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LENS MATERIAL &
IT’S
PROPERTIES
MADE BY : SWATI PANARA
FROM : BHARTIMAIYA
COLLEGE OF OPTOMETRY
2nd YEAR 3rd SEMESTER
1
What is an Ophthalmic Lens ?
 It is the portion of the transparent medium
bounded by 2 refracting surfaces at least 1
of ...
LENS MATERIAL
 Ophthalmic lens materials means all
materials used during manufacturing, i.e.
all materials entered into t...
NATURAL
MEDIA
GLASS
PLASTIC
4
NATURAL MEDIA
 Quartz or rock made out of pure silica was
exclusively used
 Its hardness and low mist retaining
property...
Where are they today?
 Clear natural crystals
of quartz are very
rare to find
 So their use is almost
stopped
6
comparisons between glass &
plastic
Glass lenses Plastic lenses
 Heavy
 Low impact resistant
 Scratch resistant
 Cheap...
Glass lenses Plastic lenses
 Less transmission
 Aberration is low
 Higher tendency to fog

 More affected
 It not ne...
GLASS LENS MATERIAL
 Amorphous compound
 Super cooled liquid
 Becomes softer & loses viscosity when
heated
 No specifi...
TYPES
 CROWN GLASS
 BARIUM CROWN
GLASS
 FLINT GLASS
 HIGH INDEX
GLASS
10
CROWN
GLASS
FLINT
GLASS
BARIUM
CROWN
GLASS
HIGH
INDEX
GLASS
11
CROWN GLASS
 70 % Silica , 14- 16 % sodium oxide , 11-
13 % calcium oxide and small percentage
of potassium , borax , ant...
 R.I - 1.523
 Abbe value - 59
 Specific gravity – 2.54
 Transparency - 91.6%
 U.V. absorption – 280 nm
13
ADVANTAGES
 Highly scratch resistant
 Resistant to solvents & temperature
fluctuation
 Tinted by vaccum coating
 Good ...
 Low in costs
 Produce least amount of chromatic aberration
 Blanks available in all sizes
15
DISADVANTAGES
 Low impact resistance
 Heavier material
 Chips can easily form while edging &
handling
 Not appropriate...
FLINT GLASS
 Ingredients:-
45-65% lead oxide
25-45% silica
10% soda + potassium oxide
17
TYPES
DENSE
FLINT
EXTRA
DENSE
FLINT
18
Types Refractive
index
Abbe
value
Specific
gravity
Dense flint 1.649 33.8 3.90
Extra
dense flint
1.69 30.9 4.23
19
ADVANTAGE
 Used in kryptok bifocal
 High prescription
20
DISADVANTAGE
 High dispersion
 High specific gravity
 Transparency less than crown glass
21
BARIUM CROWN GLASS
 25 – 40 % barium oxide
 30 % Silica
Lime , zinc , aluminum , boron , zirconium
22
TYPES
LIGHT BARIUM
CROWN GLASS
DENSE
BARIUM
CROWN GLASS
23
Types Refractive
index
Abbe
value
Specific
gravity
Light
barium
1.573 57.4 3.21
Dense
barium
1.616 55.1 3.36
24
ADVANTAGE
 High RI with out an appreciable increase in
chromatic dispersion
25
DISADVANTAGE
 Chromatic dispersion more than crown
glass
26
HIGH INDEX GLASS
 Any lens having Refractive
Index (RI) higher than 1.523 in
glass is called high index lens
 Available ...
PLASTIC LENS MATERIAL
 Organic substances based on molecular
frame work of carbon with H, N, O
 Superior to glass in man...
• ZIESS
DURALATE
• HIGH INDEX
• POLY
CARBONATE
• PMMA
CR - 39 TRIVEX
TINT
POLARIZED
LENS
29
PLASTIC LENS MATERIAL
 It is a polymeric material of large
molecular wt. which can be shaped by flow
 Also called as org...
