The autorefractor is a device that automatically and objectively measures refractive error on patients. It has become an increasingly popular instrument in optometric practice due to its ease of use, speed, reasonable accuracy, and repeatability of results.
It offers an effective way to provide a starting place for subjective refraction when optometrists are needed to undertake a thorough eye examination in an allocated time. The most recent models will also supply keratometry measurements, displaying corneal mapping and detecting corneal irregularities. The auto refractors and keratometers include infrared source of light, a fixation target, and a Badal lens system.
Source of light
An infrared light source (around 800-900nm) is employed because it allows good ocular transmission at the sclera. It is invisible to the eye, so there is no impact on patient comfort, pupil diameter, or accommodation response; however, reflection from the choroid and sclera means that a -0.50D adjustment to the ultimate results must achieve accuracy.
To relax accommodation, most autorefractors use a ‘logging target to relax accommodation, which has previously been a source of error for autorefractor measurements. Targets range from simple pictures to scenic fixation charts for optimal results making fixation easier. Initially, a patient may report that just before measurement, the perspective is blurred; this is the aftereffect of the fogging lens.
Badal lens system
Most autorefractors use an automated Badal optical system to ascertain the refractive state of the eye. This technique essentially allows an item to be considered through a Badal lens that moves toward or far from the attention without changing the magnification of the retinal image but allows a big change of optical vergence.
In an autorefractor, an infrared light source is projected with a beam splitter and the Badal lens system to make the perfect focus of the slit image on the retina. The reflection of the slit image passes out via the beam splitter to achieve the light sensor.
Calculation of refractive error
The refractive error is calculated by analyzing the way the patient’s eye influences the infrared light source. Three common methods for analysis are image quality analysis, Scheiner disc principle, and retinoscopy:
Image quality analysis. This technique has become rarely noticed in modern instruments. A picture quality analyzer determines the perfect positioning of the Badal lens system, which is determined by the output signal of the light sensor.
Scheiner disc principle. The principle is dependant on a double pinhole being placed facing the patient’s eye to ascertain the level of ametropia present. A myopic eye will see the image viewed through the two holes as crossed, whereas the hyperopic eye will see the images uncrossed.
Occluding the pinholes individually and asking the in-patient which image has disappeared identifies the crossed and uncrossed images. Most autorefractors use the Scheiner disc principle in an altered fashion, whereby two infrared light sources are imaged in the plane of the pupil to simulate the Scheiner pinhole apertures.