Objective refraction

details about autorefractometer and other objective methods of assessing visual acuity

1. Dr Sneha Thapliyal OBJECTIVE REFRACTION

2. CLINICAL REFRACTION Determines and corrects refractive errors Objective refraction Subjective refraction Essential in: Young children Mental disability Language difficulty Deaf or mute

3. OBJECTIVE METHODS OF REFRACTION Retinoscopy Autorefractometry Photorefraction Electrophysiological method of objective refraction

4. RETINOSCOPY/SKIASCOPY/SHADOW TEST Objective method of finding out the error of refraction by means of retinoscope utilizing the technique of NEUTRALIZATION

5. Far point concept The far point of eye is defined as the point in space that is conjugate with the fovea when accomodation is relaxed. Emmetropia : Parallel rays focus on fovea. Retina conjugate with infinity. Far point is at infinity . Ammetropias : Parallel rays do not focus on retina. Ammetropic eyes require a correcting lens to make retina conjugate with infinity, i.e., to move far point to infinity Hyperopia : Deficient refractive power. Parallel rays focus behind retina. Far point is beyond infinity. Plus lens converges rays on to retina and conjugate fovea with infinity.

6. Far point concept Myopia: Excessive refractive power. Parallel rays focus in front of retina. Far point is between infinity and eye. Minus lens diverges rays on to the retina and conjugate fovea with infinity. Aspherical ammetropias : This indicates different types of astigmatism. This type of errors have two far points . As a set of rays converge at one place and other at different place due to cornea not having same radius of curvature in all the meridians.

7. Principle of retinoscopy : To bring the far point of patient at the nodal point of the observer at the working distance

8. NEUTRALIZATION No movement of red reflex = NEUTRAL POINT Reversal with overcorrection by 0.25 D On alternating the working distance, slight forward, ‘with’ movement and an ‘against’ movement by slight backward movement.

9. PREREQUISITES FOR RETINOSCOPY

10. PREREQUISITES FOR RETINOSCOPY Dark room: 6 m long A trial set: 64 pairs of Spherical lenses(plus and minus) 0.12D 0.25-4.0D 4.0-6.0D 6.0-14D 14-20D 0.25 0.5 1D 2D

11. 20 pairs of Cylindrical lenses (plus and minus) 0.25-2.00D 2.0-6.0D 10 Prisms: ½ to 6 8,10,12 0.25 0.5

12. Accessories: Plano lenses Opaque discs Pinhole S tenopaeic discs M addox rods Red and Green glasses

13. A trail frame Light weight Aluminium alloy- 30 g Adjustable- horizontally and vertically 3 compartments: spherical, cylindrical, prisms Cylindrical compartment : smooth and accurate rotation Side pieces should be joint- tilted while testing for near vision Back lens in trial frames should occupy as nearly as possible the position of spectacle lens i.e. around 12 mm in front of the cornea. Phoropter or refractor Turn the dial – change the lens before the aperture of the viewing system

14. Distance-vision chart Near vision charts Retinoscope : Invented by Dr J ack Copeland REFLECTING (MIRROR) RETINOSCOPES Priestley-Smith’s mirror Plane mirror SELF-ILLUMINATED RETINOSCOPES Spot self- illuminous Streak retinoscope

15. HEAD NECK HANDLE SLEEVE UP PEEPHOLE SLEEVE DOWN STREAK RETINOSCOPE

16. HEAD HANDLE NECK MIRROR LENS SLEEVE PARASTOP BULB BATTERIES

17. Contact point b/w neck and handle bulb

18. Dark mirror Reflecting mirror Sleeve

19. Observing the optics of retinoscope we find two main systems - Projection system: Light source Condensing lens Focusing sleeve Current source - Obsevation system: Peep hole RETINOSCOPE AND IT’S PARTS:

20. The retinoscope : how it works? PROJECTION SYSTEM : illuminates the retina Light source: a bulb with linear filament that projects a line or streak of light Condensing lens: focuses rays from bulb onto the mirror Mirror : placed in the head of instrument at 45 degree angle, it bends the path of light at right angles to the axis of the handle

21. Focusing sleeve controls Meridian : Turning the sleeve rotates the streak of light. Vergence :By varying the distance between the lens and the bulb Bulb : Moved up-Plane mirror effect(Parallel rays) Moved down-Concave mirror effect(converging rays) Lens : Moved up-Concave mirror effect Moved down-Plane mirror effect

22. In Copelands instrument , moving the sleeve up or down moves the bulb. Sleeve up creates plane mirror effect & sleeve down creates concave mirror effect Copeland type- fixed lens with moving bulb

23. In other Retinoscopes , the lens rather than the bulb moves on moving the sleeve Sleeve up creates a concave mirror effect & sleeve down creates plane mirror effect Fixed bulb type, the lens moves

24. OBSERVATON SYTEM : Allows to see the retinal reflex Light reflected by the illuminated retina enters the retinoscope , passes through an aperture in the mirror & out of the peephole at the rear end of the head.

