Bifocal lenses: types and principles

Bifocals

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

2. Contents: Introduction Manufacturing and Types 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 K ryptok 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 Manufacturing Method 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

13. 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

14. 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

15. 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

16. 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

17. 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 …

18. 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

19. 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

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

21. 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

22. r2 r1 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)

23. 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

24. 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

25. Round s egment 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

26. 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

27. 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

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

29. 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

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

31. 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

32. 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

33. 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

34. 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

35. 29 mm high Executive bifocal Distance portion Near portion

36. Shape Name Segment size(mm) Round Kryptok CFR(Lantz) RS(Sola) CFR(Visio n 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 Representative Glass Fused Bifocals

37. 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 …

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

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

40. Representative Plastic Bifocals Material Index Style Manufacturer Segment size CR-39 1.498 Round Silor Aire -o-Lite Signet Armorlite 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 Signet Armorlite 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

41. CR-39 1.498 Modified Straight -top Vision-Ease Rodenstock 25, 28 28, 40 CR-39 1.498 Executive-style American Optical Orcolite Signet Armorlite Silor --- --- --- --- Spectralite 1.537 Straight-top Sola 28 High-index 1.55 Straight-top Younger 28, 35 High-index 1.56 Straight-top Signet Armorlite 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 …

42. 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.

43. 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.

44. 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

45. 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 .

46. 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.

47. Performance Characteristics of Bifocal Lenses A. 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.

48. 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.

49. 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 T he segment OC seldom exact at the reading center,may be displaced horizontally or vertically.

50. The inward displacement of segment optical center from the distance fitting point is segment inset. Segments are inset for two purposes: To ensure that the FOV through the two segments coincide or overlap for two eyes. T o 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.

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

52. Differential displacement at the segment top In effect Three 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

53. 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.

54. 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

55. 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.

56. 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

57. Chromatic aberration For lenses of high power, Transverse CA can be a problem, and patient may complaint of “color fringes”. Transverse chromatic aberration= dF d /V Transverse chromatic aberration of eye at reading level= d D F D /V D + d A F A /V A

58. 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”

59. 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.

60. 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.

61. 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

62. 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 .

63. 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: Segment inset Segment width Segment height

64. 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

65. 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.

66. 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.

67. 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

68. 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

69. 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

70. 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 .

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

72. 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

73. References Clinical Optics by Troy E. Fannin and Theodore Grosvenor System for Ophthalmic Dispensing Ophthalmic Lenses by Ajay Kumar Bhootra Internet Thank you