1. REFRACTION Dr. Puneet Kumar Srivastava

2. Refraction Def: Method of evaluating the optical state of eye. Or The process by which the patient is guided through the use of a variety of lenses so as to achieve the best possible acuity on distance and near vision test. Acc. To the physicist, Refraction is defined as bending of light rays when it passes from one media to other having diff refractive indices.

3. Five Activities 1.Retinoscopy (Objective Refraction) 2.Cycloplegia 3.Refinement (Subjective Refraction) 4.Binocular Balancing 5.Prescription Of Spectacle Lenses

4. Ophthalmic Optics Principle of Vergence: Reciprocal of the distance from the lens to the point of convergence of light. Divergent: Light rays that are moving away from each other. Convergent: Light rays that are moving towards each other. Parallel rays have zero vergence

5. Vergence Power: Describes the ability of a curved lens to converge or diverge light rays. Divergent Power: “-” minus Convergent Power: “+” plus Diopter (D): Unit of measurement of the refractive power of a lens. Focal Length (f): Distance between the lens and the image formed by an object at infinity. f =1/D

6. Types Of Lenses 1.Sphere lens: Same curvature over its entire surface and thus the same refractive power in all meridians. Convex Sphere Lens: Converge light rays, “+”plus lens. Concave Sphere Lens: Diverge light rays, ”-”minus lens. 2. Cylinder lens: 3. Spherico-cylinder lens:

7. Convex Lens Identification of a Convex lens: 1.Thick at center, thin at periphery. 2.Object held close to the lens appears magnified. 3.When it is moved, the object seen through it moves in opposite direction to the lens. Image formed by a convex lens: Real, inverted & magnified.

8. Concave Lens Identification of concave lens: 1.Thin at center, thick at periphery. 2.An object seen through it appears minified. 3.When it is moved, the object seen through it moves in the same direction as the lens. Image formed by a concave lens: Virtual, erect and diminished.

9. D= 1/f o 1D of plus power converges / minus power diverges parallel rays of light to focus at 1 meter on opposite side / same side respectively from lens. o +/- 0.25D ---------- +/- 4 meters. o +/- 4D ---------------- +/- 0.25 meters.

10. Cylinder lens: Vergence power is in only one meridian, the one perpendicular to the axis of the cylinder. Spherico- cylinder (Compound lens, Toric lens): o Focus light in two line foci. o The shape of the light rays as they are focused by the spherico-cylinder lens called the-- conoid of sturm o Between the two line foci produced by the conoid of sturm is a point called the-- Circle of Least Confusion (point of best overall focus for a sphericocylindrical lens.

11. Prism Def: Is a wedge of refracting material with a triangular cross section that deviate light towards its base. Power of Prism (Prism Diopter/PD/ X ): 1PD deviates parallel rays of light 1cm when measured at a distance of 1m from the prism. i.e, 1cm deviates at distance of 2m =0.5 PD 1cm deviates at distance of 1/2m =2 PD

12. Refractive State of eye Emmetropia: Parallel light rays are focused sharply on retina. Ammetropia: Unable to bring parallel light rays into focus. Types: 1.Myopia 2.Hypermetropia 3.Astigmatism

13. Emmetropia o It’s a optically normal eye. o In emmetropic eye, parallel light rays are focused sharply on sensitive layer of retina with the accommodation at rest.

14. What happens to the light rays coming from the FP and entering the eye? o In emmetropia the FP is at infinity. So the rays coming from FP, and entering the eye are parallel. They are focused upon the retina (with accommodation at rest) to give a clear image of object at FP.

15. Myopia o Short sightedness, excessive convergent power o Parallel rays of light coming from infinity are focused in front of retina when accommodation is at rest.

16. o In myopia the FP is at a finite distance. o So, the rays coming from object at the far point, and entering the eye are divergent. o These rays are focused upon the retina to give a clear image (with accommodation at rest).

17. Hypermetropia o Long sightedness, Insufficient convergent power. o Parallel rays of light coming from infinity are focused behind the retina with accommodation being at rest. o The posterior focal point is behind the retina, which therefore receives a blurred image.

