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.

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Electromagnetic spectrum

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Visible light400-760

UV IR Radio & TV

waves in nm Gamma

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.4

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Refraction of light.

REFRACTION AT A CURVED SURFACEWhen parallel rays of light strike a spherical surface, each individual ray will be 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 and enclose a uniform optical medium. The first surface of such a system tends to focus parallel incident rays and then again at the second surface a further focusing occurs.

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Types of spherical/cylindrical lenses: biconvexPlano-convexconvex meniscusBiconcavePlano-concaveconcave meniscusThe formation of these lenses may be understood from the figures below…

cylindrical lenses: The action of a convex cylinder. Rays of light striking the cylinder perpendicularly to the axis are brought to a focus in the focal line. sum of these individual foci, will be the focal line. Consequently, if a point of light is placed in front of the cylinder, no sharp image as a point can be formed on a screen, but a bright line may be obtained . Conversely in the case of a concave cylinder, rays falling perpendicular to the axis are diverged according to the same principles as we have discussed in concave lenses.

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• THE NOTATION OF LENSESThe more a lens is able to refract light the more powerful we consider it to be. For all practical purposes, the power of thin glass lenses is related to the surface curvature.The focal length of a lens, we may recall, is the distance from it of the focus which it forms of rays of light parallel to the principal axis. This

• distance forms a convenient standard by which to measure the refractive power.

• A focal distance of 1 meter is taken as the unit, and a lens with a focal distance 1 meter away is spoken of as having a refractive power of 1 dioptre (1 D). Since a stronger lens has a greater refractive power, the focal distance will be shorter: it therefore follows 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.

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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.

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 and posterior poles of the eye through

which the optical axis passes; line jh, visual axis.

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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.

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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.

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Normal development of refractive state of the human eye.

• At birth, the eyeball is relatively short, having +2 to +3 hypermetropia. Most infants are hyperopic, probably because the axial length of their eyeballs is too short.

• Consequently, hyperopia decreases with growth.

• Emmetropia is considered a point on the curve of refractive status that marks the transition from hyperopia to myopia.

• It occurs when the length of the eyeball, the curvature of the cornea, and the power of the unaccommodated lens all are appropriate for focusing collimated light on the retina

• This is gradually reduced until by the age of 5-7 years the eye is emmetropic and remains so till the age of about 50 years.

• After this, there is tendency to develop hypermetropia again,

• which gradually increases until at the extreme of life the eye has the same +2 to +3 with which it started.

• This senile hypermetropia is due to changes in the crystalline lens.

Ametropia (a condition of refractive error), isdefined as a state of refraction, when the parallel

rays of light coming from infinity (with accommodation at rest), are focused either in front or behind the sensitive layer of retina, in one or both the meridians.

The ametropia includes myopia, hypermetropia and astigmatism. The related conditions aphakia and pseudophakia are also discussed here.

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ERRORS OF REFRACTIONEmmetropia & 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.

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Causes and types of Ametropia

means 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

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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.

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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.

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HYPERMETROPIA

2. Curvatural hypermetropia is the condition in

• which the curvature of cornea, lens or both is

• flatter than the normal

3. Index hypermetropia occurs due to decrease in refractive index of the lens in old age. It may also occur in diabetics under treatment.

4. Positional hypermetropia results from posterior placed crystalline lens.

5. Absence of crystalline lens either congenitally or acquired (following surgical removal or posterior dislocation) leads to aphakia — a condition of high hypermetropia.

• Clinical picture• Symptoms• In patients with hypermetropia the symptoms

vary depending upon the age of patient and the degree of refractive error. These can be grouped as under:

• 1. Asymptomatic. A small amount of refractive error in young patients is usually corrected by mild accommodative effort without producing any symptom.

• 2. Asthenopic symptoms. At times the hypermetropia is fully corrected (thus vision is normal) but due to high error or defective 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.

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HYPERMETROPIA

3. Defective vision with Asthenopic symptoms.

When the amount of hypermetropia is such that

it is not fully corrected by the voluntaryaccommodative efforts, then the patients

complain of defective vision which is more for near than distance

4. 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.

• Treatment• A. Optical treatment.

Basic principle of treatment is to prescribe convex (plus) lenses, so that the light rays are brought to focus on the retina (Fig. 3.23). Fundamental rules for prescribing glasses in hypermetropia include:

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HYPERMETROPIA

1. Total amount of hypermetropia should always be discovered by performing refraction under complete cycloplegia.

2. The spherical correction given should becomfortably acceptable to the patient.

However, the astigmatism should be fully corrected.

