VISUAL ACUITY • Visual acuity is the degree of perception of the details and contours of an object by the eye.it is the ability of the eye or the ability to see two separate object as separate • The ability to see two points as two separate points with minimal separation depends on the VISUAL ANGLE • Visual angle is the angle subtended by light rays from two adjacent objects at the nodal point of the eye. • In the normal eye, the minimal visual angle produced by light rays from two adjacent objects at which the images of the two objects are perceived as two is one minute(1\60 degree). In case the distance between the retinal images of the two object is about 2 microns, which indicates that 2 foveal cones are stimulated by one unstimulatd cone in between(since the diameter of each foveal cone is about 1.5 microns) Determination of visual acuity • The various charts used for clinical determination of the visual acuity are constructed. The commonest of these charts are the Snellen’s and landolt’s charts, which depend on reading letters drawn in black on a wellilluminated white background. • The chart has 7 rows of letters which can be read by normal person at 6, 9, 12, 18, 24, 36 & 60 respectively • The person stands at 6 meters from the chart (to avoid accommodation), each eye is tested separately from above downwards. • The result is expressed as a fraction, the numerator of which is 6 (the distance between the patient and the chart) while it’s denominator is the distance at which the smallest letters distinguished by the person can be seen by the normal eye. • RESULTS - If all marks are seen - the visual acuity is 6\24. If the patient sees up to 24 – visual acuity is 6\ 24 • Visual acuity is a mathematical fraction(the ratio of one’s acuity to that of a normal person Factors affecting visual acuity • Age –The visual acuity is most sharp in children, then it gradually declines till it becomes markedly decreased in old age due to presbyopia • The refractive power of the eye – Visual acuity is decreased if there is errors in the refractive system of the eye eg due to myopia, hypermetropia or astigmatism • The degree of illumination of the test chart and the contrast between the letters and the background • Diseases of the eyes – Visual acuity is decreased in various diseases of the eyes eg cataract, glaucoma • Viscual acuity is maximal at fovea centralis
