Why bifocals?
for presbyopic patients
- convienent than 2 separate pairs
- large reading area
- allow clear vision at 2 distances
- cheaper than varifocal
What are the disadvantages of bifocals ?
- visible line between distance and reading area
- some of the smaller segments have a smaller field of view= however can change that
- prismatic effect ?
- jump?
What is the main disadvantage of bifocals?
doesn’t have intermediate distance
for e.g tablets, computer and laptops - problematic
What is the most commonly manufactured bifocal ?
Solid bifocal - due to having always dispensing plastic lenses - so definitely would be solid bifocal
What is a solid bifocal made from ?
a single piece of material
-feel the dividing line - ridge of segment
What is another manufactured bifocal ?
Cemented bifocal - can tell its cemented because epoxy resin can yellow over time
- fused bifocal
- Franklin split
What is a cemented bifocal made from ?
- 2 pieces of material
- the segment is bonded tot the main lens and onto the back surface to prevent damage
- this segment can be glazed very thinly which means its almost to invisible and hard to feel
- feel on the back surface for the line for the segment
What is the main advantages of a cemented bifocal ?
- due to segment being a separate piece of material it can be used for more complex material
- e.g if a prism is required for near but not distance vision, this can be glazed into the segment itself.
What is a fused bifocal?
both the main lens and segment are glass
- segment composed of higher refractive index than the surrounding glass
- the higher the r.i within this SEGMENT THE higher the add
How is a fused bifocal made ?
- This segment is then fused by a high temp onto a depression curve which has been cut into the main lens
- this means that no curve can be felt between the main lens and the segment - therefore won’t feel change in curvature- feel completely smooth on front and back
- if holding glass bifocal and can’t feel dividing line- its a fused bifocal
How to work out the add for a fused glass bifocal ?
A=(F1 -Fc)/K
F1 = power of the distance portion. Fc = Power of the depression curve. K = fused bifocal blank ratio (n-1)/(ns-n).- refractive index of the main lens and the segment
- for main lens the R.I -is usually a crown glass and the segment button is usually fint.
- Ns - r.i of segment if its flint is usually 1.65
What is Franklin split ?
heavy
- composed on 2 separate Lens held together by frame
- got more freedom for any prescription in the reading and the distance
- avaialable for more complex prescriptions
What is the main lens?
- consider distance OC
What is the segment ?
- OS is in the centre of the circle in a round seg
- and approx 4mm below the sea top for a D or C seg
How to measure for near patient PD?
Near vision point – Where the
patient will look for near- this is a vertical and horizontal measurement based on the near PD and the assumption that a patient looks 8-10mm down and 2mm in to read.
What happens with the main lens and segment?
-near optical centre – Is the
optical centre of the near portion. It should coincide with NVP- but rarely the case due to prismatic effect
-If distance lens is Plano the near segment OS is the same as the segment OC.
-if distance portion has prescription- the near OS will be affected by prism from the main lens
horizontally due to horizontal prism PROVIDED by the main lens the near OS (segment ) is moved sideways
- vertically a plano distance lens therefore contributes no prism and the near OS is the same as the segment OC
- vertically IF a positive distance lens IS USED with a round segment it causes the oc to rise from centre of segment due to base up prism on the main lens and base down from the segment- cancel prism - this would be the near os- the point at which no prism is experienced
- Depending on prescription -at some point the base UP from the main lens and base down from segment may cancel each other out.
- if distance prescription is negative the near OS may not be in segment at all- due to base down prism given by main lens - same with D lens
How do we calculate the position of the near OS ?
x = SA/(F+A)
X = Distance from the distance OC to the near OC
S = Distance from the distance OC to the optical centre of the segment
A=Add
F = Power of main lens
S and A values are always positive
IF answer for X - is positive means the near OS is located below the distance OC.
-IF answer for X - Is negative the near OS is located above the distance OC
What prism is experienced in a D seg?
the segment itself doesnt introduce much prism because OC is much higher up - 4-5 mm up form dividing line
- so prism for main lens has more of an influence
- often the near os has been displaced so much by the main lens that it’s not even in the segment anymore
- important- the vertical position of the Near OS is dependant on the power of the main lens, segment and segment size
What prism is experienced in a D seg?
the segment itself doesnt introduce much prism because OC is much higher up - 4-5 mm up form dividing line
- so prism for main lens has more of an influence
- often the near os has been displaced so much by the main lens that it’s not even in the segment anymore
- important- the vertical position of the Near OS is dependant on the power of the main lens, segment and segment size
- exerts a small amount of base UP prism as they are just generally looking below the dividing line
What does a positive lens exert when looking down ?
base up prism
What is the R seg prism?
base DOWN prism- as they are looking above the OS of the lens
What does the amount of segment also depend on?
how far away the px is looking from the oc
What does OC tend to be like in a R - round segment?
in round segment- the segment OC tends to be further away
