Upload
swati-panara
View
464
Download
3
Embed Size (px)
Citation preview
May 3, 2023 1
ASPHERIC LENSES
MADE BY : SWATI PANARAFROM : BHARTIMAIYA
COLLEGE OF OPTOMETRY2nd YEAR 3rd SEMESTER
May 3, 2023 2
TERMINOLOGY
• A – NOT• SPHERIC - SPHERICAL
May 3, 2023 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 .
May 3, 2023 4
• In 1980 – Whitney , Reilly and Young patented Fulvue aspheric blended Lenticular lenses .
May 3, 2023 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.
May 3, 2023 6
May 3, 2023 7
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 11
Conic section
May 3, 2023 12
Conic section
• Four types of conic section:1. Circle
2. Ellipse
3. Parabola
4. hyperbola
May 3, 2023 13
CIRCLE CONIC SECTION
It’s formed by horizontal plane
May 3, 2023 14
Ellipse conic section
It’s formed by angle plane through cone don’t intersect the base of cone.
May 3, 2023 15
Parabola conic section Intersection of cone with plane having one side parallel to the side of cone.
May 3, 2023 16
Hyperbola conic section
Intersection by plane that makes of cone than the side of cone make with its base.
May 3, 2023 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.
May 3, 2023 18
May 3, 2023 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.
May 3, 2023 20
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 24
May 3, 2023 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 THE LENS
May 3, 2023 26
TANGENITAL & SAGITAL PLANE
May 3, 2023 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.
May 3, 2023 28
• AT A GIVEN DIAMETER,AN ASPHERIC SURFACE HAS A SHALLOWER SAG THAN THE SPHERICAL SURFACE.
May 3, 2023 29
May 3, 2023 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.
May 3, 2023 31
May 3, 2023 32
Aspheric design for plus and minus lenses
May 3, 2023 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.
May 3, 2023 34
May 3, 2023 35
• SO WE MEASURE THE LENS POWER OF AN ASPHERIC LENS IN CENTER SAGITAL AREA BY THE USE OF SPHEROMETER & SAG GAUGE.
May 3, 2023 36
CONVENTIONAL INSTRUMENTS
SAG GAUGE
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 40
• This all difficulties are solve in aspheric lens design.
May 3, 2023 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.
May 3, 2023 42
POSTCATARACT ASPHERIC LENS
May 3, 2023 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.
May 3, 2023 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’.
May 3, 2023 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.
May 3, 2023 46
PROPERTIES
• REFRACTIVE INDEX – 1.60• CENTRE THICKNESS – 5.1 mm• WEIGHT – 11 Gram
May 3, 2023 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 .
May 3, 2023 48
May 3, 2023 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.
May 3, 2023 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’.
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.
May 3, 2023 51
OFF-AXIS PERFORMANCE
• RAYS OF LIGHT FROM AN OBJECT POINT STRIKE AN ASPHERIC LENS OBLIQUELY & ARE FOCUSED INTO A SINGLE POINT FOCUS.
May 3, 2023 52
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 55
ASPHERICITY FOR OPTICAL PURPOSE
PERIPHERAL ABERRATIONSSEEN IN NORMAL SPHERICAL LENS
May 3, 2023 56
ASPHERICITY FOR OPTICAL PURPOSE
PERIPHERAL ABERRATIONSABSENT IN ASPHERIC LENS
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 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
May 3, 2023 64
May 3, 2023 65
Good fitting in frame
•Flatter base curve it’s easy to keep lens tight in frame.
May 3, 2023 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
May 3, 2023 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.
May 3, 2023 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.
May 3, 2023 69
Disadvantages
• Prisms cannot be given in aspheric lenses .
• Expensive.
• Fitting is quite difficult.
May 3, 2023 70
AVAILABLITY OF ASPHERICS IN INDIA
• SEIKO – BI-ASPHERICS• KODAK ASPHERICS• ESSILOR(NIKON)ASPHERICS• GKB ASPHERICS• SHAMEIR ASPHERICS • SOLA ASPHERICS • HOYA ASPHERICS• ZEISS ASPHERICS
May 3, 2023 71
• RODENSTOCK ASPHERICS• YOUNGER OPTICS(TRIVEX)• TRANSITIONS• DELTA ASPHERICS
May 3, 2023 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.
May 3, 2023 73
May 3, 2023 74
Indian companies supplying aspheric lenses
• Roslane meditech – Haryana
• Lensel optic pvt.ltd – Maharashtra
• Optivention international – New Delhi
• Akriti oculoplasty logistics – Andhra Pradesh
May 3, 2023 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
May 3, 2023 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
May 3, 2023 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.
May 3, 2023 78
•THANK YOU