Lens Materials

crown glass

• advantages:

  • scratch resistant

  • not easily affected by environmental factors

  • low in chromatic aberrations

• disadvantages:

  • heavy

  • not impact resistant

    • must be hardened to pass US requirements for impact resistance

F

T/F: glass does not work well for ophthalmic materials b/c it is easily affected by environmental factors

T

T/F: high index glass lenses are composed of materials w/ a high specific gravity, thus, making them heavier

PPG

CR-39 plastic lenses were developed by __________ industries

T

T/F: high index plastics are made from a variety of choices

F

T/F: a scratched lens surface does not affect the impact resistance of the overall lens

T

T/F: a retailer cannot supply a nonimpact resistant lens if a patient requests it or if the patient agrees to assume all responsibility

front, 5/8, steel, 50

the standard referee test for determining impact resistance suitability for dress ophthalmic lenses is the drop ball test, a lens is first placed ______ side up on a neoprene gasket, it must be capable of withstanding the impact of a ________ steel ball weighing 0.56oz dropped from a height of _____ inches

polycarbonate

when safety is the primary concern, what lens material has traditionally been the number 1 choice?

trivex

_____ material was originally for military use as a plastic material to provide good optics & excellent safety characteristics for windows in combat vehicles

T

T/F: clear glass lenses were sometimes laminated w/ a thin layer of tinted glass to give an even tint across the lens

decrease

the impact resistance of CR-39 plastic lenses _________ w/ the addition of scratch resistance coating

F (must be hardened but do not need to be tested)

T/F: glass lenses that could be damaged by the drop ball test do not need to be hardened nor tested

T

T/F: batch testing is permitted for plastic lenses

manufacturer

the FDA defines the __________ as the person who puts the lens in the form ready for its intended use or who alters the physical or chemical characteristics of the lens by such acts as grinding, heat treating, beveling, or cutting

T

T/F: FTC eyeglasses 2 rules that the prescribers are at liberty to include whatever they consider important for the patient’s visual welfare on the prescription, this could include lens material, specific lens styles, & instructions for wear

hardened

glass lenses are not strong enough to pass the FDA mandated impact test unless they are _______

1. clean the lenses

2. place the lenses in the lens holder

3. place the lens holder above the bath to allow the lens to preheat

4. immerse the lenses in the molten salt bath for 16hrs

5. place the lens holder above the bath for post bath cooling

6. once cooled at room temp, rinse the lenses in hot water to remove the salt

list the steps of the chemical tempering process

heat treating

the advantage to _________(heat treating/chemical tempering) is that it is fast

F

T/F: the disadvantage of chemically-tempering lenses is that it is not as impact resistant as a lens that is heat tempered

T

T/F: when checking whether a glass lens has been hardened, a perfectly shaped Maltese cross pattern does not mean that the lens is any more impact resistant than a lens showing a misshapen Maltese cross

1. place the edged glass lens into a small kiln where the temp is high enough to almost bring the glass to the softening point

2. the lens is left in the kiln for 2-3min

3. remove the lens from the kiln

4. cool the lens rapidly by blowing forced air against both the front & back surfaces

list the steps of the heat-treating process

1. heat treating

2. chemically tempering

what are the 2 ways to increase the impact resistance of a glass lens?

1. remove lens from frame

2. immerse lens in solution of glycerin

3. view b/t 2 crossed polaroids

4. if tempered, will show halo-like, bright band around edge of lens

how do you tell if a lens has been chemically tempered if there is no notification enclosed from the lab?

1.523

what is the index of refraction of crown glass?

high index glass

• advantages:

  • reduced lens thickness for higher powered prescriptions

  • readily available in spherical & aspheric designs & segmented MFs & PALs

• disadvantages:

  • heavy

  • a prescription must be pretty high to meet thickness requirements for impact resistance

• similar Abbe value to polycarbonate

Abbe value

what are chromatic aberrations measured in terms of?

higher

the lower the Abbe value, the ______ the chromatic aberration

chromatic aberration

_______________ can result in color fringes being visible at high contrast borders

CR-39

• advantages:

  • scratch resistant (with scratch resistant coatings)

  • low weight

  • do not fog as easily as glass

  • welding or grinding spatter does not adhere

• disadvantages:

  • segmented MF & PALs that have to be surfaced are less likely to have an anti-scratch coating

  • lowest refractive index of materials used for spectacles

chemically tempered glass lenses

what performs better for low velocity, large mass objects? (chemically tempered glass or CR-39)

CR-39

what performs better for smaller, high velocity, sharply pointed objects? (chemically tempered glass or CR-39)

1.498

what is the index of refraction for CR-39?

high-index plastics

• advantages:

  • can be made lighter weight & with reduced thickness

  • cosmetics are better

polycarbonate

• advantages:

  • high impact resistance (#1 choice for safety)

• disadvantages:

  • must be made with an anti-scratch coating

trivex

• advantages:

  • very impact resistant

  • easy processing

  • tints easily

  • ultra-light weight

  • great optics

  • lens of choice for drill-mounted lenses

  • solvent resistant

  • low chromatic aberration

NXT material

• being used in sun & sport eyewear, helmet visors, airline cockpit view ports, ballistic police shields, vehicle door armor

• advantages:

