In the last of three articles under the heading of anisometropia, Andrew Keirl discusses single vision and multifocal spectacle lenses for the compensation of vertical differential (relative) prismatic effects. Module C39369, one CET point for optometrists and dispensing opticians
Prism-compensated lenses
The methods of eliminating vertical differential (relative) prismatic effect at the near visual points (NVPs) in spectacle lenses are well known. They are listed here for completeness:
- Single vision lenses
- Unequal bifocal round downcurve invisible segments
- Bi-prism single vision and multifocal lenses
- Split bifocals
- Presto (Norville)
- Bonded (cemented) segments.
Until, recently, glass, solid prism segment bifocals would have been included in the above list. They are however, no longer available.
Two separate pairs of single vision lenses
With this method, each pair is correctly centred, one pair centred for distance and the second pair centred for near vision. This is the best optical solution but is usually outweighed by the inconvenience factor. Single vision lenses may be used for relatively lengthy periods of close work or be combined with bifocals or progressive lenses for general use. Single vision lenses can be decentred downwards to allow the patient to use areas of each lens that are closer to the optical centre. For each millimetre of downward decentration, the pantoscopic tilt will of course need to be increased by 2°.
Unequal diameter bifocal; segments
This method uses round, invisible, downcurve segments, and works on the principle that the larger the segment, the more base down prism is produced, for example, a 40mm round segment produces more base down prism at the NVP than a 22mm round segment (Figure 1).
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The assumption here is that the segment positioning and the near additions are the same in the right and left lenses. The larger diameter round segment has its optical centre further away from the NVP than the smaller diameter segment and therefore introduces more base down prism when compared to the smaller segment. The larger segment is therefore used to neutralise or balance the vertical differential prism by adding prism to one lens. In the case of myopic anisometropia, the lower powered lens introduces more base up prismatic effect at the NVP and will therefore need the greater base down effect of the larger segment. In hypermetropic anisometropia the larger diameter segment is required for the higher-powered lens. The difference in segment diameters required (d1 – d2), is obtained by using the expression:
d1 – d2 = 20 x ??
Add
where ?? is the vertical differential (relative) prism in prism dioptres. This of course is simply another version of
c = P
F
where c is the difference in the segment radii, P is the differential (relative) prism at the NVP and F is the addition. Twenty in the above expression is a result of multiplying by two (segment radius to segment diameter) and then by 10 (cm to mm). When using round downcurve segments the maximum difference in segment diameter is currently
45-22 = 23mm and the larger segment diameter is always incorporated in the more positive or less negative of a pair. A 15mm round segment bifocal is available from the Norville Optical Group which can also supply freeform IRS blended segments in any diameter from 1mm to 50mm.
Example
Right + 3.00D Left + 1.00D
Add + 1.75D Add + 1.75D
The above lenses are to be dispensed as a pair of bifocal lenses and the NVP is assumed to be 10mm below the optical centre of the distance portion. Using
P = cF the vertical prismatic effects at the NVPs are 3.00? base up in the right eye and 1.00? base up in the left eye. The vertical differential (relative) prismatic effect is therefore 2? base up in the right eye (or 2? base down in the left eye). The difference in segment diameters required is therefore:
d1 – d2 = 20 x 2 = 22.8mm
1.75
In this example, we need to neutralise the base up vertical differential prismatic effect in the right eye. This can be achieved by adding base down prism. The right lens would therefore incorporate the larger diameter segment as the larger diameter produces more base down prism. The segment diameters used would be right round 45mm and left round 22mm. This method has optical limitations, but often the differential prism can be brought within tolerable limits. Cosmetically this is perhaps not the best solution. With regard to D-segment bifocals, there is generally insufficient difference in the vertical positioning of the segment optical centre of D-segment bifocals to have a significant effect on vertical differential (relative) prism as most D-segment bifocals have their segment optical centres (OS) very nearly the same distance from the segment top.
