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OD2 Semester 1 - Applied Clinical Training > D: Ophthalmic Lens Materials - Week 4 > Flashcards

D: Ophthalmic Lens Materials - Week 4 Flashcards

1
Q

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

A

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
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2
Q

Outline the 3 step process used to surface CR-39

A

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)

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3
Q

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)

A

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

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4
Q

Are there any alternative methods we can use to carve out a lens shape? (Explain the second method)

A

Yes, we can also cut out a particular angle by sweeping the cutting head through a particular radius of curvature

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5
Q

Explain how the fining process works. How does this help get you closer to the lens curve you want?

A

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.

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6
Q

How would a 2-step fining process work? Name a lens material that uses this

A

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.

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7
Q

Explain the process of surfacing a lens

A

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

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8
Q

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?

A 
Ft(nm-1)           
Fm = -------------         
            (nt-1)
** Fm -- tool power, Ft -- true power
** nm -- tool R.I, nt -- lens R.I
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9
Q

Example: If we want to surface a -10D spherical curve in CR-39 using crown glass tooling, how powerful a tool would we need?

A

-10 (1.5231 - 1)
Fm = ———————-
(1.498 -1)

  = - 10.50 Diopters
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10
Q

Compare general plastic vs glass lens materials in the following categories:

  • weight
  • impact resistance
  • ease/cost of tinting
  • fogging
A
  • 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)
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11
Q

Compare the processes involved in tinting plastic vs glass

A

plastic: dying in hot chamber (92deg) until required density achieved = simple/inexpensive
glass: thin metal oxide layer deposited in vacuum chamber = expensive

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12
Q

Compare the absorption/blockage of UV-A (315-400nm) between plastic and glass lens materials

A

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)

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13
Q

List the Abbe’ values of the following lens materials from lowest to highest:

  • crown glass
  • CR-39
  • Polycarb
  • High index plastic
  • High index glass
A 
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
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14
Q

What does the Curve Varation Factor tell us? Write the formula used to calculate it

A

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
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15
Q

Describe 2 potential uses for the curve variation factor (CVF)

A
  1. 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)
  2. Allows you to calculate the expected thickness of a lens (due to expected curve being flatter for higher R.I)
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16
Q

What does specific gravity measure and what is important about it in regards to lens materials?

A

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)

17
Q

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
A 
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
18
Q

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
A

polycarb: 50% lighter
Hi Index plastic: 45-50% lighter
CR-39: 30% lighter
LHI 1.6: only 5% lighter

19
Q

Compare the weight of LHI 1.7 to crown glass at different lens powers (3, 4.50 and 6 diopters)

A

+/-3D: LHI is 15% heavier
+/-4.50D: same as crown
+/-6D: 5% lighter

(heavier at lower powers and lighter at higher powers comparitively)

20
Q

Compare the weight of THI to crown glass at different lens powers: 3 and 10 diopters

A

+/-3D: THI is 30% heavier

10D: THI is same as crown

21
Q

Compare the weight of Zeiss 1.9 to crown glass at different lens powers (3 and 10 diopters)

A

+/-3D: Zeiss is 40% heavier

+/-10D: 5% heavier

22
Q

Use a formula to describe the relationship between transverse chromatic aberration and abbe number

A

TCA =cF/V = p/V

  • *p – amount of prism
  • *V – abbe number
23
Q

What TCA value is the threshold for perception of chromatic colour fringes? What value is sufficient to cause a 1 line drop in acuity?

A

perception: TCA = 0.1 x amount of prism

1 line drop: TCA = 0.16 x amount of prism

24
Q

What Abbe values are considered low dispersion, medium dispersion, and high dispersion?

A

Low dispersion: V greater or equal to 45
Medium: V greater than 39, less than 45
High: V less than 39

25
Q

What 2 tests are used to test impact resistance of glasses?

A
  1. Standard test: dropping 5/8 inch steel ball onto lens from height of 50cm
  2. Another common test: 3/4 inch steel ball fired at various speeds to determine breakage speed of lens
26
Q

Name 5 tests used by modern manufacturers to test scratch resistance of lenses

A
  1. Taber test: abrasion wheel applied to surface under a pressure
  2. Bayer test: lens secured to oscillating pan with abrasive material
  3. Steel wool test: pad of steel wool applied to surface at fixed pressure
  4. Tumble test: lenses tumbled in barrel containing abrasive mix
  5. Eraser test: abrasive eraser rubbed on surface at given pressure
27
Q

What is “Mohs hardness scale”

A

A scale from 1-10 to measure a material’s surface hardness

28
Q

Which lens material has noticeably bad scratch resistance (in comparison to others) without coating?

A

polycarbonate

(also very high index plastic, greater than 1.85 index, is a bit worse than others, but still not as bad as polycarbonate)

29
Q

What is the formula used to determine the amount of reflection of a lens (relating to transparency)? What does this formula imply?

A

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

30
Q

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
A

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

31
Q

What 3 benefits does increased transparency provide?

A
  1. reduced ghost images
  2. reduction in the glassy appearance
  3. increased transmittance
32
Q

Why is UV-C filtering not that important for lens materials?

A

Because UV-C (100-290nm) is mainly filtered out by the atmosphere

33
Q

How can we improve the UV-B absorption of CR-39 to maximum?

A

By using a (clear) UV coating

34
Q

Which is better at UV absorption, glass or plastic lens materials?

A

Plastic

35
Q

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?

A

Crown/CR-39
HI index glass
Polycarbonate

(though all are relatively decent, sometimes it matters though in certain professions)

36
Q

Is CR-39 thermoplastic or thermosetting?

A

thermosetting

OD2 Semester 1 - Applied Clinical Training (47 decks)

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