Aspheric lenses

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Aspheric lenses

  1. 1. ASPHERIC LENSES MADE BY : SWATI PANARA FROM : BHARTIMAIYA COLLEGE OF OPTOMETRY 2nd YEAR 3rd SEMESTER 9 May 2014 1
  2. 2. TERMINOLOGY • A – NOT • SPHERIC - SPHERICAL 9 May 2014 2
  3. 3. HISTORY • In 1909 – Zeiss – Katral lenses. • In 1958– David Volk – Volk conoid lenses. • In 1970s – Robert Welsh – Welsh 4 drop lenses. • 1976 – Davis and Fernald – patented a series of aspheric lenses . 9 May 2014 3
  4. 4. • In 1980 – Whitney , Reilly and Young patented Fulvue aspheric blended Lenticular lenses . 9 May 2014 4
  5. 5. INTRODUCTION • The literal meaning of “aspheric” is simply “not spherical”, which means cylinder surfaces or toroidal surfaces used for astigmatic correction. • In terms of “lens designing” the term “aspherical” usually refers to a surface that is rotationally symmetrical, but at the same time not spherical. 9 May 2014 5
  6. 6. 9 May 2014 6
  7. 7. 9 May 2014 7
  8. 8. • A change in curvature is noticed over the lens surface, rather than constant curvature like a spherical surface. • The change is the same in all direction or meridians of the lens. 9 May 2014 8
  9. 9. • The “asphericity” is achieved by flattening the periphery of the front surface in plus lens and by steepening the periphery of the front surface in minus lens and/or by flattening the periphery of the back surface to reduce the edge thickness. 9 May 2014 9
  10. 10. PRINCIPLE • THE PRINCIPAL USE OF ASPHERIC LENS DESIGN IS THE REDUCTION OR ELIMINATION OF OPTICAL ABERRATIONS PRODUCED BY LOOKING THROUGH AN OPHTHALMIC LENS OBLIQUELY. 9 May 2014 10
  11. 11. Conic section 9 May 2014 11
  12. 12. Conic section • Four types of conic section: 1. Circle 2. Ellipse 3. Parabola 4. hyperbola 9 May 2014 12
  13. 13. CIRCLE CONIC SECTION It‟s formed by horizontal plane 9 May 2014 13
  14. 14. Ellipse conic section It‟s formed by angle plane through cone don‟t intersect the base of cone. 9 May 2014 14
  15. 15. Parabola conic section Intersection of cone with plane having one side parallel to the side of cone. 9 May 2014 15
  16. 16. Hyperbola conic section Intersection by plane that makes of cone than the side of cone make with its base. 9 May 2014 16
  17. 17. WHY ASPHERIC • Unfortunately several lens aberrations affect the quality of peripheral vision through a spectacle lens. • Oblique astigmatism is the principal lens aberration that is considered to be the most serious defect in designing the ophthalmic lens. 9 May 2014 17
  18. 18. 9 May 2014 18
  19. 19. • When the rays of light strike the periphery of the lens obliquely, two focal lines are produced from each single object point. • The dioptric difference between these two focal lines is known as the astigmatic error of the lens. 9 May 2014 19
  20. 20. 9 May 2014 20
  21. 21. • CHOOSING A FLATTER BASE CURVE WILL MAKE THE LENS LOOK LESS BULBOUS & ALSO REDUCE THE MAGNIFICATION.COSMETICALLY, THE LENS LOOK MUCH. • THEY ARE EASILY RETAINED IN THE FRAME AS WELL. 9 May 2014 21