Solid visible prism segment glass bifocals
These lenses were withdrawn from supply last year and are no longer available. However, some patients will of course still be wearing these lenses and an alternative product will need to be supplied when new spectacles are required. The segment on a prism segment blank is depressed from the back surface of the distance portion, thus allowing prism in any direction to be worked on the segment. The base direction of the prism which has been incorporated into the segment of a solid visible prism segment bifocal can always be identified by looking for the thin (or level) edge of the segment ridge. Figure 2 shows a solid prism segment bifocal with base down prism incorporated into the segment. These lenses were particularly useful if the patient’s prescription called for bifocal lenses with different prism in the distance and near portions, for example, base in prism at near but not at distance. In such cases a split or Presto bifocal would be possible alternatives.
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Split bifocals
Split bifocals are essentially distance and near single vision lenses, which are cut in two and bonded together along the straight edge (Figures 3 and 4).
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Independent centration of the two parts is easily achieved, so any vertical differential (relative) prism at the NVPs is eliminated. When neatly produced, split bifocals look very similar to E-type bifocals. Split bifocals (and trifocals) are available to special order, and may be used when the prescription required is outside the range of other prism-compensated bifocals. This is a very versatile lens as the near prescription can be completely independent of the distance prescription. The author has used a split bifocal lens for a patient who requested a pair of bifocals with a photochromic lens for the distance portion and a white lens for the reading portion.
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Split bifocals can therefore be made exactly as required for centration, prism, prescription, tint etc. The segment top position is determined in the usual way. The disadvantages with this lens are that the dividing line between the two halves can be quite noticeable (depending on the prescription), the ridge can collect dirt and the lenses can separate even though they are bonded at the joint. Split bifocals are better glazed into a metal frame as the tension produced reduces the risk of the lenses coming apart. Split bifocals are often referred to as Franklin Split Bifocals.
A variation on the theme of a split bifocal is the Presto lens (Norville). The manufacturing process for this lens was originally developed for the fitting of spectacle-mounted LVA telescopic units into a carrier lens. It is now used for the manufacture of single vision and bifocal spectacle lenses that are no longer available or outside available semi-finished ranges. With regard to a Presto bifocal lens, the main or carrier lens essentially has a hole cut into it in the shape of a bifocal segment.
This hole then accepts an independently surfaced segment which is simply inserted into the hole. This method of manufacture means that the power, centration and colour of the segment can be completely different to that of the main lens. The Presto bifocal can also be used to provide unusual segment sizes or positions for specific occupational or vocational requirements. It can also be used to solve the problems of out of range bifocals or trifocals. A single vision Presto lens is available up to ±35.00D with a minimum bowl size of 20mm. Mixed refractive indices for the carrier and segment elements can be used which are useful in balancing lens thickness. High addition Presto bifocal lenses with a plano or powered (up to ±25D) distance carrier lens and segment prism to 15? are available. The usual segment shape is a flat-top bifocal with diameters from 25-35mm in 1mm steps. The segment can be positioned anywhere on the carrier lens. Figures 5 and 6 show a Presto bifocal lens with base in prism incorporated into the segment.
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Bi-prism lenses
Traditionally a bi-prism lens is a single vision, bifocal, trifocal or progressive power lens that has had base down prism removed (or base up prism added) by using the surfacing technique of ‘slabbing off’ (or ‘slabbing on’) (Figure 7).
[CaptionComponent="2840"]
This is usually done on one lens only, that is, the most negative or the least positive of the pair. A horizontal line on the lens results, this line being made to coincide with the top of the segment for bifocal lenses (Figure 8).
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If a bi-prism single vision lens is to be dispensed the slab line should coincide with the patient’s lower limbus. The use of freeform technology has revolutionised the production of bi-prism lenses and most lens types are now available as bi-prism lenses. Bi-prism progressive lenses are available in traditional (front surface slab) or digital freeform (rear surface slab) designs.
[CaptionComponent="2842"]
Freeform designs work well if the prism is split, which helps to address the physical band of prism ridge (change), resulting in an invisible ‘blend-line’ (Figure 9) as opposed to a sharp slab-line (Figure 10).
