Ophthalmic lens material and design

1. OPTOM FASLU MUHAMMED

2.  Ophthalmic lenses are made from
Glass
Plastic
 Glass lenses are often referred to as mineral
lenses, whereas if a lens is made from plastic, it is
said to be from organic material.

3. Crown Glass
 Glass works well for ophthalmic materials
because it resists scratching and is not easily
affected by environmental factors.
Disadvantages
 Glass are weight
 Impact resistance

4.  The most commonly used clear glass lens
material is made from a type of crown
glass .
 Having an index of refraction of 1.523.
 This material is low in chromatic
aberration.

5.  Will reduce lens thickness for higher powered
prescriptions.
 Index 1.60 lenses are readily available
 There are fused flat-top bifocals available in
indexes up to 1.70
 There is a 1.90 index glass available.

6.  High-index glass lens materials generally have
Abbé values close to that of polycarbonate
 The lower the Abbé value, the higher the
chromatic aberration.

7.  Resin lenses are made up of small molecular units
called monomers which link together to form long
chain known as polymers and the process of
linking the monomers together is known as
polymerization.
 There are two processes to make resin lenses
1.Thermosetting
2. Thermoplastic

8. CR-39
 For years the most commonly used plastic lens
material was CR-39.
 CR-39 was developed by PPG Industries.
 “CR” stands for Columbia Resin, and the
number 39 denotes the type of Columbia Resin
used.

9.  Plastic lenses are roughly half the weight of crown
glass lenses.
 For low velocity, large mass objects, such as
permanently stick to glass lenses, it does not
adhere to plastic lens material

10.  CR-39 plastic lenses have an index of refraction of
approximately 1.498.
 This is the lowest refractive index material used
for spectacle lenses.

11.  Polycarbonate lens material is soft and requires
an anti scratch coating.
 When safety is the primary concern,
polycarbonate has traditionally been the number
one choice.
 Safety is the primary concern for children,
monocular individuals, people with good acuity in
one eye only, and those who are
purchasing safety or sports eyewear.

12.  Trivex lens material is a very impact-
resistant lens material. It was
developed by PPG Industries.

13.  PPG Industries promotional material s
attribute the “tri” in Trivex to a tri performance lens
material; meaning it offers a triple combination of
superior optics, impact resistance, and ultra light
weight.
 It maintains a good resistance to damage from
chemicals.

14.  Trivex rivals polycarbonate in impact
resistance.
 It is the lens of choice for drill-
mounted lenses because it does not
crack or split at the drilled hole.

15.  The lens is very light weight.
 The Abbé value of 43 to 45 is less than
CR-39, but higher than its rival,
polycarbonate

16.  The resulting lens is a light-weight material that is
extremely strong and also compatible with
photochromic pigments and with polarization.
 It has already been used in sun and sport
eyewear, helmet visors for motorcycles, airline
cockpit door view ports, ballistic police shields,
and vehicle door armor.

17.  NXT has an index of refraction of 1.53, a density
of a 1.11, an Abbé value of 45, and is highly
flexible.

18.  Lenses that are made from two or more layers of
material are called laminated lenses.

20. PROPERTIES
OF OPHTHALMIC
LENSES

21. •The measure of the
proportion of light
reflected from the surface
is called reflectance.
• The measure of
proportion of light
absorbed is the
absorption and the
measure of the proportion
of light transmitted is the
transmittance

22.  If the intensity of the beam of light is represented
by the number 1, reflectance by R, absorption by
A and transmittance by T, the intensity may be
expressed as follows:
R + A + T = 1

23.  Abbe Value
Abbe value is the number that is the
measure of the degree to which light is dispersed
when entering a lens material.
 Standard plastic lenses have an abbe value of 58.
Most high index materials have a much lower
Abbe value.

24.  Reflectance is the phenomenon of light reflection
occurs at each of the lens surfaces.
 The reflectance of the lens surface is calculated
from the refractive index of the material.
Reflectance = 100 (n – 1)² / (n + 1)²%

25.  The higher the refractive index, the greater the
proportion of light reflected from the surfaces.

26.  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.

27.  The refractive index of a transparent medium is
the ratio between the velocities of the light in air to
the velocity of light in the given medium and is
denoted by ‘n’.
 n = Velocity of light in air / Velocity of
light in the medium

28.  Specific Gravity
Specific gravity is the measurement of physical
density or weight of the material in grams per
cubic centimeter.

30.  In 1971, the Food and Drug Administration (FDA)
adopted a procedure to ensure the level of
protection provided to the consumer for “street
wear.
 The standard involves dropping a 5/8” steel ball
from a height of 50” onto the lens and
the lenses that survive this test are deemed
impact resistant.

32.  The basic lens design is determined by the “base
curve selection.
 The base curve of the lens is the surface that
serves as the basis or starting point from where
the remaining curves will be calculated.