Final: Properties of Ophthalmic Lenses

Question 1

How to find index of refraction (n)

Answer 1

n = velocity of light in a vacuum / velocity of light in material

Question 2

What are the ranges for normal index values (n)? What lenses have a normal index?

Answer 2

≥ 1.48 but ‹ 1.54

Crown Glass, CR-39, Trivex

Question 3

What is the range of a Mid Index (n)? What lens has a mid index?

Answer 3

≥ 1.54 but ‹ 1.64

Polycarbonate

Question 4

High dispersion = _____ Abbe number

Answer 4

low

Question 5

What Abbe value range is considered to be low dispersion? What lenses are low dispersion?

Answer 5

Greater than or equal to 45

Crown glass and CR-39 have LOW DISPERSION

Question 6

What Abbe value range is considered to be medium dispersion? What lenses have medium dispersion?

Answer 6

Equal or greater to 39 but less than 45.

Trivex is medium dispersion

Question 7

What Abbe value range is considered to be high dispersion? What lenses have high dispersion?

Answer 7

‹ 39

Polycarbonate has high dispersion

Question 8

Axial/ Longitudinal chromatic aberration is found where? What is the formula?

Answer 8

Along the optical axis (on-axis).

ACA= Power of lens / Abbe number

Question 9

Where is Transverse/ Lateral chromatic aberration found? What color is closer to the optical axis? Equation?

Answer 9

Found off-axis. Red is closest to center.
TCA = Prism / Abbe Number
TCA = (Prism Power x distance decentralization) / Abbe number

Question 10

What is the FDA regulation code for impact resistance testing?

Answer 10

Title 21; Chapter I; Subchapter H; Part 801; Subpart H; Section 410

or

FDA Regulation 21 CFR 801.410

Question 11

Drop ball test conditions for “Dress Eyewear”

Answer 11

Steel ball
5/8th inch diameter
0.56 ounces
drop heigh 50 inches

Question 12

What is in glasses that absorb UV?

Answer 12

cerium oxide

Question 13

What is in glasses that absorb infrared?

Answer 13

iron oxide

Question 14

Characteristics of crown glass compared to other lenses

Answer 14

lowest chromatic aberration
least warpage
difficult to scratch

BUT 
high specific gravity
low refractive index
fog
shatter

Question 15

Is polycarbonate thermoplastic or thermosetting?

Answer 15

Thermoplastic

Question 16

Is CR-39 thermoplastic or thermosetting?

Answer 16

Thermosetting

Question 17

Thermoplastic characteristics

Answer 17

Soften when heated
Stretched, pressed, or molded
less stable
more susceptible to deformation by heat

Polycarbonate is thermoplastic

Question 18

Thermosetting characteristics

Answer 18

once hardened cannot be softened
Made from heated monomers allowing to dry
stable
hard
flame resistant
insensitive to heat or chemicals

CR-39 is thermosetting

Question 19

Lens most resistant to chemicals

Answer 19

Trivex

Question 20

Is polycarbonate resistant to acetone and rubbing alcohol?

Answer 20

No

Question 21

What weight of ball for basic impact testing?

Answer 21

2.4 ounces

Question 22

What weight of ball for high-impact testing?

Answer 22

17.6

Question 23

What lens is more problematic for chromatic aberration? Is the polycarbonate aberration problem significant?

Answer 23

Plus lenses; No

Question 24

Thermal tempering increases ___________

Answer 24

Impact resistance

Question 25

Chemical tempering makes glass ____ more impact resistant

Answer 25

2-3x