[CaptionComponent="2843"]
With a traditional front surface slab the minimum amount of vertical differential prism that could be removed was 1½ - 2? with harder lens materials (ie CR39) being preferred to softer (higher index) materials. With free-form designs there are no restrictions on prism, lens type or material. When dispensing bi-prism progressive power lenses, positioning the slab-line 4mm below the prism reference point has been suggested as a default position. This is particularly useful if the patient’s priority is reading. However, for use with a computer, it is a good idea to lower the line slightly if the patient is a laptop user and raise it slightly if the patient is a desktop user. The use of very short corridor designs of course raises the start of the intermediate zone. This means that the slab-line should be placed closer to or perhaps coinciding with the prism reference point. Current bi-prism availability is summarised as follows:
Single vision
- Sharp line inner slab off (traditional surfacing)
- Blend line inner slab off (freeform)
- Sharp line outer slab on (traditional surfacing).
Flat-top bi-prism bifocal
- Sharp line inner slab off (traditional surfacing)
- Blend line inner slab off (freeform)
- Sharp line outer slab on (segment side) (traditional surfacing).
E-style bifocal
- Inner sharp line (traditional surfacing)
- Inner blend line (freeform).
Progressive power lens
- Sharp line inner slab off (traditional surfacing)
- Blend line inner slab off (freeform).
Readers should note that the term ‘slab-off’ has been deprecated from current lens vocabulary.
The author was recently presented with the Hospital Eye Service prescription for dispensing shown below.
The patient had undergone recent cataract surgery and intraocular lens implantation which unfortunately resulted in acquired anisometropia. She was delighted with her distance vision but was distressed because she was told by the optometrist who performed the post-operation refraction that she would no longer be able to wear progressive power lenses. The patient was successfully dispensed by the author with a pair of bi-prism Ultor freeform progressive power lenses (Norville) with the compensating prism split between the two lenses (1.5? base down RE and 1.5? base up LE). This resulted in an almost invisible blend line on the inner surface of both lenses which was very difficult to see during verification. The patient was delighted!
When dispensing PPLs to anisometropic patients some practitioners advocate the use of a short corridor design in one eye and a long corridor design in the other. If we assume a difference in corridor length of 3mm along with a relatively high reading addition of say 2.50D, the maximum vertical differential (relative) prism that can be compensated is only 0.75?. This approach may also compromise the binocular design performance of the lenses for intermediate and near vision. In cases of anisometropia some manufacturers of PPLs suggest using a short corridor design in both eyes in order to keep the magnitude of any vertical differential (relative) prism to a minimum.
Cemented (bonded) prism segment
With this technique, the addition and prism required are combined in a segment, which is cemented to the back surface of the lens (Figure 11).
[CaptionComponent="2844"]
If this method is used for the compensation of vertical differential (relative) prism the segment is at its most cosmetically acceptable when base down prism is used, as the minimum ridge depth is then at the top of the segment. Any size or position of segment can be used, along with an ordinary non-prismatic element in the other lens. The prismatic element would be incorporated in the more positive or less negative of a pair. This technique has many uses in practice and is often used as a tool in problem solving. Some other applications of bonded lenses include prescriptions that request bifocal lenses with different prism in the distance and near portions, high reading additions, reading additions with out of range prescriptions and unusual segment sizes or positions.
On a personal note, the author would, if possible for the compensation of vertical differential (relative) prism, choose a bi-prism lens.
