CONTINUING CET EDUCATION & Sponsored by: TRAINING This issue CET: Pay as you learn  To gain more standard CET points for this year’s PAYL series, enter online at: www.otcet.co.uk or ¥ 0207 878 2412

COURSE CODE: C-8180 2 CET POINTS Dispensing II: Complex lens dispensing

Andrew Keirl and Richard Payne

With reference to the terminology used by the General Ophthalmic Services 28 (GOS), the expression complex appliance is defined as an optical appliance with at least one lens which has a power in any one meridian of plus or minus 10.00 or more dioptres, or is a prism-controlled bifocal lens. This definition provides

CET us with a point at which a GOS complex lens voucher may be issued. For some practitioners it also signals the threshold at which simple dispensing becomes complex dispensing. The previous article in this series discussed lenses up to ±5.00 D in power. Although the title of this article is “Complex Lens 22/02/08 Dispensing”, reference will be made to lenses of less than ±10.00 D in power.

Initial Considerations means that a minus lens becomes prescription of ± 5.00 D and above, he Whenever we are presented with a weaker (and therefore a stronger lens has three options. These are: complex prescription to dispense needs to be ordered to compensate for 1.Ensure that the chosen frame sits at there are several important factors that the change due to effectivity) and a the prescribed vertex distance; demand initial consideration. plus lens becomes stronger (a weaker 2.Chose a frame that sits at a different lens is therefore required) when vertex distance, but alter the power Vertex Distance moved away from the eye. The of the lenses accordingly; Vertex distance is defined as “the opposite occurs when the lens is 3.Choose another frame, which fits at distance from the corneal apex to the moved towards the eye. If a the required vertex distance. visual point on the lens”. BS 2738- practitioner is presented with a Point two above can be achieved by 3:2004 states that a vertex distance the use of: should accompany a spectacle a. Knowledge of focal length and focal prescription if the power of the lens in power; any meridian is ± 5.00 D and above, b.Appropriate conversion tables and for example, -5.00/-1.00 x 130 at charts; 12mm where “at 12mm” refers to the c. Appropriate formulae; vertex distance of the trial frame or d.Step-along. refractor head. The vertex distance of a spectacle The vertex distance can be lens plays an important part in the measured using a vertex distance spectacle correction of ametropia as measuring gauge or by simply using a the effective power of a spectacle lens millimetre ruler (Figures 1 and 2). used for distance vision depends on Tunnacliffe has provided a detailed its position relative to the eye. These account regarding the correct effects are often used by presbyopes measurement of vertex distance1. and in particular aphakes to enhance Most trial frames employ a millimetre near vision. So why is a vertex scale positioned along each side distance necessary for medium and which can be used to measure the high prescriptions? The effective vertex distance assigned to a given power of a lens changes as it is moved < spectacle refraction (Figure 3). away from or closer to the eye and all Figure 1 Differences between the vertex lenses become more positive if they The measurement of vertex distance distance of the trial frame or refractor using a vertex distance measuring gauge are moved away from the eye. This head and the final vertex distance

CONFUSED ABOUT CET REQUIREMENTS? www.cetoptics.com/cetusers/faqs/ IMPORTANT INFORMATION Under the new Vantage rules, all OT CET points awarded will be uploaded to its website by us. All participants must confirm these results on www.cetoptics.com so that they can move their points from the “Pending Points record” into their “Final CET points record”. Full instructions on how to do this are available on their website. CONTINUING CET EDUCATION & Sponsored by: TRAINING This issue CET: Pay as you learn  To gain more standard CET points for this year’s PAYL series, enter online at: www.otcet.co.uk or ¥ 0207 878 2412

astigmatic prescription, each principal power must be compensated in turn. The final result can then be re-written in the usual sph-cyl form. A change in vertex distance will also affect the spectacle magnification produced by a correcting lens. An increase in vertex distance will be accompanied by an increase in spectacle magnification. This means 29 that both the minifying effect of minus lenses and the magnifying

