D: Ophthalmic Lens Materials - Week 4 Flashcards
List 3-4 modern plastic lens materials and their refractive indices in order from lowest to highest
Then list 3-4 modern glass lens materials
Plastic:
- CR-39: 1.49
- Polycarbonate: 1.59
- High index plastic: 1.54-1.74
- zeiss plastic: 1.74
Glass:
- Crown: 1.52
- Light high index: 1.6 or 1.7
- Thin high index: 1.8
- zeiss: 1.9
Outline the 3 step process used to surface CR-39
Step 1: Grinding/Surfacing/Generating - the rough curved surface is carved out (back of lens is grinded)
Step 2: Fining - gets you closer to desired curvature
Step 3: Polishing - makes the surface smooth so it exhibits specular reflection (rather than diffuse)
How does the grinding/generating process work to carve out a lens shape? Is there a formula involved that can help us here? (Explain the first method)
We orient/change the angle that the cutting head of our tool makes to the surface of the lens.
The curve is given by R = r/sin(theta), where angle theta of the cutting head will determine what R we are cutting out
Are there any alternative methods we can use to carve out a lens shape? (Explain the second method)
Yes, we can also cut out a particular angle by sweeping the cutting head through a particular radius of curvature
Explain how the fining process works. How does this help get you closer to the lens curve you want?
The lens blank is swept across a toric (or spherical) metal tool with an opposite shaped curve to the curve you want to surface, which rotates rapidly.
This helps eliminate any elliptical errors made from the grinding/generating step.
How would a 2-step fining process work? Name a lens material that uses this
CR-39 has a 2-step process where the procedure is done twice, first with a rough emery sheet attached to the tool, then a smoother emery sheet.
Explain the process of surfacing a lens
It’s just lens polishing. The tool from before is covered by soft cloth or paper, and a polishing compound is used to polish the surfce
A tool made of a particular refractive index material requires a certain power (in diopters) to cut a particular lens surface material. What is the formula outlining this?
Ft(nm-1)
Fm = ————-
(nt-1)
** Fm – tool power, Ft – true power
** nm – tool R.I, nt – lens R.I
Example: If we want to surface a -10D spherical curve in CR-39 using crown glass tooling, how powerful a tool would we need?
-10 (1.5231 - 1)
Fm = ———————-
(1.498 -1)
= - 10.50 Diopters
Compare general plastic vs glass lens materials in the following categories:
- weight
- impact resistance
- ease/cost of tinting
- fogging
- plastic is lighter than glass
- glass shatters more easily (less impact resist)
- glass is more scratch resistant
- plastic is simple/cheaper to tint, while glass is expensive to tint
- glass fogs more easily (b/c thermal conductivity)
Compare the processes involved in tinting plastic vs glass
plastic: dying in hot chamber (92deg) until required density achieved = simple/inexpensive
glass: thin metal oxide layer deposited in vacuum chamber = expensive
Compare the absorption/blockage of UV-A (315-400nm) between plastic and glass lens materials
Plastic: absorbs UV-A well, CR-39 absorbs 90% UV-A and Polycarb absorbs 100%
Glass: does not absorb UV-A well. However coating and tinting can help it do so.
(Both absorb UV-B)
List the Abbe’ values of the following lens materials from lowest to highest:
- crown glass
- CR-39
- Polycarb
- High index plastic
- High index glass
Polycarb: 30.0 – highest dispersion
High-index glass: 35.0 or 42.0
High-index plastic: 36.0
Cr-39: 58.0
Crown glass: 58.9 – lowest dispersion
What does the Curve Varation Factor tell us? Write the formula used to calculate it
tells us how much the curve changes/has varied relative to a material standard (usually crown glass).
