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![Introduction
Multifocal lenses meet wearer’s need for focusing light at more than one ,
or multiple distances (presbyopia).
Segmented multifocals are lenses having distinctly different powers in
sharply demarcated areas of the lens.
(in other words lenses with visible segments may be referred as
Segmented multifocal lenses).
If the wearer does have a correction for distance ,the extra required
power for near must be added on to the power found in the distance
prescription already being worn, hence the term near addition.
[ Near Power = Distance power + Near addition ]](https://image.slidesharecdn.com/segmentedmultifocallenses-220617080419-cc37934a/75/Segmented-Multifocal-Lenses-pptx-3-2048.jpg)
![Centration point or optical centre
For clear vision the visual axis of the eye should coinside with the optical
centre [OC] or centration point [CP] of the lens.
The near addition is usually placed on inferonasal part of the spectacle
because the eyes are lowered and converged while reading .
The near centration point lies 2MM nasal and 8MM below the distance
centration point.](https://image.slidesharecdn.com/segmentedmultifocallenses-220617080419-cc37934a/75/Segmented-Multifocal-Lenses-pptx-6-2048.jpg)
![Construction
Franklin’s bifocal
Fused bifocals
Solid bifocal
Cemented bifocal
Split bifocal [Franklin Bifocal]](https://image.slidesharecdn.com/segmentedmultifocallenses-220617080419-cc37934a/75/Segmented-Multifocal-Lenses-pptx-13-2048.jpg)
![Cemented Bifocal
Morck invented cemented bifocal in 1888.
A thin glass wafer is cemented onto the back surface of a single vision
major lens with canda balsam . [Later epoxy resin was used]
The front surface of the wafer had the same curvature as the back surface
of the major lens hence no power change occurred between them.
Disadvantages
• Dividing line collected dirt.
• The cement had a tendency to darken with use.
• The wafer had a tendency to fall.](https://image.slidesharecdn.com/segmentedmultifocallenses-220617080419-cc37934a/75/Segmented-Multifocal-Lenses-pptx-17-2048.jpg)
![Fused Bifocal
The first fused bifocal was the fused kryptok invented by John Borsch in the
year 1908.
The fused bifocal is produced by fusing a higher index flint button to the
countersink area in the front surface of main crown glass lens , under high
temperature [600-700 degree C].
After the segment button was fused to the major lens, the same curvature
was ground on the entire front surface of the lens.
Cylinder component is grounded on the back surface of the lens.](https://image.slidesharecdn.com/segmentedmultifocallenses-220617080419-cc37934a/75/Segmented-Multifocal-Lenses-pptx-18-2048.jpg)
![Round segment
They vary in size from small lens of 22 mm to the largest 40 mm.
A 22 mm round segment is often called a Kryptok.
The optical centre [OC] is always at the centre of the segment.
Segment names
Ultex bifocal
Blended bifocals (invisible segments)
Rede- Rite (Minus add upcurve)](https://image.slidesharecdn.com/segmentedmultifocallenses-220617080419-cc37934a/75/Segmented-Multifocal-Lenses-pptx-21-2048.jpg)
![Flat-top Segments
Ribbon and bar segments
Ribbon segments are basically round segments with the top and bottom cut off
The first straight-top bifocal to be patented was the "B" or bar segment. It was
designed by two Englishmen, Watson and Culver and was patented in 1931.
The "R" or ribbon segment was developed by Silverman and patented in 1932.
‘B’ [9 mm deep] and ‘R’[14 mm deep] segment.
These lenses are 22 to 25 mm wide.
The OC of ‘B’ seg is 4.5 mm below and for ‘R’ seg its 7 mm .](https://image.slidesharecdn.com/segmentedmultifocallenses-220617080419-cc37934a/75/Segmented-Multifocal-Lenses-pptx-24-2048.jpg)
![D-segment
In an unusual development, four men (Clement, Price, N.H. Stanley, and V.
Hancock) working independently almost simultaneously filed substantially
identical patent applications for the "D"-style segment (which looks like a capital
"D" lying on its back)
They are basically round segments with the top cut off [4.5 to 5.0 mm above the
OC].
