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Ophthalmics > Properties of Lens Materials > Flashcards

Properties of Lens Materials Flashcards

1
Q

What is chromatic aberration?

A

Type of aberration that causes light of different wavelengths (colors) to be refracted differently through optical system.

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2
Q

The ___________ is the most commonly used number for identifying the amount of chromatic aberration for a given lens material.

A

Abbe Value

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3
Q

When the Abbe value of a material is high, the chromatic aberration present is ______.

A

Low

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4
Q

When the Abbe value of a material is low, the chromatic aberration present is ______.

A

High

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5
Q

Abbe’ value is the __________ of ω (dispersive power) and is symbolized by the Greek letter nu or ν.

A

reciprocal

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6
Q

Abbe’ Value equation

A

Abbe value (nu or v) = 1 /dispersive power (w)

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7
Q

What is an abberation? When does it occur?

A

The degradation of an image.

Occurs when a point source of light does not result in a single-point image after going through the lens or lens system.

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8
Q

What is an antireflection coating? What is its purpose?

A

A thin layer or series of layers of material is applied to the surface of a lens to reduce unwanted reflections from the lens surface and thus increasing the amount of light that passes through to the eye.

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9
Q

Antiscratch Coating (AKA Scratch Resistant Coating)

A

A thin, hard coating is applied to plastic lens surfaces to make them more resistant to scratching.

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10
Q

What are the two types of chromatic aberration?

A

1) Transverse / Lateral chromatic aberration

2) Longitudinal / Axial Chromatic Aberration

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11
Q

What is Lateral/ Transverse Chromatic Aberration? How do you find it? What are the units?

A

An aberration that produces images of slightly different sizes at the focal length of the lens, depending upon the color of the light.

TCA= power of lens x cm off center / Abbe number
Units are in prism DIOPTERS

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12
Q

What is Longitudinal / Axial Chromatic Aberration? How do you find it? What are the units? What lenses is this more of a problem with?

A

Occurs when a point light source that is composed of several wavelengths (such as white light) forms a series of point images along the optical axis by looking laterally. Each of these images is a different color, and has a slightly different focal length. Red closest to center, blue furthest away.

  • ACA = Power of lens / Abbe number
  • Units in DIOPTERS
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13
Q

Crown Glass Characteristics (n, specific gravity, Abbe number, materials, traits)

A
  • n=1.523
  • Specific gravity= 2.54
  • Abbe = 59
  • Main ingredient is silica
  • Very stable
  • Best optics (optically superior)
  • low chromatic aberration
  • least warpage
  • scratch resistant
  • heavy
  • low refractive index
  • fog easily
  • shatters easily
  • test individual lens for impact resistance
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14
Q

CR-39 (plastic) Characteristics (n, specific gravity, production, Abbe number, traits)

A
  • n= 1.5
  • specific gravity = 1.32
  • Abbe = 58
  • Columbia Resin 39
  • made by thermosetting
  • better impact resistance than glass
  • lighter than glass
  • scratches easier than glass
  • shatters less than glass
  • tested by impact resistance in batches
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15
Q

What does chemical tampering do? How is it done?

(AKA Chemtempering, Chem Hardening)

A

Ion-exchange method
The process of increasing the impact resistance of glass lenses by immersing them in a bath of molten salt. Exchanges the smaller ions in the glass (K+) with larger ions in the salt bath (K+). Then it is compressed with a temperature much less than thermal tempering. This results in less warpage.
-Resurfacing and re-edging are possible
-takes a lot longer than heat tempering

-MORE IMPACT RESULTS THAN HEAT TEMPERING

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16
Q

What is does heat treating do? How is it done?

AKA Air Hardening, Heat Hardening, Heat Tempering

A

The process of hardening a glass lens.
First heat it in a kiln, then quickly cool by blowing forced air against both front and back surfaces.
-no resurfacing or re-edging
-much faster than chemical

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17
Q

What is drop-ball test? What is used? Who developed this law? What is the regulation called?