PLASTIC LENS MATERIAL
TYPES
THERMOSETTING
THERMOPLASTIC
31
CR - 39
 It is also called as Columbia resin or allyl
diglycol carbonate.
 CR – 39 or allyl diglycol carbonate is a
plas...
 CR stands for Columbia Resin & it is 39th
formula of a thermosetting plastic
developed by the Columbia resin project of
...
 For years CR- 39 was used without anti
scratch coating.
 Now, however, most CR-39 lenses come
with an anti scratch coat...
 CR-39 plastic lenses do not fog up as
easily as glass lenses.
 Whereas welding or grinding spatter will
pit or permanen...
36
 Refractive index : 1.498
 Abbe value : 59.3
 Density : 1.31 g/cm3
 UV cutoff : 355 nm
 Specific gravity : 1.32 Gram/...
 The material is highly impact resistant and
carries high transparency.
 Multiple tinting and coating is possible.
 onl...
39
characteristics
 Lightness
 Impact resistance
 Chemical inertness
 Resistance to pitting
 Resistance to fogging
 Tin...
Disadvantages
 Surface abrasion
 Warpage upon glazing
 Increased thickness
 Inferior photochromic properties
 Index v...
POLYCARBONATE
 Petroleum
derivative and
plastic polymer
 It is a high index
plastic lens
 Gives extra level of
protecti...
POLYCARBONATE
 It is a synthetic material.
 Because of their unique molecular structure they
can flex easily without get...
History
 It was developed in the 1970s for
aerospace application, and is currently used
for the helmet visors of astronau...
45
 R.I. :- 1.586
 Abbe value :- 30
 Specific gravity :- 1.20
 U.V. absorption :- 380 nm
Density : 1.2 g / cm3
46
ADVANTAGE
 Lightest of all materials
 Most impact resistance material
 30-40% thinner than CR-39
 Absorbs nearly 100% ...
DISADVANTAGES
 High chromatic aberration
 Poor optical qualities
 More scratches are formed
 Transparency is less
 Mo...
 Very soft material – low scratch resistance
 Distortions in vision away from optical
center
 Not as easy to tint – dif...
ZIESS DURALATE LENSES
 UV protected lenses
 Coated with fine layer of silicon
 Make scratch resistance
 CR-39 lenses i...
HIGH INDEX
 Available in 2 forms
1) Glass high-index
2) Plastic high-index
 GHI are made up of titanium oxide, barium ox...
52
High index plastics
 Refractive index
:1.640 – 1.740
 Abbe value : 42 –
32
 Density : 1.3 – 1.5
 UV cutoff : 380 –
400...
Types Refractive
Index
Abbe value Specific
gravity
Glass
High
Index
1.600 36 1.45
Plastic
High
Index
1.597 37 1.34
54
ADVANTAGE
 Good cosmesis
 Magnification is reduced
 Available in range of lens types including
photochromic , multifoca...
DISADVANTAGE
 Greater distortions away from optical centre
 Not available in larger blank sizes
 Less impact resistant
...
POLY METHA
METHACRYLATE
 Also known as Perspex or transpex
 Also known as PMMA.
 Polymerization of metha methacrylate
...
58
59
Properties of PMMA
 Refractive index :- 1.49
 Velocity :- 58
Density :- 1.119
 Half of the weight of crown glass
60
Trivex
 Also known as phoenix or triology
 Similar to polycarbonate but have higher
quality optics & thus provide cleare...
 Refractive index : 1.532
 Abbe value : 43-45
 Density : 1.1 g / cm3
 UV cutoff : 380 nm
62
ADVANTAGE
 Trivex is lighter in
weight than standard
plastic but not quite as
thin as polycarbonate.
 Trivex is a more r...
DISADVANTAGE
 get scratch easily
64
POLARIZED LENS
 Polarized filter is made by heating and stretching
thin sheet of PVA (poly vinyl alcohol) to about 4
time...
Tinted lens
 Available in
variety of tints.
