(ă-kū'ĭt-ē )

[Fr. fr. L. acuere, to sharpen]

1. Clearness, sharpness of a sensory function, e.g., visual acuity.
2. In emergency and critical care medicine, the severity of a hospitalized patient's illness and the level of attention or service he or she will need from professional staff.

distance a.The ability to register optical details of objects that are far from the eye, e.g., on a chart positioned 20 ft away from the viewer in a test of visual acuity.

near a.The ability to register optical details when objects are only 12–14 in from the eye.

visual a.A measure of the resolving power of the eye. It is usually determined by one's ability to read letters of various sizes at a stand ard distance from the test chart. The result is expressed as a comparison: 20/20 is normal vision, meaning the subject has the ability to see from a distance of 20 ft (6.1 m) what a person with normal vision should see at that distance. Visual acuity of 20/40 means that a person sees at 20 ft (6.1 m) what a person with normal vision sees from a distance of 40 ft (12.2 m).

The Snellen chart used for most purposes ranges from 20/10 to 20/200. People who can see no better than 20/200 are said to be legally blind. People with worse vision, e.g., 20/400 or worse, are said to have “count fingers” (CF) vision: they may not be able to read any of the letters or figures on an eye chart, but they can often see (and count) how many fingers are held in front of them from a specified distance. Those who cannot identify fingers from any distance may still see “hand motion” (HM), or they may have light perception (LP). People who are completely blind are said to have “no light perception” (NLP).

Young children or people who do not know the Roman alphabet may be able to demonstrate their visual acuity with eye charts on which symbols, e.g., of animals or other commonly recognized shapes, are drawn to similar sizes and specifications.