General optics, refractive errors, and its correction. Dr.Ali.A.Taqi. Fifth year students.
The Light• Is that part of the electro-magnetic spectrum to which the human eye is sensitive i.e. the visible part of the electro-magnetic spectrum.• It’s wavelength range is 400-760 nm.• In space, light maintains a constant speed of about 186 000 miles per second, but as it travels through the substance of such a transparent body , it will encounter more resistance , this retards its progress. When a beam of light strikes a glass plate with parallel sides, it is retarded while traversing the plate, and then travels on unaffected.
Electromagnetic spectrumwaves in nm Gamma UV Visible IR Radio & light TV 400-760
Light light travels through space in straight lines. If a ray of light meets a body in its passage through space, one of three things may happen to it: 1-Absorbtion: opaque materials for example black bodies, absorb the light which falls on them;2-Reflection; materials such as mirror surfaces, reflect the light backwards;3-Transmition : transparent materials such as glass, transmit the light; a considerable proportion of it, allowing to pass through them but it’s direction will be changed (Refraction).Many substances combine these effects to some degree.
REFRACTION AT A CURVED SURFACEWhen parallel rays of light strike a spherical surface, each individual ray willbe bent to a different degree and the rays may then all meet at a focus.The distance of this focus from the surface depends on - curvature of the surface - optical density of the two media concerned, - wavelength of the lightRefraction by lenses lens is composed of two smooth curved surfaces that are aligned andenclose a uniform optical medium. The first surface of such a system tends tofocus parallel incident rays and then again at the second surface a furtherfocusing occurs.
Types of spherical/cylindrical lenses:biconvexPlano-convexconvex meniscusBiconcavePlano-concaveconcave meniscusThe formation of these lenses may beunderstood from the figures below…cylindrical lenses: The action of a convexcylinder. Rays of light striking the cylinderperpendicularly to the axis are brought to afocus in the focal line. sum of these individualfoci, will be the focal line. Consequently, if apoint of light is placed in front of the cylinder,no sharp image as a point can be formed on ascreen, but a bright line may be obtained .Conversely in the case of a concave cylinder,rays falling perpendicular to the axis arediverged according to the same principles aswe have discussed in concave lenses.
• THE NOTATION OF LENSES • distance forms a convenient standard by The more a lens is able to refract light the which to measure the refractive power. more powerful we consider it to be. For all • A focal distance of 1 meter is taken as the practical purposes, the power of thin glass unit, and a lens with a focal distance 1 meter lenses is related to the surface curvature. away is spoken of as having a refractive The focal length of a lens, we may recall, is power of 1 dioptre (1 D). Since a stronger the distance from it of the focus which it lens has a greater refractive power, the focal forms of rays of light parallel to the principal distance will be shorter: it therefore follows axis. This that a lens of a refractive power of 2 D will have a focal distance of 0.5 meter, while a lens of 0.5 D will have a focal distance of 2 meters. The strength in diopters is therefore the reciprocal of the focal length expressed in meters.
Refraction by the eye• to generate accurate vision by the eye, light must be correctly focused on the retina. This focus is done by refraction of the light.• The eye is a compound optical system:• The cornea, or actually the air/tear interface is responsible for two-thirds of refractive power of the eye, because of the large difference in index of refraction of both media.• the crystalline lens is responsible for one-third of the focusing (refracting) power of the eye. 9
Optical system of the eyeFig. 1. Optical system of the eye: a, anterior surface of cornea;b, posterior surface of cornea;c, anterior cortex; d, anterior core,e, posterior cortex; f, posterior core; v and g, anterior andposterior poles of the eye through which the optical axis passes; line jh, visual axis.
Visual Acuity• Normal VA is 6/6 or 20/20• the Snellen test is a test of minimum separable acuity, it is the clinically preferred acuity test.• A rating of 6/24 means that a letter that normally should be read at 24 METER has to be brought to within 6 METER before it is recognized by the patient.