effect of positive lenses will be CET increased if the vertex distance is increased. It is worth remembering that the corresponding reduction in 22/02/08 retinal image size may result in a drop in visual acuity for the high myope if < the vertex distance is increased by a Figure 2 < Figure 3 The vertex distance measured using a significant amount. It is therefore helpful if the prescription contains The vertex distance measuring scale on a millimetre ruler trial frame the corrected visual acuities. should always be taken into account be located in the absence of for lens powers over ±5.00 D. Any Frame Selection, Centration and prescribed prism or after any significant difference between these Decentration prescribed prism has been two distances will change the The reasons for the correct centration neutralised. The standard optical effective power of the lens, with of spectacles lenses are well known centre position is a reference point greater changes occurring with higher and include:3 specific to each individual lens shape power lenses. When dispensing • The positioning of the zone of the and is situated on the vertical line complex prescriptions it is vital that lens where the paraxial prescription that passes through the boxed centre. the two vertex distances are measured is most effective; However, if the manufacturer has not and compared, along with • Reducing unwanted differential published information to the appropriate modification of the final prismatic effects; contrary, the standard optical centre prescription to be ordered. In • Reducing the possibility of the is assumed to coincide with the boxed practice, the easiest way to formation of ghost images. centre. Correct centration is of course compensate for a change in vertex important for both complex and distance is to consult one of the The term decentration is used to simple dispensings. However the available tables2 or to use a computer describe any displacement, horizontal mechanical consequences of spreadsheet that compensates for and/or vertical of the centration point decentration become more of an issue changes in vertex distance. from the standard optical centre when high-powered lenses are For those who are mathematically position. The centration point is the involved as the amount of minded, the necessary equations to point at which the optical centre is to decentration required to achieve compensate for a change in vertex distance are given below.

If the vertex distance is decreased: Fold Fnew = 1- (dFold )

If th e vertex distance is increased: Fold Fnew = 1+ (dFold )

In both expressions, d (in metres) represents the change in vertex distance and not the vertex distance itself. Regardless of the method used to compensate for a change in vertex < Figure 4 distance, when dealing with an The boxed centre distance and inter-pupillary distance CONTINUING CET EDUCATION & TRAINING Sponsored by: This issue CET: Pay as you learn  To gain more standard CET points for this year’s PAYL series, enter online at: www.otcet.co.uk or ¥ 0207 878 2412

correct centration can dramatically influence lens thickness and weight, resulting in less than optimum cosmesis. In addition, the published tolerances for centration are a cause for concern as the prescription becomes more complex. Decentration is necessary when the box centre distance of the frame and 30 the subject’s inter-pupillary distance (or near centration distance in the case of near vision) are not the same

CET (Figure 4). The exception to this is when decentration is used to produce < Figure 5. a given amount of prescribed prism. A centred and decentred minus lens The effects of inward decentration on 22/02/08 high-powered lenses are well-known, with increased temporal edge thickness in minus lenses and increased nasal edge thickness in plus lenses (Figures 5 and 6). To ensure that decentration is kept to a minimum it is important to select a frame with a boxed centre distance that is close to the required distance (or near) centration distance. Once the patient’s inter-pupillary distance has been measured, appropriate frames can be used to establish a < suitable bridge size (distance between Figure 6. lenses or DBL). Alternatively, this can A centred and decentred plus lens be measured using a facial ruler. Once this is known, a theoretical eye size think carefully about the patient line. On one hand we have the (horizontal boxed lens size) can be sitting in front of them as the above prescription and inter-pupillary determined by subtracting the DBL guide may not work for a patient with distance. On the other we have lens from the inter-pupillary distance. For a head width of 165mm and an inter- material, lens form, frame style, example, if the patient’s inter- pupillary distance of 58mm! In this material, fitting and centration pupillary distance is 68mm and a example a frame could be judged requirements. In many instances we 16mm DBL is required the “ideal” unsuitable because its dimensions are will have to play one off against the horizontal lens size of frame to avoid too small. Further considerations other, sacrificing frame size and zero decentration will be 68 – 16 = 52mm. have to be made when dispensing decentration for higher refractive If the DBL is increased to 18mm we multifocal lenses to ensure that there index materials or special lens forms. would need to reduce the horizontal is sufficient depth for positioning the Whichever methods we employ it is eye size to 50mm. Additional segment or progression. vital to try and imagine the finished reduction in lens substance can be It is often helpful to consider frame result. Will it be optically and achieved by further reducing the selection in complex dispensings as cosmetically desirable? Once again, horizontal and vertical boxed lens the patient choosing the frame we this is an area in which computer size. This of course needs to be offset want them to have. Explain the aided dispensing comes into its own. with a wider DBL so as to maintain a reason for suggesting certain frames suitable box centre distance and thus and point out the benefits of reduced High Myopia avoid decentration. It should be decentration. Dispensing aids such as Lens material and form pointed out that the lens powers do computer programmes that display When faced with the challenge of not need to be particularly high for the profile of the finished lens are dispensing lenses for the highly this to become an issue and poor very useful in this context. myopic patient three aspects frame selection can instantly remove immediately come to mind. These the benefits of using a lens with a Frame and Lens Harmony are: higher refractive index to reduce lens Complex dispensing is often a • Selection of lens material; substance! balancing act resulting in a • Special lenses for high myopia; However, practitioners must always compromise somewhere along the • Selection of a suitable frame. CONTINUING CET EDUCATION & Sponsored by: TRAINING This issue CET: Pay as you learn  To gain more standard CET points for this year’s PAYL series, enter online at: www.otcet.co.uk or ¥ 0207 878 2412