18. o In hypermetropia object can't be placed at the FP ( It is a virtual point behind the retina). o Here the converging rays directed towards the FP behind the retina can be focused upon the retina to give a clear image by the dioptric (optical) system of the eye (with accommodation at rest). o The Far Point and the Point of Focus on the retina are Conjugate Foci.

19. Astigmatism o Refraction varies in different meridia. o The rays of light entering the eye cannot converge to a point focus but form focal lines. TYPES: o Regular astigmatism: When refractive power changes uniformly from one meridian to another. o Irregular astigmatism: Irregular changes of refractive power in different meridian.

20. Types Depending upon the axis and the angle between two principal meridia, 4 types- 1.With the rule Astigmatism: Vertical meridia is more curved than horizontal. Correction- concave cylinder at 180 +/- 20 or convex cylinder at 90 +/- 20 axis. 2.Against the rule Astigmatism: Horizontal meridia is more curved than vertical. Correction- convex cylinder at 180 +/-20 or concave cylinder at 90 +/- 20 axis.

21. 3. Oblique astigmatism : Regular astigmatism but two meridia are not horizontal and vertical though they are at 90 to each other. (e.g.45 and 135 ) Symmetrical- Cylinder lens required at 30 in both eyes. Complementry- Cylinder lens required at 30 in one eye and at 150 in the other eye. 4. Bi-oblique astigmatism: Regular astigmatism but the two principal meridia are not at 90 to each other.

22. Presbyopia o Progressive loss of accommodative ability of the crystalline lens caused by natural process of aging.

23. Retinoscopy o Objective Refraction o Skiascopy or Shadow test o The goal of retinoscopy is to determine the nature of pat.’s refractive error (if any) and approx. lens power that will diminish (neutralize) that error and approach clear vision. o Types : -Manual -Automated

24. Prerequisites for Manual Retinoscopy 1.Dark room 2.Leister’s bulb( 50W / inside Hg, outside black / aperture -1 big, 2 small 2cm) 3.Retinoscope – Priestly – Smith R. - Plane mirror R. - Streak R. 4. Trial Frames / Halberg clip 5. Trial set

25. Retinoscopes Priestley–Smith retinoscope: o Plane mirror at one end /concave mirror at other end. o Hole- 2.5mm ant. ; 4.0mm post.. Fitted with low polar convex lens. o Projects a circle of light in the patient’s pupil.

26. Streak retinoscope o Self illuminating, projects a streak. o We can rotate the streak in diff angles.

27. Trial Frame

28. Trial Set Typical “Trial Set” will have:- o Spheres - every quarter of a diopter to 4D, every half to 6D, every diopter to 14D, every 2D to 20D. o Cylinder – every quarter of a diopter to 2D. Every half to 6D. o Prism – upto 10 PD, additional 15 & 20 PD. o Plano lenses o Opaque disc o Pin hole o Stenopaeic discs o Maddox rods o Red & Green glasses

29. Principle Of Retinoscopy o Retinoscope illuminates an area of the retina,the image of this area formed at the pat.’s far point. - observer views rays of light emanating from the illuminated retina through pat.’s pupillary area from a working distance. -Depending upon the behavior of reflex the observer knows whether emerging rays are – - Convergent, Divergent or Parallel. o Lenses from the trial set are used to neutralize the movement (neutralization point = subject’s far point coincides observer’s nodal point.)

30. Optics of Retinoscopy o In retinoscopy an illuminated area of retina serves as an object, & the image at the far point of the eye located by moving the illumination across the fundus & noting the behavior of the luminous reflex in the pupil. o The observer does not see the illuminated area of the pat.’s fundus, but only the rays emanating from it to form an illuminated area of the pupil.

