Autorefractors project near-infrared or infrared light into the eye via a beam splitter and employ various optical principles to reveal the eye’s refractive characteristics from the measured light reflected from the ocular fundus.
The foundation for most of the optical principles employed in modern autorefractors (including automated lensmeters), and aberrometers was laid by Christopher Scheiner nearly 400 years ago. Using an opaque disk perforated with 2 pinhole apertures (known as the Scheiner disk), he demonstrated that an eye’s spherical error could be measured. This Scheiner principle relies on the fact that a double-pinhole aperture placed before the eye evokes different responses in an ametropic eye than in an emmetropic eye (Fig 8-3). When looking at a small, distant light source, a normal (emmetropic) eye will see a single spot of light with or without the double-pinhole aperture upfront. In case of either myopia or hyperopia, on the other hand, 2 separate spots are seen when the double-pinhole aperture is in place. This is because in a myopic eye (for example), the 2 bundles of light that have been created by the double-pinhole aperture come to focus in the vitreous, crossing over and creating 2 separate spots on the retina; in a hyperopic eye, the 2 bundles of light come to focus beyond the retina, which also leaves 2 separate spots on the retina. One can refract an eye by placing diverse lenses in front of the eye, or by moving the point source axially, until a single spot of light is achieved on the retina.
Figure 8-3 The Scheiner principle. Double pinhole apertures placed before the pupil isolate 2 small bundles of light. An object not conjugate to the retina appears doubled instead of blurred.
(Modified from Duane TD, ed. Clinical Ophthalmology. Vol 1. Hagerstown, MD: Harper & Row; 1983:2.)
A modified, objective form of this principle is still used today in most autorefractors. For example, in some instruments, the pinhole apertures are effectively replaced by 2 light-emitting diodes (LEDs) imaged in the pupillary plane. From the axial position of the LEDs required to achieve a single image on a camera, the patient’s refractive error is determined. This may be repeated in various meridians to determine any astigmatic component of the refractive error.
Many recent instruments use a stationary source of light to “flood” the eye and then isolate the light emerging from the eye into multiple beams (eg, via a Hartmann screen—essentially a Scheiner disk with multiple holes, or a multi-lenslet array, as described by Shack). They then measure the deflection or deviation from their ideal reference positions of the emerging individual light rays to infer the eye’s refractive error. This method is commonly referred to as the ray-deflection principle. It enables, not just the measurement of spherocylindrical errors but also the measurement of higher-order aberrations, if multiple parts of the pupil are analyzed, as is done with aberrometers.
The main difficulties with autorefractors are the result of human factors, such as poor fixation and accommodative fluctuation, including so-called instrument myopia; that is, the tendency to accommodate when looking into instruments. Various methods of fogging and automatic tracking have been developed to overcome the latter, with some success.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.