Lens Materials and Their Properties

OPTOMETRY · SEMESTER 1

Lens Materials and Their Properties

Ophthalmic Dispensing Theory

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_ LENS MATERIAL &

IT’S

. \ PROPERTIES

What is an Ophthalmic Lens ?

, * It is the portion of the transparent medium
bounded by 2 refracting surfaces at least |

of which is curved.

LENS MATERIAL

, * Ophthalmic lens materials means all
materials used during manufacturing, i.e.
all materials entered into the composition
of the basic ophthalmic lens.
ji # * There are three types of lens material.

} + (1) NATURAL MEDIA

* (2) GLASS

> * (3)PLASTIC

NATURAL

MEDIA
GLASS

PLASTIC

NATURAL MEDIA

) * Quartz or rock made out of pure silica was
exclusively used
property made it ideal for spectacle lens
s * Not used in optical instruments since it is

doubly refracting material

, ° Clear natural crystals

of quartz are very
rare to find
? « So their use is almost

stopped

comparisons between glass &

plastic
Glass lenses Plastic lenses
f° Heavy * Light weight
Low impact resistant ¢ High impact resistant
Scratch resistant * Highly Scratch resistant

Cheaper * Costlier

3 piece can’t be use * 3 piece can be use

Not proffered for children * good proffered for children

Glass lenses

Less transmission
Aberration is low
Higher tendency to fog
More affected
It not necessary required
coating
Plastic lenses
2% greater transmission
Aberration is high
Lower tendency to fog
Less affected
must necessary required

coating

GLASS LENS MATERIAL

', * Amorphous compound
¢ Super cooled liquid
J * Becomes softer & loses viscosity when
heated

DVIRES

CROWN GLASS

BARIUM CROWN

GLASS

FLINT GLASS

HIGH INDEX

GLASS
°

CROWN |}

GLASS

BARIUM HIGH

CROWN INDEX

GLASS GLASS

CROWN GLASS

) * 70 % Silica , 14- 16 % sodium oxide , 11-
13 % calcium oxide and small percentage
of potassium , borax , antimony , arsenic
( * Used for single vision.
ji * * Distant portion of bifocal, trifocal.

¢ Most widely used in ophthalmic industry.

Wy) ¢ R.A- 1.523

¢ Abbe value – 59
§ * Specific gravity — 2.54

* Transparency – 91.6%

C ¢ U.V. absorption — 280 nm

ADVANTAGES

i 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 |,

* Low in costs

* Produce least amount of chromatic aberration

* Blanks available in all sizes

DISADVANTAGES

ij, * 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)

FLINT GLASS

if, * Ingredients:-
45-65% lead oxide
25-45% silica

10% soda + potassium oxide

dense flint

Refractive |Abbe Specific

index value gravity
Dense flint | 1.649 33.8 3.90 |

Extra 1.69 30.9 4.23 |

ADVANTAGE

i, * Used in kryptok bifocal

* High prescription

DISADVANTAGE

i, * High dispersion
¢ High specific gravity

§ * Transparency less than crown glass

BARIUM CROWN GLASS

, * 25—40 % barium oxide

* 30 % Silica

Lime , zinc , aluminum , boron , zirconium

TYRES

LIGHT BARIUM

CROWN GLASS

DENSE

BARIUM

CROWN GLASS

Refractive |Abbe Specific

index value gravity

E573) 57.4 S22

1.616 55.1 3.36

ADVANTAGE

, * High RI with out an appreciable increase in

chromatic dispersion

DISADVANTAGE

i, * Chromatic dispersion more than crown

glass

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,&
19)
* Contents- high % of Titanium
oxide
* Transmission- less than 92%
* Useful for reducing the
thickness for high powered

lenses 2

PLASTIC LENS MATERIAL

¢ Organic substances based on molecular
frame work of carbon with H, N, O
| 4 °* Superior to glass in many aspects except
their softness
= * They are comparatively more attractive
* Can be easily tinted for cosmetic appeal

and sun wear

PLASTIC LENS MATERIAL

¢Itisa polymeric material of large
molecular wt. which can be shaped by flow
formed by combining various organic
ingredients with inorganic materials such
as carbon, hydrogen, ,nitrogen, chlorine &

Sulphur.

PLASTIC LENS MATERIAL

TYPES

CR – 39

| * It is also called as Columbia resin or allyl
diglycol carbonate.
plastic polymer commonly used in
* manufacture of eyeglass lenses.
} It was developed by Pittsburgh Plate Glass
industries.

¢ It is the thermosetting material.

* CR stands for Columbia Resin & it is 39%

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.