31. Retinoscopy Reflex Clues we get from features of retinoscopy reflex - 1. Intensity: In high refractive errors we get a faint reflex and in low refractive errors we get a brighter reflex. 2. Speed: In high refractive error we get a slow movement, and in low refractive error a rapid movement of the reflex. As the neutral point is reached the movement of the reflex is fast. 3. Size: In high refractive error we get a narrow reflex. Reflex will fill the pupil when the neutral point is reached. In very high refractive errors we may not get a reflex or may get a faint reflex with negligible movement. We will not get a good reflex with low power lenses in these cases. So try with high plus or minus lenses

32. Working Distance o The distance between the examiner and the patient’s eye is measured and converted to diopter. o Usually, most examiners uses a working distance of a arm length i.e, approx. 66cm / 2/3m / 26 inches. o D=1/f i.e, D=1/2/3 i.e, D=3/2 i.e, D=1.5D o This power is then substracted from the final dioptric amount that is measured. o The working distance must remain constant through out the examination.

33. Far Point o Far Point (FP) is the furthest point at which objects can be seen clearly by the eye. Object kept at FP of the eye can be seen clearly with accommodation at rest. o Position of FP depends on the optical state (static refraction) of the eye. o In emmetropia it is at infinity. o In myopia it is at a finite distance. o In hypermetropia it is a virtual point behind the retina.

34. o Type of mirror we are using to reflect the rays from the original source of illumination is also a factor deciding the optics. o If we are using a Plane mirror, the virtual image of the original source of illumination is formed as far behind the mirror as the original source is in front of it. o So, the tilt of the mirror to one side will shift the image (immediate source of illumination) to the other side. o Tilt of the plane mirror and the shift of illuminated patch on the retina are in the same direction.

35. o If we are using a Concave mirror, the real image of the original source of illumination is formed in front of the mirror ( position depends on the focal length of the mirror). o So, the tilt of the mirror to one side will shift the image (immediate source of illumination) to the same side itself. o Tilt of the concave mirror and the shift of illuminated patch on the retina are in opposite directions.

36. o If the image is formed between pat. & observer- Against movement. Seen in- 1. Myopia > 1.5 D o If the image is formed either behind the pat.’s eye or behind the observer’s- With movement. Seen in- 1. Emmetropia 2. Hypermetropia 3. Myopia < 1.5 D o The point of reversal or neutral point of retinoscopy is reached when the subject’s far point coincides with the observer’s nodal point & examiner see a diffuse bright red reflex in the pat.’s pupil & no movement of the reflex is discernible.

37. The detail optics is most easily considered in three stages: 1.Illumination Stage: The illumination of the subject’s retina. 2.Reflex Stage: The reflex imagery of the illuminated area formed by the subject’s dioptric apparatus. 3.Projection Stage: The projection of image by the observer.

38. ILLUMINATION STAGE

39. REFLEX STAGE o The S1 is a finite distance from the subject and the eye is emmetropic, an image of S1 will be formed behind the retina. o The illuminated patch can now be considered as an object in its own right and will form an image at the far point of eye, which in emmetropia is at infinity o Reflex stage is so called because the light is actually reflected from the fundus.

40. PROJECTION STAGE

42. o In Optics Direction of Light Ray is Reversible. o What happens to the light rays coming out of the eye from a point on retina? o They meet at the FP of the eye (when the accommodation is at rest). They meet at the FP of the eye o In emmetropia light rays coming out from a point on the retina through the optical system of the eye will be parallel (when the accommodation is at rest). o These rays can meet only at infinity ( ie. where the Far Point is located in emmetropia).

43. o In myopia light rays coming out from a point on the retina through the optical system of the eye will be convergent. o These rays meet at the FP of the eye (at a finite distance) when the accommodation is at rest

44. o In hypermetropia light rays coming out are divergent. o They will only meet "beyond infinity" when the accommodation is at rest. o They can meet only at the FP of the eye, which is a virtual point behind the retina, by extrapolating the divergent rays in the reverse direction to meet behind the retina.

45. o If we can locate the Far Point of subject's eye, then we can calculate the refraction of that eye. But observer moving towards FP is not practical. (In emmetropia FP is at infinity and in hypermetropia FP is virtual point behind patient's eye). o The method used for calculation is keeping the subject and the observer at fixed places, and bringing (shifting) the FP of the subject to the position of the nodal point of observer's eye. o This is done by using converging or diverging lenses. o Now we know the exact distance of the FP from patient's eye (ie. exact distance at which we are sitting) and also the power of lens used to bring the FP to this position. o From the measurement of distance we can calculate the power required to bring the FP to this position in emmetropia – o I call it observer factor - The power used other than this power, gives the refractive error - patient factor.

