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A color perception test is a type of visual screening used to test for colorblindness. The most common color perception test is the Ishihara test, named for its developer Dr. Shinobu Ishihara. This color perception test consists of a series of 38 cards printed with circles made of variously colored dots. Contrasting dots in the center of each circle form a numerical figure perceivable by those with normal color vision. People who suffer from colorblindness will not usually be able to perceive the numbers on the color perception test accurately.
The colored circles used in the Ishihara color perception test, which has been in use since 1917, are typically made up of dots in shades of red, green, yellow, blue, and orange. While people with normally colored vision will almost always be able to see the numerical figure formed by contrasting dots in the center of the circle. People with colorblindness will perceive it incorrectly or not at all.
There are several types of colorblindness, with the most common being red-green colorblindness, which inhibits the ability to distinguish shades of green or red. The two sub-types of red-green colorblindness are deuteranopia and protanopia. People with protanopia can usually tell green from red because green objects look less darkly colored to them.
Mostly men seem to be affected by red-green colorblindness. Scientists believe this is because the gene for red-green colorblindness is found on the X chromosome. Men have only one X chromosome, while women have two. While women can inherit the faulty gene that leads to red-green colorblindness, they will usually only have it on one X chromosome. The unaffected gene on the other X chromosome will normally end up dictating the woman's ability to see shades of red and green, so most scientists believe that women only develop red-green colorblindness when they inherent the gene from both parents.
There are other types of colorblindness that are less common. Blue-yellow colorblindness affects the ability to see shades of yellow and blue. This type of colorblindness usually appears with equal prevalence in both sexes. Dichromacy is a type of colorblindness in which a person can't see colors at all, but perceives everything in shades of gray.
Photoreceptors within the eye, known as cones and rods, are typically responsible for allowing the eye to perceive colors and light. Rods are generally responsible for perceiving light. There are two types of cones in the normal eye. One type is responsible for perceiving shades of yellow and blue, while the other type is responsible for perceiving shades of green and red. Colorblindness typically occurs when the eye lacks one or both of these types of cones.