 Metallic oxides
are added to
the molten
glass material
to get desired
tints...
A tinted lens could be available in:-
GLASS
 Solid glass tint
 Glass photo chromic tint
 Laminated tints
 Vacuum coate...
ADVANTAGE
 Reduce of glare across the visible spectrum
 Protection against harmful radiations
 Improved cosmetic appear...
Glass CR-39 Polycarbonat
e
Trivex
Specific
gravit
y
2.54 1.32 1.20 1.11
R.I 1.523 1.498 1.586 1.532
Abbe
value
58.9 58 30 ...
PROPERTIES
OF LENSES
70
71
OPTICAL PROPERTIES
ABBE VALUE REFLECTANCE
REFRACTIVE
INDEX
ABSORPTION
72
Abbe number
 Definition :- Reciprocal of the dispersive
power of the material and gives a measure of
the material ability...
 Abbe number of 60 is considered to have
the least chromatic aberrations and abbe
number of 30 is for the most chromatic
...
 The amount of prism created together with
the dispersion value of the lens material
affects the amount of “colour fringe...
REFLECTANCE
 Reflectance is the phenomenon of light
reflection occurs at each of the lens
surfaces.
 The result is the l...
 The reflectance of the lens surface is
calculated from the refractive index of the
material.
 When the light is normal ...
 The higher the refractive index, the greater
the proportion of light reflected from the
surfaces.
78
 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%
79
Refractive Index
 Definition :- the ratio of speed of light in a
vacuum to the speed of light in a given medium
 The hig...
 n = Velocity of light in air
 Velocity of light in the medium
81
ABSORPTION
 The amount of light which goes through a
lens can be reduced because of absorption
by the lens material.
 Th...
 Absorption of an ophthalmic lens generally
refers to its internal absorption, i.e. to the
percentage of light absorbed b...
MECHANICAL PROPERTIES
SPECIFIC
GRAVITY
IMPACT
RESISTANCE
SCRATCH
RESISTANCE
84
Specific gravity
 Definition :- It is the ratio of 1 cubic cm of a
material to 1 cubic cm of water
 The higher the speci...
Impact resistance
 Definition :- Relatively susceptibility of plastics
to fracture under stresses applied at high speed
...
Drop ball test for impact
resistance
87
SCRATCH RESISTANCE
 One of the straight features of glass lenses
is abrasion resistance.
 Plastic lenses need to be coat...
 Lenses may be dipped, or a thin layer of
resin may be spun onto the Lens surface.
 These coating layers are usually 5 m...
ELECTRICAL PROPERTIES
 Electrical properties characterize effects of
electromagnetic waves and electricity on
the materia...
CHEMICAL PROPERTIES
 chemical properties shows the reaction of
materials to the chemical substances
usually found during ...
THERMAL PROPERTIES
 Thermal properties state changes of state
and the effect of temperature on materials.
92
 Therefore, a hypothetical ideal lens
material from both the patient’s and
practitioner’s point of view – a high
refracti...