Refractive errors• Defective vision is most commonly caused by ametropia (errors of refraction) that is why ,when a patient complaints of a visual problem, it is extremely important to ask the question: Is it caused by a refractive error ?• The use of a simple ―pinhole‖ made in a piece of card will help to determine whether or not there is a refractive error. The vision will improve unless the refractive error is extremely large.
Normal development of refractive state of the human eye.• At birth, the eyeball is relatively • This is gradually reduced until by the age short, having +2 to +3 of 5-7 years the eye is emmetropic and hypermetropia. Most infants are remains so till the age of about 50 years. hyperopic, probably because the axial • After this, there is tendency to develop length of their eyeballs is too short. hypermetropia again,• Consequently, hyperopia decreases • which gradually increases until at the extreme of life the eye has the same +2 to with growth. +3 with which it started.• Emmetropia is considered a point on • This senile hypermetropia is due to the curve of refractive status that changes in the crystalline lens. marks the transition from hyperopia Ametropia to myopia. (a condition of refractive error), is defined as a state of refraction, when the• It occurs when the length of the parallel rays of light coming from infinity eyeball, the curvature of the (with accommodation at rest), are focused cornea, and the power of the either in front or behind the sensitive unaccommodated lens all are layer of retina, in one or both the meridians. appropriate for focusing collimated The ametropia includes light on the retina myopia, hypermetropia and astigmatism. The related conditions aphakia and pseudophakia are also discussed here. 14
ERRORS OF REFRACTION Emmetropia & ammetropia• Emmetropia (normal refractive state) parallel rays of light from a distant object( at infinity) are brought to a focus on the retina when the eye is at rest (not accommodating) so this individual can see sharply in the distance without accommodation.• In ammetropia (abnormal refractive state) , parallel rays of light are not brought to a focus on the retina in an eye at rest. A change in refraction is required to achieve sharp vision. 15
Causes and types of Ametropiameans defective refractive status (refractive error) it is divided as follows:-1-Myopia (short sightedness); the optical (refractive) power of the eye is too high so the parallel rays of light are brought to a focus in front of the retina, (when the eye is at rest). causes: 1-↑ ant-post diameter of the globe= axial myopia 2-↑ curvature of the cornea= curvature myopia 3-↑ refractive index of the lens= index myopia 16
2-Hypermetropia : (long sightedness); the optical power is too low so parallel rays of light converge towards a point behind the retina, (when the eye is at rest).causes: 1-↓ A-P diameter of the globe= axial hypermetropia. 2-↓ curvature of the cornea= curvature hypermetropia. 3-↓ refractive index of the lens= index hypermetropia.3-Astigmatism : the optical power of the cornea in different planes is not equal. Parallel rays of light passing through these different planes are brought to different points of focus. 17
HYPERMETROPIA• Hypermetropia (hyperopia) or long- sightedness is the refractive state of the eye wherein parallel rays of light coming from infinity are focused behind the retina with accommodation being at rest (Fig. 3.22). Thus, the posterior focal point is behind the retina, which therefore receives a blurred image.• Etiology Hypermetropia may be• axial,• curvatural,• index,• positional and• due to absence of lens.(aphakia)• Axial hypermetropia is by far the commonest form. 18
HYPERMETROPIA2. Curvatural hypermetropia is the condition in • Clinical picture• which the curvature of cornea, lens or both is • Symptoms• flatter than the normal • In patients with hypermetropia the symptoms vary depending upon the age of3. Index hypermetropia occurs due to decrease patient and the degree of refractive error. in refractive index of the lens in old age. It These can be grouped as under: may also occur in diabetics under treatment. • 1. Asymptomatic. A small amount of4. Positional hypermetropia results from refractive error in young patients is usually posterior placed crystalline lens. corrected by mild accommodative effort without producing any symptom.5. Absence of crystalline lens either congenitally • 2. Asthenopic symptoms. At times the or acquired (following surgical removal or hypermetropia is fully corrected (thus vision posterior dislocation) leads to aphakia — a is normal) but due to high error or defective condition of high hypermetropia. accommodation with age so Asthenopic symptoms appear and These include: • tiredness of eyes, frontal or fronto-temporal • headache, watering and mild photophobia. These Asthenopic symptoms are especially associated with near work and increase towards evening. 19
