ASPHERIC LENSES

ASPHERIC LENSESMADE BY : SWATI PANARAFROM : BHARTIMAIYA COLLEGE OF OPTOMETRY2nd YEAR 3rd SEMESTER18 March 20141

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TERMINOLOGYA NOTSPHERIC - SPHERICAL18 March 20142

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 .

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In 1980 Whitney , Reilly and Young patented Fulvue aspheric blended Lenticular lenses .

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INTRODUCTIONThe 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.18 March 20145

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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.

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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. 18 March 20149

PRINCIPLETHE PRINCIPAL USE OF ASPHERIC LENS DESIGN IS THE REDUCTION OR ELIMINATION OF OPTICAL ABERRATIONS PRODUCED BY LOOKING THROUGH AN OPHTHALMIC LENS OBLIQUELY.18 March 201410

Conic section

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Conic section Four types of conic section:Circle

Ellipse

Parabola

hyperbola

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CIRCLE CONIC SECTION

Its formed by horizontal plane

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Ellipse conic section

Its formed by angle plane through cone dont intersect the base of cone.18 March 201414

Parabola conic section

Intersection of cone with plane having one side parallel to the side of cone.

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Hyperbola conic section

Intersection by plane that makes of cone than the side of cone make with its base.

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WHY ASPHERICUnfortunately 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.18 March 201417

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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. 18 March 201419

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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.

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ASPHERIC LENS DESIGNThree 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.18 March 201422

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.18 March 201423

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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 LENS18 March 201425

TANGENITAL & SAGITAL PLANE

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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.18 March 201427

AT A GIVEN DIAMETER,AN ASPHERIC SURFACE HAS A SHALLOWER SAG THAN THE SPHERICAL SURFACE.18 March 201428

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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.18 March 201430

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Aspheric design for plus and minus lenses

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MEASURING AN ASPHERIC SURFACESince 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.18 March 201433

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SO WE MEASURE THE LENS POWER OF AN ASPHERIC LENS IN CENTER SAGITAL AREA BY THE USE OF SPHEROMETER & SAG GAUGE.18 March 201435

CONVENTIONAL INSTRUMENTS

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SAG GAUGE

PRISM IN ASPHERIC LENSThe geometric centre of the aspheric lens is to be aligned with the optical centre of the patients eyes. This accurate centering of the aspheric lens, throws up an issue with the prescribed prism.18 March 201437

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.18 March 201438

ASPHERIC LENS FOR APHAKIC PATIENTAphakics 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.18 March 201439

This all difficulties are solve in aspheric lens design.18 March 201440

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.18 March 201441

POSTCATARACT ASPHERIC LENS

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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. 18 March 201443

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.18 March 201444

CHECKING LENS POWER IN ASPHERIC LENSThe 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.18 March 201445

PROPERTIESREFRACTIVE INDEX 1.60CENTRE THICKNESS 5.1 mmWEIGHT 11 Gram18 March 201446

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 . 18 March 201447

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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.18 March 201449

OFF-AXIS PERFORMANCEWHEN 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.18 March 201450

OFF-AXIS PERFORMANCERAYS OF LIGHT FROM AN OBJECT POINT STRIKE THE SPHERICAL LENS OBLIQUELY & ARE FOCUSED INTO TWO SEPARATE FOCAL LINES,INSTEAD OF A SINGLE POINT FOCUS.18 March 201451

OFF-AXIS PERFORMANCERAYS OF LIGHT FROM AN OBJECT POINT STRIKE AN ASPHERIC LENS OBLIQUELY & ARE FOCUSED INTO A SINGLE POINT FOCUS.18 March 201452

PURPOSES FOR USING AN ASPHERIC DESIGNTHER 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. 18 March 201453

ASPHERICITY FOR OPTICAL PURPOSEIN 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. 18 March 201454

ASPHERICITY FOR OPTICAL PURPOSE18 March 201455

PERIPHERAL ABERRATIONSSEEN IN NORMAL SPHERICAL LENS

ASPHERICITY FOR OPTICAL PURPOSE18 March 201456

PERIPHERAL ABERRATIONSABSENT IN ASPHERIC LENS

ASPHERICITY FOR THINNING PURPOSEASPHERIC PLUS LENSES ARE MUCH THINNER AT CENTER THAN CONVENTIONAL SPHERICAL LENS.FOR THINNING APLUSLENS 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. 18 March 201457

ASPHERICITY FOR THINNING PURPOSEASPHERIC 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. 18 March 201458

ASPHERICITY FOR PRODUCING PROGRESSIVE POWER CHANGESBY DEFINATION,ANY LENS SURFACE THAT IS NOT SPHERIC IS ASPHERIC. PROGRESSIVE ADDITION LENSES ACHIEVE THEIR ADD POWER GAIN BY PROGRESSIVELY STEEPENING SURFACE CURVATURE .SO, PALS ARE ALSO CONSIDERED AS ASPHERIC LENSES. 18 March 201459

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.18 March 201460

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.18 March 201461

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.

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Flattened lens surface is sphericIts possible to get both god cosmetic & good opticsEye look like normal or naturalDecrease magnification or minification

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Good fitting in frameFlatter base curve its easy to keep lens tight in frame.18 March 201465

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 MAGNIFIER18 March 201466

BenefitsReduction 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.

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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.

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Disadvantages Prisms cannot be given in aspheric lenses .

Expensive.

Fitting is quite difficult.18 March 201469

AVAILABLITY OF ASPHERICS IN INDIASEIKO BI-ASPHERICSKODAK ASPHERICSESSILOR(NIKON)ASPHERICSGKB ASPHERICSSHAMEIR ASPHERICS SOLA ASPHERICS HOYA ASPHERICSZEISS ASPHERICS

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RODENSTOCK ASPHERICSYOUNGER OPTICS(TRIVEX)TRANSITIONSDELTA ASPHERICS18 March 201471

FITTING GUIDELINES FOR ASPHERICSUSE 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.

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Indian companies supplying aspheric lensesRoslane meditech Haryana

Lensel optic pvt.ltd Maharashtra

Optivention international New Delhi

Akriti oculoplasty logistics Andhra Pradesh

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DISPENSING TIPSPupillary 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 mm18 March 201475

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 cosmetic18 March 201476

CONCLUSIONASPHERIC 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.18 March 201477

THANK YOU18 March 201478