Vision is the clarity and sharpness of sight, allowing the brain to interpret the world through light signals. This ability exists on a spectrum, ranging from perfect clarity to the complete absence of light perception. For medical and legal purposes, specific thresholds classify degrees of vision loss. These definitions help determine eligibility for support services and define the limits of poor vision.
Understanding Visual Acuity Measurements
The standard method for quantifying the sharpness of distance vision is visual acuity, typically measured using the Snellen scale. This measurement is expressed as a fraction, such as 20/20, comparing the tested person’s vision to that of a person with normal sight. The numerator (usually 20 feet) is the distance the person stands from the eye chart.
The denominator indicates the distance at which a person with normal vision could clearly read the same line. For example, 20/20 vision means the person sees at 20 feet what a normally sighted person sees at 20 feet. A measurement of 20/40 means the person must stand at 20 feet to see what a person with normal vision sees from 40 feet away.
Defining the Low Vision Threshold
Low vision is a medical classification for sight loss that cannot be fully corrected by standard eyeglasses, contact lenses, or medical treatments. This condition significantly impacts a person’s ability to perform daily activities, even though they retain some usable sight.
The World Health Organization (WHO) defines moderate low vision as having a visual acuity between 20/70 and 20/160 in the better eye with the best correction. A person with 20/70 vision sees at 20 feet what a person with normal vision sees at 70 feet.
This level of impairment often requires specialized low vision aids, such as electronic magnifiers or telescopes, to complete tasks like reading or driving. When visual acuity falls below 20/160 but remains above the legal blindness threshold, it is considered severe low vision.
The Benchmark for Legal Blindness
Legal blindness is an official, standardized threshold used to determine eligibility for federal and state services. In the United States, a person is classified as legally blind if their best-corrected visual acuity is 20/200 or less in their better eye. The 20/200 benchmark means the person must be 20 feet away to see a letter that a person with normal vision sees from 200 feet away.
This classification also includes a criterion based on the field of vision, which is the total area a person can see without moving their eyes. An individual meets the definition of legal blindness if they have a visual field restriction where the widest diameter subtends an angle no greater than 20 degrees. This loss is often described as “tunnel vision,” where peripheral sight is severely limited.
The Absolute Limit: No Light Perception
The lowest level of vision a person can have is defined as No Light Perception (NLP). This condition, often called total blindness, means the individual is completely unable to detect any light, regardless of its intensity. NLP represents the end of the visual spectrum, where the eye and visual pathways no longer register light.
NLP is distinct from Light Perception Only (LPO), a severe impairment where a person can sense the presence or absence of light, but cannot discern shapes, movement, or colors. In clinical practice, NLP is the final classification recorded when all other measurements, such as counting fingers or seeing hand motion, are absent.
The majority of individuals classified as legally blind still retain some degree of light, shadow, or shape perception. This demonstrates that NLP is the most extreme and least common form of vision loss.