25. Optics in Retinoscopy Illumination stage : light is directed into the subject’s eye to illuminate the retina Reflex stage : image of the illuminated retina is formed at subject’s far point Observation stage : image of the far point is located by moving the illumination across the fundus and noting the behavior of the luminous reflex seen by the observer in the subject’s pupil

26. Clinician S o S 1 S 2 Patient Optical principles The light in the pupil is called the “ret reflex”

27. The final goal

28. Observer Subject Reflex disappears Reflex moves down , i.e. with direction of movement of light Mirror tilts further forwards Mirror tilts forwards S 2 S 2 Far point behind observers pupil Within pupil Outside pupil No S 2 S 1 Non-neutral point: With movement Far point behind observers pupil . With movement of reflex , gradual change from red reflex to no reflex

29. With movement

30. Observer Subject Far point in front of observers pupil Reflex disappears Reflex moves up , i.e. against direction of movement of light Mirror tilts forwards Mirror tilts further forwards S 2 S 2 Within pupil Outside pupil No S 2 S 1 Non-neutral point: Against movement Far point in front of observers pupil . Against movement of reflex , gradual change from red reflex to no reflex

31. Against movement

32. Reflecting mirror: Perforated mirror Central hole: 2.5mm anteriorly 4 mm posteriorly By a plane mirror, rays are slightly divergent causing less illumination Small hole corresponding to the central hole reduces illumination in pupillary area by causing central dark patch Sides of the small hole are blackened to prevent annoying reflexes from entering the eye Slightly concave mirror with central opening of 4mm and focal length greater than the distance between observer and patient; 150 cm

33. LUMINOUS RETINOSCOPE Streak retinoscope is the most popular Both light source and mirror are incorporated Intensity and type of beam can be controlled.

34. Two main techniques of retinoscopy are Static Retinoscopy : It is the refractive state determined when patient fixates an object at a distance of 6 m with accomodation relaxed . Dynamic Retinoscopy : The refractive state is determined while the subject fixates an object at some closer distance, usually at or near the plane of retinoscope itself with accommodation under action. RETINOSCOPY TECHNIQUES:

35. PROCEDURE OF RETINOSCOPY Patient is made to sit at a distance of 2/3 rd of a meter Accomodation should be relaxed Use right eye for patient’s right eye and left eye for left Trial frame is fitted Direct the light from retinoscope into patient’s pupil Move the retinoscope in horizontally and observe the red reflex.

36. Suitable lens is placed before the eyes to neutralize the band when the pupil will be filled with uniform light Now move vertically, if still pupil is filled with uniform light, no astigmatism. Examiner sits just enough to the side to avoid blocking patient’s fixation

37. Characteristics of the moving retinal reflex Speed and brilliance Low refractive High refractive Bright Faint Moves rapidly Moves slowly

38. 2. Width of reflex Narrow Wide High degree of ametropia Low degree of ametropia 3. Presence of astigmatism When the axis does not correspond with the movement of the mirror, the shadow appears to swirl around.

39. Examiner should scope both vertical and horizontal meridian. We correct the astigmatism with cylindrical lens. Cylindrical lens may be plus or minus, but have power in only one meridian , that which is perpendicular to the axis of the cylinder. The axis meridian is flat and has no power. ASTIGMATISM

40. CYLINDRICAL AXIS Break in the alignment between the reflex in the pupil and the band outside it when the streak is off the axis Rotate the streak until the break disappears Width of the streak appears narrowest when the streak aligns with the true axis.