46. o The Total Power of lenses we use contain two parts. o First one is the power used because of the position of the observer (Observer Factor or Induced Factor or False Factor). If the subject is emmetropic this is the only factor that will be there. This is the power used, only because of the position of the observer and not because of the patient's refractive error. o Second part is the power of lenses used because of the patient's refractive error (Patient Factor or True Factor). o From the above three values we can calculate the refraction of the subject's eye. o Total Power = Observer Factor + Patient Factor. I.e, Total Power - Observer Factor = Patient Factor (Refraction of Patient's Eye).

47. Practical Aspect o If the observer is at a distance of one metre from the subject the Observer Factor is +1.0 Dioptre. (+1 Dioptre is the power of the lens which can focus parallel rays at 1 metre.) Then...............

48. o In emmetropia the parallel rays coming out of the subject's eye can be brought to a focus at 1 metre by using converging (convex) lens of 1 Dioptre kept close to subject's eye. o In other words, if the power of the lens used to bring the FP to 1 metre is +1 D then we know that the rays coming out are parallel (which normally meet at infinity) and the eye we are examining is emmetropic.

49. o Total Power = +1 D, o ObserverFactor = +1 D. o So, Patient Factor is 0 (zero) o Total Power - Observer Factor = Patient Factor (Ref.Error) o (+1) - (+1) = 0 (zero) Subject is emmetropic.

50. o In hypermetropia the diverging rays coming out of the eye are brought to a focus at 1 metre by using converging lenses of power more than +1 D. o (+1 required for parallel rays in emmetropia). o The power used in excess of +1 D is the measure of hypermetropia of that eye.

51. o Total Power = +3 D, o Observer Factor = +1 D. o So, Patient Factor = +2 D o Total Power - Observer Factor = Patient Factor. o I.e, (+ 3) - (+1) = +2. o This means the patient is 2 Dioptres Hypermetropic.

52. o In myopia of less than 1 Dioptre the converging rays coming out are focused at 1 metre by using converging lens of power less than +1 D. o This difference in power compared with the emmetropic eye will give the exact measure of myopia.

53. o Total Power= +0.50 D, o Observer Factor = +1 D. o So, Patient Factor = -0.50D o Total Power - Observer Factor = Patient Factor (Ref. error) o (+0.50) - (+1.0) = - 0.50 o Patient is 0.50 Dioptre myopic.

54. o In myopia of 1 Dioptre the rays coming out of the eye are convergent and meet at 1 metre with out using any lens. o In other words if we are not using any lens to bring the FP to 1 metre we know that we are dealing with an eye which is 1 D myopic.

55. o Total Power = 0 (zero), o Observer Factor = +1 D, o So, Patient Factor = - 1.0 D. o Total Power - Observer Factor = Patient factor o ( 0 ) - ( + 1.0 ) = - 1.0 o Patient is 1 Dioptre myopic.

56. o In myopia of more than 1 Dioptre the converging rays coming out of the eye will focus at the FP which is less than 1 metre from the eye. (between the patient and the observer). o So, this focus (FP) can be brought to 1 metre by using diverging (concave) lens. o So this much of diverging (minus) power is in excess when compared with an eye with FP at 1 metre (ie.eye with 1 D myopia).

57. o Total Power = - 2.0 D, o Observer Factor = +1D, o So, Patient Factor = - 3.0D. Total Power - Observer Factor = Patient Factor (ref.error) o ( - 2.0 ) - ( + 1.0 ) = - 3.0 o Patient is 3 Dioptre myopic.

58. How do you know that you are at the FP of the Subject's eye ?. o The technique of Objective method of Refraction (Retinoscopy) will answer these questions. o Principle : Behavior of the luminous reflex in the pupil of the patient is studied by moving the illumination across the fundus. o This behaviour depends on the vergence of the light rays coming out of the pupil. o It also depends on the position of the observer.

