DISP - Bifocals - Week 6 Flashcards
What are the two formulae for normal accommodation?
18 - (age/3)
15 - (age/4)
What are two possible causes of presbyopia?
Loss of flexibility in the lens
Loss of ciliary muscle strength
What is the near addition?
The difference between the distance and near Rx
Describe the franklin split type bifocals, what are they joined and held together by?
They are two separate lenses that are joined by a bevel and held together by the frame.
Name an advantage and three disadvantages of the franklin split bifocals.
Advantage -independent control over distance and near horizontal centration Disadvantages -poor cosmesis -heavy -poor vision at the lens junction
List four types of bifocal lens type designs.
Round segment
Executive - half distance, half near
A segment - upcurve or downcurve, depending if a wide reading area is needed
D segment - half-moon near lens design
How are D segment bifocals measured?
Across the widest part of the segment
List the three construction types for bifocal lenses. Which is not common?
Cemented - not common
Fused
Solid
In a cemented bifocal, where is the near segment attached?
It is cemented onto the back surface of the distance lens
Consider fused bifocals. What is the segment typically made of?
Higher RI glass
Where is the near segment attached to in fused bifocals?
Always the front surface
Where is the cyl powere worked onto in fused and cemented bifocals?
Fused - back surface
Cemented - front surface
List three advantages and one disadvantage of fused bifocal lenses.
Advantage Cheap Good for unusual shapes Cosmetically good Disadvantages Increased chromatic aberrations with high RI segments (not a problem in clinical practice)
Describe how plastic and glass solid bifocals are made.
Glass - distance and near is ground
Plastic - distance and near is moulded
What construction type do all plastic bifocal lenses have?
Solid
