lenses Flashcards
<p>PLUS LENS</p>
<p>- convex
- thicker in middle
- thinner on edges
- magnifies images
- converges light
- AGAINST movement</p>
<p>MINUS LENS</p>
<p>- concave
- thicker on edges
- thin in middle
- diverges light
- minifies
- WITH movement</p>
<p>OPTICAL CENTRE</p>
<p>- where light goes through, no prismatic effect</p>
<p>1.54 - 1.64</p>
<p>mid - index</p>
<p>1,48-1.54</p>
<p>low index</p>
<p>1.64 - 1.74</p>
<p>high index</p>
<p>dispersion</p>
<p>chromatic abberation</p>
<p>high dispersion =</p>
<p>high chromatic abberation</p>
<p>high V value</p>
<p>low TCA</p>
<p>low v value</p>
<p>more TCA</p>
<p>V value 45 & above</p>
<p>low dispesion/ low tca</p>
<p>V value lower than 39</p>
<p>high dispersion / more TCA</p>
<p>CROWN GLASS (RI)</p>
<p>1.52</p>
<p>POLYCARBONATE (RI)</p>
<p>1.586</p>
<p>CR39 (RI)</p>
<p>1.498</p>
<p>CROWN GLASS ADVANTAGES</p>
<p>- high v value so low CA
| - scratch resistant</p>
<p>CROWN GLASS DISADVANTAGES</p>
<p>- brittle
- high specific gravity (heavy)
- low refractive index (more abberations)
- tendency to fog/shatter</p>
<p>Greatest impact resistance?</p>
<p>Polycarbonate</p>
<p>POLYCARBONATE DISADVANTAGES</p>
<p>LOW v value so more dispersions</p>
<p>advantages of plastic materials ( high index)</p>
<p>thinner & lighter than other plastics</p>
<p>TRIVEX qualities</p>
<p>- high impact resistance
- high v value
- low specific gravity</p>
<p>Which lens blocks 100% of UV-A and UV-B below 394nm</p>
<p>Trivex & polycarbonate</p>
<p>True or False: Trivex is more resistant to chemicals than any other plastic lens material.</p>
<p>True</p>
<p>In order for a patient to have problems with chromatic aberration, the amount must reach \_\_\_\_\_Δ.</p>
<p>0.25</p>
Chromatic Aberrations more of a problem for what type of lenses
PLUS ( thicker)
best choice for lens protection but scractes the easiest?
polycarbonate
how to reduce chromatic aberrations in polycarbonate
anti- reflective coating
how to improve the impact resistance of glass
thermal & chemical tempering
which is faster- chemical or thermal tempering
thermal
which is more resistant
chemical: 2-3 x more resistant
what is specific gravity
ratio of a material's weight compared with the weight of an equal volume of water
high index lens is heavier bc
contains more lead
thermoplastic example
polycarbonate
thermosetting example
CR39
material with highest dispersion
polycarbonate
why are plastic materials less subject to fogging with changes in temperature and humidity?
less thermal conductivity
what is a Trivex lens?
a tri-performance lens material: 1. improved impact resistance 2. high Abbe value/lower dispersion 3. low specific gravity
Anti reflection coating
- thin layer/material applied to surface of lens to reduce unwanted reflections: increases the amount of lens passing through the eye
why do plastic lenses need scratch resistant coatings?
- typically soft materials that scratch easily
how to estimate the power of a lens
1. thickness: thicker the lens, higher the power 2. speed of movement: slower the speed, higher the lens power
how to neutralise a positive lens?
- add a minus lens of equal power
optical centre
- vision is best at this point | -limited aberrations
toric lens are used for
astigmatic people
what happens if curve in lens is flat
PLANO
PLANO LENS
no powe
function of prism
deviate light from original path
prentice rule
the relationship between the power of the lens, the distance of the eye away from the optical centre and prismatic effect it gives.
prentice rule equation
P=cF P= prismatic effect c= distance from optical centre of the eye (cm) F= back vertex power of lens in Dioptres We ignore + and - This equations shows the importance of correct centration
how to measure durability
materials are tested with the drop ball test and graded in terms of impact resistance . effected by the elasticity of the material - if elasticity of material is good then when hit by an object , the lens will be able to bend and flex and absorb some of the energy from impact
refractive index of material refers to
- density of material | - higher the refractive index, denser the material
lowest index for plastic
cr39 - 1.498
lowest index for glass
crown glass: 1.523
refractive index refers to
- density of material | -high refractive index : higher density
more dense
more light refracts because flatter curve: behave like mirrors and reflect more light
flatter curvature
thinner lens
polycarbonate is a...
