Ophthalmic lens material

clinical optics

1. Ophthalmic lens material and their clinical significances MODERATOR PRESENTERS NIRAJ DEV JOSHI MANOJ MAHAT SABINA KHATUN

2. Presentation layout Introduction Types of ophthalmic lens material Glass and its varities

3. Introduction Ophthalmic lens is the portion of the transparent medium bounded by two refracting surfaces at least one of which is curved

4. Properties of ophthalmic lenses Optical properties Mechanical properties Electrical properties Chemical properties Thermal properties

5. Optical properties Defines how the material interacts with light. Includes Abbes number Refractive index Reflectance Absorption

6. 1.Abbes number Measure of the degree to which light is dispersed when entering a lens material It is the property of lens material and not affected by any surface technique Abbes number is reciprocal of dispersion Also called as constringence / V number Dispersion (w) = n F – n C / n D -1 Higher the abbes number lower is the chromatic aberration in the lens Abbes number>5ois crown glass & < 5o is flint glass

7. 2.Refractive index Ratio of velocity of light in air to the velocity of light in a medium Describe the ability of lend to bend light Higher the refractive index of material more it will bend the light That’s why high index lenses decreases the thickness of the lenses

8. 3.Reflection Light reflectance occur at each of the lens surfaces Result in the loss of lens transparency and undesirable reflections on the lens surfaces Reflectance = 100 (n -1) 2 /(n+ 1) 2 % For crown glass reflection is 4% The higher the refractive index the greater the proportion of light reflected from the surfaces

9. Refractive index % of light reflected 1.5 7.8 % 1.6 10.4% 1.7 12.3% 1.8 15.7% 1.9 18.3% That’s why antireflection coating is must in high index lenses

10. 4.Absorption The percentage of light absorbed between the front and the rear lens surface It occur according to Lamberts law and varies exponentially as the function of lens thickness i.e Iq = I (q)x

11. Mechanical properties Specific gravity Impact resistance Scratched resistance

12. 1.Specific gravity Ratio of 1 cubic cm of a material to 1 cubic cm of water Higher the specific gravity of a lens material higher will be the density and heavier will be the lens Give a rough idea about the relative weight of various lens

13. 2.Impact resistance Susceptibility of lens material to fracture under stresses applied at high speed Relative impact resistance of various material will vary depending on the size and shape of the missile in the test The standard test employed by FDA is Drop ball test

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. Thermal tempering Finished lenses is heated to 650 degree and rapidly cooled by blast of air on both sides surface quickly becomes rigid with cooling Surface is under compression with resulting tension in internal surface Birefringence under polariscope characterized as maltese cross patterns

17. Chemical tempering Ion exchange method Exchange small ions in the glass with larger ions in the molten salt bath below strain temperature Crown glass lenses the Bath consists of 100% kno3 and sodium ions na + in the lens is replaced by larger potassium ion present in bath Corning photochromatic lens bath consists of 40 % Nano3 and 60% Kno3 Temperature much less than thermal tempering (400-470degree ) Lower temperature means less lens warpage and fewer irregularities

18. Differences Thermal tempering Chemical temparing 1. No resurfacing and re-edging Requires re - edgng and resurfacing 2. Not as much impact resistance 2-3 X more impact resistance 3. Much faster and takes only mins Originally takes 16 hours 4. Maltese pattern visible under Peripheral band visible under polariscope Polariscope 5. Temperature 650 Temperature(400-450 )

19. 3.Scratch resistance Glass lenses are abrasion resistance Plastic lenses needed to be coated with an additional resin to approach the scratch resistance of glasses Premium hard coating > Ziess > Crizal > Teflon

20. Electrical properties Characterized the effects of electromagnetic waves and electricity on the material Chemical properties The reaction of the materials to the chemical substances usually found during lens manufacture Thermal properties Characterised changes of state and effect of temperature on material

21. Lens material All the material used during manufacture 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 An amorphous solid material that is obtained by cooling without crystallization , an organic mixture that has been found to fuse at high temperature Also called super cooled liquid Raw material - quartz (sand ) -Soda (sodium carbonate -Lime(carbon dioxide) - trace amount of potassium , arsenic ,antimony and borax

