Color Vision and Color Blindness


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Color vision is the ability to appreciate differences in color, as opposed to just black, white and shades of gray. It is one of the ways that visual function is measured. Other ways of testing visual function include visual field, visual acuity and contrast sensitivity.

The color of an object depends on the wavelength of the light that the object reflects or emit. When the light enters the eye, it is detected by the cone photoreceptor cells in the retina. Light of different wavelengths will stimulate different cone photoreceptors. The interaction between the cone photoreceptors and the subsequent processing in the brain provides the perception of color and gives you color vision.




PHOTORECEPTORS FOR COLOR VISION

In the retina, photoreceptor cells are vital for the detection of light. Without photoreceptors, you will not be able to see at all. There are 2 main types of photoreceptor cells: rod photoreceptors and cone photoreceptors.

Rod photoreceptors give you scotopic vision - the ability to see in dark conditions. They also allow you to detect movement and motion.

Cone photoreceptors give you photopic vision - the ability to see in bright conditions and are responsible for your color vision. They also allow you to see fine detail. The fovea (center of the macula) has the highest concentration of cones in the retina. In humans, there are 3 types of cone photoreceptor cells: L-cones, M-cones and S-cones.

(Image adapted from the internet)

- L-cone photoreceptors respond to light of long wavelengths (564-580 nm).
  They are associated with the color red.

- M-cone photoreceptors react to light of medium wavelengths (534-545 nm).
  They are associated with the color green.

- S-cone photoreceptors respond to light of short wavelengths (420-440 nm).
  They are associated with the color blue.



COLOR BLINDNESS

Color blindness is the inability to detect color or to notice differences in color. Conditions causing color blindness are usually inherited, and are due to defects in the cone photoreceptor cells. Although there is no treatment available, most people with color blindness will not experience any undue visual impairment or problem. If you have color blindness, most likely all that will happen is that you will notice colors differently.

Achromatopsia is a rare condition where the cone photoreceptors are either not present or not working. Vision will be very poor, and it will not be possible to distinguish any colors.


Protanopia occurs when the L cone photoreceptors, which are required to see red, are absent. Protanomaly occurs when the L cone photoreceptor cells are not functioning properly. In the example on the right, the image contains the number 37 in red. If you have an L cone photoreceptor deficiency (protanopic), you may not be able to see the number.

(Image adapted from Wikipedia)


Deuteranopia occurs when the M cone photoreceptors, which are required to see green, are absent. Deuteranomaly occurs when the M cone photoreceptor cells are not functioning properly. In the example on the right, the image contains the number 49 in green. If you have an M cone photoreceptor deficiency (deuteranopic), you may not be able to see the number.


(Image adapted from Wikipedia)


Tritanopia occurs when the S cone photoreceptors, which are required to see blue, are absent. Tritanomaly occurs when the S cone photoreceptor cells are not functioning properly. In the example on the right, the image contains the number 56 in purple. If you have an S cone photoreceptor deficiency (tritanopic), you may not be able to see the number.


(Image adapted from Wikipedia)


Note that diseases affecting the optic nerve, such as optic neuritis, will affect your perception of color. Colors will appear less bright and more faded. This is why your ophthalmologist will test your color vision if there is a suspicion that you have a problem with your optic nerve.



HOW IS COLOR VISION TESTED?

One of the things that can affect your perception of color is the brightness of the environment that you are in. Therefore, in order to obtain a meaningful measure of color vision, it must be tested under daylight conditions. Also, if you have myopia, hyperopia, presbyopia or astigmatism, make sure you wear your most up-to-date correction when performing the color vision test.

There are 2 main ways of assessing the ability to appreciate color. They are the Ishihara and Farnsworth-Munsell tests.

The Ishihara color test was designed by Professor Shinobu Ishihara from the University of Tokyo in the 1910s to test for red-green color deficiencies. The standard test comprises 24 Ishihara plates, each containing a series of colored circles.

Plates 1 to 17 contain a number which you must identify, while Plates 18 to 24 contain wiggly lines which you must trace correctly. Plate 1 is the test plate - everyone should be able to read the number in the test plate, even those with color blindness. If you are unable to read the test plate, then it is likely that you have some other eye condition affecting your vision.

The Ishihara plates are commonly used to evaluate for color deficiency arising from optic nerve disorders although they were not designed for use in this way. When used this way, your eyes will be tested individually. The eye with the optic nerve disorder will be slower and less accurate in identifying the correct numerals in the plates.

(Image adapted from the internet)

Left: You should be able to see the number 12, even if you are color blind.
Middle: If you see the number 15, you have normal color vision. If you see the number 17, you are likely to have red-green color deficiency. If you don't see any number, then you are likely to have total color blindness.
Right: If you do not see any number, then you have normal color vision. If you see the number 45, then you are likely to have red-green color deficiency.


The Farnsworth-Munsell 100 Hue Test comprises 4 trays containing 85 color reference caps spanning the entire visible spectrum. You will be asked to place the colored caps in order of hue. The test takes approximately 15 minutes to complete. The results are plotted on a graph which determines how well you see color. Some consider this to be the most accurate test available for color acuity.

(Image adapted from the internet)

The Farnsworth-Munsell color tests: the 100 Hue Test (left) and the D-15 test (right)


The Farnsworth-Munsell Dichotomous D-15 test is a simplified version of the 100 Hue test. It contains 15 colored caps which you will need to arrange in order of hue. The D-15 is used more as a screening test for color blindness, rather than to accurately determine how well you perceive colors.





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