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ASPHERIC LENSES

MADE BY : SWATI PANARAFROM : BHARTIMAIYA

COLLEGE OF OPTOMETRY2nd YEAR 3rd SEMESTER

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TERMINOLOGY

• A – NOT• SPHERIC - SPHERICAL

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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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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.

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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.

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PRINCIPLE

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

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Conic section

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Conic section

• Four types of conic section:1. Circle

2. Ellipse

3. Parabola

4. hyperbola

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CIRCLE CONIC SECTION

It’s formed by horizontal plane

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Ellipse conic section

It’s formed by angle plane through cone don’t intersect the base of cone.

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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 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.

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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.

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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 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.

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• 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.

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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 LENS

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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.

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• AT A GIVEN DIAMETER,AN ASPHERIC SURFACE HAS A SHALLOWER SAG THAN THE SPHERICAL SURFACE.

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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.

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Aspheric design for plus and minus lenses

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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.

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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.

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CONVENTIONAL INSTRUMENTS

SAG GAUGE

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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.

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• 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.

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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.

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• This all difficulties are solve in aspheric lens design.

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• 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.

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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.

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• 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’.

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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.

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PROPERTIES

• REFRACTIVE INDEX – 1.60• CENTRE THICKNESS – 5.1 mm• WEIGHT – 11 Gram

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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 .

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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.

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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.

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OFF-AXIS PERFORMANCE

• RAYS OF LIGHT FROM AN OBJECT POINT STRIKE AN ASPHERIC LENS OBLIQUELY & ARE FOCUSED INTO A SINGLE POINT FOCUS.

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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.

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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.

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ASPHERICITY FOR OPTICAL PURPOSE

PERIPHERAL ABERRATIONSSEEN IN NORMAL SPHERICAL LENS

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ASPHERICITY FOR OPTICAL PURPOSE

PERIPHERAL ABERRATIONSABSENT IN ASPHERIC LENS

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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.

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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.

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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.

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• 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.

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• 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.

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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 spheric• Its possible to get both god cosmetic & good

optics• Eye look like normal or natural• Decrease magnification or minification

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Good fitting in frame

•Flatter base curve it’s easy to keep lens tight in frame.

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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

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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.

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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.

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AVAILABLITY OF ASPHERICS IN INDIA

• SEIKO – BI-ASPHERICS• KODAK ASPHERICS• ESSILOR(NIKON)ASPHERICS• GKB ASPHERICS• SHAMEIR ASPHERICS • SOLA ASPHERICS • HOYA ASPHERICS• ZEISS ASPHERICS

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• RODENSTOCK ASPHERICS• YOUNGER OPTICS(TRIVEX)• TRANSITIONS• DELTA ASPHERICS

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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.

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Indian companies supplying aspheric lenses

• Roslane meditech – Haryana

• Lensel optic pvt.ltd – Maharashtra

• Optivention international – New Delhi

• Akriti oculoplasty logistics – Andhra Pradesh

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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

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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

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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.

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•THANK YOU