While the patient looks at vision charts, the examiner places various combinations of lenses in front of the patient's eye. The patient reports which of two lenses produces the sharper image. The better of the two is then compared with the next lens. This incremental method identifies the optimal correction. It is expedient to use the patient's objective refraction as the starting point for subjective testing. Refraction testing is performed either with a series of test lenses from a case or with a Phoroptor, which contains many lenses that can be automatically or manually placed before the patient's eye.
The examination proceeds in three stages: ❖ Monocular testing: The optimal refraction for achieving best visual acuity is determined separately for each eye. The weakest possible minus lens is used in myopic patients, and the strongest possible plus lens in hyperopic patients. The red-green chromatic aberration test can be used for fine refraction. In this test, the patient compares optotypes on green and red backgrounds. Fine adjustment of refraction permits precise shifting of the focal point of the light on the retina. Optotypes on both red and green backgrounds then appear equally sharply defined.
❖ Binocular testing: The objective of this stage is to achieve a balance between both eyes.
❖ Near point testing: The final stage of the examination determines the patient's near visual acuity, and, if necessary, the presbyopic addition ("Add"). Allowance is made for the patient's preferred reading and working position.
The values determined by this examination are entered in the eyeglass prescription (see Fig. 16.16). The vertex distance at which refraction was performed is an important additional parameter for the optician. This is the distance between the back surface of the test lens and the anterior surface of the cornea. If the manufactured eyeglasses have a different vertex distance, then the strength of the lenses should be altered accordingly. This is because the optical effect of eyeglass lenses varies according to the distance from the eye.
Before the lenses are fitted into the frame, the distance between the pupils must be measured to ensure that the lenses are properly centered. The center of the lens should be in front of the pupil. The prismatic effects of eccentric lenses might otherwise cause asthenopic symptoms such as headache or a burning sensation in the eyes.
H To facilitate early detection of glaucoma, intraocular pressure should be measured in any patient over the age of 40 presenting for refraction testing for eyeglasses.
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