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
Which of the three construction types of bifocals is optically superior?
Solid is superior to fused and cemented
How are solid bifocals generated?
By having two curves on one surface
List 4 advantages of bifocals.
Allows quick change of focus with small eye movements
Less likely to lose anternative spectacles
No need to constantly switch spectacles
Cheaper
What are 6 disadvantages of bifocals?
Restricted reading area compared to SVN (larger segment sizes are possible though)
Adaptation - initial discomfort with walking, especially down stairs
Cosmetic issues
Poor intermediate vision
Prismatic jump
Hearsay
Name 4 kinds of patients that are unsuitable for bifocals.
Unsteady on their feet
Neck/posture problems
Bedd-ridden patients
No distance refraction
What do golf glasses look like and what do they allow users to do?
Any regular bifocal placed extemely low and in the corner of one lens of a pair.
Allows golfers to read and write on the scorecard without compromising distance vision
Describe E-D trifocal design.
Has an executive lens design where the bottom half is an intermediate segment.
A smaller near segment is placed within the intermediate segment towards the bottom on the frame.
What is an inherent defect of bifocals?
Prismatic jump at the segment edge (base down)
Describe the image jump in bifocals.
Objects become displaced at the lens junction, the light does not reach the eyes from this region and the image appears to jump.
