Physics
Defects of Vision
Defects of vision refer to conditions that affect the ability of the eye to focus light properly, resulting in blurred or distorted vision. Common defects include myopia (nearsightedness), hyperopia (farsightedness), astigmatism, and presbyopia. These conditions can be corrected with eyeglasses, contact lenses, or refractive surgery.
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10 Key excerpts on "Defects of Vision"
eBook - PDF- Darlene A. Dartt, Peter Bex, Patricia D’Amore, Reza Dana, Linda Mcloon, Jerry Niederkorn, Patricia D'Amore(Authors)
- 2011(Publication Date)
- Academic Press(Publisher)
London: Butterworth-Heinemann. Read, S. A., Collins, M. J., and Carney, L. G. (2007). A review of astigmatism and its possible genesis. Clinical and Experimental Optometry 90: 5–19. 516 Visual Acuity Related to the Cornea and Its Disorders Myopia F A Vera-Diaz, Schepens Eye Research Institute, Harvard Medical School, Boston, MA, USA ã 2010 Elsevier Ltd. All rights reserved. Glossary Accommodation – The changes in optical power by the eye in order to maintain a clear image (focus) as objects are moved closer. This occurs through a process of ciliary muscle contraction and zonular relaxation that causes the elastic-like lens to round up and increase its optical power. Natural loss of accommodation with increasing age is called presbyopia. Astigmatism – An optical defect in which refractive power is not uniform in all directions (meridians). Light rays entering the eye are bent unequally by different meridians that prevent formation of a sharp image focus on the retina. Choroidal neovascularization – The creation of new, weak and leaky, blood vessels in the choroid layer of the eye. It is a common symptom of wet age-related macular degeneration and of pathological myopia. Glaucoma – The neuropathy that affects the optic nerve of the eye and involves loss of retinal ganglion cells in a characteristic pattern causing peripheral visual-field defects. Retinal detachment – A disorder of the eye in which some layers of the retina peel away from its underlying layers of support tissue. It is a medical emergency and if untreated it may cause partial or total vision loss. Definition of Myopia: Health and Economic Implications Myopia, also called near-or shortsightedness, refers to the refractive state of the eye whereby the images of distant objects are focused in front of the retina when the accommodation system is relaxed.
eBook - ePub- R.A. Edwards(Author)
- 2014(Publication Date)
- Pergamon(Publisher)
CHAPTER 22The Eye. Defects of Vision and Optical Instruments
Publisher Summary
This chapter focuses on various experimental results related to Defects of Vision and optical instruments. Image formation, in a good eye, is achieved and perfected by the action of the eye as a whole, namely, cornea, lens, aqueous, and vitreous humors. The angle subtended at the eye lens by the object is the same as that which is subtended by the image at the lens. If the eyeball is of such a length from lens to retina that parallel light from infinity is focused in front of the retina when the ciliary muscles are completely relaxed, short sight, or myopia results. Long sight or hypermetropia is a condition in which the effective power of the eye lens is too small in relation to the length so that when there is no accommodation, parallel light from infinity is focused behind the retina. Presbyopia is the loss of power of accommodation, which is normally associated with the advancing age of an individual. A photographic enlarger is a slightly less elaborate device to the projector. The photographic negative is used in place of a slide and the light-sensitive paper on which the print is to be made takes the place of the screen.22.1 Structure of the Eye
