Optics & Refraction-Dr.PrabhatDevkota.pptx

Optics & Refraction
Dr. Prabhat Kiran Devkota
MBBS(TU), MD(NAMS)
Lumbini Eye Institute

1. OPTICS & REFRACTION Dr. Prabhat Kiran Devkota MBBS(TU),MD Resident(NAMS) Lumbini Eye Institute 1 2/12/2024

2. Content Basic Terminologies Types of Lens Refractive State of Eye Refractive Component of Eye Objective Refraction Retinoscopy Autorefractometry Subjective Refraction Lens Notation, Transposition 2

3. Basic Terminologies Light: a form of energy whose interaction with retina gives the sensation of sight. Optics: the study of the behavior of light and its interactions with matter. 3

4. Reflection : a phenomenon of change in the path of light rays without any change in the medium. Refraction : the change in direction of light when it passes from one transparent medium into another of different optical density. 4

5. Interference : When two or more waves of light of the same frequency travelling in same direction superimpose, the resultant intensity is different from the sum of individual waves. This modification in the distribution of intensity of light in the region of superimposition is called interference 5

6. Diffraction: Bending of light caused by edge of the aperture or rim of the lens. Diffraction effects get less obvious as the gap gets larger 6

7. Polarization: Phenomenon of confirming the vibrations of a wave in a specific direction perpendicular to the direction of wave motion is called polarization. 7

8. Plano Lens:- Plano lenses are ophthalmic lenses that do not have any power. Used for protection. 8 Types of Lenses

9. Prism : It is a refracting medium, having two plane surfaces, inclined at an angle. Refracting angle is the angle between two surfaces. The greater the angle, the stronger the prismatic effect. Axis refers to the line bisecting the apical angle. Base refers to the surface opposite to the apical angle. The image formed is erect, virtual and displaced towards the apex of the prism. 9

10. when the prisms are arranged with their bases together, a convex lens is formed which converges the incident light to a point. when the prisms are arranged with their apices together, a diverging effect is produced and a concave lens is formed. 10

11. Spherical lens has the same curvature over its entire surface, and thus the same refractive power in all meridians. 11 Convex lens (plus lens) - Converge light rays - Thick at center & thin at periphery - Magnifies object - With movement of lens, object moves in opposite direction.

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13. 13 Concave lens (minus lens) - Diverge light rays - Thin at center & thick at periphery - Minifies object - With movement of lens, object moves in same direction.

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15. Cylindrical lens have vergence power in only 1 meridian, which is perpendicular to the axis of the cylinder. They have no power in the meridian parallel to the axis. Cylindrical lenses focus light rays to a line. Types: Convex & Concave 15

16. Rotation of the cylindrical lens causes a distortion of image. 16

17. Spherocylindrical Lens : (aka Compound or Toric lens) Spherical and Cylindrical lenses combined in 1 lens. Two focal lines formed by 2 radii of curvature. The 1st focal line is nearer the cornea, created by the more powerful corneal meridian. The 2nd focal line is farther away, created by the less powerful meridian. Between the 2-line foci produced by the conoid of Sturm is a point called the circle of least confusion , which represents the point of best overall focus. 17

18. The circle of least confusion is the circular cross-section of the Sturm's conoid , dioptrically midway between the two focal lines. The goal of refractive correction is to place the circle of least confusion on the retina. 18

19. Refractive Components of the Eye Radius of curvature : The anterior and posterior radii of curvature of the central part are around 7.7 mm and 6.8 mm respectively. About 1 mm increase in radius of curvature results in ~ 6 D of hypermetropia and viceversa . Refractive power : Net refractive power is ~+43D(anterior surface +48.83D and posterior surface -5.88D) which is 2/3 rd of the total refractive power. Refractive index = 1.376 19 Cornea:

20. The Anterior Chamber • Contains Aqueous Humor. • Depth of AC = ~2.5 to 4.0 mm • Reduction of 1 mm AC depth forward shift of lens increase the total power by ~1.4 D • Refractive index of aqueous = 1.336 20

21. Iris & Pupil Regulate amount of light entering the eye At ~2.4mm pupil size, best retinal image obtained, as aberration and diffraction are balanced optimally. Average size:2-4mm Small pupil: d epth of focus increases (pinhole effect) Large pupil: more aberration occurs 21

