Legal blindness is a specific designation used by government and medical agencies to classify a severe level of vision impairment for legal and administrative purposes. It is not an interchangeable term for total blindness, as many individuals who are legally blind retain some functional vision. This designation is established to determine eligibility for certain benefits, educational accommodations, rehabilitation services, and tax exemptions. The criteria for legal blindness are precise and standardized, focusing on two main measurements of vision: central visual acuity and the extent of the visual field.
Defining the 20/200 Visual Acuity Standard
The most common criterion for legal blindness is a severe limitation in central visual acuity, which measures the sharpness and clarity of vision straight ahead. This standard is defined as having a best-corrected visual acuity of 20/200 or less in the better-seeing eye. “Best-corrected” means the measurement is taken while the person is wearing the most effective prescription glasses or contact lenses possible.
The fraction 20/200 provides a practical comparison to normal vision, which is considered 20/20. If a person has 20/200 vision, they must stand at a distance of 20 feet to clearly see an object that a person with normal vision can see clearly from 200 feet away. This indicates a profound reduction in the ability to discern fine details at a distance.
This threshold must apply to the eye that has the best vision, even after correction. If one eye can be corrected to 20/100, for example, the person would not meet this specific legal blindness criterion, even if the other eye is 20/200 or worse. This standard emphasizes that the impairment is a permanent, uncorrectable limitation to distance vision.
Understanding Visual Field Restriction
A person can also be classified as legally blind due to a significant restriction in their peripheral, or side, vision, even if their central acuity is better than 20/200. This secondary criterion is defined as having a visual field that subtends an angle no greater than 20 degrees in the better-seeing eye. A normal horizontal visual field is typically around 160 to 170 degrees, making a 20-degree field a significant reduction.
This loss of side vision is often described as “tunnel vision,” where the individual can only clearly see a narrow cone of view directly in front of them. Conditions like advanced glaucoma or retinitis pigmentosa can cause this type of severe visual field loss. Although the person may be able to read a letter on an eye chart, the inability to perceive objects outside of this narrow field significantly impacts safe navigation and daily functioning.
The visual field measurement applies to the eye with the wider field of view, similar to how acuity is measured in the better eye. This restriction is considered just as limiting as poor central acuity for the purpose of the legal designation. The functional impact is severe, requiring the person to constantly move their head and eyes to scan their environment.
The Clinical Process for Determining Legal Blindness
The determination of legal blindness must be made by a qualified eye care professional, such as an ophthalmologist or an optometrist. This process involves a comprehensive eye examination using specific, standardized testing procedures. The first step is typically measuring visual acuity using a projected or printed Snellen eye chart, which contains rows of letters that decrease in size.
The test is performed with the patient’s best optical correction in place, and the eye doctor records the smallest line of letters the patient can read. If the patient cannot read any letters below the 20/200 line, they meet the acuity standard.
If visual acuity is above the 20/200 threshold, the eye care specialist will then proceed to test the visual field. This is commonly done using automated static threshold perimetry, such as a Humphrey Field Analyzer. This device presents lights in the peripheral vision while the patient fixates on a central point, recording the full extent of the patient’s side vision. Once the criteria are met, the eye doctor provides official documentation, often called a Certificate of Legal Blindness, which the individual can use to apply for government benefits and services.