Refraction testing means measuring the additional refractive power required to produce a sharp image on the retina. Subjective and objective methods are used. Subjective methods require information from the patient.
Subjective refraction testing: This consists of successively placing various combinations of lenses before the patient's eye until the maximum visual acuity is reached (see Correction of Refractive Errors).
Objective refraction testing: Objective testing is unavoidable when the patient is unable to provide subjective information (for example with infants) or when this information is unreliable. This method also greatly accelerates subjective refractive testing.
Retinoscopy (shadow testing): The retina is illuminated through the pupil. The examiner observes the optical phenomena in the patient's pupil while moving the light source (Fig. 16.7).
Refractometry. The measuring principle is based on ophthalmoscopic observation of a test image projected on to the patient's retina. The distance between the test figure and the eye is changed until the image appears in focus on the retina. Refraction can then be calculated from the measured values. An alternative to changing of the distance is to place various lenses in the path of the light beam.
Automated refractometry. The method measures refraction automatically with the aid of light-sensitive detectors and a computer until a focused image appears on the retina. These systems operate with infrared light.
H Any objective measurements of refraction should be verified by subjective testing whenever possible.
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