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Refractive errors


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Refractive errors

  2. 2. 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
  3. 3. OPTICS 101 “REFRACTION” Emergent ray Refracted rayIncident ray
  5. 5. 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
  6. 6. OPTICS 101: LENSES CONVEX LENS 3MParallel light raysrays (incident) Divergent light (incident) Converging light rays (emergent) 3M MORE CONVERGING POWER AMOUNT OF CONVERGENCE DEPENDENT ON NATURE OF INCIDENT RAYS
  7. 7. SO FAR• Convex Lenses converge Light• The thicker the lens the greater the convergence• Thicker lenses = More power = More convergence• Amount of convergence depends on the type of incident rays
  8. 8. OPTICS 101: LENSES CONCAVE LENS Virtual Image 3M (not really important for us) Parallel light rays (incident) Divergent light rays (incident) Divergent light rays (emergent) 3M Virtual Image MORE DIVERGING POWERAMOUNT OF DIVERGENCECE DEPENDENT ON NATURE OF INCIDENT RAYS
  9. 9. …• Concave Lenses diverge Light• The thicker the lens the greater the divergence• Thicker lenses = More power = More divergence• Amount of divergence depends on the type of incident rays
  10. 10. RECAPConvex lenses convergeThicker the lens, the greater the power, more theconvergenceAmount of convergence depends on nature of incidentrays
  11. 11. RECAPDiverging lenses divergeThicker the lens, the greater the power, more thedivergenceAmount of divergence depends on nature of incidentrays
  12. 12. Assimilate; Enjoy Ask Questions
  13. 13. The Next few slides introduces a new term“DIOPTER”It will require some attentionIf you think its too difficult, don’t worry. Mostophthalmologists are in the same boat 
  14. 14. DIOPTER• Measures the divergence (spread) or convergence of light. The ‘power’ of light• Calculated as n/d – n = refractive index of medium (1 = air) – d = distance from lens that measurements are taken (in meters)• Minus for Diverging light• Plus for Converging light
  15. 15. DIOPTER As the distance increase. The spread of light also increases. The power contained in the light beam decreases as it is* Power = n/d spread ‘too thin’* n=1* ‘-’ sign for diverging light P = -1/ 0.5. P =-2D P = -1/ 1. P =-1D P = -1/ 2. P =-.5D d .5m 1m 2m
  16. 16. DIOPTER• The Diopter also signifies the power of lens• 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)
  17. 17. DIOPTERPower of emergent rays:Algebraically add power of lightrays & power of lens Pe = -1+ (+2) =+1D + = convergent rays 1D = Power of 1D Remember: P = n/d +2D Can we find out the image +1 = 1/d location from D = 1*+1 this data? D=1m 1m 1m Power of light rays at lens: Image formed at 1m from P = -1/ 1. P =-1D lens
  18. 18. DIOPTER Power of emergent rays: Algebraically add power of light rays & power of lens Pe = -1+ (-2) =-3D - = divergent rays 3D = Power of 3D Remember: P = n/d -2D Can we find out the image -3 = 1/d location from D = 1/-3 this data? D= -.33m .33m 1mImage formed Power of light rays atat .33m from lens:lens P = -1/ 1. P =-1D
  19. 19. 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
  20. 20. Assimilate; Enjoy Ask Questions
  21. 21. THE OPTICS OF EYE Aqueous Vitreous LENS Cornea humor humorWe only consider Cornea and LensThe refractive index of aqueous humor equates to that of corneaThe refractive TRANSPARENT MEDIA ARE RESPONSIBLE of lens index of vitreous humor equates to that FORTotal Power ofREFRACTION AS(54D) ALLOW LIGHT TO PASS the eye ~ 60D THEYCornea = 40D THROUGH TO THE RETINAin refractive index) (36D) [Greater differenceLens = 20D (18D)
  22. 22. OPTICS OF THE EYE 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 This is called accommodationDifference in refractive Moreindex RefractionAir = 1.00 occurs atCornea = 1.337 Cornea-airLens = 1.38 interface
  26. 26. 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
  27. 27. EMMTEROPIAEMMTEROPIA = Equal Measure = No Refractive Error = Desirable optical system RESTING EYE ACCOMMODATING EYE
  28. 28. 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• AMMETROPIA = Not Equal Measure
  30. 30. 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
  31. 31. PRESBYOPIA What do old people do to read without glassesAs the object is moved furtherThe light rays spread more decreasingnullify power The eye will try to accommodate to their the blur(Diopter; Power = n/d) more accommodative effort left It will fail as there is noLight striking the cornea will have lower power (muscles are too weak)Less accommodative effort then can help focus the light rays
  32. 32. CORRECTING PRESBYOPIA WITH LENSESA Convex lens corrects presbyopic error. HELP BRING THE LIGHT TO FOCUS WHAT TYPE OF LENS CANIn fact any refractiveTHE RETINA? the light focuses behind the retina is corrected ON error in whichby a convex lensAs muscle weakens with increasing age so does the power of correcting lensA lens that corrects a refractive error is called a correcting lens
  33. 33. DISTANT VISION ERRORS (infinity; 6m)
  34. 34. 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)
  35. 35. DISTANT VISION ERROR I: HYPEROPIA (also known as hypermetropia)
  36. 36. How does eye react HYPERMETROPIA to ‘blur’ voluntarily (unknowing to the patient) accommodates *The eye to the blurred image For a box of candy: *Most hyperopes don’t even know they have a refractive error A common of lens canpatient’s vision a short error? What type hyperopic be used to correct this using Help fix this cause of hypermetropia iswithout eye ball *They are latent hyperopes lenses …focusing *Only when they start getting older and they loose their accommodative power do they find out the truth behind the Convex Lens *Their hyperopia then becomes retina It manifestaccommodates! Parallel light rays… …while eye is at rest… (Not Accommodating)
  37. 37. 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!
  39. 39. …accommodation MYOPIA *Inof any help? Myopia accommodation would create a blurrier image! *As it is not possible to relax accommodation *Accommodation has of be used involuntary correction of myopia A common cause no role in to long eye ball What type of lens can myopia is acorrect this error? *A corrective aid must be used to fix myopia …focusing in front of the Concave Lens retina The eye alwaysreacts to a blurred image! Parallel light rays… …while eye is at rest… (Not Accommodating)
  40. 40. 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
  41. 41. 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!