13 - AR Coatings (Exam 3) Flashcards
When were AR coatings introduced and who originally used them?
1935 by Zeiss Optics
German military
What 2 conditions must an AR coating meet to reduce reflections?
- Path condition
2. Amplitude condition
What does the path condition determine?
Optical thickness a single layer coating must be
What does the amplitude condition require?
Amplitude of light waves in lens material and film are equal
How is the desired effect of the path condition reached?
Film must be an odd multiple of 1/4th the wavelength of a wave of light
What causes the path condition to reduce reflections?
Destructive interference
What causes destructive interference?
Waves being out of phase with one another
How does the amplitude condition relate to the path condition?
Ensures that the destructive interference caused by the path condition is complete
What must the index of the film be equal to so the amplitude condition is fulfilled?
Square root of the index of the material being coated
Why do single layer AR coatings have a purplish cast when viewed at certain angles?
Yellow light is reflected least
Blue and red are reflected most
What is a major benefit of multi-layer AR coated lenses?
Increased transmission
What is the order of layers going from the base lens to a multi-layer AR coating?
Base layer Primer Hard coat Adhesion Layer AR
What are 2 primary materials used to fabricate AR coatings?
- Magnesium fluoride
2. Silica
What is the purpose of a gripper coat?
Prevents slippage during the edging of the lens
In what fashion are multiple layers of AR coating applied to a lens?
Alternating high and low refractive index
What is an AR coating typically sealed in by?
Hydrophobic top coat AND/OR
Oleophobic top coat
How are hydrophobic lenses measured?
The angle they form with a drop of water on the surface (larger angle = more hydrophobic)
For what type of lenses is multi-layer AR coating most important and why?
High-index lenses
They transmit less light
What is the technique of design an AR coating for the material it will be place on?
Substrate matching
If a lens does not have uniform reflex color across all sections, what does this indicate?
Unevenly applied AR coating thickness
What is a challenge in creating an AR coating that has no reflex color?
Difficult to determine if the lens has an AR coating
What is the result of applying an AR coating over a tint?
Lightens the tint
What must you do to lens tint if you plan on applying an AR coat if you want to keep the desired tint?
Tint the lens 10-15% darker before applying AR
What are 2 major cons to AR coated lenses?
- Cost
2. Smudges are more noticeable
What 3 things should you avoid with AR coated lenses?
- Salt/bead warmers
- Caustic chemicals (acetone, ammonia, etc.)
- Dramatic temperature changes
What does HEV do to perceived contrast and why?
Reduces contrast
Blue light scatters more easily than other visible light
What can excessive blue light potentially cause?
Macular degeneration
What cycle is altered by blue light exposure?
Circadian rhythm
What are 4 benefits of blue light exposure?
- Boosts alertness
- Helps memory
- Increases cognitive function
- Elevates mood
What is Essilor’s most abrasion resistant AR coat?
Crizal Sapphire
What is Essilor’s AR coat that features HEV blocking?
Crizal Prevencia
