C37954: Essential course in dispensing - part 15
Closing Date: 10/10/2014
As mentioned in Part 14 of this series, the consideration of the field of view provided by a spectacle lens is important when dispensing prescriptions for the correction of high ametropia. The normal monocular static visual field for a right eye is shown in Figure 1. Assuming that there is no relevant pathology, this would be the monocular static field of view enjoyed by an emmetrope or a contact lens wearer (assuming that the contact lens or its optic zone is not very small in diameter). With reference to the field of view provided by a spectacle lens, the static field of view is the total area visible through the lens. It is usually expressed as an angular measure and is defined as the maximum angular extent of vision obtainable through the lens. Factors affecting the field of view of a spectacle lens include the aperture size, lens power and back vertex distance. To obtain the maximum field of view, whatever the size of the aperture might be, the spectacle lens should be fitted as close to the eyes as the lashes permit.
There are two common terms that are used when discussing the field of view of a spectacle lens. These are the real and apparent fields of view. The apparent field of view (Figure 2) is the angle subtended by the empty frame aperture at the eye’s centre of rotation, whereas the real field of view is the field of view obtained when a spectacle lens is glazed into the frame. If the fitting distance (the distance from the back vertex of the lens to the eye’s centre of rotation) and the size of the lens aperture are known the apparent field of view (2a) can be calculated using trigonometry.
When comparing the real and apparent fields of view it is important to note that the static real field of view provided by a positive spectacle lens is less than the apparent field of view implied by the empty spectacle frame (Figure 3). This means that hypermetropes suffer from a decrease in field of view and there will be an area around the edge of a lens from which no light can enter the eye (a ring scotoma).
However, the static real field of view provided by a minus spectacle lens is greater than the apparent field of view implied by the empty spectacle frame (Figure 4).
Myopic subjects therefore benefit from an increase in field of view but there will be annular area around the lens periphery where objects will be seen in diplopia. To calculate the real field of view the position of R' (the image of R formed by the spectacle lens) is found using paraxial ray-tracing. 2? (the real field of view) can then be determined using trigonometry if the size of the lens aperture is known. As an example, a round lens, 48mm in diameter is fitted at a distance of 25mm from the centres of rotation of the eyes of a 10.00D myope and a 10.00D hypermetrope. The apparent static field of view produced will be 87.7°. The real static field of view produced in the myopic case will be 102.7° (an increase) whereas the real static field of view produced in the hypermetropic case is 71.5° (a decrease).
Near vision effectivity error
The vergence of light (from a distance object) leaving the back surface of a spectacle lens (Figure 5) is known as the back vertex power (BVP). If L1 = 0 then BVP = L2'. When considering light arising from a distant object the form of the lens is immaterial. In other words, provided that the BVPs are the same, in distance vision, lenses of different forms are interchangeable.
Figure 5: Provided that the BVPs are the same, in distance vision, lenses of different forms are interchangeable
However, in near vision, light arriving at the spectacle lens originates from a given point at some distance in front of the lens. The vergence of light leaving the back surface of the lens will depend on the BVP of the lens but also the form and thickness of the lens and the near object distance. Consider the following example. A +10.00D lens made with a -3.00D base curve, in glass, n = 1.5 with an axial thickness of 9mm is used for near vision with the near object placed at -33.3cm from the lens. The front curve of the lens will be +12.06D. Paraxial ray-tracing will show that the vergence leaving the back surface of the lens is +6.58D (Figure 6).
Figure 6: In near vision, the vergence impressed by a lens depends not only on its BVP, but also upon its form and thickness. Here, a +10.00D lens made with a -3.00 base curve, in glass, n = 1.5 and an axial thickness of 9mm, is used for near vision at -33.3cm
The difference between the vergence of light actually leaving the lens when the light originates from a near object (L2') and the anticipated vergence obtained by adding the incident vergence and the BVP of the lens (L1 + BVP) is known as the near vision effectivity error (NVEE). If the same lens was made in plano-convex form with the flat surface facing the eye the vergence leaving the lens will be +6.69D. If the form of this +10.00D lens is changed to equi-convex the vergence leaving the lens will be +6.87D. So to summarise, we have three lenses, all with the same BVP (+10.00D as measured using a focimeter), thickness and refractive index and all three lenses are used to view a near object at a distance of -33.3cm. However, because the three lenses are made in three different forms the vergence leaving each lens is different.