Segment sizes range from 22 to 45 mm [mostly used are 28 or greater]](https://image.slidesharecdn.com/segmentedmultifocallenses-220617080419-cc37934a/75/Segmented-Multifocal-Lenses-pptx-26-2048.jpg)
![The Trifocal intermediate
Invented by John Isaac Hawkins in 1826.
The trifocal lenses have an intermediate area between distance and near
portion (arm’s length) for viewing objects that are’nt at a normal reading
distance and that are’nt possible to view clearly through the distance
portion.
The power of the intermediate portion is one half that of the prescribed
near add [i.e 50% of the near add].
At the present time almost all trifocals have a 7-mm-high intermediate
segment.](https://image.slidesharecdn.com/segmentedmultifocallenses-220617080419-cc37934a/75/Segmented-Multifocal-Lenses-pptx-33-2048.jpg)
More Related Content
Similar to Segmented Multifocal Lenses.pptx
Segmented Multifocal Lenses.pptx
- 1.
- 2. Content Introduction Terminologies Construction Types of Bifocal Bifocal fitting Trifocal intermediates Types of trifocal lenses Trifocal fitting Occupational multifocal lenses
- 3. Introduction Multifocal lensesmeet wearer’s need for focusing light at more than one , or multiple distances (presbyopia). Segmented multifocals are lenses having distinctly different powers in sharply demarcated areas of the lens. (in other words lenses with visible segments may be referred as Segmented multifocal lenses). If the wearer does have a correction for distance ,the extra required power for near must be added on to the power found in the distance prescription already being worn, hence the term near addition. [ Near Power = Distance power + Near addition ]
- 5. Terminologies CENTRATION POINT SEGMENT TOP SEGMENT HEIGHT SEGMENT WIDTH SEGMENT INSET SEGMENT DIAMETER SEGMENT DROP SEGMENT DEPTH MAJOR REFERENCE POINT SEGMENT SIZE
- 6. Centration point oroptical centre For clear vision the visual axis of the eye should coinside with the optical centre [OC] or centration point [CP] of the lens. The near addition is usually placed on inferonasal part of the spectacle because the eyes are lowered and converged while reading . The near centration point lies 2MM nasal and 8MM below the distance centration point.
- 7. Segment Top Itis the highest point of the segment in cases of curved tops. In case of flat top segment, it is the mid point of the upper straight line. Segment top should lie at the level of lower eyelid The top of the segment is usually placed 1.5-2.0mm below the distance centration point of the main lens.
- 8. Segment Width The sizeof the segment horizontally, or segment width, is measured across the widest section of the segment area. Major Referance Point The major reference point is usually also the optical centre of the distance lens when no prism has been prescribed.
- 9. Segment Depth Thelongest vertical measurement of the segment is known as the segment depth. Segment Drop The vertical distance from the segment top to the distance optical centre.
- 10. Segment Hight Itis the distance from the segment top to the lowermost point of the spectacle lens.
- 11. Inset The horizontaldistance from the geometric center of the lens to the major reference point is usually known as the inset. Segment inset The segment must be further decentered to correspond to the near PD. The horizontal distance between the major reference point and the segment centre is known as the segment inset.
- 12. Inset =Distance of decentration Total inset = Inset + seg. Inset Frame PD = A + DBL Total inset = Frame PD – Distance PD Total inset = (Frame PD – Near PD)/2 Seg. Inset = (Distance PD – Near PD)/2 Minimum blank size = ED + Total inset +2 = ED + (A+DBL)- dist PD +2
- 13. Construction Franklin’s bifocal Fused bifocals Solid bifocal Cemented bifocal Split bifocal [Franklin Bifocal]
- 14. Franklin Bifocal Thefirst recorded mention of bifocal spectacle lens is a letter written by Benjamin Franklin to his friend in 1785. He commented that he formerly wore two pair spectacle lenses of same material, one for distance and the other for near , he cut them in half and half of each kind mounted in a single frame. Franklin’s bifocal lens was similar in appearance to the executive one piece bifocal that is available today. Disadvantages • The dividing line produced annoying reflections. • The dividing line tended to collect dust and dirt. • Weak structure , that could easily come apart.
