Multifocals Lenses Flashcards
(58 cards)
2 different types of add on lenses
Cemented bifocals & fresnel D seg
What are cemented bifocals?
Small plus lens glued to the back. Good optics but collects dirt and could fall off
What are fresnel d seg lenses
Sticker on lenses can be temporary but poor optics
What are fused bifocals
Small high n glass inserted into a hole in the front of the lens. (Only in glass so aberrations)
What is an up curve bifocal? Advantages and disadvantages
Lens was ground for reading rx and the distance portion was made flatter to reduce the plus. Invisible line but aberrations and BD prism on the distance rx
What is the perfection bifocal
2 lens held to get her by a groove. Difficult to manufacture
Modern one piece bifocal
Increased plus by grinding a different curve on the front or back of the lens, can feel the ledge
What bifocals are change of curvature
On piece
Perfection
Up curve
Which bifocal has a change in refractive index
Fused bifocals
Seg width
The size of a bifocal or trifocals segment measured across its widest section
Seg depth
The longest vertical dimension of the lens segment before the lens has been edged
Seg height
Vertically measured distance from the lowest point on the frame to the level of the top of the segment
Seg insert
The lateral distance from the PRP to the OC of the segment
Seg drop
the vertical distance from the PRP to the top of the segment
Flat tops/ D segments:
Seg OC is:
Advantages
Disadvantages
Seg OC is 5mm below ledge
Cheap and light
No intermediate
Round segs
-Used to be used with hig index but now available in plastic called a blended bifocal
What is a ultex bifocal
Round segment with large diameter for a larger reading area
Rarely used
Round segs disadvantages
Narrow FOV
In the way of distance
The oc is in the middle of the round segment so high image jump
Executive bifocal?
Advantages ?
Disadvantages?
The add seg covers the whole bottom half of the lens
Front surface is ground with a smaller radius of curvature
OC is at the top of the seg
_Wide FOV , no image jump
- heaviest bifocal, edge chops, thick ledge is obvious
Curved top seg
Oc is 2 mm higher than FT which allows for easier access to the wider part of seg.
Fewer reflections than FT
But same disadvantages as FT
Ribbon seg/ b segment
Ft but with button cut off.
Small depth of seg reduced riding area
2 lines increases reflections
( can be used with one eye and other eye FT to reduce vertical imbalance)
Is an usual lens, as gaze moved away from the OC,
Prims is progressively experienced , the effect is gradual and wearer is unaware (prentice rule)
When there is a second lens attached to the main one. The Gradual increase is interrupted
Image jump
What is image jump?
The apparent displacement of an object as the eye crosses the top edge of the segment (independent if distance RX)