In each case the anticipated vergence (found using L1 + BVP) for the three forms is -3.00 + 10.00 = +7.00D. The curved lens is therefore ‘underpowered’ by 0.42D (NVEE = -0.42D), the plano-convex lens by 0.31D (NVEE = -0.31D) and the equi-convex lens by 0.13D (NVEE -0.31D). In practice, the lens dispensed to the patient is likely to be in curved form. In order to compensate for the NVEE of -0.42D, a lens with a stronger BVP needs to be ordered. A +10.50D would be appropriate in this case. NVEE values for curved plus lenses are given in Table 1. Rodenstock has produced values for NVEE compensation which are now published in Ophthalmic Lens Availability (ABDO College.)
So NVEE occurs in near vision with high plus lenses. It compares the actual vergence leaving a finished spectacle lens, with the anticipated vergence determined in the consulting room, and depends on the form of the trial lens, the form of the final lens dispensed and the near object distance. In cases of high astigmatism, NVEE can also cause a variation in the required cylinder power, resulting in different cylinders for distance and near.
The presence of a high cylinder within a prescription can potentially be a difficult aspect of spectacle dispensing and is sometimes the cause of a non-tolerance. The presence of a high cylinder combined with poor frame selection can result in a less than satisfactory finished lens with often unexpected lens edge substances. Practitioners need to be able to estimate how the cylinder will affect the edge thickness of the finished lens. To do this we must of course compare the principal meridians and principal powers with the shape and dimensions of the chosen frame. It is important to remember that the power of a cylinder is always at right angles to the cylinder axis. As an example, consider the prescription:
Right -5.00/-3.00 x 180
Left -5.00/-3.00 x 90
For the right lens the principal powers are -5.00D along 180 and -8.00D along 90. For the left lens the principal powers are -5.00D along 90 and -8.00D along 180. A round shape would of course give unequal edge thicknesses with the horizontal meridian of the left lens producing the thickest edge substance. The shape and size of the frame used to provide this prescription must be given careful consideration. If a modern shallow oval shape was employed, the smaller vertical dimension will produce the thin edge and the horizontal dimension the thick edge. In this example a 45mm x 32mm oval lens shape was considered.
For the right lens the maximum and minimum edge thickness was 4.79mm (H) and 4.09mm (V). The maximum and minimum edge thickness for the left lens was 6.47mm (H) and 3.29mm (V). The difference in the thick and thin edge substances are therefore more pronounced in the left eye because the meridian with the strongest power corresponds with the longest dimension of the lens. The maximum and minimum edge thicknesses of the right lens are much more balanced. In this case it may be sensible to use a frame that has a reduced horizontal eye size for example, a ‘rounded’ quadra. However, this may ‘unbalance’ the edge thicknesses of the right lens. The above values were calculated using the Hoyailog system.
When dealing with oblique cylinders, the finished lens can become more difficult to visualise and it is vital to match and compare the principal powers of the lens with the frame dimensions. If the prescription
RT +0.50/+5.50 x 45
LT +0.50/+5.50 x 135
was matched with a frame that had an aviator or contour shape, the nasal edges of the lens would be very thick along the 135 meridian of the right lens and the 45 meridian of the left lens. The thin edge would of course be at 90 degrees to the thick edge. If the principal powers were reversed (axis 135 in the right eye and axis 45 in the left eye) differences between the thick and thin edges would be less extreme. Due to the nature of the above prescription and in particular, the curvature along the axis meridian of the lens, it is interesting to note that the centre and thick edge substances of the lens will be almost equal.
