4.1.5 Flashcards
What type of lens is a Concave/Biconcave lens?
Minus Lens
What type of lens is a Convex/Biconvex lens?
Plus Lens
What does a Toric Lens combine?
Cylindrical + Spherical
What are Aspherics in lens design?
Lenses designed to eliminate spherical aberrations
List two advantages of Aspheric lenses.
- Thinner, Flatter, Lighter
- Better Cosmetic Appearance due to flatter front surface
What is a disadvantage of Aspheric lenses?
No usable zone in periphery
What is a typical use of a minus aspheric lens?
To reduce edge thickness
What precision is Freeform lenses generated to?
0.01D
What are Customised Freeform Lenses tailored to?
The prescription and individual viewing habits of the user
What is a key feature of a Lenticular Lens?
The optical aperture is smaller than the frame aperture
What is a disadvantage of high powered lenses, especially plus lenses, when using Lenticular Lenses?
Poor Cosmetic Appearance
What is a Standard Lenticular Lens characterized by?
Carrier (responsible for providing stable foundation to the lens) and Corrective portion (responsible for rx correction) are visibly separate as central area is thick and peripheral is thinner giving a ring appearance around pupils
What distinguishes a Blended Lenticular Lens?
Carrier and Corrective portion are blended
What is a varifocal lens design characterized by?
Stable distance portion at the top, stable reading portion at the bottom and progressive power corridor between the two zones.
What are the key features of soft design varifocals
Longer progression length (change in power is more gradual) and larger field of view (wider near and int zones due to gradual power change)
What distinguishes hard design in lenses?
Short progression length (transition from distance to near is quick) and narrower field of view because of the rapid power shift
What is the purpose of prism thinning in lens design?
Incorporated vertical prism to reduce the thickness and weight of the lens.
Fill in the blank: Soft design lenses have a _______ progression length.
[longer]
True or False: Hard design lenses have a narrower field of view compared to soft design lenses.
True
List the characteristics of soft design lenses.
- Longer progression length
- wider field of view
List the characteristics of hard design lenses.
- Shorter progression length
- Spherical distance curves
- Larger stable distance and near zones
- Larger field of view
- Higher surface astigmatism
Fill in the blank: Varifocal lenses have a stable reading portion at the _______.
[bottom]
What is the purpose of Anti-Reflective Coating?
Decreases surface reflections inside the lens by phasing reflected rays into ½ their wavelengths, causing destructive interference.
How does Anti-Reflective Coating improve optical performance?
By removing glare for the patient.
What is the process involved in Anti-Reflective Coating?
Multiple coatings are applied for different wavelengths.
What are advantages of MAR
Improved visual performance
Reduced glare
Increased contrast
What are disadvantages of MAR
Easily smeared due to hydrophobic coating
Expensive due to vacuum coating process
Prone to chemical damage from everyday products like perfume
How do u calculate blank size
Frame PD-px PD+ max lens visible dia + 2
Eg. Frame PD = eye size+ bridge dia
50.20 145 , px PD = 64, max visible dia54
50+20=70(frame PD)
70-64= 6
54+6 = 60 +2 = 62
Blank size = 62
What are considerations when picking a frame for multifocals
Not too deep as reading portion will be too far down and become harder to access
Not too shallow or differentiation between dist/int/near will become close and can cause to look through wrong portion with slight movements
Corridor lengths may not allow for a small eye size and reading portion can be cut off in production
What are the different level of thinning and when would u use them
1.6 = 20% thinner than a standard lens 2.50D-4.00D
1.67 = 33% thinner 4.00D-6.00D
1.74 = 42% thinner 6.00D+
Why do thinner lenses come with MAR coating
Because high-index lenses refract the light at a quicker rate and therefore create more reflections
What are examples of frame materials
Cellulose acetate = manufactured in blocks and cut into sheets to cut the frame shape out of
Cellulose propionate = stronger than cellulose acetate greater elasticity and flexibility/lighter
Polycarbonate= used for safety eyewear can be soft or high impact resistance
Explain how hard coating and MAR coatings are applied
Hard coating = applied to lens surface to provide scratch resistance. Makes lens surface more durable and resistant to damage. (Dipped and then baked in oven)
MAR coating = reduce reflections on lens surface and allows more light to pass through the lens (vacumed on)
Why does hard coating go on before MAR coating
The hard coating provides a protective base for the more delicate MAR. Hard coating provides a strong stable surface for MAR to adhere to and if not on first then MAR will have poor adhesion
How do photochromatic lenses work
They have silver chlorine and the chlorine oxidises which makes the lens go dark
What are considerations when dispensing an over 10D pair of gls
Edge thickness
Plastic/metal frame
Weight of lenses /thinning
Aspheric lenses - less distortion in periphery due to uniform curvature
Smaller frames can hide the thickness and have less distortion as smaller lens and better aligned Pds to px pd
What considerations for dispensing an under 4yr old
Lightweight to prevent slipping down
Flexible to reduce risk of breakage
Adjustable nose pads
Anti scratch coatings
Stay on straps
No metal as safety risk
UV protection
When would I use hard varifocal lens design
High myopia rx = sharp distance due to abrupt power change
People who prioritise distance vision / spend a lot of time outdoors.
When would I use soft varifocal lens design
New wearers/first time wearers
Frequent use of near/int vision
Elderly/mobility issues as reduced need for head tilting due to gradual change
Varied work environment
How exactly does an MAR coating reduce glare
Contains several ultra thin layers of materials like magnesium fluoride /silicone dioxide and other optical materials each layer has a specific RI designed to cancel out light reflections by causing destructive interference. This means the reflections from the lens surface are out of phase with eachother and cancel eachother out