22. Crystalline Lens Radius of curvature: Ant . surface ~ 10 mm Post. surface ~ 6 mm Refractive index: Nucleus 1.41 Pole 1.385 Equator 1.375 Accommodative power: Provides a mechanism of focusing at different distances. At age 8yr is~14D, age 28yr ~9D, age 64yr ~1D Refractive Power:~+16 to +20 D, accounts for about 1/3 rd of the refractive power. . 22

23. Vitreous Refractive index same as aqueous. ie . 1.336 About 1 mm shortening of the AP diameter of the eye results in 3D of hypermetropia and vice versa. 23

24. Schematic & Reduced Eye 24 A way to represent the complex refracting system of human eye. Gullstrand's schematic eye: - Two principal foci - Two principal points - Two nodal points Listing's reduced eye: - Two principal foci - Single principal point - Single nodal point

25. Optical Axis Optical axis : Line passing through the centre of the cornea (P) and centre of the lens (N) and meets the retina (R) on the nasal side of the fovea Visual axis (aka foveal - fixation axis): Line joining the fixation point (O), nodal point (N) and fovea (F). Fixation axis : Line joining the fixation point (O) and the centre of rotation of the eye (C). 25

26. Visual Angles Angle alpha (ONA): Angle formed between the visual axis and the optical axis at the nodal point. Normally the visual axis cuts the cornea nasally this is denoted as positive angle alpha. Angle gamma (OCA): Angle formed between optical axis and fixation axis at the center of rotation Angle kappa (OPA): Angle formed between the pupillary line and visual axis. Substituted for angle alpha as it can be measured clinically. 26

27. Refractive Index Refractive power of substance in comparision to that of air is termed as refractive index. 27

28. Total Internal Reflection The critical angle is the incident angle at which the angle of refraction is 90 degrees from normal. As rays emerging from a denser medium to a rarer medium if it strikes the interface at an angle > critical angle it is reflected back which is called total internal reflection. It occurs at the cornea:air interface, and prevents visualisation of the AC angle. 28

29. Near point: Closest point at which an object can be placed and form a focused image on retina within eye’s accommodation range. Far point: Point in space that is conjugate with fovea when accommodation is relaxed. Emmetropia : Far point is at infinity. Myopia : Far Point is between infinity and eye Hypermetropia :-Far point is beyond infinity 29

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31. Refractive States of the Eye Emmetropia : When parallel light rays are focused sharply on the retina when accomodation is at rest. Ametropia : When eye is unable to bring parallel light rays from a distant object into focus. • Myopia • Hyperopia • Astigmatism Isometropia :- The optical state with equal refraction in the two eyes. Anisometropia : When the total refraction of the two eyes is unequal. Aniseikonia : an anomaly of binocular vision in which the ocular images are unequal in size or shape or both. Antimetropia : rare refractive condition in which one eye is myopic and the fellow eye is hyperopic. 31

32. Optical Defects Physiological Optical Defects: Diffraction Chromatic aberration Spherical aberration Decentering Peripheral aberrations Coma 32 Pathological Optical Defects : Myopia Hypermetropia Astigmatism

33. Chromatic Aberration Light of different wavelength gets refracted differently. Shorter wavelengths (blue) are refracted more than longer wavelengths (red). Eye focuses light of the greatest intensity i.e. yellow. So, images formed by light of longer and shorter wavelength are less intense and are neglected. The eye is hyperopic for red and myopic for blue light. 33

34. Spherical Aberration: Periphery of a lens has more refracting power than the center. A convex spherical lens refracts peripheral rays more strongly than paraxial rays, so peripheral rays are focused closer.(d/t the prismatic effect). Seen only when the pupil is widely dilated. Small pupil cuts off the peripheral rays. 34

35. Natural mechanisms to decrease aberrations Cornea at periphery is flatter than centre. Iris blocks the peripheral rays of light. High refractive index of central nucleus of lens than cortex, Low sensitivity of the peripheral retina, and Stiles Crawford effect i.e. light passing near the edge of the pupil is less efficient at evoking sensation than through the center of the pupil. 35

36. Myopia (Nearsightedness):- The light rays focus anterior to the retina when accommodation is at rest due to excessive convergent power. A concave lens(-) is used to correct myopia. Types: Axial, Curvatural , Index, Positional, d/t excessive accomodation . 36

