Complete blindness refers to total loss of vision. This means that such people have a visual acuity of 20/200 in their good eye. So, when a person with normal vision can see an object from a distance of 200 feet, a person with complete blindness can see it not beyond 20 feet. Complete blindness can be caused by several factors, and it affects people of all ages due to different reasons, ranging from diabetes to cataracts to birth defects. Diabetic Retinopathy: Diabetes causes this type of blindness, and adults as well as children are susceptible to it. In such cases, the blood vessels present in the back of the eye burst due to diabetes-related complications. This results in impaired vision, and is quite common among those who have suffered from diabetes for more than 10 years. Unfortunately, diabetic retinopathy does not usually show early signals. But it is often seen that these patients can gain a lot of help from surgery and medications. Macular Degeneration: This ailment is mostly age-related and is generally observed in senior adults. In such cases, retinal damage causes a loss of vision in the macula, present in the center of the visual field. These people are unable to read or identify faces, but retain some peripheral vision, and therefore, are not completely visually disabled. Glaucoma: Glaucoma develops due to fluid pressure buildup in the eye. This intraocular pressure causes damage to the optic nerve, which disturbs the image transmission to the brain. If you have a family history of diabetes or glaucoma, you would be at a risk. It is advised that such individuals should seek medical advice after 40 to prevent long-term loss of vision, as symptoms of glaucoma are difficult to spot. Cataract: Cataract refers to the fogginess that develops on the crystalline lens, which results in varying degrees of blurred vision. The opacity of the lens may be minor or it could be complete. Early warning signals include short-sightedness or myopia and a reduced ability to recognize shades of the color blue. Cataract is a condition that progresses with age, and can cause complete blindness if one does not undergo treatment.

Night blindness, as most of us know, is a visual impairment which occurs in darkness. One may not categorize it as total sightlessness, but the loss of vision in these cases is quite substantial. Such people are unable to adjust their vision in dim lights or darkness. Night blindness can be present in some from the time they are born. In others, it may be a result of retinol or vitamin A deficiency or even eye injuries. Another cause may also be retinitis pigmentosa, a genetic condition in which the rod cells which are present in the retina become gradually unresponsive to light. In later years, the daytime vision of such people is also known to deteriorate. This condition is mostly observed in youngsters, and they gradually become completely blind by the time they are 40. The good news is that genetic testing can help determine the risk of retinitis pigmentosa. People with a family history of this condition can assess the chances of passing it on to their children, which can help families seek appropriate medical advice.

Color-blind individuals lack the ability to discern certain colors. Mostly considered a genetic condition, it does not completely fall under the typical definition of blindness. Sometimes, exposure to ultraviolet light may cause retinal damage, resulting in color blindness. Such people usually suffer from red-green color blindness, which means that they have problems to differentiate between certain shades of red and green. In other instances, people might be unable to see some colors, this condition is called dyschromatopsia. Achromatopsia, on the other hand, is a rare type of color blindness where the person is completely unable to see colors. Interestingly, more men are found to be color-blind as compared to women, owing to its genetic origins. It may also be noted that color-blind people tend to have normal vision otherwise, barring their inability to distinguish between some colors.