As far as lens form is concerned, the high-powered, low V-value lenses at also remembered that myopes do not available products can be broadly the zero pupil position with a zero need to use as much ocular rotation as divided in the full aperture lenses and pantoscopic tilt will reduce the emmetropes or hypermetropes to reduced aperture lenses. effects of TCA and hence avoid the observe at oblique angles in the object The primary consideration when possibility of reduced visual acuity space because of the increased field of dispensing minus lenses is often the in primary gaze; view with high minus lenses. Hence, edge thickness of the finished lens. If • Use best form designs; less ocular rotation is needed which the lens incorporates inward • Be sensible with frame selection - may mitigate the effects of TCA. horizontal decentration it will be the consider shape and size carefully; When considering mineral lenses, temporal edge that displays the • Fit the spectacle frame with as as the refractive index of the lens 31 greatest substance. When dispensing small a vertex distance as possible. increases so does the density full aperture lenses for medium to The larger the vertex distance, the (mass/volume). It is therefore true to high myopia the use of higher greater the distance from the optical say that high refractive index glass CET refractive index materials combined centre to the visual point for a materials are heavier if we compare with aspherical surfaces and a particular ocular rotation. So, like with like for example, a one- reflection free coating often provides fitting the lenses as close as possible centimetre cube of one material with 22/02/08 the lens of choice. Such lenses give to the eyes keeps the visual points another of a higher density. However, the practitioner a reasonable degree of as close to the optical centres as this does not always translate to the control over the edge substance while possible and minimises the effects finished spectacle lens. It is therefore providing good off-axis performance of TCA. incorrect to say that high refractive in oblique gaze4. Most of the major index glass lenses are heavier than lens manufacturers produce resin There are other advantages to be normal refractive index glass lenses. materials with refractive indices of gained by keeping the vertex distance This is because as a high refractive around 1.7. However, such products as small as possible. As previously index glass lens is thinner than a are invariably accompanied by low V- mentioned, the spectacle normal refractive glass lens its values5. Mineral (glass) lenses are magnification is less with smaller volume will be reduced. There is less available in refractive indices up to vertex distances. The field of view is of it! This means that even though 1.9. Once again, such products often larger with a smaller vertex distance. the lens is made from a denser display low V-values resulting in Both TCA and distortion increase material its finished weight may be transverse chromatic aberration when with the distance of the chief ray from less than the same lens made using a the patient views through points away the optical axis. So, with a smaller normal refractive index glass from the optical centre. vertex distance, both distortion and material. In lower powers however, it In order to keep effects of TCA to a TCA are marginally less because the is probably true to say that high minimum, the following points chief ray meets the lens closer to the refractive index glass lenses will be should be considered by practitioners: optical axis for a given eye rotation. heavier. If weight is the patient’s • Use materials with a high V-value; These effects are small but not priority then a resin lens has to be the • Apply correct horizontal and insignificant and can be demonstrated material of choice. A reflection free vertical centration and pantoscopic by the application of accurate coating should always be applied to a tilt. Placing the optical centres of trigonometrical ray tracing. It should higher refractive index material as

Supplier/product Index Density V-value Range Form

Seiko SPG AZ 1.74 Resin 1.47 33 To -15.00 Bi-aspheric

Essilor Lineis 1.74 Resin 1.46 33 To -18.00 Aspheric

Nikon Lite V AS 1.74 Resin 1.35 32 To -20.00 Aspheric

Norville Highlite 1.70 Glass 2.90 31 To -18.00 Spherical

Zeiss Lantal 1.80 Glass 3.62 35.4 To -20.00 Spherical

Nikon Pointal 1.80 Glass 3.65 24.4 To -20.00 Spherical

Zeiss Lantal 1.90 Glass 4.02 30.4 To -20.00 Spherical

< Table 1 Very high refractive index minus single vision lens availability CONTINUING CET EDUCATION & TRAINING Sponsored by: This issue CET: Pay as you learn  To gain more standard CET points for this year’s PAYL series, enter online at: www.otcet.co.uk or ¥ 0207 878 2412

ghost images 1, 3, 4 and 5 can be thickness for all powers is 3.5mm; problematic to wearers of high • Almost complete removal of power powered minus lenses6. rings (ghost image no. 1) due to the The ideal lens material does not flat “rim area” of the aspherical unfortunately exist. Perhaps it never surface; will if we keep moving the goal posts! • Removal of the minification of the However, with the aid of the polymer face and distortion of the facial chemist, fantastic advances in features, which is seen with full- ophthalmic lens technology have been aperture high minus lenses. 32 made during the last ten years. Lens materials chosen are generally a < Figure 7. The back surface of the Lentilux is a compromise between vision and Full and reduced aperture minus lens rotationally symmetrical aspherical

