This document provides information about bifocals, including their history, types, advantages, disadvantages, fitting considerations, and optical characteristics. Some key points: - Benjamin Franklin invented the first bifocal in 1784 by splitting a distance and near lens. Solid upcurve bifocals were the first patented design in 1837. - Types include solid upcurve, perfection, cemented, fused Kryptok, and fused bifocals. Fused bifocals permanently bond the segment using heat fusion. - Optical characteristics like jump effect, prismatic effect, and chromatic aberration must be considered. Proper fitting involves measuring segment inset, width, and height. - Both mechanical stability

1. BIFOCALS

2. INTRODUCTION
• Eyes age along with rest of us .
• A presbyopic patient requires a separate correction for distance and near.
• So that two prescriptions may be provided as one pair of spectacle in the
form of bifocal lenses.

3. HISTORY
• In 1784 ,Benjamin Franklin described a pair of spectacles that which
he called them as double spectacles.
• They were made by a very crude method of splitting a distance and
near lens.
• Mounted the top half of the distance and bottom half of the near
together , fitting edge to edge in a single rim.

4. TYPES OF BIFOCALS
SOLID UPCURVE
BIFOCALS
PERFECTION BIFOCALS FUSED BIFOCALS
CEMENTED BIFOCALS

5. SOLID UPCURVE BIFOCALS
• It was the first patented bifocal made from a single piece of glass.
• It was invented by Issac Schnaitnamm in 1837.
• It was manufactured by grounding the near prescription in lens , and
more distance correction was then grounded into the upper back
surface.

6. ADVANTAGES
• Better cosmetic appearance than franklin’s bifocal.
• Wide field of view for reading
• Less chromatic aberrations
• Structurally stronger
• Easier to manufacture

7. DISADVANTAGES
• Limited choice of surface power
• Optical center cannot be easily adjusted
• Resurfacing of upper portion of back surface moves the optical center
• Distance portion provides a strong base down prismatic effect and
image jump
• No base curve is available for surfacing?

8. PERFECTION OF BIFOCALS
• It was invented in the year 1888 by august Marick.
• The two portions were joined by bevling the contact edges , such that
they fitted together at the junction.
• The distance portion was larger than the near portion .

9. ADVANTAGES
• The lens was stronger
• It did not come out easily
DISADVANTAGES
• Difficult and expensive manufacturing
• Dividing lens tend to connect dirt
• Reflections due to dividing line

10. CEMENTED BIFOCALS
• Invented by Morick.
• A thin wafer of glass with same RI as major lens was cemented to back
surface.
• Canada balsam was used as an adhesive agent.
• Front surface of wafer has same curvature as back surface of major lens
• Back surface of wafer as made lens concave than back surface of major
lens.
• Add power is difference between the curvatures of back surface of
major lens and wafer.

11. ADVANTAGES
• Cosmetically more appealing
• Less optical aberrations
• It can be made of any power ranges and can be positioned anywhere
into the main lens
DISADVANTAGES
• Dividing lines tend to collect dirt
• Adherence of wafer is affected by changes in temperature
• Wafer had a tendency to fall off

12. CEMENTED KRYPTOK BIFOCAL
• It was invented by John.L.Borsch in 1899.
• It was first bifocal lens to make use of two different materials.
• It was manufactured by grinding a countersink curve into front
surface of the major lens of ophthalmic crown glass
• Then a wafer of flint glass was cemented into the countersink area,
and the surface was covered with a thin meniscus of glass cemented
into place.
• Add power will be because of RI of the segment and the concave
interface between the segment and major portion

13. DISADVANTAGES
• Difficult to manufacture because six surfaces had to be ground and
polished
• Cemented surfaces tend to darken
• Thin and fragile cover plate
• Wafer used to come apart after some time

14. FUSED BIFOCALS
• The first fused bifocal was the fused Kryptok invented by borsch in
the year 1908.
• The segment is permanently bonded onto the convex surface of the
lens by heat fusion process and required addition depends on:
• The refractive index of the two glass materials
• The depression curve
• The curve worked on the segment side of the lens

15. • To fuse the segmented bifocal , a countersink cavity is produced on
the front surface or the convex side of lens having a RI of 1.523.
• Then , flint button was placed on countersink curve & heated to 600-
700 c at which it softened and fused.
• Any cylindrical component has to be ground on the back surface of
the lens.

