Bifocal Lenses PDF

# Chapter Five: Bifocal Lenses ## Introduction Bifocal lenses have the distance and near correction combined on a single lens but in two distinct areas. Each area has its own focal properties: - The area containing distance correction is called the distance portion or DP. - The area containing ne...

# Chapter Five: Bifocal Lenses ## Introduction Bifocal lenses have the distance and near correction combined on a single lens but in two distinct areas. Each area has its own focal properties: - The area containing distance correction is called the distance portion or DP. - The area containing near correction is called the reading portion or RP. The major portion is usually for distance, but there are exceptions. ## Bifocal Components > DP > RP > > DP > RP > > DP > RP > > DP > RP Positive spherical power is normally added to the distance prescription to form the reading prescription and is called the reading addition or add. A bifocal consists of two lenses: - **Main Lens:** Usually has the distance prescription (except in an upcurve bifocal). - **Segment:** Added to the main lens, with a power equal to the reading addition. > Main Lens > > - DP Rx > - RP Rx - DP Rx + Add > > Segment > > Add ## Bifocal Terminology - **Dividing Line (d):** The boundary line between the distance and reading portions. - **Segment Top (T):** The highest point on the dividing line, where a tangent to the segment, drawn parallel to the datum line of the lens, meets the dividing line. - **Round Segment:** A segment with a circular dividing line. - **Shaped Segment:** A segment where the dividing line is not a single circular arc (e.g., a flat top bifocal). > Shaped segments > > Flat Top > Moment by ## Downcurve and Upcurve Bifocals Bifocals with round segments positioned in the lower part of the lens are called downcurve bifocals. Bifocals with the curve inverted are called upcurve bifocals. - **Distance Optical Centre (OD):** The optical centre of the main lens (distance Rx). - **Segment Optical Centre (Os):** The optical centre of the lens forming the segment. - **Near Optical Centre (ON):** The optical centre of the reading portion. The position of the near optical centre depends on the positions and the distance and segment optical centres and may sometimes not lie on the finished lens. > OD > > ON > > Os ## Segment Terminology Bifocal lenses are positioned before the eye with the segment top lying on or just below the lower eyelid. > N > > Op at DVP > > NVP > > d-Bifocal segment position - **Segment Diameter:** The diameter of the circle of which the boundary of the finished segment forms a part. - **Segment Height:** The vertical distance from the segment top to a horizontal line tangential to the lens periphery at its lowest point. - **Segment Depth:** The vertical distance from the segment top to a horizontal line tangential to the segment at its lowest point, the segment nowhere extending to the lens periphery. > T > > ↑ > > T > > h > > bh > > 2 > > d > > Sh = d. > > Segment terminology ## Segment Specifications - **Segment Size:** A specification consisting of the segment diameter and the segment depth, usually giving only with shaped segments. - **Segment Top Position:** The vertical distance of the segment top above or below the datum line. The segment top position can be found when the vertical box dimension of the lens and the segment height are known, since segment top position = ½ (vertical box dimension) - segment height. - **Segment Drop or Cutting Instruction:** The vertical distance from the segment top to the distance optical centre. > Segment drop > > Segment drop or cutting instruction - **Geometrical Inset:** The distance between vertical lines which pass through the distance optical centre (or the point at which the distance optical centre would be placed in the absence of prescribed prism) and the midpoint of the segment diameter. - **Optical Inset:** The horizontal distance between the distance and near optical centres, any prescribed prism being neutralised. Bifocal segment positions are specified by stating the segment diameter or size, the segment height, the geometrical inset, and the segment drop (or cut). ## Example: 22 diam. X 17 high x 2½ in, cut 5 This means that the segment diameter is 22mm, the segment height 17mm, the geometrical inset 2½ mm, and the segment drop 5mm. If the finished lens was 40mm round, the segment top position would be 3mm below the datum and the specification might have been written: 22 diam x 3 below dat. X 2½ in, cut 5. ## Mechanical Requirements of Bifocal Lenses 1. The weight of the bifocal lens should not be noticeably greater than that of the equivalent single vision lens which the subject might wear for either distance or reading purposes. The purpose of a bifocal lens is to provide two pairs of lenses combined but this combination should not be unduly heavy or unsightly. 