Year: 1968
Type: article
Abstract: VISUAL acuity is determined by the smallest angle at which the eye is able to distinguish two points as separate.It is therefore inversely proportional to the minimum visual angle.The normal visual acuity is found to correspond to an average minimum visual angle of one minute.Thus with relatively small angles one may say that the visual acuity is inversely proportional to the space which separates two points or, when the space is constant, that it is proportional to the distance which separates the eye from the two points.However, the visual acuity cannot be determined by means of two points with a variable distance alone.This is because it is difficult to reckon precisely the exact moment when the two points cease to be seen separately and blend into a single visual impression.It is for this reason that for examining the acuity of vision the test objects must be slightly compli- cated.Acuity is therefore measured in terms of the visual angle subtended by the con- stituent parts of the test object.In 1862, Snellen introduced some scientific standardization into the measurement of visual acuity.His optotypes achieved such a wide and rapid success that most of the distance test types in clinical use to-day are modelled on Snellen's original chart.These optotypes are based on the following principle:
Source: British Journal of Ophthalmology
DOI: 10.1136/bjo.52.6.489
Author: M H Emarah
Institution: Cairo University
Language: English
Subfield: Epidemiology
Field: Medicine
Domain: Health Sciences
Open Access status: bronze
OpenAlex ID: W2017479519