ppt

1. Done by,
S. Arya das
Motiwala Sadaf
2ND yr 3rd SEM
B.M.C.O

2. Terminology
• “Bi” = 2
• “focal” = focal point

3. Bifocal
Split Cemented Solid
Executive
fused
Flat top
bifocal
Curve type
bifocal
Ribbon(B-
shaped)
R seg B seg
Panoptic(P
-shaped)
Note - Round and kryptok can be obtained through fused and
cemented

4. 1. Split bifocal
• Also known as “franklin bifocal”

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Advantages
• Good optical property
• Sharp image over reading
field
Disadvantages
• Dislodged from the frame
• Accumulation of dust
• Unwanted reflection from
dividing line
• Cosmetically bad

6. 3. Cemented bifocal

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Advantage
• Easy to manufacture
• Glue was easy to apply
• Segment can be changed
when needed
Disadvantage
• Wafer easily fall off
• Discoloration of glue
• Brittleness of glue

8. 4. Solid bifocal
• Also known as one piece bifocal.
• Basically two types
(i) up curve
(ii) down curve
NOTE:- Executive bifocal is a type of solid bifocal

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Advantage
• Less chromatic abberation
• Made of single piece of lens
material
• Deviding line can be made
invisible
• Cosmetically better
• Image jump is minimised
Disadvantage
• Aberration present in the
distance portion.
• Restricted field of vision for
distance.

11. 5. Executive bifocal
• Also known as E-type bifocal.

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Advantage
• Image jump is less
• Wide field of view
• Optical property is
improved
• The entire bottom half is for
near vision
Disadvantage
• Thickness can be a problem
in case of prescription

13. Fused bifocal
• The fused bifocals are classified into 4 types
namely:-
(i) D-shaped (flat top bifocal)
(ii) C-shaped (round top bifocal)
(iii) B-shaped (ribbon segment)
(iv) P-shaped (panoptik)

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• The first fused bifocal - kryptok bifocal.
• Segment of a flint glass was fused
• Cover glass was not needed
• After fusing surfacing is done for front surface

16. Fused bifocal
Advantage
• Segment’s edge don’t
collect dust.
• Segment don’t fall out
• The segment is invisible
Disadvantage
• Transverse chromatic
aberration.

17. 6. Kryptok
Cemented
• Countersink curve is form of
depression
• Wafer is cemented in it
Fused
• Countersink surface curve
with high index button is
fused at 600 – 700°C

18. Cemented kryptok bifocal
Advantage
• R.I. Of segment > distance
Disadvantage
• Difficult to manufacture
• Segment is fragile
• Cemented surface tends to
darken after long use
• Lens easily get apart

19. Fused kryptok bifocal
Advantage
• Segment edge do not
collect dust
• Segment is relatively
invisible
Disadvantage
• Button is steeper than
countersink curve
• Chromatic aberrations is
produced while reading

20. Different types of fused
bifocal

21. RIBBON SEGMENT
B - seg
• Basically round segment
with top and bottom cut off.
• It is only 9mm deep.
• Used basically for minute
works.
R - seg
• Basically round segment
with top and bottom cut off.
• It is 14mm deep.

22. 7. Ultex bifocal
• Charles Conner – 1910
• It is a bifocal made from single piece of glass
of uniform R.I.
• Additional power is obtained by changing
curvature of back surface.

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Advantage
• Near vision portion is bigger
Disadvantage
• Prismatic effect occurs
• Optical centre is not on
proper position
• Cosmetically bad