Bifocal lenses have two optical powers, one for distance vision and one for near vision. They are useful for presbyopia. There are several types of bifocal lenses including round, flat-top, and executive styles. Benjamin Franklin is credited with inventing the first bifocal lens in the late 18th century by cutting a single lens in half. Modern bifocals are manufactured using various techniques like fusing, cementing, or making from a single piece of plastic or glass. Proper positioning and design of the near segment is important to reduce issues like image jump and chromatic aberration. Bifocals come in many styles and materials to best suit individual needs and prescription requirements.

1. Bifocal lenses: its
types and principles
Presenters Moderator
Garima Paudyal Niraj Dev Joshi
Gauri Chaudhary

2. Contents:
• Introduction
• Manufacturing andTypes of bifocal lenses
• Optical characteristics of bifocal lenses
• Performance characteristics
• Theories of bifocal selection
• Bifocal fittings
• Uses and their advantages

3. Presbyopia
A presbyopic subject requires a separate correction for
distance and near vision, the two prescriptions may be
provided as one pair of spectacle in the form of a bifocal
lenses.

4. A bifocal lens is defined as having two portions of different
focal power.
Bi –two, focal-having focus
◦ The upper portion of the spectacle lens serves distant
vision and the lower smaller segment has power for near
generally.
◦ Range of Add-
0.5-20 Ds
Most commonly in practice =0.75-3.50 Ds

5. Anatomy of bifocal
Segment
width
Segment
depth
Segment
drop
Distance
portion

6. History and development of
bifocals
• Benjamin Franklin is credited
with the invention of bifocal lens
at around 1785
• He cut the lenses in half and
mounted half of each distance
vision lens and near vision lens
in the same frame

7. • Solid upcurve bifocal was invented by Issac Schnaitmann
of Philadelphia in 1837
• Cemented bifocal was invented by Morck in 1888
• Cemented Kryptok bifocal was invented by John Borsch
in 1899
• In 1908, fused Kryptok bifocal was invented by John
Borsch Jr.

8. Types of Bifocal Lenses

9. Types of Bifocals
Round segment
D-Segment
C-Segment
Panoptik
Ribbon type
Split
Cemented
Solid
Executive
Shape
ManufacturingMethod
Fused

10. Manufacturing of bifocal
• Glass bifocals are manufactured by both fused and
one-piece construction
• Fused bifocals are constructed of two or more
separate pieces of glasses of different R.I., fused
together at a high temperature
• One-piece glass bifocals are made from a single
homogenous piece of glass
• The increase in plus power in the reading portion is
provided by a change of curvature on either the front
or the back surface of the lens

11. • Plastic bifocals are made of either
thermosetting(CR-39) material or
thermoplastic(Polycarbonate) material
• Thermosetting bifocal lenses are made by cast
molding process
• Thermoplastic bifocals are made by a process
known as injection molding

12. Franklin bifocal
• a/k/a split bifocal
• Made of two separate lenses held together by the
frame
• Can be made in either glass or plastic materials

14. • Optical centers of both the distance and near
portion is at the dividing line
• Less chromatic aberrations
• Possibility of lenses coming out
• Dust accumulation at dividing line
• Dividing line produces annoying reflections
• Lens gets heavier as the add power increases
Advantages
Disadvantages

15. Cemented bifocals
• It was invented by Morck
• A small segment having same R.I. as the distance
lens is glued onto the distance lens
• Initially, the adhesive used was Canada Balsam,
but now ultraviolet cured epoxy resin is used

17. • Less optical aberrations
• Can be made of any power and positioned
anywhere into the main lens
• Chance of segment falling off
• Dividing line tends to collect dirt
Advantages
Disadvantages

18. Cemented Kryptok bifocal
• Invented by John Borsch in 1899
• It was the first bifocal lens to make use of lens of 2 different
R.I.
• It was manufactured by grinding a countersink curve into
the front surface of the major lens of ophthalmic crown
glass
• Then a wafer of flint glass was cemented into the
countersink area of, and the surface was covered with a thin
meniscus of glass cemented into place