2. The dividing line must be as inconspicuous as possible. This is only possible if the added segment is made circular and as thin as possible with its optical centre coinciding with its geometrical centre. This type of a dividing line is inconspicuous and is said to be invisible. Generally the thinner the dividing line, the less the obstruction caused to the wearer when his eye crosses the dividing line. 3. Bifocal lenses should be permanent and mechanically stable. If the bifocal lens was made by cementing a segment to the main lens the resultant bifocal can only be as permanent as the union between the two parts. ## Optical Requirements of Bifocal Lenses 1. The reading portion should provide as clear a definition as the distance portion which will depend on the from of the lens and the contact between the components. The form of the lens should be such that aberrations are minimised in both the distance and near portion. Clarity through the near portion will be affected if the contact surface is poor. 2. The Op should coincide with the DVP unless prismatic effects are required, and the ON with the NVP (the centre of the near visual zone). With downcurve bifocals it is not difficult to place the Op at the DVP but the position of the ON will depend on the power of the lens and the positions of Op and Os. 3. When the On does not coincide with the NVP, the prismatic effects existing at the NVP due to this should still be under control. Any difference existing between the right and left NVPs should not exceed about 1 in the vertical meridian. The NVPs generally lie abut 10mm below and 2½ mm inwards from the DVPs. These values can be used to determine prismatic effects. ## Example: Consider the prescription R +2.00DS, L-2.00DS, Add 2.00DS made up in bifocal form with the following specification 30 x 15 x 2, cut 5mm. Each lens is 40mm round. RE: Power of the main lens = +2.00DS Power of the segment = +2.00DS The NVP lies 10mm below and 2 mm inwards from the OD. Therefore, CVM = 1cm and Chm = 0.25cm. Thus the prismatic effect at NVPR due to the main lens is 2 base up and 0.5 base out (using P=cF). The optical centres of the segment lenses lie 10mm directly below the NVPs. Therefore Cs = 1cm. Hence prismatic effect at NVPR due to segment lens alone is 2 base down. The total prismatic effect at NVP = 0.5 out. LE: Power of main lens = -2.00DS Power of segment = +2.00D5 The decentrations of Os and Os from the NVP are the same as for the right eye. Hence prismatic effect at NVP due to the main lens alone is 2 base down. The prismatic effect at NVPL = 4 down and 0.5 in. ## Various General Results a. A positive main lens exerts prism base up and out at the NVP. b. A negative main lens exerts prism base down and in at the NVP. c. A downcurve bifocal segment whose optical centre (i.e Os) lies below the NVP, exerts prism base down at the NVP. The segment centre is inset to lie below the NVP. d. If the reading addition and the segment diameters are the same in each eye, then the prismatic effect exerted by each segment will be the same. e. The base down prismatic effect exerted by the segment tends to reduce the base up prismatic effect exerted by a positive main lens. It may even completely neutralise it. f. The base down prismatic effect exerted by the segment increases the base down effect exerted by a negative main lens. The centration with negative downcurve bifocals is always poor. g. Even though the power of the reading portion is zero, the prismatic effect still exists in the near visual zone. > - DCO > > > ON > > G. > > > 11 > > i = geometrical inset > > > b = bodily > > o.i.optical inset decentration of Opio.i. > > Horizontal specificarions of segment ## Worked Examples Assume Op lies on the datum line. The NVP lies 10mm below and 2 ½ mm inwards from the Op. **Example 1:** Calculate the vertical and horizontal prismatic effects at the NVP of the following bifocal lens R+3.00 DS/+1-00 DC × 90. Add 1.50 Segment 22 x 4 below x 2 in. - Prism due to main lens = 3ª base up and 14 base out - Prism due to segment = 0.754 base down - Total prismatic effect = 2.254 base up and 1 base out. **Example 2:** Calculate the vertical and horizontal prismatic effects at the NVP of the bifocal lens L-6.00 DS/-2-00 DC×180. Add 2-50 Segment 45 x 5 below x 2 in. - Prism