59. Finding 1. o When the observer is at 1 metre from the patient and if the rays coming out from the patient's eye form a focus (FP of the patient) behind the observer at a finite distance or at infinity or 'beyond infinity' (virtual point behind patient's retina), then the luminous reflex in the pupil will move in the same direction of movement of the illumination across the retina ('With Movement'). o This finding we get in myopia less than 1 D, in emmetropia and in hypermetropia.

60. Finding 1. o The first ray of light entering observer's eye is from the same edge of the pupil as the first position of retinal illumination. o As the illumination on the retina moves towards the principal axis the light reflex in the pupil also moves in the same direction. o The last ray of light entering the observer's eye is from the other edge of the pupil. o This gives a with movement reflex in the pupil. o (As in emmetropia, hypermetropia and myopia less than 1D - observer at 1 metre from patient.)

63. Finding 2. o When the observer is at 1 metre from the patient and if the rays coming out from the patient's eye form a focus (FP) in front of the observer (between patient and observer) the luminous reflex in the pupil will move in the opposite direction of movement of the illumination across the retina ('Against Movement'). o This finding we get in myopia more than 1 D.

64. Finding 2. o The first ray of light entering observer's eye is from the opposite edge of the pupil when the position of the retinal illumination is considered. (Light rays cross at FP and the diverging rays are entering observer's eye). o As the retinal illumination moves towards the principal axis the light reflex in the pupil moves in the opposite direction. o The last ray of light entering the observer's eye is from the other edge of the pupil, this gives an against movement reflex in the pupil. (seen in myopia more than 1D - observer at 1 metre)

66. Finding 3. o When the observer is at 1 meter from the patient and if the rays coming out from the patient's eye form a focus (FP) at the nodal point of the observer then the pupil of the patient will appear uniformly illuminated.

68. o The rays coming out through all the parts of the pupil are focused at the nodal point of observer's eye.nodal point o By a slight shift of retinal illumination this focal point is displaced away from observer's eye making patient's pupil uniformly dark.

69. Neutral Point or End Point or Point of Reversal in Retinoscopy is reached by bringing the FP of patient's eye to the Nodal Point of observer's eye by using lenses. (except in myopia of 1 D, where it is reached without any lens).

70. Steps in Retinoscopic Procedure 1.Set the retinoscope so that the light rays emanating are parallel. 2.Adjust the pat. In appropriate testing position & distance. 3.Direct the pat. To look at a specific distant target, (optotype). If cycloplegia has been used, pat. Can directly look into the light.

71. 4. look through the examiner’s eye piece of the retinoscope & direct the light into the pat.’s pupil. - If refraction from the pat.’s pupil is not easy to see then reason are--- o Retinoscope bulb can be dim, dirty or turned off. o The pat. May have very high refractive error. o The room light may not be sufficiently dim. o Media opacity. -Extra reflection can be- Cornea, Trial lens, Extra light in examining room.

72. 5. Streak horizontal – Move up & down. Streak vertical – Move right & left. 6. Note the motion of the reflex- Against- Add “-” minus lenses in 0.5 diopter increment until the against movement lost. With- Add “+” plus lenses in 0.5 diopter until it becomes difficult to tell the direction of movement.

73. 7. Smaller sweeps are helpful as the reflex band appears widen. When the movement of the reflex fills the pupil and can not be ascertain- - The reflected light rays coming from the eyes are parallel. -The lens combination used to reach this point (with the dioptrc equivalent of working distance) is the objective measurement of the refractive error of the eye. - This is referred to as neutrality.

74. 8. Confirmation of neutrality: If our assessment is correct we will get the following results– 1. After reaching the neutral point (end point) in Retinoscopy (with plane mirror) we move slightly towards the subject, then we will get a ‘with movement’ (because the Far Point is now behind the Observer). 2.If we move slightly away from the subject we will get an ‘against movement’ (because now the Far Point is between the observer and the subject).

75. 9. Note the power of the lenses used. Substract the dioptric equivalent of working distance = Refractive error in the axis of sweeping. 10. If there is different reflexes from the horizontal & the vertical orientation of the streak = Astigmatism. - Need cylindrical eye glasses.