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. Put major ingredient & cullet in melting pot (36 inches diameter 32 inches heigth 1. Batch process Raise temperature of pot to 800-1000 & keep for 3-5 days Add ingredients at interval till temperature raised to 1400 At the end of melting and fining ,skim of unwanted materials and stir the remaining melt with long clay rods

26. Cool to temp 1200 ,pour and roll into sheet of various thickness .place it in heated annealing oven and gradually cool to room temperature Cut into small pieces ,reheat and mould into rough blanks Ground and polish the front surface of shell into desired curvature Reblocked the blanks , ground and polished on second side For production of small quantities of glass e.g high index glass

27. 2. Continuous flow process The molten glass is not poured jnto sheets Extruded by means of a continuous process and pressed into molds to make rough blanks To produce large quantities of particular type of glass

28. Types of glass Crown glass Flint glass Barium crown glass High index glass Absorptive glass Photochromic glass Polarised glass

29. Crown glass Contains 70 % silica ,14-16 % sodium oxide .11-13% calcium oxide and small percentage of potassium ,borax ,antimony ,arsenic Used for single vision glass Distance portion of bifocal and trifocal Most widely used in ophthalmic industry

30. characteristics Refractive index = 1.523 Abbes number =59 Specific gravity=2.54 Transparency=91.6% U.V abortion=280nm

31. Advantages Highly scratched resistance Resistance to solvent and temperature fluctuation Good optical quality High range curves blanks and addition available Available in photochromic sunglass option Low in cost Produce least amount of chromatic aberration Blanks available in all sizes

32. Disadvantages Low impact resistance Heavier material Chips can easily form while edging and handling Not appropriate for soldier and sport wear U.V absorption not 100%

33. Flint glass Ingrediants 45-65% lead oxide 25-45% silica 105 soda +potassium oxide

34. Types Refractive index Abbes number Specific gravity Dense flint 1.649 33.8 3.90 Extra dense flint 1.69 30.9 4.23

35. Advantages Used in kryptok bifocal High prescription Disadvantages High dispersion High specific gravity so heavier Transparency less than crown glass

36. Barium crown glass Ingrediants 25-40% barium oxide 30% silica Trace amount of lime ,zinc , alumunium , boron and zirconium Use – in segment of Nokromes series of fused bifocal

37. Types Refractive index Abbes value Specific gravity Light barium 1.573 57.4 3.21 Dense barium 1.616 55.1 3.36

38. Advantages High refractive index without an appreciable increase in chromatic dispersion Disadvantages Chromatic dispersion more than crown glass

39. High index lenses The term high index refers to the lenses whose index of refraction is higher than 1.523 in glass and 1.49 in plastic

40. classification Normal index =1.48 to 1.53 Mid index = 1.54 to 1.63 High index = 1.64 to 1.73 Very high index =1.74 to above

41. Glass high index Contents high% of titanium oxide Transmission less than 92% Useful for reducing the thickness for high powered lenses Plastic high index Some plastic high index are made from polyurethane Made up of thermosetting plastics Most cosmetically acceptable lenses

42. Optical properties of high index glasses compared to crown glass Suppliers Glass R.I Abbes no. Specific gravity Numerous crown 1.52 58.9 2.54 Hoya Lh 2 1.6 42.5 2.58 corning Photogray extra 16 1.6 42.2 2.73 Rodenstock High crown 1.604 41.8 2.67 pilkington Slimeline 750 1.704 51.0 3.38 schott High -lite 1.706 31.0 2.99

43. High index lens design It is most commonly available in multiple progressive and single designs Aspheric and atoric design are also becoming readily available to the eye care provider in high index material High index is also available with polarization or photochromatic options

44. When to recommended high index lenses High index lens materials for any patient with a prescription of +/- 3.00 diopters or more People who are self conscious about the thickness of their lenses People who wear contacts because of traditional lenses distort their eyes

45. Advantage of high index lenses Power rings round the edges of the lenses is reduced Reduce thickness Look flatter Thinner Lighter The benefit is greater peripheral vision as well as less magnification/ minification of the eye

46. Disadvantages Low abbes number so increase chromatic aberration Poor light transmission and increased backside and inner surface reflection increasing so importance of anti reflective coating More expensive than normal glass Greater distortion away from the optical centre