THERMOPLASTIC | can be moulded&remoulded by heating and cooling
what does v value influence
- how much tca patient will experience
dispersal
is looking at when incident light hits the lens , how much its split into its colour components
when is TCA experienced
when patient looks off-axis: not optical centre
lower v value =
more dispersion
How much TCA can we tolerate
0.1 prism dioptres but varies with each patient
what do surface reflections do
- dramatically reduce transmission of white light with high refractive index materials
what is UV cut off
- light that is not reflected from the lens surface passes through -some is absorbed by lens material instead of being transmitted 100-380 nm
curve variation factor
how much thinner is a material compared to crown glass
coating a lens is dependant on
material of lens
does glass require a hard coat?
no : hard enough, doesn't need it - resistant to abrasion and scratches abrasion ( rubbing with fingerprints) scratching is from larger particles
what is a hard coat
- clear coating protects from abrasion and scratches - should never refer to as scratch proof
2 types of plastic
thermoplastic: - include polycarbonate and long chain molecules within the material don't cross-link therefore they can be heated are remoulded thermosetting: set in a mould with a chemical reaction which cross-links molecules within the material e. g. high index plastic( always hard coated) and CR39
polycarbonate properties
high impact resistance | scratches easily so may choose to hard coat
hard coat properties btw dont have much flexibility: will crack if too thick
- must be 0.5-10 microns - spin coated: expensive, not used much, NO TINT - vaccum: thinner, harder coat, expensive, NO TINT - IN MOULD - dip dyed: lots of lens can be coated at once - labs clean, important to control humidity & temperature
why develop AR coating?
-light reflected from lens is light that isn't reaching the eye , this means image is not clear and this can give rise to reflections and ghost images
what does reflection of lens cause?
light scatter - which reduces contrast sensitivity - which is how well Px sees definition between target and it's background - ghost images which causes visual disturbances - reduction of light transmittance leading to asthenopia ( eyestrain )
what does anti-reflection coating do ?
clear coating added to lens to protect against reflections from artificial light which cause glare and dazzle e.g. computer , car headlights
what do ghost images cause ?
- cause visual disturbances to the Px - causes px to see images of their eye or eyelashes - can be reflections from the front or back surface - they can be distracting to wear and give glassy appearance to the lens
relationship between refractive index and ghost image
higher the refractive index higher the intensity of ghost image
ghost image will not be seen if
if outside px field of view
how do we reduce ghost images
1- reducing BVD- reducing amount of light getting to lens 2- angling the lens - to sit closer to lens 3- changing the base curve - to make lens and more curved as we know flatter curves give more reflection 4- applying anti-reflection coating
types of AR coating
single layer: can result in 5% of reflections so 95% of light getting through to patients eye multi layer: reduce reflections to 1% or even less
why do we coat both sides of lens
ghost images and reflections can come on both front and back surface
anti reflective coatings work on principle of
destructive interference
what is constructive interference
two waves combined and in phase to make 1 large wave
what is destructive interferancw
two waves of light of equal amplitude and out of phase will cancel each other out
conditions for single layer coats to work
-path condition: -wave reflections to be exactly out of phase from the front and the back of the coating it must be 1/4 wavelength thick - this can only occur for one wavelength at a time and we know that visible light is composed of many wavelength - for this coating to be most efficient the thickness of the coating must be different for all different wavelength ( not possible ) so we use light in middle of spectrum - 550 nm
how is anti reflection coating visible to people
- have a bloom which is visible to observers - so if you tilt lens back and forth in front of artificial light source you can see slight sheen of colour on surface of lens- what you're seeing is light being reflected - light being transmitted through to eye is the one you can't see - this means that you can spot single layer coating which has a purplish bloom to it because light its letting through is the central wavelength ( yellow light )
what is the conditions that must be met once path condition is satisfied for single layer coats to work ?
- once path condition is satisfied then waves will be out of phase but they need to be of equal amplitude 2- amplitude condition - ensures waves are of equal strength
what is the function of multi-layer coating ?
allow us to reduce reflectance of more than one wavelength resulting in less reflection and more light reaching px eye
when do we use multilayer coatings
on plastics
why is harder to coat plastic with single layer coating ?