due to main lens = 84 base down and 1.54 base in - Prism due to segment = 4.44 base down - Total prismatic effect = 12.44 base down and 1.54 base in. **Example 3:** Calculate the vertical and horizontal prismatic effects at the NVP of the bifocal lens R+2.00 DS/+2-00 DC×60 Add 2.00 Segment 38 x 2 below × 2 in.' - Prism due to main lens = 2.724 base up and 1.74ª base our - Prism due to segment = 2.2 base down - Total prismatic effect = 0.526 base up and 1-745 base out. ## Jump There should be no sudden introduction of prismatic effect by the segment at its dividing line, or where the eye crosses the dividing line, in transition from the distance portion to the reading portion. > Jump > > Image jump is the name given to a sudden upward displacement of the image owhen an object is viewed first through the distance portion and then through the top of the reading portion. The segment exerts prismatic effects at all points within its circumference with the base of this prismatic effect lying at Os. As the eye views from the distance portion to the near portion, as the direction of gaze is lowered the eye meets a gradually increasingly prismatic effect as the line of vision moves away from the distance optical centre but just as it crosses the dividing line into the reading portion it suddenly meets the base down effect exerted by the segment. The effect is that the wearer sees all objects through the segment having jumped to a new position. This has been found to very disconcerting. > > The magnitude of the jump depends on the distance of Os from the point in question at the dividing line and the power of the segment lines, the power of the Add. For a round invisible segment, the distance Os to the dividing line is equal to the radius of the segment and so Jump in prism dioptres is radius of segment in cm x Add. Visual field loss at the periphery is also a consequence of jump. > > If the Os is placed on the dividing line then the jump is eliminated. Os can be moved upwards by working an appropriate amount of prism onto the segment lens. The amount of prism which must be worked is equal and opposite to the jump which it is required to climinate. Thus for the prescription +2.00DS Add 3.00 22mm segment, the jump at the dividing line is 3.3 base down and the segment must include 3.3 base up to neutralise it. Bifocal lenses in which the jump has been eliminated are called no jump bifocals eg. Executive Bifocals. ## The Optical Centre of the Reading Portion The centration in the reading portion of a positive downcurve bifocal is better than with a negative downcurve bifocal. The On must lie closer to the near visual zone when the DP is positive than when the DP is negative. The optical centre can be defined as a point at which there are not prismatic effects. Consider a positive bifocal lens. In the reading portion the main lens is exerting prism base up whereas the segment lens is exerting prism base down. There must be one point between On and Os at which the base up rism exerted by the DP exactly equals the base down prism exerted by the segment. This point is On the optical centre of the RP. > OD > > ON > > 74 > > Os The distance from Op to ON is x. The DP has a spherical power of F. The power of the segment lens (equal to the reading addition) is A and the distance from Op to Os which is the separation between the optical centres of the main lens or DP and the segment is s. For there to be an optical centre at On, the prism exerted by the main lens (xF) must be equal and opposite to the prism exerted by the segment (s-x)A i.e xF = (s-x)A or x = $A/A+F (if s is substituted in mm then x will also be given in mm) In this expression s and A are always positive. The x will be positive or negative depending on the value of F. When x is positive On lies below On but when x is negative On lies above On i.e not in the reading portion at all. ## Examples **Example:** Find the position of the optical centre for near in the following bifocal lenses (i) +4.00 DS Add 2.00 22 mm segment cut 4 mm, (ii) -5.00 DS Add 1.00 30 mm segment cut 3 mm. (i) We have F = +400A X = $SA/A+ F +2.005 = 15 mm, hence = 15x2 6 = +5 mm i.e., On lies 5 mm below On and 1 mm below the segment top. This result is easily checked. At this point the main lens is exerting xFA base up, i.e. 2ª base up and the segment lens is exerting (s-x) A base down, i.e. 24 base down. The two effects neutralise one another. (ii) We have F = -5.00 A + 1.00s = 18 mm, hence x = 18 x 1 -4 = -4.5 mm, i.e., On lies 4.5 mm above Op or 7.5 mm above the segment top. This result may also be checked. At this point