Term 2

Question 1

bio based

Answer 1

leather, bone, fish, mammal, tortoiseshell
current bio based- wood eg bamboo walnut and horn which is from cattle so unethical. and tortoishell no longer killed

also caster seed oil- 100 percent bio based polymer made from oil of castre beans
natural acetate- plant based material composed of cotton and wood

hypoallergenic, renewable, sustainable, appealing, need specialist craftspeople, expensive, difficult to adjust or fix

Question 2

plastics what are the two types

Answer 2

polymerisation so small monomers to polymers
thermoplastics- soften when heated and can be reshaped
thermosetting-do not soften on reheating unless v high temps

Question 3

thermoplastics celluloses

Answer 3

cellulose nitrate- one of first, flammability issues so prohibited. tough, high shine, retained shape, stable in hot conditions- became brittle and yellows w age, not hypoallergenic, flammabe so illegal

cellulose acetate- cheap, widely used, coloured frames, lacquer to improve scratch resistance, easy to adjust and colour. not hypoallerg, may bubble or blister when heated, lacks elastic memory, can discolour if heated too much

cellulose propionate- similar to acetate, lighter stronger frames and can be thinner so even more lighter more flexible and more resistant to warping and fading w age. but prone to damage w acetone cleaning product and not hypoallergenic

Question 4

cellulose acetate more info

Answer 4

manufacturd in block form, moulded to give a stronger frame. frame can be coloured by adding water based dye during moulding, finished w hard high gloss polyurethane or acrylic lacquer to improve scratch resistance and appearance. sides usually reinforced w metal

lacks elastic memory so may become unstable if heated too much. cellulose materials arent hypoalleregenic, tr90, polyamide and carbons are

Question 5

tr90

Answer 5

thermoplastic
durable, flexible, rubber like texture used for childrens frames. lightweight, chemical and crush resistant. hypoalleregenic. can come w diff endpieces eg curled for children so better

can be difficult to adjust.

Question 6

epoxy resin what type of plastic is it as well

Answer 6

thermosetting plastic- sets permenantly when heated which gives it an elastic memory. if gets damaged can warm it up and it changes back to original

injection moulded then dyed then lacquered. lightweight, hypoallergenic, good plastic memory.

can be hard to adjust and brittle when cold`

Question 7

polyamide- thermoplastics (carbon fibre vs carbon nylon)

Answer 7

carbon fibre- has to be wrapped in a thin polymer sheet to colour it, made entirely of carbon fibre and generally more expensive.

lightweight, strong, flexible, robust, hypoallergenic, but difficult to adjust metal sides and expensive.

Question 8

different golds

Answer 8

gold- not commonly used now, hypoallergenic, unaffected by temp, acid resistant but they are heavy, bend easily, soft and expensive

rolled gold- gold layer bonded to mixed metal base which makes it stronger, more gold than gold plated. easily repaired, expensive. hypoallergenic.

gold plated- mixed metal base w electroplated gold than lacquered. gold layer thin. more cost effective than rolled gold. likely to deteriorate, colour wears off and not hypoallergenic

Question 9

titanium vs aluminium vs stainless steel

Answer 9

titanium- strong, flexible, hypoalleregenic, resistant to corrosion and perspiration, lightweight can be coloured by plating (electrolysis) but production costs are high

aluminium- light strong, stain and corrosion resistant, can be coloured by plating electrolyysis, cant be soldered or riveted so glazing and adjustment is hard, cold to touch, not hypoalleregenic

stainless steel- alloy of steel and chromium, lightweight, strong, flexible, resistant to corrosion and perspiraiton but not hypoallergenic

Question 10

nickel silver- metal alloy vs monel

Answer 10

no silver in it, cheap sturdy robust easy to solder but ppl allergic to it +dulls quickly

monel- form of nickel silver- 70 percent nickel, less corrosion

Question 11

metal alloys

Answer 11

memory metals so can be crushed or bent, lightweight, flexible, strong, can wrap around side of finger and wont break

usually titanium based with 2 structures- in unbent frames we have the martensite phase and bending them is the austenite phase. not stable at room temp so they return back to the martensite phase.