When dealing with prescriptions of this nature it is necessary to employ minimum substance surfacing techniques, resulting in an elliptical-shaped uncut lens. It is also helpful to talk to your surfacing laboratory for advice on what may and may not work! The current trend in acetate spectacle frames means that the thicker rim of a plastics frame may help to disguise variations in edge thickness. There are also some metal frames designed specifically for this purpose such as Stepper SI 60012 (Figure 7). The lens glazed into this frame includes a -11.00D cylinder manufactured using 1.7 index glass (Norville).
When considering lens thickness at points other than those corresponding to the principal meridians, it is necessary to estimate the cylinder power along the meridian in question. It should be remembered that there is no power due to the cylinder along its axis. However, maximum cylinder power occurs at right angles to the axis. It is convenient to assume that the power of a cylinder varies by the square of the sine of the angle between the cylinder axis and the meridian in question. That is:
Fn = Fcylsin2?
So, to determine the notional power of the lens at 30° from the cylinder axis, we would add one-quarter of the cylinder power to the sphere in order to determine the notional power of the lens along this meridian. If the intermediate meridian was at 45° to the cylinder axis we would add one-half of the cylinder power to the sphere and at 60° to the cylinder axis, add three-quarters of the cylinder power to the sphere.
As an illustration, consider the prescription -5.00/-2.00 x 180. If we wanted to estimate the notional power 30° from the cylinder axis we would need to add one-quarter of the cylinder power to the power of the sphere. The notional power of the lens along a meridian which is 30° to the cylinder axis is therefore -5.50D. If we wanted to find the notional power along a meridian 60° the cylinder axis we would need to add three-quarters of the cylinder power to the power of the sphere which would be -6.50D. Whenever possible, the lens shape chosen for an astigmatic prescription should have its maximum diameter coincident with the minimum power meridian of the lens.
Higher refractive index materials and thickness reduction techniques
Whenever we are required to dispense a relatively high powered lens, it is natural to consider a higher refractive index material. This approach usually works well when considering high minus spherical and cylindrical prescriptions. However, in plus powers, and also depending on the axis direction and magnitude of any prescribed cylinder, the use of high refractive index materials can have limited benefits in terms of lens thickness reduction. In fact, there are occasions, particularly with higher plus powers when the use of a higher refractive index material will result in a thicker lens. It is often better to request that a thickness reduction technique is used during the surfacing of the lens.
If thickness reduction techniques are used during surfacing, optimum results are obtained if the cylinder is equal to or higher than the sphere and the plus-cylinder axis lies close to the horizontal. This combination gives the manufacturer the best opportunity of reducing lens thickness while maintaining the required power. The further the plus-cylinder axis is from the horizontal, the thicker the finished lens. Sub-optimum results are also obtained if the cylinder is less than the sphere. As an example of the effectiveness of thickness reduction techniques if the plus-cylinder axis is horizontal, consider the following example:
Right +2.00/+4.00 x 180 Left +2.00/+4.00 x 90
The frame used was a 53mm x 32mm quadra with a DBL of 19mm. There was no decentration. Thickness reduction surfacing was used on both lenses with the following results:
Centre thickness 3.3mm, maximum edge thickness 2.0mm, minimum edge thickness 1.0mm, effective diameter 55mm.
Centre thickness 5.4mm, maximum edge thickness 4.9mm, minimum edge thickness 1.0mm, effective diameter 55mm.
Several of the major lens manufacturers have powerful computer programs that are very useful in practice if the exact thickness of a lens has to be calculated and demonstrated to the patient. They are all designed to allow the practitioner and the patient to examine various frame and lens combinations with the aim of selecting the best finished result. Once again, the above values were calculated using the Hoyailog system.
For very high cylindrical powers we occasionally encounter a manufacturing problem when it becomes physically impossible to work the full cylindrical correction onto a single surface. To overcome this, a prescription laboratory may consider using a bitoric lens. In this form a toroidal surface is worked onto both sides of the lens, effectively splitting the prescribed cylinder across both surfaces.