the main lens is exerting 2.25 base up and the segment is exerting 2.254 base down. In this case the optical centre for near is virtual and can only be located if the segment is extended in diameter, Os remaining in the same place, to include the point 7.5 mm above the original segment top. It might be of service to consider what happens to On as the power of the main lens, F, varies. We have x = SA A+ F s and A are positive. When Fis positive x is also positive and always less than s, since (A + F) > A. Then On lies between On and Os. When Fis 0xs (as we have already seen) and On coincides with Os. When Fis negative but less than A, x is positive but greater than s, i.e. On lies below Os. When F is negative and A, then (AF) = 0 and the RP cannot have an optical centre. When F is negative and greater than A, x is negative and On lies above OD. ## Segment Inset So far an inset of the segment has been ignored. If the segment is inset the horizontal displacement of the segment centre Os, from the distance optical centre Op may be called SH and we have XH = SHA/A+ F If XH is positive, On will lie on the nasal side of OD whereas when XH is negative ON will lie on the temporal side of OD. **Example:** Find the position of the optical centre for near in the following bifocal lens. L+2.00 Add 2.00, 38 mm segment, drop 3 mm, inset 3 mm. We have F=+2-00, A +2.00, sy=22 mm, SH3 mm. xy=SyA/(A+F)=22×2/4+11 mm XHSHA/(A+F)=3×2/4+1½ mm !i.c. O lies 11 mm below and 1½ mm inwards from Op. This last example neatly summarizes the principle for locating the optical centre of the reading portion of a bifocal lens. We have combined two lenses of the same power, +2-00 for distance vision with a segment element of +2.00. to produce a reading portion of power +4-00. The optical centres of these two lenses are separated by 22 mm vertically and 3 mm horizontally. The optical centre of the +4-00 reading portion must lie midway between the two original' optical centres, i.e. 11 mm down and 12 mm in. ## Differential Prismatic Effects in the Near Visual Zones It is important that with bifocal lenses there is control over the prismatic effects which exist in the near visual zones, since th eye should not be called upon the tolerate large amounts of differential prismatic effect especially in the vertical meridian where the tolerance is 1. With bifocal lenses the eyes are forced to use extra-axial zones of the lenses and if we assume that the near visual points lie 10mm below the distance centres it will be realised that 1.00D of vertical anisometropic difference between each eye produces a 1 differential prismatic effect in the vertical meridian. Fortunately many bifocal designs permit some control over these differential prismatic effects and are called prism-controlled bifocals. ## Calculating Differential Prismatic Effects Consider the prescription: R+2.00DS/+1.00DC x 90 Add 2.00 L+4.00DS/+1.00DC x 90 made up as a cement bifocals with the specification 22 x 4 below x 2 1/2 in,cut 4mm. The prismatic effects at each NVP situated 10mm below and 21/2 inwards from the distance optical centres are: RE - prism due to main lens: 2 up and 0.75 out - prism due to segment: 1 down - total prismatic effect: 1 up and 0.75 out LE - prism due to main lens: 4 up and 1.25 out - prism due to segment: 1 down - total prismatic effect: 3 up and 1.25 out The differential prismatic effect is therefore LE 2 up and 2 out. # The Evolution of Bifocal Lenses > "Although some of the archaic varieties are now of historical interest only they have been included because they are so obviously the ancestor of a particular modern design." ## Split Bifocals > Component > > Lenses > > OP > > RP > > DP > > Curved > > Split > > Bifocal > > OP > > DP > > RP > > Franklin > > Bifocal > > DP > > RP > > Split "perfection" > > Spiir bifocals The first bifocal lens was made early in the 18th century by G Hertel but is usually accredited to Benjamin Franklin (American statesman) who in 1784 described the pair of bifocals he was wearing. The story goes that Benjamin Franklin was on a train and looked at the lady across the carriage. To help her read her book and see outside the train she was using two pairs of glasses at once, one perched on top of the other. He hit upon the idea of encasing two lenses in the same frame. The bifocal was born. The Franklin split bifocal is still occasionally used today. The Franklin bifocal is generally knows as split bifocals. It consists of tow separate lenses one for distance and one for near vision cut in half and then edged so that their contact edges are parallel to one