47. Absorptive glass Made by addition of metallic oxides to the raw materials in the batch . Addition of different metal provide different color cobalt – blue chromium oxide –green magnesium oxide –violet uranium oxide – yellow cerium oxide – UV absorption iron oxide – IR radiation

48. properties Reduces the amount of transmitted light or radiant energy Acts as a filter May be uniform or neutral , absorbing light of all wavelengths equally May be selective , absorbing light of certain wavelengths more than others

49. Major forms of absorptive lenses Tinted solid glass lenses Glass lenses with surface coatings Tinted plastic lenses Photochromatic lenses Polarizing lenses

50. Photochromatic lenses( glass) Contain silver halide crystal On exposure to uv light silver halide separates into silver and halogen ions The dissociated silver halide regroup to form opaque silver colloid The opaque silver halide colloid absorb visible light and photochromatic lenses becomes dark In absence of uv light the silver and halide crystals recombine and the lenses fades

51. Advantages Helps eye to cope up as they react to different light conditions during day time. Are immune to fatigue or deterioration of their photochromic performance with extended use. Are available in single vision, bifocal & varifocal forms Gives visual comfort indoor & outdoor

52. DISADVANTAGES Do not adjust immediately Losses its darkening ability with time Cant replace sunglasses because temperature increases reaction slows Do not darken under vehical because windscreen absorb uv light

53. Polarizing lenses Polarized lenses are transparent pieces of material usually glass or plastic that blocks certain types of light waves Light reflected from surfaces like a flat road is generally horizontally polarized This horizontally polarized light is blocked by the vertically polarizer in the lenses . The result is the reduction in the annoying and sometime dangerous glare. Occurs at the Brewsters angle i.e tan i =n

54. Recommendation of polaroid lenses Day time driver Fisherman At beach /sea shores Snowfield worker Good for UV protection

55. Limitations Do not provide universal protection from glare Because the polarizing stripes reduce the amount of light entering the eye , these lenses should not be used at night can cause headache and eyestrain Do not usually work with snow glare because snow tends to reflect light equally in all directions

56. PRESENTATION LAYOUT Introduction History Types on the basis of physical properties Plastic materials Summary References

57. Plastics Polymeric material of large molecular weight which can be shaped by flow. Also called as organic material Synthetic materials formed by combining various organic ingredients with inorganic material such as carbon ,hydrogen , oxygen, nitrogen, Sulphur , etc

58. History 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 World war 2 were PMMA and later develop CR-39 for use in military aircraft. In 1947 Robert Graham, formed the Armor-lite Lens Company in Pasadena and California & later described the first ophthalmic lenses made from CR-39 (scratches more easily than glass, is much more scratch resistant than PMMA)

59. In 1957 General Electric developed a new plastic material, a polycarbonate resin , called Lexan , which has great mechanical strength and high service temperatures. In 1982 corning glass works announced the development of a lens called Corlon , a two layered ophthalmic composite material consisting of a glass lens backed by a thin layer of polyurethane film

60. Types Two types of plastic on the basis of physical properties. Thermoplastic material Thermosetting material

61. Thermoplastic material -soften when heated and can be remolded. -have their molecule arranged in long chains and usually supplied in pellet, granular or sheet form. -the material can be stretched ,pressed ,molded into complex shapes with no appreciable change in chemical structure.

62. Since no chemical changes occur so softening and hardening cycle may be repeated indefinitely Less dimensionally stable ,withstand less heat without deformation. Examples include acrylates (plexiglass, Lucite and PMMA), cellulose acetate , cellulose nitrate ,polycarbonate ,nylon , polystyrene and vinyls .

63. Manufacture Granules of raw material are melted at 320 C Molten material is injected into molds Reciprocating screw flights

64. Thermosetting plastic material Once heated and molded, these plastics cannot be reheated and remolded. The molecules of these plastics are cross linked lattice pattern in three dimensions and this is why they cannot be reshaped The bond between the molecules is very strong. Difficult to recycle.