Conicoidal and polynomial surfaces are rotationally symmetrical surfaces of revolution (they have the same degree of asphericity along all meridians) and can be used successfully for spherical prescriptions. For example a +2.00D lens made with a +5.00D front curve would be point-focal in form if the convex surface is a hyperboloid with a p-value of -0.1. The p-value of the aspherical surface would be the same (-0.1) along all meridians. When the prescription contains a cylinder, the p-value of a symmetrical hyperboloidal surface would only be correct for one principal meridian of the lens. In the other meridian, the asphericity of the surface must be increased so that it is appropriate for this meridian of the lens. We have therefore described an aspherical surface that has two p-values at right angles to each other. Carl Zeiss employed a surface of this type for its original Hypal design (1986).
This more complicated aspherical surface is not a rotationally symmetrical surface of revolution, but like a toroidal surface has a different shape along its two principal meridians. The geometry evolves from a minimum p-value along one meridian to a maximum p-value along the other. The surface employed on the original Zeiss Hypal lens was not strictly ‘atoroidal’ as the surface did not incorporate the cylindrical correction which was worked, as usual, on the concave surface of the lens. When the toroidal surface itself is aspherised, it will have both different powers and different asphericity along each principal meridian. This type of surface is particularly useful when dispensing astigmatic lenses with a high cylindrical power. Early atoric lenses certainly helped optimise the visual performance of lenses for high astigmats but with the introduction of freeform surfacing techniques, the ability to optimise the performance of the lens in both meridians has increased dramatically.
Other interesting lenses
The subject of special or interesting lenses is far too wide to include all available special lenses in an article of this nature and lenses for the correction of anisometropia will be discussed in future articles in this series. However, digital curve calculation and freeform surfacing and polishing represent a revolution in ophthalmic lens design and manufacture. The most significant application of freeform surfacing and production is of course in the manufacture of progressive power lenses as the inner surface of a PPL can now provide the progressive addition, cylindrical corrected and any prescribed prism. However, freeform surfaces can now be found on most lens types including single-vision, bifocal, trifocal and degressive lenses.
There are also a number of interesting and innovative applications of freeform surfacing available from the Norville Optical Group. The IRS is a round, blended bifocal segment with the segment ‘free-formed’ on the concave surface of the lens. In a similar fashion to inner surface progressive designs, the IRS segment also incorporates any cylindrical element. Because the design is freeform the lens has the advantage of atoral curve correction across the full lens beyond the segment limits for both the distance and outer segment areas. The lens does have a relatively wide ring of blend area surrounding the segment as opposed to a thin visible line so the segment top should be fitted 1-1.5mm lower than normal. The IRS blended bifocal can be ordered in most resin materials (excluding polycarbonate) including Transitions and Transitions XTRActive in 1.67 resin. The standard segment diameter is 28mm although bespoke segment sizes from 15mm to 40mm can be ordered. In addition, the segment can be placed anywhere on the lens. Another application of the IRS blended bifocal is ‘double seging’. This is where a second segment is free-formed onto the concave surface of a traditional downcurve bifocal with its segment moulded on the front surface. As an example, the Omega aspheric bifocal is available with additions to +3.50D. This could be boosted by creating an additional segment on the concave surface of the lens with an additional addition of say +1.50D giving a total reading addition of +5.00D.
CombiPal, as the name suggests, combines a PPL with a traditional front-surface bifocal. With this lens, freeform surfacing is used to generate a low-addition progressive surface on the concave surface of a semi-finished front surface bifocal lens. This technique creates a lens with wider aberration-free zones than is possible with traditional PPL designs particularly when high reading additions are required. The reading area in a +3.50D addition PPL is limited whatever lens design is used and dispensing a high addition PPL to a first time wearer often results in a less than satisfactory outcome. Splitting the addition between a front-surface bifocal segment and a back-surface PPL (with the bifocal segment providing the majority of the addition) maintains reasonable fields of view for near even with +4.00D additions or higher. CombiPal is available as a flat-top D35 segment or a round downcurve 40mm segment.
