal lesions such as CSR, macular oedema and shallow retinal detachment.
- Red-green deficiency is seen in optic nerve lesions such as optic neuritis, Leber’s optic atrophy and compression of the optic nerve.
- Acquired blue colour defect (blue blindness) may occur in old age due to increased sclerosis of the crystalline lens. It is owing to the physical absorption of the blue rays by the increased amber coloured pigment in the nucleus.
Causes of colour blindness
Color blindness is a genetic condition caused by a difference in how one or more of the light-sensitive cells found in the retina of the eye respond to certain colors. These cells, called cones, sense wavelengths of light, and enable the retina to distinguish between colors. This difference in sensitivity in one or more cones can make a person color blind.
Symptoms of colour blindness
The symptoms of color blindness are often observed by parents when children are young. In other cases, symptoms are so slight, they may not even be noticed. Common symptoms of color blindness include:
- Difficulty distinguishing between colors
- Inability to see shades or tones of the same color
Tests for colour blindess
These tests are designed for:
(1) Screening defective colour vision from normal;
(2) Qualitative classification of colour blindness i.e., protons, deuteran and tritan; and
(3) Quantitative analysis of degree of deficiency, i.e., mild, moderate or marked.
Commonly employed colour vision tests are as follows:
1. Pseudochromatic charts: It is the most commonly employed test using Ishihara’s plates. In this there are patterns of coloured and which reveal one pattern to the normal individuals grey dots and another to the colour deficients. It is a quick method to screen Congenital proton and deuteron (i.e., red-green defects). Another test based on the same principle are:
Hardy-Rand-Rittler plates (HRR) ( more sensitive than Ishihara since it can detect all the three congenital defects).
American optical colour plate test.
2. Edridge-Green lantern test: In this test the subject has to name the various colours shown to him by a lantern and the judgement is made by the mistake he makes.
3. Farnsworth-Munsell 100 hue test: It is a spectroscopic test in which subject has to arrange the coloured chips in ascending order. The colour vision is judged by the error score, i.e., greater the score poorer the colour vision. It is the most sensitive test for both congenital and acquired colour vision defects. It consists of 85 hue caps (not 100) and colour vision is judged by error score (higher score means poorer colour vision).
4. Farnsworth D15 hue discrimination test is similar test but utilizes only 15 hue caps.
5.City university colour vision test: It is also a spectroscopic test where a central coloured plate is to be matched to its closest hue from four surrounding colour in each of 10 plates.
6. Nagel’s anomaloscope: In this test, the observer is asked to mix red and green colour in such a proportion that the mixture should match the given yellow coloured disc. The judgement about the defect is made from the relative amount of red and green colours and the brightness setting used by the observer.
7. Holmgren’s wools test: In this, the subject is asked make a series of colour-matches from a selection to of skeins of coloured wools.
Treatment for colour blindness
There is no known cure for color blindness. Contact lenses and glasses are available with filters to help color deficiencies, if needed. Fortunately, the vision of most color-blind people is normal in all other respects and certain adaptation methods are all that is required.
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