A
  • A test to determine the impact resistance of ophthalmic lenses.
  • Either a 5/8th inch or 1-inch steel ball (0.56 ounces) is dropped onto the front surface of a lens from a height of 50 inches.
  • Developed by the FDA
  • FDA regulation 21 CFR 801.410
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18
Q

Mid/High Index plastic characteristics (pros/cons)

A

Pros:

  • considered for high prescriptions
  • thinner overall
  • flatter curvature
  • higher index of refraction
  • lighter weight than other plastics

Cons:

  • expensive
  • high distortion
  • more reflection
  • more brittle
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19
Q

___________ is an aberration that is present even when light is made up of only one wavelength (one color).

A

Monochromatic aberration

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20
Q

A _________ lens changes its transmission characteristics when exposed to light.

A

photochromic

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21
Q

Polycarbonate Characteristics , Pros/Cons (n, specific gravity, production, Abbe number, traits)

A
  • n = 1.586
  • specific gravity = 1.20
  • Abbe = 30

Pros:

  • very impact resistant (known for its strength)
  • made from thermoplastic
  • high refractive index
  • low weight
  • less fogging

Cons:

  • more dispersion than glass/CR-39
  • scratches easily (soft)
  • not resistant to chemicals like rubbing alcohol and acetone
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21
Q

What kind of glasses material would you give a child?

A

polycarbonate

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22
Q

Thermosetting materials and characteristics. What lens uses thermosetting materials?

A

-once hardened cannot be softened
-supplied as liquid monomers
-other ingredients added (dyes, etc.)
-The mixture is heated to produce polymerization, then cured
stable, rigid, hard, flame resistant, insensitive to heat and chemicals

CR-39 is thermosetting material

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23
Q

Thermoplastic materials. What kind of lens uses thermoplastic materials?

A

-A material that will bend when heated, but does not return to its original shape when reheated because it does not have a “plastic memory”.
-soften when heated
-can be stretched, pressed, or molded without changing chemical structure
may be injection molded to form ophthalmic lenses
- less stable and more susceptible to deformation by heat

Polycarbonate

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24
Q

Trivex Characteristics (n, specific gravity, Abbe number, materials, traits)

A
  • n = 1.53
  • specific gravity= 1.11
  • Abbe = 44
  • Good impact resistance (more than poly)
  • less dispersion than polycarbonate
  • light (low specific gravity)
  • Blocks UVA and UVB
  • most resistant to chemicals
  • increased tensile strength
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25
Q

3 types of ophthalmic materials

A

1) Crown Glass (glass)
2) CR-39 (plastic)
3) Polycarbonate (plastic)

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26
Q

4 important properties for lenses

A

1) Index of Refraction (n)
2) Dispersion (ω)
3) Specific Gravity
4) Impact Resistance

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27
Q

If a material has normal index, what is its range of index of refraction?

A

Normal Index n = 1.48 - 1.53

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28
Q

If a material has mid index, what is its range of index of refraction?

A

Mid Index n = 1.54 - 1.63

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29
Q

If a material has high index, what is its range of index of refraction?

A

High Index n = 1.64 to 1.73

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30
Q

If a material has a very high index, what is its range of index of refraction?

A

greater than 1.74

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31
Q

What is dispersion? What symbol?

A
  • It is the chromatic aberration of an optical material.
  • Symbol used is omega (w)
  • High dispersion = high chromatic aberration
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32
Q

Dispersion equation

A

ω = (nf - nc ) ∕ (nd - 1)

nf = blue
nc= red
nd= yellow
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33
Q

Reciprocal of dispersion is the __________ (AKA constringence)

A

Abbe Number

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34
Q

What are the Abbe number classifications? What are the values for each?

A

Low Dispersion = greater than or equal to 45
Medium Dispersion = 39 to 44
High Dispersion = Less than 39

35
Q

What is a corrected chromatic aberration?

