Aspheric Lenses.pptx

Aspheric lenses are the special type of lenses which used to correct the spherical aberration.

1. ASPHERIC LENSES SALAL KHAN M.OPTOM, FAAO(USA ), DSM(FIFA )

2. ASPHERIC SURFACE? A= Not, Spheric= Spherical When a circle is rotated about its diameter, the solid of revolution that is obtained is called a sphere and the surface of the sphere is called spherical . Any surface that is not spherical can be termed ashperical . An aspheric surface changes shape i.e., it does not have the same radius of curvature over the entire surface. INTRODUCTION

3. Aspheric surfaces are useful in ophthalmic optics, as they neutralize the oblique astigmatism caused by off-axis viewing by means of the astigmatism inherent in the surface. Thus any point apart from optical centre will have surface astigmatism. INTRODUCTION

4. In 1976- Davis & Fernald- Patented a series of aspheric lenses. In 1980- Whitney, Reilly and Young patented Fulvue aspheric blended lenticular lenses HISTORY

5. Aspheric lenses are lenses where at least one of the surfaces is made aspherical. ASPHERIC LENSES

6. The principle use of aspheric lens design is the reduction or elimination of optical aberrations produced by looking through an ophthalmic lens obliquely. PRINCIPLE

7. Aspheric are based on the surface curvature that comes from a conic section . There are 4 basic types of conic sections. These are: 1. Circle 2. Ellipse 3. Parabola 4. Hyperbola CONIC SECTION

8. Circle: It’s formed by horizontal plane Ellipse: It’s formed by angle plane through cone don’t intersect the base of cone. 2 types of ellipse present- Oblate & Prolate Parabola: Intersection of cone with plane having one side parallel to the side of cone Hyperbola : Intersection by plane that makes of cone than the side of cone makes with its base CONIC SECTION

9. CONIC SECTION

10. The type of asphericity used on a lens surface is often classified by “ p- values”. P- value refer to the value of p in the equation: y 2 = 2r o x − px 2 [r = paraxial radius of curvature p = conic coefficient of the surface] Knowing the p-value will differentiate the conic sections from each other. P-Value

11. P-Value

12. P-Value P-Value Conic Section p > 1 Oblate ellipse p = 1 Circle 0 < p < 1 Prolate ellipse p = 0 Parabola p < 0 Hyperbola

13. To optically correct lens aberrations. To allow the lens to be made flatter. To reducing magnification and making it more attractive. To produce a thinner, lighter weight lens. To ensure a good, tight fit in the frame. To make a lens with progressive optics Purposes for Using an Aspheric Design

14. Aspherics can also be recommended for: Children who are sensitive about how their glasses look ; Contact lens wearers so they will not overwear their contacts to avoid wearing thick, ugly spectacle lenses; and Older wearers to decrease lens weight. Other Possibilities for Using Aspherics

15. Use monocular interpupillary distances. Measure major reference point heights in the conventional manner. Then subtract 1 mm for each 2 mm of Pantoscopic tilt. (The OC should not be more than 5 mm below the pupil.) Remember that the laboratory cannot grind prism for decentration with aspheric lenses. Moving the OC away from the center of the aspheric zone will destroy any aspheric optical advantage. Fitting guidelines for aspherics

16. Full asherics : The changes start gradually and increase more rapidly as distance increases from the center of the lens. Non-full aspherics: some aspheric lenses are designed with a spherical central area or cap that may vary in size depending upon who makes the lens. In this central area, the lens behaves like a spherically based lens. Full Versus Non-full Aspherics

17. Thank You