3. Gradually increase the spherical correction at 6 months interval till the patient accepts manifest hypermetropia.

4. In the presence of accommodative convergent squint, full correction should be given at the first sitting.

5. If there is associated amblyopia, full correction with occlusion therapy should be started.

Modes of prescription of convex lenses

1. Spectacles are most comfortable, safe and easy method of correcting hypermetropia.

2. Contact lenses are indicated in unilateral hypermetropia (Anisometropia).

For cosmetic reasons, contact lenses should be prescribed once the prescription has stabilized, otherwise, they may have to be changed many a times.

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APHAKIA• Aphakia literally means absence of

crystalline lens from the eye. Aphakia produces a high degree of hypermetropia.

• Causes1. Congenital absence of lens. It is a rare

condition.2. Surgical aphakia occurring after removal of

lens is the commonest presentation.3. Aphakia due to absorption of lens matter isnoticed rarely after trauma in children.4. Traumatic extrusion of lens from the eye

also constitutes a rare cause of aphakia.5. Posterior dislocation of lens in vitreous

causes optical aphakia.

Clinical featuresSymptoms.1. Defective vision. Main symptom in aphakia ismarked defective vision for both far and near due tohigh hypermetropia and absence of accommodation.2. Erythropsia and cynopsia i.e., seeing red and blueimages. This occurs due to excessive entry ofultraviolet and infrared rays in the absence of

crystalline lens.Signs of aphakia include:1. Limbal scar may be seen in surgical aphakia.2. Anterior chamber is deeper than normal.3. Iridodonesis i.e., tremulousness of iris can be

demonstrated.4. Pupil is jet black in color.5. Purkinje's 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.

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APHAKIATreatmentOptical principle is to correct the error by convexlenses of appropriate power so that the image isformed on the retina (Fig. 3.23).Modalities for correcting aphakia include: (1) spectacles, (2) contact lens,(3) intraocular lens, and(4) refractive corneal surgery. Spectacles prescription has been the mostcommonly employed method of correcting

aphakia,especially in developing countries. Presently, use

ofaphakic spectacles is decreasing. Roughly, about

+10 D with cylindrical lenses for surgically induced astigmatism are required to correct aphakia in previously emmetropic patients

Contact lenses. Advantages of contact lenses over• spectacles include: (i) Less magnification of image.(ii) Elimination of aberrations and

prismatic effect of thick glasses. (iii) Wider and better field of vision.(iv) Cosmetically more acceptable. (v) Better suited for uniocular aphakia.Disadvantages of contact lenses are: (i) more cost;(ii) cumbersome to wear, especially in old

age and in childhood; and (iii) corneal complications may be

associated.

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APHAKIA Intraocular lens implantation is the best

availablemethod of correcting aphakia. Therefore, it

is thecommonest modality used.

Refractive corneal surgery , It includes:i. Keratophakia. In this procedure a

lenticule prepared from the donor cornea is placed between the lamellae of patient's 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

• PSEUDOPHAKIA• The condition of

aphakia when corrected with an

• intraocular lens implant (IOL) is referred to as pseudophakia.

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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).

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Etiological classification

1. Axial myopia results from increase in antero-posterior length of the eyeball. It is the commonest form.

2. Curvatural myopia occurs due to increased curvature of the cornea, lens or both.

3. Positional myopia is produced by anteriorplacement of crystalline lens in the eye.4. Index myopia results from increase in the

refractive index of crystalline lens associated with nuclear sclerosis.

5. Myopia due to excessive accommodation occurs in patients with spasm of accommodation.

Clinical varieties of myopia

• 1. Congenital myopia• 2. Simple or developmental

myopia• 3. Pathological or degenerative

myopia• 4. Acquired myopia which may be: (i) post-traumatic;(ii) post-keratitic; (iii) drug-induced, (iv) pseudomyopia; (v) space myopia

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2. Simple myopiaSimple or developmental myopia is the

commonestvariety. It is considered as a physiological error

notassociated with any disease of the eye

Etiology. It results from normal biological variation

in the development of eye which may or may not be

genetically determined. Some factors associated with

simple myopia are as follows:1- Axial type of simple myopia may signify just

aphysiological variation in the length of the

eyeballor it may be associated with precociousneurological growth during childhood.2- Curvatural type of simple myopia is

consideredto be due to underdevelopment of the eyeball.

