Visual acuity is measured using a test created by Dr. Herman Snellen in 1862. Dr. Snellen was from Denmark and created the charts to better be able to study visual acuity. The charts allowed him to create a definition for standard vision. This definition helped to give doctors a point to work towards in correcting and comparing vision.
The charts are designed so that the size of the letters and the white space around them are the same. This uniformity makes it easier to create letters to test the visual acuity at different distances. The charts are created with a definite measuring length in mind. This means that at that length the person with standard vision should be able to read the corresponding lines on the chart with ease. In the United States, this measurement is taken at a distance of 20 feet. Someone with good vision would have a measurement of 20/20 meaning that at twenty feet from the chart the person can read the line of letters for standard vision. Other countries use different distances to measure visual acuity. They have measurements such as 6/6 and 15/15. These numbers are still considered standard vision, since the person can accurately read the charts at the correct distance.
The first number in the fraction 20/20 is the distance from the chart. The second number is how accurately the person can read the corresponding lines on the chart. Each line on the chart represents a distance that a person with standard vision could see clearly. On many charts the first line represents the distance of 70 feet. Each line usually decreases by the degree of ten. If you have 20/70 vision, it means that you see clearly at 20 feet what someone with standard vision would see at 70 feet.
The measurement of 20/20 is used as a standard to be able to fly an airplane. Most states allow people with 20/40 vision to drive cars without corrective lenses. You may be able to receive tax benefits if your vision cannot be corrected to be better than 20/200. If you receive a vision screening test in which the score is greater than 20/40, you should visit an eye doctor. The prescription will be different than the test, since other factors, such as astigmatism and blurriness are taken into account to correct your vision.
In addition to the Snellen chart, doctors may use the Tumbling E chart to measure visual acuity. This chart has the letter E facing different directions, and is used for children and others who are illiterate. Doctors may use the Broken Wheel chart, and have patients identify which side of the circle is missing a piece. Charts have also been made involving easily recognizable symbols such as a heart, house and flower.
It is possible to have better vision than 20/20. It is rare to have vision better than 20/15. This means that at twenty feet the person can easily read what most people could read at fifteen feet. Most doctors try to write a prescription that will correct vision to standard 20/20.