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OPTOM FASLU MUHAMMED
Special Type of Lenses
Special Type of Lenses
Fresnel lenses & prisms
These lenses have been created particularly for use
in high powers.
Here a central portion of the lens is grounded to
have the power and this is called the aperture.
peripheral part of the lens act as a carrier.
The aperture is usually 30-40 mm in
Thickness of central part poses a difficulty
while fixing the lens to the frame. This
Difficulty is offset by grinding the peripheral
part ,which reduces the thickness by 1.2-2.0
mm and improves the cosmetic appearance
Various types of Lenticular lenses
Solid lenticulars : carrier has got convex slope.
Plano lenticulars: the carrier is plane and the
aperture is either convex or concave .A concave
plano lenticular is called Myodisk.
Cemented lenticulars :the aperture part carries
the sphere and is glued on a carrier on which the
cylinder is incorporated.
Profile lenticulars: aperture edge follows the
same shape as whole lens shape so that the
aperture is made as inconspicuous as possible.
Fused lenticulars:the aperture is ground on
the back surface of a plus lens and is filled with
glass of higher refractive index and heated at 600
degree C.The front surface is ground to reduce the
same amount of power.
Anisometropia is when there is a difference in
refractive power between the left and right eyes.
A significant amount of Anisometropia ends up
When Anisometropia is corrected with spectacle
lenses, problems are not always over.
Aniseikonia is a relative difference in the size
and/or the shape of the images seen by the right
and the left eyes.
Types of Aniseikonia
Anatomic Versus Optical Aniseikonia
Knapp’s Law and Axial Ametropia
According to Knapp’s law, “When a correcting lens is
so placed before the eye that its second principal
plane coincides with the anterior focal point of an
axially ametropic eye, the size of the retinal image
will be the same as though the eye were emmetropic.”
(It should be noted that for Knapp’s law to be
fulfilled, the ametropia must be purely axial, and
there must be no anatomic Aniseikonia present.)
CORRECTING ANISEIKONIA WITH
If an exact amount of aniseikonia is found,
modifications to the spectacle lenses that
change relative spectacle magnification will
be of benefit whether the anisometropia is
axial or refractive.
This is because there are specific
modifications that can be made to spectacle
lenses that will change their magnification.
There are several ways to approach the problem
1.Use a frame with a short vertex distance
2. Use a frame with a small eye size. This secondarily
reduces vertex distance.
3.Use an aspheric lens design. This usually flattens the
4.Use a high-index lens material. This will thin plus
lens center thickness
What Is an Aspheric Lens?
The term aspheric means “not spherical.”
The degree of curvature of a spherical lens is
continuously uniform with a consistent radius of
curvature throughout its entire surface, like that of a
ball or sphere.
An Aspheric lens surface changes shape. It does not
have the same radius of curvature over the entire
Purposes for Using an Aspheric Design
1.The first reason is to be able to optically correct
2.To allow the lens to be made flatter, thereby reducing
magnification and making it more attractive.
3. To produce a thinner, lighter weight lens.
4.To ensure a good, tight fit in the frame.
5. To make a lens with progressive optics.
Asphericity for Optical Purposes
Once lens powers go beyond the +7.00 D to−23.00 D
range, however, it is necessary to use an aspheric
Asphericity for Flattening Purposes
For lenses with spherical base curves, higher plus
power always results in steeper base curves
Unfortunately, for high plus lenses the steeper the
base curve, the worse the lenses look.
Choosing a flatter base curve will make the lens look
less bulbous and also reduce magnification.
Cosmetically the lens looks much better. It even
looks considerably thinner.
To Ensure a Good, Tight Fit in the Frame
Asphericity for Producing Progressive
Asphericity for Thinning Purposes
A Fresnel lens is similar to a series of
concentric prisms, each with a slightly
higher prismatic effect.
When Are Fresnel Lenses Used?
Fresnel lenses are not just used for spectacles
Large minus Fresnel lenses are sometimes applied to
a window to create a wider field of view, or are used
for the warning beams of seaside lighthouses.
High density lens
The eye-care industry differentiates lens materials by
their "refractive index." Simply put, this is the
material's ability to displace light .
Density and refractive index are not the same, but
they go hand-in-hand. As material density increases,
so does its refractive index.
There are higher index glass lens materials available
that will reduce lens thickness for higher powered
Plastic and Glass Lenses
Plastic is the most common material used in
spectacle lenses. Standard plastic lenses have an
IR of 1.49 or 1.50.
Glass lenses are not as popular as plastic because
glass is about twice as heavy. Standard glass
lenses have an IR of 1.52.
A lens with an IR higher than 1.52 is considered
to be a Hi-index lens. The most common range
for hi-index lenses is between 1.54 and 1.74.
Index of Refraction
The index of refraction (IR) is the difference in
the speed of light as it passes through air and into
lens material, and is expressed as a number.
Regular plastic lenses have an IR of 1.50. High
index lenses bend light more efficiently because
the lens material is compressed. Thinner lenses
have a higher IR number.
Polycarbonate lenses are lighter than
plastic and recommended for children,
safety and sports because the material is
Polycarbonate is a hi-index material with
an IR of 1.586. The lenses are lighter but,
in certain types of prescriptions, the optics
are not as crisp or clear as with plastic or
High-index glass lens materials generally have Abbé
values close to that of polycarbonate.
Unfortunately, high-index glass lenses are composed
of materials with a higher specific gravity, making
Varifocal lens is also called as progressive
addition lenses (PAL), progressive power
lenses, graduated prescription lenses, and or
multifocal lenses, are corrective lenses used in
eyeglasses to correct presbyopia.
They are characterized by a gradient of increasing
lens power, added to the wearer's correction for the
other refractive errors.