DISP - Coatings and Tints - Week 7

Question 1

From what two sources can ghost images form?

Answer 1

The front of the lens

The back of the lens

Question 2

Does reflectance increase or decrease with refractive index?

Answer 2

Increases

Question 3

Describe how antireflective coatings work.

Answer 3

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

Question 4

Are antireflective coatings a single layer or multilayer?

Answer 4

Can be multilayered for multiple wavelength interference

Question 5

How are antireflective coatings applied?

Answer 5

Vacuum deposited

Question 6

Is dirt more or less visible with an antireflective coating?

Answer 6

More visible

Question 7

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

Answer 7

Dip or spin method

Cured by heat or UV

Question 8

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

Answer 8

Improves adhesion
Improves impact resistance
Improves ability to hold tint

Question 9

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

Answer 9

Thermal - higher abrasion resistance, but longer cure times

UV - lower abrasion resistance, but shorter cure times

Question 10

Which produces a more uniform coating, dip or spin?

Answer 10

Spin

Question 11

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

Answer 11

Dip coating

Question 12

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

Answer 12

UVB

Question 13

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

Answer 13

The whole spectrum

Question 14

What wavelengths do glass and CR-39 block?

Answer 14

Glass - 300nm

CR-39 - 350nm

Question 15

Who neess UV protection (5)?

Answer 15

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

Question 16

List and describe the three types of glare.

Answer 16

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

Question 17

Who needs tinting (4)?

Answer 17

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

Question 18

What types of glare does tinting have no effect?

Answer 18

Discomfort and disability glare

Question 19

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

Answer 19

Advantage - very stable, cannot be scratched off

Disadvantage - lens thickness affects density

Question 20

What surface are tints applied to crown glass and how?

Answer 20

Vacuum coating on the back surface

Question 21

List two disadvantages of surface tinting crown glass.

Answer 21

Easy to scratch off

With very thick lenses, the fact that its only on the back surface is noticeable

Question 22

How are plastic lenses tinted?

Answer 22

Surface coating

Question 23

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

Answer 23

No

Question 24

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

Answer 24

No, because the tint penetrates reasonably deep

Both surfaces are tinted

Question 25

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

Answer 25

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

Question 26

What percentage tint density range is for cosmetic use only?

Answer 26

15-20%

Question 27

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

Answer 27

30%

Question 28

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

Answer 28

50%

Question 29

What is the most common tint colours?

Answer 29

Grey/brown

Question 30

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

Answer 30

Colours other than grey

Question 31

What are 4 advantages of polarising lenses?

Answer 31

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

Question 32

What are 4 disadvantages of polarising lenses?

Answer 32

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

Question 33

What are three advantages of photochromic lenses over fixed tint?

Answer 33

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

Question 34

Do plastic photochromic lense sdeteriorate with age?

Answer 34

Yesd

Question 35

What is preferred for photochromic lenses, glass or plastic?

Answer 35

Plastic

Question 36

Do AR coatings increase transmission?

Answer 36

Yes