  22. 22. ASPHERIC LENS DESIGN • Three dimensional aspheric surfaces are produced by rotating non- circular curves about an axis of symmetry. • The central curvature or vertex curvature of an aspheric surface is nearly spherical which is the front curve value utilized for lens power and surfacing calculations. 9 May 2014 22
  23. 23. • Away from the vertex curvature, the amount of surface astigmatism smoothly increases. • The rate of increase in surface astigmatism depends upon the degree or type of asphericity. 9 May 2014 23
  24. 24. 9 May 2014 24
  25. 25. • DEMONSTRATION OF THE SURFACE CREATED BY ROTATING AN ELLIPSE ABOUT AN AXIS OF SYMMETRY. • NOTICE THE CHANGING RADII OF CURVATURE IN BOTH THE TANGENTIAL & SAGGITAL PLANES OF LENS.THIS SURFACE ASTIGMATISM IS DESIGNED TO NEUTRALIZE THE OBLIQUE ASTIGMATISM PRODUCED AS THE WEARER LOOKS AWAY FROM THE9 May 2014 25
  26. 26. TANGENITAL & SAGITAL PLANE 9 May 2014 26
  27. 27. • Since, flattening a lens introduces astigmatic and power error, the peripheral curvature of the aspheric surface should change in a manner that neutralizes this effect. 9 May 2014 27
  28. 28. • AT A GIVEN DIAMETER,AN ASPHERIC SURFACE HAS A SHALLOWER SAG THAN THE SPHERICAL SURFACE. 9 May 2014 28
  29. 29. 9 May 2014 29
  30. 30. • Proper base curve selection as recommended by the manufacturer is critical for an aspheric surface. • Even the smaller increments of surface power affect the base curve selection quite considerably. • Consequently, aspheric lenses have more base curves options. 9 May 2014 30
  31. 31. 9 May 2014 31
  32. 32. Aspheric design for plus and minus lenses 9 May 2014 32
  33. 33. MEASURING AN ASPHERIC SURFACE • Since the curvature of an aspheric surface varies from the centre to the periphery, normal measuring instruments cannot measure the front curve value or vertex curvature of an aspheric lens accurately. 9 May 2014 33
  34. 34. 9 May 2014 34
  35. 35. • SO WE MEASURE THE LENS POWER OF AN ASPHERIC LENS IN CENTER SAGITAL AREA BY THE USE OF SPHEROMETER & SAG GAUGE. 9 May 2014 35
  36. 36. CONVENTIONAL INSTRUMENTS 9 May 2014 36 SAG GAUGE
  37. 37. PRISM IN ASPHERIC LENS • The geometric centre of the aspheric lens is to be aligned with the optical centre of the patient‟s eyes. • This accurate centering of the aspheric lens, throws up an issue with the prescribed prism. 9 May 2014 37
  38. 38. • WHEN PRISM IS GROUND IN AN ASPHERIC LENS TO DECENTER THE OPTICAL CENTER,THE OPTICAL CENTER IS MOVED AWAY FROM THIS REFERENCE POINT-WHICH IS TYPICALLY LOCATED AT THE GEOMETRIC CENTER OF THE LENS. • HENCE,GRINDING PRISM TO DECENTER THE OPTICAL CENTER OR DECENTERING THE OPTICAL CENTER TO INDUCE PRISM IS NOT RECOMMENDED. HOWEVER, PRESCRIBED PRISM CAN BE GROUND. 9 May 2014 38
  39. 39. ASPHERIC LENS FOR APHAKIC PATIENT • Aphakics spectacle correction requires powers of + 8.00D to + 15.00D. • Such a high power correction produces a number of difficulties like: • Magnification • Decreased field of view • Aberration and swimming of object in the field of view • “Popeye” appearance of patients • Sensitivity to exact position of the lenses. • Lens weight and thickness. 9 May 2014 39
