astigmatism

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astigmatism

(əstĭg`mətĭz'əm), type of faulty vision caused by a nonuniform curvature in the refractive surfaces—usually the cornea, less frequently the lens—of the eye. As a result, light rays do not all come to a single focal point on the retina. Instead, some focus on the retina while others focus in front of or behind it. The condition may be congenital, or it may result from disease or injury; it can occur in addition to nearsightednessnearsightedness
or myopia,
defect of vision in which far objects appear blurred but near objects are seen clearly. Because the eyeball is too long or the refractive power of the eye's lens is too strong, the image is focused in front of the retina rather than upon it.
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 or farsightednessfarsightedness
or hyperopia,
condition in which far objects can be seen easily but there is difficulty in near vision. It is caused by a defect of refraction in which the image is focused behind the retina of the eye rather than upon it, either because the eyeball is
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. The spherical lenses used to correct nearsightedness and farsightedness must be specially adapted to correct the out-of-focus plane of vision of the astigmatic eye. When the patient observes a pattern of straight lines placed at various angles, those running in one direction appear sharp while those in other directions (particularly at right angles to the sharp lines) appear blurred. A special cylindrical lens is placed in the out-of-focus axis to correct the condition. In many cases contact lenses are the most effective means of correcting astigmatism.
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Astigmatism: astigmatism due to concave mirrorclick for a larger image
Astigmatism: astigmatism due to concave mirror

astigmatism

(ă-stig -nă-tiz-ăm) An aberration of a lens or mirror system that occurs when light falls obliquely on the system and is focused not as a single point image but as two perpendicular and separated lines. In the reflecting system shown in the illustration rays from points A and B on the mirror converge to the vertical line image ab; rays from C and D converge to the horizontal line image cd. The pencil of reflected rays, elliptical in cross section, cannot produce a sharp image anywhere along its path; the plane of optimum focus occurs between ab and cd where the pencil has its smallest cross section. Astigmatism is not as severe an aberration as coma.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

astigmatism

[ə′stig·mə‚tiz·əm]
(electronics)
In an electron-beam tube, a focus defect in which electrons in different axial planes come to focus at different points.
(medicine)
A defect of vision due to irregular curvatures of the refractive surfaces of the eye so that focal points of light are distorted.
(optics)
The failure of an optical system, such as a lens or a mirror, to image a point as a single point; the system images the point on two line segments separated by an interval.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

astigmatism

, astigmia
1. a defect of a lens resulting in the formation of distorted images; caused by the curvature of the lens being different in different planes
2. faulty vision resulting from defective curvature of the cornea or lens of the eye
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
Among measurements, this common thread serves to determine their matrices that can separate regular from irregular astigmatism. Matrices can now be determined for principal meridians of arbitrary location for an astigmatic eye and various medical and engineering fields.
SAI measures differences in corneal power within each ring along the entire corneal surface, and it increases as irregular astigmatism increases and the decentralized cone becomes steeper.
The search equation was (keratoconus [mh] OR KC OR pellucid marginal degeneration OR PMD OR irregular astigmatism) AND (scleral contact lens OR scleral contact lenses OR corneoscleral contact lenses OR RGP OR rigid gas permeable OR rigid contact lens OR soft contact lenses OR silicone hydrogel OR toric soft contact lenses OR piggyback contact lens OR piggyback system).
* Post-treatment increase in ametropia, anisometropia and / or irregular astigmatism can make the patient unhappy even though the cornea is clear and BCVA has improved (5,16) Refractive changes arise due to the location and extent of tissue removal from the cornea.
(1,2) An increase in myopia, irregular astigmatism, higher order ocular aberrations and corneal scarring can lead to reduced visual acuity (VA).
Cases with previous ocular surgery, scars, corneal degenerations, glaucoma and irregular astigmatism were excluded.
The fluid beneath rigid lenses, irrespective of design, will correct anterior corneal asymmetry and irregularity, whether in the form of regular corneal astigmatism or those that cause higher order aberrations and irregular astigmatism. Unlike soft toric lenses the correction does not, as a rule, depend on the orientation of the contact lens so the astigmatism is consistently and reliably corrected.
A recurrent pterygium can be associated with decreased visual acuity due to involvement of visual axis and/or irregular astigmatism, extra ocular motility restriction and Symblepharon formation.
Exclusion Criteria: Patients with previous history of glaucoma, traumatic cataract, corneal pathology, poorly dilating pupils, corneal scarring or degeneration, irregular astigmatism were excluded.
(3) However, there are still occasions in practice when soft toric lenses will not provide the ideal solution, that is irregular astigmatism, keratoconus and other irregularities of corneal topography.
But also less easy to deal with are those with irregular astigmatism. The main challenges are people who have less sphere than cyl in their spectacle prescription (Rx) as lens stability becomes more important because the astigmatism is the major part of what needs correcting.