Ametropy
Ametropy (from ancient Greek ἀμετρία ametría "disproportion" and ὤψ ōps "eye") - misleadingly called " ametropia " - describes the condition of an eyeball in medical optics that does not depict an object that is optically infinite on the retina with relaxed accommodation . It is the opposite of emmetropia .
Contrary to that of the term " refractive error " suggested inaccuracy is the fact that the vast majority is ametrop all human eyes. Ametropia is usually not an abnormal condition, but a normal variant .
Forms of ametropy
Hyperopia, clarity
As hyperopia ametropia is known, in which the image of a lying in the optical infinity object and parallel incident light beams in a relaxed accommodation behind comes to lie the retina. The majority of all human eyes are hyperopic . The average refraction of the human eye is about +0.5 diopters .
The term "farsightedness", which is often misused, is not identical to hyperopia. This is how presbyopia - that is, "presbyopia" - is correctly designated.
Myopia, nearsightedness
As myopia ametropia is known, in which the image of the infinitely far object in a relaxed accommodation before comes to lie the retina.
Hyperopia and myopia are grouped together as "axial ametropia". Myopia predominates among these.
Astigmatism, astuteness
When astigmatism is called an eyeball of the state where the parallel incident light beams depending on their plane of incidence are refracted to different degrees. The planes with maximum and minimum refractive power are usually perpendicular to one another. The strength of the astigmatism is given as the difference in refractive power between these two planes. Astigmatism up to 0.5 D is normal. An astigmatism in which the planes with the maximum and minimum refractive power are not perpendicular to one another is referred to as "irregular" astigmatism.
More ametropies
There are a number of other ametropies in which parallel incident light rays are refracted differently not only depending on their plane of incidence, but also depending on their distance from the optical axis. Such ametropia, called high-order aberrations (HOA), occur in almost every eye, but cannot be corrected with spectacle optics. The most important of these aberrations are coma, three-leaf defects (triaxial astigmatism) and spherical aberration. In addition, diseases of the cornea (e.g. keratoconus ), injuries or eye operations (e.g. keratoplasty ) can result in further imaging errors.
Correction of ametropia
Ametropies can mean that the eye cannot achieve its full visual acuity without correction . Glasses , contact lenses and refractive surgery are available for correction . Axial ametropia and regular astigmatisms can be corrected with glasses. Contact lenses complete this spectrum with irregular astigmatisms and suitable cases of other ametropies. Depending on the method used, refractive surgery can correct high axial ametropia and irregular corneal situations under suitable conditions. However, there are numerous limitations here.
Individual evidence
- ^ Wilhelm Gemoll : Greek-German school and hand dictionary. 9th edition, reviewed and expanded by Karl Vretska . Freytag et al., Munich et al. 1965.