65. Once hardened the material can’t be softened*. If subjected to high temperature ,material decomposes without melting. Good dimensional stability , rigid , relatively insensitive to heat ,relatively hard surface. Example include Columbia Resin-39, Bakelite , melamine(used for tableware.)

66. Commonly used plastic material Polymethyl methacrylate (PMMA) Corlon Polycarbonate CR-39 Trivex High index plastics

67. 1. PMMA PMMA - Poly Methyl Methacrylate (C₅O₂H₈)n It is a thermoplastic and transparent plastic. has a refractive index of 1.490 Chemically, it is a synthetic polymer of Methyl Methacrylate Preparation: Suspension polymerization and Radical polymerization

68. Clinical advantages High degree of transparency Half the weight of glass Disadvantages Became soft and deformed in hot climates More liable to abrasions

69. Transparent Property: Density of PMMA: 1150 - 1190 kg/m³ Density of Glass: 2400 - 2800 kg/m³ It can transmit 97% of visible light ( 3-mm thickness)

70. Contd.. Daylight Redirection : PMMA can be used in laser - cut panels to redirect sunlight into a light pipe and then spread it into a room.

71. 2. Corlon Special type of lens made from plastic and glass lens Front is glass* and back is plastic* Lenses are manufactured as C-Lite lenses Advantages 25% thinner than conventional lenses 25% lighter than all glass lenses * glass-crown *Plastic- polyurethane

72. 3.Polycarbonate The first alternate plastic material was polycarbonate It is a high index plastic lens Gives extra level of protection to the lens wearers Usually preferred for children and sportswear

73. Manufacturing process Polycarbonate is a thermoplastic material so it begins as solid and is melted down and is then injected into a mold at a temperature of 320 C In the injection process, the polycarbonate conforms, under pressure, to the highly polished metal surfaces of the injection mold A device will squeeze the lens to prevent lens shrinkage and ensure optical accuracy After removal from the molds, the lenses are inspected and processed through a coating machine

74. Because the surface hardness of polycarbonate is much softer than CR-39, all polycarbonate lenses receive hard coating After the coating process is completed, a heat curing process produces polymerization and cross linking of the coating The coating solvents evaporate, leaving behind a silica based solid coating approximately 4-6 µm thick

75. Clinical Advantages 1.High Refractive index i.e 1.586 So lens are thinner 2.Low specific gravity i.e 1.20 So lighter and thinner lens in high prescription 3.Polycarbonate lenses absorb ultraviolet radiation upto 380nm without additional treatment 4.High impact resistance so tremendiously beneficial where safety is the primary concern I.e for children, monocular individuals, industries and sportsperson

76. Disadvantages Due to high index increase lens reflections Difficulty in molding to free from waves and blemishes Difficulty in edging and beveling

77. Polycarbonates are soft and scratches very easily Having a higher index of refraction may produce more reflections than ophthalmic crown glass or CR-39 Low Abbe number(30) so produces undesirable lateral chromatic aberration upon peripheral gaze.

78. Impact Resistance of Polycarbonate Polycarbonate is impact resistance because its molecules are extremely long chains of atoms that can slide back and forth on each other, with the result that the material can be flexed and even deformed without breaking Stephens and Davis showed that polycarbonate has 21 times the impact resistance of CR-39 for ¼ inch and 1/8 inch steel balls fired at high speed

79. 4.CR-39 CR stands for Columbia Resin developed by the Columbia resin project of (PPG) industries back in 1940. Number 39 denotes the type of Columbia Resin used. • For years CR- 39 was used without anti-scratch coating • It was developed by (PPG) Pittsburgh Plate Glass industries. Good optical quality, with a high V-value and is easy to surface, edge, drill and coat . .

