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1
LENS MATERIAL &
IT’S PROPERTIES
Qazi Muhammad OmairLecturer OD
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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.
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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
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NATURAL MEDIA
GLASS
PLASTIC
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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
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WHERE ARE THEY TODAY? Clear natural crystals
of quartz are very rare to find
So their use is almost stopped
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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
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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
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GLASS LENS MATERIAL Amorphous compound Super cooled liquid Becomes softer & loses viscosity when heated No specific melting point
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TYPES CROWN GLASS BARIUM CROWN
GLASS FLINT GLASS HIGH INDEX
GLASS
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CROWN GLASS
FLINT GLASS
BARIUM CROWN GLASS
HIGH INDEX GLASS
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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.
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R.I - 1.523 Abbe value - 59 Specific gravity – 2.54 Transparency - 91.6% U.V. absorption – 280 nm
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ADVANTAGES Highly scratch resistant Resistant to solvents & temperature fluctuation Tinted by vaccum coating Good optical qualities Available in photochromic sunglass option
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Low in costs Produce least amount of chromatic aberration Blanks available in all sizes
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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)
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FLINT GLASS Ingredients:- 45-65% lead oxide 25-45% silica 10% soda + potassium oxide
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TYPES
DENSE FLINT
EXTRA DENSE FLINT
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Types Refractive index
Abbe value
Specific gravity
Dense flint 1.649 33.8 3.90
Extra dense flint
1.69 30.9 4.23
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ADVANTAGE Used in kryptok bifocal Kryptok Lenses. Also referred to as a round lens,
the magnifying area in the lower part of the lens is a complete circle. The lens can restrict viewing up top, due to the round shape. There is less significant transition between reading and distance vision.
High prescription
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DISADVANTAGE High dispersion High specific gravity Transparency less than crown glass
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BARIUM CROWN GLASS 25 – 40 % barium oxide 30 % Silica Lime , zinc , aluminum , boron , zirconium
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TYPES
LIGHT BARIUM
CROWN GLASS
DENSE BARIUM
CROWN GLASS
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Types Refractive index
Abbe value
Specific gravity
Light barium
1.573 57.4 3.21
Dense barium
1.616 55.1 3.36
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ADVANTAGE High RI with out an appreciable increase in
chromatic dispersion
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DISADVANTAGE Chromatic dispersion more than crown glass
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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
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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
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• ZIESS DURALATE
• HIGH INDEX
• POLY CARBONATE
• PMMA
CR - 39
TRIVEX
TINTPOLARIZED LENS
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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.
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PLASTIC LENS MATERIAL TYPES
THERMOSETTING
THERMOPLASTIC
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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.
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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.
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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.
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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.
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Refractive index : 1.498 Abbe value : 59.3 Density : 1.31 g/cm3 UV cutoff : 355 nm Specific gravity : 1.32 Gram/cc
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The material is highly impact resistant and carries high transparency.
Multiple tinting and coating is possible. only drawback is weak resistant to abrasion.
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CHARACTERISTICS Lightness Impact resistance Chemical inertness Resistance to pitting Resistance to fogging Tintability Versatility in optical design
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DISADVANTAGES Surface abrasion Warpage upon glazing Increased thickness Inferior photochromic properties Index variability with temperature
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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
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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.
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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.
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R.I. :- 1.586 Abbe value :- 30 Specific gravity :- 1.20 U.V. absorption :- 380 nm Density : 1.2 g / cm3
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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
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DISADVANTAGES High chromatic aberration Poor optical qualities More scratches are formed Transparency is less More lens distortion Difficult to dye More expensive
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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
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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.
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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.
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HIGH INDEX PLASTICS Refractive
index :1.640 – 1.740
Abbe value : 42 – 32
Density : 1.3 – 1.5 UV cutoff : 380 –
400 nm
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Types Refractive Index
Abbe value Specific gravity
Glass High Index
1.600 36 1.45
Plastic High Index
1.597 37 1.34
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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
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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 .
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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
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PROPERTIES OF PMMA Refractive index :- 1.49
Velocity :- 58
Density :- 1.119
Half of the weight of crown glass
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TRIVEX Also known as phoenix or triology Similar to polycarbonate but have higher quality
optics & thus provide clearer vision
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Refractive index : 1.532 Abbe value : 43-45 Density : 1.1 g / cm3
UV cutoff : 380 nm
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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.
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DISADVANTAGE get scratch easily
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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.
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TINTED LENS Available in
variety of tints.
Metallic oxides are added to the molten glass material to get desired tints.
METAL OXIDES
COLOUR
Iron GreenCobalt BlueGold RedNickel BrownSilver YellowManganese Pink
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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
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ADVANTAGE
Reduce of glare across the visible spectrum Protection against harmful radiations Improved cosmetic appearance
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Glass CR-39 Polycarbonate
Trivex
Specific
gravity
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
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PROPERTIES OF LENSES
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OPTICAL PROPERTIES
MECHANICAL PROPERTIES
ELECTRICAL PROPERTIES
CHEMICAL PROPERTIES
THERMAL PROPERTIES
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OPTICAL PROPERTIES
ABBE VALUE REFLECTANCE
REFRACTIVE INDEX ABSORPTION
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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
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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.
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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.
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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.
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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%
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The higher the refractive index, the greater the proportion of light reflected from the surfaces.
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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%
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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.
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n = Velocity of light in air Velocity of light in the medium
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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..
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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.
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MECHANICAL PROPERTIES
SPECIFIC GRAVITY
IMPACT RESISTANCE
SCRATCH RESISTANCE
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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.
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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.
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DROP BALL TEST FOR IMPACT RESISTANCE
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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.
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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.
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ELECTRICAL PROPERTIES Electrical properties characterize effects of
electromagnetic waves and electricity on the materials.
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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.
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THERMAL PROPERTIES Thermal properties state changes of state and the
effect of temperature on materials.
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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.
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Unfortunately, there is no lens material that fits this description, and we have to use compromise
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THANK YOU