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OD2 (S1&2) > DISP - Coatings and Tints - Week 7 > Flashcards

DISP - Coatings and Tints - Week 7 Flashcards

1
Q

From what two sources can ghost images form?

A

The front of the lens
The back of the lens

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2
Q

Does reflectance increase or decrease with refractive index?

A

Increases

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3
Q

Describe how antireflective coatings work.

A

A signle film is designed to be a quarter of a given wavelength thick
For that wavelength, destructive interference will occur

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4
Q

Are antireflective coatings a single layer or multilayer?

A

Can be multilayered for multiple wavelength interference

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5
Q

How are antireflective coatings applied?

A

Vacuum deposited

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6
Q

Is dirt more or less visible with an antireflective coating?

A

More visible

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7
Q

In what two ways are scratch resistance or hard coatings applied?

A

Dip or spin method
Cured by heat or UV

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8
Q

What purpose does a primer have when applying a hard coating (3)?

A

Improves adhesion
Improves impact resistance
Improves ability to hold tint

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9
Q

Consider UV and thermal cured hard coats. Describe their abrasion resistance and cure times.

A

Thermal - higher abrasion resistance, but longer cure times
UV - lower abrasion resistance, but shorter cure times

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10
Q

Which produces a more uniform coating, dip or spin?

A

Spin

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11
Q

What is more suitable for batch processing, dip or spin coating?

A

Dip coating

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12
Q

What type of UV light is considered to cause the most damage?

A

UVB

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13
Q

How much UV light is absorbed by the time light reaches the cornea?

A

The whole spectrum

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14
Q

What wavelengths do glass and CR-39 block?

A

Glass - 300nm
CR-39 - 350nm

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15
Q

Who neess UV protection (5)?

A

Everyone
Patients exposed to high levels of UV
Patients with cataracts or post-cataract surgery
Patients with pterygia
Patients with previous episodes of herpes simplex keratitis

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16
Q

List and describe the three types of glare.

A

Absolute glare - luminances when most patients experience discomfort
Discomfort glare - feeling of discomfort associated with a luminaire that is significantly brighter than the surrounding field
Diability glare - reduced contrast and vision associated with a luminaire that is significantly brighter than the surrounding field

17
Q

Who needs tinting (4)?

A

Patients bothered by absolute glare
Patients with eye diseases influenced by high illumination
Patients exposed to high levels of UV radiation
Patients who want it for cosmesis

18
Q

What types of glare does tinting have no effect?

A

Discomfort and disability glare

19
Q

List an advantage and disadvantage of solid tints for crown glass.

A

Advantage - very stable, cannot be scratched off
Disadvantage - lens thickness affects density

20
Q

What surface are tints applied to crown glass and how?

A

Vacuum coating on the back surface

21
Q

List two disadvantages of surface tinting crown glass.

A

Easy to scratch off
With very thick lenses, the fact that its only on the back surface is noticeable

22
Q

How are plastic lenses tinted?

A

Surface coating

23
Q

Consider plastic lenses. Does tint density vary from the centre of the lens to the rim?

A

No

24
Q

Are scratches more noticeable on a tinted plastic lens than a clear untinted one? Explain.

A

No, because the tint penetrates reasonably deep
Both surfaces are tinted

25
Q

What material, plastics or glass, is more popular for tints? List 4 reasons why.

A

Plastics
-lighter than glass
-safer than glass
-easier to tint
-plastic provides inherently good UV protection

26
Q

What percentage tint density range is for cosmetic use only?

A

15-20%

27
Q

What percentage tint density is the maximum that can be worn indoors or at night?

A

30%

28
Q

What is the minimum percentage tint density that is required for glare reduction?

A

50%

29
Q

What is the most common tint colours?

A

Grey/brown

30
Q

What tints should be avoided in patients with colour vision defects?

A

Colours other than grey

31
Q

What are 4 advantages of polarising lenses?

A

General reduction in glare
Specific reduction of specular reflections from horizontal surfaces (water)
Details under water surfaces are improved
Sky is darkened

32
Q

What are 4 disadvantages of polarising lenses?

A

Non-prescription lenses frequently use single layers of polarising material, which is soft and scratches easily
Prescription lenses are only available in a limited range of materials/designs
Slight head rotations can darken or lighten surroundings
Some LCD displays may be difficult to see

33
Q

What are three advantages of photochromic lenses over fixed tint?

A

Financial - separate tinted/clear glasses not required
Convenience - do not have to swap back and forth
Safety - UV/glare protection always there

34
Q

Do plastic photochromic lense sdeteriorate with age?

A

Yesd

35
Q

What is preferred for photochromic lenses, glass or plastic?

A

Plastic

36
Q

Do AR coatings increase transmission?

A

Yes

OD2 (S1&2) (174 decks)

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