Measurement of Visual Acuity
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Throughout the preceding chapters we have discussed various measurements and corrections of vision. In the last chapter we shall study the techniques used to estimate the state of refraction of the eye and the correction that is suitable for it. This is the primary goal of optometry. The methods that we use are either subjective or objective, according to whether the measurement uses only the light which penetrates the subject’s eye (in which case the subject is the only judge of what is projected onto his retina) or whether the light reflected by the retina out of the eye also plays a role; in this case a clinician can measure the refraction of the subject without the subject’s participation, other than by his patience and relaxation of his accommodation). Among the subjective methods, we shall first study Donders’ method. Then we shall briefly describe various devices, called subjective optometers, which are sometimes used. The most often used objective method is retinoscopy, but even here we can use ingenious devices, objective optometers or refractometers, of which we shall study the main types.
KeywordsVisual Acuity Test Pattern Spherical Lens Good Acuity Ciliary Muscle
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