CET thickness or weight. It is also forms curve, both in the central and apparent that mid-index materials peripheral zones, which has been will soon become the “standard” refractive indices of 1.7 and 1.8 with calculated by the lens designers to material. It is of course important to 1.9 being available in early 2008. A provide the optimum conditions for 22/02/08 ensure that the material best suited to photochromic tint (photobrown in 1.6 the wearer to achieve good visual the patient’s requirements is always index), cosmetic tints and multi- acuity. The diameter of the central dispensed. In some cases, this coatings are also available. The lens optical zone reduces by stages as the material may be glass and while the is decentred so that the optical centre power increases. For every 2.00 D use of glass as an ophthalmic lens is 4mm above the geometrical centre increase in power the diameter of the material is in decline, there are some of the uncut, which is an advantage optically effective zone reduces by areas where glass can outperform when it comes to frame selection. The 2mm. The diameter of the optical plastics. The demise of glass lens should be fitted as a progressive zone of a -6.25 D lens is 40mm spectacle lenses over recent years has so monocular distance centration whereas for a -18.50 D lens, the left many entrants into the optical distances and vertical centration data diameter is only 30mm. Rodenstock profession with little knowledge or must be specified. It should be noted states that the field of view provided understanding of the attributes of that as the power and refractive index by the Lentilux is almost the same for glass as an ophthalmic lens material. increases the diameter of the central all back vertex powers. The outer This has probably contributed the aperture decreases. If the lens were circumference of the optical zone current lack of recommendation for glazed centred on the pupil, in the connects at all points to a “rim area” glass lenses. As with any product, primary position, a zero pantoscopic in which the strong concave there are always advantages and tilt would be required. With higher curvature of the power area disadvantages. A selection of power, and therefore smaller continually decreases. It is this products suitable for single vision apertures, it is best to fit with zero reduction in surface curvature that high minus dispensing are listed in pantoscopic tilt. In addition, the lens contributes to the considerable Table 1. should be fitted as close to the eye as reduction in edge substance. The For very high myopic prescriptions, possible. aspherical surface of the Lentilux has a reduced aperture lens can be Benefits to the patient include: been optimised in the area considered. This type of lens employs • Good visual acuity; surrounding the optical centre so that various techniques to reduce the edge • Good field of vision; the patient is provided with the best thickness using smaller effective lens • Controlled peripheral aberrations; possible off-axis acuity for a range of apertures while still maintaining a • Good edge thickness and weight; eye movements. The designers of viable overall lens size for glazing • Allows for a wide range of frame Lentilux have given the “rim area” of (Figure 7). selection; the lens an aspherical form whereas • Reasonable cosmesis; high minus lenses are traditionally Reduced aperture lenses • Removal of the minification of the made using flat or spherical, concave Super Lenti (Norville) face which is seen with full surfaces. This is a custom manufactured, aperture lenses. A further thickness reduction is aspherised lens for high myopic achieved by the fact that the lens is prescriptions, which attempts to Lentilux (Rodenstock) made using a high-refractive index eliminate the obvious disadvantages The Lentilux is an aspherical, single glass material (Perfalux 1.7) ne = of high minus lenses (thick edges and vision lens for the correction of high 1.707, Ve = 39.2 and density = 3.21 the unsightly “bottle bottom” ring myopia (up to -24.00 D). The stated g/cm3. The use of a high-refractive appearance). This lens is advantages of this lens are: index material means that a multi- recommended for powers of -11.00 • Good visual acuity; layer anti-reflection coating should and above. The lens is available in a • Thin and light lenses. The edge always be applied. resin material with a refractive index thickness will not exceed 4.50mm As with all aspheric lenses correct of 1.55 and in glass materials with even at -24.00 D. The mean edge centration is important and the usual CONTINUING CET EDUCATION & Sponsored by: TRAINING This issue CET: Pay as you learn  To gain more standard CET points for this year’s PAYL series, enter online at: www.otcet.co.uk or ¥ 0207 878 2412