- single layer coatings are quite brittle and they tend to crack when applied to a flexible plastic material
what does a multilayer anti-reflection coating contain ?
will include hard coat so px doesn't need to pay extra
what is adhesion coat ?
on top of hard coat and will help the thick and thin layer coating stick
what are thick and thin pair of coating made of ?
formed of titanium oxide and silicone dioxide
thick pair function
cancel central spectrum of light
thin pair function
cancel blue and red reflections
what are the multilayer coating in order ?
hydrophobic ( lens will be easier to clean ) thick pair ( cancel the central spectrum of light ) thin pair ( cancel out reflections from blue and red end of spectrum ) adhesion coat hard coat
why are multi layer coating harder to produce ?
- require a sterile environment for manufacture | - can be vacuum coated , evaporation coated or sputter coated ( simpler process and smaller equipment )
what are the disadvantages of anti-reflection coating ?
- any dirt , water or skin oil will reduce the effectiveness of the coating - AR coating exaggerate the contrast between clear and dirty area smudges on lens reduce transmission by 4% - deteriorate in heat - coating needs to be fully intact to give good vision
why do we dispense tints
- protection from uv light - ^ glare/discomfort glare: light that causes discomfort- watery eyes - visual comfort therpeutic + medical - cosmetic -contrast filters: increasing contrast of objects so can see clearer for e.g in a sport like a football use a green tint etc
Who can benefit from a tint?
-a patient that could have early signs of cataract- which elevate their discomfort.
What do tints protect against?
certain wavelengths of light such as infrared- which can be damaging to the eye.
Why do we need protection from the different wavelengths of light - UVA ?
linked to cataracts and retinal damage: deeply penetrating
UVB :
cataracts and photokeratitis
Why do we need protection from the visible spectrum ?
disability and comfort glare
Why do we need protection from Infrared ?
800nm + causes heat cataracts and retinal/choroidal burns
Do all sunglasses have UV protection ?
No | -UV is a separate clear coating and you can then go onto adding a tint to it as it is not always provided.
Why are we more concerned about UV coating a tinted lens?
-pupil size increase due to dark tint and more light is let into the eye- change the ocular dynamics of the eye
What can the amount of light entering the eye be altered by ?
- absorption | -reflection
What are mirror coatings supplied with?
Supplied with a dark tint to prevent the wearer seeing themselves in the mirror coating.
What do mirror coatings do?
- they have Tint depth which also influences the amount of reflection - they Don’t wear well- can easily be scratched affecting them
How can you describe the tint?
- State the tint by colour - Depth in percentage - Light transmission factor (LTF) - FOR how light or dark you want the tint to be so by telling how much light you want transmitted through from this factor - Absorbance (ABS) - to tell how much light is absorbed by the lens 85% LTF = light lens as 85% of light being transmitted
ABS (ABSORBANCE) for 85% ltf
15% of light absorbed by lens
What colour tint does a patient choose?`
-To enhance contrast? Drivewear Sports tints -Personal preference? -Cosmesis? Frame choice -Protection Certain colours provide protection
```N tint
neutral, grey tint
why is N tint good
filter out all wavelengths roughly the same they don't distort colours much whereas a contrasting tint will. -So when a patient sees through them there is no distortion
Who is a N tint good for ?
drivers- don't want traffic light colours being distorted | -Architectures
what tints are available
- Fixed - Graduated - Photochromic - Polarised
fixed tints
same colour throughout lens - tint absorbs 1mm into material - just front surface
What are the methods of tinting fixed tints?
-Plastic- dip dying - Glass- solid or laminated glass material = cannot dip dye
-Plastic- dip dying | - Glass- solid or laminated
Solid - the tint is introduced to the lens when it's molten- a tint is solid throughout the material/ lens - if patient wants green tint add iron to molten mix -Laminate the lens- a laminated tint is a thin sheet which is laminated or vacuum coated onto one surface of the lens- a tint is only one surface of the lens.
What do we do when deciding between a solid and laminated tint?
Lamberts law
what is lamberts law
thicker the material, darker the tint - 8 : laminiated is better plano: solid tint
what is a solid tint
dispersed in the whole lens/material
what is a laminated tint
only at one surface of the lens. | -not affected by tint at all - equal across the lens
What is a graduated tint ?
- single OR dual coloured - darker at top, GRADUATING to a lighter colour at bottom advantage: good for reading for elderly patients
What is a photochromic tint?
