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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