even extremely high powers can Product Index Density V-value Range up to result in very neat and lightweight lenses, particularly if the profile Norville Lenticular 1.498 Resin 1.30 58 up to -23.00 design is chosen. A selection of reduced aperture lenses for high Zeiss Profile 1.501 Resin 2.55 58.3 up to -25.00 minus dispensing is given in Table 2. Lenticular The strategies for obtaining a cosmetically acceptable edge Norville Lenticular 1.523 Glass 2.50 58 up to -23.00 thickness in high minus powers are therefore: 33 Zeiss Profile 1.706 Glass 3.19 39.3 up to -30.00 • Little or no decentration Lenticular • Small horizontal and vertical eye

sizes CET Norville Lenticular 1.701 Glass 2.90 31 up to -33.50 • Very high index lenses • The use of aspherical surfaces • Specialist lenticular lens forms Norville Super Lenti various various various up to -45.00 22/02/08 Frame Selection and Field Rodenstock Lentilux 1.707 Glass 3.21 39.2 up to -24.00 of View Thick edge substances can occur even < Table 2 when using very high index lenses Reduced aperture high minus lenses and the frame must be able to mechanically support these lenses. rules apply. These are: thickness has been reduced or Plastics rimmed frames and thicker • Centre horizontally according to the “flattened”, resulting in an aperture rimmed metal frames provide such subject’s monocular (distance or and a margin. A step usually exists support and help to obscure some of near) centration distance. between the flattened margin and the the edge thickness. Care should • Centre vertically so that the position aperture. The aperture shape can be always be taken when selecting of the optical centre corresponds to round, oval or profile with the frames with adjustable pad bridges as the required pantoscopic tilt for the margins being convex or plano. The excessive nasal edge thickness can fitting. For every two degrees of major benefit of lenticular lenses is obstruct the correct placement of the pantoscopic tilt, position the optical the reduction in volume and pad arms and pads. Smaller eye sized centre so that it lies 2mm below the consequently a reduction in weight of frames with wider bridges and swept pupil centre. For a fitting with a the finished lens. The real field of out lugs are ideal, as these help to pantoscopic tilt of 10°, the optical view offered by minus lenticular reduce the required decentration and centre should be positioned 5mm lenses is also very good. Most ove rall edge thickness while retaining below the centre of the subject’s manufacturers offer high minus lenses a reasonable adult sized head width pupil. in lenticular forms. Zeiss, for in the frame. Lenticular lenses with example, offers high minus plano- their reduced edge substances and Exact centration is important for all margin profile lenticular lenses in a larger blank size availability can lenses of moderate to high power in choice of 1.5 and 1.7 glass materials make frame selection far easier. order to avoid non-tolerances due to or Clarlet 1.5. With care, attention to It is well known that the real field induced vertical differential prismatic detail and sensible frame selection, of view obtained with a minus lens is effects.

Lenticular lenses A good cosmetic appearance with very high minus lenses can be achieved by the use of lenticular lenses along with careful dispensing and rational frame selection. When compared to modern aspheric forms manufactured using very high index resin materials, lenticular lenses are often considered by some to be “old fashioned” but they still have a place in modern optometric practice. < In high minus forms, lenticular Figure 8 lenses are lenses in which the edge Ring diplopia in high minus lenses CONTINUING CET EDUCATION & TRAINING Sponsored by: This issue CET: Pay as you learn  To gain more standard CET points for this year’s PAYL series, enter online at: www.otcet.co.uk or ¥ 0207 878 2412

greater than the apparent field of view. distance prescriptions are able to Even when reduced aperture lenses delay the need for a reading addition. are used for the correction of high This phenomenon occurs in spherical myopia the real field of view is still lenses because the mean oblique wider than the apparent field of view. image vergence of the lens reduces It is interesting to note that -15.00 D with rotation away from the optical lens dispensed with a 28mm aperture centre. Therefore if a -15.00 D < Figure 9. gives the same real field of view as a distance lens is used for near vision, Plus uncut lenses +5.00 D lens with a 45mm aperture7. with the eye rotating to view a near 34 Another and somewhat theoretical object placed 30° from the principal point to consider regarding the field of compared to the primary position of axis, the oblique power obtained will view in minus lenses is that of ring gaze. When dispensing single vision be 0.67D less than the distance

