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Ophthalmic Optics - Block 11 > Lecture 9 - Lens Material And Design > Flashcards

Lecture 9 - Lens Material And Design Flashcards

1
Q

Another name for CR-39

A

Hard resin

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2
Q

Is hard resin glass or plastic?

A

Plastic

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3
Q

What’s the difference between high index glass and high index plastic?

A

Weight (glass is heavier)

More impact issues (glass)

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4
Q

What lens used to be the industrial standard?

A

Glass (crown glass)

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5
Q

Primary components of glass

A

Silica
Soda
Lime

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6
Q

Advantages of glass

A

Transparent and colorless
Resistant to heat and chemicals
Scratch resistant
High abbe number with low CA

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7
Q

Disadvantages of glass lenses

A 
Heavy weight
Poor impact resistance (safety issue)
Poor UV protection
Can't be tinted after manufacturing
Lengthy processing time
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8
Q

2 types of plastic lenses

A

Thermoplastic (polycarbonate)

Thermosetting (CR-39)

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9
Q

Which plastic lens softens when heated and can be re molded?

A

Thermoplastic (polycarbonate)

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10
Q

Which plastic lens when harden end can not be softened even at high temps?

A

Thermosetting (CR-39)

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11
Q

Advantages of CR-39

A 
Lighter than glass
High abbe number
Low price
Tintable
Many designs and options 
Really good optics
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12
Q

Disadvantages of CR-39

A 
Scratches and chips easily
Recommend scratch coatings
Thicker than hi-index plastic
Warpage
Not for children
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13
Q

Hi index plastic advantages?

A

High prescriptions (thinner, not necessarily lighter due to higher density)

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14
Q

Hi index disadvantages

A 
Expensive
High refractive index = low abbe number
Optics can be compromised with chromatic aberration (need anti-reflective coating)
Need scratch coating
Contain sulfur (not popular with labs)
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15
Q

Polycarbonate advantages

A

Excellent impact resistant
UV protection (block all wavelengths below 370nm)
Many designs and options

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16
Q

Polycarbonate disadvantages

A 
Not great for optics (high CA, low abbe)
Not great for tinting
Soft surfaces (scratches easily)
Most avoid exposure to heat
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17
Q

What type of lens is commonly used in children’s glasses or sportswear?

A

Polycarbonate (or trivex)

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18
Q

Advantages of trivex lenses

A 
Tintable 
Good UV protection
Lightweight
Extremely high impact resistant
Fewer hear issues than polycarb
Thinner and lighter than CR-39
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19
Q

How would you describe trivex lenses?

A

Combination of thermosetting (polycarb) and thermoplastic (CR-39)

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20
Q

List in order the thickest to thinnest lenses (primarily edge thickness)

A

CR-39
Trivex
Polycarb
Hi index

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21
Q

Which type of lens is not available in all lens types?

A

Trivex

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22
Q

Which primary type of lens has a low abbe number, with optical clarity not optimal for some users, even though its strong, thinner, and impact resistant?

A

Polycarb

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23
Q

Which lens is the thinnest available, but is brittle and not tintable, with possible lower optics

A

Hi index plastic 1.74

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24
Q

What are the consequences for not using minimum blank size on lens appearance?

A

It won’t be as thin as it could be

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25
Q

Three multifocal types

A

Bifocal
Trifocals
Progressive

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26
Q

Are minus lenses lower or higher base curves?

A

Flatter, so less base curve

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27
Q

With higher perscriptions, do you want a higher or lower base curve?

A

Higher

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28
Q

If you flatten the base curve of a single vision lens, will it get clearer or more blurry?

A

Blurry b/c changes effective power away from optical center

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29
Q

Distance in surface power produces what?

A

Produces unwanted astigmatism

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30
Q

As astigmatism increases as you go away from the center of the lens, what would an aspheric lens do?

A

It combats the off-axis astigmatism, so you can see better/clearer peripherally as well as centrally

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31
Q

How do aspheric lenses make distortion free lenses?

A

Change where light is focusing in the periphery, so all light rays meet at one point, all while getting thinner and flatter lenses

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32
Q

What manufacturer does he reference a lot with aspheric lenses?

A

SACO (say-co)

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33
Q

What reduces distortion while also making the lens even thinner than using an aspheric lens?

A

Double-side aspheric design

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34
Q

What single vision lens design offers a larger area of clear, natural and distortion-free vision, also allowing customization?

A

Digital freeeform distance lenses

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35
Q

Longest horizontal dimension of the segment

A

Seg width

36
Q

Longest vertical dimension of the segment

A

Seg depth

37
Q

Distance from the lowest point on the lens to the top o the seg

A

Seg height

38
Q

Vertical distance b/w the major reference point and the top of the seg

A

Seg drop

39
Q

The distance portion of the multifocal lens is decenered from the geometrical center to correspond with what?