HYPERMETROPIA • Treatment • A. Optical treatment. Basic principle3. Defective vision with Asthenopic of treatment is to prescribe convex symptoms. (plus) lenses, so that the light rays areWhen the amount of hypermetropia is brought to focus on the retina (Fig. such that 3.23). Fundamental rules for prescribing glasses in hypermetropiait is not fully corrected by the voluntary include:accommodative efforts, then the patients complain of defective vision which is more for near than distance4. Defective vision only. When the amount ofhypermetropia is very high, the patients usuallydo not accommodate (especially adults) and there occurs marked defective vision for near and distance. 20
HYPERMETROPIA1. Total amount of hypermetropia should Modes of prescription of convex always be discovered by performing lenses refraction under complete cycloplegia. 1. Spectacles are most2. The spherical correction given should be comfortable, safe and easycomfortably acceptable to the patient. method of correcting However, the astigmatism should be fully hypermetropia. corrected.3. Gradually increase the spherical correction 2. Contact lenses are indicated in at 6 months interval till the patient unilateral hypermetropia accepts manifest hypermetropia. (Anisometropia).4. In the presence of accommodative For cosmetic reasons, contact convergent squint, full correction should lenses should be prescribed be given at the first sitting. once the prescription has5. If there is associated amblyopia, full stabilized, otherwise, they may correction with occlusion therapy should have to be changed many a be started. times. 21
APHAKIA• Aphakia literally means absence of Clinical features crystalline lens from the eye. Aphakia Symptoms. produces a high degree of hypermetropia. 1. Defective vision. Main symptom in aphakia is marked defective vision for both far and near due• Causes to1. Congenital absence of lens. It is a rare high hypermetropia and absence of condition. accommodation. 2. Erythropsia and cynopsia i.e., seeing red and2. Surgical aphakia occurring after removal of blue lens is the commonest presentation. images. This occurs due to excessive entry of3. Aphakia due to absorption of lens matter is ultraviolet and infrared rays in the absence of crystalline lens.noticed rarely after trauma in children. Signs of aphakia include:4. Traumatic extrusion of lens from the eye 1. Limbal scar may be seen in surgical aphakia. also constitutes a rare cause of aphakia. 2. Anterior chamber is deeper than normal.5. Posterior dislocation of lens in vitreous 3. Iridodonesis i.e., tremulousness of iris can be demonstrated. causes optical aphakia. 4. Pupil is jet black in color. 5. Purkinjes image test shows only two images (normally four images are seen- Fig. 2.10). 6. Fundus examination shows hypermetropia small disc. 7. Retinoscope reveals high hypermetropia. 22
APHAKIATreatment Contact lenses.Optical principle is to correct the error by convex Advantages of contact lenses overlenses of appropriate power so that the image is • spectacles include:formed on the retina (Fig. 3.23). (i) Less magnification of image.Modalities for correcting aphakia include: (ii) Elimination of aberrations and prismatic effect(1) spectacles, of thick glasses. (2) contact lens, (iii) Wider and better field of vision.(3) intraocular lens, and (iv) Cosmetically more acceptable.(4) refractive corneal surgery. (v) Better suited for uniocular aphakia. Spectacles prescription has been the most Disadvantages of contact lenses are:commonly employed method of correcting (i) more cost; aphakia, (ii) cumbersome to wear, especially in old age andespecially in developing countries. Presently, use in childhood; and of (iii) corneal complications may be associated.aphakic spectacles is decreasing. Roughly, about +10 D with cylindrical lenses for surgically induced astigmatism are required to correct aphakia in previously emmetropic patients 23
