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Ophthalmic Optics Block 11 > Lens Material And Design > Flashcards

Lens Material And Design Flashcards

1
Q

CR-39, another name for it

A

Hard resin

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2
Q

High index glass 1.6 vs high index plastic 1.6

A

Weight difference

Impact issues

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3
Q

Is glass common or not?

A

No

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4
Q

Refractive index of glass in general glass

A

1.523

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5
Q

Used to be the industrial standard but not now

A

Glass

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6
Q

Manufacturing of glass

A

Names for shape glass attains during its manufacturing
Heated to 1000-16000C
Stirred, conditioned, and molded, annelaing to avoid cracking

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7
Q

Advantages of glass

A

Highly transparent and colorless, resistance to heat and chemicals, highly scratch resistance, high abbe number, low CA

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8
Q

Disadvantages of glass

A 
Weight 
Poor impact resistance 
Poor UV 
Can’t be tinted 
Length processing time
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9
Q

Why is glass still the best choice in some cases

A

Chemical resistance, but impact resistance issues

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10
Q

Two types of plastics

A

Thermoplastics and thermosetting

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11
Q

Thermoplastic lenses (polycarbonate)

A

Softens when heated (can be remolded)
-injection molded under controlled pressure, can be melted back into liquid form, no cross linking (gives ability to be liquefied and remolded

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12
Q

Thermosetting plastics (CR-39)

A

When hardened cannot be softened even at high temps

  • liquid monomer, cast in molds
  • mix monomer with initiator
  • cured in oven
  • molecules from cross inks (cannot be melted back)
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13
Q

CR-39 (hard resin) advantages

A 
Light weight than glass
 High abbe number 
Lowest price point 
Tintiable 
Many designs 
Good optics
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14
Q

Disadvantage of CR-39

A 
Scratches and chips easily
Don’t put in semirimless 
Not good for children 
Recommend scratch coating 
Thicker than hi index plastic 
Warpage
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15
Q

Good for higher Rxed (thinner but not necessary lighter due to higher density)

A

Hi index plastic

1.60, 1.67, 1.74

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16
Q

Optics of hi-index plastic

A

Can be compromised with chromatic aberration, need to have anti reflective coating
-fresnels law
-need scratch coating
Not popular with labs

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17
Q

Polycarbonate is what type of plastic

A

Thermoplastic

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18
Q

Advantages of polycarbonate

A
  • Excellent impact resistant
  • UV protection (block all wavelengths below 370nm)
  • many designs, many options
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19
Q

Disadvantage of polycarbonate

A
  • not great optics
  • not great tinting
  • soft surface
  • must avoid exposure to heat
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20
Q

What is polycarbon usually used in

A

Children’s glasses and sportswear

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21
Q

What kind of plastic is trivet

A

Combination of thermosetting and thermoplastic

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22
Q

Tribes weight

A

Lighter than polycarbonate and Lower chromatic dispersion

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23
Q

Thickness of trivet

A

Similar to CR-39

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24
Q

Advantages of trivex

A
  • tintable
  • good UV protection
  • very lightweight
  • extremely high impact resistant
  • fewer heat issues than polycarbonate
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25
Q

Polycarb and drill mount

A

Not a good because it needs to be heated and you cant heat polycarbonate

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26
Q

Trivex and drill mount

A

Can use this because it tolerates heat well

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27
Q

Low cost, excellent optics

A

CR39

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28
Q

Chemical resistant, not commonly used

A

Glass

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29
Q

Most expensive, more chromatic dispersion, softer, scratches easily, strong Rx

A

Hi-index Rx

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30
Q

Good for children, sports, hazardous jobs, anyone wanting thinner, lighter lenses

A

Polycarbonate and trivex

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31
Q

Why doesn’t the center thickness change when you increase RI in a minus lens

A

Because we are changing the edge thickness in a minus lens

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32
Q

Weight in glasses as we increase RI

A

Gets a little lighter, then levels off

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33
Q

Minimum edge thickness in + lens when you go up in index

A

Levels off quickly, but drops the center thickness

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34
Q

List the materials in order from low RI to high RI

A

CR-39
Trivex
Polycarb
Hi index

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35
Q

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

A

Geometrical center (GC)

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36
Q

Horizaontl length of the box

A

Eye size or lens size (A)

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37
Q

When do we use the term eye size

A

When referring to the frame

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38
Q

When do we use the term lens size

A

When referring to the lens

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39
Q

Vertical length of box

A

B distance (B)

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40
Q

Shortest horizaontl distance between the lenses

A

Bridge size (DBL)

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41
Q

Horizaontal distance between the geometrical centers of the two lenses

A

Frame PD

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42
Q

The longest diamter of a lens

A

Effective diameter (ED)

43
Q

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

A

Major reference point (MRP)

44
Q

Consequences of not using the proper minimum blank size on lens appearance

A

Giving the same power but making it thinner to look nicer

45
Q

Minus lenses and base curves

A

Flatter in the front, so smaller base curve

46
Q

Plus lenses and base curve

A

Higher base curve, lens bulge

47
Q

If there is a some wrap to a frame, how does that affect plus or minus lenses?

