Week 5: Frame Materials/Adjustments & Multifocals Flashcards
List seven different plastic frames
- Cellulose acetate
- Cellulose propionate
- Polyamide
- Optyl
- Carbon fibre
- Nylon
- Polycarbonate
Describe Cellulose Acetate
- Best quality plastic
- Frames are cut from an acetate sheet
- Sheets are pre-coloured prior to cutting
- Labour intensive – 120 steps in the manufacturing process
Advantages & disadvantages of Cellulose Acetate
Advantages:
- Non-flammable
- Polishes well (maintains glossy appearance for long time)
- Colour-fast
- Easily repaired
- Acetate can be marginally shrunk if lenses have been cut too small
Disadvantages
- Plasticisers will deteriorate with time, forming a white film on the surface
- Small cracks may develop in frame
- Frame becomes brittle after exposure to sun or extreme heat
- May return to normal flat shape with time, or with extreme heat
Describe Cellulose Propionate
- Injected moulded
- Less labour intensive
- Less ‘off-cuts’ - ↓ waste material
- Requires more plasticisers, hence less stable & more heat sensitive
Advantages & disadvantages for Cellulose Propionate
Advantages:
- Slightly thinner & lighter than acetate
- Less expensive to produce
- Frames can be made & coloured later
- Better shape retention
- Hypo-allergenic
Disadvantages:
- Needs more heat than acetate for adjustments
- Cannot be shrunk
- Surface dye may become buffered off
- Tends to become brittle with age, sun & excessive heat
Describe Polyamide
- Injection-moulded, however plasticisers are not required
- Nylon-based material (strong & flexible)
- Do not need to be heated to make fitting/adjustments
Advantages & disadvantages of Polyamide
Advantages:
- Very lightweight
- Good shape stability
- Hard surface
- Heating not required for adjustments and fitting
- Hypo-allergenic
Disadvantages:
- Overheating can result in shrinkage
Describe Optyl
- Thermo-setting material
- Difficult to overheat
- Approximately 30% lighter than cellulose acetate
Advantages & disadvantages of Optyl
Advantages:
- Returns to original shape regardless of heating
- Does not age & become brittle
- Much lighter than most other plastics
- Hypo-allergenic
- Patterned/coloured after production
Disadvantages:
- Can be difficult to adjust
- Should not be cooled too quickly
- Adjustments will be lost if frame left in car/heat
- Ageing and exposure to sunlight decreases strength
- Colour can fade
Describe Carbon Fibre
- Injection-moulded
- Flexible nylon compound mixed with a fibre or whisker
Advantages & disadvantages of Carbon Fibre
Advantages:
- Lightweight
- Good shape retention & strength
- Hypo-allergenic
Disadvantages:
- Carbon fibre is quite brittle when dropped
- Difficult to fit and adjust carbon fibre frames
- Poor range of colours available
- Can break frame when fitting new lenses
Describe Nylon
- Most basic of the nylon-plastics
- Injection-moulded
- Typically used for safety specs and sunglasses
Advantages & disadvantages of Nylon
Advantages:
- Very strong and flexible
- Lightweight
- Not affected by heat and cold
Disadvantages:
- Poor surface finish
- Extremely difficult to adjust & to fit lenses
- Can be brittle
- Can only be manufactured in dark colours
Describe Polycarbonate
- Primarily used for safety specs and sports glasses
- If made with no prescription, the lens + frame are often moulded into one unit
Advantages & disadvantages of Polycarbonate
Advantages:
- Very impact resistant
- Can be used as shield over top of specs
Disadvantages:
- Poor surface finish – dull and not polished
- Extremely difficult to adjust and to fit lenses
- Can be brittle
List 5 different metal frame material
- Gold-filled
- Gold-plated
- Nickel Silvers
- Titanium
- Stainless steel
Describe Gold Frames
- Gold is mixed with other metals to produce an alloy
- Most ‘gold frames’ contain very little gold
- There are two processes for attaching the gold layer to the base metal;
- Gold-filled
- Gold-plated
Describe gold filled frames
- Layer of gold is brazed onto the surface of the base metal & becomes bonded. The frame is then produced
Advantages & disadvantages of gold filled frames
Advantages:
- High quality – tarnish & perspiration resistant
- Easy to adjust & align
- Very durable & maintain appearance
Disadvantage:
- Quite expensive
Describe Gold-Plated frames
- The base metal frame is produced, then suspended in an electrolyte bath with gold applied by electrolysis to the frame
Advantages & disadvantages of Gold-Plated frames
Advantages:
- High quality – tarnish & perspiration resistant
- Easy to adjust & align
- Very durable, & maintain appearance
- Easier and cheaper to produce than gold-filled frames
Disadvantages:
- Slightly more susceptible to corrosion from perspiration
- Still relatively expensive
Describe Nickel Silvers
- Contains more than 50% copper, 25% nickel and rest zinc – no silver
- Copper gives pliability, zinc adds strength, and nickel gives the whitish appearance
- May be most commonly found in hinges, end pieces, bridges and the inner core of temples
Describe titanium frame
- Metal frame of choice today!
- Very light, very strong, and very expensive
What are the advantages & disadvantages of Titanium frames?