37. Hyperopia (Farsightedness):- the light rays focus posterior to the retina when accommodation is at rest due to insufficient convergence power .A convex lens(+) is used to correct hyperopia Types: Axial, Curvatural , Index, Positional, Aphakic Consecutive 37

38. Astigmatism : type of refractive error wherein the refraction varies in different meridian. Corrected with cylindrical lens . Regular Astigmatism : when principal meridians are perpendicular & the refractive power changes uniformly. against-the-rule astigmatism (0-30)°: Horizontal meridian steeper oblique astigmatism (30-60)° with-the-rule astigmatism (60-90)°: vertical meridian steeper b) Irregular Astigmatism : irregular change of refractive power in different meridia . it cannot be corrected by spectacles 38

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40. Presbyopia is a progressive loss of accommodative ability of the crystalline lens caused by the natural process of aging. It manifests after the age of 40years. It can be corrected optically with use of plus lenses for near work. 40

41. Refraction Phenomenon of bending of light as it passes from one transparent medium into another of different optical density. Snell’s Law of Refraction: The incident ray, refracted ray and the normal all lie in same plane. Ratio of sine of angle of incident to sine of angle of refraction is constant in any two transparent mediums, which is termed as refractive index 41 n1 sin θ 1 = n2 sin θ 2

42. Definition:- Process of measuring patient’s refractive error & determining the optical correction needed to focus light rays from distant and near objects onto retina and providing the patient with clear and comfortable vision. 42

43. Refraction Methods Objective Refraction Retinoscopy Refractrometry Keratometry Photorefraction Subjective Refraction I. Subjective adjustment of refraction II. Subjective refinement of refraction III. Binocular balancing 43

44. Retinoscopy aka ‘ Skiascopy ’ or ‘Shadow Test’ or ‘ pupilloscopy ’. An objective method of assessing error of refraction. Utilizes technique of neutralization. Focault's Principle - when light is reflected from a mirror into the eye, the direction in which light will travel across the pupil will depend upon the refractive state of the eye. aim of retinoscopy is to convert the far point of the subject's eye to the nodal point of the observer's eye. It is performed with Retinoscope . 44

45. Objective of retinoscopy To locate the far point of the eye conjugate to the retina. Myopia or hyperopia Bring far point to the infinity by using appropriate lenses. Determines amount of ametropia 45

46. Retinoscopy - Optics Part of fundus illuminated by light reflected into patient’s eye with retinoscope. Illuminated area – serves as an object. The rays which emanate from this illuminate the pupillary area and forms its image at the far point of the eye When the immediate source of light is moved across the eye, behavior of the luminous reflex in pupil will depend upon the refractive status of the eye. 46

47. Retinoscopy - Stages 1.Illumination Stage :- Light is directed into the patients eye to illuminate the retina. 2.Reflex Stage :- An image of the illuminates retina is formed at the far point of the eye. 3.Projection Stage :- The image at the far point is located by moving the illumination across the eye( fundus ) and noting the behaviour of the reflex in the patients pupil 47

48. Types of Retinoscope Reflecting mirror retinoscope Plane mirror Priestley-smith’s mirror Self illuminated retinoscope Spot retinoscope Streak retinoscope 48

49. Consists of:- Projection System:- light source (bulb) condensing lens mirror focusing sleeve current source Observation system:- peephole 49 Parts of Streak Retinoscope

50. By sliding the sleeve up or down, the distance between the bulb and lens is changed to converge or diverge the light streak. Plane mirror effect : apparent light source behind retinoscope , creating parallel light rays. Concave mirror effect : apparent light source in front of the retinoscope , creating concave light rays. 50

51. Pre-requisites for Retinoscopy : 1. Dark room 2. A trial set consisting of: -32 Spherical lenses (plus & minus) from 0.12 upto 20 -20 Cylindrical lenses (plus & minus) from 0.12 upto 6 -Prism ( 1/2,1,2,3,4,5,6,8,10,12pd ) -Accessories: Plano lens, Pinhole, Occluder , Stenopaeic slit, Maddox rod & Red and Green lenses. 3. A trial frame 51

