Lens Material And Design Flashcards
1
Q
CR-39, another name for it
A
Hard resin
2
Q
High index glass 1.6 vs high index plastic 1.6
A
Weight difference
Impact issues
3
Q
Is glass common or not?
A
No
4
Q
Refractive index of glass in general glass
A
1.523
5
Q
Used to be the industrial standard but not now
A
Glass
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
7
Q
Advantages of glass
A
Highly transparent and colorless, resistance to heat and chemicals, highly scratch resistance, high abbe number, low CA
8
Q
Disadvantages of glass
A
Weight Poor impact resistance Poor UV Can’t be tinted Length processing time
9
Q
Why is glass still the best choice in some cases
A
Chemical resistance, but impact resistance issues
10
Q
Two types of plastics
A
Thermoplastics and thermosetting
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
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)
13
Q
CR-39 (hard resin) advantages
A
Light weight than glass High abbe number Lowest price point Tintiable Many designs Good optics
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
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
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
17
Q
Polycarbonate is what type of plastic
A
Thermoplastic
18
Q
Advantages of polycarbonate
A
- Excellent impact resistant
- UV protection (block all wavelengths below 370nm)
- many designs, many options
19
Q
Disadvantage of polycarbonate
A
- not great optics
- not great tinting
- soft surface
- must avoid exposure to heat
20
Q
What is polycarbon usually used in
A
Children’s glasses and sportswear
21
Q
What kind of plastic is trivet
A
Combination of thermosetting and thermoplastic
22
Q
Tribes weight
A
Lighter than polycarbonate and Lower chromatic dispersion
23
Q
Thickness of trivet
A
Similar to CR-39
24
Q
Advantages of trivex
A
- tintable
- good UV protection
- very lightweight
- extremely high impact resistant
- fewer heat issues than polycarbonate
25
Polycarb and drill mount
Not a good because it needs to be heated and you cant heat polycarbonate
26
Trivex and drill mount
Can use this because it tolerates heat well
27
Low cost, excellent optics
CR39
28
Chemical resistant, not commonly used
Glass
29
Most expensive, more chromatic dispersion, softer, scratches easily, strong Rx
Hi-index Rx
30
Good for children, sports, hazardous jobs, anyone wanting thinner, lighter lenses
Polycarbonate and trivex
31
Why doesn’t the center thickness change when you increase RI in a minus lens
Because we are changing the edge thickness in a minus lens
32
Weight in glasses as we increase RI
Gets a little lighter, then levels off
33
Minimum edge thickness in + lens when you go up in index
Levels off quickly, but drops the center thickness
34
List the materials in order from low RI to high RI
CR-39
Trivex
Polycarb
Hi index
35
Point on the datum line halfway between two vertical lines which are tangent to the edges
Geometrical center (GC)
36
Horizaontl length of the box
Eye size or lens size (A)
37
When do we use the term eye size
When referring to the frame
38
When do we use the term lens size
When referring to the lens
39
Vertical length of box
B distance (B)
40
Shortest horizaontl distance between the lenses
Bridge size (DBL)
41
Horizaontal distance between the geometrical centers of the two lenses
Frame PD
42
The longest diamter of a lens
Effective diameter (ED)
43
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
Major reference point (MRP)
44
Consequences of not using the proper minimum blank size on lens appearance
Giving the same power but making it thinner to look nicer
45
Minus lenses and base curves
Flatter in the front, so smaller base curve
46
Plus lenses and base curve
Higher base curve, lens bulge
47
If there is a some wrap to a frame, how does that affect plus or minus lenses?
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
Which Rx is more rounded
Plus
49
Which Rx is flatter
Minus
50
Only one Rx
Single vision
51
Sometimes a patients needs more plus power for neat vision,which can be accomplished by using a near add
Multiofocal
52
The plus peer in a multi focal lens segment that is added to the power in the distanveportion of the lens
Near addition
53
Types of single vision
Distance
Computer/intermediate (arms length)
Reading (40cm)
54
Distance single vision
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
Off center in glasses
Changes the power a lot and introduces cyl
56
Designed for one Rx correction, types of a single vision
Aspheric
57
Aspheric lenses
Helps clear the periphery
58
Asphericity of plus lenses
Front: the surface will become flatter away from center.