CVF = (nc - 1)/(nm-1)
**nc – R.I of base material (usually crown glass)
**nm – R.I of lens material
Describe 2 potential uses for the curve variation factor (CVF)
- Can be used to ensure you select the correct lab tool for the material you’re trying to grind (if only tools of a certain material are available)
- Allows you to calculate the expected thickness of a lens (due to expected curve being flatter for higher R.I)
What does specific gravity measure and what is important about it in regards to lens materials?
is a measure of the physical density of the material (in grams per cubic cm)
Density would typically mean more mass, however, more dense materials usually have a higher n (i.e. amount in moles), therefore they have smaller mass in finished product. So the final lens may be lighter (sometimes even despite higher density material)
Rank the following lens materials from lowest to highest density:
- Crown glass
- CR-39
- LHI 1.6
- THI (thin high index) 1.8
- Zeiss
- Polycarb
- High index plastic
polycarb: 1.20
Hi-index plastic: 1.22-1.27
CR-39: 1.32
Crown: 2.54
LHI 1.6: 2.60
THI 1.8: 3.47
Zeiss: 4.02
Compare the following lens materials to crown glass in terms of lens weight:
- polycarb
- hi-index plastic
- CR-39
- LHI (light high index) 1.6
polycarb: 50% lighter
Hi Index plastic: 45-50% lighter
CR-39: 30% lighter
LHI 1.6: only 5% lighter
Compare the weight of LHI 1.7 to crown glass at different lens powers (3, 4.50 and 6 diopters)
+/-3D: LHI is 15% heavier
+/-4.50D: same as crown
+/-6D: 5% lighter
(heavier at lower powers and lighter at higher powers comparitively)
Compare the weight of THI to crown glass at different lens powers: 3 and 10 diopters
+/-3D: THI is 30% heavier
10D: THI is same as crown
Compare the weight of Zeiss 1.9 to crown glass at different lens powers (3 and 10 diopters)
+/-3D: Zeiss is 40% heavier
+/-10D: 5% heavier
Use a formula to describe the relationship between transverse chromatic aberration and abbe number
TCA =cF/V = p/V
**p – amount of prism
**V – abbe number
What TCA value is the threshold for perception of chromatic colour fringes? What value is sufficient to cause a 1 line drop in acuity?
perception: TCA = 0.1 x amount of prism
1 line drop: TCA = 0.16 x amount of prism
What Abbe values are considered low dispersion, medium dispersion, and high dispersion?
Low dispersion: V greater or equal to 45
Medium: V greater than 39, less than 45
High: V less than 39
What 2 tests are used to test impact resistance of glasses?
- Standard test: dropping 5/8 inch steel ball onto lens from height of 50cm
- Another common test: 3/4 inch steel ball fired at various speeds to determine breakage speed of lens
Name 5 tests used by modern manufacturers to test scratch resistance of lenses
- Taber test: abrasion wheel applied to surface under a pressure
- Bayer test: lens secured to oscillating pan with abrasive material
- Steel wool test: pad of steel wool applied to surface at fixed pressure
- Tumble test: lenses tumbled in barrel containing abrasive mix
- Eraser test: abrasive eraser rubbed on surface at given pressure
What is “Mohs hardness scale”
A scale from 1-10 to measure a material’s surface hardness
Which lens material has noticeably bad scratch resistance (in comparison to others) without coating?
polycarbonate
(also very high index plastic, greater than 1.85 index, is a bit worse than others, but still not as bad as polycarbonate)
What is the formula used to determine the amount of reflection of a lens (relating to transparency)? What does this formula imply?
Fresnel’s Reflection Factor
r = [(n’-n)/(n’+n)]^2
** n’ – R.I of material (avg. 1.498), n – R.I of air
This means that as R.I increases, more light is likely to be reflected at each surface
Out of the following, which lens materials are the most transparent?
- crown glass
- 1.806 glass
- CR-39
- poly
- 1.580 plastic
- 1.80 plastic
- over 1.85 plastic
Crown, CR-39 and 1.580 plastic are the most transparent
(though note that all of them are decently transparent with at least 87% transparency)
What 3 benefits does increased transparency provide?
- reduced ghost images
- reduction in the glassy appearance
- increased transmittance
Why is UV-C filtering not that important for lens materials?
Because UV-C (100-290nm) is mainly filtered out by the atmosphere
How can we improve the UV-B absorption of CR-39 to maximum?
By using a (clear) UV coating
Which is better at UV absorption, glass or plastic lens materials?
Plastic
Rank the following in terms of chemical resistance from lowest to highest:
- polycarb
- CR-39
- Crown glass
- HI index glass
When would these differences in resistance matter?
Crown/CR-39
HI index glass
Polycarbonate
(though all are relatively decent, sometimes it matters though in certain professions)
Is CR-39 thermoplastic or thermosetting?
thermosetting