CET diplopia8. Minus lenses produce an high-powered distance lenses, it is prescription. The lens effectively acts overlap of fields at and around the recommended that the optical centres like a weak progressive power lens. lens margin (Figure 8). Although this are fitted about 3mm below the pupil This phenomenon, however, is not as has the potential to cause diplopia, it centre with 6º of pantoscopic tilt in noticeable with aspheric lens forms 22/02/08 is rarely if ever noticed by patients as the frame. This provides a reasonable and can lead to complaints about poor the rim is so far away from the compromise for distance and near near vision through distance aspheric primary line of sight that the wearer vision use. It is also very important to lenses. In addition, myopes can would need to maintain a rotation of examine the fit of any current create a reading addition by some 45 to 50 degrees which would be spectacles, looking at aspects such as increasing the vertex distance. very uncomfortable to sustain. Ring vertex distance, centration, diplopia is also rarely noticed in pantoscopic tilt and the overall fit of High Hypermetropia lenticular lenses as the power the frame. When using a “specialist” Lens material and form difference between the optic and lens design, it is also important to High powered plus lenses present the marginal zones is so great. follow any specific recommendations practitioner with a different set of provided by the lens manufacturer. problems to overcome. Nasal edge Centration and Fitting thickness, centre thickness and Correct vertical and horizontal Presbyopia overall weight are areas of most positioning of the lenses is of course For the highly myopic presbyope we concern to both the practitioner and vital when dispensing high encounter a new problem, one of lens the patient. prescriptions. Once the frame and availability. Although the basic frame The problems associated with high lens selection has been completed selection and decentration criteria plus powered lenses are: accurate binocular and monocular remain the same, the only lens • Weight and thickness of the inter-pupillary distances must be options available are very high index finished lenses; obtained. Spectacle wearers generally full aperture lenses. Examples of such • Oblique performance when viewing tend to spend most of their time lenses are given in Table 3. off axis; looking through the lenses with their It is interesting to note that some • Magnification; heads slightly depressed when early presbyopes with high minus • Lack of accommodation if aphakic; • Restricted field of view; • Centration and prescribed prism. Product Index Density V-value Range

Norville Solid 1.701 Glass 2.90 31 To -12.00 Unlike high minus lenses, the 30mm bifocal finished blank size of a high powered Zeiss Tital Gradal 3 1.706 Glass 3.19 39.5 To -20.00 plus lens plays a vital role in dictating the final thickness of the Essilor Panamic lens when glazed. The avoidance of Lineis 1.740 Resin 1.46 33 To -20.00 unwanted decentration is essential Nikon Presio GI 1.740 Resin 1.46 33 To -20.00 and by combining minimum substance surfacing techniques along Zeiss Lantal Gradal 1.800 Glass 3.62 35.4 To -20.00 with higher refractive index materials Top E and aspherical surfaces, surprisingly good results can be obtained (Figure Essilor Comfort 1.810 Glass 3.65 34 To -20.00 9). The use of aspheric lens forms in Essilor Panamic 1.810 Glass 3.65 34 To -18.00 very high plus powered lenses, for example, in the spectacle correction < Table 3 of aphakia, can dramatically improve Very high refractive index bifocal/progressive lens availability for high myopia the optical performance and cosmetic CONTINUING CET EDUCATION & Sponsored by: TRAINING This issue CET: Pay as you learn  To gain more standard CET points for this year’s PAYL series, enter online at: www.otcet.co.uk or ¥ 0207 878 2412

to obtain a surface which flexes back • Little sensitivity to fitting distance on itself at larger diameters (Figure change; 11). Such surfaces are known as • Increased field of view; polynomial surfaces and were first • Reduction in the “Jack-in-the-box employed for aspheric post-cataract effect”; lenses in 1978 (AO Fulvue Aspheric • Flatter; lens)10. The main advantage of this • Less magnification; type of lens is the absence of ring • Less TCA. scotoma and Jack-in-the-box effect which normally occurs at the edge of Although still available, traditional 35 a strong plus lens. These two effects lenticular lenses for the correction of are shown in Figure 12 and are due to high hypermetropia have become far

