Properties of Ophthalmic lens materials

FDA Drop ball test for ophthalmic lenses

- 5/8" - 1" steel ball - weighing 0.56 ounces - dropped from 50" - lens must not fracture

general characteristics: polycarbonate

- GREAT impact resistance - higher refractive index and lower specific gravity than glass and CR-39 - BUT more dispersion = low abbe # = more chromatic aberration

general characteristics: CR-39

- better impact resistance than untreated glass - lower specific gravity than glass

non-visible radiation absorption (glass)

- cerium oxide: ultraviolet (UV) light - iron oxide: infrared (IR) light

Other glass materials for ophthalmic lenses

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

mid/high index plastic disadvantages

- more expensive - generally higher dispersion (lower abbe number) - more reflective/less transmission (less light to eye) - may be more "brittle"

Trivex overview

- new lens material introduced by PPG in 2001 - Tri-performance lens material: 1. improved impact resistance 2. high abbe value compared to polycarbonate 3. low specific gravity - only licensed to be produced by select lens producers

plastic

- polymeric material of large molecular weight - synthetic material formed by combining organic ingredients with inorganic materials

Impact resistance

- prior to 1971, no impact resistance standards were required - in 1972, the FDA implemented the "drop ball test" - FDA Regulation 21 CFR 801.410

Basic ingredients for crown glass

- silica (sand) 70% - sodium oxide (soda) 14-16% - calcium oxide (lime) 11-13%

index of refraction (n)

- speed of light in a vacuum compared to its speed in an optical material - standard: based on yellow helium line, wavelength 587.6nm (n_d) = Fraunhofer D line - varies with different wavelengths

Thermosetting process

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

mid/high index plastic advantages

- thinner overall - thinner edges with myopic lenses - may be flatter (in center) with hyperopic lenses - lighter than other plastics *great for higher Rx's (+/-4D or more)

What is a lens?

- transparent material between two polished surfaces - has refractive index different from the surround medium - must be curved to bend light to correct ametropia

Specific gravity

- weight of a given volume of material compared to the weight of the same volume of water - specific gravity of water = 1 - depends on density of the material - high specific gravity = heavier material

trivex drop test: "dress" eye wear standards

0.56 oz ball

CR-39: 1. refractive index 2. Abbe value 3. specific gravity

1. 1.499 2. 58 3. 1.32

Trivex: 1. refractive index 2. abbe value 3. specific gravity 4. minimum central thickness

1. 1.53 2. 45 3. 1.11 4. 1.0mm

polycarbonate: 1. refractive index 2. Abbe value 3. specific gravity

1. 1.586 2. 30 3. 1.20

polycarbonate: 1. refractive index 2. abbe value 3. specific gravity 4. minimum central thickness

1. 1.586 2. 30 3. 1.20 4. 1.5mm

two main groups of plastic materials

1. Thermoplastic (ex. polycarbonate) 2. Thermosetting (ex. CR-39)

Trivex advantages

1. blocks 100% of UV-A and UV-B below 394nm (like polycarbonate) 2. more resistance to chemicals than any other plastic lens material 3. withstands 2.2 pound steel ball dropped 50" 4. increased tensile strength 5. ideal for three piece or drilled-mount frames

Main categories of ophthalmic lenses

1. crown glass (glass) 2. CR-39 (plastic) 3. Polycarbonate (plastic)

disadvantages of crown glass

1. high specific gravity (heavy) 2. relatively low refractive index 3. tendency to fog (can be dangerous, ex. factory workers, police, etc.) 4. tendency to shatter (many tini pieces)

4 important properties of ophthalmic lenses

1. index of refraction (n) 2. dispersion (ω) 3. specific gravity 4. impact resistance

Advantages of crown glass

1. offers the most optically superior and stable lens material 2. lower chromatic aberration 3. least warpage 4. difficult to scratch

Thermosetting materials

1. once hardened, cannot be softened (can't be remolded) 2. stable, rigid, hard, flame resistant, insensitive to heat and chemicals ex) CR-39

Thermoplastic materials

1. soften when heated 2. can be stretched, pressed, or molded without changing chemical structure 3. may be injection molded to from ophthalmic lenses (can be melted down and remolded) 4. BUT less stable and more susceptible to deformation by heat ex) polycarbonate

What were some issues with the original CR-39 lenses?