A

The wearer’s PD

40
Q

Distance from geometric center to major reference point

A

Distance decentration

41
Q

Inset accounting for near PD. Distance from Major reference point to center of seg

A

Seg inset

42
Q

Inset of seg as measured from the optical center of the lens. Distance from geometric center to center of seg (or near reference point)

A

Total inset

43
Q

How do you find total inset?

A

Seg inset + distance decentration

44
Q

How many millimeters do you move in for the average near power?

A

2 mm per eye (4 mm total)

45
Q

What type of lens is used to correct all refractive errors plus the error or condition of presbyopia?

A

Bifocals

46
Q

First one-piece bifocal. Designed by starting with required near power and adding a second curvature to upper portion and back of surface

A

Solid upcurve

47
Q

Small segment cemented to back surface of lens. Segment could be replaced as add changed. However brittle when cold and moved when hot

A

Cement

48
Q

Similar to Franklin bifocal but 2 lenses held together with groove

A

Perfection

49
Q

First to use 2 kinds of glass. Three components

A

Cemented kryptok

50
Q

Components fused together by heat instead of cement. Mostly glass until recent intro of new materials

A

Fused Kryptok

51
Q

One-piece back surface bifocal

A

Ultex “A”

52
Q

Similar to Franklin, but one piece

A

Executive

53
Q

In fused Kryptok, where it’s ground into the main lens where the segment is to be placed

A

Curvature of the countersink

54
Q

Which modern bifocal gives you a really wide reading area?

A

Executive or E-style

55
Q

Trifocals eliminate what from using a bifocal?

A

The “dead zone”

56
Q

Trifocals have an additional intermediate add, usually how much power of the near add?

A

One half or 60%

57
Q

For someone who plays cards a lot, what type of lens might be best for them?

A

Trifocal

58
Q

28 mm is a good measurement for viewing what?

A

iPad

59
Q

35 mm is a good measurement for viewing what?

A

Magazine

60
Q

What lens can you turn into a quadrifocal?

A

Trifocal

61
Q

Advantages of round seg

A

Least visible

Light

62
Q

Disadvantages of round seg

A

Takes a while to get down to a useful width

Most jump, particularly in larger segs

63
Q

Advantages of flat top

A

Immediate broad field
Minimal jump
No prism, 5 mm below top
Light

64
Q

Disadvantages of flat top

A

Visible ledge in larger segs

65
Q

Advantages of executives

A

Widest field

No jump

66
Q

Disadvantages of executive

A 
Thick
Heavy
Ugly
Chips
Collects debri
67
Q

Which type of lens uses a “corridor” between distance and near powers?

A

Progressive (PAL)

68
Q

Do progressive lenses have image jump?

A

No

69
Q

How is blending achieved for a PAL lens?

A

By incorporating various amounts of surface astigmatism oriented at an oblique axis

70
Q

The cylindrical power lateral to the umbilic of a progressive surface increases twice as rapidly as the addition power

A

Minkwitz’s Theorem

71
Q

Vertical meridian of a PAL that is spherical at every point

A

Umbilic

72
Q

Progressive lens characterized by wider area of stable optics at distance and near along with a shorter, narrower corridor (steeper gradient)

A

Hard

73
Q

Progressive lens characterized by narrower area of optimal power at distance and near, but offer a longer, wider intermediate zone (shallower gradient)

A

Soft

74
Q

Which progressive design is better for sustained viewing tasks?

A

Hard designs

75
Q

Which progressive design is better for dynamic viewing tasks?

A

Soft designs

76
Q

What are short corridor PALS for?

A

Can be worn in frames with small B measurements

77
Q

Uses computer-aided design and processing to create high-level, customized lenses unique to your prescription

A

Free-form processing (digital surfacing)

78
Q

Advantages of PALs

A

No img jump
Cosmetically appealing
Perceived as more youthful and fashionable
Uninterrupted vision from distance to near

79
Q

Disadvantages of PALs

A

Expensive
Adaptation time (point nose at what you see)
Unwanted astigmatism in periphery (distortion, swimming sensation)

80
Q

Point on the datum line halfway between the two vertical lines which are tangent to the edges

A

Geometrical center

81
Q

Horizontal length of the box, when referring to the frame

A

Eye size

82
Q

Horizontal length of the box, when referring to the lens itself

A

Lens size

83
Q

Vertical length of the box

A

B distance

84
Q

Shortest horizontal distance between the lenses

A

Bridge size

85
Q

Horizontal distance b/w the geometrical centers of the two lenses

A

Frame PD

86
Q

Longest diameter of a lens

A

Effective diameter

87
Q

Point on the lens through which the line of sight (visual axis) passes. Would correspond to the optical axis if no prism power were needed

A

Major reference point

Ophthalmic Optics - Block 11 (2 decks)

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