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What is a Snellen eye test? What resources are required for a Snellen eye test? How should you administer a Snellen eye test? At what age is a Snellen eye test administered? What is a Snellen chart? How do you use a Snellen chart? How many lines are there in Snellen chart? What are the symbols or letters of the alphabet on a Snellen chart used in ophthalmology? What should be the dimensions, color, and quality of material of a wall-mounted Snellen chart? What is the standard distance and height at which a Snellen chart is placed? What should be the intensity of light in the room a Snellen eye test is administered? Can a person declared legally blind read any letters of a Snellen chart? What minimum particle size can a normal adult human eye detect? In what situations is a Snellen eye test not applicable? What alternative measures should you take when a patient is illiterate? What alternative measures should you take when a person has another medical condition and a Snellen eye test is not applicable? When are you declared legally blind? How to use the eye chart: The Snellen Eye Chart is read while standing 20 feet from the chart. Be sure the room you are in is well lit but is not in full sunlight. Test one eye at a time (cover the other eye with your hand or card etc.). The lowest line that you can read correctly is your visual acuity. At 20 feet (6 meters) you should try to read the letters on the row 6th from the top. If you can read that line ok it means you have visual acuity of 20/40 or better: 1/2 normal. Standing at 10 feet from the eye chart if the smallest letters you can read were on the 6th row from the top, this would give you an acuity of 10/40: 1/4 normal. If you are nearsighted, your vision will become clearer the closer you stand to the eye chart. Visual acuity (VA) is acuteness or clearness of vision. Especially form vision, which is dependent on the sharpness of the retinal focus within the eye, the sensitivity of the nervous elements, and the interpretative faculty of the brain. VA is a quantitative measure of the ability to identify black symbols on a white background at a standardized distance as the size of the symbols is varied. The VA represents the smallest size that can be reliably identified. VA is the most common clinical measurement of visual function. A visual acuity of 20/20 is frequently described as meaning that a person can see detail from 20 feet away the same as a person with normal eyesight would see from 20 feet. If a person has a visual acuity of 20/40, he is said to see detail from 20 feet away the same as a person with normal eyesight would see it from 40 feet away. Someone with 20/20 visual acuity does not have "perfect" vision, since it is quite possible to see better than 20/20. The maximum acuity of the human eye without visual aids (such as binoculars) is generally thought to be around 20/10 (6/3). Recent developments in optometry have resulted in corrective lenses conferring upon the wearer a vision of up to 20/10. Some birds, such as hawks, are believed to have an acuity of around 20/2; in this respect, their vision is much better than human eyesight. Many humans have one eye that has superior visual acuity over the other. When are you declared legally blind? If a person cannot achieve a visual acuity of 20/200 (6/60) or above in the better eye, even with the best possible glasses, then that person is considered legally blind in the United States. A person with a visual field narrower than 20 degrees also meets the definition of legally blind. |