cannot be repaired, not completely memory metal, not hypoallergenic

Question 12

smaller vs larger frames

Answer 12

smaller- better for higher spherical powers bc thats smaller lenses. better optically as they reduce aberrations, lighter and thinner so thickness differences will be less noticeable in high anisometropia

larger- cosmetically preferred, bifocal lenses, much thicker, aberrations, msu, lens weight and thickness, ring scotoma if the patient can see their frame

Question 13

what does the lens shape need to be to make the thickness differences less noticeable

Answer 13

The thickest edge is 90 degrees to the negative cylinder axis, lens shapes need to be similar to the meridian containing the thickest edge to make the thickness differences less noticeable.

Question 14

fronts with deep lens shapes and thinner fronts and what do we have to check when dispensing glasses

Answer 14

Fronts with deep lens shapes suit wide faces and can conceal bags under eyes
Thinner fronts suit faces w delicate features. No part of front should touch the face

If the patient has high cheekbones ask them to smile= make sure not touching face
Check that there is adequate ventilation btwn face and frame at upper and lower rims to avoid condensation.

The frame head width should be compatible w frontal width, sides shouldn’t be in contact w the temple. No strain on joint.

Question 15

crown glass vs barium crown glass vs flint glass

Question 16

crown glass vs barium crown glass vs flint glass

Answer 16

crown glass- most used in ophthalmic work, soda lime silica glass containing silica, caO, naO, K, borax, antimony and arsenic which improves the quality of glass. all these things are heated to a high temp.

barium crown glass- soda lime glass containing Bao which increases n with minimal increase in dispersion. less material needed for same effect. 30 percent siliva, some lime, zn, ant, zirconium= improvoes quality of glass

flint glass- soda lime glass containing pbo 60 percent of it= increases n but also increases dispersion= chrom abb. used for crystalware. 30 percent siliva, arsenic antimony. pbo pollutant so difficult to dispose of

Question 17

individual batch method vs continous flow process

Answer 17

individual- used when small quantity of glass is required
continuous- large quantities of glass required- 5 precisely controlled stages.

MFSFA fine before form

Question 18

continous flow process- how to make a glass lens

Answer 18

melting- molten glass formed frm raw materials in a furnace
fining- furnace temp increased to refine molten glass+remove bubbles
stirring- molten glass cooled whilst stirred= more viscous suitable for moulding
forming- molten glass moulded into lens blank before passing into press
annealing- lens blank slowly cooled to room temp to avoid internal stresses

then lens undergoes inspection, max possible lens power determined by curves and thickness of blank, lens surfacing= conversion of blank into uncut spectacle lens. lenses are left as semi unfinised uncuts w a spherical front surface curve generated

Question 19

glass vs plastic

Answer 19

maintains form better, more scratch resistant, better optical clarity, less chrom abb. higher n= thinner lens, heavier, poor impact resistant, brittle, can shatter.

plastics- cr39 most common plastic material.

glass moulds can be reused but sealing rings cannot. both semi finished and funished blanks for plastics can be produced by cascting, high index plastics can be made using diff monomers- polyaddition is polymer from more than 1 monomer. produc time may take up to 48 hours

Question 20

lens manufacture for plastics

Answer 20

monomers and polymers
casting process used and liquid monomers are poured into a mould (initiator can be added to trigger polymerisation)
after filling moulds are subjected to a thermal process called curing and the heat activates the initiator and triggers polymerisation.

after polymerisation sealing ring is removed and plastic lens is sep from dye. further curing process= releases internal strain in lens