The human eye (Fig. 22.1 ) contains a converging lens L of a gelatinous, transparent material which is not of uniform refractive index throughout and the surfaces of which have different curvatures. The power of the lens is controlled by the ciliary muscles M which act in order to increase the curvature of the lens surfaces. When these muscles are fully relaxed the eye is said to be unaccommodated and it is adjusted for viewing objects at a great distance. When the muscles are fully tensed the eye is fully accommodated for close vision. The point closest to the eye at which an object may be focused clearly when the eye is fully accommodated is called the near point. The distance from the eye of the near point varies considerably from one person to another but is often regarded as being at about 25 cm. The far point is at the greatest distance for which vision is clear when the ciliary muscles are completely relaxed. Ideally this point should be at infinity. The hard, opaque coating of the eyeball, called the sclerotic S , becomes transparent at the front of the eye in order that light may enter. This transparent “window” is called the cornea C . In fact most of the refraction of the light occurs at the cornea, the lens being employed largely for accommodation. Behind the cornea and in front of the lens is the aperture through which light is permitted to enter the lens. This is the pupil P , the size of which is adjusted by a diaphragm called the iris I , which is the familiar coloured part of the eye. The pupil has its largest diameter when illumination is poor, and vice versa. The space A between the lens and the cornea is filled with a salt solution called the aqueous humour . The front of the cornea is also moistened with salt solution and the action of blinking with the eyelids keeps the surface of the cornea clean. The inner wall of the sclerotic at the back of the eye forms the light-sensitive surface R on which the light entering the eye is focused to form images. This surface is called the retina and is covered with light-sensitive cells situated at the ends of nerve fibres which form a network over the surface of the retina and which all leave the the eyeball in a bundle known as the optic nerve O . This leads to the brain which interprets the image on the retina resulting in “sight” or vision. The point at which the optic nerve leaves the eyeball is insensitive to light and is called the blind spot B . In contrast to this is the fovea , or yellow spot Y , at which the retina is most sensitive. This point lies at the intersection of the principal axis of the lens and the retina. The space V between the retina and the lens is filled with a jelly-like substance called the vitreous humour
eBook - PDF- Gerhard K. Lang(Author)
- 2015(Publication Date)
- Thieme(Publisher)
Such exercises cannot influence the sharpness of the ret -inal image; they can only seemingly improve uncorrected visual acuity by training the patient to make better use of additional visual information. However, after puberty no late sequelae of chron-ically uncorrected vision are to be expected. 16.3.2 Hyperopia (Farsightedness) Definition In hyperopia, there is a discrepancy between the refractive power and axial length of the eye such that parallel incident light rays converge at a focal point posterior to the retina ( ▶ Fig. 16.10a). ▶ Epidemiology. Approximately 20% of persons between the ages of 20 and 30 years have refraction exceeding +1 diopter. Most newborns exhibit slight hyperopia ( newborn hyperopia ). This decreases during the first few years of life. In advanced age, refraction tends to shift toward the myopic side due to sclerosing of the nucleus of the lens. ▶ Etiology. The mechanisms that coordinate the development of the eyeball so as to produce optic media of a given refractive power are not yet fully understood. ▶ Pathophysiology. In farsighted patients, the vir-tual far point of the eye lies posterior to the retina ( ▶ Fig. 16.10b). Only convergent incident light rays can be focused on the retina ( ▶ Fig. 16.10b). This is due either to an excessively short globe with normal refractive power ( axial hyperopia ; ▶ Fig. 16.10d) or, less frequently, to insufficient refractive power in a normal-length globe ( refractive hyperopia ; ▶ Fig. 16.10e). Axial hyperopia is usually congenital and is characterized by a shallow anterior chamber with a thick sclera and well developed ciliary mus-cle. ! Note Hyperopic eyes are predisposed to acute angle closure glaucoma because of their shallow ante-rior chamber. This can be provoked by diagnostic and therapeutic mydriasis. ▶ Special forms of refractive hyperopia • Absence of the lens (aphakia) due to dislocation.
