2. Nature herself provides us with various
transparent minerals from which spectacles might
be made. Most of these natural media are gradually
supposed to have been formed by solidification
during the cooling of the earths crust.
3. In very general terms, a glass is essentially an
amorphous compound, sometimes referred to as a
super cooled liquid. When glass is heated it
becomes softer and softer as the temperature
increases but it has no melting point. It loses its
viscosity or becomes more and more liquid.
Similarly on cooling its viscosity increases, the
liquid becomes so rigid that for practical purposes
it may now be regarded as a solid. Its high viscosity
prevents it from crystallizing as water would
crystallize at 0 degree C to form ice.
GLASS
4. Crown glass, the variety of glass most used in
ophthalmic work, is a Soda-lime-Silica glass
containing about
70% Silica,
12% Calcium oxide( CaO) or lime and
15%Sodium Oxide (NaO).
Added to these batch materials are small percentages of
potassium, borax, antimony and arsenic which improve
the quality of glass by acting as refining agents during
the melt.
5. Barium Crown glass contains about
35% barium oxide (BaO),
30% Silica and
small percentage of lime, Zinc, alumina,
boron, antinomy and Zirconium.
The effect of barium oxide is to increase
the refractive index of the glass without
greatly increasing the mean dispersion.
6. Flint glass – so called because its sand constituents used
to be obtained from powered flint. It contains about
60% lead oxide (Pb0),
30% Silica,
8% Soda and potash, and
small percentage of arsenis and antinomy.
The effect of the lead content is to increase the refractive
index of the glass but at the same time it also increases the
mean dispersion and specific gravity or weight. Because of
its high dispersion, luster and specific gravity, flint glass is
not ideal for ophthalmic use.
7. The higher the refractive index, the more dense the
glass is said to become. The lead content of
various flint glasses is approximately:
light flint 35%,
dense flint 45%,
extra dense flint (EDF) 55%,
double extra dense flint (DEDF) 65%.
8. The two principal plastics which emulate crown
glass are
1. “polymethyl methacrylate”( PMMA) which is
known as Perspex.
2. Allyl diglyocol Carbonate or CR39.
Refractive index of PMMA=1.49,
V=58,
density is 1.19(half the weight of crown glass).
Refractive index of CR39=1.498,
V=57.8
density =1.32.
9. The first alternate plastic material was
polycarbonate,
refractive index = 1.586,
V=30,
density 120,
It also has U.V. absorption.
Polycarbonate is extremely difficult to break.
POLYCARBONATE
10. Trivex is a very new lens material that exhibits three
very good lens characteristics, therefore its name
Trivex. Trivex is also known as Trilogy and phoenix.
I. First, it is the lightest weight of all plastic materials.
II. Second, it has impact resistance at least equal to
polycarbonate.
III. Third, it has an excellent abbe value.
Refractive index = 1.530,
V=45,
Density=1.11
It also has similar U.V. absorption as
polycarbonate. Like polycarbonate, Trivex is
extremely difficult to break.
TRIVEX
11. High Index Plastic can be considered as any
plastic material with an index of 1.60 or
above. Plastic lenses started with an index of
1.498 and they have now reached as high as
1.74.
HIGH INDEX PLASTIC
