Refraction Techniques

Refraction techniques are used by optometrists to determine the prescription for corrective lenses such as glasses or contacts. Some techniques are subjective, meaning the results are determined by the patient stating his or her perception, and some are objective, meaning the examiner or a machine determines the result. Some techniques look at spherical refraction, meaning the portion of a patient's vision that is determined by the degree to which the cornea and lens over- or under-focus light, and some techniques look at the astigmatic refraction, which is due to the cornea having an oval shape.

Visual Acuity

• The first refractive technique an optometrist uses in an exam determines visual acuity, which gives a subjective, quantitative measurement of how good or poor a patient's vision is. This is done with an eye chart with letters of decreasing size down the chart. The farther down the chart the patient can read at a standard distance (usually 20 feet), the better their vision. Using the Snellen system of measurement, each row of letters has a number rating, and the patient's Snellen acuity is equal to the testing distance divided by the rating of smallest detected letter. So a patient who has 20/20 vision can read, at a distance of 20 feet, the row with a rating of 20.

Direct Ophthalmoscopy

• Optometrists use an ophthalmoscope to view the retina. While this is mostly done to ensure retinal health, it can also be used as a refraction technique to give an approximate, objective spherical correction. The arrangement of prisms and lenses in the ophthalmoscope is such that a corrective lens is required for the optometrist to get a clear view of the retina. The properties of the patient's eye determine how strong of a corrective lens is needed; this gives an estimate of how strong their prescription will need to be.

Keratometry

• Keratometry is used to determine the cornea's degree of curvature, as this represents its ability to focus an image. An image is reflected off the cornea, which works like a convex mirror, and the size of the reflected image, magnified for easier viewing, goes into calculations that determine the degree of curvature of the cornea. In addition to giving this objective spherical measurement, keratometry helps determine the correction needed for astigmatism, as the image is ovoid rather than circular for astigmatic patients.

Refractors

• The most time-consuming part of an eye exam is the refractive technique of using refractors in trial clip to hold the lenses up to the patient's eyes as the optometrist goes through different options. This is a highly subjective test, as it relies on the patient replying whether changes to the lenses in the trial clip make their vision better or worse.