¢ For years CR- 39 was used without anti

scratch coating.
with an anti scratch comings making the
material much more scratch resistant.
: ¢ For smaller, high velocity, sharply pointed
objects, CR-39 lenses perform better than

chemically tempered glass.

) * CR-39 plastic lenses do not fog up as

easily as glass lenses.
pit or permanently stick to glass lenses, it

does not adhere to plastic lens material.

i, * Refractive index : 1.498

* Abbe value : 59.3
§ * Density : 1.31 g/cm?
¢ UV cutoff: 355 nm

) ¢ Specific gravity : 1.32 Gram/cc

", ¢ The material is highly impact resistant and

carries high transparency.

characteristics

i, * Lightness
* Impact resistance
#3 * Chemical inertness
¢ Resistance to pitting
: ¢ Resistance to fogging
* Tintability
¢ Versatility in optical design

Disadvantages

', * Surface abrasion
* Warpage upon glazing
@§ * Increased thickness
¢ Inferior photochromic properties
: ¢ Index variability with temperature

POLYCARBONATE

° Petroleum
derivative and
plastic polymer
* Itis a high index
plastic lens
protection to the
lens wearers
* Usually preferred
for children and

sportswear R

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.

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.

i] ¢ RL: 1.586

¢ Abbe value :- 30
§ * Specific gravity :- 1.20
¢ U.V. absorption :- 380 nm

Density : 1.2 g/cm:

ADVANTAGE

i 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 .,

DISADVANTAGES

i, * High chromatic aberration
* Poor optical qualities
§ * More scratches are formed
¢ Transparency is less
< ¢ More lens distortion
* Difficult to dye

= * More expensive

¢ 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

ZIESS DURALATE LENSES

j, * 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.

°

ooo @

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. 31

High index plastics

i
A * Refractive index
N&e
tage 21.640 — 1.740
* Abbe value : 42 —

Rp? Density :1.3-1.5

32
sx:¢ UV cutoff : 380—

400 nm

Refractive | Abbe value | Specific

Index gravity
1.600 36 1.45
1.597 37 1.34

ADVANTAGE

, * Good cosmesis
* Magnification is reduced
4 * 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 ,,

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 .
ooe
°

POLY METHA

METHACRYLATE

ij, * 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

Properties of PMMA

ii) * Refractive index :- 1.49

J * Velocity :- 58

Density :- 1.119

¢ Half of the weight of crown glass

Trivex

, * Also known as phoenix or triology
¢ Similar to polycarbonate but have higher

quality optics & thus provide clearer vision

i, * Refractive index : 1.532

¢ Abbe value : 43-45
§ * Density: 1.1 g/cm

¢ UV cutoff : 380 nm

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. 6

DISADVANTAGE

i), * get scratch easily

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.

Tinted lens

a } * Availablein | METAL COLOU
SA variety of tints. OXIDES R
* Metallic oxides Cobalt {Blue |
eee) are added to
P themotten [Gold Red
“oe

ee OE

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

ADVANTAGE

* Reduce of glare across the visible spectrum
J * Protection against harmful radiations

) * Improved cosmetic appearance

Glass CR-39 Polycarbonat

W| Specific 2.54 1.32 1.20

gravit
y_| |
RI 1.523 ad 1.586

Abbe 58.9 58 30

value | |
Trivex
1532

=)

; rT

ae
wy, *PROPERTIES

OF LENSES

SSAC.

OPTICAL PROPERTIES

| ABSORPTION

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
5 * Used to calculate axial and transverse

chromatic aberration

¢ 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.

¢ 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.

REPEECTANCE

, * Reflectance is the phenomenon of light
reflection occurs at each of the lens
surfaces.
and undesirable reflections on the lens

surfaces.

, * 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)?/ (n+ 1?%

, * The higher the refractive index, the greater

the proportion of light reflected from the

surfaces.

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.1. the thinner the lens can be
made
* Ifa 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.

i, * n= Velocity of light in air

¢ Velocity of light in the medium

ABSORPTION

, * The amount of light which goes through a
lens can be reduced because of absorption
by the lens material.
lens, but constitutes an intrinsic function of

a tinted or photo chromatic lens..

¢ 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.

} MECHANICAL PROPERTIES

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.

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.

Drop ball test for impact

resistance

SCRATCH RESISTANCE

, * One of the straight features of glass lenses
is abrasion resistance.
additional resin to approach the scratch
resistance of glasses.
] + These resin coatings can be applied in a

number of ways.

¢ 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.

ELECTRICAL PROPERTIES

, * Electrical properties characterize effects of
electromagnetic waves and electricity on

the materials.

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.

THERMAL PROPERTIES

; * Thermal properties state changes of state

and the effect of temperature on materials.

* 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.

¢ Unfortunately, there is no lens material that

fits this description, and we have to use

compromise