- 15. Solid bifocals Inventedby Isaac Schnaitmann in 1838. Solid bifocal can be considered as one piece bifocal, made from single piece of material. This lens was made by grinding flatter curvature on the upper portion of the back surface of the lens, thereby reducing the plus power sufficiently for distance vision. Disadvantages • Due to the limited choice of surface power, significant amount of aberrations were present in the distance portion , which restricted the field of vision. • The resurfacing of the upper portion of the back surface moved the optical centre of the distance portion below its original position which produces a strong base down prismatic effect.
- 17. Cemented Bifocal Morckinvented cemented bifocal in 1888. A thin glass wafer is cemented onto the back surface of a single vision major lens with canda balsam . [Later epoxy resin was used] The front surface of the wafer had the same curvature as the back surface of the major lens hence no power change occurred between them. Disadvantages • Dividing line collected dirt. • The cement had a tendency to darken with use. • The wafer had a tendency to fall.
- 18. Fused Bifocal Thefirst fused bifocal was the fused kryptok invented by John Borsch in the year 1908. The fused bifocal is produced by fusing a higher index flint button to the countersink area in the front surface of main crown glass lens , under high temperature [600-700 degree C]. After the segment button was fused to the major lens, the same curvature was ground on the entire front surface of the lens. Cylinder component is grounded on the back surface of the lens.
- 20. Types of bifocal Round segments Flat top segments Curve top and panoptic segment Ultex bi focal Franklin or Executive style segment Blended bifocal
- 21. Round segment Theyvary in size from small lens of 22 mm to the largest 40 mm. A 22 mm round segment is often called a Kryptok. The optical centre [OC] is always at the centre of the segment. Segment names Ultex bifocal Blended bifocals (invisible segments) Rede- Rite (Minus add upcurve)
- 22. Ultex bifocal In1910 Charles Conner, an optician from Indianapolis, developed Ultex bifocal. For the Ultex lens, the additional plus power for the reading segment is obtained by Using a large lens blank, the back surface is ground in the form of a saucer having a round disk át the center (which forms the near portion of the lens) and a flatter or less concave curvature in the periphery (forming the distance portion). After grinding, the blank is cut into two lenses, each having a segment height of 19 mm . If a segment height greater than 19 mm is required, only one lens is cut from the blank.
- 23. The Ultex-stylebifocal is currently available in the A, AA (or AL), and B segments. Ultex-A have a seg hight of 19mm. Ultex-AA or AL have 32mm seg hight, Ultex-B have have 22mm seg hight.
- 24. Flat-top Segments Ribbon andbar segments Ribbon segments are basically round segments with the top and bottom cut off The first straight-top bifocal to be patented was the "B" or bar segment. It was designed by two Englishmen, Watson and Culver and was patented in 1931. The "R" or ribbon segment was developed by Silverman and patented in 1932. ‘B’ [9 mm deep] and ‘R’[14 mm deep] segment. These lenses are 22 to 25 mm wide. The OC of ‘B’ seg is 4.5 mm below and for ‘R’ seg its 7 mm .
- 26. D-segment In anunusual development, four men (Clement, Price, N.H. Stanley, and V. Hancock) working independently almost simultaneously filed substantially identical patent applications for the "D"-style segment (which looks like a capital "D" lying on its back) They are basically round segments with the top cut off [4.5 to 5.0 mm above the OC]. Segment sizes range from 22 to 45 mm [mostly used are 28 or greater]
- 27. Curve-top and Panoptic Modifications of the D style have included the Panoptik, designed by Hammon; and the curved-top version, called the Widesite, designed by Price. Curve-top segments look similar to flat tops, except That the upper line is arched, rather than flat The curve top has distinct points on the either corner. Panoptic Segments are curved as well, but the corners are rounded. Segment widths available from 22 to 28 mm. OC is 4.5mm below.
