Multifocals Lenses Flashcards
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)
What else is seen during image jump?
Scotoma of visual information
What bofical has no image jump
Executive because oc is right at the edge
Which bifocals show the most image jump
Round segs/ ultex
Because oc is in the center of the circle ( far for top of seg)
Where is the OC in flat bifocals
5mm from top
Where is the OC in round bifocals
Diameter/2 (different for all kinds)
Oc of ultex is usually
19 (huge image jump)
In image jump, the further the separation between the segment OC and the top of the segment
The more pronounced the effect (image jump)
What is the typical intermediate power in a trifocal?
1/2 of the near add power, but can depend on their intermediate needs
FOV of trifocals is
Decreased so used with caution for computer add
When can trifocals be considered
> +1.50 add struggle to see intermediate
Mechanics/ card players
What are the different types of trifocals and the numbers use to describe them?
- Flat top trifocals ( first # how tall the intermediate seg is, second # how wide the segment )
- E/D trifocal ( executive with a D seg) executive is for intermediate, D seg is for reading)
- Executive trifocal (franklin)
How do we fit trifocals?
Lower pupil margin to the deepest part of the frame
Bifocal- lower eyelid
What are occupational multifocals?
Lens styles that are specifically designed with certain work circumstances in mind
What is a double segment lens
Occupational bifocal
Pt can view overhead tasks at near viewing distanced
Top and bottom usually have same power
Quadrafocal lens
4 different powers
Golfers bifocal
Near is at the inferior temporal of the lens usually on one eye
Can write down score but see ball when looking down
Rede rite bifocal
Distance is a circle at the top. Rest is near rx
In a PALS lens the add increases in the central region which is called
Corridor or channel
Progressive/intermediate Zone of a PALS?
Transition between distance and near zones where clear vision is possible
How is the additional power achieved in a PALS ?
Variable curvature/ continually shrinking spheres
Horizontally the curves are different radii
The amount of cyl is related to the rate of power change
Base curve of PALS
Min value at upper distance and max value at lower near portion
Prism thinning
Equal amounts of vertical prism can reduce the overall thickness/thickness difference
BU is removed
Vertical prism is equal in each eye -> no prism imbalance
What are disadvantages of variable curve PALS
Unwanted astigmatism/distortIon at the bottom
Blur/swimming sensation
The degree of distortion in a PALS lens is influenced by
Add power (higher add-narrower corridor) Length of the corridor (shorter corridor) Width of the distance and near zones (affect size of corridor)
The amount of unwanted cyl is represented as an ______ plot. Which are unique to___.
Isocylinder plot
Unique to the brand of PALS and the rx
2 design policies of PALS
Hard and soft
What are hard pals
Concentrates distortion into smaller areas expanding the area of clear vision requiring less head/eye movement. But creates higher levels of blur and narrow corridor
What are soft pals
Spreads distortion across larger areas reducing blur and creates larger corridors. But narrows the distance and near zones of clear vision requiring more head movement
What type of design is the newer PALS
Between short and hard design
Horizontal symmetry in PALS
Minimizes difference in induced cylinder between corresponding points on the nasal and temporal sides of the corridor
PAL disadvantages include
- Adaptation period to peripheral distortion
- having a narrow intermediate area
- need head movement
OPLs /occupational progressives
Computer progressives have a wide intermediate and near zones with low levels of peripheral astigmatism
How to fit a pt in a PAL
Monocular PD
Seg height is measure from the center of pupil to deepest part of frame
What are additional measurements to individualize PAL lenses?
Frame
Patient ( arm length)
Measure w mm ruler
