Lecture 8 coating and tints

Q: Why are lens coatings used?

To improve lens performance by:

Increasing visual clarity

Reducing reflections

Improving durability

Providing UV protection

Improving resistance to scratches, dust, fog and smudges

Q: What is a hard coat and why is it used?

A hard coat (scratch-resistant coating) creates a harder lens surface, improving scratch and abrasion resistance and increasing lens longevity.Not scratch-proof.

[8) Ophthal...gs & Tints | PowerPoint]

Q: Compare Anti-Reflective (AR) and Anti-Glare (AG) coatings.

AR coating

Uses destructive interference

Increases light transmission

Reduces reflections

AG coating

Uses diffuse scattering

Can produce haze

Common on screens, not spectacles

Q: How does an AR coating work?

AR coatings use destructive interference, where reflected light waves cancel each other out, reducing lens reflections.

Q: What are the advantages of AR coatings?

Increased light transmission

Improved contrast

Reduced ghost images

Reduced night-driving glare

Better cosmetic appearance

Q: What is an anti-static coating?

A conductive coating that prevents static charge build-up and reduces dust attraction.

Q: Compare the two anti-fog mechanisms.

Surfactant method

Lowers water surface tension

Hydrophilic method

Absorbs water

Prevents droplet formation

Q: What is a hydrophobic/oleophobic coating?

The outer coating layer that repels water and oil by increasing the wetting angle.

Benefits:

Easier cleaning

Fewer smudges

Better water repellence

Q: Blue-light coating and blue-light filtering lenses: what must you know?

Filter part of the blue-violet spectrum.

NOT the same as AR coating.

Digital screens produce much less blue light than daylight.

Current evidence does not strongly support benefits for eye strain, sleep, macular protection or visual performance.

Q: Compare UVA, UVB and UVC.

Q: Which patients should always receive UV protection?

Especially:

Paediatric patients

Aphakic patients

Patients with albinism

What is the difference between absorption and transmission?

Absorption

Percentage of light absorbed by the lens.

Transmission

Percentage of light passing through the lens.

Example: 80% absorption = 20% transmission

Q: Compare glass and plastic tints.

Glass tints

Incorporated into material

Do not fade

Thickness affects appearance

Plastic tints

Dip dyed

Cheap and easy

Can fade over time

Q: What is a uniform tint?

A tint with evenly distributed colour across the whole lens.

Q: What is a gradient tint?

A lens that is darker at the top and gradually lighter toward the bottom.

Q: Know the sunglass categories.

Q: What factors affect photochromic lenses?

UV level

Type of radiation

Temperature

Lens thickness

Colder temperatures = darker activation.

Q: How do polarising lenses work?

They block horizontally polarised reflected glare while allowing vertically polarised light to pass.

Test: Rotate two polarising lenses 90° to each other → light is blocked.

Q: Explain ophthalmic coatings and tints.

Coatings improve lens performance and durability.

Hard coats improve scratch resistance.

AR coatings use destructive interference to reduce reflections and increase transmission.

AG coatings use diffuse scattering and may cause haze.

Anti-static coatings reduce static and dust.

Anti-fog coatings work by surfactant or hydrophilic methods.

Hydrophobic/oleophobic coatings repel water and oils.

Blue-light coatings filter part of the blue-violet spectrum but evidence for clinical benefit is limited.

UV protection is essential.

UVC is absorbed by the atmosphere.

UVB mainly damages anterior eye tissues.

UVA penetrates deeper and contributes to cataract formation.

Absorption + transmission = 100%.

Glass tints are permanent; plastic tints may fade.

Uniform tints are evenly coloured.

Gradient tints are darker at the top.

Category 0-4 tints are based on transmission.

Category 4 is unsuitable for driving.

Photochromic lenses darken in response to UV and are affected by UV level, temperature and thickness.

Polarising lenses reduce reflected glare by blocking horizontally polarised light.

Yellow/orange improve low-light contrast, brown improves depth perception, green balances contrast, and grey preserves colour perception.

Q: Match tint colour to its main use.

Yellow/Orange → improved low-light contrast

Brown/Copper → enhanced contrast & depth perception

Green → contrast + colour balance

Grey → neutral colour perception