Process of color vision
The mechanism by which color is perceived is a complex physiological process, made possible thanks to the retina.
The retina is covered by millions of photoreceptors: the rods and the cones.
Rods enable vision in grey because they are only sensitive to the luminous intensity, thus they play a key part in low light vision.
Cones enable vision in color, there are 3 types of cones: S sensitive to blue, M to green and L to red.
They convert light energy to nervous signals and transmit it to the brain through the optic nerve. In the brain these signals are decoded to create the image being seen.
Perception of color could be, in some cases, incomplete, or totally absent.
This is referred to as dyschromatopsia.
Hereditary dyschromatopsia : daltonism
Hereditary dyschromatopsia is also known as daltonism. It is characterized by an absence of color perception or an incapacity to distinguish certain color tints.
Frequency of color blindness in people is on average, 8% of men and 0.5% of women.
Translated from : Orthoptie
A colour blind person does not have the 3 normal cones needed for colour perception.
- either one of the cones is missing and the person is "dichromatic".i.e. he only perceives 2 color ranges.
- if the red is missing, the patient is named as a protanope, if green is missing, he is a deutranope (most frequent), if blue is missing (extremely rare), he is a tritanope
- if either one of the other cones are deficient, the daltonic is also abnormal trichomatic
- if red is deficient, he is named protanomal
- if it's green, he is deuteranomal
- if it's blue, he is tritanomal
Translated from SNOF
As visual cells don't renew, these visual disorders are definitive and incurable.
Acquired dyschromatopsia are color vision disorders consecutive to a deterioration of cones, optical nerves or of the visual cortex, after a disease.
They evolve in time and can be associated to a degradation of the visual field.
Eye diseases responsible for such symptoms include: retinal detachment, macular degeneration, trauma of the optical nerve, Leber's disease, intense solar exposure.
Contrary to hereditary dyschromatopsia, these anomalies can progress positively, thanks to the treatment offered against the initial disease.
Pseudo isochromatic plates : Ishihara test
This test was created in 1917 by Ishihara Shinobu.
It is formed by 38 plates called "pseudo isochromatic", plates made of little colored circles, forming a background on which appears a symbol (letter or number).
Patients who do not have any dyschromatopsia clearly distinguish the symbol, while those who do suffer from the condition will have difficult.
Its purpose is to highlight the existence of a potential dyschromatopsia, but don't enable the detection of its seriousness.
These tests use samples of colors and they enable the classification of the condition severity.
In the Holmgren test, the patient has to select a certain number of similar colors from amongst a bunch of colored threads.
In the Farnsworth test, the patient has to group or put in order some coloured tokens.