16. FUSED BIFOCAL DESIGNS

17. ADVANTAGES
• Segment edges do not collect dirt
• Segment do not fall out, discolour or chip
• Lenses can be produce in large quantity with low cost
• Relatively it is light weight
DISADVANTAGES
Chromatic aberrations are because of flint glass
Strains and tension produced

18. BIFOCAL SEGMENT SHAPES
• Segments vary from tiny B segment to E style in which the whole
bottom half of the lens is used for near portion.
• The most commonly used bifocal shape are D segment, E style, and
round segment bifocal.
• B segment and C segment are relatively rare.

19. ROUND SHAPED BIFOCAL
• They are also called down curve bifocal lenses.
• They have a segment with a dividing line that is single circular are
which is least visible compared to others.
• As this Round segment placed much below from top rise to “jump” at
the edge of the segment when the line gaze passes from distance to
near.

20. D SHAPED BIFOCALS
• Available in solid plastics and fused glass forms
• Wide reading area at the point of visual entry into segment
• C shaped segment is the modified D shaped segment which has
advantage of glazing and surfacing over D shaped segment.

21. E-STYLE BIFOCAL
• It is one piece bifocal lens with two different curves ground usually on
the front surface
• The optical centre for both distance and near are lie on the dividing
line.
• It is classified as monocentric lens.
• These are popularly known as Executive bifocals.
• The big advantage of E-style is that entire bottom half of lens is for
near vision.
• Thickness can be problem in hypermetropic prescription

22. ARRANGEMENT OF E STYLE LENS OPTIC
CENTRE:

23. OPTICAL CHARACTERISTICS OF BIFOCAL
LENSES :
• Jump effect
• Prismatic effect
• Chromatic aberration
• Magnification

24. JUMP EFFECT:
• When looking from distance to near portion of a bifocal lens, sudden
change in the prismatic effect due to the introduction of base down
prism by the segment causes the world to “jump”.
• Jump = Segment radius (cms)x reading addition

25. PRISMATIC EFFECT:
• When a wearer uses single vision lens and he gazes down to read
through the lens, he encounters a prismatic effect.
• Prism controlled bifocals

26. CHROMATIC ABERRATION:
• Transverse chromatic aberration (TCA) arising out of the oblique rays
is potential problem for the wearers of the single vision lenses as well
as bifocal lenses.
MAGNIFICATION:
• Small magnification difference between the distance and the reading
portion.

27. BIFOCAL FITTING:
• Bifocal segments must be positioned such that the distance and near
positions of the lens provide adequate fields of view for distance and
near vision respectively.
• While fitting the bifocal into a spectacle frame three factors must be
considered :
1) Segment inset
2) Segment width
3) Segment height

28. Segment inset:
• Difference between the subjects distance and near PD(2mm for each
lens)
• To ensure:reading fields for two segments will coincide with one
another.
Segment width:
• Different segment widths are usually available for straight toped
segments 22,25,28 and 35 mm respectively.
Segment height:
• Segment top is tangential to the lower edge of the iris.

29. PROCEDURE FOR TAKING MEASUEMENTS:
• Choose the final frame and adjust it to fit the subject correctly.
• If the frame is without demonstration lens,attach a vertical strip of
transparent adhesive tape to each eye of the frame to enable
reference points to be marked.
• Replace the frame on the subjects face and direct the subject to look
straight into your eyes.
• Your eyes are on exactly the same level as those of the subjects.
• Place dots at the same height as lower edge of subjects left and right
iris margin.

30. • IPD marking: draw a straight line from dot to another.
• Segment top: straight line to the lower eye wire of the frame(mm).
• Put back the frame on the face and verify.

31. Mechanical requirement of bifocal lenses:
• Spectacles should mot be unduly heavy or unsightly.
• Dividing line should be as inconspicuous as possible.
• Mechanically stable.
Optical requirement of bifocal lenses:
• Reading portion should be as clear as distance portion.
• Distance optic centre should coincide with distance visual portion.
• Near optic centre should coincide with near visual portion.
• There should not be introduction of prismatic effect by the segment
dividing line.

32. Points to be considered while dispensing
bifocals:
• Frame size and shape
• Type of bifocal and lens material
• Distance and near power
• Optical centre and segment inset
• Pantascopic tilt and facial wrap

33. Reference:
Books:
• Ophthalmic lenses – By Ajay Kumar Bhootra.
• The principles of ophthalmic lenses – By Mr.M.Jalie.