Which measurement is primarily used to determine legal blindness?

The measurement primarily used to determine legal blindness is 20/200 or worse. This standard indicates that at a distance of 20 feet, a person with this level of visual acuity can only see what a person with normal vision (20/20) can see at 200 feet. Legal blindness is often defined in terms of visual acuity because it reflects a significant impairment in the ability to see which affects daily functioning.

This criteria is established to help provide a uniform measure for eligibility for disability benefits, access to certain resources, and accommodations for those with severe visual impairment. The definition also emphasizes the impact of vision loss on an individual's ability to perform routine tasks, thus justifying the legal designation of blindness for access to support services and assistance. Other measurements, like 20/100, 20/70, or 20/50, do indicate varying levels of visual impairment but do not meet the specific standard for legal blindness set at 20/200.