Test 1 Flashcards
Tinted solid glass absorptive lenses
- introduces _ or _ during manufacturing.
- How are spectral transmission characteristics controlled?
Problems
- Metals or metallic oxides during manufacturing.
- Controlled by the quantities of the metals used.
Transmission is greatly reduced with tint.
Different areas of lens (thick/thin areas) will have varying shades.
Photochromic lenses contain
Darkening rate is dependent onn
Silver halide crystals that darken when exposed to long wavelength UV radiation.
Temperature- cold works better to activate, warmth returns them to clear.
ideal activation: cold temp and high UV. on top of a mountain
Do photochromic glass lenses wear out?
No, but may need to be broken in
Corning photochromic filter (CPF) lenses - glass
RELIEVE GLARE for patients with severe light sensitivity. Filter rout the shorter (blue) wavelengths. Selective! Ex: block all wavelengths under 511 nm
Ex: retinitis pigmentosa. Bad glare issue, albinism, aniridia.
Photochromic compounds used in plastic lenses
Spiropyrans. UV breaks the bond between the spiro carbon and oxygen. The new open compound strongly absorbs light in the visible region. Reverse when the UV source is removed.
Disadvantage to plastic photochromic lenses
May wear out/fatigue over time. Ex; Couple years
Imbibed photochromic plastic lenses
Darkens consistently across the lens, regardless of rx. Available in wide range of material/design.
4 types of glare
Relatively bright light which interferes with optimal vision or produces discomfort
Distracting, discomfort, disability, reflected
Distracting: Annoying. Caused by lens reflections- when you look at someone and you see yourself reflecting off their glasses. Fix with AR.
Discomfort: Sensation of irritation or pain from sources of light in the field of view. Stray light that causes visual discomfort but DOES NOT interfere with resolution.
Fix by changing environment aka get out of sun
Disability: Causes objects to have a lower contrast than they would if there was no glare. INTERFERES with resolution. Background brightness is increased and object brightness decreases.
Ex: turning on lights while watching a movie.
Light sources within the central 10 degree of the visual field contribute to this glare
Reflected: Caused by reflected light sources.
Ex: Glare off a shiny page in a book when holding it at the wrong angle.
Fix with polarized lenses.
methods to reduce surface reflections
Base curve tilt Pantoscopic tilt Smaller frame Change vertex distance AR coating
Low minus Rx has most reflection
What determines the index of refraction for the coating?
ideal index of refraction for coating = square root of index of ref of lens material
Why should coating be made with an optical thickness of 1/4 wavelength
So the wave will be 1/2 wavelength out of phase
The thickness of the AR coating is going to be 0.25 wavelength of whatever wavelength you are processing. If the wavelength of interesting is 500, the thickness of the coating would be a quarter thickens of that, so 125nm thick
For an AR coating to work properly, we have to satisfy two conditions
Amplitude condition and path control
What is typically used as an AR for glass lenses
magnesium fluoride
3 properties of AR that would be important to the patient
Hydrophobic
Olephobic
Anti-particulate
Production sequence of glass lenses
Clean lens
Heat in vacuum
Add magnesium fluoride
Production sequence of plastic lenses
Clean Baked to remove H20 Heat in vacuum Add AR Sodium dioxide used on outer layer
How many layers is a typical AR coating
5-6 layers
Brewsters angle formula
Tan(angle of incidence) = index of medium to which the light is incident
Tan(i) = n’