19. • This lens was difficult to manufacture because six
surfaces had to be ground and polished
• The cover plate was thin and fragile, the cemented
surfaces tended to darken and the lens easily came apart
Contd…

20. Solid bifocals
• a/k/a one piece bifocals
• Invented by Issac Schnaitnamm in 1837
• Can be made from glass or plastic
• Made from one lens material
• The change in segment power is d/t change in
segment curvature

22. F1
F3
F2
F4
Approx. power of distance portion ,
Fapprox. = F1 + F2
Power of reading addition,
Fadd = F3 + F4
But, F3 = -F2
So,
Fadd = F4 – F2

23. • Better cosmetic appearance
• Less chromatic aberrations
• Limited choice of surface powers
• Optical center cannot be easily adjusted
Advantages
Disadvantages

24. Fused bifocals
• Invented by John Borsch Jr. in 1908
• Available only in glass
• The segment of the lens is made from glass having
higher R.I. than the carrier lens.
Eg. Flint glass segment in crown glass as carrier lens
• The segment cannot be felt

26. r2r1
r3
Major lens = п’
Segment = п’’
Air = п
Power of distance portion, Fd = (п’ – п’’/r1) +( п – п’/r2)
Power of reading portion, Fr = (п’’ – п/r1) + (п’ – п’’/r3) + (п –
п’/r2)
Near add = Fr – Fd = (п’’ – п’/r1) + (п’ – п’’/r3)

27. F1 = Fc + (п’ – п/п’’ – п’) × F near add
п’ – п / п’’ – п’, is the ratio
If ophthalmic crown and flint glasses are used,
then it is 3 : 1 i.e. the add will change by 1 D if F1
changes by 3D
Toric power is always added on the surface other
than that on which segment has been added

28. • The segment does not fall out
• The segment edges do not collect dirt and dust
• Lenses can be produced in large quantities at low
cost
• Segment was relatively invisible
• Flint segment produces high chromatic
dispersion
• The two materials should be fused at appropriate
temperature
Advantages
Disadvantages

29. Round segment bifocals
• Vary in size from a small lens of 22 mm upto the
largest 40 mm
• Most commonly used size is 22 mm
• The optical center is at the center of the segment

30. • A versatile lens because the segment can be
rotated and still not look tilted
• Segment can be located at odd locations on the
lens
Advantages
Blended bifocals are round-segment bifocals with the
border smoothened out to keep the segment from
being seen

31. D-Segment bifocals
• a/k/a flat-top segments
• They are round segments with the top cut off
• Segment sizes range from 22 upto 45 mm
• The optical center of segment is about 5 mm below
the segment line

32. • It allows the lens segment to have maximum
reading width where a person will be reading
• Less thicker then E-segment bifocals
Advantages

33. Curve-Top and Panoptik Segments
• Curve-top is also known as C-segment
• Similar to flat top except that the upper line is
arched with two distinct points
• In Panoptik segments, the top is curved but the
corners are round
• The optical center is located 4.5 mm below the
top

35. Ribbon segment
• Round segments with the top and bottom cut off
i.e. both top and bottom are flat
• Types
B-segment
R-segment

36. • Resembles brick layer
• B-segment is 9 mm deep
• The optical center is located 4.5 mm below the top
• Good for someone who needs distant vision below
the bifocal area
• Available only in glass
• It is 14 mm deep and optical center is located 7 mm
below the top
B-Segment
R-Segment

38. Executive bifocal
• Franklin style lenses
• It is a one-piece lens with the segment extending
the full width of the lens
• Monocentric i.e. optical centers for distance and
near portion occupy exactly the same spot on the
lens that is located on the line of segment
• Advantage of a very wide near-viewing area
• Gets bulky as the add power increases

