Correction of refractive errors

Correction of refractive errors
For correct vision it is necessary that a sharp image of sufficient size for registration is formed on the retina and that the images formed on the retinas of both eyes are of the same size.

A good correction should ensure emmetropia while keeping the images on the retina as large as possible, and it must take into account the convergence of the eyes, the visual field and the visual field. The field of vision is the set of all directions in space determined when the eye looks at a fixed point and is an expression of the sensitivity of the peripheral retina. The field of view is the set of directions in space that can be seen by a freely moving eye and depends on the mobility of the eye about its geometric center.

Correct correction of myopia is ensured by a scatterometer of optimal optical power, at which the examinee can see sharply to infinity. Farsightedness is corrected with a clutch of the smallest optical power at which the examinee can see letters clearly at a distance of 25 cm. Astigmatism is corrected with toric (cylindrical) glasses.

Correction of refractive errors is carried out with spectacles, the lenses of which are placed about 12 mm in front of the cornea, or with contact lenses attached to the cornea.

The advantage of contact lenses is the widening of the field of vision and less distortion when looking sideways. The lenses are made of a highly hydrophilic material. Today, soft contact lenses are mainly used to correct myopia, hypermetropia, astigmatism (toric) and presbyopia (bifocal). Hard (gas permeable) contact lenses, which used to be used, are nowadays indicated only in rare cases. Lenses are designed to be worn for different periods of time depending on the type. There are lenses for daily wear (up to 18 hours) and lenses for continuous wear (up to 30 days continuously).

Refractive errors can also be corrected surgically using high-power lasers. For farsightedness, laser thermoplasty is performed, where the laser irradiated peripheral fibres of the cornea are shrunk, thus achieving a bowing of the central optical zone. For the correction of myopia and astigmatism, excimer lasers are mainly used, which can remove hundredths of a millimetre of thin corneal layer (radial keratotomy), thus changing the curvature of the front surface of the eye.

Related articles

 * Optical system of the eye
 * Refractive errors
 * Use of lasers in medicine