Why do we dispense this?
- to provide protection from uv light
- to provide protection from glare (disability and discomfort)
- to provide visual comfort
- therapeutic and medical - certain condition eg cataract cause light scatter
- contrast and filter - certain sports
- cosmetic
EM Spectrum
generally interested in visible light spectrum
UV 100-400nm
Visible spectrum 400-800nm
Infrared (IR) 800+
Spectrum and damage
- UVA linked to cataracts and retinal damage - its quite deeply penetrating
- UVB linked to cataracts and photokeratitis
- Visible spectrum can cause disability and discomfort glare
- IR causes heat cataract and retinal/choroidal burns
UV Protection
- not always provided with tinted lenses
- its a separate coating
- clear coating
- amount of light entering the eye can be altered by absorption or reflection
Mirror Coatings
- supplied with a dark tint to prevent the wearer seeing themselves in the mirror coating
- tint depth also influences the amount of reflection
- dont wear well - can get scratched
- protection from absorption and reflection
Describing Tints
-colour
-depth in percentage - light transmission factor (LTF) and absorbance (ABS)
eg an 85% LTF lens is light
What colour
- to enhance contrast eg drivewear and sports tints
- personal preference
- cosmesis - frame choice
- protection - certain colours provide protection
Tints Available
- fixed
- graduated
- photochromic
- polarised
Fixed Tints
- standard tints
-same colour throughout the lens - methods of tinting
1- Plastic dip dyeing - important lenses are done in pairs
-diff materials have diff porosities so accept/reject tint easier
-newer/older lens absorb tint differently
2- Glass - solid/laminated
solid - tint is introduced to lens when its molten
- tint provided depends on metal oxides used in molten mix
laminated - v thin extra sheet
Lamberts Law
Thicker the material, the darker the tinting is
Graduated Tints
- can be single or dual coated - theyre darker at top graduating to a lighter colour at the bottom
- easy to do in plastic
- advantages: reading, fashion, can be one colour at the top another at the bottom
Photochromic Tints
- darken due to energy provided by UV
- darker quicker than they turn back to light
- no separate sunglasses require 0 convenient as no swapping pairs
- tint variable so comfort at all light levels
- temp dependent - work better in cold enviros
- full uv400 protection
- less reactive behind a uv filter eg car windscreen so they may not get as dark when driving
- residual colour with age - lenses will look yellow after time, may affect vision in darker conditions
Photo chromic tints for glass
- silver halides - separate when activated by UV
- equi tint (laminate) can be used on high rx
Photo chromic tints for plastic
- pyrans and oxazines in the 100-150 microns of the front surface of the lens
- molecules rotate on activation by UV and become darker
Polarised
unpolarised light - light which vibrates in all directions
polarised light - vibrations restricted to one meridian, can cause glare, polarised in the plane parallel to the reflecting surface
- axis of filter set vertical
-bs tolerance 5 degrees
- done by stretching iodine crystals
- if px complains glare concerns them, use polarised instead of standard tint
IR Lenses
- exposure from sun, furnaces and explosions
- linked to heat, cataract, retinal and choroidal lesions
- cant protect with CR39
- Rayban G15- ferrous oxide - produce a green tint
