Lens Coatings

Question 1

Why hard coat a lens?

Answer 1

• lenses tend to suffer abrasion, caused by fine particles and scratching caused by larger particles
• scratches dont look good cosmetically, reduce VA and vision that px gets through the lens
• glass generally not hard enough not to require a hard coat
• we hard coat a plastic lens
• must be thin 0.5-10 microns and are quite brittle

Question 2

Two ways to Hard coat:

Answer 2

1- Thermo setting

• aet in a hard mould, the cross chains link and they’re set in that position
• mid to high index lenses and CR39s tend to be this type of plasi=tic

2- Thermoplastics

• example is a polycarbonate lens
• could reheat them and mould it into a diff shape, extremely soft and have advantages in terms of impact resistance

Question 3

Hard coats can be:

Answer 3

Spin Coated - not really done, requires a lot of machinery, high cost, no real advantages
Vacuum - results in a slightly thinner coat, extremely expensive and not tintable
In Mould - made in mould, good adhesion as part of the layers itself, not tintable
Dip Dyed - most common, easy to do, no relas expertise needed, allows you to do multiple lenses at once so cheaper and quicker form a productive point of view. It needs control in terms of humidity and temp

Question 4

Reflections from Lens surface

Answer 4

• cause light scatter which reduces contrast sensitivity - has an effect on how px judges contrast eg good contrast is black and white
• cause ghost images which lead to visual disturbance
• reduction of light transmittance which causes asthenopia (eye strain)
  (talking about artificial light)

Question 5

Conditions for ghost images to be visible (have to be met simultaneously)

Answer 5

• intensity of ghost image is brighter than background illuminance
• vergence of the light which forms the image must be close to the vergence of light refracted by lens
• position of the refracted image of the source must be close to the object

Question 6

Can reduce ghost images by

Answer 6

• reducing BVD - distance bet lens and front surface of cornea
• angling the lens - reduces g1, 4 and g5
• change the base curve
• applying anti reflection coating

Question 7

Anti reflection coatings

Answer 7

1 - single layer

2- multiple layer

Question 8

Constructing Interference

Answer 8

If waves are lined up correctly then it leads to an increase in amplitude

Question 9

Destructive Interference

Answer 9

• Peaks and troughs are lined up in a way that they cancel each other out
• it takes the light thats being reflected and reducing that reflection to 0 so light is entering the eye

Question 10

Single layer coats (path condition)

Answer 10

• wave reflections to be exactly out of phase from the front and back of the coating
• must be 1/4 wavelength thick
• peaks have to match troughs - so 1/2 a wavelength out

Question 11

Single layer coatings (amplitude condition)

Answer 11

ensures waves are of equal strength
nf=squareroot ng
f is film coating and g is material

Question 12

Multi Layer Coatings

Answer 12

Can reduce reflectance of more than one wavelength
Thick and thin pairs work against diff parts of the wavelength - thick for central wavelengths and thin for short and long wavelengths
- easiest to produce
- require a sterile enviro for manufacture
- can be vacuum coated, evaporation coated or sputter coated

Question 13

Features and benefits

Answer 13

• lets through 99% of light - px can see clearer
• reduction in ghost images - enhanced cosmesis (people can see your eye)
• reduction in reflections - less glare and dazzle
• hydrophobic top coat - easy to clean

Question 14

Disadvantages

Answer 14

• any dirt, water or skin oil will reduce the effectiveness of the coating
• AR coatings exaggerate the contrast bet clean and dirty areas
• deteriorate in heat
• coating needs to be fully intact to give good vision