Clinical pictureSymptomsPoor vision for distance (short-sightedness) isthe main symptom of myopia. Asthenopic symptoms may occur in patients withsmall degree of myopia. Half shutting of the eyes may be complained byparents of the child. The child does so to achievethe greater clarity of stenopaeic vision.Signs Prominent eyeballs. The myopic eyes typicallyare large and somewhat prominent. Anterior chamber is slightly deeper than normal. Pupils are somewhat large and a bit sluggishly

reacting. Fundus is normal; rarely temporal myopic crescent

may be seen. Magnitude of refractive error. Simple myopia usually occur between 5 and 10 year of age and it keeps on increasing till about 18-20 years of

age at a rate of about –0.5 ± 0.30 every year. Insimple myopia, usually the error does not exceed6 to 8.

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3. Pathological myopia

Pathological/degenerative/progressive myopia, as the name indicates, is a rapidly progressive error which starts in childhood at 5-10 years of age and results in

high myopia during early adult life which is usually associated with degenerative changes in the eye.

Etiology. It is unequivocal that the pathological

myopia 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 is

definitely linked with (i) heredity and (ii) general growth process.

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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 converse

of 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.

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Treatment of myopiaModes of prescribing concave lenses are

spectacles and contact lenses. Their advantages and disadvantages over each other are the same as described for hypermetropia.

Contact lenses are particularly justified in cases of high myopia as they avoid peripheral distortion and minification produced by strong concave spectacle lens.

Surgical treatment of myopia is becoming very popular now-a-days.

Low vision aids (LVA) are indicated in patients

of progressive myopia with advanceddegenerative changes, where useful

vision cannotbe obtained with spectacles and contact

lenses.Prophylaxis (genetic counseling). As thepathological myopia has a strong genetic

basis,the hereditary transfer of disease may

bedecreased by advising against marriage

betweentwo individuals with progressive myopia.

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ASTIGMATISM

• Astigmatism is a type of refractive error wherein the refraction varies in the different meridia. Consequently, the rays of light entering in the eye cannot converge to a point focus but form focal lines. Broadly, there are two types of astigmatism: regular and irregular.

Refractive types of regular astigmatism Depending upon the position of the two focal lines in

relation to retina, the regular astigmatism is further

classified into three types:1. Simple astigmatism,simple myopic astigmatismsimple hypermetropia

astigmatism

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2. Compound astigmatism

the rays of light in both the meridia are focused either in front or behind the retina and the condition is labeled as compound myopic or compound hypermetropia astigmatism, respectively

3. Mixed astigmatism the light rays in one meridian are

focused in front and in other meridian behind the retina

Thus in one meridian eye is myopic and in another hypermetropia.

Symptoms• Symptoms of regular astigmatism

include: (i) defective vision; (ii) blurring of objects; (iii) depending upon the type and

degree of astigmatism, objects may

appear proportionately elongated; and

(iv) asthenopic symptoms, which are marked especially in small amount of astigmatism, consist of a dull ache in the eyes, headache, early tiredness of eyes and sometimes nausea and even drowsiness.

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Treatment

1. Optical treatment of regular astigmatism comprises

the 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 quite

effective.

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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.

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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.

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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.

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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

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TYPES OF REGULAR ASTIGMATISM

1-Simple hyperopic astigmatism.

2-Simple myopic astigmatism 3-compound myopic

astigmatism4- compound hypermetropic

astigmatism5-mixed astigmatism

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ASTIGMATISM• Corneal topography demonstrates with-the-rule astigmatism. The purple lines

drawn suggest the pattern for Limbal relaxing incisions.

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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

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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).

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Intraocular lens implantation to correct a phakia.

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IOL implantation

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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).

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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.

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Convex(plus) lenses in the form of reading glasses therefore are needed

to 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 crystalline

lens that reduces the elasticity of the lens capsule and the plasticity of the

lens core.

The physiologic part of accommodation is the innervations and

contraction of the ciliary muscles. Some hold that sclerosis of the ciliary

body reduces its ability to constrict, and the lens does not sufficiently

obtain the conditions required for changing its shape

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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

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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.

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Intrastromal Corneal Ring Segments (INTACS)

• The polymethylmethacrylate ring segments

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Intrastromal Corneal Ring Segments (INTACS)

• Segments are inserted one at a time into the channels.

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Intrastromal Corneal Ring Segments (INTACS)

Intrastromal corneal ring segments after insertion.

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Laser refractive surgery

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Thank you.

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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 20033-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.