1. they had a slightly yellow tinge 2. they had a much softer surface than glass lenses so they scratched easily

Normal index range

1.48 - 1.53

What is the range of index of refraction for all ophthalmic lenses?

1.48 - 1.9 (anything outside of this range can be ruled out as incorrect answers math problems)

Mid index range

1.54 - 1.63

High index range

1.64 - 1.73

Very high index range

1.74 - 1.9

tensile strength of trivex

130 to 180 lbs of pulling forces vs 30lbs for plastic

trivex drop test: high impact standards

17.6 oz

Absorptive glasses

2 types: 1. non-visible radiation absorption 2. photochromic

trivex drop test: basic impact standards

2.4 oz ball

Medium dispersion (medium Abbe #)

39 - 44.9

High dispersion (low Abbe #)

<39

how do dispersion and Abbe values relate?

Abbe value is the reciprocal of dispersion, so high dispersion = low Abbe value - We want lower dispersion, thus higher Abbe values are better

History of CR-39 plastic used in ophthalmic lenses

At the end of WWII, PPG had a railroad tank car full of CR-39 monomer sitting in a rail yard without a buyer. PPG had produced CR-39 (Columbia Resin; 39th lab formulation) under government contract for the military as clear transparent sheets used in airplanes, but the war had ended. Dr, Graham, a chemist who had worked with CR-39, had the idea to use the material to cast spectacle lenses. Dr. Graham purchased the tank car full of CR-39 and started a new company (Armorlite) which began casting spectacle lenses. The major turning point for the acceptance of CR-39 as a spectacle lens material occurred with the introduction of impact testing standards for spectacle lenses enforced by the FDA. Years later, Dr. Graham later sold Armorlite to 3M for $60 million.

Is CR-39 a thermosetting or thermoplastic material? Polycarbonate? Trivex?

CR-39 = thermosetting Polycarbonate = thermoplastic Trivex = "quasi-thermosetting/thermoplastic" material; combines polyurethane chemistry with nitrogen to produce a reinforced structure

FDA Regulation 21 CFR 801.410

FDA regulation that governs impact resistance requirements for ophthalmic lenses

polycarbonate

a 1.586 index lens material known for its strength

crown glass

a commonly used glass lens material having an index of refraction of 1.523

photochromatic lens

a lens that changes its transmission characteristics when exposed to light

high-index lens

a lens with an index of refraction that is at the upper end of the range of available indices of refraction for lenses, yielding a lens that is thinner than other lenses of the same size and power

CR-39

a registered trademark of Pittsburg Plate Glass Co. for an optical plastic known as Columbia Resin 39 - it has been the standard material from which conventional plastic lenses are made

Thermoelastic

a term used to describe a material that will bend when heated and will return to its original shape when reheated

thermoplastic

a term used to describe 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"

drop-ball test

a test to determine impact resistance of ophthalmic lenses whereby either a 5/8th inch or 1 inch steel ball is dropped onto the front surface of a lens from a height of 50 inches

antireflection coating

a thin layer or series of layers of material applied to the surface of a lens for the purpose of reducing unwanted reflections from the lens surface and thus increasing the amount of light that passes through the eye

antiscratch coating (aka scratch resistant coating)

a thin, hard coating applied to plastic lens surfaces to make them more resistance to scratching

Crown glass a) refractive index b) Abbe # c) specific gravity

a) n=1.523 b) 59 c) 2.54

Glass

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

monochromatic aberration

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

lateral chromatic aberration (chromatic power, or transverse chromatic aberration)

an aberration that produces images of slightly different sizes at the focal length of the lens, depending upon the color of the light - ex) lens at screen focal point will have aberration up and down the screen

Chromatic aberration: which color light focuses first?

blue light focuses first, then green, then red **this is alphabetical in order

Meril lenses

glass lenses

Ophthalmic crown glass

has trace amount of potassium, borax, antimony, and arsenic

how do dispersion values and chromatic aberration values relate?

