REFRACTIVE ERRORS SPS Y2-2012

WHAT ARE WE GOING TO DO TODAY? Illustrate how light is brought into focus on the retina Associate refractive errors with poor vision Draw ray diagrams to show refractive errors (hyperopia, myopia, astigmatism) Describe Accommodation, Presbyopia and its correction Formulate a plan of vision correction (by spectacles) for refractive errors (distance & near) Demonstrate Snellen’s Visual Acuity test [Hands-on]

WHAT ARE WE GOING TO DO TODAY? A little bit of optics Refractive media of the eye How the eye interacts with light Refractive Errors

OPTICS 101 Incident ray Refracted ray Emergent ray “REFRACTION”

OPTICS 101: REFRACTION

OPTICS 101: ACCEPT IT! Light rays coming from infinity (e.g. sun, stars, galaxies) are parallel All other light rays that originate from an object (TV, bulb, book) are divergent Us ophthalmologists consider a distance of 6 meters or more as infinity All distances are measured in meters

OPTICS 101: LENSES CONVEX LENS Parallel light rays (incident) Converging light rays (emergent) MORE CONVERGING POWER 3M Divergent light rays (incident) Converging light rays (emergent) 3M AMOUNT OF CONVERGENCE DEPENDENT ON NATURE OF INCIDENT RAYS

SO FAR Convex Lenses converge Light The thicker the lens (= more power) the greater the convergence Amount of convergence depends on the type of incident rays

OPTICS 101: LENSES CONCAVE LENS Parallel light rays (incident) Divergent light rays (emergent) Virtual Image MORE DIVERGING POWER Virtual Image 3M Divergent light rays (incident) AMOUNT OF DIVERGENCECE DEPENDENT ON NATURE OF INCIDENT RAYS

… Concave Lenses diverge Light The thicker (= more power) the lens the greater the divergence Amount of divergence depends on the type of incident rays

RECAP Convex lenses converge Thicker the lens, the greater the power, more the convergence Amount of convergence depends on nature of incident rays

RECAP Diverging lenses diverge Thicker the lens, the greater the power, more the divergence Amount of divergence depends on nature of incident rays

Assimilate; Enjoy Ask Questions

The Next few slides introduces a new term “DIOPTER” It will require some attention If you think its too difficult, don’t worry. Most ophthalmologists are in the same boat

DIOPTER Measures the divergence (spread) or convergence of light. The ‘power’ of light Calculated (notation ‘D’) as: n/d – n = refractive index of medium (1 = air) – d = distance from object that measurements are taken (in meters) DO NOT CONFUSE ‘D’ (DIOPTER) with ‘d’ (distance) Minus for Diverging light Plus for Converging light

DIOPTER The Diopter also signifies the power of lens Calculated just as before (n/f. f = focal length) The more the power the more the converging or diverging ability of the lens Minus for Diverging lens (just like light rays) Plus for Converging lens (just like light rays)

DIOPTER –POWER OF OBJECTS.5m1m2m - P = -1/ 0.5. P =-2D P = -1/ 1. P =-1D P = -1/ 2. P =-.5D * Power = n/d * n=1 * ‘-’ sign for diverging light d As the distance increase. The spread of light also increases. The power contained in the light beam decreases as it is spread ‘too thin’

DIOPTER –POWER OF LENSES HOW WILL THE INCIDENT LIGHT RAYS INTERACT WITH THE LENS THE LIGHT RAYS WILL BE CONVERGED BY THE LENS THE AMOUNT OF CONVERGENCE IS THE POWER OF THE LENS FOCAL POINT THE POINT OF LIGHT RAYS CONVERGE IS CALLED THE FOCAL POINT FOCAL LENGTH DISTNACE BETWEEN THE LENS & FOCIAL POINT IS THE FOCAL LENGTH Focal Point Focal length ‘f’ n/f POWER OF THE LENS IS CALCULATED AS: n/f (n= Refractive Index) If f= 1m; P lens ? P lens = 1/1 = +1D + (POSITIVE) IF A LENS CONVERGES LIGHT RAYS ITS POWER IS + (POSITIVE) Converges light rays

DIOPTER –POWER OF LENSES Focal Point Focal length ‘f’ - (NEGATIVE) IF A LENS DIVERGES LIGHT RAYS ITS POWER IS - (NEGATIVE) If f= 1m; P lens ? P lens = 1/1 = -1D Diverges light rays

OBJECT – LENS INTERACTIONS Incident light rays from objects (aka object light rays) interact with lenses to form images Light rays that exit the lens are emergent light rays (aka image light rays) The point where image light rays meet is the where image is formed The location of Image is determined using this simple formula: P image_rays = P obj_rays + P lens (P = Power)

INTERACTION OBJECT INCIDENT RAYS (from object) LENS EMERGENT RAYS ( to image ) IMAGE P obj_rays + P lens = P image_rays n/d + P lens = P image_rays RI Location of image (d) =n/P image_rays

