10. coatings and tints

what is the use of a hard coat

scratch resistant

what are the advantages of a hard coat

1. scratch resistant

2. increased abrasion resistance

3. greater longevity of lenses

what are the disadvantages of hard coated lenses

1. clear coating is hard to detect

2. does not replace lens care

what is the difference between anti glare and anti reflection coatings

Anti glare : suppress reflections completely by diffusing scattering of light

Anti reflection: reduce reflections by destructive interference of reflected light

which side of the lens are anti glare and anti reflection coatings applied to

both the front and back surface

What are the advantages of anti reflection coatings

1. increased transmission of light

2. increased contrast

3. decreased ghost images

4. decrease night driving glare

5. cosmetically better looking glasses

6. reduce smudges on lenses

what are the disadvantages of anti reflection coatings

1. finger prints and smudges are visible on the lens and interfere with vision

2. require constant cleaning

what is the purpose of a anti static coating

inhibits static charges and prevents dust from accumulating

What is the purpose of a hydrophobic coating

reduces fogging

what are the advantages of hydrophobic coatings

1. repels water

2. repels dust and dirt

3. resists stains and repels grease

what are the disadvantages of hydrophobic coatings

1. doesn’t completely clean the lenses

2. less effective when oils get onto the lenses

3. careful cleaning still required

what is the purpose of a oleophobic coating

repels oils

which type of patient generally require uv coating glasses

patients who have had cataract surgery

what is a hard coat

• scratch resistant

• standard for plastic lenses

• ensures constant visual quality

What is anti reflection coating

• reduces reflections from surfaces

• Destructive interference

what is destructive interference

when two waves travelling in the direction are aligned at the crest of one wave and trough of the other causing the waves to cancel out

what is antiglare coating

• suppresses reflections by diffuse scatter

• cheaper than antireflection

• used on screens

• applied to both the front and back surface

what are the two conditions of destructive interface

1. path condition

2. amplitude

how is amplitude condition calculated

n coating = square root n lens

what are the advantages of anti reflective coating

• Increased transmission of light

• Increased contrast due to increased transmission

• Decrease ghost images

• Decrease night driving glare

• Overall lens looks cosmetically better

• Some coatings are also Oleophobic reducing smudges

what is a disadvantages of anti reflection coating

Finger print and smudges are visible on the lens and interfere with the vision

What is anti static coating

n electrically conductive coating that is designed to disperse or inhibit static charges and their effects. It also helps to prevent dust from accumulating on coated lenses

what are the properties of a hydrophobic coating

1. reduces fogging

2. repels dust and dirt

3. repels water to form droplets

4. less prone to abrasions

what are the two methods of making lenses Anti fog

1. surfactants

2. hydrophilic method

what are EMS blocking coatings

blocks electromagnetic waves which may damage the eye

what are the types of ultraviolet coating

UVC , UVB , UVA

ophthalmic absorptive tints are referred to as

• absorption

• transmission

The higher the grade of the absorption tint the

darker the lens

what type of tint is present on a glass tint

solid tint

what are advantages of plastic lenses being tinted

1. cheap

2. easy

3. quick

what are disadvantages of plastic lenses being tinted

1. tint will fade

2. doesnt absorb infrared and uv

3. difficult to match tint

what is the percentage light transmission for light gradient tinted lenses

43-80%

what is the percentage light transmission for medium gradient tinted lenses

18-43%

what is the percentage light transmission for dark gradient tinted lenses

less than 8%

what are the types of photochromic lens tints

• glass

• plastic

what factors affect the photochromic process

◦The amount of radiation present: In general, the greater the amount of light/radiation, the darker the photochromic lens will go (i.e. will go darker on a bright sunny day vs a cloudy day).

◦The type of radiation present: Activation depends on short wavelength radiation (esp. UVA/blue region), which is greater at higher altitudes, so the process is more effective.

◦Temperature: photochromic performance tends to increase (i.e. lens goes darker) as temperature decreases (e.g. cold weather/snow).

◦Thickness of the material: an increase in density of the photochromic material will tend to produce a darker lens in the activated state (certain types of lens).

what is the difference between photochromic and polarising tints

Polarized lenses are designed to reduce glare, while photochromic lenses are designed to darken when exposed to UV light.

which colour contrast is used in low light

yellow and orange

which contrast colour lenses heighten contrast in partly cloudy and sunny conditions

amber , pink red (but may cause significant colour imbalances.)

which colour contrast lenses block high amounts of blue light to heighten contrast and visual acuity. They Particularly useful to improve contrast on grass and against blue skies

Amber , copper brown ( melanin lenses)

which colour contrast tints heighten contrast while preserving colour balance.

green ( used in sports eg cricket)

which colour contrast tint reduces overall brightness while preserving 100 percent normal colour perception

Grey