How do Eye Doctors Check or Measure Vision?
Many patients ask what do the numbers mean such as 20/20 or 20/40. The eye charts that are used are standardized and the vision is tested at a certain distance. The most common method of testing vision is using the Snellen eye chart. A Dutch ophthalmologist Herman Snellen developed the eye chart in 1862. It only uses Sloan letters C, D, E, F, L, N, O, P, T, and Z. The thickness of the lines is equal to the thickness of the white spaces between the lines and the thickness of the gap in a C. The height and width of the letter is five times the thickness of the line. Ian Bailey and Jan Lovie developed an eye chart that has five letters on each row with equal size and spacing. These charts are known as Log-MAR charts. Charts can also contain Landolt Cs, Tumbling Es, Jaeger chart for near vision, and pictures for children. Most eye doctors use Snellen charts in their offices. The eye chart is usually positioned at 20 feet or in metric terms 6 meters.
Reporting or Recording Visual Acuity
The vision in the United States is usually recorded as 20/20, 20/40, 20/60, and etc… In metric, it is recorded as 6/6.0, 6/9.5, 6/12, and etc… Other methods are in decimal form and Log-MAR. If a person cannot see the eye chart their vision still needs to be recorded.
NLP or no light perception- unable to see any light or total blindness
LP or light perception- only able to tell that a light is present
LP with P or light perception with projection- able to see light and tell from which direction the light is coming from
HM or hand motion- able to see a hand moving in front of the eye and is designated in distance such 1, 2, or 3 feet
CF or count fingers- able to count fingers and is recorded in feet such as 1, 2, 3, and etc. feet
What do the Numbers Mean?
We usually measure distance vision at 20 feet and that is the first number in the 20/20 recording of vision. The second number tells the size of the letter. It was determined that a 20/20 letter was what a normal person could see at 20 feet. A 20/20 letter is of the size that there is I minute of arc between the letter O and a letter C. Then 20/40 vision means that a person can see a size of letter of 20/40 size at 20 feet where a normal person would be able to that letter at 40 feet. Vision of 20/200 or worse is considered legally blind.
Method of Testing Vision
Vision is typically tested without glasses or correction and with correction if the person wears glasses. It is usually tested in subdued lighting and one eye at a time. Near vision is tested with bright light using a reading card. This is usually written in Jaeger terminology such as J1, J2, J3, and etc… In children, the chart contains pictures of typical things a child can recognize such as birthday cake, duck, phone, and etc… Rotating Es can be used in people where there is a language problem and they can point in which direction the Es are pointing.
Another method of measuring vision is called contrast sensitivity. This is a chart in which lines or figures of different shades of grey are placed next to each other. The size of the lines and the amount of difference in the shades of grey are depicted. This method provides a higher level measuring a person’s vision. A person may see 20/20 but not have very good contrast sensitivity. Then driving at night with less contrast sensitivity may cause problems for some people. In the future we start measuring vision with this type of testing.
By using standards that all eye doctors use, the level of visual acuity measured is always going to be the same no matter where the vision is tested. This way eye doctors know if a person’s vision is changing even if was checked by another eye doctor.