L9 Lens Coatings Flashcards Preview

Opthalmic Lenses and Dispensing > L9 Lens Coatings > Flashcards

Flashcards in L9 Lens Coatings Deck (14):
1

Why hard coat a lens?

- 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

2

Two ways to Hard coat:

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

3

Hard coats can be:

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

4

Reflections from Lens surface

- 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)

5

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

- 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

6

Can reduce ghost images by

- 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

7

Anti reflection coatings

1 - single layer
2- multiple layer

8

Constructing Interference

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

9

Destructive Interference

-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

10

Single layer coats (path condition)


- 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

11

Single layer coatings (amplitude condition)

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

12

Multi Layer Coatings

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

13

Features and benefits

- 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

14

Disadvantages

- 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