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Special type of lenses

Special type of lenses

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Special type of lenses

  1. 1. OPTOM FASLU MUHAMMED Special Type of Lenses
  2. 2. Special Type of Lenses Lenticular lenses Aniseikonic lenses Aspheric Lenses Fresnel lenses & prisms
  3. 3. Lenticular lenses 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 diameter .
  4. 4. Lenticular lenses
  5. 5. 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 as well.
  6. 6. 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.
  7. 7. 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.
  8. 8. Lenticular lenses
  9. 9. Anisometropia Anisometropia is when there is a difference in refractive power between the left and right eyes. A significant amount of Anisometropia ends up creating problems. When Anisometropia is corrected with spectacle lenses, problems are not always over.
  10. 10. ANISEIKONIA 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 Physiologic Aniseikonia Symmetrical Aniseikonia Anatomic Versus Optical Aniseikonia
  11. 11. 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.)
  12. 12. CORRECTING ANISEIKONIA WITH SPECTACLE LENSES 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.
  13. 13. There are several ways to approach the problem of Aniseikonia:- 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 base curves. 4.Use a high-index lens material. This will thin plus lens center thickness
  14. 14. ASPHERIC LENS
  15. 15. 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 surface.
  16. 16. Purposes for Using an Aspheric Design 1.The first reason is to be able to optically correct lens aberrations. 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.
  17. 17. 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 design.
  18. 18. 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.
  19. 19. To Ensure a Good, Tight Fit in the Frame Asphericity for Producing Progressive Power Changes
  20. 20. Asphericity for Thinning Purposes (Geometric Asphericity)
  21. 21. FRESNEL LENS A Fresnel lens is similar to a series of concentric prisms, each with a slightly higher prismatic effect.
  22. 22. When Are Fresnel Lenses Used? Nonspectacle Uses 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.
  23. 23. Short-Term Wear Creating Adds
  24. 24. 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 prescriptions.
  25. 25. 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.
  26. 26. 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.
  27. 27. Polycarbonate Polycarbonate lenses are lighter than plastic and recommended for children, safety and sports because the material is impact-resistant. 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 glass lenses
  28. 28. 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 them heavier.
  29. 29. Varifocal lens 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.

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