What does it mean to be Legally Blind?

Poor Vision versus Legal Blindness
People with strong eyeglass or contact lens needs frequently make the mistake of telling their eye doctors that they are "legally blind." But legal blindness in the United Stated is based on a person's inability to drive and perform daily activities.
Legal blindness is defined by United States federal law, and is defined as having either one of the following:
  1. Central visual Acuity worse than 20/200
  2. Visual field restriction of less than 20 degrees
What is Central Visual Acuity?
The Snellen Visual Acuity Chart, invented by Herman Snellen in 1862, remains the most common method used to measure central visual acuity. A level of 20/200 means that a person with poor eyesight sees at 20 feet away, even with eyeglasses, what a normally-sighted person can see at 20 feet away.
Common causes for severely declined visual acuity include age-related macular degeneration, congenital retinal scar, end-stage glaucoma, corneal scar or swelling, diabetic retinopathy, central retinal vein or artery occlusions, ocular tumors, and more.
Snellen visual acuity chart
What is meant by visual field limitation?
Computerized instruments, including the Humphrey-Zeiss visual field analyzer, are used to measure a patient's peripheral, or side vision. Lights are presented to each eye, and the patient acknowledges seeing the light by pressing a button. A person can have 20/20 central visual acuity but severe side vision problems that can still result in legal blindness. Common causes include glaucoma, stroke, multiple sclerosis and optic nerve tumors.
Constriction of peripheral vision to less than 20 degrees in both eyes also meets the definition of legal blindness, even if the central vision is 20/20.
constricted visual field perimetry test

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