Question 21

polycarbonate- thermoplastic material

Answer 21

lens made by injection moulding
pellets of polycarbonate are heated to 300 degrees then this viscous liquid is forced inside steel moulds
chilled water is passed around each mould to cool and solidify the polycarbonate w moulds being compressed in case of shrinkage
v quick process
20 seconds

polycarbonate is v soft so scratch resistant coating is applied

Question 22

lens surfacing

Answer 22

blocking- lens blank attached to metal button using soft metal alloy. generating- lens blank surface brought to approx thickness using diamond impregnated cutting wheel

diamong impreg cutting wheel rotates to face of lens and moves side to side and lens blank rotates= curved surface. rotating cutting wheel through angle enables a curved surface to be generated. lens surface polished to optical quality using finer abrasive surface ground against a block w same curvature of finished lens

most lenses have their front surface powers finished and require back one worked onto lens. this can be done using diamond tipped point which gives a more refined surface so no need for smoothing but needs polishing.

Question 23

lens edging

Answer 23

lens blank needs to be cut into the frame- must ensure optical position is correct at either centre or away

grinding wheel used to shape lens so it fits into frame and to create an edge so its secured in the frame.

frame tracer used to determine lens shape needed and then lens cut into shape and then required edge is ground onto shaped lens to hold it in the frame

Question 24

increasing refractive index

Answer 24

achieved by increasing density of a lens material so it gets heavier, higher n is thinner but its denser especially for glass for plastics no differnce really

as n increases gravity increases as more dense so a lens of higher index glass will be thinner but will only be lighter if the decrease in volume makes up for the increase in specific gravity or density.

low power lenses of higher n will be thinner but heavier than crown glass
high powered of higher n will be thinner and lighter than crown glass
plastics doesnt rlly matter

increasing n reduces the edge thickness for neg lenses and reduces centre thickness for positie lenses. reduces lens thickness in higher powerd glass and lens weight.

Question 25

aberrations

Answer 25

cause light to be split into a spectrum (dispersion) due to prismatic effect of light. edge of lens= prismatic effect at periphery px may see coloured fringes. degree to which a lens material exhbits transverse chromatic aberration= abbe number lower abbe number- greater aberrations px prone to notice dispersion when abbe numbers is less than 40= large lenses as well.

Question 26

abbe number and n

Answer 26

in general abbe numbers decrease as n increases so yes thinner and may be lighter but aberrations would be more noticeable in these high index materials so patients should be warned about possibility of seeing coloured fringes in these high index materials more than 1.7

Question 27

glass lenses and polycarbonate and trivex

Answer 27

glass- higher abbe numbers than plastic so better optical performance for same n polycarbonate- low abbe number mid infex plastic. abbe number 30 good for childrne though strong lightweight trivex- similar n to polycarbonate strong lightweight but has a higher abbe number of 46 but may be more expensive

Question 28

reflections

Answer 28

index increasing also increases lens surface reflections. for plastics 8 percent of light is lost by reflection and for glass 18 percent is lost. optically not as good can use anti reflection coatings on lenses. reflections reduces contrast and makes lenses more unsightly. lens of n more than 1.5 must be treated w anti reflection coating which increases light transmission to more than 99 percent

Question 29

plastics vs glass

Answer 29

plastic- approx 50 percent lighter than glass, better impact resistance, safer (polycarbonate and trivex even more so glass- can be toughened chemically or thermally, better scratch resistance bc harder surfaces. also have treatments to make surface of plastic lenses harder

Question 30

visible spectrum light and what can the lens let in

Answer 30

Visible spectrum from 38-780nm w peak at 555nm Light split into ultraviolet: c,b,a and infrared. C is absorbed by earth’s atmosphere, B is 280 -315, A is 315-380nm and infrared is less than 780nm. Uv The lens lets through more light or uv as we age so artificial lens needs to have uv blocker Uva and Uvb can damage the cornea= cataract and amd. IR is hazardous and over 1400nm can lead to glass blowers cataract.

Question 31

how to know what wavelengths of light were travelling through a lens

Answer 31

If we wanted to know which wavelengths of light were travelling through a lens we could use a hoya tint metre which tells us how much light is transmitted through the lens in the uv infrared spectrum. Curve that shows the lens material and what light is transmitted through, if it lets through infrared but not much visible light= sunglasses.

