Properties of Lens Materials Flashcards
What is chromatic aberration?
Type of aberration that causes light of different wavelengths (colors) to be refracted differently through optical system.
The ___________ is the most commonly used number for identifying the amount of chromatic aberration for a given lens material.
Abbe Value
When the Abbe value of a material is high, the chromatic aberration present is ______.
Low
When the Abbe value of a material is low, the chromatic aberration present is ______.
High
Abbe’ value is the __________ of ω (dispersive power) and is symbolized by the Greek letter nu or ν.
reciprocal
Abbe’ Value equation
Abbe value (nu or v) = 1 /dispersive power (w)
What is an abberation? When does it occur?
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.
What is an antireflection coating? What is its purpose?
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.
Antiscratch Coating (AKA Scratch Resistant Coating)
A thin, hard coating is applied to plastic lens surfaces to make them more resistant to scratching.
What are the two types of chromatic aberration?
1) Transverse / Lateral chromatic aberration
2) Longitudinal / Axial Chromatic Aberration
What is Lateral/ Transverse Chromatic Aberration? How do you find it? What are the units?
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
What is Longitudinal / Axial Chromatic Aberration? How do you find it? What are the units? What lenses is this more of a problem with?
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
Crown Glass Characteristics (n, specific gravity, Abbe number, materials, traits)
- 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
CR-39 (plastic) Characteristics (n, specific gravity, production, Abbe number, traits)
- 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
What does chemical tampering do? How is it done?
(AKA Chemtempering, Chem Hardening)
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
What is does heat treating do? How is it done?
AKA Air Hardening, Heat Hardening, Heat Tempering
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
What is drop-ball test? What is used? Who developed this law? What is the regulation called?
- 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
Mid/High Index plastic characteristics (pros/cons)
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
___________ is an aberration that is present even when light is made up of only one wavelength (one color).
Monochromatic aberration
A _________ lens changes its transmission characteristics when exposed to light.
photochromic
Polycarbonate Characteristics , Pros/Cons (n, specific gravity, production, Abbe number, traits)
- 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
What kind of glasses material would you give a child?
polycarbonate
Thermosetting materials and characteristics. What lens uses thermosetting materials?
-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
Thermoplastic materials. What kind of lens uses thermoplastic materials?
-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
Trivex Characteristics (n, specific gravity, Abbe number, materials, traits)
- 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
3 types of ophthalmic materials
1) Crown Glass (glass)
2) CR-39 (plastic)
3) Polycarbonate (plastic)
4 important properties for lenses
1) Index of Refraction (n)
2) Dispersion (ω)
3) Specific Gravity
4) Impact Resistance
If a material has normal index, what is its range of index of refraction?
Normal Index n = 1.48 - 1.53
If a material has mid index, what is its range of index of refraction?
Mid Index n = 1.54 - 1.63
If a material has high index, what is its range of index of refraction?
High Index n = 1.64 to 1.73
If a material has a very high index, what is its range of index of refraction?
greater than 1.74
What is dispersion? What symbol?
- It is the chromatic aberration of an optical material.
- Symbol used is omega (w)
- High dispersion = high chromatic aberration
Dispersion equation
ω = (nf - nc ) ∕ (nd - 1)
nf = blue nc= red nd= yellow
Reciprocal of dispersion is the __________ (AKA constringence)
Abbe Number
What are the Abbe number classifications? What are the values for each?
Low Dispersion = greater than or equal to 45
Medium Dispersion = 39 to 44
High Dispersion = Less than 39
What is a corrected chromatic aberration?
theoretical perfect lens with no chromatic aberration
What happens in axial/ longitudinal chromatic aberration?
Some light waves converge on your retina, some behind, and some in front of retina. (optical axis)
How do you find the power of prism in a lens?
multiply the power of the lens by the displacement(cm) from the center
What is specific gravity? What is the value for water? What does a higher specific gravity mean? What does it depend on?
- 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
Who regulates ophthalmics?
FDA and Code of Federal Regulations (CFR)
What is the code for ophthalmic regulation?
FDA Regulation 21 CFR 801.410
Title 21; Chapter I; Subchapter H; Part 801; Subpart H; Section 410
When did impact testing get involved?
1971.