another. This combination is then placed into a frame, usually a metal frame because it is difficult with a plastic frame. In a metal frame they are easily screwed up against one another. In the curved split bifocal, the reading segment is downcurved. An improved version of the split bifocal was introduced towards the end on the 19th century and is known as the "perfection" bifocal which has one of its contact edges bevelled to fit into a groove cut into the edge of the other component. Split bifocals meet most of the optical requirements of bifocal lenses in that optical centres of each portion can be placed in any desired position and any prescription requirement can be met. Furthermore, prisms can be incorporated independently in the distance and near portions. Mechanically however these designs are poor in that the contact is edge is very visible and can splinter in time, and dirt tends to accumulate along the ledge formed at the dividing line. ## The Upcurve Bifocal > DP > > DP > > DVP > > B > > B > > RP > > RP > > The upcurve bifocal > > ON This lens is primarily a near vision lens into the upper half of which is depressed a concave segment for distance vision. The main lens is therefore the reading portion and the segment lens added to this to form the distance portion is negative. The lowest point on the dividing line is called the segment bottom and the vertical distance of the segment bottom above or below the datum line is called the segment bottom position. The reading portion can be centred so that the NVP coincides with the near optical centre ON. The centration at the distance visual point depends on the power of the main lens and the concave segment. Since the major portion is used for near vision, the reading prescription is written followed by the power required to produce the distance prescription - prefixed by a minus sign. eg. Dist: R: +3,00DS NV: R: +5.00DS L: -1.00DS L: +1.00DS is written as follows: R +5.00DS L: +1.00D5 Add: -2.00DS for distance vision Generally upcurve bifocals are only prescribed for negative prescriptions since the poor centration produced in the distance portion when the main lens is convex permits the DP to be used only for casual vision. ## Cemented Bifocals > DP > > RP > > DP > > RP > > -6,00 > > +6,00 > > +7.00 > > -4,00 > > +8,00 > > +1.00DS/+1.00DC x 90 > > Add 2.00 > > Cemented bifocals The cemented bifocal consists of a segment lens cemented to the mains lens with the contact surfaces being of identical curvature. Previously Canada balsam was used but has now been replaced by epoxy resins. This was because Canada balsma used to discolour with age. Any size or shape segment can be cemented to the main lens and any desired centration can be obtained. As early as 1906 the improved cemented bifocal was produced and called "film wafer" since the segment was so thin that it was difficult to handle. From an optical standpoint, cemented bifocals are excellent because of the size, shape and position of the segment together with the optical centration in both portions of the lens, can be controlled at will. They are thus particularly useful for vocations whosc visual demands lie outside that provided by the more popular types of bifocals. ## Cement Kryptok bifocals: > Main lens > > Cover lens > > Cement kryptok or inset cement bifocal This bifocal consists of a segment of high refractive index glass cemented into a depression worked on the main lens. The back surface of the segment is then covered by a plano lens cemented tot he back surface of the whole lens. These inset cement bifocals are named "kryptok" bifocals. (kruptos = hidden (greek)). ## Fused bifocals: > a) > > Button > > Crown > > blank > > > > b) > > Wire > > feeler > > Gum > > Fused blank > > Semi-finished > > fused blank > > ८) > > aseu b > > Refractory block > > ↑ > > ↑ > > MANUFACTURE OF FUGED BIFOCAL > > One of the most popular designs used. The reading portion is almost invisible in wear since the dividing line can be made knife-edge when the segment is circular. The reading addition is produced by fusing a segment lens of high refractive index glass into a depression worked on the main lens, the addition being made up partly by the difference between the contact surface powers and partly by the curve chosen to work the outer surface of the segment. The segment portion is then ground and polished to the same curvature as the main lens. If the main lens required an astigmatic correction then this is worked on the opposite surface from that of the segment. The glass used to form the segment is chosen carefully for its physical