lens coating Flashcards
(38 cards)
what does coating a lens dependent on ?
dependent on the material of lens
why does glass not require a hard coat ?
- glass is hard enough not to require a hard coat
- resistant to abrasion and scratches
what is difference between scratching abrasion ?
- rubbing with the fine particle
- scratching is from a larger particle
what are hard coat?
- clear coating that help protect against abrasion and scratching
- hard coat should never be referred to as scratch proof coating
why do plastic materials require ?
plastic material require hard coat
what are the two types of plastic ?
- thermosetting plastic - set in a mould with a chemical reaction which cross-links molecules within the material
e. g. high index plastic( always hard coated) and CR39
2.thermoplastic - include polycarbonate and long chain molecules within the material don’t cross-link therefore they can be heated are remoulded
what is the property of poly-carbonate ?
- high impact resistance material
- but scratches easily so may chose to hard coat
what are the properties of hard coat ?
- must be thin 0.5-10 microns
- they don’t have much flexibility and would crack if they’re too thick
can be 1 - spin coated - high production cost non tintable 2 - vacuum - vacuum coating reduce in a thinner , harder coat non tintable high cost equipment
3 - in mould
4 - dip dyed - popular as a lot of lenses can be tinted at once
- conditions in lab must be very clean
- must control humidity and temperature
why develop anti-reflection coating ?
- all lens material reflect some of light which is due to enter the eye at both the front and back surface - light reflected from lens is light that isn’t reaching the eye , this means image is not clear and this can give rise to reflections and ghost images
what does reflection of lens cause ?
- light scatter - which reduces contrast sensitivity - which is how well Px sees definition between target and it’s background
- ghost images which causes visual disturbances
- reduction of light transmittance leading to asthenopia ( eyestrain )
what does anti-reflection coating do ?
clear coating added to lens to protect against reflections from artificial light which cause glare and dazzle
e.g. computer , car headlights
what do ghost images cause ?
- cause visual disturbances to the Px
- causes px to see images of their eye or eyelashes
- can be reflections from the front or back surface
- they can be distracting to wear and give glassy appearance to the lens
what are the 3 conditions that must be met simultaneously for ghost images to be visible ?
1- intensity of the ghost image is brighter than the background illuminance
intensity is affected by angle of light incident on the surface and refractive index of material
2- vergence of the light which forms the image must be close to the vergence of light refracted by the lens
- vergence of reflected ghost image must be close to the power of lens
3- position if the refracted image of the source must lie close to the object
relationship between refractive index and ghost image ?
higher refractive index = higher intensity of ghost image
when will ghost image not be seen ?
- ghost image won’t be seen if its overlapping the object its ghosting from
- outside the field of view of the patient
how can we reduce ghost images ?
1- reducing BVD- reducing amount of light getting to lens
2- angling the lens - to sit closer to lens
3- changing the base curve - to make lens and more curved as we know flatter curves give more reflection
4- applying anti-reflection
coating
what are the types of anti-reflection coating ?
- single layer - can result in around 5% of reflections so 95% of light is getting through each of the px eye
- multiple layer - reduce the reflection even further to 1% or even less than 1%
how can measure the amount of light being reflected ?
by measuring on a spectrophotometer
why do we coat both sides of lens ?
because we get ghost images and reflections on both front and back surface
what do anti-reflective coating work based on ?
anti-reflection coating work on the principle of destructive interference
what is constructive and destructive interference ?
destructive = two waves of light of equal amplitude and out of phase will cancel each other out
constructive = two waves are combined and in phase which will result in one large wave
what are the conditions that must be met for single layer coats to work ?
1- path condition - wave reflections to be exactly out of phase from the front and the back of the coating it must be 1/4 wavelength thick - this can only occur for one wavelength at a time and we know that visible light is composed of many wavelength
- for this coating to be most efficient the thickness of the coating must be different for all different wavelength ( not possible ) so we use light in middle of spectrum
- 550 nm
what do anti-reflection coating have that is visible to observers ?
- have a bloom which is visible to observers - so if you tilt lens back and forth in front of artificial light source you can see slight sheen of colour on surface of lens- what you’re seeing is light being reflected
- light being transmitted through to eye is the one you can’t see - this means that you can spot single layer coating which has a purplish bloom to it because light its letting through is the central wavelength ( yellow light )
a purple bloom coating is caused by which wavelength ?
the yellow middle wavelengths are being let through , but coating is less efficient for shorter and longer wavelength either side giving rise to purple bloom on lens what are the conditions that must be met for single layer coats to work