41. STRADDLING Confirmation of axis Retinoscope streak- 45 degree off axis in both directions Correct axis- width of streak equal in both positions Axis not correct- width is unequal Narrow reflex is guide towards which cylinder’s axis should be turned

42. PROBLEMS IN RETINOSCOPY: Red reflex may not be visible or maybe poor Small pupil Hazy media High degree of refractive error Overcome by causing mydriasis and/or use of converging light with concave mirror retinoscope Changing retinoscopic findings Abnormally active accomodation Overcome by fogging retinoscopy and cycloplegic Spherical aberrations Dilated pupils

43. Conflicting shadows Irregular astigmatism Triangular shadows Conical cornea

44. www.phacotube.com Scissoring shadows Astigmatism Overcome by: undilated pupil

45. Cycloplegics in retinoscopy When retinoscopy is performed after instilling cycloplegic drugs it is termed as Wet Retinoscopy Atropine 1% ointment Used in children < 7yrs age Dose:1% eye ointment 3times daily for 3days Effect lasts for 10-20days Deduction for cycloplegia with atropine is 1D

46. 2. Homatropine 2% drops Used in hypermetropes between 7 to 35yrs Dose:1drop instilled every 10mins for 6times & retinoscopy is performed after 1-2hrs Effect lasts for 48-72hrs Deduction for cycloplegia with homatropine is 0.5D 3. Cyclopentolate I% drops Used in persons between 7 to 35yrs Dose:1drop instilled every 10mins for 3 times & retinoscopy is performed after 1- 1 1/2hrs Effect lasts for 6 to 18hrs Deduction for cycloplegia with Cyclopentolate is 0.75D

47. Only mydriatic [ 10% phenylephrine ] may is used in elderly patients to enhance the pupillary reflex when the pupil is narrow or media is slightly hazy Later it should be counteracted by use of miotic drug[ 2% pilocarpine ]

48. Clinician S 2 Patient Working distance We now have neutral We have also introduced negative vergence due to our working distance (WD) = 1/d (m) Where d = distance in m, measured between your ret and patient’s eye We have added lenses To get the right prescription we need to compensate Rx = lens power – 1/d So to get neutral, we needed: lens power = Rx + 1/d

49. Working distance compensation Calculation For example, if neutrality is achieved with a +3.00DS lens and your working distance is 50cm Rx = +3.00DS – (1/0.50) = +3.00 – 2.00 =+1.00DS Working distance lens Before you begin, add a “working distance lens” A +ve lens to cancel out the negative vergence As in eg., WD = 50cm WD lens = 1/0.50 = +2.00DS Neutrality for the same patient is still +3.00DS WD lens = +2.00DS Lens power = +1.00DS Rx = lens power - 1/d

50. AUTOREFRACTOMETRY Alternative method of assessing error of refraction by use of an optical equipment called refractometer or optometer OBJECTIVE SUBJECTIVE

51. History Collins(1937) developed the first semi automated electronic refractionometer. Safir (1964) automated the retinoscope and this work led to the first commercial autorefractor-the Ophthalmometron . 6600 Autorefractor was the second commercial autorefractor (1969). Munnerlyn (1978) design using a best contrast principle with moving gratings led to the Dioptron , an automated objective refractor.

52. OBJECTIVE SUBJECTIVE SOURCE OF LIGHT TIME REQUIRED Infra-red light 2-4 min Visible light 4-8 min INFORMATION PROVIDED Less informative More informative (corrected visual acuity) PATIENT CO-OPERATION Required less (only has to look straight at target) More co-operation (patient has to turn knob, focus target, answer questions about appearance of target) OCULAR FACTORS Better in macular diseases with clear ocular media Can be done in hazy ocular media OVER REFRACTION CAPABILITY WITH SPECTACLES, CL, IOL Difficult No problem EXPECTED RESULTS Preliminary refractive findings Gives refined subjective result.

53. TOPCON RM 8900/8800 Monitor Control lever with measurement switch Forehead rest Chin rest Examination window Power switch 1.Print switch 2.Menu switch 3.IOL switch

54. SPECIFICATIONS AND PERFORMANCE

55. OPTICAL PRINCIPLES The Scheiner principle The optometer principle Retinoscopic principle Knife edge principle Ray- deflection princile Image size principle

56. Basic design Infrared source (IR-LED) Fixation target Badal lens system Polarized beam splitter Light sensor

57. Autorefractors : Primary and secondary source of electromagnetic radiation: Primary- NIR (Near Infrared Radiation): 780-950nm Efficiently reflected back from the fundus Essentially invisible to the patient Secondary- back scatter from the fundus Determine sphere power, cylinder power, and cylinder axis

58. Fixation target Accomodation is most relaxed when target is- Of low spatial frequency Same as typically seen at distance

59. Nulling vs open-loop measurement principle NULLING PRINCIPLE REFRACTOMETERS: Change their optical system until the refractive error of the eye is neutralized. OPEN-LOOP PRINCIPLE REFRACTOMETERS: Makes measurements by analysing the characteristics of the radiation exiting the eye Do not alter their optical system