high dispersion = high chromatic aberration

What types of chemical is trviex resistant to?

household chemicals such as bleach, salt water, windex, sun tan lotion, and nail polish remover - also acetone and rubbing alcohol

should ophthalmic lenses have high or low dispersion?

low dispersion because we want as little chromatic aberration as possible

index of refraction equation

n = C of vacuum/V of medium

measuring index of refraction for red light

n_c = 656nm is used

measuring index of refraction with blue light

n_f = 486nm is used

is polycarbonate resistant to acetone and rubbing alcohol?

no! but trivex is

longitudinal aberration (axial chromatic aberration)

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. Each of these images is a different color, and has a slightly different focal length - each color of light will have its own focal point

organic material lenses

plastic lenses

are glass or plastic lenses less subject to fogging? why?

plastic, b/c less thermal conductivity than glass

Abbe value equation

reciprocal of dispersion V = 1/ω ω is dispersion V is the Abbe value

photochromic (glass)

silver halide crystals degrade in the presence of UV light exposure, changing the chemical makeup of the material, so the lenses change color -> once UV light exposure is removed, the silver halide crystals reform -> original chemical makeup is restored -> original color of lens is restored

trivex

the brand name for a PPG Industries plastic lens material known for its high impact resistance and ability to be processed in a manner similar to that of other plastic lenses

crown glass index and weight

the higher the index of the glass, the more lead in the glass; therefore, the heavier the glass

Doctor driven dispensing

the idea that the doctor should talk to the patient about their recommendations for lens material/coatings prior to sending them to the optical - this helps the patient understand the doctors recommendations - doesn't seem like optician is upselling them

Abbe' Value

the most commonly used number for identifying the amount of chromatic aberration for a given lens material - the higher the abbe' value, the less chromatic aberration present in the lens - Abbe' value is the reciprocal of ω (dispersive power) and is symbolized by the Greek letter nu or v

heat treating (air hardening, heat hardening, or heat tempering)

the process of hardening a glass lens by first heating it in a kiln, then quickly cooling by blowing forced air against both front and back surfaces

Chemical tempering (chemtempering, or chem hardening)

the process of increasing the impact resistance of glass lenses by immersing them in a bath of molten salt

refractive index

the ratio of the speed of light in a medium (such as air) to the speed of light in another medium (such as glass)

absolute refractive index

the ration of the speed of light in a vacuum to the speed in another medium

aberration

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

Dispersion (dispersive power)

the spreading of white light into all its colors, each with a different focal point; chromatic aberration of an optical material

chromatic aberration

the type of aberration that causes light of different wavelengths (colors) to be refracted differently through the same optical system

"corrected" chromatic aberration

theoretical "perfect" lens with no chromatic aberration -> doesn't exist

why do plastic lenses need scratch resistant coatings?

they are typically very soft materials that scratch too easily

How can we add bifocal to glass lens?

two ways: 1. increase the index of the bifocal material 2. increase the curvature of the bifocal material

Axial chromatic aberration formula

units: Diopters

Transverse/Lateral Chromatic aberration formula

units: prism diopters

Why is plastic now more common that glass lenses?

up through the 1960's glass greatly outsold plastic, but with the introduction of impact resistance requirements by the FDA, lenses shifted more towards plastic So what changed: 1. FDA impact resistance 2. fashion changes (bigger lenses) 3. fashion tints became more popular 4. improved plastic materials

prior to plastic lenses, when would we have used a high index glass?

when increasing the index (and weight) would balance with the weight of the lower index, but thicker lens for high power - generally when Rx: ±7.00 D or higher

What is the standard wavelength for measuring index of refraction?

yellow helium line, wavelength 587.6nm (n_d) - aka Fraunhofer D line

Fraunhofer D line

yellow helium line, wavelength 587.6nm (n_d) used as the standard for measuring index of refraction

Dispersion equation

ω = dispersion n_f = index of refraction for blue light n_c = index of refraction for red light n_d = index of refraction for yellow light

prism formula

∆ = cF F is lens power c is decentration (in cm)

Low dispersion (high Abbe #)

≥45 *this is what we want in ophthalmic lenses