Multifocals Flashcards
How to find horizontal prism in pt’s glasses
Dot PDs and place each in the lensometer. Note prism in each lens independently.
Circles on reticle indicate 1, 2, and 3 prism
How to find vertical prism in pt’s glasses
Start with strongest lens first. Center illuminated mires over the reticule in the first lens. Do not move the stage. Now slide the spectacles along the lensometer to the other eye and note the position of the illuminated mires relative to the reticule of the second lens. If they are above or below the reticle lines, there is vertical prism present.
Cemented bifocal
Thin glass wafer with the same index of refraction as the major lens was cemented onto the back of the surface of the major lens. Used widely in the 20th century.
Then Kryptok made a more blended cemented bifocal. Supposed to be invisible. First bifocal with reading due to higher index.
Fused Kryptok
Button fused to the countersink area. No glass required. Front surface spherical. Precursor of modern fused bifocals.
Ultex bifocal (no longer used)
One piece of glass. Add power is produced by a change in curvature.
Executive bifocal
Bottom half of lens is more curved. straight line across center dividing lens.
Current fused bifocal designs:
Round segments
Straight top
Ribbon segment
Round segment. Where is the OC
Segment optical center is half way below the segment top.
Ex: Seg at 22. OC at 11
Straight top. Where is the OC
OC is 5mm below seg top. Like a round with top cut off.
Ribbon segments. Where is SOC
OC midway between the top and bottom segment boundary cut offs.
Current one piece bifocal designs:
Round segments
Straight top segments
Double semgne bifocals for occupational lenses. Pilot, electrician, auto worker
Trifocals
Provides clear vision at intermediate distance. Flat on top.
Where to make bifocal measurements when fitting for kids and adults.
What about trifocals?
Bifocals:
Adults- Lower lid, lower limbus
Kids- lower pupillary border
Trifocal:
Adults- Lower pupillary border
Optical performance of a bifocal lens can be partially described as
- Image jump. Differential displacement at the segment top.
- Differential displacement at the reading level.
- Total displacement at the reading level. Most relevant.
What is image jump
Prism induced as the line of sight moves from the distance portion of the lens into the segment area. Usually describes the phenomenon occurring at the top of the segment.
Independent of distance lens power.
how to calculate image jump at top of seg
Image jump = dFa
d= distance from the segment top to the OC in cm
Fa= power of add
will be base down.
How to calculate differential displacement reading level
Don’t care about distant power. Need to know where pt is reading from.
If the reading level is above the segment center, produces a base down prism effect.
If the reading level is below the segment center, it produces a base up prism effect.
Differential displacement= vertical distance between the segment OC and the reading level in Cm x power of add
Image jump and differential displacement are independent of
Distance powers. Only concerned with reading level.
Total displacement
Total displacement at the reading level depends on both the power of the distance lens and the power of the add.
CHOOSE DISTANCE POWER AT 90 MER
Total displacement formula
(Distance OC of the lens to reading level x power of the distance portion of the lens) + (segment OC to the reading level)(power of the add)
How to prevent differential displacement, and total displacement?
No differential displacement at reading level if OC is at reading level.
No total displacement at reading level if you chose a seg that will induce the same amount of vertical prism as the major lens, but in the opposite direction.
