Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

DISPENSING > Lens materials > Flashcards

Lens materials Flashcards

1
Q

what would the choice of lens material influence ?

A

the choice of lens material will influence

  • vision
  • comesis - look of lens
  • weight- therefore the comfort
  • safety
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

what the lens properties ?

A 
. durability 
. refractive index
. density and specific gravity 
. curve variation factor 
. V value 
. reflectance 
. UV cut off
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what is durability ?

A

the ability of a material to withstand wear and tear , pressure and damage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

how to measure durability ?

A

. impact resistance - which is the ability of a material to withstand a high force or shock applied to it over a short period of time
. 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
. elasticity is also effected by glazing as the lens needs some room to flex
. greatly effects the safety of the lens

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

what also is durability in relation with ?

A
  • durability is also in relation to how hard/soft a material is - so how much it scratches
  • can also be in relation to chemical resistance
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what type of lens has high impact resistance but soft and scratches easily ?

A

polycarbonate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

what do we need to take into account when thinking about durability ?

A
  • which aspect of durability is important to our patient
    . impact resistance
    . hard/soft material
    . chemical resistance
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

what does the refractive index of material refer to ?

A
  • refractive index of material refers to the density of material
  • the higher the refractive index the denser the material
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

what happen in a dense material ?

A

. denser material = higher refractive index
. the more dense the material the more light will reduce in speed and the more light will refract as it enters the material

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

how to achieve the same overall power lens ?

A

less curvature unless lens material is required

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

what does flatter curvature mean ?

A

flatter curvature = thinner lens

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

what is the equation of refractive index ?

A

n= velocity of light in air / velocity of light in medium

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

what does it mean when higher refractive index are denser ?

A

. this means less high index material is required than a lower refractive index material to refract light by the same amount

. therefore a higher refractive index lens is thinner than a lower refractive index lens of the same power

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

how is refractive index expressed ?

A

refractive index is expressed as a ratio relative to air n=1

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

what is the lowest refractive index available in plastic ?

A

CR39 = 1.498

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what is the lowest refractive index available in glass ?

A

crown glass = 1.523

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

what is polycarbonate ?

A

it’s a thermoplastic polymer and this means that it can be moulded and remoulded by heating and cooling

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

what else do we look at when looking at refractive index ?

A

we look at reflectance

  • higher refractive lenses , refract more light because they have flatter curve and have denser material so they behave like mirrors and reflect more light
  • high refractive lens should also have anti-reflective coating
  • surface reflections dramatically reduce the transmission of white light with high refractive index materials
19
Q

what is density ?

A
  • measured in grams

- density describes how heavy material is and it’s based on 1cm^3 of the material

20
Q

what is the density of water ?

A

1 gram

21
Q

what is heavier glass or water ?

A
  • glass is 2.5x heavier than water
22
Q

what type of density do we need in spectacles ?

A
  • spectacle lens should be as light as possible

- the density of the material should be as low as possible

23
Q

what is specific gravity ?

A
  • relates the density of the material to the density of water
  • specific gravity = density material / density of water
24
Q

what does the weight of a finished lens depend on ?

A
  • depends on the density of the material and how much we’re using
  • the more material we use the heavier the overfull product
25
Q

what happens when curves are flatter ?

A
  • less material is required to provide a prescription when using a high index material , the overall finished lens can be lighter even if the density of the material is greater
26
Q

what is the difference in weight of high index glass material and crown glass ?

A
  • the weight of the finished lens in high index glass material will often not be heavier than the crown glass equivalent
  • for plastics , the weight of high index and CR39 will be 50% of crown glass equivalent
  • the higher index plastics being the lightest as you are using less material and have a thinner centre substance
27
Q

what is the curve variation factor ?

A
  • the ratio of the curvature of a materials surface compared to the surface providing the same power in spectacle crown glass
  • CVF = ( 1.523 -1 ) / ( n-1 )
  • how much thinner the material chosen compared to crown glass
28
Q

what is crown glass equivalent equation ?

A

F X CVF

29
Q

what is V value ?

A
  • measurement of how much light is dispersed by the lens material
  • V value is important when thinking of the vision that a patient gets through lens we prescribe
30
Q

what we use when calculating refractive index ?

A
  • when we calculate refractive index we use yellow light to calculate velocity
  • other spectrum colours all have their own index
  • manufactures also give the indices of the end of the spectrum , red and blue light and give a ratio of the indices known as V value
31
Q

what is the equation for V value or abbe number /

A

Vd = nd - 1 / nF - nC

32
Q

what does v value influence ?

A
  • influences how much a patient is going to suffer from transverse chromatic aberration or TCA
  • more light is dispersed , the greater the problem may be perceived by PX
33
Q

what is dispersal ?

A

is looking at when incident light hits the lens , how much its split into its colour components

34
Q

when is TCA experienced ?

A

when a PX looks off-axis , so its not experienced at the optical centre

35
Q

how is white light dispersed ?

A
  • §when light is dispersed by a lens into its monochromatic constituents because shorter blue wavelength are deviated more than red long wavelengths and this is influenced by the prism the patient is experiencing away from the centre of the lens
  • this means that patients may see coloured fringes when they are looking at high contrast object when looking at the edge of a lens
36
Q

what is the equation for transverse chromatic aberration ?

A 
TCA = p/Vd 
p= prismatic effect of the lens 
V= v-value of yellow light for the lens material
37
Q

how is TCA experienced by a patient when looking at high contrast image ?

A
  • colour fringing on one side giving a blue hue and on the other side giving red hue
38
Q

what happens when PX is looking at low contrast image ?

A
  • grey on white background , then they may describe the transverse chromatic aberration as a general blurring
39
Q

when is TCA noticed ?

A

. this aberration is noticed by the eye when viewing through spectacle lenses especially when the V-value is low
. it is more significant in higher power lenses
. it is more significant in lower V - value materials - often high index lenses
. lower V value = more dispersion

40
Q

what is the amount of TCA we can tolerate with out any issue ?

A

. tolerance 0.1 prism D - varies from Px

41
Q

what is the reflectance at each surface of lens equation ?

A

P =( n-1 / n+1 )^2 x 100%

42
Q

what do surface reflections do ?

A

. surface reflection dramatically reduce the transmission of white light with high refractive index materials
. some anti- reflection coating is therefore essential when dispensing higher lenses ( 1.67 or above )

43
Q

what is UV cut off ?

A
  • light that is not reflected from the lens surface passes through - here some is absorbed by the lens material instead of being transmitted
  • lenses have protection from some wavelengths even if there is no added UV protection
  • UV light is found between 100-380 nm

DISPENSING (19 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions