Bimal Opthalmic Lens material properties.pptx

I'm an Optometrist and I want to share my knowledge about properties of opthalmic lens material that is used in prescription glass for human eye.

1. LENS MATERIALS Bimal Kumar Thakur Consultant Optometrist

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

3. PROPERTIES OF LENSES

4. OPTICAL PROPERTIES 4 ABBE VALUE REFLECTANCE RE F R A CT IVE INDEX ABSORPTION

5. 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. Also called as constringence / V number

6. 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.

7. That’s why antireflection coating is must in high index lenses

8. 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.

9. 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. It occur according to Lamberts law and varies exponentially as the function of lens thickness

10. MECHANICAL PROPERTIES 10 SPECIFIC GR A VITY IMPACT RESIS T ANCE SCRATCH RESIS T ANCE

11. 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.

12. 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. The standard test employed by FDA is Drop ball test

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14. Drop ball test 5/8 inch steel ball Weight approx. 0.56 ounch Dropped from a height of 50 inches Strike within a 5/8 inch diameter circle located at he geometrical center of the lens To pass this test lens must not fracture Polycarbonate > high index plastic lenses > CR -39 > ordinary glass lenses

15. Tempering Any treatment that places the lens under compression provides the lense with greater impact resistance than that possessed by ordinary annealed glass Types Thermal tempering Chemical tempering

16. 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.

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

19. 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.

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

21. LENS MATERIAL Ophthalmic lens materials means all materials used during manufacturing, i.e. all materials entered into the composition of the basic ophthalmic lens. Types of lens material. Natural Media Glass Plastic

22. Natural Media It includes quartz or rock made out of pure silica Its hardness and low mist retaining property made it ideal for spectacle lens Not used in optical instrument since it is double refracting medium Clear natural crystal are very rare to find So their use is almost stopped

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

24. Manufacture Glasses are manufactured by two process Batch process Continuous process Both of these process involve the following steps molting , fining, stirring , Anealing ,inspection ,ground & polising

25. CROWN FLINT GLA SS G L A S S BARIUM HIGH CROWN IND E X GLASS G L A S S 27 TYPES OF GLASS MATERIAL

26. 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

27. Cont... Refractive index- 1.523 Abbe value - 59 Specific gravity- 2.54 Transparency- 91.6% UV Absorption 280nm

28. 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

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

30. 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.

31. 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

32. 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

33. 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

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35. 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

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

37. 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.

38. C R39 POLYCARBONATE TRIVEX HIGH LENSES IND E X PLASTIC 40 TYPES OF PLASTIC MATERIAL

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40. 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.

41. 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.

42. 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.

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

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

45. 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.

46. 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

47. 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

48. 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.

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

50. 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

51. 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.

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

53. THANK YOU