D: Ophthalmic Lens Materials - Week 4

Question 1

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

Answer 1

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

Question 2

Outline the 3 step process used to surface CR-39

Answer 2

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)

Question 3

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)

Answer 3

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

Question 4

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

Answer 4

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

Question 5

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

Answer 5

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.

Question 6

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

Answer 6

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.

Question 7

Explain the process of surfacing a lens

Answer 7

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

Question 8

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?

Answer 8

Ft(nm-1)
Fm = ————-
(nt-1)
** Fm – tool power, Ft – true power
** nm – tool R.I, nt – lens R.I

Question 9

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

Answer 9

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

  = - 10.50 Diopters

Question 10

Compare general plastic vs glass lens materials in the following categories:
- weight
- impact resistance
- ease/cost of tinting
- fogging

Answer 10

• 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)

Question 11

Compare the processes involved in tinting plastic vs glass

Answer 11

plastic: dying in hot chamber (92deg) until required density achieved = simple/inexpensive

glass: thin metal oxide layer deposited in vacuum chamber = expensive

Question 12

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

Answer 12

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)

Question 13

List the Abbe’ values of the following lens materials from lowest to highest:
- crown glass
- CR-39
- Polycarb
- High index plastic
- High index glass

Answer 13

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

Question 14

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

Answer 14

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

Question 15

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

Answer 15

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)

Question 16

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

Answer 16

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)

Question 17

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

Answer 17

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

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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

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

Question 20

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

Answer 20

+/-3D: THI is 30% heavier
10D: THI is same as crown

Question 21

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

Answer 21

+/-3D: Zeiss is 40% heavier
+/-10D: 5% heavier

Question 22

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

Answer 22

TCA =cF/V = p/V

**p – amount of prism
**V – abbe number

Question 23

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

Answer 23

perception: TCA = 0.1 x amount of prism
1 line drop: TCA = 0.16 x amount of prism

Question 24

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

Answer 24

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

Question 25

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

Answer 25

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

Question 26

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

Answer 26

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

Question 27

What is “Mohs hardness scale”

Answer 27

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

Question 28

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

Answer 28

polycarbonate

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

Question 29

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

Answer 29

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

Question 30

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

Answer 30

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)

Question 31

What 3 benefits does increased transparency provide?

Answer 31

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

Question 32

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

Answer 32

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

Question 33

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

Answer 33

By using a (clear) UV coating

Question 34

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

Answer 34

Plastic

Question 35

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?

Answer 35

Crown/CR-39
HI index glass
Polycarbonate

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

Question 36

Is CR-39 thermoplastic or thermosetting?

Answer 36

thermosetting