INTERACTION 2m +2D P obj_rays + P lens = P image_rays = +1.5 D (Diopters) (P obj_rays = -1/2) n/d (Location of image (d) =n/P image_rays ) Location of image (d) = 1/1.5 =.67m.67m

INTERACTION -2D.40m2m P obj_rays + P lens = P image_rays (P obj_rays = -1/2) n/d (-)2 = -2.5D (Diopters) Location of image (d) = -1/2.5 = -.40m (Location of image (d) =n/P image_rays ) minus (-) sign: Image on same side as object

DIOPTER *Remember: 1/f = 1/p + 1/q *This is the same as: P lens =P obj + P image * Flipping: P image =P obj + P lens p q f Focal Length

QUICK TEST 4m 1D ? USE BOTH FORMULAE TO FIND IMAGE POSITION 1/f = 1/p + 1/q P image =P obj + P lens Power = 1/d 1/f = 1/p+ 1/q f (focal length)=1/power of lens =1 1/1 =1/4 + 1/q 1/1 – 1/4 = 1/q = 1/q.75 = 1/q q = 1/.75 = 1.33m P obj_rays + P lens = P image_rays P obj_rays = -1/4 = -.25D P lens = 1D P image_rays = =.75 d image = 1/.75 = 1.3m 1.3m 1D

DIOPTER Measures power of light rays as well as lenses Is ‘+’ for Converging light rays and lenses Is ‘-’ for Diverging light rays and lenses Light ray lens interaction is calculated algebraically

Assimilate; Enjoy Ask Questions

THE OPTICS OF EYE TRANSPARENT MEDIA ARE RESPONSIBLE FOR REFRACTION AS THEY ALLOW LIGHT TO PASS THROUGH TO THE RETINA Cornea Aqueous humor LENS Vitreous humor * We only consider Cornea and Lens as refractive media *Total Power of the eye ~ 60D (54D) Cornea = 40D (36D) [Greater difference in refractive index) Lens = 20D (18D)

VISION FOVEA MACULA LIGHT RAYS ARE FOCUSED ON THE FOVEA

OPTICS OF THE EYE More Refraction occurs at Cornea-air interface Difference in refractive index Air = 1.00 Cornea = Lens = 1.38 *The lens is able to change its shape *The stimulus is a blurred image *The lens tries to bring the image into focus just like a camera *For near tasks (reading) the lens ‘thickens’ increasing power accommodation *This is called accommodation

ACCOMODATION

ACCOMMODATION Ciliary Muscles strength largely determine amount of accomodation At Birth it is about 14 Diopters (i.e. the lens’ power can be increased by 14D by accommodation to about 34D). At 40 this is effectively reduced to 2 Diopters

NEAR REFLEX When a person focuses for near Three distinct changes occur – Accommodation – Pupillary constriction – Eyes converging (moving inwards) These three together are called the ‘near reflex’ The accommodation reflex sometimes is used synonymously with near reflex

RECAP Light rays coming from infinity (>6m) are focused by a resting (non- accommodating) eye on the retina Light rays coming from a finite distance (<6m) produce a blur image on the retina This blur image forces the eye to accommodate Accommodation involves constriction of ciliary muscles to make the lens thicker (more power) Thicker lens then focuses the light rays back onto the retina Amount of accommodation depends on the distance of the near object The eye will always try to focus a blurred image on the retina Remembering this simple fact will help clear A LOT of confusion

EMMTEROPIA EMMTEROPIA = Equal Measure = No Refractive Error = Desirable optical system RESTING EYE ACCOMMODATING EYE

REFRACTIVE ERRORS A fault in the mechanism of Refraction Produces a blur image on the retina Can be for far (infinite; >6m)objects As well as for near (finite; <6m) objects AM-METROPIA AM-METROPIA = Not Equal Measure

NEAR VISION ERROR (READING ERRORS)

PRESBYOPIA Presby = Old + Hard (Rigid) Opia = sight An age related phenomena (40 Years) Weakness in ciliary muscles Can’t contract as much as before Lens can’t accommodate as well Thus can’t focus for near objects Becomes worse with advancing age

PRESBYOPIA * The eye will try to accommodate to nullify the blur *It will fail as there is no more accommodative effort left (weak muscle) What do old people do to read without glasses *As the object is moved further *The light rays spread more decreasing their power (Diopter; Power = n/d) *Light striking the cornea will have lower power *Less accommodative effort then can help focus the light rays

CORRECTING PRESBYOPIA WITH LENSES WHAT TYPE OF LENS CAN HELP BRING THE LIGHT TO FOCUS ON THE RETINA? * A Convex lens corrects presbyopic error. *As muscle weakens with increasing age so does the power of correcting lens *A lens that corrects a refractive error is called a correcting lens

DISTANT VISION ERRORS (infinity; 6m)

DISTANT VISION ERRORS: RULES ALL ERRORS ARE DEFINED WITH THE FOLLOWING CONDITIONS – Light rays are parallel (coming from distance) – The eye is at rest (not accommodating) – The error is defined based on where the light focuses Hyperopia/ Hypermetropia (Long measure) Myopia (Short measure)