- Darken due to the energy provided by UV.- use energy to alter the structure- when hit UV light turns dark - Darken quicker than they return back to light - on a sunny day will turn dark MORE and less UV means less dark. - No separate sunglasses required - Tint variable - Temperature dependent- work better in cold environments than hot - Full UV400 protection - if remove this will limit how it works - not as well - Less reactive behind a UV filter- e.g. car windscreen.
What benefits and advice do you think you should give your patients regarding Darken quicker than they return back to light
- may think they are faulty so make sure u tell patient first - possibly health and safety hazard if they come inside - wouldnt be able to suddenly see so take care when walking into buildings
No separate sunglasses required (tint)
-save money | -conviencince - no swapping glasses
Less reactive behind a UV filter- e.g. car windscreen
- get glare - better outside the car | - may not get as dark when driving
Residual colour with age
- older styles they get old after time (get yellow tint after a while ) - may affect vision in darker conditions.
What can photochromatic lenses be made in ?
-Glass- which contain Silver halides- separate when activated by UV - when separated they go darker and recombine and get lighter Equi-tint can be used on high prescriptions -Plastic- use Pyrans and oxazines in the 100-150 microns of the front the surface of the lens Molecules rotate on activation by UV- don't split and as they rotate they become darker
Why Polarised lenses?
-Unpolarised light- light which vibrates in all directions - Polarised light- it maintains in the one direction- which can cause glare and mirror effect - Brewster angle- if angle between the refracted and reflected beam is 90° then the reflected beam will be completely plane polarised.
Why is polarised light not good?
can cause glare and mirror effect e.g when driving
What is polarized light?
polarized in the plane parallel to the reflecting surface e.g surface of water
How can we polarise lenses?
- Axis of the filter set vertical - BS tolerance 5 degrees - Stretched iodine crystals - darker tint more polarisation occurring -
How can we tell our patient is wearing a polarised lens?
- do that by thinking about the effect of holding up another polarised lens in front of them - if you have 2 polarised lens - it will let some light through - the picture will be less clear- contract reduced by still able to see the image - if by rotating the filters at right angles to one another - block both vertical and horizontal meridian- won't see a image
What are IR lenses?
- need to use with glass lens - Exposure from sun, furnaces and with explosions! - Linked to heat cataract, retinal and choroidal lesions - Cant protect with CR39 - Rayban G-15- ferrous oxide- green tint - glass lens
Can UV damage be reversed
no - protection required all year round: summer & winter
are children more vulnerable to UV
yes
do darker tinted lenses provide more protection
no
field of view
total area patient can see when eye is in primary position- i.e. patient is looking ahead
what is real field of view ?
it is obtained when a spectacle lens is glazed into the frame
what is field of view influenced by ?
- frame and therefore lens shape - vertex distance - power of the lens - only in real field of view
what are frame markings found in spectacle lenses ?
- horizontal lens size x distance between right and left lens ( mm ) e.g. 50x18 50*18 50 □ 18 - total length of side ( mm ) from screw to the tip taking out the bend e. g. 135 mm - manufacturer / design / suppliers name or trademark - frame model - frame colour
what box centre distance ?
- measuring from the centre of one box to the centre of another box - add horizontal lens size and distance between lenses - this measurement that can be used to work out any decentration that we are going to use to place the optical centres of the lenses in front of the patient's pupil. - The patients pupillary distance would match the box centre distance.
what are the three groups for lens mounting ?
1. full rim - rim surrounds whole of lens 2. rimless - no rim surrounding lens 3. supra - frame covers part of lens when covers bottom and not top - this can be good for reading
what does mounting type determine ?
determines the edge of lens which we need to glaze to allow lens to remain securely in frame and not fall out
when do we use BEVEL and MINI BEVEL lens edge ?
used for full frames
when do we use flat lens edge
rimlesss flat style
when do we use supra lens edge ?
edge not covered must be held in place this can be done by a carving a groove into lens edge and thin clear wire is placed into groove and secured to frame
what are ways rimless lens mounting can be produced ?
rimless edge can be flat scalloped facetted inlaid
```what are frame components ?
-pads - bridge - rim - incases lens - side - extension from front of frame that fits around the ear - joint - where side of frame and front of frame meet
what is bridge ?
the bridge forms the main connection between the right and left rims . it may or may not have pads different designs for plastics and metal frames
what is pads
extension or attachments to the bridge which form the bearing surface of the frame for the nose larger pad = greater weight distribution
what are the bridge types ?
