Visual acuity, Visual field & Binocular vision วัตถุประสงค์ ... อธิบายและบอกวิธีการหา visual acuity, visual field ได้ อธิบายกลไกของ binocular vision

Visual Acuity Visual acuity is the degree to which the details and contours of objects are perceived, usually defined in terms of the minimum separable Minimum separable - the shortest distance by which 2 lines can be separated and still be perceived as 2 lines (visual angle ~ 1 minute) Snellen letter charts - viewed at a distance of 20 ft (6 m)

Factors affecting the visual acuity Dioptric Normal errors Refractive errors (myopia, hyperopia, astigmia) Pupillary size Compostion of light Stimulus Size of detail Brightness contrast Illumination Exposure time

Neural factors Fovea centralis - is the point where visual acuity is greatest slender, densely packed cones rod-free no blood vessels & nerve fibers overlying the receptors “private line” to optic nerve (cone-to-optic nerve fiber ratio = 1:1) Periphery cone-to-rod ratio decreases increased convergence

Temporal Discrimination Capability Critical fusion frequency The time-resolution ability of the eye is determined by measuring the critical fusion frequency (CFF), the rate at which stimuli can be presented and still be perceived as separate stimuli. Stimuli presented at a higher rate than the CFF are perceived as continuous stimuli. An instantaneous flash of light (1/one million seconds) excites visual receptors as long as 1/10 – 1/5 second. Motion picture - 24 frames/second TV - 60 frames/second

Visual Fields & Binocular Vision The visual field of each eye is the portion of the external world visible out of that eye. The peripheral portions of the visual fields are mapped with a perimeter, and the process is referred to as perimetry. The central parts of the visual fields of the 2 eyes coincide; therefore anything in this portion of the field is viewed with binocular vision. The impulses in the 2 retinas are fused at the cortical level.

Corresponding Points = the points on the retina on which the image of an object must fall if it is to be seen binocularly as a single object. Binocular vision adds appreciation of depth and proportion. Diplopia - If the image on the retina of the eye does not fall on the corresponding points, double vision (diplopia) results.

Left eye Right eye