80. • It is the thermosetting material • 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. For smaller, high velocity, sharply pointed objects, CR-39 lenses perform better than chemically tempered glass

81. Manufacture Ophthalmic lenses made of CR-39 are cast from allyl di-glycol carbonate monomer Supplied as yellow viscous liquid Some add a copolymer or other additives, such as anti yellowing agents, ultraviolet absorbers and mold releasers These copolymers may reduce lens shrinkage, make casting easier and make lens tougher and lighter After catalyst and other ingredients are added, the liquid resin is poured into a glass mould having a concave and a convex surface

82. The inside surface of the mold are highly polished to produce finished lens surfaces of high quality The entire mold is then placed in an oven and subjected to a controlled time/temperature relationship : the cure cycle After completion of the cure cycle, the molds are removed from the oven, dismantled and separated from the finished lens Lower temperatures and longer curing times result in lenses having superior rigidity, dimensional stability, impact resistance and scratch resistance

83. Liquid monomer is mixed with a catalyst, placed in molds & heated

84. Optical properties of CR39 Refractive index = 1.498 Abbe value = 58 Specific gravity = 1.320 Clinical advantages 1.Lightness Low specific gravity 2.Impact resistance Pass FDA impact resistance test

85. Contd.. 3.Chemical inertness Resistance to all chemical solvents except highly oxidizing agents 4.Resistance to fogging Due to low thermal conductivity 5.Tintability Tint or re-tint or remove

86. 6. Versatility in optical design: examples include aspheric surfaces for cataract lenses, magnifiers with aspheric surfaces, progressive addition multifocal lenses etc. 7.Resistance to pitting Welding spatter or particles

87. Disadvantages 1.Surface abrasion Low resistance to surface abrasion 2. Warpage upon glazing 3. Increased thickness

88. 4. Inferior photochromic properties 5. Index variability with temperature Change in RI as a function of temperature is 50 times greater than of optical glass making CR-39 less stable in thermally fluctuating environments

89. Impact Resistance of CR-39 With ballistics tests using a 1/8 inch steel ball, it was found that a 3mm CR-39 lenses were more impact resistant than air tempered glasses Welsh et al tested untreated glass, air tempered glass, chemically tempered glass and CR-39 plastic lenses with a ballistic device using a 3/8 inch missile and found the CR-39 to be the most impact resistant and untreated glass to be the least

90. 5. TRIVEX Trivex lenses developed by PPG industries, Inc 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. Commercial brands- Hoya as the Phoenix material, Younger Optics as Trilogy®, Augen Optical as Trinity™, Thai Optical as Excelite TVX™,

91. Optical properties • Refractive index – 1.523 • Abbe value – 35 to 43 • Density – 1.11   Trivex is a very new lens material that exhibits three very good lens characteristics, therefore its name Trivex . Trivex is also known as Trilogy and phoenix. I. First, it is the lightest weight of all plastic materials. II. Second, it has impact resistance at least equal to polycarbonate. III. Third, it has an excellent abbe value.

92. Excellent Optics incorporates UV protection. Edges can be polished. the material can be coated for scratch protection and anti-reflection and tinted Available in single vision, aspheric, bifocal, and a limited number of progressive lens designs.  It is the lens of choice for drill- mounted lenses because it does not crack or split at the drilled hole.

93. Minimal disadvantages. inability to be tinted darker because it has a 1.53 index, lenses may have very slightly thicker edges than other materials,

94. 6. HIGH INDEX PLASTIC LENSES High Index lenses Lenses of plastic or glass material having higher index of refraction. Characteristics Light weight Thinner Comfortable Attractive

95. Contd … • High index Plastics are made up of Thermosetting plastics. • These lenses have higher than standard refractive index. • Most cosmetically acceptable lenses. High index lenses are available in glass or plastics.

96. • It enable the lens to bend light “faster” than other. The higher the index of lens material, the thinner and light-weight the lens is. The thinnest, light weight lenses are typically the most expensive.

97. Optical characteristics Refractive index – 1.640-1.740 • Abbe value – 42-32 • Density – 1.3-1.5 UV cutoff- 380nm-400nm Advantages • Cosmetically good • Magnification is reduced • Thinner and lighter than other lenses Available in range of lens type including photochromatic , multifocal etc • Impact resistant

98. Disadvantages • Greater distortion • Not available in larger blank size • More chromatic dispersion • Off axis aberration are seen • Expensive

99. From crown glass (n=1.523) to high index lenses (n=1.90), the annoying reflection rises 8% to 20%. Twice as much as crown glass. An AR coating is a must for lenses with high index materials. Refractive index and reflectance per lens surface