 Unfortunately, there is no lens material that
fits this description, and we have to use
compromise
94
THANK
YOU
95
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Lens material and its propertes

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Lens material and its propertes

  1. 1. LENS MATERIAL & IT’S PROPERTIES MADE BY : SWATI PANARA FROM : BHARTIMAIYA COLLEGE OF OPTOMETRY 2nd YEAR 3rd SEMESTER 1
  2. 2. What is an Ophthalmic Lens ?  It is the portion of the transparent medium bounded by 2 refracting surfaces at least 1 of which is curved. 2
  3. 3. 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 three types of lens material.  (1) NATURAL MEDIA  (2) GLASS  (3)PLASTIC 3
  4. 4. NATURAL MEDIA GLASS PLASTIC 4
  5. 5. NATURAL MEDIA  Quartz or rock made out of pure silica was exclusively used  Its hardness and low mist retaining property made it ideal for spectacle lens  Not used in optical instruments since it is doubly refracting material 5
  6. 6. Where are they today?  Clear natural crystals of quartz are very rare to find  So their use is almost stopped 6
  7. 7. comparisons between glass & plastic Glass lenses Plastic lenses  Heavy  Low impact resistant  Scratch resistant  Cheaper  3 piece can’t be use  Not proffered for children  Light weight  High impact resistant  Highly Scratch resistant  Costlier  3 piece can be use  good proffered for children 7
  8. 8. Glass lenses Plastic lenses  Less transmission  Aberration is low  Higher tendency to fog   More affected  It not necessary required coating  2% greater transmission  Aberration is high  Lower tendency to fog  Less affected  must necessary required coating 8
  9. 9. GLASS LENS MATERIAL  Amorphous compound  Super cooled liquid  Becomes softer & loses viscosity when heated  No specific melting point 9
  10. 10. TYPES  CROWN GLASS  BARIUM CROWN GLASS  FLINT GLASS  HIGH INDEX GLASS 10
  11. 11. CROWN GLASS FLINT GLASS BARIUM CROWN GLASS HIGH INDEX GLASS 11
  12. 12. CROWN GLASS  70 % Silica , 14- 16 % sodium oxide , 11- 13 % calcium oxide and small percentage of potassium , borax , antimony , arsenic  Used for single vision.  Distant portion of bifocal, trifocal.  Most widely used in ophthalmic industry. 12
  13. 13.  R.I - 1.523  Abbe value - 59  Specific gravity – 2.54  Transparency - 91.6%  U.V. absorption – 280 nm 13
  14. 14. ADVANTAGES  Highly scratch resistant  Resistant to solvents & temperature fluctuation  Tinted by vaccum coating  Good optical qualities  High range curves blanks & addition available  Available in photochromic sunglass option 14
  15. 15.  Low in costs  Produce least amount of chromatic aberration  Blanks available in all sizes 15
  16. 16. DISADVANTAGES  Low impact resistance  Heavier material  Chips can easily form while edging & handling  Not appropriate for children & sport wear  U.V. absorption not 100 % (upto 280nm) 16
  17. 17. FLINT GLASS  Ingredients:- 45-65% lead oxide 25-45% silica 10% soda + potassium oxide 17
  18. 18. TYPES DENSE FLINT EXTRA DENSE FLINT 18
  19. 19. Types Refractive index Abbe value Specific gravity Dense flint 1.649 33.8 3.90 Extra dense flint 1.69 30.9 4.23 19
  20. 20. ADVANTAGE  Used in kryptok bifocal  High prescription 20
  21. 21. DISADVANTAGE  High dispersion  High specific gravity  Transparency less than crown glass 21
  22. 22. BARIUM CROWN GLASS  25 – 40 % barium oxide  30 % Silica Lime , zinc , aluminum , boron , zirconium 22
  23. 23. TYPES LIGHT BARIUM CROWN GLASS DENSE BARIUM CROWN GLASS 23
  24. 24. Types Refractive index Abbe value Specific gravity Light barium 1.573 57.4 3.21 Dense barium 1.616 55.1 3.36 24
  25. 25. ADVANTAGE  High RI with out an appreciable increase in chromatic dispersion 25
  26. 26. DISADVANTAGE  Chromatic dispersion more than crown glass 26
  27. 27. HIGH INDEX GLASS  Any lens having Refractive Index (RI) higher than 1.523 in glass is called high index lens  Available in 1.6 , 1.7 , 1.8 , & 1.9  Contents- high % of Titanium oxide  Transmission- less than 92%  Useful for reducing the thickness for high powered lenses 27
