This PowerPoint included all of the optical and non-optical aspects, uses, advantages, and disadvantages, as well as detailed notes on how to use each aspect.

1. ASPHERIC LENS
Jithin Johney, Assistant professor of Optometry

2. What Is an Aspheric Lens?
 The term aspheric means “not spherical.”
 The degree of curvature of a spheric 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 surface.
 Aspherics are, generally speaking, based on a surface curvature that comes from a
conic section.
 A conic section is a slice through a cone. There are 4 basic types of conic sections.
These are:

3.  1. A circle: A circle is the shape formed by a horizontal plane, or slice through an
upright cone.
 2. An ellipse: An ellipse is a shape formed by an angled plane through a cone that
does not intersect the base of the cone.
 3. A parabola: A parabola is a curve that is formed by the intersection of a cone
with a plane having one side parallel to the side of the cone.
 4. A hyperbola: A hyperbola is a shape formed when a cone is intersected by a
plane that makes a greater angle with the base of the cone than the side of the cone
makes with its base.

4. Conic sections create the curves that are often used for lens surfaces. The circle is
used for spherically based lenses. The ellipse, parabola, and hyperbola are used for
aspheric surfaces.
When these shapes are used as the shape for the front of a lens, they compare
as shown in Figure

5.  The type of asphericity used on a
lens surface is often classified by
“p-values.”
 Thus knowing the “p-value” of an
aspheric surface helps to
understand which type of
asphericity is being used and how
far the surface departs from a
circular or spherical shape.
 For example, a surface having a
p-value of −3.0 is a hyperbolic
surface.

7.  Aspheric surfaces have a changing radius of curvature and thus a varying amount
of surface astigmatism everywhere except at the center of the lens surface.
 This means that it is possible to select a specific type of aspheric surface that will
neutralize unwanted oblique astigmatism.

8. Flattening a +5.00 D lens from a +10.00 D spherical base curve lens back to a +6.50
makes it look more like a low-powered plus lens. With this flat curve, it is no longer
optically sound. Even though the center may produce 20/20 vision, the periphery
suffers from both power error and oblique astigmatism.

9. Properly using aspherics, it is possible to flatten a lens and still overcome peripheral
aberrations. Here, this +5.00 D lens has been flattened to have a +6.50 front curve,
yet because the front curve is aspheric, vision remains clear in the periphery.

10. Purposes for Using an Aspheric Design
 There are at least five good reasons for producing a lens that has an aspheric
surface.
 1. Optically correct lens aberrations.
 2. Allow the lens to be made flatter, thereby reducing magnification and making it
more attractive.
 3. Produce a thinner, lighter weight lens.
 4. Ensure a good, tight fit in the frame.
 5. Make a lens with progressive optics.

11. Asphericity for Optical Purposes
 As stated earlier, for most powers, it is possible to produce a lens that is optically
sound using regular, spherical surfaces.
 Once lens powers go beyond the +7.00 D to −23.00 D range, however, it is
necessary to use an aspheric design.

12. For a plus lens
When using asphericity for the purpose of thinning a plus lens, the front
surface is flattened to give the edge more thickness. For a plus lens, center
thickness is limited by edge thickness. If edge thickness can be added with
asphericity, then the whole lens can be thinned, and center thickness will be
reduced.

13. For a minus lens
Asphericity can be used to thin the edge of a high minus lens. This is done by
steepening the periphery of the front and/or flattening the periphery of the
back curve.

14. A plus lens may be thinned by decreasing the overall diameter of the lens
(A), increasing the refractive index of the lens (B), and changing from a spherical
surface to an aspheric surface (C).

15.  Asphericity for Producing Progressive Power Changes By definition, any lens
surface that is not spheric is aspheric.
 Progressive addition lenses achieve their add power gain from a progressively
steepening surface curvature.
 So progressive addition lenses are also aspheric lenses.
 Most progressive addition lens designs continue to follow the same rules as
spheric base curve lens designs.

16. Thankyou