DISP - Coatings and Tints - Week 7 Flashcards
From what two sources can ghost images form?
The front of the lens
The back of the lens
Does reflectance increase or decrease with refractive index?
Increases
Describe how antireflective coatings work.
A signle film is designed to be a quarter of a given wavelength thick
For that wavelength, destructive interference will occur
Are antireflective coatings a single layer or multilayer?
Can be multilayered for multiple wavelength interference
How are antireflective coatings applied?
Vacuum deposited
Is dirt more or less visible with an antireflective coating?
More visible
In what two ways are scratch resistance or hard coatings applied?
Dip or spin method
Cured by heat or UV
What purpose does a primer have when applying a hard coating (3)?
Improves adhesion
Improves impact resistance
Improves ability to hold tint
Consider UV and thermal cured hard coats. Describe their abrasion resistance and cure times.
Thermal - higher abrasion resistance, but longer cure times
UV - lower abrasion resistance, but shorter cure times
Which produces a more uniform coating, dip or spin?
Spin
What is more suitable for batch processing, dip or spin coating?
Dip coating
What type of UV light is considered to cause the most damage?
UVB
How much UV light is absorbed by the time light reaches the cornea?
The whole spectrum
What wavelengths do glass and CR-39 block?
Glass - 300nm
CR-39 - 350nm
Who neess UV protection (5)?
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
List and describe the three types of glare.
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
Who needs tinting (4)?
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
What types of glare does tinting have no effect?
Discomfort and disability glare
List an advantage and disadvantage of solid tints for crown glass.
Advantage - very stable, cannot be scratched off
Disadvantage - lens thickness affects density
What surface are tints applied to crown glass and how?
Vacuum coating on the back surface
List two disadvantages of surface tinting crown glass.
Easy to scratch off
With very thick lenses, the fact that its only on the back surface is noticeable
How are plastic lenses tinted?
Surface coating
Consider plastic lenses. Does tint density vary from the centre of the lens to the rim?
No
Are scratches more noticeable on a tinted plastic lens than a clear untinted one? Explain.
No, because the tint penetrates reasonably deep
Both surfaces are tinted
What material, plastics or glass, is more popular for tints? List 4 reasons why.
Plastics
-lighter than glass
-safer than glass
-easier to tint
-plastic provides inherently good UV protection
What percentage tint density range is for cosmetic use only?
15-20%
What percentage tint density is the maximum that can be worn indoors or at night?
30%
What is the minimum percentage tint density that is required for glare reduction?
50%
What is the most common tint colours?
Grey/brown
What tints should be avoided in patients with colour vision defects?
Colours other than grey
What are 4 advantages of polarising lenses?
General reduction in glare
Specific reduction of specular reflections from horizontal surfaces (water)
Details under water surfaces are improved
Sky is darkened
What are 4 disadvantages of polarising lenses?
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
What are three advantages of photochromic lenses over fixed tint?
Financial - separate tinted/clear glasses not required
Convenience - do not have to swap back and forth
Safety - UV/glare protection always there
Do plastic photochromic lense sdeteriorate with age?
Yesd
What is preferred for photochromic lenses, glass or plastic?
Plastic
Do AR coatings increase transmission?
Yes