  40. 40. • This all difficulties are solve in aspheric lens design. 9 May 2014 40
  41. 41. • Various design approaches have been applied to cataract lenses. • There are two main approaches the “foveal philosophy” and the “peripheral philosophy”. • Both use the aspheric curve designing. • Foveal philosophy parallels the standard lens design philosophy, trying to give the patient the largest possible dynamic field of view, which is not possible with the spherical curves. • Therefore, aspheric curves are used. 9 May 2014 41
  42. 42. POSTCATARACT ASPHERIC LENS 9 May 2014 42
  43. 43. ASPHERICS & HIGH-INDEX • ASPHERICS MAKES THE LENS SLIGHTLY THINNER THAN THE CONVENTIONAL SPHERICAL LENS. • MAKING AN ASPHERIC LENS WITH A HIGHER-INDEX MATERIAL WILL MAKE THE LENS FURTHER MORE THINNER. 9 May 2014 43
  44. 44. • HIGH-INDEX LENS MATERIAL WILL GIVE THE PROBLEM OF CHROMATIC ABERRATIONS & MORE SURFACE REFLECTIONS. • HOWEVER , THE SURFACE REFLECTIONS CAN BE REDUCED WITH THE HELP OF „ANTIREFLECTION COATING‟. 9 May 2014 44
  45. 45. CHECKING LENS POWER IN ASPHERIC LENS • The lens power in case of aspheric lenses can be measured on the focimeter so that it is aligned through the geometric centre or middle of aspheric side of the lens. • In this area of the lens, the correct power as per the prescription can be seen. • As we move away from the geometric centre of aspheric surface, the prescription alters- incline with the way the lens surface changes power. 9 May 2014 45
  46. 46. PROPERTIES • REFRACTIVE INDEX – 1.60 • CENTRE THICKNESS – 5.1 mm • WEIGHT – 11 Gram 9 May 2014 46
  47. 47. HOW OBLIQUE ASTIGMATISM IS REDUCED BY ASPHERIC SURFACE ? • THE POWER CHANGES ALONG THE RADIAL / TANGENTIAL DIRECTION IS ABOUT 3 TIMES AS GREAT AS THE CHANGE IN SAGGITAL / CIRCUMFERENTIAL DIRECTION.THUS ALL ASPHERIC SURFACES ARE INHERENTLY ASTIGMATIC AWAY FROM THE CENTER . THE INHERENT ASTIGMATISM OF ASPHERIC SURFACE IS USED TO BALANCE THE OBLIQUE ASTIGMATISM CAUSED BY GAZING OR SEEING OBLIQUELY THROUGH THE LENS . 9 May 2014 47
  48. 48. 9 May 2014 48
  49. 49. • WITH MOST ASPHERIC SURFACE USED FOR OPHTHALMIC LENSES,THE CURVATURE EXTENDING TANGENTIALLY/RADIALLY IN ALL DIRECTIONS FROM CENTER CHANGES PROGRESSIVELY & THE CURVATURE IN THE OPPOSITE MERIDIAN I.e. SAGGITALLY / CIRCUMFERENTIALLY ALSO CHANGES PROGRESSIVELY.BUT THE CHANGE IS MUCH RAPID IN ITS RADIAL DIRECTION. 9 May 2014 49
  50. 50. OFF-AXIS PERFORMANCE • WHEN THE RAYS OF LIGHT FROM THE OBJECT IN THE PERIPHERY STRIKE ANY SPHERICAL LENS OBLIQUELY,IT RESULTS IN ASTIGMATIC FOCUSING ERROR. • THE TWO FOCAL LINES ARE PRODUCED FROM EACH SINGLE OBJECT POINT.THE DIOPTRIC DIFFERENCE BETWEEN THESE TWO FOCAL LINES IS KNOWN AS „THE ASTIGMATIC ERROR OF THE SPHERICAL LENS‟. 9 May 2014 50
  51. 51. OFF-AXIS PERFORMANCE • RAYS OF LIGHT FROM AN OBJECT POINT STRIKE THE SPHERICAL LENS OBLIQUELY & ARE FOCUSED INTO TWO SEPARATE FOCAL LINES,INSTEAD OF A SINGLE POINT FOCUS. 9 May 2014 51
  52. 52. OFF-AXIS PERFORMANCE • RAYS OF LIGHT FROM AN OBJECT POINT STRIKE AN ASPHERIC LENS OBLIQUELY & ARE FOCUSED INTO A SINGLE POINT FOCUS.9 May 2014 52