76. Determining Cylinder Steps to determine the presence of astigmatism: 1.Position the pat. & yourself, illuminate the pat.’s pupil with retinoscope. Horizontal – up & down Vertical – Right & left 2.Compare the intensity & direction of the reflex. - If both are comparable in intensity & direction- An insignificant Astigmatism is present. - If streak is brighter in one direction than its in other, or if the reflex moves in opposite directions – A cylindrical, or astigmatic, refractive error is present.

77. 3. Neutralize with a retinoscope. - If we are using “-” minus cylinder – First neutralize that is moving slowest or the most “with” the steak. Then neutralize the reflex, perpendicular to it. - If we are using “+” plus cylinder – First neutralize the reflex that is the most “against” & then neutralize the one perpendicular to it. 4. Rotate the sleeves of the retinoscope so that the streak oriented perpendicular to the direction of the initial orientation.

78. - If we are only using spheres for retinoscopy:- o Neutralize in one direction & note the power of lens in that axis. o Repeat the process in axis 90 to the first, and note the power of the lens in that axis o Now add the two sphere powers. 5. If the neutrality can not be achieved with streak oriented horizontally & vertically, an oblique astigmatism can be present. - Adjust the streak acc. to the direction of movement of reflex & neutralize in the axis of movement & second at 90 to the axis.

80. 6. Note the power of the lens combinations that used at each axis. - In writing the eye glasses orescription in “plus” cylinder form – Write the power & axis of the least – plus lens that used to neutralize the reflex. - Horizontal streak neutralizes vertical meridian. - Vertical streak neutralizes horizontal meridian.

81. Say, o From 1m pat. Need -1.00 D in horizontal direction. (up & down movement) o I.e, -2.00 D at 180 Similarly, o From 1m pat. Need -0.5 D in vertical direction. (Right & left movement) o I.e, -1.50 D at 90 So, o Cylinder = +0.5 D Therefore, o Prescription = -2.00 D + 0.50D 90

82. Cycloplegic Refraction o Can be useful as an adjunct to refraction for almost any pat., but it is especially helpful for a pat. who has a active accommodation. (most pat. <18 yrs). o Should be done once for every pat., preferably during an initial evaluation. o Pat.s should be asked about allergies, use of other medications & existence of other medical conditions before cycloplegic drugs are given.

83. Effect of Cycloplegia Latent Hypermetropia: o Normal tone of ciliary muscle is affected by the cycloplegics. When the tone is lost (Cycloplegia) the lens zonules are taut flattening the lens. o This leads to less convergence of light rays by the lens. So there is a shift of refraction of the eye to the hypermetropic side (light rays focused posterior to the previous focus). o This value of hypermetropia, made manifest by cycloplegia is the latent hypermetropia. o So, an emmetropic eye will become hypermetropic, myopic eye becomes less myopic and hypermetropic eye will become more hypermetropic. o As a rule the latent hypermetropia amounts to only one Dioptre.

84. o So, when we do retinoscopy under cycloplegia the values we get are to be corrected for this hypermetropic factor. o I call it ‘Cycloplegic Factor’. o This extra power of converging “+” lens (cycloplegic factor) used to reach the point of reversal must also be subtracted from the total power. Now, o Correction Factor = Observer F. + Cycloplegic F. o Total power – Correction F. = Pat.’s F. (ref. error)

85. o When the observer is at 1 metre from the subject and the cycloplegia is with atropine then the Observer Factor is +1.0 D and the Cycloplegic Factor is +1.0 D. So the Correction Factor is +2.0 D. o Total Power – Correction Factor = Patient Factor o Total Power – (+1 + +1) = Patient Factor o Total Power – (+2) = Patient Factor Eg. (+2) - (+2) = 0 (zero) Eye is Emmetropic (+3) - (+2) = +1.0 Eye is 1D Hypermetropic (+1) - (+2) = - 1.0 Eye is 1D Myopic ( 0 ) - (+2) = -2.0 Eye is 2D Myopic (-1) - (+2) = - 3.0 Eye is 3D Myopic