A

theoretical perfect lens with no chromatic aberration

36
Q

What happens in axial/ longitudinal chromatic aberration?

A

Some light waves converge on your retina, some behind, and some in front of retina. (optical axis)

37
Q

How do you find the power of prism in a lens?

A

multiply the power of the lens by the displacement(cm) from the center

38
Q

What is specific gravity? What is the value for water? What does a higher specific gravity mean? What does it depend on?

A
  • weight of a given volume of material compared to the weight of the same volume of water.
  • 1 for water
  • higher specific gravity = heavier material
  • depends on density of the material
39
Q

Who regulates ophthalmics?

A

FDA and Code of Federal Regulations (CFR)

40
Q

What is the code for ophthalmic regulation?

A

FDA Regulation 21 CFR 801.410

Title 21; Chapter I; Subchapter H; Part 801; Subpart H; Section 410

41
Q

When did impact testing get involved?

A

1971.

42
Q

Describe glass

A

amorphous, supercooled liquid with high viscosity and no discrete melting point

43
Q

1 ingredient for crown glass

A

Silica (sand) 70%

44
Q

What is found in trace amounts in crown glass?

A

-trace amounts of potassium, borax, antimony, and arsenic

45
Q

Other glass materials found in crown glass

A

flint: lead oxide
barium crown: barium oxide
high-index: titanium oxide

46
Q

Describe absorptive glasses and what they contain. 2 types

A

Non-visible radiation absorption:

  • cerium oxide: ultraviolet (UV)
  • iron oxide: infrared (IR)

Photochromic:
-silver halide crystals

47
Q

In general, _________ offers the most optically superior and stable lens material

A

Crown glass

48
Q

The higher the index of the glass, the more ____ in the glass; therefore, the heavier the glass.

A

lead

49
Q

Before plastic lenses, high index was generally used when prescriptions were _____D or higher.

A

-7.00

50
Q

Crown glass refractive index, Abbe number, and specific gravity

A

Index: 1.523
Abbe: 59
Specific Gravity: 2.54

50
Q

Polycarbonate refractive index, Abbe number, and specific gravity

A

Index: 1.586
Abbe: 30
Specific Gravity: 1.2

51
Q

Describe the different parts of the lens with a multifocal

A

Crown glass = carrier of multifocal
Light barium = intermediate segment of trifocal
Dense barium = near segment of bifocal

52
Q
  • polymeric material of large molecular weight

- synthetic material formed by combining organic ingredients with inorganic materials

A

Plastic

53
Q

How thermoplastic materials are made

A
  • soften when heated
  • can be stretched, pressed, or molded without changing chemical structure
  • may be injection molded to form ophthalmic lenses
  • less stable and more susceptible to deformation by heat
54
Q

Thermosetting process

A

Liquid monomer is mixed with a catalyst, placed in molds & heated

  • supplied as liquid monomers
  • other ingredients added (dyes, etc.)
  • mixture is heated to produce polymerization, then cured
55
Q

What are some problems with CR-39 original formulation?

A
  • Aged “poorly” with a slightly yellow tinge

- Much softer surface (so it scratched easier)

56
Q

What changed that led to glass being sold more than plastic in the 1960’s?

A
  • FDA requirements for impact resistance
  • Fashion changes (bigger lenses)
  • Fashion tints became more popular
  • Improved plastic materials
57
Q

CR-39 has a ____ impact resistance than untreated glass and a _______ specific gravity than glass.

A

Greater; lower

58
Q

Polycarbonate has _____ impact resistance.

A

great

59
Q

Compared to CR-39, polycarbonate has a _______ refractive index and a _____ specific gravity, BUT _______ dispersion.

A

Higher; lower; higher

60
Q

Why is plastic less subjective to fogging?

