ASPHERIC LENSES by optom.jithin johney

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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.

13. 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).

14. 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.

15. Thankyou