Pilotor is a similar idea to CombiPal but the bifocal segment is located at the top of the lens. By inverting a front-surface bifocal semi-finished lens and generating a progressive surface on the concave surface of the lens, the original Essilor Varilux Pilot is reborn! The segment area of the lens is used for viewing objects at near and intermediate distances at eye level whereas the PPL element provides the usual distance and near vision. The segment is usually a round 40mm segment, although an E-style segment or even a trifocal lens is possible. Double progressive designs are PPLs with both an up and down progressive surface. This can be achieved by inverting a traditional front-surface PPL and free-forming a second progressive surface on the concave surface of the lens (a similar idea to the Pilotor lens). There must, however, be enough space to accommodate the distance portion of the lens. In addition, freeform technology enables the production of an up and down, double progressive design (Auto-Pilotor), both on the concave side of the lens. Different additions for the top and bottom progressions can be specified along with different corridor lengths for each half of the lens and different PPL designs for example, an indoor PPL at the top and a general purpose PPL at the bottom. Freeform technology has further changed the production of bi-prism lenses. Bi-prism lenses will be considered later in this series.
Another new addition to Norville’s ever-expanding freeform range of lenses is Digitor, described by Norville as a ‘dual surface multifocal’. This lens is based on a design patented by Owen Aves in 1907, which features a variable front surface base curve providing the optically ideal base curve for all viewing zones. The variable base curve front surface continually increases in power from top to bottom (an increase in curvature in the lower part of the lens compared to the upper part of the lens) and provides benefits to wearers in both the distance and near zones of the lens.
It is important to note that while lens manufacturers tend to categorise products by base curve, they also tend to restrict the range of base curves available so that it is only possible to provide best-form performance in a limited number of prescriptions and only one power per base curve will have the ideal design, all the others being a compromise. However, freeform surfacing (of the concave surface) can be used to ‘clean up’ the optical performance of these compromise lens forms.
Essentially the use of variable base front surface means that there is less ‘cleaning up’ (compensation) to be done and the digital design power can be used to refine and personalise the design for each individual eye. Full customisation is possible if the usual fitting and as-worn parameters are provided. Digitor is fitted in the same way as a traditional PPL.
The author was involved in the initial trial of this lens. He found that changing over from his existing PPL to the Digitor lens was not instantaneous, particularly for distance vision. However, after a day or two, the author was happily wearing the lenses which provided wider fields of near and intermediate vision than his existing PPLs. The Digitor can best be described as a ‘Multifocal E-Style’ and could be considered as an alternative to large diameter D segment and E-style bifocals. Digitor is available in a good range of resin materials (up to n = 1.67) which includes photochromic and polarising options.
Another relatively new and interesting lens that uses freeform technology is a ‘bifocal’ lens from Shamir called Duo. The disadvantages of traditional bifocal lenses are well known and are associated with the visible dividing line (appearance and image jump) and a lack of intermediate vision. By using freeform technology Shamir Duo (Figure 8) offers both surface and optical continuity as the loss of the visible dividing line improves the cosmetic appearance of the lens and eliminates image jump.
Another disadvantage that practitioners are facing with traditional bifocal lenses is the ever-reducing range of bifocal types and materials, particularly in photochromic options.
With reference to the Norville catalogue, traditional bifocals in Transitions are only available in refractive indices of 1.5 and 1.53 (Trivex). Laboratories that do not supply Trivex are only able to supply Transitions bifocals in a 1.5 material. The use of traditional bifocals therefore limits lens materials and treatments that can be used, allowing less versatility and offering patients fewer choices. Shamir Duo is available in a full range of materials in refractive indices from 1.5 to 1.74 (including Trivex) and in Transitions and polarising options to 1.67. Shamir Duo has an impressive prescription range (-18.75 to +15.50) and additions are available to +4.00D.
