Review

Question 1

What are the characteristics of an ideal lens?(6)

Answer 1

• High index of refraction
• High ABBE
• low specific gravity
• Low UV transmittance
• high light transmittance
• high impact resistance

Question 2

What type of lens materials are available to recommend to patients?

Answer 2

• glass
• plastic
• high index

Question 3

Different type of plastic lenses include:

Answer 3

Thermosetting
Thermoplastic
Trivex

Question 4

Different kinds of high index lenses include

Answer 4

Plastic and glass

Question 5

Physical requirements that lenses must meet before they are able to be worn?

Answer 5

-meet standard impact resistance requirements

Question 6

Physical requirements for dress eyewear:

Answer 6

No minimum thickness requirement

Lens must not fracture on resistance test

Question 7

Who supplies the standards that ophthalmic products should follow?

Answer 7

ANSI: American national standards institute

Question 8

n for CR39

Answer 8

1.50

Question 9

n for crown

Answer 9

1.523

Question 10

n for poly

Answer 10

1.586

Question 11

n for 1.7 index

Answer 11

1.701

Question 12

Specific gravity for 1.7 index

Answer 12

2.99

Question 13

Specific gravity for crown

Answer 13

2.54

Question 14

Specific gravity for cr39

Answer 14

1.32

Question 15

Specific gravity for poly

Answer 15

1.20

Question 16

Abbe value for crown

Answer 16

58.9

Question 17

Abbe value for CR39

Answer 17

57.8

Question 18

Abbe value for 1.7 index

Answer 18

30.8

Question 19

Abbe value for poly

Answer 19

30

Question 20

Has exceptional impact resistance

Answer 20

Polycarbonate

Question 21

How does the reflectance of a lens material vary with its index of refraction?

Answer 21

Based on the equation: higher index means higher reflection

Question 22

Why do optometrist care about the light reflected from ophthalmic lenses?

Answer 22

The reflection can cause ghost images, it can be aesthetically frustrating in photos, cause haze or decrease contrast

Question 23

Which lens materials are/is thermosetting?

Answer 23

CR39

Question 24

Which material is/are thermoplastic

Answer 24

Polycarbonate

Question 25

What is the difference between thermosetting and thermoplastic?

Answer 25

- Thermosetting cannot be melted back while melting and cooling are the cycles used in thermoplastic lenses - thermosetting uses cross linking while thermoplastic so dont

Question 26

Compare and contrast trivex and polycarbonate

Answer 26

- trivex abbe value = 43-45 far better than poly carb = 30 - trivex rivals poly in terms of impact resistance - trivex is very lightweight just like poly - 2x more scratch resistance over poly

Question 27

What is a lens blank

Answer 27

Circular ophthalmic lenses at any stage (unfinished, semi, or finished) prior to being edged

Question 28

What is lens surfacing?

Answer 28

When a prescription is ground onto the surface of a lens blank

Question 29

What is lens edging

Answer 29

Cutting a lens blank to fit an ophthalmic spectacle frame

Question 30

Compare semi-unfinished with finished lenses

Answer 30

- semi-finished: unfinished back surface, typically used for multi focal and high-powered single vision lenses - finished: finished optical surfaces on both the front and back -> ready for edging

Question 31

What lenses would you recommend to patients with high prescriptions and/or requesting thin/ner lenses

Answer 31

High index

Question 32

What ophthalmic lens material is a patient likely viewing through if they “see rainbows”?

Answer 32

Some type of low ABBE value meaning there is more chromatic aberration (poly)

Question 33

How does curvature relate to the radius of a surface

Answer 33

The curve has an inverse relationship to the radius

Question 34

Compare minus cylinder and plus cylinder form lenses

Answer 34

Plus cyl= toric portion on the front of the lens | Minus cyl= toric portion on the back of the lens

Question 35

Tsercherning ellipse

Answer 35

The locus of points showing the relationship between the power of a surface and the power of the lens for which oblique astigmatism is eliminated

Question 36

WTR

Answer 36

150-180 and 180-030

Question 37

ATR

Answer 37

060-120

Question 38

Oblique

Answer 38

030-060 and 120-150

Question 39

What is the optical axis

Answer 39

Line going through the center of the lens

Question 40

Optic center

Answer 40

Literally a point of the center of the lens

Question 41

Refract light ray to meet at one common point

Answer 41

Spherical

Question 42

Refract rays to a focal line

Answer 42

Cylindrical

Question 43

Refract light rays to form 2 line images perpendicular to each other

Answer 43

Spherocylindrical

Question 44

For a cyl lens the axis meridian has most___ power and power meridian has most ___ power

Answer 44

Plus | Minus

Question 45

How do we determine the power of a obliquely crossed toric lenses

Answer 45

The sin squares law of curvature | If alpha is 30 degrees make sure to go 30 degrees from the axis meridian

Question 46

Generalizations about the resultant power of obliquely crossed toric lenses

Answer 46

- both cyl with same sign= axis is between the original axes - 2 cyl are of equal power = axis is between the 2 - if the 2 cyl are of unequal power= axis is shifter toward the stronger of the 2 cylinders

Question 47

Pantoscopic tilt

Answer 47

180 degree axis

Question 48

Face form tilt

Answer 48

090 axis

Question 49

How is the optic center adjusted for purposely pantoscopic lens tilt

Answer 49

Optical center must be lowered from the pupillary level an amount in mm - equal to one half the pantoscopic tilt in degrees