The term “sphere” (or SPH) in ophthalmology refers to the primary lens power, measured in diopters, used to correct conditions like nearsightedness or farsightedness on a standard eyeglass prescription. However, the criteria for being designated legally blind in the United States do not rely on this lens power number. The definition is a legal and administrative standard set by federal law, based instead on two distinct measurements: central visual acuity and the total extent of the visual field. This designation does not signify total or absolute blindness, as most individuals classified as legally blind still possess some usable vision. The purpose of this federal standard is to determine eligibility for specific benefits and services.
Defining Legal Blindness: The 20/200 Standard
The most recognized criterion for legal blindness relates to central visual acuity, which measures the sharpness of vision at a distance. An individual meets this standard if their best-corrected vision is 20/200 or worse in the better-seeing eye. The critical detail is the requirement for “best correction,” meaning the measurement must be taken while the person is wearing the most effective glasses or contact lenses possible.
This 20/200 measurement is derived from the standard Snellen eye chart used during eye examinations. The fraction 20/200 means that a person must stand 20 feet away to see an object clearly, which a person with normal, 20/20 vision could see from 200 feet away. Recent updates to the testing standards allow for newer low-vision charts, where a patient who cannot read any letters on the 20/100 line is considered to have acuity of 20/200 or less.
The Role of Peripheral Vision Restriction
A person can also be classified as legally blind based solely on a severe restriction of their peripheral vision, even if their central visual acuity is better than 20/200. This alternative criterion addresses conditions that significantly narrow the field of view, often resulting in what is commonly described as “tunnel vision.” The federal standard requires that the widest diameter of the visual field subtend an angle of no greater than 20 degrees in the better eye.
A normal visual field typically spans 160 to 170 degrees horizontally. A reduction to 20 degrees represents a profound loss of side vision, making navigation and environmental awareness extremely difficult. The measurement of this remaining visual field is performed using specialized procedures known as perimetry testing. This testing can involve either automated static threshold perimetry or manual kinetic perimetry to accurately map the extent of the vision loss.
What the Designation Means for Individuals
The designation of legal blindness serves a distinct administrative purpose, unlocking access to a range of non-medical support and resources. The primary implication is eligibility for various government programs, including disability benefits from the Social Security Administration. These benefits provide a necessary financial safety net for those whose vision loss prevents them from engaging in substantial gainful activity.
Beyond direct financial assistance, the status provides access to specific tax advantages, such as an increased standard deduction on federal income taxes. This designation also connects individuals to specialized services designed to foster independence and vocational success, including rehabilitation training and assistance with adaptive technology.