. fixed pad bridge - plastic frames only , pads moulded into rim and same material as frame . regular bridge - plastic frames only no moulded pad shapes . keyhole bridge - plastic frames only only pads resting against patient's nose . pads on arms - metal frame only
what are pad on arms ?
. metal frame only . pads can be screw on or push in . can be saddle / comfort / strap bridge with same material attachment . greater surface area touching nose = more comfortable
what are pads on arms made from ?
. silicone - flexible , light weight , soft | . vinyl/acetate - light weight , rigid , cheaper
what are some examples of sides ?
what are some examples of sides ?
protective eyewear protect against
penetrating trauma & chemical burns
what is impact resistance?
the ability of a material to withstand a high force or shock applied to it over a short period of time
how are materials tested?
- materials are tested with the drop ball test and graded in terms of impact resistance
what is the affect of elasticity?
- the elasticity of material is what give the material its impact resistance - elastic material is able to absorb some of the shock/force that hits it and lens doesn't break
PROTECTIVE EYEWEAR | how should the frame and lens be ?
- the frame needs to be loose enough that if the lens is flexing during this period of elasticity that its got somewhere to go - lens isn't glazed too tight
what are types of protective eyewear
- spectacles that are glazed with prescription - some can be worn on top of spectacles ( temporary measure ) - googles ( protect around ocular area ) - can also be ventilated - visors - protect the whole head
A screw comes out of the side of a PX safety glasses that he uses for work and asks to put a new one in - what do you say?
no, as this will void the manufacturer guarantee | - you need to send back to manufacturer who will re-test the unit and ensure that its meeting standards
what are lens markings
lens markings show that if a lens has met standards but only found if applicable
what are examples of lens markings?
. scale number ( filter lenses ) . manufacturers mark - always relevant . optical class . mechanical strength symbols . field of use . scratch resistance . resistance to fogging . radiant heat
```what is protective eyewear considered as ?
considered as a single unit- must be tested as such
protective eyewear what are scale numbers
- higher numbers= darker filters | - consists of code number and a shade number
what is optical class
- 1,2 or 3 - indicates the optical quality of the ocular - how close the prescription is what is should be 1= in tolerance with what the prescription is expected to be . +/- 0.66 D - as numbers get bigger tolerance is lower
what is mechanical strength ?
- looks at impact resistance e. g bearing test - can be marked with various codes - most safety spectacles are marked with an S or F - S= increased robustness e. g. CR39- not very tough - F= low energy impact e. g. polycarbonate - B= medium energy impact e. g. goggles - A = high energy impact e. g visors or face shield or some goggles
what materials make a good safety lens material?
polycarbonate- trivex- glass - must be treated to meet S standard
what are the pros and cons of using PLASTIC lenses as protective eyewear?
- more elastic than crown glass - impact resistance depends on material - impact resistance is affected by coating - CR39 can break into hooked shards
what are pros and cons of using GLASS as protective eyewear?
- untreated is not suitable for protective lenses - can be heat or chemically toughened - laminated
what are pros and cons of using TRIVIEX as protective eyewear?
higher v value compared to polycarbonate - UV cut off to 400nm - tinting not possible - good chemical resistance - n=1.54 - lowest specific gravity
what is a compression envelope ?
- outside of the lens is put into compression - molecules at the lens surface closer together compared to the centre - allows lens to become stronger - heat and chemical toughening both produce compression envelopes
what does force cause in an untreated glass?
untreated glass- force causes compression at blow and release tension on the other side
what are the two main methods of toughening a glass lens ?
- heat toughened | - chemically toughened
How is heat toughening carried out?
- lenses must be glazed before toughening - requires a minimum centre/edge thickness of 2.4 mm - lens is heated just below melting point 650 c - thicker lens= heated longer - variable heating time - outer glass cools more rapidly than centre - if it does smash then blunt pieces - rejection rate compared to chemically toughened - can be recognised by Maltese cross on a polarising filter
what is compression envelope caused by ?
- caused in difference in cooling between the surface and centre
what should you remember when supplying protective eyewear?
- if protective eyewear is supplied as industrial prescription protective eyewear then the employer is required to assess the workplace risk - you need ti supply to the company order form - if you do perform a workplace assessment you must keep detailed case records of the assessment