A

Provide less thermal conductivity than glass, therefore less subject to fogging with changes in temperature and humidity

61
Q

Mid/High Index Plastic
Disadvantages

A
  • More expensive!!
  • Generally higher dispersion (i.e. lower Abbe number)
  • More reflective
  • May be more “brittle”
62
Q

Trivex characteristics

A
  • Improved impact resistance
  • High Abbe value
  • Low specific gravity
63
Q

Type of lens only licensed to be produced by select lens producers

A

Trivex

64
Q

Is trivex thermosetting or thermoplastic?

A

Kind of both

65
Q

What do manufacturers combine to make trivex?

A

Combines polyurethane chemistry with nitrogen to produce a reinforced structure

66
Q

True or False:

Trivex blocks 100% of UV-A and UV-B below 394 nm (like polycarbonate).

A

True

67
Q

True or false:

Trivex is more resistant to chemicals than any other plastic

A

True.

  • Resistant to household chemicals such as bleach, salt water, WINDEX, sun tan lotion and nail polish remover
  • Resistant to acetone and rubbing alcohol (unlike polycarbonate)
68
Q

Drop ball weights for different categories of lenses

A

Dress Eyewear = 0.56 ounce
Basic Impact = 2.4 ounces
High Impact = 17.6 ounce

69
Q

Trivex can withstand a ___ pound steel ball drop at 50 inches.

A

2.2 pound

70
Q

Tensile strength of trivex compared to plastic

A 
Trivex = 130 - 180 pounds of pulling force
Plastic = 30 pounds of pulling force
71
Q

What kind of frames are ideal for trivex?

A

three piece or drilled-mount frames

72
Q

PPG Trivex refractive index, abbe number, and specific gravity

A

Index = 1,527
Abbe number = 44
specific gravity = 1.11

73
Q

Assumption: In order for a patient to have problems with chromatic aberration, the amount must usually reach _____.

A

0.25 prism diopters

74
Q

By optimizing the lens material with aspheric design and AR coating, problems with _________ and the Abbe number become minimal issues

A

polycarbonate

75
Q

For most glasses and some plastics, as refractive index increases, _____ number decreases
and _____________ increases.

What is the exception?

A

Abbe ; specific gravity

Polycarbonate

76
Q

What is the strength of glass? How can it be improved?

A 
-Brittle material
Impact resistance (strength) improved by heat and chemical tempering
77
Q

Process of thermal tempering of glass

A
  • Finished lens is heated to 650°C, then rapidly cooled
  • Surface quickly becomes rigid with cooling
  • Surface is under compression, with resulting compression of internal lens
  • Produces birefringence, or double refraction, of the lens
  • Maltese cross” pattern
  • Observable with a polariscope
  • Not “hardened” glass
  • Increases impact resistance
78
Q

Process of Chemical Tempering of Glass Lenses

A
  • Ion-exchange method
  • Exchange small ions in the glass (Na+) with larger ions in the salt bath (K+)
  • Compression of the surface layers
  • Temperatures much less than thermal tempering (400°C- 470°C)
  • Lower temperature means less lens warpage and fewer irregularities
79
Q

Thermal vs. Chemical Tempering

A

Thermal:

  • No resurfacing or re-edging
  • Not as impact resistant
  • Much faster, takes only minutes

Chemical:
Resurfacing and re-edging are possible (must be chemically tempered again)
2-3X More impact resistant
Originally took 16 hours to complete

80
Q

__________________ glass is superior for large mass, low velocity objects

A

Chemically tempered

81
Q

_______ is superior for small, high-velocity, sharply pointed objects

A

CR-39

82
Q

What is lens of choice for eye protection? Why?

A

Polycarbonate

  • Representative of the most impact resistant lens material category
  • Material is able to flex and deform
  • Protection for fast-moving large or small missiles
83
Q

Why choose plastic over glass for safety?

A
  • When CR-39 fractures, it breaks into fewer pieces with less-sharp edges
  • Surface scratching and pitting does not reduce the strength of CR-39
  • Minute surface and edge imperfections in glass lenses from grinding, edging and handling reduce impact resistance (Griffith Flaws)

Ophthalmics (7 decks)

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