Properties of Ophthalmic Lens Materials

Question 1

Which lens to stat with when doing lensometry?

Answer 1

The lens with most power.

Question 2

Define index of refraction

Answer 2

Speed of light in a vacuum compared to its speed in an optical material.

Based on yellow helium line, wavelength 587.6nm, nd, also called the Fraunhofer D line

Question 3

index of refraction general ranges

Answer 3

Normal to mid ranges: 1.48 to 1.64
High: 1.64 to 1.74
Very High: Greater than 1.74

1.4-1.9 is the average range.

Question 4

Define dispersion (dispersive power)

Answer 4

Chromatic aberration of an optical material. Spread of white light when passing through a material.

Represented as an omega “w”

Question 5

Dispersion is represented with which symbol?

Answer 5

w (omega)

Question 6

nd
nf
nc

Answer 6

nd is yellow light
nf is blue
nc is red

Question 7

How are dispersion and chromatic aberration related

Answer 7

High dispersion results in high chromatic aberration

Want dispersion and chromatic aberration to be low

Question 8

Formula to determine dispersion, “w”

Answer 8

w= (nf-nc) / (nd-1)

nf- blue
nc- red
nd- yellow

Question 9

Reciprocal of dispersion is the

Answer 9

Abbe number, AKA constringence.

Dispersion low, abbe number high

Question 10

Abbe number can also be called

Answer 10

Constringence

Question 11

Abbe number is represented by which symbol

Answer 11

V “nu”

Question 12

abbe number, V= ?

Answer 12

V= 1/w (dispersion)

Question 13

Do you want abbe number to be high or low

Answer 13

High. high abbe number means low dispersion AKA low chromatic aberration

Question 14

Abbe number range

Answer 14

Low dispersion is greater than 45 or 50
Medium dispersion is 39-45
High dispersion (BAD) is less than 39

Overall range is 20-80

Question 15

Two types of chromatic aberration. Which one is more of a problem?

Answer 15

Axial/longitudinal and transverse/lateral

Transverse is more of a problem

Question 16

Formula for axial/longitudinal chromatic aberration

Answer 16

Axial= lens power/ abbe number

Units: Diopters

Question 17

What is axial aberration?

Answer 17

colors converge at different points on or off the retina due to their different wavelengths. Technically, colors will never be able to focus on the retina at the same time. Ex: Gas station color lights

Question 18

What is transverse/lateral chromatic aberration?

Answer 18

Looking in periphery of glasses.

Question 19

Formula for transverse/lateral chromatic aberration

Answer 19

TCA= F x d / abbe number

F x d = prentices rule, prism.
F is the lens power
d is the distance off center in cm

Units: prism diopters

Question 20

Define specific gravity

Answer 20

Weight of a given volume of material compared to the weight of the same volume of water

Depends on the density of the material.

Usually between 1 and 3

Question 21

Impact resistance is governed by

Answer 21

The FDA.

Question 22

Prior to the year ___, there were no impact resistance standards for lenses. In ___, began the drop ball test.

Answer 22

1971.

In 1972, they started the drop ball test.

Question 23

Requirements of the drop ball test

Answer 23

5/8 inch steal ball
weighing 0.56 ounces
dropped from 50 inches

lens must not fracture

Question 24

Characteristics of glass

Answer 24

Amorphous (non crystal), supercooled liquid with high viscosity and no discrete melting point

Question 25

Basic ingredients in glass

Answer 25

Silica (sand) 70%
Sodium oxide (soda) 14%
Calcium oxide (lime) 11%

Question 26

Advantages to using glass lenses

Answer 26

Superior and stable optics
Lowest chromatic aberration AKA highest abbe number
Least warpage
Difficult to scratch

Question 27

Disadvantages to glass lenses

Answer 27

Shatter easily, tendency to fog, low refractive index, high specific gravity

Question 28

The higher the index of glass, the more ___ in the glass. Therefore, it will be heavier

Answer 28

Lead

Question 29

In general, what happens to the specific gravity and abbe number as you increase the index?

Answer 29

Increase index, decrease specific gravity (thinner and lighter), and decrease abbe number resulting in more chromatic aberrations or dispersion.

Question 30

“mineral” lenses vs “organic” lenses

Answer 30

Mineral- glass

Organic- plastic

Question 31

Plastic characteristics

Answer 31

Polymetric material of large molecular weight

Synthetic lateral formed by combining organic ingredients with inorganic materials

Question 32

Two main groups of plastic

Answer 32

Thermoplastic, which is polycarbonate

Thermosetting, which is CR39.

Question 33

Describe thermoplastic materials and give an example

Answer 33

Polycarb
Soft when heated
Can be stretched, pressed, or molded.
May be injection molded to form lenses

LESS stable and more susceptible to deformation by heat. When it cools, it could change size

Question 34

Describe thermosetting materials and give an example

Answer 34

CR39
Once hardened, cannot be softened
Stable, rigid, hard, flame resistant, and insentsitive to heat and chemicals.

Question 35

Manufacturing processes of thermosetting lenses like CR39

Answer 35

Supplied as liquid monomers
Other ingredients added such as dyes or catalysts
Mixture is heated to produce polymerization, then cured with UV light.