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Optical Instruments for Viewing Applications
Published in Abdul Al-Azzawi, Photonics, 2017
Another common eye defect, astigmatism, occurs when the cornea is not spherical but is more curved in one plane then in another. As a result, the focal length of the astigmatism eye is different in one plane than in the perpendicular plane. When light rays that lie in one plane are in focus on the retina of an astigmatism eye, those in the perpendicular plane will be in focus either in front or in back of the retina. Astigmatism is corrected by a lens that converges (or diverges) rays in one plane, while not effecting rays in the perpendicular plane. Such a lens is a cylindrical lens, which is curved in one direction but not in the perpendicular direction, as if cut out of a cylinder of glass. More modern techniques, such as laser surgery, are being used to correct common vision defects.
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Published in Splinter Robert, Illustrated Encyclopedia of Applied and Engineering Physics, 2017
[optics] The formation of a focal image of a narrow bundle of light formed by a converging lens generally results in two lines, that are both sharp images, respectively, which are orthogonal to each other in two planes carved out by the optical axis and perpendicular to each other, however, not necessarily in the same location on the optical axes, generally a finite distance apart. The images convolve into an oval shape proceeding down the optical path from the primary to the secondary focal line. The image formation yields a distance between the sharp projections of the two perpendicular lines. The distance between these two points on the optical axis is referred to as the astigmatic difference. There is, however, a circular image formed in between the two respective locations, but his image is not sharply defined but accepted as the focal point. In the eye, astigmatism represents a geometric contraction in one or more directions, resulting in a distorted image on the retina. The distorted image can be corrected by astigmatic lenses that are defined with a cylindrical axis of correction to the normally applied symmetrical concave or convex lens surface (see Figure A.96).
How we see
Published in Karl H.E. Kroemer, Fitting the Human, 2017
Another frequent problem in the aging eye is having cataracts, patterns of cloudiness inside the normally clear lens. When vision is severely impaired, an eye surgeon may remove the cloudy lens and replace it with an artificial implant. Two other vision deficiencies, usually not related to aging, can usually be improved by surgery or lenses: Astigmatism occurs if the cornea is not uniformly curved, so that, depending on its position within the visual field, an object is not sharply focused on the retina. Often, the astigmatism is a spherical aberration, meaning that light rays from an object located at the side are more strongly refracted than those from an object at the center of the field of view, or vice versa.Chromatic aberration is fairly common: An eye may be hyperopic for long waves (red) and myopic for short waves (violet or blue).Color perception
Vecto-keratometry: determination of anterior corneal astigmatism in manual keratometers using power vectors
Published in Expert Review of Medical Devices, 2023
Raquel Salvador-Roger, Rosa Vila-Andrés, Vicente Micó, José J. Esteve-Taboada
Characterization of the anterior corneal surface is an important aspect in the clinical management of refractive data. Since 1619 when the first attempts were made to measure corneal curvature using glass spheres [1], the study of anterior corneal curvature has evolved from some rudimentary methods to today’s modern measurement [2]. Included in this cornea characterization, corneal astigmatism (understood as dioptric power difference between principal meridians of the anterior surface of the cornea) plays a determinant role for many optometric and ophthalmic procedures and methodologies such as total astigmatism determination in a patient’s eye by using Javal’s or Grosvenor’s rules [3,4], intraocular lens power determination, and contact lens fitting processes [5,6]. Thus, measurement of complete and accurate corneal astigmatism is of capital importance in optometry and ophthalmology practice.
Quantification for biomechanical properties of human cornea by using acoustic radiation force optical coherence elastography
Published in Journal of Modern Optics, 2022
Yanzhi Zhao, Yongbo Wang, Yueyuan Xu, Yunjiang Zhang, Hongwei Yang, Xiao Han, Yirui Zhu, Yubao Zhang, Guofu Huang
The cornea, lying in front of the iris and pupil, is a totally transparent avascular connective tissue which provides three-fourths of the total refractive power of the human eye along with the tear film on the ocular surface [1,2]. Most refractive errors, such as myopia, hyperopia and astigmatism can be attributed to the altered biomechanical properties of the cornea [3]. Corneal refractive surgeries such as laser-assisted in-situ keratomileusis (LASIK) and small incision lenticule extraction (SMILE) have gained considerable popularity in recent years, offering a long-term solution to refractive errors. However, these surgeries are always accompanied by the alteration of cornea elasticity between pre- and post-operation, and there is a lack of good characterization of this change [4–7]. In addition to the above-mentioned factors, other ocular diseases like keratoconus and keratomalacia are also associated with the biomechanical properties of cornea [8,9]. Therefore, assessment of cornea elasticity is important for better understanding corneal pathologies, accurate diagnosing the related diseases, as well as improving the safety and efficacy of current refractive surgeries.
Visual fatigue induced by watching virtual reality device and the effect of anisometropia
Published in Ergonomics, 2021
Sang Hyeok Lee, Martha Kim, Hyosun Kim, Choul Yong Park
The study was conducted on healthy applicants aged between 20 and 50. Prior to participating in the study, all applicants underwent an ophthalmic screening examination. Applicants were excluded from the study if they have eye diseases, such as glaucoma or cataracts, that could affect their vision, if they have strabismus with 8 prism dioptres or higher, if the refractive error (spherical equivalent) exceeded −6 dioptres or +6 dioptres, if astigmatism exceeded 2 dioptres, if the best spectacle-corrected distance visual acuity of any eye was lower than 0.6 (Snellen acuity), and if they have undergone a surgery on the cornea or crystalline lens except for past laser refractive surgeries, such as LASIK, PRK, and LASEK. Subjects who passed the screening test and participated in the study underwent surveys on the presence of subjective presbyopia symptoms, prior experience on the use of VR devices, and presence of motion sickness.