  53. 53. PURPOSES FOR USING AN ASPHERIC DESIGN • THER E ARE 4 PRIMARY REASONS FOR PRODUCING A LENS THAT HAS AN ASPHERIC SURFACE. 1.TO BE ABLE TO OPTICALLY CORRECT LENS ABERRATIONS 2.TO ALLOW THE LENS TO BE MADE FLATTER,THEREBY REDUCING MAGNIFICATION & MAKING IT MORE ATTRACTIVE. 3.TO PRODUCE A THINNER & LIGHTER LENS 4.TO MAKE A LENS WITH PROGRESSIVE OPTICS.9 May 2014 53
  54. 54. ASPHERICITY FOR OPTICAL PURPOSE • IN HIGH POWERED LENSES IT IS NECESSARY TO USE AN ASPHERIC LENS DESIGN,TO REDUCE OR ELIMINATE THE PERIPHERAL ABERRATIONS. • IN THE MIDDLE,AN ASPHERIC LENS SURFACE STARTS OUT AS ANY OTHER SPHERICAL SURFACE STARTS.THEN,AT A CERTAIN DISTANCE FROM THE OPTICAL CENTER,THE LENS SURFACE GRADUALLY CHANGES ITS CURVATURE AT A RATE CALCULATED TO OFFSET PERIPHERAL ABERRATIONS. 9 May 2014 54
  55. 55. ASPHERICITY FOR OPTICAL PURPOSE 9 May 2014 55 PERIPHERAL ABERRATIONS SEEN IN NORMAL SPHERICAL LENS
  56. 56. ASPHERICITY FOR OPTICAL PURPOSE 9 May 2014 56 PERIPHERAL ABERRATIONS ABSENT IN ASPHERIC LENS
  57. 57. ASPHERICITY FOR THINNING PURPOSE • ASPHERIC „PLUS‟ LENSES ARE MUCH THINNER AT CENTER THAN CONVENTIONAL SPHERICAL LENS. • FOR THINNING A‟PLUS‟LENS EITHER OR BOTH THE LENS FRONT OR BACK SURFACES ARE FLATTENED QUITE A BIT TOWARDS THE EDGE. • FLATTENING THE PERIPHERY MAKES IT POSSIBLE TO GRIND THE WHOLE LENS THINNER. 9 May 2014 57
  58. 58. ASPHERICITY FOR THINNING PURPOSE • ASPHERIC „MINUS‟ LENSES ARE MUCH THINNER AT PERIPHERY THAN CONVENTIONAL SPHERICAL LENSES. • FOR THINNING A „MINUS‟ LENS, THE LENSES FRONT SURFACE IS STEEPENED OR THE BACK SURFACE IS FLATTENED TOWARDS THE PERIPHERY OR BOTH,WHICH REDUCES THE EDGE THICKNESS CONSIDERABLY. 9 May 2014 58
  59. 59. ASPHERICITY FOR PRODUCING PROGRESSIVE POWER CHANGES • BY DEFINATION,ANY LENS SURFACE THAT IS NOT SPHERIC IS „ASPHERIC‟. PROGRESSIVE ADDITION LENSES ACHIEVE THEIR ADD POWER GAIN BY PROGRESSIVELY STEEPENING SURFACE CURVATURE .SO, PAL‟S ARE ALSO CONSIDERED AS ASPHERIC LENSES. 9 May 2014 59
  60. 60. • PAL DESIGNS CONTINUE TO THE SAME RULES AS DO SPHERIC BASE CURVE DESIGNS I.e. , THEIR DISTANCE PORTION WILL HAVE THE SAME BASE AS THE ONE WOULD EXPECT FOR SPHERICALLY BASED CORRECTED LENSES. 9 May 2014 60
  61. 61. • PALS CAN ALSO BE MADE WITH A FLATTER BASE ON THE DISTANCE PORTION.TO AVOID UNWANTED ABERRATIONS,THE FRONT SURFACE SHOULD BE ASPHERICALLY COMPENSATED FOR THE OTHER NONPROGRESSIVE ASPHERIC LENS,WHICH WILL GIVE RISE TO A MORE COMPLEX DESIGN. 9 May 2014 61
  62. 62. Asphericity for flattering purpose • High plus lenses steeper base curve worse lenses look • Choosing flatter base curve make the lens less bulbous & decrease magnification. • Cosmetically look much better. 9 May 2014 62
  63. 63. • Flattened lens surface is spheric • Its possible to get both god cosmetic & good optics • Eye look like normal or natural • Decrease magnification or minification 9 May 2014 63
  64. 64. 9 May 2014 64
  65. 65. Good fitting in frame • Flatter base curve it‟s easy to keep lens tight in frame. 9 May 2014 65