Question 32

uv blocking filters and cr39 blocking infrared spectrum

Answer 32

filters can be incorporated into spectacle lenses, it doesn’t have to be a tinted lens to stop uv coming through. These uv filters should block wavelengths less than 360nm so blocks the uv bit of the spectrum. Coated lenses have a pale brown or yellow appearance. CR 39- Blocks some uv light but not all of it, doesn’t block uva so we put a uva absorbing coating onto the lens, polycarbonate blocks uva. If we wanted to block infrared spectrum: IR blocking filters are usually supplied eg plano glasses or face shields. Best protection is IR absorbing glass (addition of ferrous oxide) usually a pale green appearance.

Question 33

tinted lenses

Answer 33

noticeable colour, eye protection, glare reduction, psych or cosmetic Types of tint: graduated, rainbow, polarising, mirror, photochromic 0-4. 4 cant use when driving. 2-3 cant be used when driving in twilight or at night Transmission depends on the thickness of a material, if transmission less than 75 percent cant be used to drive in twilight or at night, 8 percent= cant drive.

Question 34

equitint vs solid tint (tinting glass)

Answer 34

equitint- tint same across lens. vacuum coated onto surface of lens or a layer of tinted glass is bonded or laminated onto the top surface of the lens. difficult to reproduce bc they dip them in dye so might not match properly. Tint density is uniform over lens surface. Wide range of colours solid tint- Made by metallic oxides into glass and depending on what oxide it is changes the colour. Material is incorporated within the lens matrix, tint density is variable and dependant on lens thickness. Repeatable but depends on thickness. Feo= green tint. Manganese oxide= pink tint.

Question 35

tints plastic

Answer 35

dip into bath of hot dye, lens absorbs tint into surface layer, longer in bath= darker tint. Normally done by machine, can be lightened using bleaching agent. Graduated tints can be made by lifting lens so some are tinted for longer, difficult to reproduce and high index plastics may be harder to tint. Some hard coats may also prevent tinting.

Question 36

photochromic lenses

Answer 36

lenses darken when exposed to uv. Indoors lets light through outdoors exposure= transmission drops and they look like sunglasses. Temp can affect these: cold they react faster and go darker faster. Anti reflection coatings on the front they go darker faster as more light goes through as less is reflected. Not recommended for driving, these lenses are uv blocking. Plastic photochromic lenses do slow down after approx. 2.5 years. car windscreen- Standard photochromic lenses don’t darken in a car bc windscreen contains uv blockers anyway, photochromic reaction requires uv action. So they made a slight change to the photochromics where blue visible light triggers them ont uv.

Question 37

glass photochromic

Answer 37

glass photochromic- Clear to dark: Uv light causes a breakdown of clear silver halide crystals to dark silver crystals and halogen. Dark to clear: Removal of uv light causes silver and halogen to form clear silver halide cystals again. Silver halide distributed evenly throughout glass lens (solid tint) tint may be uneven

Question 38

plastic photochromic

Answer 38

clear to dark- uv light causes a portion of carbon molecules to rotate increasing absorption of radiation= lens darken Dark to clear: removal of uv light= portion of organic molecules to flip back again causing the lens to lighten. Photochrom mols eg spiroxazines applied by equitant so tint is even. Different comb of mols alter colours of plastic photochromic lenses.

Question 39

polarising lenses and what do polarising filters do

Answer 39

light is reflected off surfaces eg water, road, surfaces, car windscreens, snow->polarised. The intensity of reflected polarised light acts as a glare source which can be uncomfortable for eye and impair vision. Polarising filter reduces effect of glare and reflections, polarising lenses transmit light in one orientation only so cuts out some light waves reducing intensity of surface reflections- driver less distracted by light reflec from a wet road.