APHAKIA Intraocular lens implantation is the • PSEUDOPHAKIA best available • The condition of aphakia whenmethod of correcting aphakia. corrected with an Therefore, it is the • intraocular lens implant (IOL) iscommonest modality used. referred to as pseudophakia.Refractive corneal surgery , It includes:i. Keratophakia. In this procedure a lenticule prepared from the donor cornea is placed between the lamellae of patients cornea.ii. Epikeratophakia. In this procedure, the lenticule prepared from the donor cornea is stitched over the surface of cornea after removing the epithelium.iii. Hyperopic Lasik 24
MYOPIA• Myopia or short- sightedness is a type of refractive• error in which parallel rays of light coming from infinity• are focused in front of the retina when• accommodation is at rest (Fig. 3.24). 25
Etiological classification Clinical varieties of myopia1. Axial myopia results from increase in • 1. Congenital myopia antero-posterior length of the eyeball. It is the commonest form. • 2. Simple or developmental2. Curvatural myopia occurs due to myopia increased curvature of the cornea, lens • 3. Pathological or degenerative or both.3. Positional myopia is produced by anterior myopiaplacement of crystalline lens in the eye. • 4. Acquired myopia which may4. Index myopia results from increase in the be: refractive index of crystalline lens associated with nuclear sclerosis. (i) post-traumatic;5. Myopia due to excessive accommodation (ii) post-keratitic; occurs in patients with spasm of accommodation. (iii) drug-induced, (iv) pseudomyopia; (v) space myopia 26
2. Simple myopiaSimple or developmental myopia is the Clinical picture commonest Symptomsvariety. It is considered as a physiological Poor vision for distance (short-sightedness) is error not the main symptom of myopia.associated with any disease of the eye Asthenopic symptoms may occur in patients with small degree of myopia. Half shutting of the eyes may be complained byEtiology. It results from normal biological variation parents of the child. The child does so to achieve the greater clarity of stenopaeic vision.in the development of eye which may or may not be Signs Prominent eyeballs. The myopic eyes typicallygenetically determined. Some factors associated with are large and somewhat prominent. Anterior chamber is slightly deeper than normal.simple myopia are as follows: Pupils are somewhat large and a bit sluggishly1- Axial type of simple myopia may signify reacting. just a Fundus is normal; rarely temporal myopicphysiological variation in the length of the crescent may be seen. Magnitude of eyeball refractive error. Simple myopia usually occur between 5 and 10 year of age and it keeps onor it may be associated with precocious increasing till about 18-20 years ofneurological growth during childhood. age at a rate of about –0.5 ± 0.30 every year. In2- Curvatural type of simple myopia is simple myopia, usually the error does not exceed considered 6 to 8.to be due to underdevelopment of the eyeball. 27
3. Pathological myopiaPathological/degenerative/progressive myopia, as the name indicates, is a rapidly progressive error which starts in childhood at 5-10 years of age and results inhigh myopia during early adult life which is usually associated with degenerative changes in the eye.Etiology. It is unequivocal that the pathologicalmyopia results from a rapid axial growth of the eyeball which is outside the normal biological variations of development. To explain this spurt in axial growth various theories have been put forward. So far no satisfactory hypothesis has emerged to explain the etiology of pathological myopia. However, it isdefinitely linked with(i) heredity and(ii) general growth process. 28
Treatment of myopia• 1. Optical treatment of myopia constitutes prescription of appropriate concave lenses, so that clear image is formed on the retina (Fig. 3.29). The basic rule of correcting myopia is converseof that in hypermetropia, i.e., the minimum acceptance providing maximum vision should be prescribed.In very high myopia under correctionis always better to avoid the problem of near vision and that of minification of images. 29
Treatment of myopiaModes of prescribing concave lenses are spectacles and contact lenses. Their Low vision aids (LVA) are indicated in advantages and disadvantages over patients each other are the same as described of progressive myopia with advanced for hypermetropia. degenerative changes, where usefulContact lenses are particularly justified vision cannot in cases of high myopia as they avoid be obtained with spectacles and contact lenses. peripheral distortion and minification Prophylaxis (genetic counseling). As the produced by strong concave pathological myopia has a strong genetic spectacle lens. basis,Surgical treatment of myopia is the hereditary transfer of disease may be becoming very popular now-a-days. decreased by advising against marriage between two individuals with progressive myopia. 30
