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Optom. Fakhruddin S. Barodawala

M. Optom,

ASPHERIC
LENSES

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Aspheric Lenses


Aspheric
• A lens designed to reproduce images better by
having its edges flattened/steepened so that it is
not a perfect sphere, hence: "a" (not) "spherical"
(a sphere).
• A spheric lens surface has a one specific radius of
curvature. An aspheric lens surface changes
shape: it does not have the same radius of
curvature over the entire surface.

Aspheric
• The aspheric lens design mimics nature— the surface of a
normal human cornea is aspheric.
• In an aspheric lens design, the radius of curvature changes,
either flattening or steepening across the surface of the lens.
• Plus-power aspheric lenses become flatter toward the edge of
the lens whereas minus power aspheric curves become
steeper.
• The aspheric curvature on high plus lenses are ground on the
anterior side of the lens, whereas the aspheric curvature of
high minus lenses are ground onto the posterior side of the
lens.

Aspheric – for Plus
• Since the curve of a hyperopic aspheric lens
flattens as it moves away from the center,
center
the lens profile does not protrude as much as
it would if the lens were spherical.
• Nor does the aspheric plus lens magnify the
eye as much as a regular lens design would.
• Thus, the eye looks more natural.


Aspheric – for Minus
• In the minus-power aspheric lens, the curve
steepens toward the edge of the lens.
lens
• This results in a thinner lens edge, which is
particularly desirable in a strong prescription.
• There is also less of a tendency toward the
“small eyes” look that high myopic
prescriptions can create.

Aspheric Lenses
• Aspheric lenses are now
available for plus and minus
lens prescriptions
• Front surfaces of aspheric
lenses are flatter than
conventional lenses
• Aspheric lenses are thinner and
do not protrude or bulge as
much as ordinary lenses

Purposes for using an aspheric surface
1. First reason is to be able to optically
correct lens aberrations.
2. Allows the lens to be made flatter,
thereby reducing the magnification and
making it more attractive.
3. To produce a thinner, lighter-weight lens.
4. To make the lens with progressive optics

Asphericity for Optical Purposes
• Once the power go beyond +7.00 to -23.00D
range it is necessary to use a aspheric design
• In the middle the aspheric lens surface starts
out as any other spherical surface starts out.
• Then at a certain distance from the OC, the
lens surface gradually changes its curvature at
a rate calculated to offset peripheral
aberrations.

Asphericity for Flattening Purposes
• For lenses with spherical base curves , higher plus power
results in steeper base curves.
• But steeper base curves, worse the lenses look and it is
easier to dislogde the lens from the metal frame
• Choosing a flatter base curve - lens looks less bulbous reduces magnification - cosmetically better as wearer’s
eye does not look big.
• But just flattening the regular lens results in bad optics
• In the periphery the there will be unwanted cylinder if just
using spherical lenses,
• Therefore flattened base curve with aspheric lens surface
– both good cosmesis and good optics.

Asphericity for Thinning Purposes
• For plus lenses either or both the lens front and back
surfaces are flattened quite a bit towards the edge.
Flattening the periphery makes it possible to grind the
whole lens thinner.
• To thin minus lenses, the lens front surface is
steepened, back surface is flattened towards the
periphery or both. This reduces the edge thickness.


Asphericity for Producing Progressive
Power changes
• Any surface not spherical is aspheric
• PALs achieve their add power gain from a progressively
steepening surface curvature
• Therefore progressives are also aspheric lenses.
• It can be made with a flatter base curve for the distance
portion , to avoid unwanted aberrations front surface
should be aspherically compensated as is any other nonprogressive aspheric lens.


Advantages
• Aspheric design makes possible thinner, flatter, and
lighter lenses, with greatly improved appearance,
especially for patients with higher prescriptions.

Optical:
• Lighter
• Thinner
• Flatter


Advantages
• Improve cosmetic appearance, eye looks more
natural due to reduction in magnification.


Advantages
Visual:
• Good off-axis visual acuity

Advantages
Visual:
• Low distortion and
aberrations
• Thinner periphery and less
minifiction in case of minus
lenses.
• Oblique astigmatism and
distortions are minimized

Best Choice
• Best combination is Aspheric with high index material.
• Aspheric design excels when combined with a highindex material for mid-to-high prescriptions
• A high-index material alone allows a flatter (more
attractive) base curve, but the flatter base curve
increases off-axis distortion (peripheral distortion).
• Aspheric design compensates for this, making for a
thin, light, attractive lens with superb optics—an effect
that increases in importance as the prescription
increases
• In high prescriptions, Peripheral distortions are
increased which are minimized with the use of
aspheric.

Grinding Prism for de-centration
• In conventional lenses grinding prisms moves for
decentration moves the OC away from the center of the
lens blank without creating any optical problems.
• Aspheric design prohibits grinding prism for decentration
• But if we move the OC of the aspheric lens, the asphericity
will be misplaced relative to the position of the eye.
• When the eye looks one way, it reaches the aspheric
portion too soon. When it looks the other way, aspheric
area is not reached soon enough.
• In short the OC of the aspheric lens must be locked to one
position on the lens blank.

Fitting guidelines for Aspherics
• Use monocular IPDs
• Measure major reference points height in the
conventional manner
• Then subtract 1mm for each 2degree of pantoscopic
tilt
• Aspheric areas concentrically surrounds the lens OC.
Therefore, do not move the MRP more than 5mm
below the pupil
• Therefore it is not advisable to use pantoscopic tilt of
more than 10 degrees
• Remember that the lab cannot grind prism for
decentration with aspheric lenses. Moving the optical
center away from the center of the aspheric zone will
destroy the optical advantage.

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