52. 4. Retinoscopes 5. Phoroptor (alternative to trial set & trial frame) 6.Distance vision chart: commonly Snellen’s self illuminated vision box. 7. Near Vision charts: Jaeger’s chart, Reduced Snellen’s test types & Times Roman type face. 52

53. Trial Frame & Phoroptor : 53

54. Refractive States of the Eye Optics of reflex and projection stage when performed at 1 meter distance from subject with plane mirror effect depends upon the refractive status of eye as: Emmetropia or Hypermetropia or Myopia <1D: with movement Myopia of 1D: no movement Myopia >1D: against movement 54

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56. Optics of Movement of Reflex Emmetropia : with movement . Light rays emerging out of eye from illuminated spot on fundus (F1) are parallel to each other . Hypermetropia : with movement . Light rays emerging out of eye from illuminated spot on fundus (f1) are divergent . 56

57. Myopia (<1D): with movement. Light rays emerging out of fundus are convergent & meet behind observer sitting at 1 m from patient & examiner intercepts them before they meet. Myopia (1D): no movement. Light rays emerging out of fundus are convergent & meet at a point 1 m infront of patient i.e. at pupillary plane of observer. 57

58. Myopia >1D: against movement. Light rays emerging out of the patient’s eye are convergent & meet in space between patient & observer. 58

59. Procedure Patient made to sit at an arm length distance ( harmon’s distance) from examiner. The eye level of the patient and examiner should match. Sitting at the rightside of patient hold the retinoscope with right hand, using your right eye look for patients right eye & vice versa. Patient is instructed to look at far point (to relax the accommodation) in dry retinoscopy or, directly into light (if cycloplegics is used). 59

60. Examiner observes red reflex in pupillary area of patient through a peephole in retinoscope . Retinoscope is moved in horizontal and vertical meridians and characteristics of moving retinal reflex are noted. 60

61. Finding Neutrality "Against" movement Myopic neutralizes with minus lenses "With" movement Hyperopic neutralizes with plus lenses 61

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63. Characteristics of Reflex Speed: Large refractive errors have slow reflex whereas small refractive errors have faster reflex. Brilliance: - Reflex is dull when far point is away from the observer and becomes clearer as it comes closer to neutrality. -Against reflex are usually dimmer than with reflex. Width: Streak is narrow when the far point is away, becomes wider and at neutrality it fills the entire pupil . 63

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65. Finding Cylinder Axis Before power the axis of meridians must be determined Characteristics of streak reflex : Break : is seen when the streak is not oriented to the principle axes and it disappears when it is rotated correctly. Width : Appears narrowest when streak aligns with principle axis. Intensity: Brighter when streak is on correct axis. Skew : is the oblique motion of the reflex. The streak moves slightly differently than the reflex when the axes are not aligned. It can be used to refine axis in small cylinders. 65

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67. Confirmation of Axis The axis may be confirmed with straddling technique Streak is turned 45° off axis in both direction If axis is correct, width of reflex should be equal in each of two positions If axis is not correct, widths will be unequal in two positions 67

68. Finding Cylinder Power +3.00 +2.00 68 With 2 Spheres : Neutralize one axis with appropriate sphere Keep on changing sphere till second axis is neutralized Difference between two sphere =Cylindrical power For Eg : If 90° is neutralized with +3.00 sphere and 180° is neutralized with +2.00 sphere Gross retinoscopy : +2.00 /+1.00× 180° Examiner's working distance +1.50ds(for 66cm) is subtracted from sphere Final refractive correction: +0.50 /+1.00× 180°

69. With a Sphere and Cylinder Neutralize one axis with spherical lens Neutralize other axis with a cylindrical lens at appropriate orientation Spherical cylindrical gross retinoscopy can be read directly from the trial lens apparatus 69

70. End Point of Retinoscopy : Neutralization point i.e. no apparent movement of the reflex in any meridian & pupil filled with red glow. Verified by following maneuvers: - over correction by 0.25 D should cause reversal of movement. - on altering the working distance by forward or backward movement of the head ‘with’ or ‘against’ movement should be observed. 70

71. Working Distance D istance between the examiner and patient’s eye must be measured and converted into diopters. Subtract d ioptric equivalent of working distance from neutralization point reached in retinoscopy . Eg : working distance of arm’s length ( ̴ 66cm) requires +1.50 D , 50cm distance requires +2D and 1 meter distance requires +1D Working Distance must remain constant throughout the examination. 71