A

Plus lenses can do a wrap pretty well. Minus lenses will flatten it out or you will have to pick a way off base curve to make it thick

48
Q

Which Rx is more rounded

A

Plus

49
Q

Which Rx is flatter

A

Minus

50
Q

Only one Rx

A

Single vision

51
Q

Sometimes a patients needs more plus power for neat vision,which can be accomplished by using a near add

A

Multiofocal

52
Q

The plus peer in a multi focal lens segment that is added to the power in the distanveportion of the lens

A

Near addition

53
Q

Types of single vision

A

Distance
Computer/intermediate (arms length)
Reading (40cm)

54
Q

Distance single vision

A

Measured at 20ft or 6m, but covers distance beyond intermediate to infinity, because at that point light rays emitted from their same point are funcationally parallel

55
Q

Off center in glasses

A

Changes the power a lot and introduces cyl

56
Q

Designed for one Rx correction, types of a single vision

A

Aspheric

57
Q

Aspheric lenses

A

Helps clear the periphery

58
Q

Asphericity of plus lenses

A

Front: the surface will become flatter away from center.
Back: surface be EOMs steeper

59
Q

Minus lenses asphericity

A

Front: beceoms steeper
Back: becomes flatter

60
Q

A positive side effect of flattening a lens with aspchericity

A

Leads to thinner and lighter lenses. The effect is most evident with high plus powers. Reduction int he center thickness leads to a reduced mag effect

61
Q

Freeform distance lens

A

Same application as aspheric

  • another level up from it
  • allows for customization of lenses for optics and comfort
62
Q

Longest horizaontl dimesion of the segment

A

Seg width

63
Q

Longest vertical dimension of the segment

A

Seg depth

64
Q

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

A

Seg height

65
Q

Vertical distance between the major reference point and the top of the seg

A

Seg drop

66
Q

The distance portion of the lens and the wearers PD

A

Degenerated from the geometric center to correspond to the wearers PD
-segment is also further deceneterd to correspond to the near PD

67
Q

The distance from the GC to the MRP

A

Distance decentration

68
Q

The isn’t accounting for a near PD. The distance from the MRP to the center of the seg

A

Seg inset

69
Q

The inset of the seg as measured from the optical center of the lens, distance from the geometric center to the center of the seg

A

Total inset

70
Q

Total inset=

A

Seg inset + distance decentration

71
Q

Near PD should be ____ than distance PD

A

Smaller

72
Q

Rule of thumb for near PD

A

2mm less than far PD per eye

73
Q

Used to correct all refractive errors plus the error or condition of presbyopia

A

Bifocal lens

74
Q

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

A

Solid upcurve

75
Q

Small segment cementedto back surface of lens. Segment could be replaces as add changed. However, brittle cement when cold and moved when hot

A

Cement

76
Q

Similar to Franklin bifocal, but two lenses held together with groove

A

Perfection

77
Q

First to use two kinds of glass. Three components

A

Cemented kryptok

78
Q

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

A

Fused kryptok

79
Q

Ultex A

A

Fused kryptok

-one piece back surface bifocal

80
Q

Executive bifocal

A

Fused Kryptok

-similar to franklin, but one piece

81
Q

Start as semi finished blanks with the front surface already has the base curve and segment. The lab would grind the back surface

A

Bifocals

82
Q

Used to correct all refractive errors plus the error or condition of presbyopia, these lenses will have three distinct areas of clear vision

A

Trifocals

83
Q

Intermediate add on trifocals

A

Usually one half or 60% the power of ther near add

84
Q

Advantages to round seg

A

Least visible, light

85
Q

Disadvantage of round seg

A

Takes a while to get down to a useful width

Most jump particularly in larger segs

86
Q

Flat top advantages

A

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

87
Q

Flat top disadvantages

A

Visible ledge in larger segs

88
Q

Executive advantages

A

Widest field

89
Q

Executive disadvantage

A

Thick, heavy, ugly
Chips
Collects debris
No jump

90
Q

Unlike tris and bis, has a smooth continuous building of power form the distance to the near through a corridor, no image jump

A

Progressive lenses

91
Q

Two spherical lenses connected by two blending regions

A

PALs

92
Q

Prism thinning in PALs

A

Proces by which a PAL can be made thinner be removing some BU prism. This leaves residual base down prism and a substantially thinner lens

93
Q

The cyl power lateral to the umbilic (center of PAL corridor) of a PAL increases twice as rapidly as the add power

A

Minkwitz’s theorem

94
Q

If you double the add, what happens to cyl

A

Twice the unwanted cyl

95
Q

Lens designed with shorter corridors in PALs do what to cyl

A

More unwanted cyl in the periphery or narrower viewing zones

96
Q

Hard PALs

A

Characterized by a wider area of stable optics at distance and near long with a shorter, narrower corridor
-better for sustained tasks

97
Q

Soft PAL designs

A

Narrower areas of optimal power at distance and near, but offer a longer, wider intermediate zone
-better for dynamic viewing tasks

98
Q

Multi-design PALs

A

Today many lenses are designed this way, meaning they may be softer with decreasing add power and harder with increasing add power

99
Q

PALs designed by Rx

A

To account for variation in spectacle mag associated with BC and back vertex power

100
Q

Short corridors PALs

A

So PAL can be worn in frames with small B measurements

-faster transition from distance to near

101
Q

Free form processing (digitical surfacing) in PALs

A

Uses computer aided design and processing to create high level, customized lenses unique to your Rx
Freeform not the same as customization

102
Q

Advantages of PALS

A

No image jump
Cosmetically appealing
Perceived as more youthful and fashionable

103
Q

Disadvantages of PALs

A

Expensive
Adaptation time
Unwanted cyl in periphery

Ophthalmic Optics Block 11 (4 decks)

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