A retinoscope is an instrument used by an optometrist or ophthalmologist to assess the range of refraction error in the eyes as part of a comprehensive optical examination. The device passes a beam of light directly into the patient's eye while he or she views a distant object, which permits the clinician to examine how light is reflected off the retina. The degree of refraction is indicated by the field of vision in which the image points of the light no longer converge and accurate focus is lost. In the jargon of geometrical optics, this is known as the circle of confusion, the blur circle, or the shadow effect. The error of refraction is then corrected by using a phoropter, which introduces a series of lenses of various optical strengths until the retinal reflex is observed as being in normal range through the retinoscope.
While retinoscopy may sound complicated and highly technical, it's actually quite simple and painless. In fact, most people aren't bothered at all by the light shining in their eyes, and may only be remotely aware of the intrusion. That's because the patient is usually paying close attention to the eye chart in order to correctly recite the letters shown in as many succeeding rows as possible. If cycloplegic drops are put into the eyes prior to being examined with a retinoscope to dilate the pupils and enhance retinal reflex, however, the patient can expect to continue to experience these effects for a few hours.
A retinoscope also tells the clinician what type of visual correction is needed based on whether the refraction error is spherical or cylindrical. A high degree of refractive power indicates that the light focus remains in front of the retina, resulting in myopia, a condition more commonly known as nearsightedness. Conversely, hyperopia, or farsightedness, occurs when there is too little refractive power and the light focus remains behind the retina. Those fortunate enough to evidence no refraction error can be thankful for being blessed with emmetropic eyes.
Examination with a retinoscope also detects cylindrical refraction errors that occur with astigmatism. While these visual abnormalities are also caused by a refractive power that is too strong or too low, the distinction here is that the focus of the object may differ between the two planes or meridians of the eye. In other words, the image may be sharp across one meridian and blurry on another. This has the effect of producing a curvature in the refractive power, possibly due to a misshapen cornea or lens.
