coatings

Question 1

what is a hard coat

Answer 1

commonly used
plastics are too soft
not applied to glass
not scratch resistant 
must have thin layer applied as its rigid

Question 2

what are the two types of hard coat

Answer 2

thermosetting: set in mould and cross links molecules within the material.
can’t redo it
used for cr39 and high index plastic

thermoplastic: long chain materials that don’t cross link so can heat and remould
used on polycarbonate

Question 3

different ways hard coat is applied?

Answer 3

spin coated- costly and not used
in mould coated- coat is deep within lens. not tintable
vacuum coated- costly, under set conditions, thinner and lighter but can’t tint
dip dye coated- popular, tintable,do more glasses.

Question 4

what is single antireflection coating

Answer 4

used for glasses only
wave reflection need to be out of phase from the front and back of the lens.
coating of 1/4 wavelength thick is used to cause half a wavelength difference
waves need to be of equal condition
purple bloom on lenses

Question 5

what is multilayer AR

Answer 5

layers - hard coat
adhesion coat
thick and thin layer made from titanium oxide or silicon dioxide
hydrophobic layer

Question 6

benefits of mar

Answer 6

reduce reflectance of more than one wavelength 
easy to clean
reduce ghost images
people can see your eyes
less glare

Question 7

why are antireflection coatings used

Answer 7

reflections cause light scatter which reduces contrast of images
causes ghost images
reduces light transmission which leads to eye strain
higher index materials have more reflections

Question 8

how to reduce ghost images

Answer 8

reduce bvd- ghost image 4/5
angle the lens
change the base curve- flatter lenses reflect more

Question 9

disadvantages of ar coatings

Answer 9

dirt or oil will reduce effectiveness

deteriorate in heat