  • lightweight

  • extremely strong

  • compatible w/ photochromic pigments & polarization

  • highly flexible

  • compatible w/ low-powered sphere & cyl prescriptions

  • ideal for rimless or half-rim frames

  • low chromatic aberration

  • solvent resistant

laminated lenses

made from 2 or more layers of material, can be used to even a tint, cut reflected glare, or increase impact resistance

crown glass

what lens material has the highest Abbe value?

polycarbonate

what lens material has the lowest Abbe value?

dress eyewear

eyewear designed for everyday use

safety eyewear

eyewear designed to meet higher standards of impact resistance since it will be worn in situations that could be potentially hazardous to the eyes

sports eyewear

eyewear that is designed to protect the eyes &/or enhance vision in specific sports situations

no

is there a minimum thickness requirement for the various lens materials?

performance

impact resistance requirements are _______ based

referee test (drop ball test)

lens must withstand an impact of 5/8ths inch steel ball weighing 0.56oz dropped on the front of the lens from 50in

yes

can plastic lenses be tested for impact resistance in “uncut-finished” stage before edged?

after, before

glass lenses are impact resistance tested _____ the lens is edged & hardened & _____ being placed in a frame

yes

are plastic lenses exempt from impact testing?

1. raised MF lenses (executive lenses)

2. prism segment MFs

3. slab-off lenses

4. lenticular cataract lenses

5. iseikonic lenses

6. depressed-segment one-piece MFs

7. biconcave, myodisc, & minus lenticular lenses

8. custom laminate lenses (polarizing lenses)

9. cement assembly lenses

what glass lenses are exempt from impact testing?

when an optometrist determines that impact-resistant lenses will not fulfill the visual requirements of a particular patient

when can retailers dispense lenses that are not impact-resistant?

3

FDA requires that records be kept for ___ yrs after the purchase of eyeglasses

no

do records have to be kept for individuals buying non-prescription eyewear?

2.0mm

heat tempered lenses must be at least ______ thick

stable

in regular sunglasses, the tint in a glass lens is _______ over time

as long as the lens lasts

how long does the scratch resistance last on a glass lens?

yes

is glass suited for AR coating?

no

is untreated crown glass used for ophthalmic eyewear in the US?

more

for small, high velocity objects, a badly scratched untreated glass lens is ______ impact resistant than a badly scratched, heat treated lens

decrease

coatings _____ the impact resistance of a plastic lens

45%

what is the average loss of strength caused by a hard coating?

20%

what is the average loss of strength caused by an AR coating?

1. surfaced lens

2. hard coating

3. AR coating

4. rimless edging

5. thick coating

what things weaken a lens?

polycarbonate, trivex

what lens materials are the best in terms of impact resistance?

FTC eyeglass 1

• prescriptions must be given immediately after the eye exam is complete, whether or not the pt asks

• must include sphere, cyl, axis, prism, & signature

FTC eyeglass 2

• prescribers are at liberty to include whatever they consider important for the patient’s visual welfare on the prescription

• can include: lens material, lens styles, instructions for wear

CPF550, coated polycarbonate, uncoated CR-39, crown glass

rank the following lenses in order from most to least short wavelength & UV radiation absorption:

coated polycarbonate, crown glass, CPF550, uncoated CR-39

T

T/F: a -2.50D high-index glass lens is heavier than a crown glass lens of equal thickness & power

F

T/F: a high index plastic lens is heavier than a lower index crown glass lens

to create stress within the lens & increase impact resistance

the purpose of blowing air against both sides of a glass lens after heating the lens in a thermal-hardening unit is:

polycarbonate, CR-39, chemically hardened crown glass, heat-tempered crown glass, untreated crown glass

arrange these new, unscratched lenses in order from the most to the least impact resistant:

CR-39, untreated crown glass, chemically hardened crown glass, heat-tempered crown glass, polycarbonate

CR-39, crown glass, trivex, polycarbonate, corning clears 16

arrange these materials in order from the material w/ lowest index of refraction to the material w/ the highest index of refraction:

crown glass, polycarbonate, CR-39, corning clears 16, trivex

trivex, polycarbonate, CR-39, crown glass, index 1.80 glass

arrange these materials in order from the material w/ the lowest density to the material with the highest density:

crown glass, polycarbonate, CR-39, index 1.80 glass, trivex

colmascope

what is the name of the instrument used to check to see if a glass lens has been heat treated?

F

T/F: listing a lens material as part of an eyeglasses prescription is inappropriate

F

T/F: b/c the lab takes full responsibility for the accuracy of safety & sports prescription eyewear, verification of lens thickness by the dispenser is unnecessary

none

what is the minimum thickness requirements mandated by the FDA for dress eyewear?

F

T/F: plano sunglasses manufactured in quantity do not have to be impact resistant

back

a lens that has been scratched on the ______ surface is most likely to break

profesisonal

the duty to inform someone of the safest options for their eyewear needs is a ____________ responsibility

3.2mm (_3.00D power and above though have min thickness of 2.5mm)

what are basic impact safety eyewear minimum thicknesses?

2.0mm

what is the minimum thickness for high impact safety eyewear?

1in, 50in

the standard referee test for determining impact resistance suitable for basic impact prescription safety lenses is a _____ steel ball dropped on the front surface of the lens from a height of _____

F

T/F: putting 2.0mm thick CR-39 lenses in a safety frame but not marking the lenses for safety is acceptable if the person just wants the glasses for regular wear

chemtempered

a ___________ thick crown glass lens is the most impact resistant

F

T/F: a lens may be identified as having been chemically tempered by placing it in a colmascope