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Aspheric lenses
This document discusses aspheric lenses. It begins with a brief history of aspheric lens development from 1909 to 1980. It then covers terminology, the introduction of aspheric lenses which aim to reduce optical aberrations compared to spherical lenses. The document discusses various aspheric lens designs and how they can reduce peripheral aberrations and make lenses thinner. It also covers measuring aspheric lenses, uses of aspheric lenses, and benefits such as reduction of oblique astigmatism and thinner lens designs.
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Aspheric lenses
- 1. ASPHERIC LENSES MADE BY: SWATI PANARA FROM : BHARTIMAIYA COLLEGE OF OPTOMETRY 2nd YEAR 3rd SEMESTER 9 May 2014 1
- 2. TERMINOLOGY • A –NOT • SPHERIC - SPHERICAL 9 May 2014 2
- 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. • In 1980– Whitney , Reilly and Young patented Fulvue aspheric blended Lenticular lenses . 9 May 2014 4
- 5. INTRODUCTION • The literalmeaning 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
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- 8. • A changein 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. • 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. PRINCIPLE • THE PRINCIPALUSE 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. Conic section 9 May2014 11
- 12. Conic section • Fourtypes of conic section: 1. Circle 2. Ellipse 3. Parabola 4. hyperbola 9 May 2014 12
- 13. CIRCLE CONIC SECTION It‟s formedby horizontal plane 9 May 2014 13
- 14. Ellipse conic section It‟sformed by angle plane through cone don‟t intersect the base of cone. 9 May 2014 14
- 15. Parabola conic section Intersectionof cone with plane having one side parallel to the side of cone. 9 May 2014 15
- 16. Hyperbola conic section Intersectionby plane that makes of cone than the side of cone make with its base. 9 May 2014 16
- 17. WHY ASPHERIC • Unfortunatelyseveral 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
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- 19. • When therays 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
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- 21. • CHOOSING AFLATTER 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. 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. • Away fromthe 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
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- 25. • DEMONSTRATION OFTHE 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. TANGENITAL & SAGITAL PLANE 9May 2014 26
- 27. • Since, flatteninga 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. • AT AGIVEN DIAMETER,AN ASPHERIC SURFACE HAS A SHALLOWER SAG THAN THE SPHERICAL SURFACE. 9 May 2014 28
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- 30. • Proper basecurve 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.
- 32. Aspheric design forplus and minus lenses 9 May 2014 32
- 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
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- 35. • SO WEMEASURE THE LENS POWER OF AN ASPHERIC LENS IN CENTER SAGITAL AREA BY THE USE OF SPHEROMETER & SAG GAUGE. 9 May 2014 35
- 36. CONVENTIONAL INSTRUMENTS 9 May2014 36 SAG GAUGE
- 37. PRISM IN ASPHERICLENS • 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. • WHEN PRISMIS 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. ASPHERIC LENS FORAPHAKIC 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. • This alldifficulties are solve in aspheric lens design. 9 May 2014 40
- 41. • Various designapproaches 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.
- 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. • HIGH-INDEX LENSMATERIAL 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. CHECKING LENS POWERIN 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. PROPERTIES • REFRACTIVE INDEX– 1.60 • CENTRE THICKNESS – 5.1 mm • WEIGHT – 11 Gram 9 May 2014 46
- 47. HOW OBLIQUE ASTIGMATISM ISREDUCED 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.
- 49. • WITH MOSTASPHERIC 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. OFF-AXIS PERFORMANCE • WHENTHE 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. OFF-AXIS PERFORMANCE • RAYSOF 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. OFF-AXIS PERFORMANCE • RAYSOF LIGHT FROM AN OBJECT POINT STRIKE AN ASPHERIC LENS OBLIQUELY & ARE FOCUSED INTO A SINGLE POINT FOCUS.9 May 2014 52
- 53. PURPOSES FOR USINGAN 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. 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. ASPHERICITY FOR OPTICAL PURPOSE 9May 2014 55 PERIPHERAL ABERRATIONS SEEN IN NORMAL SPHERICAL LENS
- 56. ASPHERICITY FOR OPTICAL PURPOSE 9May 2014 56 PERIPHERAL ABERRATIONS ABSENT IN ASPHERIC LENS
- 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. 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. ASPHERICITY FOR PRODUCING PROGRESSIVE POWERCHANGES • 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. • PAL DESIGNSCONTINUE 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. • PALS CANALSO 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. Asphericity for flatteringpurpose • 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. • Flattened lenssurface 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.
- 65. Good fitting inframe • Flatter base curve it‟s easy to keep lens tight in frame. 9 May 2014 65
- 66. USES • (1) ZOOMLENS • (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. Benefits • Reduction ofoblique 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. • The lenslooks 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. Disadvantages • Prisms cannotbe given in aspheric lenses . • Expensive. • Fitting is quite difficult. 9 May 2014 69
- 70. AVAILABLITY OF ASPHERICSIN INDIA • SEIKO – BI-ASPHERICS • KODAK ASPHERICS • ESSILOR(NIKON)ASPHERICS • GKB ASPHERICS • SHAMEIR ASPHERICS • SOLA ASPHERICS • HOYA ASPHERICS • ZEISS ASPHERICS 9 May 2014 70
- 71. • RODENSTOCK ASPHERICS •YOUNGER OPTICS(TRIVEX) • TRANSITIONS • DELTA ASPHERICS 9 May 2014 71
- 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.
- 74. Indian companies supplyingaspheric lenses • Roslane meditech – Haryana • Lensel optic pvt.ltd – Maharashtra • Optivention international – New Delhi • Akriti oculoplasty logistics – Andhra Pradesh9 May 2014 74
- 75. DISPENSING TIPS • Pupillarydistance: 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. SUMMARY Flatter than thebest 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. CONCLUSION • ASPHERIC LENSESARE 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.