Advantages:
- Very strong
- Lightest of metals (40% lighter than most metals)
- Hypo-allergenic
- Holds adjustments well
Disadvantages:
- Very expensive
- Difficult to repair – cannot be soldered
- Restricted range of colours
Describe Fexon – Memory Metal
- Titanium-based alloy
- Usually found in the shaft of the temple, and bridge, allowing twisting without breakage
Describe Fexon – Memory Metal
- Titanium-based alloy
- Usually found in the shaft of the temple, and bridge, allowing twisting without breakage
What are the advantages & disadvantages of Fexon – Memory Metal
Advantages:
- Return to original shape, even after twisting or bending
- Lightweight
- Hypollargenic
- Corrosion-resistant
- 25% lighter than standard metals
Disadvantages:
- Can be difficult to adjust
Describe Stainless Steel frames
- Alloy of iron, nickel, manganese, chromium
- Not too expensive
Advantages & disadvantages of stainless steel frame
Advantages:
- Strong, so can be made extremely thin and light weight
- Highly flexible/springy temples
- Low toxicity
- Hypoallergenic (if no nickel)
- Resistant to corrosion, abrasion & head
Disadvantages;
- Difficult to adjust, solder or weld
Describe multifocals
- These are lenses in which there are two (bifocals) or three (trifocals) distinct portions of different focal power
Advantages & disadvantages of Franklin Bifocal
Advantages:
- Distance and near optical centres being able to be placed in any desired position
Disadvantages:
- Not very durable
- Poor cosmetically
- Collects dirt along the ridge between the segments
What are 7 different types of bifocal?
- Round seg
- Flat top
- Executive
- Curved top
- Ribbon Seg
- A seg
- Blended seg
Describe round seg
- The reading portion forms a perfect circle
- Optical centre of the segment is at the exact centre of the seg
- Typically the seg diameter is 24mm, although can be from 22 to 40mm
Describe Flat-Top/D-Seg
- Most popular design
- Similar to a round seg, with the top ‘cut-off’
- Is made in glass (fused) or plastic (one- piece) – hence the ridge in CR-39
- OC typically 5mm below the seg line
- Typically D-28 (mm) or D-35
Describe Curved-Top/Panoptik
- Look similar to flat-top, however the upper line is curved
- Much less common than flat tops
Describe Ribbon Seg
- Like a flat-top, with the bottom of the seg also cut-off
- 2 types;
- B-seg (9mm deep)
- R-seg (14mm deep)
- May be used for near prism control
Describe Executive for bifocals
- Always one-piece construction (glass or CR-39)
- ‘Shelf’ between the two sections
- May be better to prescribe a large flat-top (D-35
Describe A segment for bifocals
- Like a large round seg
- Two spherical surfaces on one side of the lens
- Always one-piece
- Plastic: reading curve moulded on front of lens
- Glass: bifocal placed on back, therefore needs a plus-cyl on front surface
- Diameter is 38mm for glass & 40mm for plastic
Describe blended segment for bifocals
- Very much the same as a round seg, with the edges blended for cosmetic appearance
Who would wear Bifocals?
- Presbyopes or people with accommodative problems
- Previous wearers
- Unsuccessful progressive lens wearers
- Some children with accommodative problems, esophoric myopes
- Patients with economic restrictions
- Some occupations
Who are not Suitable to wear Bifocals?
- New presbyopes
- Image conscious people
- Unstable walkers
- Successful progressive or trifocal lens wearers
Who are not Suitable to wear Bifocals?
- New presbyopes
- Image conscious people
- Unstable walkers
- Successful progressive or trifocal lens wearers
Advantages vs Disadvantages for Bifocals
Advantages:
- Convenience – not changing between specs
- Cheaper than buying two separate pairs of specs
- Less likely to lose specs
- More convenient for handicapped presbyopes
- Children who use lenses for accommodative control can find near seg more easily
Disadvantages:
- Aesthetically not ideal
- Implicitly associated with age
- Image jump
What is Seg Magnification
- The seg being a positive lens will magnify an image, with higher the seg power, more the resultant magnification
What is an image jump?
- Is also known as “differential displacement”
- It is the difference in location of objects observed when transitioning between distance & near segments
- If a px wearing bifocals move their gaze from the distance portion to near portion of the lens, the image will appear to shift up
What are the four types of trifocals?
- Flat top
- Executive
- E-D
- Round seg
Describe Flat-Top for trifocals
- Depth of intermediate portion is typically 7mm
- If trifocal is fused, the intermediate portion will have a refractive index between that of distance & near glass
Describe Executive Trifocal
- Depth of intermediate portion is typically 7mm
- If the trifocal is fused, intermediate portion will have a refractive index between that of the distance & near glass
Describe E-D Trifocal
- Combines an executive with a flat-top lens
- Excellent for working at a desk
Describe round seg for trifocals
- ‘Bulls-eye’ appearance
Who would wear Trifocals?
- Presbyopes
- Older patients
- Handymen
- Chefs
- Computer use
What are the two main methods for construction of glass bifocals?
- Fused bifocal
- One-piece bifocal
Describe Cemented Bifocal
- The main lens contains the distance power, and a wafer is the near segment
- Near power is the power of the distance portion plus the power of the segment
Describe One-Piece Bifocal
- Bifocals may be constructed as a one piece of same refractive index
- Usually made of plastic
- These lenses are constructed out of a single piece of lens material, with the increase in near power resulting from a change in the curvature in the front surface of the lens
Describe Fused Bifocal
- Segment of the bifocal is made of a glass of higher refractive index than the distance ‘carrier’ lens
- The segment is fused into the lens such that no change in curvature can be felt on the front surface