- 28. Executive Executive isthe trade name of the Franklin style bifocal. It’s a one-piece lens with the segment extending the full width of the lens. Very wide near viewing area. As the add power increases , the segment ledge gets bigger making the lens thicker and heavier. The seg OC is on the segment line . The distance and seg OCs occupy exactly the same spot on the lens , this is why its also called monocentric bifocal.
- 29. Vertical Placementof the Optical Center of the Segment An important respect in which various bifocal styles differ from one another has to do with the distance from the seg centration point to the segment top. • the distance from the segment optical center to the segment top in the case of a 22-mm round fused segment is 11 mm. • the corresponding distance for a 38 x 19 mm Ultex A-style segment is 19 mm. • for a 22 x 16 mm Univis D-style segment, the distance from the segment optical center to the segment top is 5 mm. • For the Executive-style one-piece bifocal, the segment optical center is located at the segment top.
- 30. Lateral Placementof the Optical Center of the Segment The amounts of segment inset for various lens powers and interpupillary distances, as determined by Sterling, are shown in Table . The use of this table is recommended.
- 31. Inset =Distance of decentration Total inset = Inset + seg. Inset Frame PD = A + DBL Total inset = Frame PD – Distance PD Total inset = (Frame PD – Near PD)/2 Seg. Inset = (Distance PD – Near PD)/2 Minimum blank size = ED + Total inset +2 = ED + (A+DBL)- dist PD +2
- 32. A typical routineprocedure for taking measurement is 1. Choose the final frame and adjust it to fit the subject correctly. 2. Replace the frame on the subjects face and direct the subject to look straight into your eyes. 3. your eyes are on exactly the same level as those of the subject. 4. 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. 5. Remove the frame and put the frame face down with dots coinciding with a straight line. Draw a straight line from one dot to another 6. The position of the segment top is usually specified in millimeter from the marked straight line to the lower eye wire of the frame. 7. Put back the frame on the face and verify.
- 33. The Trifocal intermediate Invented by John Isaac Hawkins in 1826. The trifocal lenses have an intermediate area between distance and near portion (arm’s length) for viewing objects that are’nt at a normal reading distance and that are’nt possible to view clearly through the distance portion. The power of the intermediate portion is one half that of the prescribed near add [i.e 50% of the near add]. At the present time almost all trifocals have a 7-mm-high intermediate segment.
- 34. Types of Trifocal Split trifocals Cemented trifocals Fused trifocals Solid trifocals
- 35. Split trifocal Sameas franklin type bifocals Different power lenses were cut and fitted into the single frame.
- 36. Cemented trifocal Smallsegments of different powers will be cemented to the main lens. For positioning distance portion above, a main lens having the power of the Intermediate is taken then for distance a concave seg on above and a convex seg below is cemented. For positioning intermediate zone above, a a main lens having distance power is chosen, and two convex lenses are cemented at the upper and lower position having the required power.
- 37. Fused trifocal Commontype nowadays . Same like fused bifocals, composite buttons with different materials will be fused on to the main Distance Portion Lens Here the Intermediate portion is positioned in between Distance and Reading portion. Invisible Segments so – cosmetically looks good.
