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The Human Eye and Its Aberrations
Published in Daniel Malacara-Hernández, Brian J. Thompson, Advanced Optical Instruments and Techniques, 2017
Defocus of the human eye is termed spherical refractive error. Defocus accounts for the bulk of the loss in visual performance of the eye. If the wavefront error W(r,θ) in the presence of defocus is given by W(r,θ)=W020r2, where (r,θ) are the polar coordinates in the exit pupil, then the spherical refractive error, φs, of the eye is given by φs=−2000W020, where φs is in units of diopters for W020 in units of mm−1. For W020>0, the spherical refractive error is negative and the eye is said to be myopic. In this case, a real object point in front of the eye is conjugate to the retina. For W020<0, the spherical refractive error is positive and the eye is said to be hyperopic. In this case, a virtual object point lying behind the eye is conjugate to the retina. Finally, an eye where the retina is conjugate to infinity is described as emmetropic.
Introduction to Quantum Mechanics
Published in Caroline Desgranges, Jerome Delhommelle, A Mole of Chemistry, 2020
Caroline Desgranges, Jerome Delhommelle
Let us add that refraction (or refractometry) is also the name of a clinical test during an eye exam. The idea is to determine if the eye has a refractive error, and if so, what best corrective lenses should be prescribed. In practice, several test lenses with different optical powers are presented to the patient. The one giving the sharpest, clearest, vision is then selected, and the refractive error can be determined. For example, an eye that has no refraction error when viewing a distant object 6 meters or 20 feet away is said to have emmetropia (Greek: en- (in) + metron (measure) + -ops (eye)). It means that the eye can focus parallel rays of light in the retina without using any accommodation. Therefore, one will score “6/6” (meters) or “20/20” (feet). For example, a person with “20/80” vision can only see clearly an object at 20 feet whereas others who do not need corrective lenses can see at 80 feet, meaning that this person is nearsighted or has myopia. The purpose of the corrective lenses is to compensate for refractive errors of the eyes including myopia, hyperopia and astigmatism (see Figure 3.2). The concept of corrective lenses is to help focus the ray of light on the retina, thus allowing a person to have a clear image of the object. More specifically, for an eye exhibiting myopia, the eye focuses the ray of light before it actually reaches the retina, giving a blurred image. Therefore, the corrective lens should be concave in order to make the ray diverge, allowing the eye to focus exactly on the retina. For hyperopia, the eye focuses beyond the retina surface. To compensate for that, the corrective lens should be convex in order to make the ray converge, again helping the eye to focus on the retina. In practice, the corrective lenses are characterized by their optical power measured in diopter units (D) with 1 D = 1 m–1. For example, –2.0 D corresponds to a power for the corrective lens of 2 m–1 meaning that it creates a focal point at 0.5 m or 50 cm. The negative sign indicates that the focal point is located 50 cm before the eye, implying that this prescription is intended to correct myopia. On the other hand, there is no negative sign in the case of corrective lenses for hyperopia, since the focal point should be after the eye. But the story does not end there. How about distance and depth perception? 3D vision is achieved through our ability to have binocular vision (two eyes). Both eyes help us see an image from two different points and angles. Then, the brain processes the information from optic nerves and uses different techniques to assess distance and depth perception. For example, convergence and accommodation are two different techniques the brain uses to estimate distances, based on efforts made by eye muscles. For instance, an object close to us requires more effort than the accommodation needed to see an object further away. More information can be obtained through parallax and geometry pattern recognition, which we acquire as we grow up. This gigantic database is then used by the brain to apprehend the world surrounding us, in 3D. Let us not forget that 33% of our cerebral cortex plays a role in processing vision!
Effect of Correlated Color Temperature and S/P-ratio of LED Light Sources on Reaction Time in Off-axis Vision and Mesopic Lighting Levels
Published in LEUKOS, 2023
E. G. Vicente, B. M. Matesanz, M. Rodríguez-Rosa, A. M. Sáez, S. Mar, I. Arranz
Eleven young subjects (24 (9.3), mean and standard deviation; 2 males and 9 females) with no history of ocular disease were enrolled in this study and underwent a complete ocular examination, including long-distance refraction, color test assessment, examination of anterior and posterior segments with slit lamp and fundoscopy, respectively. Exclusion criteria were myopia greater than 6.00 diopters (D), hyperopia greater than 4.00 D and astigmatism greater than 3.00 D; as well as color-vision defects or the presence of ocular pathology. All subjects, excluding the emmetropic ones, were compensated with a contact lens with the refraction that provides the highest monocular visual acuity. The monocular visual acuities ranged from 0.00 to −0.16 logMAR (Minimum Angle of Resolution).
Vision though afocal instruments: generalized magnification and eye-instrument interaction
Published in Journal of Modern Optics, 2018
William F. Harris, Tanya Evans
Only the top block row is needed below. Because of the top-left entry, , the compound system of afocal instrument G and emmetropic eye E reduces an incident pencil of parallel rays to a point focus on the retina; GE behaves as an emmetropic ‘super eye’ as it were.