properties, especially its coefficient of expansion since it must cool at the same rate as the crown glass to which it will be fused at high temperature. If this could not be fulfilled the greater contraction of one component on cooling would cause the other component to fracture. > > The original bifocals used flint glass which had a high dispersion resulting in chromatic aberration noticed at the dividing line of the segment. This effect is more pronounced with a larger segment diameter and increase in power. However with the introduction of higher refractive index glass with low dispersion, this problem is considerably reduced and segments of larger diameter are readily available. The disadvantage of high 'n' though is that they are more prone to scratching and damage, and one cannot work prisms onto the segments. The advantages of fused bifocals are that the segment is inconspicuous and is available in a wide variety of shapes. There are also inexpensive to manufacture. A variety of shapes are available and the most common being the rounded segment and the flat-top. - **Rounded Seg:** Adv: seg dividing line not always visible therefore cosmetically appealing. Disadv: small field of view, flint glass used for near seg is of high 'n' which causes increased chromatic aberrations and surface reflections. - **Flat Top:** Adv: optical centre of seg closer to dividing line therefore less jump and available in larger seg sizes which mean larger field of view. Disadv: increased chromatic abberations and surface reflections, more visible dividing line therefore cosmetically unappealing. ## Shaped fused segments: In the 1920s a series of fused bifocals with shaped segments was introduced under the name Univis (Universal Visibility). A portion of the round segment lens is removed to produce the particular shape and also results in an improvement in the optical performance of fused bifocals. Different forms include Univis A, B, C, D and differed in the positioning of the segment optical centre Os. The production of shaped segments was made possible by the introduction of the composite button which is made up of both crown and flint glass, the crown component having the same refractive index as the crown main lens to which the button is fused. The crown portion of the button therefore disappears into the main lens when the blank is semi-finished. ## Invisible solid bifocals: Solid bifocals were introduced in 1905 and are ground from one piece of glass, the reading addition being produced by grinding a second curve onto one surface of the lens. The change in curvature at the segment dividing line can be felt with the fingers, a slight ridge which is formed marking the point where the curvature changes. The introduction of the invisible bifocal was made possible by the construction of special surfacing equipment and the development of new surfacing techniques. ## Visible solid bifocals: The invisible solid bifocal offers little control over the centration of the reading portion once the diameter of the reading segment has been fixed. The advantages of solid bifocals are the availability of larger diameter segments, chromatic aberration is negligible, more scratch resistant (glass) and control over the centring of the segment by the use of worked prisms. The solid bifocal available now is solid '38'. ## Other: - **Executive:** Has a straight visible dividing line situated on the front surface of the lens. The optical centre of the segment is positioned on the dividing line thus fulfilling the condition of "No jump". It also provides a larger field of view due to the size of the segment portion. The disadvantages include high cost, cosmetically unappealing and the dividing line collects dust. - **Seamless bifocals:** Introduced by Younger Optical, and has a dividing line which is truly invisible since the change in curvature between the distance portion and reading portion is made continuous by a third curve. They are thus sometimes called "blended" bifocals. - **Lenticular bifocals:** These have been devised to overcome the mechanical disadvantages inherent in high powered bifocal lenses. In addition to improving the mechanical requirements of high powered lenses, some designs also attempt to fulfil the optical requirements of bifocal lenses especially to improve the centration of the reading portion. The eyes are however forced to use extra-axial zones of the lenses. > Univis A > > Univis B > > C > > C > > C > > C > > F > > F > > F > > F > > C > > C > > Univis B > > Univis D > > Univis C > > Univis D > > Composite buttons > > - > > = > > Uniris segments