  28. 28. PLASTIC LENS MATERIAL  Organic substances based on molecular frame work of carbon with H, N, O  Superior to glass in many aspects except their softness  They are comparatively more attractive  Can be easily tinted for cosmetic appeal and sun wear 28
  29. 29. • ZIESS DURALATE • HIGH INDEX • POLY CARBONATE • PMMA CR - 39 TRIVEX TINT POLARIZED LENS 29
  30. 30. PLASTIC LENS MATERIAL  It is a polymeric material of large molecular wt. which can be shaped by flow  Also called as organic lenses.  Most plastics are synthetic materials formed by combining various organic ingredients with inorganic materials such as carbon, hydrogen, ,nitrogen, chlorine & Sulphur. 30
  31. 31. PLASTIC LENS MATERIAL TYPES THERMOSETTING THERMOPLASTIC 31
  32. 32. CR - 39  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 eyeglass lenses.  It was developed by Pittsburgh Plate Glass industries.  It is the thermosetting material. 32
  33. 33.  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 39 denotes the type of Columbia Resin used. 33
  34. 34.  For years CR- 39 was used without anti scratch coating.  Now, however, most CR-39 lenses come with an anti scratch coating, making the material much more scratch resistant.  For smaller, high velocity, sharply pointed objects, CR-39 lenses perform better than chemically tempered glass. 34
  35. 35.  CR-39 plastic lenses do not fog up as easily as glass lenses.  Whereas welding or grinding spatter will pit or permanently stick to glass lenses, it does not adhere to plastic lens material. 35
  36. 36. 36
  37. 37.  Refractive index : 1.498  Abbe value : 59.3  Density : 1.31 g/cm3  UV cutoff : 355 nm  Specific gravity : 1.32 Gram/cc 37
  38. 38.  The material is highly impact resistant and carries high transparency.  Multiple tinting and coating is possible.  only drawback is weak resistant to abrasion. 38
  39. 39. 39
  40. 40. characteristics  Lightness  Impact resistance  Chemical inertness  Resistance to pitting  Resistance to fogging  Tintability  Versatility in optical design 40
  41. 41. Disadvantages  Surface abrasion  Warpage upon glazing  Increased thickness  Inferior photochromic properties  Index variability with temperature 41
  42. 42. POLYCARBONATE  Petroleum derivative and plastic polymer  It is a high index plastic lens  Gives extra level of protection to the lens wearers  Usually preferred for children and sportswear 42
  43. 43. POLYCARBONATE  It is a synthetic material.  Because of their unique molecular structure they can flex easily without getting deformed.  All polycarbonate lens absorb U.V radiation up to 380nm without additional treatment.  Polycarbonate lens produce more reflection than ophthalmic crown glass. 43
  44. 44. History  It was developed in the 1970s for aerospace application, and is currently used for the helmet visors of astronauts and for space shuttle windshields.  Eyeglass lenses made from polycarbonate were introduced in the early 1980s in response to demand for lightweight impact resistance lenses. 44
  45. 45. 45
  46. 46.  R.I. :- 1.586  Abbe value :- 30  Specific gravity :- 1.20  U.V. absorption :- 380 nm Density : 1.2 g / cm3 46
  47. 47. ADVANTAGE  Lightest of all materials  Most impact resistance material  30-40% thinner than CR-39  Absorbs nearly 100% of UV-A and UV-B radiation  Can be hard coated for scratch resistance  Center thickness can be ground to 1.2mm  Absorbs ultra violet radiation up to 380nm 47
  48. 48. DISADVANTAGES  High chromatic aberration  Poor optical qualities  More scratches are formed  Transparency is less  More lens distortion  Difficult to dye  More expensive 48
  49. 49.  Very soft material – low scratch resistance  Distortions in vision away from optical center  Not as easy to tint – difficult to match colours  Produces undesirable lateral chromatic aberration upon peripheral gaze 49
  50. 50. ZIESS DURALATE LENSES  UV protected lenses  Coated with fine layer of silicon  Make scratch resistance  CR-39 lenses is higher than heat tampered glass lenses but equal to chemical tampered glass lenses. 50