Describe glass
amorphous, supercooled liquid with high viscosity and no discrete melting point
1 ingredient for crown glass
Silica (sand) 70%
What is found in trace amounts in crown glass?
-trace amounts of potassium, borax, antimony, and arsenic
Other glass materials found in crown glass
flint: lead oxide
barium crown: barium oxide
high-index: titanium oxide
Describe absorptive glasses and what they contain. 2 types
Non-visible radiation absorption:
- cerium oxide: ultraviolet (UV)
- iron oxide: infrared (IR)
Photochromic:
-silver halide crystals
In general, _________ offers the most optically superior and stable lens material
Crown glass
The higher the index of the glass, the more ____ in the glass; therefore, the heavier the glass.
lead
Before plastic lenses, high index was generally used when prescriptions were _____D or higher.
-7.00
Crown glass refractive index, Abbe number, and specific gravity
Index: 1.523
Abbe: 59
Specific Gravity: 2.54
Polycarbonate refractive index, Abbe number, and specific gravity
Index: 1.586
Abbe: 30
Specific Gravity: 1.2
Describe the different parts of the lens with a multifocal
Crown glass = carrier of multifocal
Light barium = intermediate segment of trifocal
Dense barium = near segment of bifocal
- polymeric material of large molecular weight
- synthetic material formed by combining organic ingredients with inorganic materials
Plastic
How thermoplastic materials are made
- 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
Thermosetting process
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
What are some problems with CR-39 original formulation?
- Aged “poorly” with a slightly yellow tinge
- Much softer surface (so it scratched easier)
What changed that led to glass being sold more than plastic in the 1960’s?
- FDA requirements for impact resistance
- Fashion changes (bigger lenses)
- Fashion tints became more popular
- Improved plastic materials
CR-39 has a ____ impact resistance than untreated glass and a _______ specific gravity than glass.
Greater; lower
Polycarbonate has _____ impact resistance.
great
Compared to CR-39, polycarbonate has a _______ refractive index and a _____ specific gravity, BUT _______ dispersion.
Higher; lower; higher
Why is plastic less subjective to fogging?
Provide less thermal conductivity than glass, therefore less subject to fogging with changes in temperature and humidity
Mid/High Index Plastic Disadvantages
- More expensive!!
- Generally higher dispersion (i.e. lower Abbe number)
- More reflective
- May be more “brittle”
Trivex characteristics
- Improved impact resistance
- High Abbe value
- Low specific gravity
Type of lens only licensed to be produced by select lens producers
Trivex
Is trivex thermosetting or thermoplastic?
Kind of both
What do manufacturers combine to make trivex?
Combines polyurethane chemistry with nitrogen to produce a reinforced structure
True or False:
Trivex blocks 100% of UV-A and UV-B below 394 nm (like polycarbonate).
True
True or false:
Trivex is more resistant to chemicals than any other plastic
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)
Drop ball weights for different categories of lenses
Dress Eyewear = 0.56 ounce
Basic Impact = 2.4 ounces
High Impact = 17.6 ounce
Trivex can withstand a ___ pound steel ball drop at 50 inches.
2.2 pound
Tensile strength of trivex compared to plastic
Trivex = 130 - 180 pounds of pulling force Plastic = 30 pounds of pulling force
What kind of frames are ideal for trivex?
three piece or drilled-mount frames
PPG Trivex refractive index, abbe number, and specific gravity
Index = 1,527
Abbe number = 44
specific gravity = 1.11
Assumption: In order for a patient to have problems with chromatic aberration, the amount must usually reach _____.
0.25 prism diopters
By optimizing the lens material with aspheric design and AR coating, problems with _________ and the Abbe number become minimal issues
polycarbonate
For most glasses and some plastics, as refractive index increases, _____ number decreases
and _____________ increases.
What is the exception?
Abbe ; specific gravity
Polycarbonate
What is the strength of glass? How can it be improved?
-Brittle material Impact resistance (strength) improved by heat and chemical tempering
Process of thermal tempering of glass
- 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
Process of Chemical Tempering of Glass Lenses
- 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
Thermal vs. Chemical Tempering
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
__________________ glass is superior for large mass, low velocity objects
Chemically tempered
_______ is superior for small, high-velocity, sharply pointed objects
CR-39
What is lens of choice for eye protection? Why?
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
Why choose plastic over glass for safety?
- 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)