60. ALLOWANCE NIR = 800-900nm The reflectance of the fundus increases towards the red end of the spectrum Into the infra red, there will be multiple reflections of scattered radiation within the eye which will degrade the image A 800-900nm will create hypermetropia of 0.75-1.00 DS relative to 550nm

61. Basic Principle Infrared light Rotating chopper Badal lens To PC Polarised filter Slit image more minus 0 more plus Slit mask Light sensor

62. BADAL OPTOMETER: Infrared light is collimated passes through rectangular masks housed in a rotating drum beam splitter Optometer system Moves laterally to find the optimal focus of the slit on the retina

63. Optimal Focus: Optimal focus is achieved when a peak signal is received from the light sensor .

64. SCHEINER DOUBLE PIN-HOLE The original Scheiner double pin-hole was invented in the 16th century. In a clinical setting, the double pin-hole identifies the level of ametropia in a subject by placing it directly in front of the patient’s pupil

65. In a myopic eye, the patient sees crossed diplopic images, whereas in hyperopia , the patient sees uncrossed images. Crossed and uncrossed doubling can easily be differentiated by asking the patient which image has disappeared, when either top or bottom pin-hole is occluded.

66. OPTOMETER PRINCIPLE Aim: to study the number of dioptres of correction on a scale with fixed spacing when the light from the target on the far side of the lens enter the eye with vergence of different amounts- - + The vergence of the light in the focal plane of the optometer lens is linearly related to the displacement of the target.

67. Modern optometers Limitations of early optometers : Alignment problem : both pin-hole apertures must fit within the pupil. Irregular astigmatism : the best refraction over the whole pupil may be different in contrast to the two small pin hole areas of the pupil. Accommodation : the instrument myopia alters the actual refractive status of the patient .

68. Scheiner’s Principle: Two LEDs (light emitting diodes) are imaged to the pupillary plane. These effectively act as a modified Scheiner pin-hole by virtue of the narrow pencils of light produced by the small aperture pinhole located at the focal point of the objective lens.

69. 2 LEDs imaged to the pupillary plane Ocular refraction l/t doubling of LED if refractive error is present Reflection of LEDs from retina back out of the eye Reflection by a semi-silvered mirror Dual photodetectors

70. RETINOSCOPIC PRINCIPLE Autoretinoscopes Source of optical train corresponds to streak retinoscope . Source of light is drum rotating about a source of NIR which produces incident rectangular beam. Neutralization is by badal optometer Unwanted reflexes filtered by polarization of incoming NIR Meridional refractors

71. Autoretinoscope based on direction of fundus streak motion are nulling refractors . Autoretinoscope based on speed of fundus streak motion are non nulling refractors.

72. BEST FOCUS PRINCIPLE: Aim: to find the best focus of an image on retina through the analysis of maximum contrast that can be captured by autorefractor . Both meridional and nulling Neutralization is by badal optometer Filters unwanted reflexes by polarization

73. Knife-edge principle Aim: to evaluate the refractive uniformity of the lens. These are nulling autorefracors that are not meridional Based on principle of reciprocity All light returning through the system passes completely back into the source. Theoretically, no light should escape past the knife-edge.

74. Knife edge principle Foucault Knife edge test for an emmetropic eye. The reflex on the detector moves over most of the surface Knife edge test for myopic eye . The motion of the reflex across the detector provides information on the nature of the refractive error. The speed of the reflex describes the magnitude of refraction

75. Ray deflection principle Aim: to measure Linear deflection of the fundus image in 3 or more meridia at a fixed distance from the eye Angular deflection of rays Position of far point in those meridia trignometrically Open loop (non- nulling ) meridional refractors Corneal reflexes removed by central aperture in the plane Coaxial reflexes removed by polarization

76. Image size principle Aim: to measure the size of fundus image in 3 or more different meridia and calculate the full refractive error on the basis of ocular magnification or minification of the image relative to emmetropia . Non nulling Detection system : fundus camera- CCD camera, video imaging of the fundus reflex, analysis by sophisticated computer programme

77. Conclusion  Autorefraction is a valuable tool in determining a starting point for refraction. Modern technology has resulted in improvements in design, size, speed and accuracy. There are primarily two principles utilised in current autorefractors the Scheiner principle Retinoscopic principle. Improvements in target design (auto-fogging distance targets and open view autorefractors ) attempt to relax accommodation in patients. Autorefractor should not be used as the final refractive correction without further confirmation.