What does 20/20 mean? Measuring visual acuity
The number to the left of the slash (/) is the testing distance. Most people are tested at 20 feet, resulting in a reading of 20/X. The number to the right of the slash (/) indicates the distance at which a person with normal vision can see the testing object. For example, if a person’s vision was determined to be 20/40, that would indicate that at a testing distance of 20 feet, the patient was able to read what people with normal vision could read at 40 feet.
- Ideal vision, or acuity, is considered to be 20/20. Those with 20/20 vision can see an object 20 feet away as they are intended to see it.
- Low Vision is acuity of 20/70; the lowest visual acuity acceptable to retain a daytime only restricted driver’s license in the state of Illinois. Prevent Blindness America has state-by-state standards for visual acuity requirements for driving privileges.
- Legally Blind is acuity of 20/200; a statutory definition created by the Social Security Administration to determine eligibility for disability benefits. Although there is a significant loss of detail vision, legally blind individuals usually retain some level of functional sight
- Severe Vision Loss is an acuity between 20/200 to 20/400.
- Profound Vision Loss is an acuity below 20/400. Generally, a person with this degree of vision loss cannot read a standard eye chart and low vision testing such as counting fingers is used to measure visual acuity. For example, a notation of “CF 3” on an eye report indicates an ability to count fingers on the examiner’s hand at a distance of three feet.
- Light Projection is the ability to discriminate light from dark and to determine the light source such as a window or lamp
- Light Perception is the ability to distinguish light from dark, but the source of the light cannot be determined.
- No Light Perception is total blindness.