Lens material

Published on 02/01/2019

1. LENS MATERIALS PRESENTED BY : SHOAIB AKHTAR

2. OBJECTIVE Introduction to ophthalmic lens and its material Glass material –history and its types Plastic material – history and its types Properties of ophthalmic lenses - its classifications and sub classification

3. WHAT IS OPHTHALMIC LENS ? It is the portion of the transparent medium bounded by two refracting surfaces at least one of which is curved.

4. LENS MATERIAL Ophthalmic lens materials means all materials used during manufacturing, i.e. all materials entered into the composition of the basic ophthalmic lens. There are two types of lens material. Glass Plastic

5. GLASS MATERIAL Glass material often referred to as mineral lenses . Advantages Absorb heat and cold Are scratch resistant Highest optical clarity Low Aberration

6. HISTORY OF GLASS MATERIAL Early optical glasses were known as either crown or flint glass. The adjective crown was originally applied to window glass used seventeenth and eighteenth century for houses in England. Before the development of optical glass the pieces of this glass is used for ophthalmic lenses and the lenses were known as crown lenses.

7. Cont... Today Crown glass refers to glass whose primary ingredients are silica, soda or potash and lime. 1676 , George Ravenscorft , an Englishman, used ground flints as a source of silica and incorporated a relatively large quantity of lead as one of the basic constituents of glass. The glass became known as Flint glass.

8. Cont... It was softer, heavier, clearer and more brilliant than any other glass available and was used for drinking vessels, bowls and quality crystal wear. Beginning of in 1876 Ernst Abbe and Otto Schott of Jena, Germany collaborated in experimenting with large number of chemical oxides in the manufacture of glass and developed a wide range of new glasses.

9. Cont... In 1880 , the invention of barium crown by Abbe introduced a glass of high index without an appreciable increase in dispersive power. Today Pittsburgh Plate Glass industries and Scott glass Technologies comprise the major glass manufacture.

10. CROWN FLINT GLA SS G L A S S BARIUM HIGH CROWN IND E X GLASS G L A S S 10 TYPES OF GLASS MATERIAL

11. CROWN GLASS 70 % Silica , 14- 16 % sodium oxide , 11- 13 % calcium oxide and small percentage of potassium , borax , antimony , arsenic. Used for single vision lenses Most common used in ophthalmic industry

12. Cont... Refractive index- 1.523 Abbe value - 59 Specific gravity- 2.54 Transparency- 91.6%

13. Cont... Advantages Highly scratch resistant Good optical qualities Available in single vision and multifocal lenses Available in photochromic sunglass option Low cost Reduce chromatic aberration

14. Cont... Disadvantages Low impact resistance Heavier material Chips can easily form while edging

15. FLINT GLASS Lead oxide 45% to 65% Silica 25% to 45% Mixture of soda and potassium oxide Used for bifocal segments for fused bifocals and single vision lenses of high power because the high index of refraction makes the lenses thinner.

16. Cont... Refraction index- Light flint : 1.580 - D ense flint : 1.690 Abbe value - 30 to 40 Advantages Available in single vision and bifocal lenses Disadvantages Transparency less than crown glass Distortion of image

17. BARIUM CROWN GLASS Barium oxide 25% to 45% Barium oxide which has the same effect as lead oxide in increasing refractive index but without great increase in chromatic dispersion. Refractive index - 1.514 to 1.616 Abbe values - 55 to 59

18. HIGH INDEX GLASS There are higher index glass lens materials available that will reduces thickness for higher powered lenses. Contents- High % of Titanium oxide Refractive index - Available in 1.6 , 1.7 , 1.8 , & 1.9 Abbe value – 33 , 32 , 31 , 30

19. Cont...

20. Cont... Advantages Available in range of lens types including photochromic , multifocal etc. Thinner than other lens materials Can be multicoated Available in wide range of blank size & finished as well as semi finished lens types

21. Cont... Disadvantages Greater distortions Not available in larger blank sizes Less impact resistant More chromatic dispersion Off axis aberration are seen

22. PLASTIC LENS MATERIAL It is a polymeric material of large molecular wt. which can be shaped by flow. Also called as organic material . Most plastics are synthetic materials formed by combining various organic ingredients with inorganic materials such as carbon, hydrogen, nitrogen, chlorine & Sulphur.

23. HISTORY OF PLASTIC MATERIAL Just as World war 1 served as the impetus for the development of optical glass industry, World war 2 served as the impetus for the development of the plastic industry. Plastic material developed during the 2 World war were PMMA develop CR39 for use in military aircraft. In 1947 Robert Graham formed Armorlite lens company in California later described the first ophthalmic lens made from CR39.

24. Cont... In 1957 General Electric developed a new plastic material a Polycarbonate called Lexan . In 1982 Corning glass work the development of a lens called Corlon .

25. C R39 POLYCARBONATE TRIVEX HIGH LENSES IND E X PLASTIC 25 TYPES OF PLASTIC MATERIAL

26. Cont...

27. CR39 It is also called as Columbia resin or allyl diglycol carbonate. CR – 39 or allyl diglycol carbonate is a plastic polymer commonly used in manufacture of eye lenses. It was developed by (PPG) Pittsburgh Plate Glass industries. It is the thermosetting material.

