Refractive errors happen when the cornea of the eye is shaped differently from the normal circular form and cannot receive light rays properly. Vision errors include myopia or near-sightedness, hyperopia or far-sightedness and astigmatism, all of which can be treated by using prescription eye glasses, contact lenses or in some cases, eye surgery like LASIK or PRK. What corrective lenses do is compensate for the errors in the cornea so that light rays focus onto the retina and not in front as with myopia, or behind as with hyperopia. Refractive eye surgery entails the use of a laser to adjust the cornea, allowing light rays to enter the eye properly.
The optometrist or optician can use several ophthalmic instruments to measure refractive errors and make prescription glasses accordingly.
Working like a phoropter, the focometer is a cylindrical ophthalmic equipment used to measure refractive errors. Developed by Drs. Ian Berger and Larry Spitzberg, this monocular instrument is hand-held and lightweight, specially made for easy diagnosing of vision defects in order for the optometrist or optician to be able to know what kind of lenses are needed as prescription glasses for the patient. Designed so that the person using it won’t need to be a trained optometrist or optician, the focometer is a useful instrument for measuring refractive errors in remote parts of the world where the necessary equipment are unavailable.
The usual procedure in measuring vision disorders and in making prescription glasses begins with the optometrist or ophthalmologist using an autorefractor or keratometer to see how light rays refract in the cornea of the eye. Once this is done and the optometrist finds out the visual acuity of each eye, a phoropter is sometimes used to find out which lenses would suit the patient and give them best corrected vision. Only then can the optometrist fill out the prescription for eyeglasses that would help in correcting the patient’s eye disorder. Such a procedure requires a trained ophthalmic professional to operate the optical equipment.
A focometer works by rotating a band around the body while the patient is looking through the eyepiece, until the image that is being viewed starts to focus. This is done at natural lighting and even without the presence of a trained optometrist or optician. A focometer is usually used to measure myopia or hyperopia, but using a target with the device can enable it to detect astigmatism. Measurements of astigmatism using a focometer aren’t as accurate as those taken by an autorefractor, however. A study on the accuracy of the focometer is done in comparison to an autorefractor, and it was found that the discrepancy between the findings from each ophthalmic instrument was within limits and that measurements using a focometer are mostly valid. Focometer results improved after the patient had done a couple of test runs before making the actual measurements.
Findings coming from the focometer are then going to be used as basis for the lenses that the optician is going to make for the correction of the refractive disorder diagnosed. This ophthalmic equipment has been proven to be useful in getting the correct prescription for the corrective lenses.