  51. 51. HIGH INDEX  Available in 2 forms 1) Glass high-index 2) Plastic high-index  GHI are made up of titanium oxide, barium oxide or lead oxide  PHI are made up of Thermosetting plastics  These lenses have higher than standard R.I.  Most cosmetically acceptable lenses  It enables the lens to bend light "faster".  Hence they are lighter and thinner than standard lenses. 51
  52. 52. 52
  53. 53. High index plastics  Refractive index :1.640 – 1.740  Abbe value : 42 – 32  Density : 1.3 – 1.5  UV cutoff : 380 – 400 nm 53
  54. 54. Types Refractive Index Abbe value Specific gravity Glass High Index 1.600 36 1.45 Plastic High Index 1.597 37 1.34 54
  55. 55. ADVANTAGE  Good cosmesis  Magnification is reduced  Available in range of lens types including photochromic , multifocal etc.  Thinner and lighter than conventional lens materials  Can be multicoated , tinted ,UV etc  Available in wide range of blank size & finished as well as semi finished lens types 55
  56. 56. DISADVANTAGE  Greater distortions away from optical centre  Not available in larger blank sizes  Less impact resistant  More chromatic dispersion  Off axis abberation are seen  Fragile, expensive  Incases of plus lenses, the periphery become too thin and hence easily breakable.  Higher RI, more the reflections, hence reducing transmission . 56
  57. 57. POLY METHA METHACRYLATE  Also known as Perspex or transpex  Also known as PMMA.  Polymerization of metha methacrylate  MMA its prepared by hydralysis & esterification of acetone cyanohydrins  Cyanohydrins produced by action of hydro cyanic add on acetone 57
  58. 58. 58
  59. 59. 59
  60. 60. Properties of PMMA  Refractive index :- 1.49  Velocity :- 58 Density :- 1.119  Half of the weight of crown glass 60
  61. 61. Trivex  Also known as phoenix or triology  Similar to polycarbonate but have higher quality optics & thus provide clearer vision 61
  62. 62.  Refractive index : 1.532  Abbe value : 43-45  Density : 1.1 g / cm3  UV cutoff : 380 nm 62
  63. 63. ADVANTAGE  Trivex is lighter in weight than standard plastic but not quite as thin as polycarbonate.  Trivex is a more rigid material making it a better selection for rimless or dril mount frames.  Dose not break easily. 63
  64. 64. DISADVANTAGE  get scratch easily 64
  65. 65. POLARIZED LENS  Polarized filter is made by heating and stretching thin sheet of PVA (poly vinyl alcohol) to about 4 times of its original length.  Sheet is then passed through weak & iodine sol.  Iodine molecules diffuse into the PVA layer and thus creates a polarizing filter.  This thin sheet is laminated between 2 layer of coated cellulose acetate butyrate and then pressed to the desired curvature. 65
  66. 66. Tinted lens  Available in variety of tints.  Metallic oxides are added to the molten glass material to get desired tints. METAL OXIDES COLOU R Iron Green Cobalt Blue Gold Red Nickel Brown Silver Yellow Manganese Pink 66
  67. 67. A tinted lens could be available in:- GLASS  Solid glass tint  Glass photo chromic tint  Laminated tints  Vacuum coated tints PLASTIC  Deep tint  Plastic photo chromic tint 67
  68. 68. ADVANTAGE  Reduce of glare across the visible spectrum  Protection against harmful radiations  Improved cosmetic appearance 68
  69. 69. Glass CR-39 Polycarbonat e Trivex Specific gravit y 2.54 1.32 1.20 1.11 R.I 1.523 1.498 1.586 1.532 Abbe value 58.9 58 30 40 69
  70. 70. PROPERTIES OF LENSES 70
  71. 71. 71
  72. 72. OPTICAL PROPERTIES ABBE VALUE REFLECTANCE REFRACTIVE INDEX ABSORPTION 72
  73. 73. Abbe number  Definition :- Reciprocal of the dispersive power of the material and gives a measure of the material ability to hold spectrum together  The higher the abbe value less will be the transverse chromatic aberration at periphery  Used to calculate axial and transverse chromatic aberration 73
  74. 74.  Abbe number of 60 is considered to have the least chromatic aberrations and abbe number of 30 is for the most chromatic aberrations.  When the wearer moves the eyes away from the centre and looks through the periphery of the lens, the prism is created. 74
  75. 75.  The amount of prism created together with the dispersion value of the lens material affects the amount of “colour fringes” the wearer sees.  Standard plastic lenses have an abbe value of 58.  Most high index materials have a much lower Abbe value. 75