< Figure 10. the abrupt change in power at the less popular since the introduction of CET A convex prolate ellipsoïdal surface edge of the lens. With polynomial polynomial designs. Table 4 lists a designs the surface power reduces range of products that can be used for appearance of a high plus spectacle from the centre of the lens to the edge, high plus dispensing. 22/02/08 lens. As an example, consider a where the periphery is afocal. These It is also worth noting that although +12.00 DS lens made using spherical lenses usually perform well out to we rarely see aphakic patients today, surfaces with a back surface power of about 30º from the optical axis. when dispensing for aphakia an -3.00 D, centre thickness 10mm and a Lenses such as the Essilor Omega, ultraviolet absorbing filter should be centre of rotation distance of 25mm. Zeiss Clarlet Aphal and Rodenstock ordered as the natural crystalline lens If an eye rotates 30° away from the Perfastar use a continuous polynomial is very good at absorbing UVA optical axis the oblique power will be aspheric front surface to give the radiation. UV protection to 400 nm is +12.00DS/+1.50DC. In other words, effect of a blended lenticular. The advisable and there are many the refracted pencil will include +1.50 chief advantage of these polynomial filters/coatings on the market that D of unwanted cylinder. If the same designs is the absence of any ring provide this level of protection. lens were made using a convex prolate scotoma or Jack-in-the-box effect When dispensing high plus lenses, ellipsoidal surface (Figure 10) the off- (Figure 13). The lens therefore offers particularly for the spectacle axis performance would be +11.33 D awareness of objects in the peripheral correction of aphakia, there must be at 30°. The lens is now free from field. However the field of clear cooperation and communication aberrational astigmatism. However, it vision is quite small and wearers between the prescriber and the is not an ideal design as the mean often turn their head to obtain a wide dispensing optician to ensure that the oblique error (MOE) at 30º will be - field of vision. Polynomial lens trial lens prescription is reproduced 0.68D. Another improvement in the designs combine the advantages of as accurately as possible. Points that performance obtained by the use of a both lenticular and full-aperture must be considered include: prolate ellipsoidal surface is a lenses. To summarise, the advantages • The form of the trial lenses used; reduction in distortion as the of the polynomial designs are: • The pantoscopic angle of the trial distortion produced by the aspheric • No visible dividing line frame; lens will be around 30% less than in • Good mean oblique power when • The vertex distance of the trial the spherical design9. viewing off-axis; frame. Higher order aspherical surfaces can • Reduced distortion; be obtained by deforming a conicoid • Very slightly thinner; A diligent optometric practitioner

< Figure 12. < Figure 13. < Figure 11. Ring scotoma and the Jack-in-the-box The absence of ring scotoma and the A Polynomial surface effect Jack-in-the-box effect CONTINUING CET EDUCATION & TRAINING Sponsored by: This issue CET: Pay as you learn  To gain more standard CET points for this year’s PAYL series, enter online at: www.otcet.co.uk or ¥ 0207 878 2412

may adjust and fit the chosen frame vision and exacerbates the loss in obstruct correct placement of the pad before the refraction takes place. He power resulting from NVEE. arms and pads resulting in poor can then adjust the trial frame to fitting. Smaller eye sized frames with match the characteristics of the final Frame Selection and Field of View wider bridges and swept out lugs are frame as opposed to the other way As with high minus lenses, high plus ideal, as these help to reduce the round! lenses need to be mechanically required decentration and overall supported by the frame. The position thickness while retaining a Near vision effectivity error of the centre of gravity of a plus lens reasonable adult sized head width in Near vision effectivity error arises can often lead to the frame being the frame. 36 because lenses of the same power but “front heavy” and therefore prone to When dispensing high plus powers, manufactured in different forms are slipping. Regular, fixed pad bridge frames selected should: not interchangeable for near vision. and saddle bridge metal frames offer • Be lightweight to avoid heavy/thick

CET Potential NVEE problems will only the best support and although rims adding to a ring scotoma; occur when dealing with medium to adjustable pad bridges provide for • Possess adjustable pads to allow high hypermetropic patients. As an useful refinement, excessive nasal some vertical movement of the example, consider a +10.00 D lens edge thickness in the lens can optical centres; 22/02/08 made in plano-convex form and used for near vision (convex surface facing the eye) with an object placed at a Supplier/ Index V-value Range Form distance of 33cm from the lens. The product vergence incident at the lens will be - 3.00 D and paraxial ray tracing will Norlite Aspheric 1.498 58 To +15.50 Aspheric show that the vergence leaving the lens is +7.02 D. This value is known Norville Hi Drop 1.498 58 To +15.00 Aspheric as the actual exit vergence. The anticipated exit vergence is found Essilor Orma 1.500 58 To +20.00 Aspheric from -3.00 + (+10.00) and is +7.00 D. Omega Near vision effectivity error (NVEE) is defined as the actual vergence leaving Zeiss Clarlet 1.501 58 To +23.00 Spherical a lens minus the anticipated exit vergence. In this case the NVEE is Zeiss Aphal 1.501 58 To +23.00 Aspheric +0.02 D and a value this small is of little consequence to the patient’s Rodenstock 1.502 58.2 To +22.00 Aspheric visual comfort. If we now compare Perfastar three more lenses (Table 5) all of back vertex power +10.00 D but in different Seiko SSV 1.67 32 To +16.00 Aspheric forms, we can see the how lens form effects NVEE. Rodenstock 1.67 31.4 To +12.00 Aspheric The bottom line here is that we have Supersin four lenses, all of the same back vertex power when measured on a focimeter, Rodenstock 1.67 31.4 To +12.00 Aspheric but providing four different effects for Cosmolit the wearer. These NVEE values Norville Highlite 1.701 31 To +12.00 Spherical illustrate how lenses of the same power but manufactured in different forms are not interchangeable for near Zeiss Tital 1.706 39.3 To +25.00 Spherical vision. It also illustrates the problem of labelling the lenses in terms of their Norlite 1.498 58 To +48.00 Spheric back vertex power! NVEE is of Lenticular significance with medium/strong plus spectacle lenses only, never minus Norlite 1.498 58 To +22.00 Aspheric lenses and should always be Lenticular considered for compensation as appropriate. Norville Lentic 1.701 31 To +28.00 Button As mentioned in the section on high minus lenses, a loss in mean oblique Norville 1.701 31 To +20.00 Spheric image vergence occurs when of axis Bi-Lentic zones of the lens. This occurs when < Table 4 an aspheric lens is used for near High plus single vision lens availability CONTINUING CET EDUCATION & Sponsored by: TRAINING This issue CET: Pay as you learn  To gain more standard CET points for this year’s PAYL series, enter online at: www.otcet.co.uk or ¥ 0207 878 2412

