Bifocals L2 Flashcards
Why Bifocals?
- For presbyopes there are definite advantages:
- more convenient than 2 separate pairs
- wide reading area
- allow clear vision at any two distances
- For children that have problems with accommodation
- tend to be cheaper than varifocals.
Disadvantages of Bifocals?
- Visible line between distance vision and reading area which makes patient look ‘old’
- Smaller segment diameters have limited Field Of View compared to single vision (however this can be changed)
- Px over 55 years will start to require intermediate correction on top of distance and near ( we use tablets and screens at intermediate distances so this matters more now).
- Prismatic effect
- Jump
Describe Solid Bifocals ( i.e. structure, feel, etc)
- Made from a single piece of material
- Can feel the segment dividing line on front surface.
- All solid lenses are plastic
- The rx is different on near on a solid lens due to a change in curvature
Describe Cemented bifocals (i.e. structure, feel, etc)
- Made from 2 pieces of material, segment is bonded to the main lens, usually onto the back surface to protect from damage.
- you would feel the line on the back surface
- Held together by epoxy resin - but this can yellow with time
Describe Fused bifocals (i.e. structure, feel, etc)
- Both main lens and segment are glass
- Cant feel segment or line on any surface
- Main segment is made from crown glass (1.523) and there’s a depression curve cut into the glass, and a piece of glass of higher refractive index ( usually flint 1.65) is placed within that (see attached picture)
- So change of rx comes from change in refractive index of the materials
How do you work out the add in the seg of a FUSED bifocal?
A = (F1-Fc)/K
F1 = power of distance portion Fc = power of depression curve K = fused bifocal blank ratio (n-1)(ns-n)
Describe Franklin Split bifocals
- Composed of 2 separate lenses which are held together by the frame
- Held by epoxy resin
- Can incorporate two diff rxs within distance and near
What is the geometrical inset?
Distance between the distance OC (Od) and he midpoint of the segment (Os)
BASICALLY HOW MUCH IN THE SEGMENT IS.
What should the distance Optical Centre coincide with?
The px’s distance pd
Where is the seg Optical Centre (OC)?
- Approx 4mm below seg top for D/C seg
- Centre of the circle for R (round) seg
Where is the Near Vision Point?
Where the px will look for near - this is a vertical and horizontal measurement based on the near pd and the assumption that a px looks 8-10mm down to read and 2 mm in to read
(however if you have calculated the patient’s inset you may want to change this)
What is the Near Optical Centre?
If distance lens is plano then what is the near optical centre the same as?
If distance portion has a prism to it then will the near optical centre be affected?
- OC of the near portion. Should coincide with the NVP
- If distance lens is plano then near optical centre is the same as the same as segment optical centre for a round seg , or 4mm below seg top for a D seg.
- YES
- Horizontally its rarely the case due to the displacement form horizontal prism in the distance lens
- Vertically its often not if the distance rx is minus due to base down prism even if a D seg is used and a plus dist rx causes the OC to rise from the centre of the seg due to base up prism
Disadvantage of Bifocal - Prism
- Prism is exerted by the distance lens and ther eading seg
- If its a plus rx, opt for the round seg as it induces less prismatic effect when they look down to read - there is a base up prism exerted when reading in a positive lens
Disadvantage of Bifocal - Jump
Jump = sudden intro of base down prism by the prismatic effect of the dividing line
- Causes the image to move up as they move from one image to the other
- the bigger the diff bet the seg top to the OC of the near seg, the more jump there is
How to calculate jump experienced by a R seg?
Jump = seg radius (cm) x Add
