A vision screening can be performed using the Snellen eye chart to measures your visual acuity. This is usually one of the first tests performed during a routine eye exam. The Snellen eye chart, which was first developed in 1862 by Dutch ophthalmologist Hermann Snellen, sets a standard for what an individual with normal vision can see at a distance of 20 feet. The chart consists of eleven lines of block letters. The first line has one very large letter and in each line that follows the letters decrease in size with multiple letters on each line.

Reasons why a vision screening is performed

The Snellen eye chart is used during a vision screening to determine whether prescription lenses are needed to improve your vision. While this screening is an initial part of a routine eye exam, the test may also be performed outside the optometrist's office. For instance, you must pass a vision screening test at the DMV when applying for or renewing your driver's license.

How a vision screening is performed

During a vision screening, you will typically be asked to sit or stand 20 feet away from the eye chart. In instances where the optometrist's office is not 20 feet long, the chart may be located behind you and a mirror may be placed on the opposite wall to simulate a distance of 20 feet. The optometrist will then ask you to cover one eye and read the letters on the chart aloud beginning at the top and working your way to the bottom. The test will be performed again with the other eye covered and then with both eyes uncovered.

The smallest row of letters that you can see clearly will indicate your visual acuity. Numbers on the side of the chart indicate how many feet away a person with normal acuity could be to read that line on the chart. For instance, if you can read the line labeled 20/40, you need to be at a distance of 20 feet to read something that individuals with a normal visual acuity could read from 40 feet away.

If you are having difficulty seeing, your optometrist can perform a vision screening with the Snellen eye chart to determine if you need prescription lenses or a change in your prescription. Further tests may also be required to confirm the specific prescription needed to help you achieve clear vision. Contact your optometrist today to schedule an appointment.