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DISPENSING > lens coating > Flashcards

lens coating Flashcards

1
Q

what does coating a lens dependent on ?

A

dependent on the material of lens

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2
Q

why does glass not require a hard coat ?

A
  • glass is hard enough not to require a hard coat

- resistant to abrasion and scratches

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3
Q

what is difference between scratching abrasion ?

A
  • rubbing with the fine particle

- scratching is from a larger particle

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4
Q

what are hard coat?

A
  • clear coating that help protect against abrasion and scratching
  • hard coat should never be referred to as scratch proof coating
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5
Q

why do plastic materials require ?

A

plastic material require hard coat

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6
Q

what are the two types of plastic ?

A
  1. thermosetting plastic - 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

2.thermoplastic - include polycarbonate and long chain molecules within the material don’t cross-link therefore they can be heated are remoulded

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7
Q

what is the property of poly-carbonate ?

A
  • high impact resistance material

- but scratches easily so may chose to hard coat

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8
Q

what are the properties of hard coat ?

A
  • must be thin 0.5-10 microns
  • they don’t have much flexibility and would crack if they’re too thick
can be 
1 - spin coated - high production cost 
non tintable 
2 - vacuum - vacuum coating reduce in a thinner , harder coat 
non tintable 
high cost equipment

3 - in mould

4 - dip dyed - popular as a lot of lenses can be tinted at once

  • conditions in lab must be very clean
  • must control humidity and temperature
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9
Q

why develop anti-reflection coating ?

A
  • all lens material reflect some of light which is due to enter the eye at both the front and back surface - 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
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10
Q

what does reflection of lens cause ?

A
  • 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 )
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11
Q

what does anti-reflection coating do ?

A

clear coating added to lens to protect against reflections from artificial light which cause glare and dazzle
e.g. computer , car headlights

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12
Q

what do ghost images cause ?

A
  • 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
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13
Q

what are the 3 conditions that must be met simultaneously for ghost images to be visible ?

A

1- intensity of the ghost image is brighter than the background illuminance
intensity is affected by angle of light incident on the surface and refractive index of material

2- vergence of the light which forms the image must be close to the vergence of light refracted by the lens
- vergence of reflected ghost image must be close to the power of lens

3- position if the refracted image of the source must lie close to the object

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14
Q

relationship between refractive index and ghost image ?

A

higher refractive index = higher intensity of ghost image

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15
Q

when will ghost image not be seen ?

A
  • ghost image won’t be seen if its overlapping the object its ghosting from
  • outside the field of view of the patient
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16
Q

how can we reduce ghost images ?

A

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

17
Q

what are the types of anti-reflection coating ?

A
  • single layer - can result in around 5% of reflections so 95% of light is getting through each of the px eye
  • multiple layer - reduce the reflection even further to 1% or even less than 1%
18
Q

how can measure the amount of light being reflected ?

A

by measuring on a spectrophotometer

19
Q

why do we coat both sides of lens ?

A

because we get ghost images and reflections on both front and back surface

20
Q

what do anti-reflective coating work based on ?

A

anti-reflection coating work on the principle of destructive interference

21
Q

what is constructive and destructive interference ?

A

destructive = two waves of light of equal amplitude and out of phase will cancel each other out

constructive = two waves are combined and in phase which will result in one large wave

22
Q

what are the conditions that must be met for single layer coats to work ?

A

1- 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
23
Q

what do anti-reflection coating have that is visible to observers ?

A
  • 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 )
24
Q

a purple bloom coating is caused by which wavelength ?

A

the yellow middle wavelengths are being let through , but coating is less efficient for shorter and longer wavelength either side giving rise to purple bloom on lens what are the conditions that must be met for single layer coats to work

25
Q

what is the conditions that must be met once path condition is satisfied for single layer coats to work ?

A
  • 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

26
Q

what is amplitude condition equation?

A

nf = √ng
equation states that refractive index of coating or film should be equal to the square root of the refractive index of the glass

27
Q

what material does a magnesium fluoride work best on ?

A

n=1.38

1.38 squared gives 1.9 so the material which works best for magnesium chloride coating in a 1.9 glass material

28
Q

what is the function of multi-layer coating ?

A

allow us to reduce reflectance of more than one wavelength resulting in less reflection and more light reaching px eye

29
Q

when do we use multi layer coating ?

A

on plastic material

30
Q

why is harder to coat plastic with single layer coating ?

A
  • single layer coatings are quite brittle and they tend to crack when applied to a flexible plastic material
31
Q

what does a multilayer anti-reflection coating contain ?

A

will include hard coat so px doesn’t need to pay extra

32
Q

what is adhesion coat ?

A

on top of hard coat and will help the thick and thin layer coating stick

33
Q

what are thick and thin pair of coating made of ?

A

formed of titanium oxide and silicone dioxide

34
Q

what is the function of thick pair ?

A

cancel the central spectrum of light

35
Q

what is function of thin pair ?

A

cancel out reflections from blue and red end of spectrums

36
Q

what are the multilayer coating in order ?

A

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

37
Q

why are multi layer coating harder to produce ?

A
  • require a sterile environment for manufacture

- can be vacuum coated , evaporation coated or sputter coated ( simpler process and smaller equipment )

38
Q

what are the disadvantages of anti-reflection coating ?

A
  • 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

DISPENSING (19 decks)

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