What is low vision? A re-evaluation of definitions

Purpose. To re-evaluate definitions of low vision, visual impairment, and disability. Methods. We review current definitions of legal blindness and low vision and how these definitions are variably based on disability or impairment. We argue for a definite distinction being made between criteria for visual impairment and visual disability, low vision being defined as the presence of a visual impairment that results in a disability. Visual impairment is defined according to population norms and a statistical cut- off is used. Visual disability is defined by consideration of the level of visual measures which result in measurable or reportable disability. We consider the evidence that contrast sensitivity should be a criterion for visual disability in addition to visual acuity and visual field. Conclusions. According to the current information, we define visual impairment as best monocular or binocular visual acuity < (worse than) 6/7.5, total horizontal visual field <146°(Goldmann III-4e) or <109°(III-3e), and contrast sensitivity <1.5 (Pelli-Robson); we define visual disability as best monocular or binocular visual acuity <6/12 or contrast sensitivity < 1.05.