![]() Test objects used here are large enough that detection is not a limiting factor (figure 3), but careful letter choice and chart design are required to ensure that letter recognition tasks are uniform for different letter sizes and chart working distances (Bailey and Lovie, 1976).įigure 3. Target recognition tasks, which are most commonly used in clinical visual acuity measurements, require the recognition or naming of a target, such as with Snellen letters. The task required here is to detect the location of the gap (figure 2).įigure 2. The Landolt C and the Illiterate E are other forms of detection used in visual acuity measurement in the clinic. (b) Dark test object on a bright background (a) Bright test object on a dark background. The task of detection involves stating whether the spot or line is present. Target detection requires only the perception of the presence or absence of an aspect of the stimuli, not the discrimination of target detail (figure 1).įigure 1. Apart from these limitations, a number of factors also affect visual acuity such as refractive error, illumination, contrast and the location of the retina being stimulated. Visual acuity is limited by diffraction, aberrations and photoreceptor density in the eye (Smith and Atchison, 1997). There are various ways to measure and specify visual acuity, depending on the type of acuity task used. This may be thought of as the ability of the eye to see fine detail. Visual acuity is the spatial resolving capacity of the visual system.
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