Here is a picture of an apple. How do people know if the color of this apple is red? First, the light containing multiple wavelengths hits the surface of the apple. Then, the apple absorbs some of the wavelengths from the light, reflecting the specific wavelengths. Those Reflected wavelengths will represent red color in this case. Those wavelengths comes into the interior of the eyeball and reach the retina that contains rods and cones. The rods and cones are the photoreceptor cells that detect the color of the wavelength and send the neural signals to the brain. Rods participates in detecting brightness.
Cones are specialized in detecting colors in bright conditions, and there are three types of cones each responsible for recognizing blue, green, and red. S-cones are responsible for detecting the short wavelengths (blue), M-cones detect medium wavelengths (green), and L-cones detect long wavelengths (red). When a wavelength enters, two or three cones will receive the wavelength, and each of them will rate different sensitivity values. Because the wavelength of the red that represents red color is about 650nm, M-cones and L-cones will detect the wavelength and by comparing the ratio of sensitivity each of them got, our brain will determine the color of the wavelength.
However, not all people can recognize all the colors properly. There are some people who have problems with perceiving certain colors. For example, some people have difficulty in distinguishing the colors green and red. The symptom of having abnormalities in sensing particular colors is called ‘Color Vision Deficiency’. Color vision deficiency can be further classified into several categories depending on its severity.
First one is called ‘Color Blindness’. People with color blindness cannot recognize one or more colors. As mentioned above, there are three types of conical cones which are red conical cells, green conical cells, and blue conical cells, and each of them contributes in detecting red, green, and blue colors.
Monochromacy is diagnosed when the person lacks two types of cones. These people cannot recognize any colors but brightness. However, only 0.003% of the whole population has monochromacy. The most common type of color blindness is green-red color blindness. People who don’t have one of green or red conical cells are called green-red color blinds, and they cannot tell the difference between red color and green color. More precisely, people who don’t have green conical cells see green and reddish purple color as no color, and people who don’t have red conical cells see red and bluish green color as no color.
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That is because when we lack one of the M-cones and L-cones, we have problems in comparing the ratio of two sensitivity. For example, if a person lacks a green conical cell which is M-cones, it would be hard to determine if the sensitivity value from the L-curve is red or green. There is also yellow-blue color blindness. It works in the same mechanism with green-red color blindness, and it is much more rare compared to green-red color blindness.
Second, much mild level of color deficiency is called ‘Color Weakness’. People with color weakness has partial inability in one of their three conical cells. This partial inability means that one of the S, M, and L cones curve was shifted from the original position. If the cones curve shifts, they will have different sensitivity value of the same wavelengths. They can recognize most of the colors, but if the partially unable color is mixed with the colors with similar sensitivity value, they have hard time distinguishing the colors. For example, a person with green-red color weakness can distinguish the color red and green when they are separated or only one of them is present.
That is because the ratio of sensitivity value went indistinguishable due to the shift. When there exists pink and light green right next to each other, it would be hard for them to separate those colors because the sensitivity value ratio is so similar with the shifted cones curve. Similar to color blindness, there are also two types of color weaknesses which are green-red color weakness and yellow-blue color weakness.
The cause of these color deficiencies is mostly related to genes, but in some cases retina disease, occipital nerve damage, and drug abuse can cause the symptom as well. There is no clear treatment for dealing with color deficiencies, but special glasses and applications were invented to help those people. The glass invented by EnChroma helps the people with color deficiencies to have more distinct wavelengths of colors, so the cones can have more clear ratios of sensitivity values. Also, the application that helps color deficient people does the role of adjusting the colors of the picture into the detectable colors, so that color deficient people can perceive the color more distinctively. Even though the treatments for the innate color deficiencies are known to be impossible, the scientists are trying to come up with a way to cure color deficiency by inserting photopigment genes into the retina. It already succeeded in fixing the monkey's color deficiency. Although there are some safety problems, it can be said that the future of color deficiency treatment is bright.
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