The author recently trialled this lens on an existing bifocal lens wearer who requested a Transitions lens in a high refractive index bifocal form. She was not keen on trying PPLs as she had heard bad things about them! When the patient collected the completed spectacles she was obviously impressed with the single-vision appearance of the lenses. Adaptation was almost instantaneous. Distance vision was good with minimal blur at the nasal and temporal edges of the distance area. The patient managed N5 on the reading chart without difficulty and appeared to locate the reading area without excessive head movement. She immediately commented on the loss of the dividing line and found the improved continuity between the distance and near zones helpful. Although not marketed as a PPL, Shamir Duo does appear to assist in viewing objects at intermediate distances. A follow-up telephone call to the patient 10 days after collection confirmed that she had fully adapted to the lenses and was happy. Shamir Duo is fitted and ordered in exactly the same way as a traditional PPL.
Complex dispensing presents us with additional challenges to those encountered in simple dispensing. These include selecting an appropriate frame in order to avoid decentration and the prudent consideration of lens material and form. But what exactly is a complex lens and who can legally dispense them?
With reference to BS EN ISO 13666:2012 (Ophthalmic optics-Spectacle lenses Vocabulary) there is no formal definition of a complex lens or a high powered lens. With respect to the GOS a complex lens is defined 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. If the distance prescription is below 10.00D but the reading addition takes it to 10.00D dioptres or more, the complex lens definition would apply to the reading spectacles. The term prism-controlled bifocal has historically been used to describe a glass, solid visible prism segment bifocal. However, these lenses have recently been discontinued. 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. It states that this can include a ‘slab-off’ or bi-prism lens where, for example, the near portion of one lens contains a prism to reduce the vertical prismatic difference that would otherwise occur in anisometropia.
Using the GOS definition, who can dispense a complex lens?
Non-registered staff cannot dispense spectacles to patients under the age of 16 years unless under the supervision of a registered optometrist or dispensing optician. If such a dispensing is supervised the actual work of supplying the spectacles can be delegated but the supervising practitioner remains responsible for the whole process of supervised dispensing. The supervising practitioner must be on the premises at key stages of the dispensing (sale and supply). With regard to the dispensing of industrial safety spectacles, hospital prescriptions and complex lenses to adult patients, providing the spectacles are dispensed to a written prescription only which is valid, complies with the recall period (if not specified the prescription should be less than two years old) and is signed and dated by a registered optometrist or registered medical practitioner there are no restrictions placed on unregistered staff as long as the patient is not registered sight impaired or severely sight impaired.
It is of course important that practitioners keep up to date in terms of their product knowledge regarding lens design and availability. Question the patient carefully, listen to the responses and make a careful, considered decision.
(The correct answer is in bold text)
1 For the prescription -2.00/-2.00 x 60, what would the notional power be along the 180 meridian?
A -2.00 D
B -3.00 D
C -3.50 D
D -4.00 D
2 Which of the following statements regarding near vision effectivity error (NVEE) is correct?
A The amount of NVEE produced depends only on the form of the trial lens used during the refraction.
B The amount of NVEE produced depends on the form of the trial lens used during the refraction and the form of the final lens dispensed.
C The amount of NVEE produced depends on the form of the trial lens used during the refraction, the form of the final lens dispensed and the near working distance.
D NVEE is of no consequence to the aphakic patient who is corrected using spectacles.
3 Which of the following prescriptions would be ideal for thickness reduction by surfacing?
A +2.00/+4.00 x 90
B +2.00/+4.00 x 180
C +2.00/+4.00 x 45
D +2.00/+4.00 x 135
4 Which of the following statements is correct?
A Complex lenses, as defined by the GOS can only be dispensed by or under the supervision of a registered practitioner.
B The prescription +6.25/+0.75 x 90 Add +4.00 D dispensed as a reading lens can be defined with regard to the GOS as “complex”.
C A bi-prism bifocal is not defined by the GOS as a complex lens.
D Prescriptions requiring a back vertex distance can only be dispensed by or under the supervision of a registered practitioner.
5 Which of the following lenses would be most appropriate for use by an electrician?
A Shamir Duo
B Norville Pilotor
C Norville Digitor
D Norville Combipal
6 Which of the following would be the real field of view produced by a round, 50 mm diameter -5.00 D lens fitted 25 mm from the eye’s centre of rotation?
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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