9,10,11 - Ophthalmic Lenses Flashcards
- Constringence is measured in…
- Increase n value changes thickness how? What happens to weight? What happens to dispersion?
- Weight/density measured in…
- Abbe number (V) (Higher no. = less dispersion)
- Decrease thickness. But often heavier and more dispersion as a result.
- Specific Gravity (SG)
CVF is?
Curve Variation Factor measures proportional change in thickness relative to
CVF = (1.523 - 1)/(n –1)
How to measure change in thickness between two lens type?
ΔLT = (old n – 1)/(new n – 1)
If ΔLT = 0.85, can say it is 15% thinner than old glass.
Describe 3 optical glass materials.
- What are they made of/How are they made?
- Advantages?
- Disadvantages?
Crown glass = soda-lime
Light flint glass = lead
“Hi Index” glass = rare earth materials (propriatary)
Advantages:
- Scratch resistance
- Chemical resistance
- Can have higher n
Disadvantages:
- Fragile
- Require more saftey
- Weight
Usually only for high index materials for those with higher prescriptions. Prescribing them should come with warnings.
Describe CR-39
- What are they made of/How are they made?
- Advantages?
- Disadvantages?
- Liquid resin + catalyst poured into mould and cured in oven
Advantages:
- Light
- Impact resistant
- Chemically inert
- Pitting resistance
- Fogging resistance
- Tintable dye
Disadvantages:
- Surface abrasion
- Increased thickness
- Less mechanically stable
Describe polycarbonate
- What are they made of/How are they made?
- Advantages?
- Disadvantages?
- Injection moulded at 320°C into polished metal moulds
Advantages:
- Lighter than CR39
- Thinner than CR39
- More impact resistance
- Built in UV filter
Disadvantages”
- Low scratch reisistance
- Low abbe
- More reflections
- Difficult to work with
- Affected by solvents
Use when impact resistance and higher index are required. Often for children because of the impact reisistance and the low scratch resistance is countered with a hard coating.
Describe trivex/triology
- What are they made of/How are they made?
- Advantages?
- Disadvantages?
Advantages
- Lighter than CR39 and polycarbonate
- Thinner than CR39
- Chemical resistance
- Impact resistance similar to polycarbonate
Disadvantages
- Moderate scratch resistance
Main material for children.
Describe high index plastic materials in general.
Abbe no. in general is bad. SG doesn’t actually change much and is pretty good for this refractive index. Often different types are referred to by their n value.
Describe PMMA.
Shatterproof but scratched easily. Mainly used for fashion sunglasses, not optical lenses.
Describe hardcoats
- What is it?
- Which materials always need one and why?
- Thickness?
- Methods of putting on hard coat?
- Layer of scratch resistance
- All plastics need this due to low scratch resistance (glass are inherently scratch resistant)
- 0.002 - 0.01mm thick (pretty thick for a coating
Quartzing method:
- Use vacuum, place silica on top.
- The lens and lens coating will move relative to one another causing crazing. Thermal shock causes this. This was the original method but we don’t in NZ much so crazing may happen from lenses overseas that used cheaper methods like quartzing.
Lacquer coats:
- Applying a thin hard coat via dipping or spinning.
- Problem was they found that it was often not thin enough.
Varnishes:
Most hard coats use this. Either dipped or spin, but they are small nanoparticles. They don’t craze because they float on surface.
- Scratch resistant
- High flexibility
- Low friction coefficient
Describe the type of reflections that can be seen when glasses are worn
Wearer:
- Ghost images
- Dividing line of flat top bifocal
Observer:
- Veiling glare
- Power rings
Anti-reflection coating?
- Importance?
- How it works?
- What are mirror coatings?
- Single layer vs double vs multi-layered AR?
- Increases transmittance
- Match path and amplitude conditions
- Mirror coatings use constructive interference instead (requires tinting)
- Single layer gives colour reflex as only one wavelength is absorbed.
- Double layer makes light go through lower index layer then higher index layer then lens. Double layer is used not to change reflex colour but rather because we have issue attaching AR coat onto plastic lens. The second layer helps attach.
- Multilayered reduce all wavelength reflections and uses alternating low and high index coating. Final hydrophobic layer at top possible.
What are hydrophobic coatings?
- High wetting angle causes fluid to build up as beads which smoothly run off
- Very thin, reduces effectiveness over time
Filter lenses
- Different types and their purpose?
Each reduces glare and total light entering eye
UV contributes to pathologies like cataract, pterygium, AMD and can be absorbed by anterior structures. Always good to block out UV, especially if tinted glasses.
Blue
Pros:
- Reduce retinal exposure to blue light (ARMD)
- Reduce disruption of circadian rhythm from evening exposure to blue.
Cons:
- CV and scotopic vision worse
- Daytime exposure requires blue light to set circadian rhythm
IR causes heating and high exposures causes glassblower’s cataract whilst low exposure causes dry-eye symptoms
Describe solid tint glasses
- Lenses as a whole is tinted.
Pros: - Low cost
- Surface scratching doesn’t ruin it
- No extra modification required
Cons: - Different shading at different parts of lens (due to changing thickness)
- Permanent tint cannot be modified.