100. Nikon Plastic High Index lenses Nikon Lite DXII The world's lightest lens. This is a mid-index lens that is 40% lighter and 28% slimmer than conventional plastic. Nikon Lite IV Lightweight, super-thin high index lens. 29% lighter and 40% thinner than conventional plastic. Nikon Lite V 50% thinner than conventional plastic. Nikon Pointal Ultra slim glass is 45% thinner for the best cosmetic appearance

101. Plastic photochromic lens 1 st commercially successful photochromic lens produced by Transition optical in 1991,the plastic versions are getting increasingly commercialized. Rather of silver halide crystals , it uses organic dyes ( indolino spirorazines i.e. INS) When activated by UV rays, the INS molecules scatters absorbing sunlight & reducing amount of visible light . When removed chemical reaction reverse & lens return to its clear state.

102. Materials used in Contact lens Hard lens material Manufacture from plastic or thermoplastic material. The first commercially available contact lens material was PMMA (poly methylene meth acrylate). Rigid gas permeable lens material – Earlier Cellulose acetate butyrate –thermoplastic Silicone Styrene

103. Presently RGP material A. Silicone acrylate Commonly manufactured from copolymer of PMMA and silicone containing vinyl monomer B. Fluoropolymers Made from copolymer containing fluorine High oxygen permeability Not prone to surface deposits

104. Hydrophilic soft lens material Soft lens are made of hydrogels containing hydroxyl groups A.HEMA lenses Original soft lens material Resistant to biodegradation and withstand chemical and thermal sterilization B.HEMA VP lenses Mixture of HEMA and Vinyl pyrro-lidone (VP) Tendency to color with age

105. C. MMA-PVD lenses Made of hydrophilic polymer PVP ,monomer VP and hydrophobic MMA D. Glycidyl Methacrylate Combination of HEMA and VP or with MMA and VP

106. Intraocular lenses material 1 .Rigid PMMA IOLs Commonly used material for manufacture of IOL is PMMA. Rigid ,non -foldable and hydrophobic(water content<1%) Chemically stable compound Specific gravity is much low( ie 1.2), much lower than heavier glasses Disadvantages include variation in temperature and water absorption Because of the required large incision, PMMA IOLs are seldom preferred today .

107. 2. Foldable IOLs Became popular after the success of phaco -emulsification Available in different designs and are made up of: A . Silicone IOLs: Made up of Polyorganosiloxane Low Index of refraction- 1.43 Silicone lenses have been suspected to favor bacterial adhesion, with increased risk for postoperative infection, so not in use in modern days

108. B. Hydrophobic Foldable Acrylic Hydrophobic foldable acrylic materials are a series of copolymers of acrylate and methacrylate derived from rigid PMMA, with the purpose of making them foldable and durable. refractive index between 1.44 and 1.55. C.Hydrophilic Foldable Acrylic Hydrophilic acrylic materials are composed of a  mixture of hydroxyethylmethacrylate (PHEMA) and hydrophilic acrylic monomer Hydrophilic acrylic lenses are soft, somewhat compressible, and have excellent biocompatibility because of their hydrophilic surface. Hydrophilic acrylic IOLs are the most popular worldwide, especially in the US because of the FDA approval.

109. Comparisons between glass & plastic S.N Glass lens Plastic lens 1 Heavy Light weight 2 Low impact resistance High impact resistance 3 Lower Scratch resistant Highly scratch resistant 4 Cheaper Costlier 5 3 piece cant be used 3piece can be used 6 Aberration is low Aberration is high 7 Higher tendency to fog Lower tendency 8 Less transmission 2% greater transmission 9 Coating isn’t a necessity Coating is must 10 Not preferred for children Preferred for children

110. Plastics or Glass So due to inherent lightness and safety, plastics has almost completely replaced glass as the first choice of spectacle lens material Today in most of the developed world, some 95% of spectacle lenses are made from plastics materials However glass is still in use with very high indices in excess of 1.80, and to photochromic lenses with specialized properties, such as Corning’s CPF glasses.

111. References Clinical Optics- Troy E. Fannin , Theodore Grosvenor System of ophthalmic dispensing- Clifford W. Brooks , Irvin M. Borish Essentials of ophthalmic lens finishing- Clifford W. Brooks CET articles Internet

112. Happy Holi