eBook - PDFOphthalmology
A Pocket Textbook Atlas
- Gerhard Lang(Author)
- 2006(Publication Date)
- Thieme(Publisher)
There is also the possibility of implanting an anterior chamber intraocular lens (diverging lens) anterior to the natural lens to reduce refrac-tive power. See Chapter 5 for additional surgical options. Popular health books describe exercises that can allegedly treat refractive errors such as nearsightedness without eyeglasses or contact lenses. Such exercises cannot influence the sharpness of the retinal image; they can only seemingly improve uncorrected visual acuity by training the patient to make better use of additional visual information. However, after puberty no late sequelae of chronically uncorrected vision are to be expected. Hyperopia (Farsightedness) Definition: In hyperopia, there is a discrepancy between the refractive power and axial length of the eye such that parallel incident light rays converge at a focal point posterior to the retina (Fig. 16. 10 a ). Epidemiology. Approximately 20% of persons between the ages of 20 and 30 have refraction exceeding + 1 diopters. Most newborns exhibit slight hyperopia ( newborn hyperopia ). This decreases during the first few years of life. In advanced age, refraction tends to shift toward the myopic side due to sclerosing of the nucleus of the lens. 16 Optics and Refractive Errors 449 Refraction in hyperopia Fig. 16. 10 a The focal point of parallel light rays entering the eye lies posterior to the retina. b Divergent light rays are focused on the retina. The virtual far point lies posterior to the eye (dashed line). c To bring the focal point onto the retina, a far-sighted person has to accommodate even when gazing into the distance. d Axial hy-peropia. The refractive power is normal, but the globe is too short (more common). e Refractive hyperopia. The globe has a normal length, but the refractive power is in-sufficient (less common). f A special form of refractive hyperopia is aphakia (absence of the lens).
eBook - PDFPhysics of the Body
Revised 2nd Edition
- John R. Cameron, James G. Skofronick, Roderick M. Grant(Authors)
- 2017(Publication Date)
- Medical Physics Publishing(Publisher)
Ametropia affects over half of the population of the United States. It is often possible to correct it completely with glasses or by the use of laser surgery to change the shape of the cornea. There are four general types of ametropia: myopia (near-sightedness), hyperopia or hypermetropia (far-sighted- ness), astigmatism (asymmetrical focusing), and presbyopia (old sight) or lack of accommodation. Figure 12.23 illustrates these conditions schematically and shows the regions where blurring occurs. For each eye, we define the near point as the closest distance at which it can see clearly; the far point is the greatest dis- 1/F = 1/P + 1/Q (12.1) (12.2) D far = 1/F far = 1/ + 1/Q = 0 + 1/0.02 m = 50 D (12.3) D near = 1/F near = (1/0.25) + (1/0.02) = 4 + 50 = 54 D (12.4) necessary accommodation = D far = (54 D) (50 D) = 4 D 295 Physics of the Eyes and Vision Table 12.2 A summary of various focusing problems and their characteristics Focusing Problem Common Name Usual Cause Corrected With* Myopia Near-sightedness Long eyeball or cornea too curved Negative lens Hyperopia Far-sightedness Short eyeball or cornea not curved enough Positive lens Astigmatism – Unequal curvature or cornea Cylindrical lens or hard contact lens Presbyopia Old-age vision Lack of accommodation Bifocals or trifocals *All except presbyopia can be corrected with laser surgery. Figure 12.23 Schematic of normal and defective focusing. Wavy lines indicate a blurred image on the retina. Physics of the Body 296 tance at which it has good vision. The various focusing problems and their char- acteristics are summarized in Table 12.2. The myopic individual usually has too long an eyeball or too much curvature of the cornea; distant objects come to a focus in front of the retina, and the rays diverge to cause a blurred image at the retina (Figure 12.24b). This condition is easily corrected with a negative lens. A hyperopic eye has a near point further Figure 12.24 Focusing properties of the eye.