72. Cycloplegic Refraction Causes: paralysis of accomodation & dilatation of pupil. When cycloplegics is used - Wet Retinoscopy . Atropine : < 5 yrs, 1% , TDS x 3days, Retinoscopy on 4 th day, Effect 10-20 days, PMT after 3wks, Tonus allowance 1D Homatropine : 5–8 yrs,2% Gtt. every 10min x 6times, Retinoscopy after 90 min, Effect for 48-72hrs, PMT after 3days, Tonus allowance 0.5D Cyclopentolate : 8–20 yrs,1% Gtt every 15min x 3times, Retinoscopy after 90 min, Effect for 6-18 hrs, PMT after 3days, Tonus allowance 0.75D 72

73. Indications:- Childrens (<15yr) Patient with active accommodation Hyperopic patients upto 35yr (if symptomatic) Early Presbyopia (if using glasses) Variable powers on dry refraction Symptoms disproportionate to refractive error Accommodative Esotropia Bilateral refractive asymmetry Mentally disabled or uncooperative patients Suspected Malingering 73

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75. Postmydriatic Test It refers to repeating the refraction after the effect of cycloplegia has worn off. Ideally, it is done in all cases of cycloplegics refraction if there is a difference in the correction in dry and cycloplegics refraction. The next visit depends on the cycloplegics agent used. Eg . For atropine it is done after 3 weeks, for homatropine and cyclopentolate it is done after 3 days. 75

76. Rough Estimate of Refractive Error after Retinoscopy : Gross Retinoscopy : retinoscopic value Net Retinoscopy = Gross Retinoscopy - [Working Distance + Tonus Allowance(if used)] If retinoscopic finding is +7 Ds at 1 meter with Atropine then, Refractive Error = +7 – (1+1) = +5 D 76

77. Autorefractometry Alternative method of detecting refractive error using optical equipment called Refractometer or Optometer . Based on 2 principles: - Scheiner Principle - Optometer Principle Advantage: -Faster -No training required Disadvantages: -Expensive -Need electricity -Wrong reading on irregularities & opacities 77

78. Lensometer An instrument used to measure the power of a patient’s present spectacle lenses. Both manual and automated lensometers are available. Measures 4 principal properties : spherical and cylindrical power axis if cylinder is present presence and orientation of prism optical centration 78

79. Subjective Refraction Should proceed as: Subjective adjustment of refraction Subjective refinement of refraction Binocular balancing 79

80. A. Subjective Adjustment of Refraction Trial & Error technique Fogging Technique 80

81. 1. Trial & Error technique This employs use of different spherical & cylindrical lenses. SPHERICAL LENSES: -the strongest convex lens and the weakest concave lens providing best vision should be chosen for hypermetropia and myopia respectively. 81

82. CYLINDRICAL LENSES Need verification of axis and power. Verification of axis : - Best done by simply rotating the cylinder in steps of 5 or 10 ˚ in either direction and asking whether acuity improves. Verification of Power -by changing the cylindrical lens in the trial frame. 82

83. 2. Fogging 83 Principal: to make eye artificially myopic by increasing plus power (or decreasing minus power). Fog eye by adding 1 to 1.50D power more than retinoscopy finding. Then unfog by reducing 0.25D till best Snellen’s visual acuity is attained

84. B. Subjective refinement of refraction First refine sphere than cylinder 1. Refining Sphere i . Fogging test ii. Duchrome test iii. Pinhole test 2. Refining cylinder i . Jackson’s cross cylinder technique ii. Astigmatic fan & block technique 84

85. 1. Refining Sphere Done by:- i . Fogging technique ii. Duchrome test iii. Pinhole test 85

86. Duchrome Test Determines too much minus or too much plus correction. Due to chromatic aberration, the shorter green wavelength is focused in front longer red wavelength Patient is asked to read letters with red and green background Emmetropic : both colors equally sharp Hypermetropic : green is sharper, indicates over minused , plus should be added. Myopic: red is sharper, indicates over plused , minus should be added. 86

