The practice of optometry involves numerous measurement and evaluation of the eyes, its related structure and visual system and information processing. This is made possible by the use of optometric instruments that assist in the examination of a patient’s vision and other visual functions. One of the most commonly used is the refractor, an instrument that measures each eye’s refraction.

The refractor is a device that offers objective measurement of a patient’s refractive error. This instrument helps optometrists determine what visual problem the patient has, whether it’s myopia (nearsightedness), hyperopia (farsightedness) or astigmatism – distorted vision problems that may be corrected with the use of contact lenses or eyeglasses. This instrument is the most commonly used to produce a starting prescription.

The refractor is often used as the initial test to obtain the initial settings required in a phoropter, another optometric instrument that measures the patient’s problems with visual refraction. The optometrist can then use the phoropter to change settings and lenses to fit the patient’s eyeglass prescription. Unlike the refractor, the phoropter requires subjective feedback from the patient, who provides the information about whether a particular combination of lenses offers the best vision or not.

How a refractor works
To understand how a refractor works, it is important to know how the eye processes images. Light that reflects from an object enters the eye where it is focused by optical structures such as the cornea, pupil, iris and lens. When light finally reaches the retina, the upper half of the image is focused on the retina’s lower half while the lower half of the image is focused on the retina’s upper half. When processed by the brain, the image it produces or the image that we see, looks right side up.

During an eye examination, the way the light changes as it enters the patient’s eye determines the kind of vision problem he has or if he has distorted vision at all. Using the refractor, an optometrist will be able to measure those changes and determine the initial readings of the patient’s prescription.

The auto refractor
An advancement to the manually operated refractor, the auto refractor is a machine that is used during eye examination. This optometric instrument is operated and controlled using a computer.

A patient sits in front of the auto refractor and rests his chin on the attached rest. Using one eye, they peer into the refractor to look at a picture. This picture moves into focus and then out again. This is done several times to provide an average reading of the person’s vision. How the image registers on the patient’s retina is read by the auto refractor. Using the average reading, a prescription is formed. The auto refractor then prints out the approximate measurement of the patient’s prescription for use with eyeglasses and/or contact lenses.

During the eye examination, the patient remains passive and does not actively interact with the test. While the auto refractor is a commonly used optometric instrument for most offices and clinics, it is often the preferred method to use with patients who are extremely young, have certain disabilities or those who are difficult to communicate with.

The auto refractor may also be used as the first step to a subjective refraction test where the patient is an active participant during the eye examination. During this test, lenses are used with a phoropter to determine which combination allows the patient the best visuals. The patient determines the right combination, which makes the test a more refined measurement of the patient’s prescription.

The future of the refractor
Despite changes and developments in optometry, the area of refraction has not had some significant changes. Although the auto refractor already offers a more advanced way of examining the patient’s visual functions and vision, newer technologies, such as wavefront aberrometers, may pave the way for better eye examinations and evaluations.

Wavefront analysis provides a way to quantify eye aberrations objectively. First used with telescopes in the 1960s, wavefront has since moved to optometry, where it is capable of measuring higher order aberrations. For the majority of patients using the most common methods of refraction, wavefront aberrometers can access optical aberrations that were difficult to examine before. Wavefront aberrometers are capable of examining a wider area of the pupils compared to auto refractors. This allows them to produce a more refined refraction data based on the premise that refraction is a part and function of the pupil’s size.

While most experts believe the advances shown by wavefront aberrometers can be helpful in providing a more sophisticated way of providing prescription for problems with vision, they still do not see refractors becoming redundant. Wavefront aberrometers may make eye examinations faster and more polished, but they cannot completely replace the more subjective method the refractor instrument offers.