eBook - PDFDoors to Hidden Worlds
The Power of Visualization in Science, Media, and Art
- Alfred Vendl, Martina R. Fröschl, Alfred Vendl, Martina R. Fröschl(Authors)
- 2023(Publication Date)
- De Gruyter(Publisher)
Our results showed that the distances between signs prescribed by norms and stand- ards at the time did not sufficiently consider people with vision impairments. Challenge 2: Eye Diseases Cause Multiple Symptoms The WHO estimates that about 2.2 billion people worldwide are affected by vision impairments. These impairments include presbyopia (1.8 billion) and other refractive errors such as myopia and hypermetropia (123.7 million), cataracts (65.2 million), AMD (10.4 million), glaucoma (6.9 million), corneal opacities (4.2 million), diabetic retinopathy (3 million), trachoma (2 million), and other eye diseases.31 Some impairments, such as refractive errors, are very common and, in most cases, are easy to correct with glasses or contact lenses. Other eye diseases, such as AMD, have a sustained impact on visual function and can lead to central vision loss.32 While highly treatable, cataracts are one of the leading causes of vision impairment (33 percent) after refractive errors (43 percent) and are, at 51 percent, the leading cause of blindness.33 All these vision impairments and eye diseases have one thing in common: they have a negative impact on a person’s vision — but in very different ways. Refractive Errors Refractive errors create blurry vision for affected people and are the major cause of vision impairments worldwide.34 The most common reason for this blurry vision is an increase or decrease in the axial length of the eye. This condition is called myopia (also known as nearsightedness or shortsightedness) if the eye grows too long or hyperopia (farsightedness or longsightedness) if the eye grows too short from front to back.35 Myopia causes blurry vision due to images being focused at a point in front of the retina, which creates a blurred image on the retina.
- Kevin Hayes(Author)
- 2020(Publication Date)
- Omnigraphics(Publisher)
Discuss this matter with your eye-care professional. Refractive surgery aims to change the shape of the cornea per-manently. This change in eye shape restores the focusing power of the eye by allowing the light rays to focus precisely on the retina for improved vision. There are many types of refractive surgeries. Your eye-care professional can help you decide if surgery is an option for you. HYPEROPIA What Is Hyperopia? Hyperopia, also known as “farsightedness,” is a common type of refractive error where distant objects may be seen more clearly than objects that are near. However, people experience farsightedness differently. Some people may not notice any problems with their vision, especially when they are young. For people with significant farsightedness, vision can be blurry for objects at any distance, near or far. 106 | Visual Impairment and Rehabilitation Sourcebook, First Edition What Is Refraction? Refraction is the bending of light as it passes through one object to another. Vision occurs when light rays are bent (refracted) as they pass through the cornea and the lens. The light is then focused on the retina. The retina converts the light rays into messages that are sent through the optic nerve to the brain. The brain interprets these messages into the images we see. What Are Refractive Errors? In refractive errors, the shape of the eye prevents light from focusing on the retina. The length of the eyeball (longer or shorter), changes in the shape of the cornea, or aging of the lens can all cause refractive errors. How Does Hyperopia Develop? Hyperopia develops in eyes that focus images behind the retina instead of on the retina, which can result in blurred vision. This occurs when the eyeball is too short, which prevents incoming light from focusing directly on the retina. It may also be caused by an abnormal shape of the cornea or lens. Who Is at Risk for Hyperopia? Hyperopia can affect both children and adults.
eBook - PDF- Vernon H. Smith, F. E. James(Authors)
- 2014(Publication Date)
- Butterworth-Heinemann(Publisher)
36 Eyes and Education by a moat of blindness, outside which is another circle of vision that extends to the periphery. This type of defect is often associated with night blindness and is caused by a rare progressive hereditary condition called Retinitis Pigmentosa. (ii) Common causes of visual field defects in schoolchildren. The causes of visual field defects are many and varied. We have mentioned Retinitis Pigmentosa already. This disease is often present in several members of one family, and unfortunately there may be very little that can be done to arrest its progress. It is unusual for it to cause severe incapacity before the age of ten or eleven, but it could be well advanced by the time the pupil is taking his 'O' or 'A' levels. The macular or central scotoma is unfortunately more common. Some types of infection, which may even be present at birth, affect the central part of the retina in one eye (if both eyes are affected it is extremely unlikely that the child will see well enough to attend a normal school) and sometimes a severe blow on the eye can cause damage that destroys central vision. Unhappily also not uncommon in children is the game of looking at the sun—sometimes through binoculars or a telescope. This cannot be con-demned too strongly, because the rays of the sun are concentrated on the macula and produce a solar burn which has a disastrous effect on central vision producing a severe central scotoma. Defects affecting the peripheral visual field are perhaps more common than scotomata. Sometimes they are due to congenital malformation of the eye. A child born with a Keyhole pupil—the so-called coloboma of the iris may have a similar defect in his retina, and as the deficiency in the iris is usually below, he will have a visual field defect in the upper part of his visual field. Children are also sometimes born with a defect in one half of their visual field.