87. Pinhole Test Pinhole size varies 1 - 2mm (ideal size 1.2 mm). Can correct ref. errors between +5.0D and -5.0D. Pinhole admits only central rays of light which do not require refraction by the cornea/lens, thus cutoff the scattered rays increasing the depth of focus. So that the blur created by optical irregularities or refractive error becomes reduced. It confirms whether the optical correction is correct or not. An improvement in vision through a pinhole indicates that more refractive correction is still possible. 87

88. 2. Refining Cylinder Done by:- Jackson’s cross cylinder technique Astigmatic fan & block technique 88

89. Jackson’s cross cylinder technique Combination of 2 cylinders of equal strength, with opposite sign placed at right angles (90 ° ) to each other. C ross cylinder is placed with its axis 45 ° to axis of cylinder in trial frame. If no change, axis of correcting cylinder is correct. If improvement occurs correcting cylinder should be rotated. Should be repeated several times until neutral point reached. 89

90. Refinement of Cylinder Power Align the JCC axis with cylinder axis. Flip the cross cylinder and ask the patient “which is better?” Add or subtract cylinder power according to preferred position of JCC For every change in -0.50 dcyl add +0.25dsph and viceversa . 90

91. Astigmatic Fan & Block Test Consists of series of radiating lines spaced at 10 o interval & arranged in a manner of rays of a rising sun around a central panel carrying a V and two sets of mutually perpendicular lines ( the blocks). ‘V’ & ‘block’ simultaneously can be rotated through 180 o . 91

92. Patient is asked to see the fan after fogging by + 0.5 D Normal patients can see all the lines clearly. In astigmatism , some lines are seen more sharply defined. Concave cylinder is then added with each axis at right angle to the clearest line until all the lines are equally sharp 92

93. C. Binocular Balancing Process sometimes known as equalizing accommodative effort or equalization of vision It allows both eyes to have retinal image simultaneously in focus Methods: Alternate Occlusion Prism dissociation 93

94. Near Refraction Usually done at age >40years or, pseudophakic eyes (irrespective of age). Extra plus sphere is added to distant correction usually started with +1DS Patient’s range of clear vision is determined by using near vision chart If range is too close: power is reduced, add -0.25DS in steps If range is too far: power is increased, add +0.25DS in steps 94

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96. Lens Notation A written spectacle prescription follows a standard format Power of sphere( Sph ) is recorded first with sign(+/-), followed by power of cylinder( Cyl ) with its sign(+/-) and axis( ° ) Prescription is recorded for each eye using abbreviation OD ( oculus dexter ) for the RE, OS ( oculus sinister) for the LE. 96

97. -To change the lens to another optically equivalent form. -Usually spectacle are transposed & made as minus cylinders, even if the prescription was written in plus cylinder form. WHY?? Minus cylinder (concave) lens is thinner, lighter and more cosmetic. Minus cylinder reduces some amount of aberrations. Plus cylinder produce more magnification than the equivalent minus-cylinder form. Plus cylinder is often difficult to adjust for the wearer. 97 Lens Transposition

98. 98 Optically Equivalent Lens Form

99. Cylindrical power can be recorded in either plus or minus form . 3 Steps:- Add cylinder power to sphere power Sign of cylinder power is reversed Rotate axis of cylinder through 90° For Eg : +2.50 ds / +0.50dc x 180 ° +3 ds / -0.50dc x 90 ° -1.00ds /+1.50dc× 90° +0.50ds/ -1.50dc ×180° 99

100. Optical Cross into Sphero -Cylinder: Small power (sphere) Big-small power difference (cylinder) Axis of small power Sphero -Cylinder into Optical Cross: Sphere (given axis) Sphere+Cylinder (opposite axis) 100 +3 +2.50 Eg :- +2.50 dsph / +0.50dcyl x 180 ° +3 +2.50 Eg :- +2.50 dsph / +0.50dcyl x 180 °

101. Spherical Equivalent It is average power of a spherocylindrical lens . It represents the dioptric position of the circle of least confusion of the conoid of Sturm. -to compare or balance both eyes , and -to reduce an excessive cylindrical correction . Calculated as: Spherical equivalent = power of the sphere + (cylinder power/2) 101

102. References Borish’s Clinical Refraction 2 nd Edition Duke-Elder’s Practice of Refraction 8 th Edition AAO-Clinical Optics 2022-2023 A. R. Elkington , Clinical optics (1999) Practical Ophthalmology AAO 8thEd 102

103. Thank You!! 103