Clinically, a person may be born with a color deficiency from poorly developed photoreceptors or due to blind zones in the central vision. It's well known that patients with optic nerve damage, especially injury to the central portion of the optic nerve, experience moderate or severe color vision reduction. A person may not perceive any colors if there is a large, dense scotoma affecting central vision. Preserved peripheral vision does not contribute to color vision as the peripheral retina contains only rod photoreceptors, no cone photoreceptors.
Color blindness is the term most used to describe a person having a color deficiency – very few individuals are truly color blind in the sense that they only perceive different shades of gray (a condition known as complete achromatopsia). With color deficiency, one or more of the three cone subtypes is either absent, malfunctioning, or has altered wavelength sensitivity. When this occurs, a person perceives a different color than normal in response to a particular wavelength or color of light. The most common form of color deficiency is red-green.
Many different versions of color vision testing exist such as Ishihara, Farnsworth-Munsell, D-15 Color Arrangement Test, and the RGB anomaloscope. Fedorov Restore Vision Clinic uses the Ishihara for color vision assessment. Most of these tests consist of visualizing a picture composed of colored dots, arranging a series of colored plates in a particular order, or matching color images. Color vision is not expected to change after birth. Determination of proper color vision is necessary as certain careers require normal color vision (ex. pilots). Clinicians may also assess color vision to gauge retinal and optic nerve health.