- 38. Types of fusedtrifocal Univis ‘Trivis’ Trifocal Tillyer ‘S724’ Trifocal Tillyer ‘Sovereign Curve Top’ Trifocal Panoptik Trifocal Shuron ‘Round Top’ trifocal Cromwell ‘Round top’ Trifocal Wiseman ‘Trivue’ trifocal Univis ‘Ultra CV’ Trifocal Double Semgment Trifocal
- 39. Univis trivis – The intermediate portion may be 6mm, 8mm, or 10 mm deep Seg. Diameter- 22mm. IP/RP ratio- 63%. Tillyer “S724” – Similar to the trivis design accept the intermediate seg.depth is 7mm. IP/RP ratio- 50%. Overall seg size- 24 X 22mm Tillyer “Soveraign curve top”- Intermediate depth- 7mm IP/RP ratio- 50% Overall seg size- 24 X 19 mm Panoptic trifocal- For +1.50 and +1.75 D it is 35% Intermediate depth – 6.8mm +2.00 to +2.75 D it is 55% IP/RP ratio varies with RP add +3.00 to +3.50 D it is 66%
- 41. Shuron Round top- Intermediate depth- 8mm Seg dia- 21.5 mm Cromwell round top- Intermediate depth- 7.5 mm Seg dia- 22 mm IP/RP ratio- 50% Wiseman Trivue trifocal- Its produced by fusing the reading seg into the intermediate Seg dia- 22.5 mm Intermediate depth- 6.5mm IP/RP- 50% Ultra C.V- The D shaped near seg is fused into the intermediate segment. IP/RP ratio- 50 %
- 43. Solid trifocals Thename SOLID – because all the distance, Intermediate and Reading portions made from the same material and Single Blank with different radius of curvature. TYPES ARE : Up and down curve , concentric and band trifocals Up curve and Down curve trifocal- Up and down curve trifocals are produced by working an upcurved distance portion into the upper portion of down curved solid bifocal. The diameter of the upcurve and its position in relation to the downcurve seg and the IP/RP ratio can be varied.
- 44. Concentric solid trifocal These are produced by working a second smaller segment in the centre an invisible solid down curve bifocal ,the bifocal blank remaining, at this stage , in its uncut saucer form. The periphery of the larger segment represents the boundary between distance and intermediate portions. The periphery of the smaller segment represents the boundary between intermediates and reading portion, The width of the intermediate concentric band is equal to the difference between the radii of the concentric segments. Band trifocals It has straight visible dividing lines between each portion. The optical centre of each segment is made to coincide with its dividing line thus eliminating jump between each portion.
- 47. Occupational multifocal Occupationalbifocals and trifocals are special multifocal lenses that are designed to help people over age 40 who are experiencing presbyopia perform specific work-related tasks, hobbies and other activities that require clear intermediate and near vision. Because occupational multifocal lenses typically have large intermediate and near zones (with only a small area of the lens devoted to far vision), these lenses should not be worn for driving and other tasks that require clear, unobstructed distance vision.
- 48. Double-D Lenses ForOverhead Near Work The Double-D design is an example of eyewear that has a D-shaped bifocal segment at the bottom of the lens and an upside-down "D-seg" at the top of the lens. The rest of the lens area provides distance correction. These lenses are designed for occupations like plumbers, pharmacists, librarians, electritians, auto mechanics ets.
- 50. Quadra focal lens The quadra focal lens is a double segment lens with a flat top trifocal in the bottom and an upside down flat top segment on the top. Rede- Rite bifocal It’s a so called upcurve bifocal because it has a large round segment at the top , most of it is cut off after edging. It’s a minus add , which means that the segment at the top has more minus power than the rest of the lens. This lens is a bifocal with a huge add area at the bottom and a small distance viewing area at the top.
- 52. E-D Lenses E-Dtrifocal. This lens has a relatively large distance zone, a full-width zone for intermediate vision and a D-shaped near segment. The E-D trifocal is suitable for someone who needs a clear, wide field of view at an intermediate distance and who also needs to see clearly both close up and in the distance.
- 53. Golfer's bifocal. Around-top bifocal can be used to create an occupational lens called a "golfer's bifocal. " The small, round reading segment is placed low and in the outside corner of one lens only. Typically the right lens has this segment for right-handed golfers, and the left lens has the segment for left-handed golfers. Referance - https://www.allaboutvision.com/lenses/multifocal.htm
- 54. Image jump Thefurther from the OC the eye looks , the greater will be the prismatic effect. For a single vision lens as the eye moves downwards from the OC the prismatic effect increases. Now if there is a near add lens is present, while the eyes moves downwards , when crossing the border of the segment the prism induced by the distance portion is suddenly changed by the prism induced by the seg. This abrupt change causes the object ti be suddenly displaced. This displacement of the image is called image jump.
- 55. For calculatingimage jump Prentice’s rule is used. Δ = cF
- 56. Referance- • Clinicaloptics • System for Ophthalmic Dispensing • The Principles of Ophthalmic Lens • https://www.allaboutvision.com/lenses/multifocal.htm es
- 57.