The lowest level of vision a person can have is defined by the complete absence of light perception, but vision loss exists along a broad spectrum before that final point. Visual acuity is the primary measurement used to define this range, describing the clarity or sharpness of central vision. It is a metric for determining functional ability, though it is only one component of overall visual health. Understanding the numeric scores and the clinical thresholds for severe loss provides a clear picture of what the lowest levels of sight mean.
Understanding Visual Acuity Scores
Visual acuity is most commonly measured using the Snellen chart, the familiar eye chart with rows of letters that decrease in size. The score is expressed as a fraction, such as 20/20 or 20/40, which compares the patient’s vision to what is considered a normal standard. The first number, 20, represents the testing distance in feet, while the second number represents the distance at which a person with normal vision can clearly read the same line of letters.
A score of 20/20 is the benchmark for normal visual acuity. By contrast, a score of 20/40 means the person being tested must stand at 20 feet to read letters that a person with normal vision could read from 40 feet away. Visual acuity is considered low vision when it is measured at 20/70 or worse, even with the best possible correction from glasses or contact lenses. This score indicates a degree of permanent visual impairment that can significantly interfere with daily activities like driving or reading.
The Criteria for Legal Blindness
The concept of legal blindness is a specific administrative definition used primarily to determine eligibility for government benefits, special services, and tax exemptions. It is a defined threshold of severe visual impairment, not a clinical term for total darkness. This designation is met when a person’s best-corrected central visual acuity is 20/200 or less in their better-seeing eye.
A person also meets the criteria for legal blindness if their visual field, or peripheral vision, is severely restricted. Specifically, a visual field that subtends an angle of 20 degrees or less is considered legally blind, regardless of the central visual acuity score. Many individuals who are considered legally blind still retain some functional vision, such as the ability to perceive light, movement, or large objects. This distinction highlights that the term defines a level of disability rather than a total inability to see.
Vision Below the Chart
When a person’s vision is too poor to read even the largest letter on the Snellen chart—which typically corresponds to 20/400 or worse—eye care professionals use a different, sequential set of qualitative measurements. The first step beyond the chart is testing for the ability to count fingers (CF) at a very close distance, such as one to five feet. If the individual cannot accurately count fingers, the examiner then tests for hand motion (HM), which means the individual can only detect movement of a hand waved in front of the face.
If hand movement cannot be detected, the next stage of assessment is light perception (LP), where a bright light is shone directly into the eye to see if the individual can perceive the illumination. This is the lowest level of residual sight that can be clinically recorded. The absolute lowest level of vision is documented as no light perception (NLP), which means the eye cannot detect any light. This designation of NLP is the clinical definition of total blindness.