Color blindness (color vision deficiency, or CVD) affects approximately 1 in 12 men (8%) and 1 in 200 women in the world. It is usually genetic, but it can be acquired later in life. Change in color vision can signify a more serious condition. Anyone who experiences a significant change in color perception should see an ophthalmologist.
In the retina (the light-sensitive tissue lining the back of the eye), there are two types of cells that detect light: rods and cones. Rods detect only light and dark and are very sensitive to low light levels. Cone cells detect color and are concentrated near the center of your vision. There are three types of cones that see color: red, green and blue. The brain uses input from these three color cone cells to determine our color perception.
Color blindness can occur when one or more of the color cone cells are absent, nonfunctioning, or detect a different color than normal. Severe color blindness occurs when all three cone cells are absent, and mild color blindness occurs when all three cone cells are present, but one cone cell functions abnormally to detect a different color than normal.
There are different degrees of color blindness. Some people with mild color deficiencies can see colors normally in good light but have difficulty in dim light. Others cannot distinguish certain colors in any light. The most severe form of color blindness, in which everything is seen in shades of gray, is uncommon. Color blindness usually affects both eyes equally and remains stable throughout life.
In the human eye, some 6 million cones are responsible for filling in the details and colors in a visual image. The cone photo pigment is called iodopsin. There are three types of cones (red, green, and blue) based on their maximum sensitivity to the visible light spectrum, which is determined by the particular variety of iodopsin produced by that cone. It is important to note that the sensation of any color can be derived from the proper mix of these three primary colors. Cones require a higher light intensity to illicit a response than the rods because they have a higher threshold for stimulation.