Color blindness (color deficiency) is an inability to distinguish between certain colors. Partial color deficiencies are more common than complete color deficiencies (rod monochromats). The most common forms of partial color deficiencies are the red-green defects (protanopia and deuteranopia).
Color vision is a function of the cones in the retina that are stimulated by light and transmit impulses to the brain. Persons with the red-green defects have difficulty distinguishing between red, green and yellow. More rare is the blue-yellow defect (tritanopia) that results in an inability to distinguish between blue and yellow.
A person with red-green color deficiency will see red and green objects as shades of gray. In mild cases, people may be unaware of their color deficiencies until specifically tested. In more severe cases, people will experience difficulty with everyday tasks, such as responding to traffic lights while driving. There are regulations barring individuals severely affected by color deficiencies from certain occupations where normal color vision is vital (e.g., pilots).
Normally, color deficiency is an inherited defect and approximately 1 out of every 10 men and 1 out of every 200 women have some degree of color deficiency. In rare cases, color deficiency may result from injuries, diseases or as a side effect of certain medications. There are currently no known cures for color deficiencies.