  76. 76. 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. 76
  77. 77.  The reflectance of the lens surface is calculated from the refractive index of the material.  When the light is normal on the lens surface, the percentage of light reflected at each surface is given  by:  Reflectance = 100 (n – 1)² / (n + 1)²% 77
  78. 78.  The higher the refractive index, the greater the proportion of light reflected from the surfaces. 78
  79. 79.  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% 79
  80. 80. 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  If a material has a greater ability to refract light, less curve is required to obtain a specific power hence resulting in a thinner lens.  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. 80
  81. 81.  n = Velocity of light in air  Velocity of light in the medium 81
  82. 82. 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 intrinsic function of a tinted or photo chromatic lens.. 82
  83. 83.  Absorption of an ophthalmic lens generally refers to its internal absorption, i.e. to the percentage of light absorbed between the front and the rear lens surfaces.  Lens absorption occurs according to Lambert’s law and varies exponentially as a function of lens thickness. 83
  84. 84. MECHANICAL PROPERTIES SPECIFIC GRAVITY IMPACT RESISTANCE SCRATCH RESISTANCE 84
  85. 85. Specific gravity  Definition :- It is the ratio of 1 cubic cm of a material to 1 cubic cm of water  The higher the specific gravity of a lens material, the higher will be the density and heavier will be the lens  SG will give a rough idea about the relative weight of various lens  It cannot accurately predict relatively weights of finished lenses as the denser material normally have higher R.I. and thus have a smaller mass. 85
  86. 86. Impact resistance  Definition :- Relatively susceptibility of plastics to fracture under stresses applied at high speed  Relative impact resistance of various materials will vary, depending on the size and shape of the missile used in the test  The standard test employed by FDA involves dropping a 5/8 inch steel ball on to the lens from a height of 50 inches  Polycarbonate is the most impact resistance followed by high index plastic lenses, CR-39, and finally the ordinary glass lenses. 86
  87. 87. Drop ball test for impact resistance 87
  88. 88. SCRATCH RESISTANCE  One of the straight features of glass lenses is abrasion resistance.  Plastic lenses need to be coated with an additional resin to approach the scratch resistance of glasses.  These resin coatings can be applied in a number of ways.  88
  89. 89.  Lenses may be dipped, or a thin layer of resin may be spun onto the Lens surface.  These coating layers are usually 5 micron thick.  While abrasion resistance is an important property for spectacle lenses, it is not crucial to the normal use of the product. 89
  90. 90. ELECTRICAL PROPERTIES  Electrical properties characterize effects of electromagnetic waves and electricity on the materials. 90
  91. 91. CHEMICAL PROPERTIES  chemical properties shows the reaction of materials to the chemical substances usually found during lens manufacture, in every day life, or to certain extreme conditions to which materials can be subjected.  These substances are usually hot or cold water, acids and organic solvents. 91
  92. 92. THERMAL PROPERTIES  Thermal properties state changes of state and the effect of temperature on materials. 92
  93. 93.  Therefore, a hypothetical ideal lens material from both the patient’s and practitioner’s point of view – a high refractive index with low dispersion, unbreakable, unscratchable, low density, available with aspherical surfaces in all multifocal form, easy to tint and to add “inexpensive”, and from the financial point of view, we do not really want them last forever. 93
  94. 94.  Unfortunately, there is no lens material that fits this description, and we have to use compromise 94
  95. 95. THANK YOU 95
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