Normal vision depends on a multi-faceted, complex process. Light enters the eye through the cornea and lens, with the iris helping to focus the image. The light is projected onto the back wall of the eye, where it is perceived by millions of tiny nerve endings that make up the retina. From here, the retina translates the images into nerve impulses that are transmitted to the brain through the optic nerve.

When any of these eye parts are damaged, either through illness or injury, blindness can occur:

The lens may cloud, obscuring the light entering the eye.
The eye's shape can change, altering the image projected onto the retina.
The retina can degrade and deteriorate, affecting the perception of images.
The optic nerve can become damaged, interrupting the flow of visual information to the brain.

What Qualifies as Blindness?

People who are going blind often first deal with vision impairment, which then progresses into blindness. Blindness can affect one or both eyes, and doesn't necessarily cause total darkness. Many people who are considered blind can still see some light or shadows, but cannot see anything clearly.

Likewise, "legal blindness" does not mean that a person cannot see anything, but that their vision is so impaired that they need a lot of help perceiving images.

The United States typically defines someone as legally blind when the person’s central vision has degraded to 20/200, or the person has lost peripheral vision so that he sees less than 20 degrees outside of central vision. Normal vision is 20/20, and people can usually see up to 90 degrees with their peripheral vision. An estimated 1.1 million people in the United States are considered legally blind.

Leading Causes of Blindness

Nearly all cases of blindness in the United States are caused by eye diseases, with less than 4 percent of blindness caused by eye injury or trauma. About 77 percent of people who have eye injuries fully recover, while another 11 percent have mild impairment.

Though the following eye diseases are common causes of blindness, you should not assume you are going blind if you have any of these conditions. There are treatments available for each condition — some more treatable than others.

Cataracts

Cataracts occur when the normally crystal clear lens of the eye becomes cloudy. This causes blurry vision, faded colors, and problems seeing through glare. Cataracts are the world's No. 1 blindness cause, and more than 22 million Americans have cataracts in one or both eyes. A person's risk of developing cataracts increases as he grows older: By the time they are 80 years old, more than half of all Americans either have a cataract or have undergone cataract surgery.

People with cataracts can deal with the disease at first through the use of special glasses, magnifying lenses, and brighter lighting. Those with advanced cataracts can undergo surgery to replace the clouded natural lens with an artificial one.

Glaucoma

Glaucoma usually occurs when the fluid pressure inside one or both eyes slowly begins to increase. This pressure damages the optic nerve and the retina, causing a gradual decrease in peripheral vision. Experts estimate that about 2.3 million people in the United States have been diagnosed with glaucoma, while another 2 million have glaucoma but are not aware of their deteriorating eyesight.

Vision loss due to glaucoma cannot be reversed, but the disease can be managed through the use of prescription eye drops or surgery. It is important to have regular eye exams so you can catch glaucoma early, because treatment can save your vision.

Macular Degeneration

Macular degeneration involves the gradual deterioration of the macula, or the nerve endings in the retina that are crucial for sharp central vision. People with this condition deal with blurring and blind spots in their central vision. This is the most common cause of blindness in seniors, affecting more than 10 million Americans.

There is no cure for macular degeneration, but treatments are available to slow its progress. These include combination vitamin therapy, laser surgery, photodynamic therapy, and special medications that are injected into the eye.

Diabetic Retinopathy

Diabetic retinopathy occurs when the systemic damage caused by diabetes begins to affect the retina. Specifically, the blood vessels that nourish the retina can be negatively affected by diabetes, causing vision loss through bleeding and damage to the retina. More than half of the 18 million Americans who have diabetes are affected by diabetic retinopathy to some extent.

The best treatment for diabetic retinopathy is close control of diabetes. If the disease becomes more advanced, patients can undergo eye surgery to protect their sight.

What Causes Blindness?