40. Double segment bifocals
• They have 2 segments : one below eye level and
the other above eye level
• The separation between the two segments is 13
or 14 mm
• Useful for those who require close work above
eye level
• Useful for electricians, painters, auto mechanic
• a/k/a Occupational bifocal

42. Minus add bifocal
• Designed predominantly for close work
• Has a relatively small distance-vision window at top
• Has segment top above the center of the pupil
• Available in Ultex-style one-piece form called the
Rede-Rite bifocal
• Useful for those who require a large field for near
vision work. Eg, Barber, Postal clerk

43. 29 mm high Executive bifocal
Distance portion
Near
portion

44. Shape Name Segment
size(mm)
Round Kryptok
CFR(Lantz)
RS(Sola)
CFR(Vision Ease)
CFR(X-cell)
24
24, 25
22
22, 25
22, 25
Straight-top FT(Vision Ease)
D(X-cell)
Kote-Line(Lantz)
FT(Sola)
22, 28, 35
22, 25, 28, 35
25, 28, 35
25, 28, 35
Modified 22-C(Vision-Ease) 22 × 16
RepresentativeGlass Fused
Bifocals

45. Straight-top 25-C(Vision Ease)
28-C(Vision Ease)
P-24(Vision Ease)
25 × 17.5
28 × 19
24 × 16.5
Ribbon R-14(Vision-Ease) 22 × 14
Segments B-9(Vision-Ease)
Ribbon Seg(X-Cell)
Ribbon Seg(X-Cell)
22 × 9
22 × 14
22 × 9
Contd…

46. RepresentativeGlass One-Piece Bifocals
Shape Name Segment Size
Round Philtex One-Piece B, Style CC
segment on back surface
22
Hemispheric 1.Philtex One-PieceA, Style
CC; segment on back surface
2.Philtex One-PieceA, Style
CX;segment on front surface
3.Philtex One-PieceAL, Style
CC; segment on back surface
4.Philtex One-PieceAL, Style
CX; segment on front surface
5.Plus Base Ultex; segment
on front surface
6.UltexA; segment on front
surface
38 × 19
40 × 19
38 × 30
40 × 35
40 × 20
40 × 19

47. Contd…
Straight-top Executive
Bi-Line
Bifield
E-Style
E-Style High seg
25
25
25
25
29

48. Representative Plastic Bifocals
Material Index Style Manufacture
r
Segmen
t size
CR-39 1.498 Round Silor
Aire-o-Lite
SignetArmorlite
Sola
Vision-Ease
X-Cel
Younger
American optical
Philips
22, 24, 25
22, 25, 28
22, 24
22, 24, 40
22, 25, 40
22
22, 40
24
26
CR-39 1.498 Straight-top SignetArmorlite
Vision-Ease
American Optical
Silor
X-Cel
Younger
22,25,28,35
22,25,28,35
22,28
25,28,35
25,28,35
25,28,35,45

49. CR-39 1.498 Modified Straight
-top
Vision-Ease
Rodenstock
25, 28
28, 40
CR-39 1.498 Executive-style American Optical
Orcolite
SignetArmorlite
Silor
---
---
---
---
Spectralite 1.537 Straight-top Sola 28
High-index 1.55 Straight-top Younger 28, 35
High-index 1.56 Straight-top SignetArmorlite
Vision-Ease
Younger
28, 35
28, 35
28, 35
High-index 1.57 Straight-top X-Cel 28, 35
Polycarbonate 1.586 Straight-top
Executive-style
Aire-o-Lite
Vision-Ease
Gentex
Orcolite
Gentex
25, 28, 35
25, 28, 35
28, 35
25, 28, 35
---
High-index 1.60 Straight-top Optima
Silor
28
28
Contd…

50. Optical Characteristics of bifocal
lenses
• These vary widely from one bifocal style to other so they
should be taken into consideration when selecting a bifocal
segment style for a given patient.
Image jump
 Sudden change in prismatic effect due introduction of the
base down prism by the segment causes the world to
“jump”.
 Occurs when looking from distance portion to the near
portion of a bifocal lens.