Question 40

lens reflections- frontal, backwards, internal, bifocal segment reflections

Answer 40

- frontal (reflections like a mirror which isn’t cosmetically nice) reduces light transmission into eye and light incident on front of a lens surface is reflected back to a person looking at spectacle wearer. Lens reflections- backward (light from behind spectacle wearer is reflected from the back lens surface and into the wearers eye) this causes ghost images that can reduce contrast+ difficulties when reading in sunglasses/ using a computer. Lens reflections- internal (light reflected between two spectacle lens surfaces) causes ghost images that can reduce contrast+ reduce cosmetic appearance by producing power rings in negative spectacle lenses. Lens reflections- bifocal segment reflections more noticeable where thickness of segment boundary is widest, this bounces straight into patient’s eye.

Question 41

anti reflection coating how do they work

Answer 41

- More reflection for glass. Anti reflection coatings stop reflection. How: For one wave destructive interference occurs when there is an optical path difference of half a wavelength between the reflected ray at the surface of the coating and the reflected ray at the surface of the lens. The optical thickness of coating must be a quarter wavelength to bring ab destructive interference. There is no net reflected light energy so lens transmission increases= unwanted frontal reflec reduced more light->eye The coating is a metal oxide, a single anti-reflection coating can only eliminate one wavelength so approx. 550nm can be chosen. For complete destructive interference to occur, the amp of the waves must be equal to completely cancel each other out so the coating must have a refractive index of the square root of the refractive index of lens. !!!!!!!!

Question 42

single ar

Answer 42

One wavelength stopped but all other wavelengths can still be reflected. Single coat chosen to be effective is 550nm. Left with a hue of purple which is the residual hue (determined by the wavelengths that are still reflected) Ar coating gold residual hue could be recommended for night driving due to a shift in spectral sensitivity. Photopic sensitivity curve has a peak at 500nm. Choosing what wavelength we’re blocking out has an effect on px glasses.

Question 43

multiple mar

Answer 43

Reduces reflectance over a wider area of visible spectrum, layers of different thicknesses work to cancel reflec at diff wavelengths. Most MAR coatings have a green, blue or gold residual hue. Dimmer hue= better coating performance. Improved visual performance (glare reduced, contrast increased, night driving) improved cosmetic app esp for high powers, necessary for high index materials otherwise too much light loss, improves photochromic performance due to increased uv transmission, difficult to clean smear easily, vacuum coated so expensive, very thin so can be scratched off and prone to chem damage=hairspray

Question 44

hard coatings and how can they be put onto lenses

Answer 44

10 micrometres thick, increases surface hardness of plastic makes them scratch resistant but not proof. Resists general wear and tear due to cleaning. Scratches->diffraction which reduces contrast on a severely scratched lens and scratches can’t be polished out so not economical. dipped or spin coated- applied to both lens surfaces. spin coated only to one side. mould hard caoting- lacquer coated on one side of lens mould and polymerised at same time vacuum coating- addss thin layer of silica, expensive unless applying another coat in the same process

Question 45

top coatings and other coatings

Answer 45

top- avoid smearing, visible in prescence of ar coatings other- reduce smearing, repel water or fat or grease, hydrophobic, water repellent properties, oleophobic (fat repellent properties)

Question 46

mirror coatings

Answer 46

Here you want lots of reflections, you want constructive interference, coating here is thicker. Thickness is ½ wavelength so doubles reflection. Cosmetic choice- when needed to greatly reduce light levels eg snow water and ice. Have a normal absorption tint to further reduce transmission and glare and this determines the colour the patient sees. May need a back surface anti reflection coating to reduce the view of their own eyes, hard or top coat to protect mirror layer.

Question 47

blue control coatings

Answer 47

Anti reflection coatings designed to reflect visible blue light. These may be applied to one surface only with a standard MAR on the other or to both surfaces. Logically the residual reflection is a bright violet or blue and these are marketed as blue light may be damaging to the eye. This may induce visual fatigue but there is limited evidence to support this.