Lens material and its propertes

1. LENS MATERIAL &
IT’S
PROPERTIES
MADE BY : SWATI PANARA
FROM : BHARTIMAIYA
COLLEGE OF OPTOMETRY
2
nd
YEAR 3
rd
SEMESTER
1

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. 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. NATURAL
MEDIA
GLASS
PLASTIC
4

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. Where are they today?
Clear natural crystals
of quartz are very
rare to find
So their use is almost
stopped
6

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. 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. GLASS LENS MATERIAL
Amorphous compound
Super cooled liquid
Becomes softer & loses viscosity when
heated
No specific melting point
9

10. TYPES
CROWN GLASS
BARIUM CROWN
GLASS
FLINT GLASS
HIGH INDEX
GLASS
10

11. CROWN
GLASS
FLINT
GLASS
BARIUM
CROWN
GLASS
HIGH
INDEX
GLASS
11

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. R.I-1.523
Abbe value-59
Specific gravity–2.54
Transparency-91.6%
U.V. absorption–280 nm
13

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. Low in costs
Produce least amount of chromatic aberration
Blanks available in all sizes
15

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. FLINT GLASS
Ingredients:-
45-65% lead oxide
25-45%silica
10% soda + potassium oxide
17

18. TYPES
DENSE
FLINT
EXTRA
DENSE
FLINT
18

19. Types Refractive
index
Abbe
value
Specific
gravity
Dense flint1.649 33.8 3.90
Extra
dense flint
1.69 30.9 4.23
19

20. ADVANTAGE
Used in kryptok bifocal
High prescription
20

21. DISADVANTAGE
High dispersion
High specific gravity
Transparency less than crown glass
21

22. BARIUM CROWN GLASS
25 –40 % barium oxide
30 % Silica
Lime , zinc , aluminum , boron , zirconium
22

23. TYPES
LIGHT BARIUM
CROWN GLASS
DENSE
BARIUM
CROWN GLASS
23

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. ADVANTAGE
High RI with out an appreciable increase in
chromatic dispersion
25

26. DISADVANTAGE
Chromatic dispersion more than crown
glass
26

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. 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. •ZIESS
DURALATE
•HIGH INDEX
•POLY
CARBONATE
•PMMA
CR -39 TRIVEX
TINT
POLARIZED
LENS
29

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. PLASTIC LENS MATERIAL
TYPES
THERMOSETTING
THERMOPLASTIC
31

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. 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 denotes the type of Columbia
Resin used.
33

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. 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

37. Refractive index : 1.498
Abbe value : 59.3
Density : 1.31 g/cm
3
UV cutoff : 355 nm
Specific gravity : 1.32 Gram/cc
37

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

40. characteristics
Lightness
Impact resistance
Chemical inertness
Resistance to pitting
Resistance to fogging
Tintability
Versatility in optical design
40

41. Disadvantages
Surface abrasion
Warpage upon glazing
Increased thickness
Inferior photochromic properties
Index variability with temperature
41

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. 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. 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

46. R.I.:- 1.586
Abbe value:-30
Specific gravity:-1.20
U.V. absorption:-380 nm
Density : 1.2 g / cm
3
46

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. DISADVANTAGES
High chromatic aberration
Poor optical qualities
More scratches are formed
Transparency is less
More lens distortion
Difficult to dye
More expensive
48

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. 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. HIGH INDEX
Available in 2 forms
1) Glass high-index
2) Plastic high-index
GHI are made up of titanium oxide, barium oxideor
lead oxide
PHI are made up of Thermosettingplastics
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

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. TypesRefractive
Index
Abbe value Specific
gravity
Glass
High
Index
1.600 36 1.45
Plastic
High
Index
1.597 37 1.34
54

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. 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. 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

59. 59

60. Properties of PMMA
Refractive index :-1.49
Velocity :-58
Density :-1.119
Half of the weight of crown glass
60

61. Trivex
Also known as phoenix or triology
Similar to polycarbonate but have higher
quality optics & thus provide clearer vision
61

62. Refractive index : 1.532
Abbe value : 43-45
Density : 1.1 g / cm
3
UV cutoff : 380 nm
62

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. DISADVANTAGE
get scratch easily
64

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. 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. 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. ADVANTAGE
Reduce of glare across the visible spectrum
Protection against harmful radiations
Improved cosmetic appearance
68

69. Glass CR-39Polycarbonat
e
Trivex
Specific
gravit
y
2.54 1.32 1.20 1.11
R.I1.5231.498 1.586 1.532
Abbe
value
58.958 30 40
69

70. PROPERTIES
OF LENSES
70

71. 71

72. OPTICAL PROPERTIES
ABBE VALUE REFLECTANCE
REFRACTIVE
INDEX
ABSORPTION
72

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. 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. 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 abbevalue
of 58.
Most high index materials have a much
lower Abbe value.
75

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. 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. The higher the refractive index, the greater
the proportion of light reflected from the
surfaces.
78

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. 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. n = Velocity of light in air
Velocity of light in the medium
81

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. 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. MECHANICAL PROPERTIES
SPECIFIC
GRAVITY
IMPACT
RESISTANCE
SCRATCH
RESISTANCE
84

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. 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. Drop ball test for impact
resistance
87

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. 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. ELECTRICAL PROPERTIES
Electrical properties characterize effects of
electromagnetic waves and electricity on
the materials.
90

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. THERMAL PROPERTIES
Thermal properties state changes of state
and the effect of temperature on materials.
92

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. Unfortunately, there is no lens material that
fits this description, and we have to use
compromise
94

95. THANK
YOU
95