28. Cont... CR stands for Columbia Resin & it is 39 th formula of a thermosetting plastic developed by the Columbia resin project of ( PPG) industries back in 1940 . “CR” stands for Columbia Resin, and the number 39 th denotes the type of Columbia Resin used. For years CR- 39 was used without anti scratch coating.

29. Cont... Now, however, most CR-39 lenses come with an anti scratch coating, making the material much more scratch resistant. CR-39 plastic lenses do not fog up as easily as glass lenses.

30. Cont... Refractive index - 1.498 Abbe value - 58 Specific gravity - 1.32 Advantages Lighter weight than glass lenses Impact resistance Resistance of fogging

31. Cont... Disadvantages Can yellow with age Scratches more easily Surface abrasion

32. POLYCARBONATE Polycarbonate is a thermoplastic. The lens material is soft and requires an anti-scratch coating. It was developed in the 1970s for aerospace application, and is currently used for the helmet visors of astronauts and for space shuttle windshields.

33. Cont... Eyeglass lenses made from polycarbonate were introduced in the early 1980s in response to demand for lightweight impact resistance lenses. Refractive index - 1.586 Abbe value - 29 Specific gravity - 1.20

34. Cont... Advantages Lightest of all material Most impact resistant material 30-40% thinner than CR39 Absorb 100% UV rays Disadvantages High chromatic aberration More scratches and more epensive Transparency less

35. HIGH INDEX PLASTIC Plastic are made up of Thermosetting plastics. These lenses have higher than standard refractive index. Most cosmetically acceptable lenses. It enable the lens to bend light “faster” than other. Hence they are lighter and thinner than standard lenses.

36. Cont... Refractive index – 1.66 Abbe value – 32 Density – 1.35 Advantages Cosmetically good Magnification is reduced Thinner and lighter than other lenses

37. Cont... Available in range of lens type including photochromatic , multifocal etc Impact resistant Disadvantages Greater distortion Not available in larger blank size More chromatic dispersion Off axis aberration are seen Expensive

38. TRIVEX LENSES Trivex lenses developed by PPG industries. The lens material was originally for military use. The lens material to provide excellent safety for window in combat vehicles and good optics. Similar to polycarbonate but have higher quality optics & thus provide clearer vision.

39. Cont... Refractive index – 1.523 Abbe value – 35 to 43 Density – 1.11 Advantages Impact resistance Lighter in weight Disadvantages More scrach

40. PROPERTIES OF LENSES

41. OPTICAL PROPERTIES 41 ABBE VALUE REFLECTANCE RE F R A CT IVE INDEX ABSORPTION

42. ABBE VALUE DEFINITION : Reciprocal of the dispersive power of the material and gives a measure of the material ability to transmit the light . Abbe number of 60 is considered to have the least chromatic aberrations and A bbe number of 30 is for the most chromatic aberrations. Most high index materials have a much lower Abbe value.

43. REFLECTANCE Reflectance is the phenomenon of light reflection occurs at each of the lens surfaces. The result is the loss of lens transparency and undesirable reflections on the lens surfaces. The reflectance of the lens surface is calculated from the refractive index of the material.

44. REFRACTIVE INDEX DEFINITION : The ratio of speed of light in a vacuum to the speed of light in a given medium. The higher the R.I. the thinner the lens can be made. Materials with an index between 1.523 and 1.57 are considered as mid-index, while 1.58 and greater is considered as high-index.

45. ABSORPTION The amount of light which goes through a lens can be reduced because of absorption by the lens material. This is negligible in case of a non–tinted lens, but constitutes an function of a tinted or photo chromatic lens.

46. MECHANICAL PROPERTIES 46 SPECIFIC GR A VITY IMPACT RESIS T ANCE SCRATCH RESIS T ANCE

47. SPECIFIC GRAVITY DEFINITION : The ratio of the density of a substance to the density of some substance (such as pure water) taken as a standard when both densities are obtained by weighing of air. Which tell us about the relative weight of the material with respect to same amount of water. The higher the specific gravity of a lens material, the higher will be the density and heavier will be the lens.

48. IMPACT RESISTANCE DEFINITION : is the ability of a material withstand a high force or shock applied to it over a short period of time. Relative impact resistance of various materials will vary, depending on the size and shape of the missile used in the test. Polycarbonate is the most impact resistance material.

49. Cont...

50. SCRATCH RESISTANCE Lenses that are treated front and back surface with a clear, scratch resistance coating have a much harder surface that is more resistance to scratching. Front and back surface features of glass lenses is scratch (abrasion) resistance. Plastic lenses need to be coated with an additional resin to approach the scratch resistance of glasses.

51. Cont...

52. ELECTRICAL PROPERTIES Electrical properties characterize effects of electromagnetic waves and electricity on the materials.

53. CHEMICAL PROPERTIES Chemical properties shows the reaction of materials to the chemical substances usually found during lens manufacture, and in every day life. These substances are usually hot or cold water, acids and organic solvents.

54. THERMAL PROPERTIES Thermal properties changes of state and the effect of temperature on materials.

55. REFERENCE System for ophthalmic Dispensing Clifford W. Brooks and Irvin M. Borish Internet

56. THANK YOU