eBook - PDF- José Alonso, José A. Gómez-Pedrero, Juan A. Quiroga(Authors)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
5 The Lens-Eye System 5.1 Introduction The eyes are optical systems whose function is the formation of real images of the environ- ment that the brain can detect, process, and interpret as visual information. Like any optical system, natural or artificial, eyes cannot produce perfect images; they have aberrations, the most common of which are spherical defocus and astigmatism, both known as refractive errors. Defocus and astigmatism are also known as second-order aberrations. They are nothing but errors of curvature of the wavefront refracted through the eye, and we already know that curvature is determined by the quadratic – second order – terms of the polynomial expansion used to describe the wavefronts. As eyes are biological organs, the percentage of individuals presenting significant second-order aberrations is known as the prevalence of refractive errors. Although prevalence is a clinical term typically used to measure the extent a disease affects a population, refractive errors are not diseases. An eye affected by refractive error can be perfectly healthy; it simply has a mismatch between its power and its size. Refractive errors can be corrected by surgery, not free from risks, that changes the curvature of the cornea, henceforth changing the power of the eye. But still today a majority of refractive errors are compensated with lenses that, once annexed to the eye, create a new optical system, the lens-eye system, that should be free from second-order aberrations and should produce a sharp image on the retina. There are basically three types of these ophthalmic compensations: spectacle lenses, contact lenses, and intraocular lenses (IOL). Probably, the reader already has a general idea about the differences between the three types of lenses and may think that they mainly differ in their location with respect to the eye.
eBook - PDFAstigmatism
Optics, Physiology and Management
- Michael Goggin(Author)
- 2012(Publication Date)
- IntechOpen(Publisher)
Fig. 8. Two examples of retinal images having basic (sharp) retinal images of the same size (b) but different degrees of blurring (DB) due to the different sizes of the discs of least confusion. Astigmatism – Optics, Physiology and Management 42 Fig. 9. Degree of blurring (DB) for far (3 m) and near (40 cm) objects as a function of the astigmatism (A) (D: dioptres). Equations 3 and 6 show that, for a given entrance pupil size, the size of the DLC corresponding to a certain amount of astigmatism is only approximately half the size of the DLC generated by the same amount of spherical ametropia. Therefore, patients with astigmatism and the DLC focused on the retina theoretically have better vision than those with the same amount of spherical ametropia. This paraxial approximation supports the fact that a visual acuity of 6/18 was traditionally thought to indicate spherical ametropia of about 1.00 D or astigmatism of approximately 2.00 DC, provided that the DLC is focused on the retina (Rabbets, 2007). However, Section 3.3 will show that when all aberrations of the eye are considered—rather than the paraxial approximation—the difference in vision between spherical ametropia and astigmatism of the same amount is in fact much lower. Finally, in eyes with myopic astigmatism, far vision cannot be improved by accommodation, so the patient would be expected to have vision similar to that of an eye with spherical ametropia equal to the corresponding spherical equivalent. 3.2 Lens rotation and mismatches in the cylinder This section analyses the effects of lens rotation or mismatches in the cylindrical power of the lens used to correct astigmatism. It should be noted that the rotation of a cylindrical lens with respect to the eye results in a residual refraction consisting of a cylinder (C) and a sphere (S) due to the combination of two obliquely crossed cylinders.
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