GOS definitions
The General Ophthalmic Services (GOS) defines a ‘complex lens’ as either a lens with a power in any one meridian of plus or minus 10.00D or more or a prism-controlled bifocal lens. Historically, the term ‘prism-controlled bifocal’ has been used to describe a glass, solid visible prism segment bifocal which of course has been recently discontinued. However, BS EN ISO 13666:2012 defines a prism-controlled bifocal (or multifocal) as a lens whose method of construction permits some independent control of prismatic effect or optical centration of the various portions of the lens. This document clearly states that this can include a ‘slab-off’ or bi-prism lens to reduce the vertical differential (relative) prismatic effect in anisometropia. With reference to GOS, a bi-prism lens can therefore be classed as a ‘complex lens’ and the appropriate NHS voucher can be issued. For patients entitled to a full voucher this would be a voucher H in all cases regardless of the distance or reading power. For patients who are not entitled to a full voucher a complex lens voucher can be issued. It is important to note that this is not a supplement. It can only be claimed in isolation and cannot be combined with a normal voucher.
Model answers
(The correct answer is shown in bold text)
1 This question relates to the information provided in Prescription A. Which of the following options would probably be the best choice of lens in order to control the vertical differential prism at near?
A A single vision bi-prism lens for the right eye.
B A single vision bi-prism lens for the left eye.
C Two pairs of correctly centred single vision spectacles.
D A split bifocal lens for the left eye.
2 This question relates to the information provided in Prescription B. Assuming that the near visual points are 10 mm below the distance optical centres, what will be the vertical differential prismatic effect produced?
A 2? base down in the right eye
B 2? base down in the left eye
C 2? base up in the right eye
D 3? base down in the right eye
3 This question relates to the information provided in Prescription B. The vertical differential prism in this case is to be controlled using bifocals with unequal diameter round downcurve segments. Which of the following statements is correct?
A The difference between the segment diameters needs to be 20 mm with the largest diameter segment used for the right eye.
B The difference between the segment diameters needs to be 20 mm with the largest diameter segment used for the left eye.
C The diameter of the segment used for the right eye needs to be as close to 20 mm as possible.
D The diameter of the segment used for the left eye needs to be as close to 20 mm as possible.
4 This question relates to the information provided in Prescription C. Which of the following lens options would be the best choice for controlling the vertical differential prismatic effect produced in this case?
A A pair of unequal diameter round segment bifocal lenses.
B A cemented prism segment used for the left eye.
C An E-type bi-prism bifocal used for the left eye.
D A pair of split bifocals.
5 Which of the following statements is correct?
A Differential (relative) prism should not be split when dispensing a bi-prism progressive power lens.When dispensing a cemented prism segment to control vertical differential prismatic effect, it is always preferable to use a segment with base up prism.A bi-prism E-type bifocal will display jump.
B When dispensing a cemented prism segment to control vertical differential prismatic effect, it is always preferable to use a segment with base up prism.
C bi-prism E-type bifocal will display jump.
D With reference to a bi-prism progressive power lens, 2? is the minimum amount of prism that can be removed by surfacing.
6 Which of the following statements is correct?
A It is always preferable to dispense a split bifocal into a plastics frame.
B When dispensing a single vision bi-prism lens, the slab line should bisect the optical centre of the distance portion.
C When dispensing a progressive bi-prism lens, the slab-off line should be placed 4 mm below the prism reference point.
D Unequal diameter D-segment bifocals are often used to control vertical differential prismatic effect.
Acknowledgement
The author would like to thank Mr Frank Norville for his helpful comments on an earlier version of this article.
Further reading
Fowler C and Latham Petre K. Spectacle Lenses: Theory and Practice Butterworth Heinemann Oxford UK, 2001.
Jalie M. Principles of Ophthalmic Lenses 4th edition The Association of British Dispensing Opticians London UK, 1984.
Jalie M. Ophthalmic Lenses & Dispensing 3rd Edition Butterworth Heinemann Oxford UK, 2008.
Norville Optical Prescription Companion 2014.
Ophthalmic Lens Availability.) The Association of British Dispensing Opticians London UK, 2014.
Tunnacliffe A H. Essentials of Dispensing 2nd Edition ABDO, 2003.
Andrew Keirl is an optometrist and dispensing optician in private practice, associate lecturer in optometry at Plymouth University, ABDO principal examiner for professional conduct in ophthalmic dispensing, ABDO practical examiner and external examiner for ABDO College