86. o The value of Total Power can be plus or minus. So for the ease of calculation instead of subtracting +2 we can add -2 with the Total Power. o Total Power + (-2) = Refraction of Patient’s eye Eg. (+2) + (-2) = 0 (zero) Eye is Emmetropic (+3) + (-2) = +1.0 Eye is 1D Hypermetropic (+1) + (-2) = - 1.0 Eye is 1D Myopic ( 0 ) + (-2) = -2.0 Eye is 2D Myopic (-1) + (-2) = - 3.0 Eye is 3D Myopic

87. Commonly used Cycloplegics And there ability to change in refractive power Atropine sulphate (0.5 – 1 %):- +1.0 D Homatropine hydrobromide (1-2%):- +0.50 D Cyclopentolate hydrochloride (0.5-1%):- +0.50 D Tropicamide (0.5-1%):- +0.50 D

88. To simplify the calculation step – o Keep the “Correction Factor lens" in the trial frame before starting Retinoscopy. o After reaching the point of reversal remove this lens. o The power of the remaining lens is the measure of refraction. o No need of correction for the Observer factor and cycloplegic factor. o So, we can avoid the confusion of calculation. o Correction Factor depends on the distance we are from the subject and the cycloplegic used.

89. Subjective Refraction o Should be proceed as: 1.Subjective verification of refraction. 2.Subjective refinement of refraction. 3.Binocular balancing. o Usually performed without the use of cycloplegic drops = Manifest or Dry refraction. o In some cond.s cycloplegics are used = Cycloplegic refraction. We can assess an accurate measurement of the non accommodative refractive state of eye.

90. Subjective verification of refraction: 1.Properly place a trial frame, Visual acuity is tested for both eyes, separately. 2.Place one of the following lens prescription in the trial frame – o Retinoscopy finding. o Previous eye glass prescription. o Previous manifest refraction (useful for pat. undergone cataract surgery or penetrating keratoplasty) 3.Occlude the other eye with opaque disc. 4.Measure the visual acuity using Snellen’s chart.

91. 5. At this juncture, a pin-hole test may give some indication of the best vision attainable by lenses. (the standard of vision obtained with a pin-hole in a pat. with opacities in the media may not be attainable with lenses.) o The subjective verification of the refraction can be performed by – - The “Trial & Error” technique - The Fogging technique

92. Trial & Error technique o Trial of different Spherical & Cylindrical lenses. 1.Sphere lenses: - Add +0.25 D and ask- Which lens makes the letter clearer no.1(original) or no.2(+0.25 D) or they are the same? a. If pat. Prefers no.2 – Repeat the process (add +0.25D more). Again ask which is better – no.2 or no.3(+0.25 D + +0.25 D) or they are same? Continue adding increments of +0.25 D sphere until pat. says the two choices are equivalent.

93. b. If pat. prefers choice no.1 – Subtract -0.25 D sphere and ask which is better no.1(original) or no.4(-0.25D) or they are same? Continue subtracting in increment of -0.25 D sphere until the pat. says that the two choices are equivalent. o Advise myopic pat. To choose the lens that lets them actually see the letters more clearly, not the one that makes the letter smaller & darker. o If the pat. says the letters are just smaller & darker, too much power has been added.

94. 2. Cylinder lenses: Verification of axis: It is best done by simply rotating the cylinder in steps of 5 or 10 degrees in either direction & asking whether the acuity improves. When the power of cylindrical lens is small, pat. may have difficulty in deciding at which axis the vision is better. (Stronger cylinder is used to verify the axis).

95. Verification of cylinder power: - Add +0.25 D cyl in axis of error and ask the pat. which is better - no.1(original) or no.2(+0.25 D cyl) or they are same? a. If pat. chooses no.2, Add +0.25 D cyl Continue adding +0.25 D cyl until pat says the two choices equivalent. b. If pat chooses no.1, Subtract -0.25 D cyl, And ask which is better, no.1(original) or no.3(-0.25 D cyl) or both are same? Continue subtracting -0.25 D cyl until pat says the two choieses are equivalent.