ASTIGMATISM• Astigmatism is a type of Refractive types of regular refractive error wherein astigmatism Depending the refraction varies in upon the position of the the different meridia. two focal lines in Consequently, the rays of relation to retina, the light entering in the eye regular astigmatism is cannot converge to a further point focus but form focal classified into three types: lines. Broadly, there are two types of astigmatism: 1. Simple astigmatism, regular and irregular. simple myopic astigmatism simple hypermetropia astigmatism 31
2. Compound astigmatism Symptomsthe rays of light in both the • Symptoms of regular astigmatism meridia are focused either in include: front or behind the retina and (i) defective vision; the condition is labeled as (ii) blurring of objects; compound myopic or (iii) depending upon the type and compound hypermetropia degree of astigmatism, objects astigmatism, respectively may appear proportionately elongated;3. Mixed astigmatism andthe light rays in one meridian are (iv) asthenopic symptoms, which are focused in front and in other marked especially in small meridian behind the retina amount of astigmatism, consist of a dull ache in the eyes, headache,Thus in one meridian eye is early tiredness of eyes and myopic and in another sometimes nausea and even hypermetropia. drowsiness. 33
Treatment1. Optical treatment of regular astigmatism comprisesthe prescribing appropriate cylindrical lens,discovered after accurate refraction.i. Spectacles with full correction of cylindrical powerand appropriate axis should be used for distance and near vision.ii. Contact lenses. Rigid contact lenses may correct up to 2-3 of regular astigmatism, while soft contact lenses can correct only little astigmatism.2. Surgical correction of astigmatism is quiteeffective. 35
How we Correct Ametropia???what are our choices???• All three types of ametropia can be corrected by wearing spectacle lenses. these diverge the rays in myopia(minus- minifying lens), converge the rays in hypermetropia(plus-magnifying lens) and correct for the non-spherical shape of the cornea in astigmatism(minus or plus cylinder lens).• Spectacle correction of myopia BY SPHERICAL MINUS LENS CORRECTION• This requires a lens at the eye that will diverge collimated light so that it appears to come from the far-point. Such a lens is a minus lens.• Minus lenses cause image minification and ―barrel‖ distortion in addition to prismatic image displacement. 36
Spectacle correction of myopia.Fig. a. Rays from the far point are focused on the retina. b. A negative lens whose second focal point coincides with the far point forms a virtual image of rays from infinity at the far point. c. Rays from the infinity strike the eye with a vergence as if from the far point and are focused on the retina. 37
Spectacle correction of hyperopia BY PLUS SPHERICAL LENS. a. The far point lies behind the eye. Rays converging to the far point lies behind the eye. Rays from the far point are focused on the retina.b. A plus lens focuses rays from infinity at its second focal point, which is coincident with the far point.c. Convergent rays strike the eye and are focused on the retina. 38
ASTIGMATISM & CORRECTION BY CYLINDRICAL LENSES• Cylinders, have a maximum curvature along their circumferential direction and zero curvature along their length, that is, parallel to the cylinder axis. The zero curvature is 90 degrees to the maximum curvature. A cylindrical refracting surface will form a line image of a point parallel to the cylinder axis 39
Irregular Astigmatism• In the previous examples of types of regular astigmatism, the axes were at 90 and 180 degrees. In reality, the axes may be at any meridian. If the maximum and minimum curvatures are 90 degrees apart, the astigmatism is regular—for example, 45 degrees and 135 degrees, or 65 degrees and 155 degrees.• If, however, the two principal meridians of curvature are not 90 degrees apart or the corneal curvature is not axially symmetric, the condition is irregular astigmatism. This may be due to injury, corneal diseases that leave scars, Keratoconus, or congenital abnormalities 43
