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OV2021 dispensing > Bifocals > Flashcards

Bifocals Flashcards

1
Q

Why bifocals?

A

for presbyopic patients

  • convienent than 2 separate pairs
  • large reading area
  • allow clear vision at 2 distances
  • cheaper than varifocal
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2
Q

What are the disadvantages of bifocals ?

A
  • 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?
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3
Q

What is the main disadvantage of bifocals?

A

doesn’t have intermediate distance

for e.g tablets, computer and laptops - problematic

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4
Q

What is the most commonly manufactured bifocal ?

A

Solid bifocal - due to having always dispensing plastic lenses - so definitely would be solid bifocal

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5
Q

What is a solid bifocal made from ?

A

a single piece of material

-feel the dividing line - ridge of segment

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6
Q

What is another manufactured bifocal ?

A

Cemented bifocal - can tell its cemented because epoxy resin can yellow over time

  • fused bifocal
  • Franklin split
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7
Q

What is a cemented bifocal made from ?

A
  • 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
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8
Q

What is the main advantages of a cemented bifocal ?

A
  • 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.
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9
Q

What is a fused bifocal?

A

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
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10
Q

How is a fused bifocal made ?

A
  • 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
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11
Q

How to work out the add for a fused glass bifocal ?

A

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
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12
Q

What is Franklin split ?

A

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
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13
Q

What is the main lens?

A
  • consider distance OC
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14
Q

What is the segment ?

A
  • 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

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15
Q

How to measure for near patient PD?

A

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.

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16
Q

What happens with the main lens and segment?

A

-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

17
Q
A

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
18
Q

How do we calculate the position of the near OS ?

A

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

19
Q

What prism is experienced in a D seg?

A

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
20
Q

What prism is experienced in a D seg?

A

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
21
Q

What does a positive lens exert when looking down ?

A

base up prism

22
Q

What is the R seg prism?

A

base DOWN prism- as they are looking above the OS of the lens

23
Q

What does the amount of segment also depend on?

A

how far away the px is looking from the oc

24
Q

What does OC tend to be like in a R - round segment?

A

in round segment- the segment OC tends to be further away

25
Q

What does OC tend be like in a D seg?

A

in D seg- close to the top of segment- results in little prism

26
Q

What happens if px is looking below the OC of the D seg ?

A

they will experience base UP prism

27
Q

What happens if px is looking above the OC of the R seg?

A

Base DOWN prism

28
Q

What is a better choice segment for a positive prescription px ?

A

A Round segment - R seg

29
Q

What happens the larger the R seg?

A

larger- the more prism it will exert

30
Q

What happens when px experience prismatic effect at near?

A

it displaces the object form where they expect it to be

-so important - that the amount of prism the px experiences at near is as small as it can possibly be

31
Q

E.g FOR prism at NVP

need to calculate horizontally and vertically

P=CF

LE +2.00DS R25 seg ADD +2.00DS GI 2mm Seg top 2mm below distance OC.
How much prism is experienced at a point 8mm below and 2mm in?

Calculate the horizontal first

A

check horizontal measurements because that’s what we will use
- +2.00DS , ADD= +2.00DS , GI 2mm (HORIZONTAL MEASURMENTS of inset), and px is looking 2mm inwards
P=CF
Prism from distance lens
§Horizontally –( Dist) P = 0.2 x 2 = 0.4 base OUT - DRAW OUT TO UNDERSTAND

  • Prism from near lens = ?
    No prism from near lens because = the Geometrical inset (GI) is 2mm and the px is looking 2mm inwards- therefore the NVP is going to coincide with the GI therefore no C in the P=CF equation- therefore no prism added in the segment in itself

-Therefore the total amount of horizontal prism experienced by the px only has contribution from the distance Lens = 0.4 Base OUT

Horizontal total prism = 0.4 base OUT

NOW Look at VERTICAL PRISM

-WHAT has a impact on vertical ?
+2.00DS , R25 seg ADD +2.00DS, seg top 2mm below distance OC, px is looking 8mm below

Vertically- need to work out how far down the patient is looking into the segment.
draw diagram

told segment is R25 - and segment OS of a round segment is at its centre of the segment - therefore is 12.5mm
-also told that the segment top lies 2mm below the distance OC
- As px looks 8mm down when reading
8-2 = 6mm into the segment looking
and so means they are looking 6.5mm upwards form the OC of the segment itself
C= 6.5mm
Vertically:
Distance-
P = 2x0.8= 1.6 Base UP

Near - P = 2 X 0.65 = 1.3 BASE down- because the px is looking above the OS of the segment - R seg experience base down Prism

1.6 base up and 1.3 base down = 0.3 base UP
and 0.4 base OUT

WHY Is it base up ?^^???

32
Q

What is a disadvantage of bifocal “?

A

jump
- the sudden introduction of base down prisms by the effect of the dividing line
-beacsue the px is experiencing base down as they enter the segment the image appears to move upwards to where the px expects the image to be.
The image px is looking at jumps upwards

33
Q

How to calculate amount of jump experienced by R seg?

A

jump = segment radius (cm) x Add

34
Q

How to reduce px jump?

A

give smaller segments

35
Q

How to reduce px jump?

A

give smaller segments

36
Q

How to reduce px jump even if given smaller segment ?

A

provide an executive lens as they have no jump as the OC of the near vision segment is on the dividing line.
-D and C segs have less jump as their near OCs are 4-5 mm from the top of the segment.

37
Q

What segment experiences more jump ?

A
  • R segs have more jump as the near OC of the segment is far from the distance OC as it is in the centre of the segment. EG for a R22 segment the OC is 11mm.
38
Q

What are the adv and disc?

A
  • Cosmesis- R segs are less noticeable.
  • Cost- Larger segments are more expensive.
  • Availability- E segs are harder to get hold of

OV2021 dispensing (7 decks)

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