DISTANT VISION ERROR I: HYPEROPIA (also known as hypermetropia)

HYPERMETROPIA Parallel light rays… …while eye is at rest… (Not Accommodating) …focusing behind the retina What type of lens can be used to correct this error? A common cause of hypermetropia is a short eye ball Convex Lens For a box of candy: Help fix this hyperopic patient’s vision without using lenses How does eye react to ‘blur’ It accommodates! * The eye voluntarily (unknowing to the patient) accommodates to the blurred image *Most hyperopes don’t even know they have a refractive error *They are latent hyperopes *Only when they start getting older and they loose their accommodative power *Their hyperopia then becomes manifest

HYPEROPIA Hyperopia has also been dubbed as – Long sightedness (‘layman’ term) This is because they can involuntarily accommodate for distance However this assumption is wrong The definition says: Eye at rest DO NOT USE THIS TERM! IT CREATES CONFUSION!

RECAP: HYPEROPIA A distance vision refractive error Parallel light rays focus behind the retina in a resting eye The eye automatically compensates for the error by accommodating Corrected with a convex lens, just like presbyopia (a near vision refractive error) Just because both these types of errors are corrected by the same lens mean they are the same error!

DISTANT VISION ERROR II: MYOPIA

MYOPIA Parallel light rays… …while eye is at rest… (Not Accommodating) …focusing in front of the retina What type of lens can be used to correct this error? Concave Lens A common cause of myopia is a long eye ball …accommodation of any help? The eye always reacts to a blurred image! *In Myopia accommodation would create a blurrier image! *As it is not possible to relax accommodation *Accommodation has no role in involuntary correction of myopia *A corrective aid must be used to fix myopia

MYOPIA Myopia has also been dubbed as – short sightedness (‘layman’ term) This is because they are said to have normal near vision However this assumption is wrong Even hyperopes has normal near vision! DO NOT USE THIS TERM! IT CREATES CONFUSION!

RECAP: MYOPIA A distance vision refractive error Parallel light rays focus in front of the retina in a resting eye The eye CAN NOT automatically compensate for the error by accommodating Corrected with a concave lens, unlike presbyopia (a near vision refractive error) and hyperopia

ASTIGMATISM X Y X Y EQUALLY CURVED IN ALL DIRECTION CURVATURE VARIES

ASTIGMATISM Curvatures of surfaces Spheres have similar curvature Ovals have different curvature If curvature is different refraction of light through those surfaces will be different

X Y ASTIGMATISM THIS IS EXACTLY AS THE WORKING OF CONVEX LENS WE HAVE TALKED ABOUT POINT FOCUS

*A-STIGMA-TISM = NOT- POINT - FOCUS TWO FOCII ASTIGMATISM X Y

CORRECTING ASTIGMATISM FOCUS 1 FOCUS 2 WE NEED A LENS THAT WORKS IN ONE MERIDIAN ONLY X Y RETINACORNEA

CYLINDIRCAL LENS

CORRECTING ASTIGMATISM FOCUS 1 FOCUS 2 WE NEED A LENS THAT WORKS IN ONE MERIDIAN ONLY X Y Cylindrical Concave Lens A COMBINATION OF SPHERE AND CYLINDICAL LENS PRODUCES A STIGMATIC (POINT FOCUS) IMAGE

WHAT IF X Y X Y CYLINDRICAL AXIS NOT ONLY HAVE A POWER VALUE BUT ALSO A AXIS VALUE THE CYLINDRICAL LENS MUST BE ROTATED TO ‘FIT’ THE CORRECT AXIS

RECAP Occurs when the cornea is shaped like a ‘rugby’ ball Can be present with hyperopia or myopia or both Is corrected using a cylindrical lens The lens has a axis as well as a power value

WAIT THERE IS MORE… It is possible to reshape the surface of the cornea with laser The cornea can be reshaped into a convex or concave lens using ultra-violet laser The laser used is called EXCIMER Three types of procedures: – Photo Refractive Keratectomy (PRK; Common) – Laser insitu Keratomelusis (LASIK) – Laser Intra-epithelial Keratomelusis (LASEK) Oh, and Contacts lenses can also used!

PRK FOR MYOPIA & HYPER OPIA

AND FINALLY! Lens can change its shape in response to blur This is called accommodation Accommodation allows for focusing near (<6m) as a part of near reflex Accommodation also involuntarily corrects hyperopia Accommodation decreases with age This is called presbyopia Presbyopia is corrected by a convex (+) lens

AND FINALLY! Refractive errors for distance vision are – Hypermetropia & Myopia Hypermetropia = focus behind retina Myopia = focus in front of retina Hypermetropia can be involuntarily corrected by accommodation Myopia can not be involuntarily corrected

… LONG Sightedness SHORT Sightedness MISLEADING TERMS USE THEM KNOWING WHAT THEY MEAN BETTER DO NOT USE THEM AT ALL!