96. Subjective Refinement of Refraction Refining the Cylinder: o Its always better to refine first cylinder and then sphere. o Method: 1.Jackson’s cross cylinder 2.Astigmatic fan & Maddox V test

97. Jackson’s cross cylinder test o To verify the strength and axis of cylinder. o Two cylinders at 90 to each other & of equal strength but of opposite sign. o Commonly +/-0.25 D and +/-0.50 D. o Following steps are used in cross cylinder refraction- 1.Adjust sphere to the most plus or least minus: Fogging- decrease the fog to best visual acuity. The goal(if astigmatism is present) is to place the circle of least diffusion of the conoid of sturm on the retina, thus creating mixed astigmatism.

98. 2.Discovering the astigmatism: Place cross cylinder at 90 & 180. If no preference is found check at 45 & 135. 3.Refinement of the axis: -0.5D & +0.5D at 45`, ask pat of any change in visual acuity. No difference – axis is correct. If improves in one direction – Rotate towards plus (for plus cyl) or towards minus (for minus cyl). The test is repeated several times until the neutral point is reached.

99. 4. Refinement of Cylinder power: Add +0.25 D & -0.25 D alternately, ask pat which is clear? No improvement – power is correct Improve in one condition – add +0.25 D (improves in +0.25 D) or add -0.25 D (improves in -0.25 D). Again check until no improvement is seen.

100. Maddox V test (Fan & Block technique): o To confirm cylindrical correction. o Astigmatic fan consist of a dial of radiating lines at 10 interval to one another (rising sun) around a central panel carrying a V & two sets of mutually perpendicular lines( the blocks). o Pat. Is asked to see the fan after fogging by +0.5 D. o Normal person will see all the lines equally clear. o In Astigmatism, some lines will be seen more sharply defined. o Concave cylinder added at 90 to the clearest line until all line are equally sharp.

101. Subjective refinement of sphere o Final step in monocular refraction is the refining of the sphere which can be done using – 1.Snellen’s visual acuity chart 2.Duochrome test

102. Finalizing sphere with Snellen’s visual acuity chart: o Fogging o un

104. Confirmation of subjective Refraction Duochrome Test: o Pat. Asked to read red & green letters. o In emmetropic eye: Green light anteriorly to retina. Red light posteriorly to retina. So, Equally sharp. o Myopia: Red letters more clear than green. o Hypermetropia: Green letters are more clear than red. o Principle: Shorter wavelength are refracted more than longer wavelength.

108. Mechanism of Production o Axial myopia: Increased antero-posterior length. Commonest. o Curvatural myopia: Increased curvature of cornea lens or both. o Positional myopia: Anterior placement of crystalline lens in the eye. o Index myopia: Increased refractive index of crystalline lens. o Myopia due to excessive accommodation: In spasm of accommodation.

109. Aetiology o Axial hypermetropia: Axial shortening of eyeball. (Approx. 1mm shortening of the anteroposterior diameter of eye results in 3D of hypermetropia.) o Curvatural hypermetropia: Curvature of cornea, lens or both are flatter than normal. (Aprox. 1mm increase in radius of cuvature results in 6D of hypermetropia.) o Index hypermetropia: Due to change in refractive index of the lens. o Positional hypermetropia: Posterior placed crystalline lens. o Absence of crystalline lens: High hypermetropia.

110. Regular Astigmatism Aetiology: o Corneal astigmatism: Abnormality of curvature of cornea. Commonest. o Lenticular astigmatism : 1.Curvatural: Cong. Abnormalities of curvature of lens. Marked lenticular astigmatism in Lenticonus. 2.Positional: Cong. tilting or oblique placement of lens. 3.Index astigmatism: Variable refractive index of lens in different meridia. o Retinal Astigmatism: Oblique placement of macula

111. Irregular Astigmatism Aetiological types: o Corneal irregular astigmatism: In patient with extensive corneal scars or keratoconus. o Lenticular irregular astigmatism: due to variable refractive index in different parts of the crystalline lens. Mature cataract. o Retinal irregular astigmatism: Due to distortion of the macular area due to scarring or tumors of retina and choroid pushing the macular area.