  66. 66. USES • (1) ZOOM LENS • (2) ASTRONOMICAL TELESCOPE • (3) DVD PICK-UP • (4) HIGH POWER LASER • (5) COLLIMATORS • (6) LED LENS • (7) GLASSES • (8) HIGH QUALITY MAGNIFIER 9 May 2014 66
  67. 67. Benefits • Reduction of oblique astigmatism. • Magnification of the eye and surrounding structure as seen by the observer is reduced. • Spectacle magnification for the patient is also reduced. • It is flatter , thinner and lighter lens. 9 May 2014 67
  68. 68. • The lens looks flatter which makes the eyes look more natural due to reduced magnification or minification. • Also flatter lens fits better ensuring the attractive look of the spectacles, thereby, offering the subjects a wider variety of frames to choose from. • If the aspheric lens design is coupled with high index material, it will definitely provide thinner, lighter and flatter lens that looks cosmetically the best. 9 May 2014 68
  69. 69. Disadvantages • Prisms cannot be given in aspheric lenses . • Expensive. • Fitting is quite difficult. 9 May 2014 69
  70. 70. AVAILABLITY OF ASPHERICS IN INDIA • SEIKO – BI-ASPHERICS • KODAK ASPHERICS • ESSILOR(NIKON)ASPHERICS • GKB ASPHERICS • SHAMEIR ASPHERICS • SOLA ASPHERICS • HOYA ASPHERICS • ZEISS ASPHERICS 9 May 2014 70
  71. 71. • RODENSTOCK ASPHERICS • YOUNGER OPTICS(TRIVEX) • TRANSITIONS • DELTA ASPHERICS 9 May 2014 71
  72. 72. FITTING GUIDELINES FOR ASPHERICS • USE MONOCULAR PD. • MEASURE MAJOR REFERENCE POINT HEIGHTS IN THE CONVENTIONAL MANNER.THEN SUBTRACT 1mm FOR EACH 2mm OF PANTASCOPIC TILT. • MOVING THE OPTICAL CENTER,FOR PRISMATIC EFFECT,AWAY FROM THE CENTER OF THE ASPHERIC ZONE WILL DESTROY ANY ASPHERIC OPTICAL ADVANTAGE. 9 May 2014 72
  73. 73. 9 May 2014 73
  74. 74. Indian companies supplying aspheric lenses • Roslane meditech – Haryana • Lensel optic pvt.ltd – Maharashtra • Optivention international – New Delhi • Akriti oculoplasty logistics – Andhra Pradesh9 May 2014 74
  75. 75. DISPENSING TIPS • Pupillary distance: Precise monocular PD measurements position is essential to point the pole of the lens in front of pupil. • Vertical height: The vertical height of each pupil centre is marked on the dummy lens of the selected frame. • Rotation: Rotation of the eye must be considered. This requires lowering the optical centre based on the pantoscopic angle of the frame. • The rule is to lower the optical centre 1 mm for every 2 degrees of pantoscopic tilt. • However, the maximum drop is 5 mm 9 May 2014 75
  76. 76. SUMMARY Flatter than the best form spherical surface • Elimination of large amounts of oblique astigmatism • Decreased spectacle magnification/ minification as shape factor is reduced • Increased field of view • Reduced lens thickness • Reduced edge substance in negative lenses • More lightweight • Good off-axis optical performance • Controlled distortion • Better cosmetic 9 May 2014 76
  77. 77. CONCLUSION • ASPHERIC LENSES ARE A HOGHLY USEFUL GROUP OF LENSES WHICH PROVIDE VISUAL & COSMETIC BENEFITS FOR PATIENT. • ASPHERIC LENSES CAN BE ORDERED IN HIGH INDEX MATERIAL FOR THE ULTIMATE IN ATTRACTIVE THIN LENS. 9 May 2014 77
  78. 78. 9 May 2014 78

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