Back: surface be EOMs steeper
59
Minus lenses asphericity
Front: beceoms steeper
Back: becomes flatter
60
A positive side effect of flattening a lens with aspchericity
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
Freeform distance lens
Same application as aspheric
- another level up from it
- allows for customization of lenses for optics and comfort
62
Longest horizaontl dimesion of the segment
Seg width
63
Longest vertical dimension of the segment
Seg depth
64
Distance from the lowest point on the lens to the top of the seg
Seg height
65
Vertical distance between the major reference point and the top of the seg
Seg drop
66
The distance portion of the lens and the wearers PD
Degenerated from the geometric center to correspond to the wearers PD
-segment is also further deceneterd to correspond to the near PD
67
The distance from the GC to the MRP
Distance decentration
68
The isn’t accounting for a near PD. The distance from the MRP to the center of the seg
Seg inset
69
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
Total inset
70
Total inset=
Seg inset + distance decentration
71
Near PD should be ____ than distance PD
Smaller
72
Rule of thumb for near PD
2mm less than far PD per eye
73
Used to correct all refractive errors plus the error or condition of presbyopia
Bifocal lens
74
First one piece bifocal. Designed by starting with required near power and adding a second curve to upper portion and back of surface
Solid upcurve
75
Small segment cementedto back surface of lens. Segment could be replaces as add changed. However, brittle cement when cold and moved when hot
Cement
76
Similar to Franklin bifocal, but two lenses held together with groove
Perfection
77
First to use two kinds of glass. Three components
Cemented kryptok
78
Components fused together by heat instead of cement. Mostly glass until recent introduction of new materials
Fused kryptok
79
Ultex A
Fused kryptok
| -one piece back surface bifocal
80
Executive bifocal
Fused Kryptok
| -similar to franklin, but one piece
81
Start as semi finished blanks with the front surface already has the base curve and segment. The lab would grind the back surface
Bifocals
82
Used to correct all refractive errors plus the error or condition of presbyopia, these lenses will have three distinct areas of clear vision
Trifocals
83
Intermediate add on trifocals
Usually one half or 60% the power of ther near add
84
Advantages to round seg
Least visible, light
85
Disadvantage of round seg
Takes a while to get down to a useful width
| Most jump particularly in larger segs
86
Flat top advantages
Immediate broad field
Minimal jump
No prism 5mm below top
Light
87
Flat top disadvantages
Visible ledge in larger segs
88
Executive advantages
Widest field
89
Executive disadvantage
Thick, heavy, ugly
Chips
Collects debris
No jump
90
Unlike tris and bis, has a smooth continuous building of power form the distance to the near through a corridor, no image jump
Progressive lenses
91
Two spherical lenses connected by two blending regions
PALs
92
Prism thinning in PALs
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
The cyl power lateral to the umbilic (center of PAL corridor) of a PAL increases twice as rapidly as the add power
Minkwitz’s theorem
94
If you double the add, what happens to cyl
Twice the unwanted cyl
95
Lens designed with shorter corridors in PALs do what to cyl
More unwanted cyl in the periphery or narrower viewing zones
96
Hard PALs
Characterized by a wider area of stable optics at distance and near long with a shorter, narrower corridor
-better for sustained tasks
97
Soft PAL designs
Narrower areas of optimal power at distance and near, but offer a longer, wider intermediate zone
-better for dynamic viewing tasks
98
Multi-design PALs
Today many lenses are designed this way, meaning they may be softer with decreasing add power and harder with increasing add power
99
PALs designed by Rx
To account for variation in spectacle mag associated with BC and back vertex power
100
Short corridors PALs
So PAL can be worn in frames with small B measurements
| -faster transition from distance to near
101
Free form processing (digitical surfacing) in PALs
Uses computer aided design and processing to create high level, customized lenses unique to your Rx
Freeform not the same as customization
102
Advantages of PALS
No image jump
Cosmetically appealing
Perceived as more youthful and fashionable
103
Disadvantages of PALs
Expensive
Adaptation time
Unwanted cyl in periphery
