Antireflective Coating Flashcards
What is used to measure the thickness of the tear film
Interference fringe
________ coatings on lenses are thin films that work according to the principles of interference
Anti reflective
How much light is lost by a spectacle lens?
- 4% of light incident on spectacle is lost. This reduces the illumination at the retina and is cosmetically unattractive. Light reflected into the eye by the back surface of the spectacle from the face is unwanted glare, especially at night
(4. 2% for each surface)
How does antireflective film on glass work?
Front and back surface reflections destructively interfere
How do you get destructive interference for the front and back of the lens?
- Reflections from film must travel a difference of lambda/2
- Ideal refractive index is 1.234 (for film), the best we can do is magnesium fluoride (1.38)
What is the ideal coating for antireflective?
Magnesium fluoride
With just the right _______ of a thin film, the second reflection interferes destructively with the first reflection
Thickness
The minimum thickness of an anti reflecting thin film on glass is
A quarter of the wavelength in the thin film
tn=lambda/4
The product of the thickness and the refractive index, tn2, is called
The optical thickness
Magnesium fluoride coating reduces the front surface reflection from ______ to ______
4.3% to 1.5%
A MgFl (n-1.38) AR coating is placed on a glass lens. What is the minimum thickness of the film for incident light of wavelength 552nm?
Use t=lambda/4n
100nm
A magnesium fluoride coating will reduce the reflection, but to reduce it further requires a _________
Multilayer coating
Percentage of light through an uncharted surface?
4.3%
Percentage of light through single coating on a surface
1.3%
Percentage of light getting through a multi coat film on single surface
0.5%
Advantages of anti reflection to patients
- sees details more crisply
- environment appears brighter
- cosmetic benefit
Disadvantages of AR coating
- exaggerated appearance of dirt
- elimination of reflection by multilayer AR coating in not cosmetically beneficial (dead gray)
What is the solution to the cosmetically unattractive dead grey
Residual color allows some reflection that is cosmetically pleasing.
-bluish green, blue, gold color are preferred as residual colors
Interference filter
A high transmittance narrow band that transmits by constructive interference while reflecting all other wavelengths
Types of filters
- long pass interference filters
- short pass interference filters
- band pass interference filters
Long pass interference filter
Only permits longer wavelengths to pass through
Short pass interference filters
Only permit shorter wavelengths to pass through
Band pass interference filters
Only permit a certain wavelength band to pass through
Hot mirror
Reflect IR and transmit visible
Cold mirror
Reflect visible and transmit IR
Biological mirror that reflects light which passes through rods and cones
Tapetum lucidum
What is the difference between tapetum lucidum and AR coatings on specs?
OPL between reflections is lambda instead of lambda/2, thus constructive, rather than destructive interference occurs
Tapetum increases the sensitivity of the cats vision by ____
44%
How does tapetum lucidum act to increase retinal sensitivity
By reflecting light back through the photoreceptors layer
Which is commonly used single layer anti reflection coating material
Magnesium fluoride
Single layer anti reflection coating reduces light reflection from 4.3% to about ____
1.5%
How muc does a multi layer anti reflection thin film coating reduce the light reflection to about
0.5%
The minimum thickness of an antireflecting thin film on glass is ______ of the wavelength in the thin film
Quarter
