DISP - Ophthalmic Lens Materials - Week 4 Flashcards
Name three modern plastic spectacle lens materials.
CR-39
High-index plastics
Polycarbonate
Name 4 modern glass spectacle lens materials.
Crown
Light high index
Thin high index
Zeiss 1.9
In what form is raw CR-39?
Liquid
Define thermosetting.
Once hardened, cannot be softened.
True or false
CR-39 is not thermosetting.
False, it is
What is the weight, impact resistance, and Abbé number of CR-39 like?
Lightweight, reasonable impact resistance, and a high Abbé number.
Is CR-39 scratch resistant?
Relatively soft, a coating often used.
Consider a CR-39 lens with a scratch resistant coating. What impact does this have on the heat resistance of the lens?
CR-39 has good heat resistance, but coatings often do not.
Name and describe the three processes of surfacing CR-39.
Grinding - preparation of the curved surfaces of the lens by grinding.
Fining - second stage of grinding, the surface is brought closer to the desired curvature.
Polishing - smoothing the surface so it exhibits specular rather than diffuse reflection.
What kind of curve is made during the first stage of surfacing a CR-39 lens, and in what stage is it corrected?
An elliptical, rather than spherical curve is made. It is corrected in the second stage, fining.
Describe the process of fining.
Toric or spherical tools made of metal with a curve of opposite shape to the curve to be surfaced is used. It is covered in an emery sheet. The lens blank is swept across it under pressure while the tool rotates rapidly.
What does the lens look like after fining?
Frosted finish
Describe lens polishing.
Like fining, but instead of an emery sheet, a soft cloth or paper is used.
Do high index plastics have a higher or lower Abbé number compared to high index glass?
Lower
Are high index plastics lightweight and impact resistant?
Yes both
Do high index plastics thermoset?
Yes
How are CR-39 and high index plastics made? Can they be made as completed lens with no further surfacing?
Made by moulding, as complete lens, with no further surfacing needed.
The back surface needs finishing.
What was once the most common spectacle lens material?
Crown glass
Does crown glass have good surface durability or is it easily scratched?
Good surface durability
Does crown glass have superior or poor optical properties?
Superior (heineprecisiongermanengineeringquality)
What three oxides can be added to high index glass, and for what purpose? Name two consequences.
Titanium oxide, barium oxide, or lead oxide, to increase refractive index. They also increase weight and lower Abbé numbers.
Compare the weight of plastic vs glass.
Plastics are lighter
Compare the impact resistance of plastics vs glass.
Glass shatters more easily and sharply
Compare the surface durability of plastics vs glass.
Glass is resilient to scratches
Compare the ease of tinting for plastics vs glass.
Plastic is simpler and inexpensive, glass is expensive
Describe fogging with glass vs plastic, and why this is the case.
Glass - fogs easily, demists slowly
Plastic - fogs slowly, demists quickly
This is because glass has higher thermal conductivity
Consider the curve variation factor. What is it used to find? What two things does it allow a practitioner to do, and what does it demonstrate the benefit of?
It is used to find the equivalent crown glass curve to the material being considered.
It enables the lab to select the appropriate tool if it had tools only for glass.
Allows the practitioner to calculate the expected thickness of a lens.
Demonstrates the benefit of increasing n.
What is meant by specific gravity?
Density in grams per cubic centimetre
It is the ratio of 1cm3 of material to 1cm3 of water (which is 1mL, which is also 1g)
Consider LHI, THI, Zeiss, and Crown glass. Order them by density (low to high).
Crown
LHI
THI
Zeiss
Consider high index plastic, CR-39, and polycarbonate. Order them by density (low to high).
Polycarbonate
High index plastic
CR-39
What is the transverse chromatic aberration threshold for perception?
TCA of 0.1Δ
What is the transverse chromatic aberration value is sufficient to cause a 1 line drop in acuity?
TCA of 0.16Δ
Define impact resistance.
Material’s ability to resist the force of a sudden impact.
Describe the Taber test for scratch resistance.
Abrasion wheel is applied to the surface under a measured amount of pressure.
Describe the Bayer test for scratch resistance.
Lenses secured to an oscillating pan containing an abrasive material.
Describe the steel wool test for scratch resistance.
A pad of steel wool of a certain grade is applied with a fixed pressure.
Describe the tumble test for scratch resistance.
Lenses are tumbled in a barrel containing an abrasive mix.
Describe the eraser test for scratch resistance.
An abrasive eraser is rubbed on the lens surface at a given pressure.
Consider a scratch resistance test. What are lenses compared to?
A control lense and a graded haze scale.
What is material transparency a function of? Describe the relationship.
Refractive index, as it increases, more light is lost.
The amount of reflection is determined by what?
Fresnel’s Reflection Factor
Consider crown glass and CR-39. How much light is reflected at each surface?
~4% per surface.
Consider high index materials. How much light is reflected at each surface.
~8% per surface.
What three things does increased transparency produce?
Reduced ghost images
Reduction in the glassy appearance
Increased transmittance
What absorbs more UV light, plastic or glass?
Plastic
What nm light does CR-39 absorb up to?
360nm
What nm light does polycarbonate absorb up to?
390nm
What two materials have poor UV absorption and ould require a UV coating?
Glass and CR-39
What test assesses chemical resistance? Describe it briefly.
Thermodyne test. It stimulates prolonged exposure to atmospheric conditions using heat and humidity.
Order the following based on their chemical resistance (from worst to best).
Polycarbonate
HI glass
Crown glass and CR-39
