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Lens material

Published on 02/01/2019

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Lens material

  2. 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. 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. 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. (1) Glass (2) Plastic
  5. 5. GLASS MATERIAL • Glass material often referred to as mineral lenses.  Advantages 1. Absorb heat and cold 2. Are scratch resistant 3. Highest optical clarity 4. Low Aberration
  6. 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. 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. 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. 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.
  11. 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. 12. Cont... • Refractive index- 1.523 • Abbe value - 59 • Specific gravity- 2.54 • Transparency- 91.6%
  13. 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. 14. Cont...  Disadvantages • Low impact resistance • Heavier material • Chips can easily form while edging
  15. 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. 16. Cont... • Refraction index- Light flint : 1.580 - Dense 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. 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. 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. 19. Cont...
  20. 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. 21. Cont...  Disadvantages • Greater distortions • Not available in larger blank sizes • Less impact resistant • More chromatic dispersion • Off axis aberration are seen
  22. 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. 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. 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.
  26. 26. Cont...
  27. 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. 28. Cont... • CR stands for Columbia Resin & it is 39th 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 39th denotes the type of Columbia Resin used. • For years CR- 39 was used without anti scratch coating.
  29. 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. 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. 31. Cont... Disadvantages • Can yellow with age • Scratches more easily • Surface abrasion
  32. 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. 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. 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. 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. 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. 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. 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. 39. Cont... • Refractive index – 1.523 • Abbe value – 35 to 43 • Density – 1.11 Advantages • Impact resistance • Lighter in weight Disadvantages • More scrach
  42. 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 Abbe number of 30 is for the most chromatic aberrations. • Most high index materials have a much lower Abbe value.
  43. 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. 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. 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.
  47. 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. 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. 49. Cont...
  50. 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. 51. Cont...
  52. 52. ELECTRICAL PROPERTIES • Electrical properties characterize effects of electromagnetic waves and electricity on the materials.
  53. 53. CHEMICAL PROPERTIES • Chemical properties shows the reaction of materials to the chemical substances usually found during lens manufacture, andin every day life. • These substances are usually hot or cold water, acids and organic solvents.
  54. 54. THERMAL PROPERTIES • Thermal properties changes of state and the effect of temperature on materials.
  55. 55. REFERENCE • System for ophthalmic Dispensing Clifford W. Brooks and Irvin M. Borish • Internet
  56. 56. THANK YOU