Anisometropia• is the condition in which the refractive error of one eye differs from the other. It may be characterized by unequal amounts of myopia or hyperopia, or one eye may be myopic and the other hyperopic, to which the special term Anisometropia is applied. Examples of :-What happen to the refractive state of the eye after cataract extraction?!!!The lens provides one-third of the refractive power of the eye so that after cataract extraction (the removal of an opaque lens) the eye is rendered highly hypermetropia, a condition termed aphakia. This can be corrected by; • the implantation of an intraocular lens (IOL) intra- operatively [pseudophakia] ; • contact lenses; • aphakic spectacles (eye glasses). 44
Intraocular lens implantation to correct a phakia. 45
CONTACT LENSESTypes: (rigid, gas permeable & soft hydrophilic CLs).• Retard the diffusion of oxygen to the cornea. Rigid gas permeable lenses are relatively more permeable to oxygen than soft lenses.• Although soft lenses are better tolerated, gas permeable lenses have certain advantages:• their ↑ oxygen permeability ↓the risk of corneal damage (from hypoxia)• their rigidity allows easier cleaning and offers less risk of infection;• their rigidity allows for a more effective correction of astigmatism;• proteinaceous debris is less likely to adhere to the lens and cause an allergic conjunctivitis.• Plane soft contact lenses may also be used as ocular bandages, e.g. in the treatment of some corneal diseases as a persistent epithelial defect.• The optical benefits of contact lenses over spectacle correction in high myopia include: – Minimum change in image size. – the elimination of prismatic object displacement with its attendant ―barrel‖ distortion. – elimination of image degradation caused by the spherical aberration of spectacle lenses with off-axis viewing (coma). 47
PRESBYIOPIA• It is not a real refractive error but failure or weak accommodation power with aging.• The rays of light from closer objects, such as printed page, are divergent, can be seen well only by the process of accommodation, at which the circular ciliary muscle contracts, allowing the naturally elastic lens to be more globular shape = greater converging power, the eyes also converge.• With age the lens gradually hardens and the lens no longer becomes globular, so the accommodation ↓,reaching a critical point after age of40years.• close work becomes gradually more difficult . Objects have to be held away to reduce the need for accommodation. 48
Convex(plus) lenses in the form of reading glasses therefore are neededto converge the light rays from close objects.This occurs earlier in hypermetropes than myopes.The physical part is related to hardening or sclerosis of the crystallinelens that reduces the elasticity of the lens capsule and the plasticity of thelens core.The physiologic part of accommodation is the innervations andcontraction of the ciliary muscles. Some hold that sclerosis of the ciliarybody reduces its ability to constrict, and the lens does not sufficientlyobtain the conditions required for changing its shape
LOW VISION AIDS• Patients with poor vision can be helped by advice on lighting conditions & low vision aids. Devices used include:• magnifiers for near vision;• Telescope for distant vision• Closed circuit TV to provide magnification & improve contrast;• large print books;• talking clocks and watches; etc 50
Refractive Surgery.• Although refractive errors are most commonly corrected by spectacles or contact lenses, laser surgical correction is gaining popularity.• The laser & non laser surgeries either modify the shape of the cornea or do an open eye surgery as in phacic IOL , clear lens extraction.• The excimer laser precisely removes part of the superficial stromal tissue from the cornea to modify its shape. Myopia is corrected by flattening the cornea and hypermetropia by steepening it.• 51
Intrastromal Corneal Ring Segments (INTACS)• The polymethylmetha crylate ring segments
Intrastromal Corneal Ring Segments (INTACS)• Segments are inserted one at a time into the channels.
Intrastromal Corneal Ring Segments (INTACS) Intrastromal corneal ring segments after insertion.
References• Refraction• 1. Duke Elder’s Optics & refraction.• Refractive errors and correction.• 1-Parson’s disease of the eye 2003. 2-Lecture notes on ophthalmology, Bruce James, Chris Chew, ninth edition, Blackwell scientific 2003 3-Atlas of ocular pathology, ocular trauma, on CD.• 2-Clinical ophthalmology Kanski J 2007• 3-ophthalmology.a short textbook.Gerhard.k.Lang.Thieme publications.2000.• 4-comprehensive ophthalmology.A.K.Khuran,fourth edition,2007,new age publishers. 57