51. • It has got both vertical and horizontal elements. Jump
occurs all around the edge of the bifocal segment, although
it is at the top part which is most frequently noticed.

52. • The amount of jump is simply the magnitude of the
prismatic effect exerted by the segment at its dividing
line i.e product of the distance from the segment top to
the segment optical centre (cm) and the power of the
reading addition.
• For Round Bifocal,
Jump = Segment radius * reading addition

53. • For B, C and D segments,
Jump = Reading addition × Distance to the centre of the
circle of which the segment is part from the top edge.
• For E segments, the jump is purely horizontal at points
away from the common centre because their distance and
near optical centres coincides at the dividing line.
The jump is completely independent of the power of the
main lens and the position of the distance optical centre.

54. • To eliminate the jump effect in a bifocal lens, it is
necessary to work the segment in such a fashion that its
optical centre coincides with the segment top.

55. Performance Characteristics of
Bifocal LensesA.Vertical placement of the optical center of the
segment
• All lenses function at their best when optic axis of the
lens passes through the center of rotation of the eye.
• The level of distance optical center of bifocal lenses is
determined on the basis of amount of pantoscopic tilt of the
lens.
• The distance optical center is lowered 1mm from the center
of the pupil for every 2 degree of the pantoscopic tilt.

56.  As a pantoscopic tilt of a 6 degree is cosmetically
desirable,the distance OC is normally lowered or located
3mm below the point in front of the center of the pupil.
 For fused and one piece round knife edged segment, OC
of segment lies at GC of the segment.
 For round kryptok segment,the distance fron segment
OC to segment top is 11mm from the top of segment,for
Ultex it is 19 mm,for straight top – 5mm,for Executive-at
the dividing line.

57. B. Lateral placement of the optical center of
the segment
• The segment OC should be
decentered inward from the
distance fitting points of two
lenses to concide with the
reading centers
• The segment OC seldom exact
at the reading center,may be
displaced horizontally or
vertically.

58. • The inward displacement of segment optical center from
the distance fitting point is segment inset.
• Segments are inset for two purposes:
i. To ensure that the FOV through the two segments
coincide or overlap for two eyes.
ii. To prevent the segment from producing any horizontal
prismatic effect at the reading center.
Note:The correct amount of segment inset depends on-
distance IPD,stop distance,fixation distance,the power of
the distance correction in the horizontal meridian.

59. Differential displacement
 Differential displacement at the segment top
 Differential displacement at the reading level
 Total displacement at the reading level
 Transverse chromatic aberration

60. Differential displacement at the
segment top
• In effectThree optical centres
i.e Distance OC , segment OC
and resultant OC
• Prentice rule
• Image jump = d Fa
d=distance from segment top to
segment pole
Fa= power of add
• Independent of distance power

61. • The amount of jump present at the top of the segment
depends on the prismatic effect of the segment at that
point.
• It is simply the differential displacement at the top pf
segment.
• This figure shows base down prismatic effect resulting in
apparent scotomatous area.

62. • Size of scotoma
• Linear height of apparent scotoma (h)= reading distance
(cm)/100cm per pd
• Size of the scotoma= image jump X h
• Zone of confusion (Blind area)
• Size of pupil
• Vertex distance
• add

63. Differential displacement at the
reading level
• Differential displacement = Da Fa
Da= vertical distance between
segment optical centre and reading
level
Fa= power of add
If the reading level is above the
segment center, a base down
prismatic effect is present.
If the reading level is below the
segment center, base up prismatic
effect is present.

