Lecture 9 - Lens Material And Design Flashcards by Haley Rambo

Another name for CR-39

Hard resin

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Is hard resin glass or plastic?

Plastic

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

Weight (glass is heavier)

More impact issues (glass)

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What lens used to be the industrial standard?

Glass (crown glass)

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Primary components of glass

Silica
Soda
Lime

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Advantages of glass

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

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Disadvantages of glass lenses

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

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2 types of plastic lenses

Thermoplastic (polycarbonate)

Thermosetting (CR-39)

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

Thermoplastic (polycarbonate)

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

Thermosetting (CR-39)

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Advantages of CR-39

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

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Disadvantages of CR-39

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

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Hi index plastic advantages?

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

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Hi index disadvantages

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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Polycarbonate advantages

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

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Polycarbonate disadvantages

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

Polycarbonate (or trivex)

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Advantages of trivex lenses

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

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How would you describe trivex lenses?

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

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

CR-39
Trivex
Polycarb
Hi index

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

Trivex

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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?

Polycarb

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

Hi index plastic 1.74

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

It won’t be as thin as it could be

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Three multifocal types

Bifocal Trifocals Progressive

Are minus lenses lower or higher base curves?

Flatter, so less base curve

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

Higher

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

Blurry b/c changes effective power away from optical center

Distance in surface power produces what?

Produces unwanted astigmatism

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

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

How do aspheric lenses make distortion free lenses?

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

What manufacturer does he reference a lot with aspheric lenses?

SACO (say-co)

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

Double-side aspheric design

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

Digital freeeform distance lenses

Longest horizontal dimension of the segment

Seg width

Longest vertical dimension of the segment

Seg depth

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

Seg height

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

Seg drop

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

The wearer's PD

Distance from geometric center to major reference point

Distance decentration

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

Seg inset

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

Total inset

How do you find total inset?

Seg inset + distance decentration

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

2 mm per eye (4 mm total)

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

Bifocals

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

Solid upcurve

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

Cement

Similar to Franklin bifocal but 2 lenses held together with groove

Perfection

First to use 2 kinds of glass. Three components

Cemented kryptok

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

Fused Kryptok

One-piece back surface bifocal

Ultex "A"

Similar to Franklin, but one piece

Executive

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

Curvature of the countersink

Which modern bifocal gives you a really wide reading area?

Executive or E-style

Trifocals eliminate what from using a bifocal?

The "dead zone"

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

One half or 60%

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

Trifocal

28 mm is a good measurement for viewing what?

iPad

35 mm is a good measurement for viewing what?

Magazine

What lens can you turn into a quadrifocal?

Trifocal

Advantages of round seg

Least visible | Light

Disadvantages of round seg

Takes a while to get down to a useful width | Most jump, particularly in larger segs

Advantages of flat top

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

Disadvantages of flat top

Visible ledge in larger segs

Advantages of executives

Widest field | No jump

Disadvantages of executive

``` Thick Heavy Ugly Chips Collects debri ```

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

Progressive (PAL)

Do progressive lenses have image jump?

How is blending achieved for a PAL lens?

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

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

Minkwitz's Theorem

Vertical meridian of a PAL that is spherical at every point

Umbilic

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

Hard

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

Soft

Which progressive design is better for sustained viewing tasks?

Hard designs

Which progressive design is better for dynamic viewing tasks?

Soft designs

What are short corridor PALS for?

Can be worn in frames with small B measurements

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

Free-form processing (digital surfacing)

Advantages of PALs

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

Disadvantages of PALs

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

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

Geometrical center

Horizontal length of the box, when referring to the frame

Eye size

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

Lens size

Vertical length of the box

B distance

Shortest horizontal distance between the lenses

Bridge size

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

Frame PD

Longest diameter of a lens

Effective diameter

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

Major reference point