64. Total displacement at the reading level
• Total displacement= dDFD +
dAFA
• dD=distance from distance OC of lens
to reading level
• FD= Power of distance portion of lens
• dA= distance from segment pole to
reading level
• FA= Power of add

65. Chromatic aberration
• For lenses of high power,Transverse CA can be a problem,
and patient may complaint of “color fringes”.
• Transverse chromatic aberration= dFd/V
• Transverse chromatic aberration of eye at reading level=
dDFD/VD+ dAFA/VA

66. Image jump in different designs
- Image jump is ZERO in executive design
- Image jump is least in D-bifocal
- Image jump is HIGHEST in Ultex design
- The lenses with the segment optical center located at or near
to the segment top are referred to as “No jump bifocals”

67. Theories of Bifocal selection
• The problems of image jump, differential displacement at
the reading level, and the total displacement at the
reading level led to three theories of bifocal selection.
i. Bifocal segment should be selected so that jump is
eliminated by choosing a bifocal with a segment pole
located at the dividing line. Ex: Straight top-one piece.

68. ii.The bifocal segment should be selected so that
differential displacement at the reading level is eliminated
by choosing a bifocal with a segment pole located at the
reading level. Ex: Straight top fused.
iii.The bifocal segment should be selected so that the total
displacement at the reading level is zero by selecting a
bifocal with a segment that provides prismatic effect
opposite that provided by the distance lens.

69. Marking of Bifocal height
• Using lower lid or limbal method by transparent
scale(tape)
• Tall person needs lower segment than usual,opposite
occurs for short person.
• More desk work- raise the segment

70. Segment top positions
• For kryptok segment(round)- at limbus level
• For D segment(straight top)-2 mm below limbus
• Executive segment-2 mm below limbus
• Curved top- 1 mm below limbus
• Trifocal- 2mm above limbus
• Smart segment- 2 mm above limbus.

71. BIFOCAL FITTING
• Bifocal segments must be positioned so that the distance
and near portions 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

72. 1. Segment inset
• It is specified as the difference between the subjects
distance PD and near PD.
• In the normal range of PDs, the near PD for a reading
distance of 40cms is 4mm less than the distance PD

73. • Segment inset is therefore usually specified as 2mm for
each lens.
• There are two reasons for insetting bifocal segment:
To ensure that the subject’s line of sight will go through
the segment at its optical center.
To ensure that the reading fields for the two segments
will coincide with one another.

74. 2. Segment width
- Different segment width are usually available for straight
top shaped segments in 22, 25, 28 and 35 mm.
- Some manufacturers also provide this option in round
shaped segments.

75. 3. Segment Height
 Bifocal lenses prescribed for general
purpose use are usually mounted before
the eyes so that the segment top is
tangential to the lower edge of the iris
 If the bifocal prescribed is mainly for near
vision, then the segment top might be
fitted little higher.
 If the lenses have been prescribed for
some vocational purpose and are to be
designed for only occasional near vision
use, then the segment top might be fitted
2-3 mm lower than the normal

76. Dispensing bifocal
Let the patient wear the actual frame that he will own
Frame is fitted on patients face such that it has optimum
tilt and cosmesis.
Segment height is taken from lower point of frame to
the lower lid of patient.
Subjective method of determining segment height

77. Counseling to patients
• Point letter not with eye but with chin.
• During walking, lean your head forward.
• Use lateral image that is out side segment as clue to walk

78. Uses of Bifocal lenses:
• Presbyopia
• Eliminates need for changing glasses for near and distance
work
• Accommodative insufficiency
• Accommodative esotropia
• Reduce the progression of myopia.
• In patients with aphakia and pseudophakia.

79. Condition for prescribing bifocals in
pediatrics
• Accommodative esotropia
• Congenital aphakia
• Esophoria
• Constant or intermittent esotropia
• Pseudophakic

80. Disadvantages of bifocal lenses
• Image jump when visual axis passes from far vision glass
to the reading segment.
• Degradation of image quality.
• Absence of intermediate vision.
• Difficulty with depth perception

81. References
• Clinical Optics byTroy E. Fannin andTheodore
Grosvenor
• System for Ophthalmic Dispensing
• Ophthalmic Lenses by Ajay Kumar Bhootra
• Internet