• Have a box centre distance as close vertical frame dimension will produce as possible to the patient’s inter- the thin edge and the vertical the thick Lens Form NVEE pupillary distance in order to edge. The difference in the thick and reduce or remove any decentration thin edge substances will be more Plano-convex +0.02 of the lens. pronounced with a shallow oval shape that a round shape. If the power Centration and Fitting meridians were reversed i.e. -8.00 D Curved form +0.42 When dispensing high plus powers along 85 and -6.00 D along 175, a the vertex distance should be kept as shallow oval would be a better idea as Equi-convex +0.13 small as possible in order to: there will be less difference between 37 • Maximise the field of view; the thick and thin edge substances. • Reduce convergence demand; Returning to the original case, it would Convex-plano +0.31

• Reduce the retinal image size; be sensible to use a frame that has a CET • Reduce spectacle magnification; reduced horizontal eye size for < Table 5 • Reduce distortion and chromatic example, a “rounded” quadra. NVEE values for four +10.00D lenses aberration; When dealing with oblique 22/02/08 • For single vision distance lenses the centration points should correspond to the monocular PDs; Supplier/product Type Seg Index Range to Add to • Near vision lenses should be centred according to the monocular Norlite 22mm Bifocal Round 1.498 To +16.00 To 20.00 NCDs; • Vertical centration and pantoscopic Omega Aspheric 22mm Bifocal Oval 1.498 To +18.00 To 3.50 tilt must correspond.

High Plus Presbyopes Orma Asph-lenti 25mm Bifocal Oval 1.50 To +17.00 To 2.75 For the high plus presbyope we again encounter the problem of lens Norlite Hyper Aspheric Bifocal Flat Top 1.498 To +15.00 To 5.00 availability with a greater range of bifocal than progressive lenses being Norlite 22mm Lenticular Bifocal Round 1.498 To +24.00 To 25.00 available. Suitable products for this category are given in Table 6. Norlite 22mm Asph-lenti Bifocal Round 1.498 To +20.00 To 6.00 Astigmatism Maximum and minimum edge Norlite 22mm Apsh-lenti Bifocal Flat Top 1.498 To +20.00 To 6.00 thicknesses The presence of a high cylinder within Perfastar Bifo 24mm Bifocal Round 1.502 To +16.00 To 4.00 a prescription means that practitioners need to be able to estimate how the cylinder will affect the edge thickness Clarlet Duopal 25mm Bifocal Curve 1.501 To +20.00 To 3.50 of the finished lens. To do this we must of course compare the principal Clarlet Alphal 22mm Bifocal Round 1.501 To +20.00 To 3.50 meridians and principal powers with the shape and dimensions of the Rodenstock Grandasin Bifocal Curve 1.525 To +13.00 To 4.00 chosen frame. As an example, consider the prescription -6.00/-2.00 x 85. The principal powers of this lens Norville Solid 30mm Bifocal Round 1.701 To +12.00 To 4.00 are -6.00 D along 85 and -8.00 D along 175. The maximum edge substance Varilux Omega Orma PPL - 1.50 To +15.00 To 4.00 will therefore be found along the 175 meridian of the lens. The shape and Rodenstock Hyperop PPL - 1.67 To +13.00 To 3.00 size of the frame used to provide this prescription must be given careful consideration. A round shape would Varilux Ipseo Lineis PPL - 1.74 To +13.00 To 3.50 of course give unequal edge thicknesses with the horizontal Varilux Physio Lineis PPL - 1.74 To +13.00 To 4.00 meridian producing the thickest edge substance. If a modern shallow oval < Table 6 shape was employed, the smaller Bifocal and progressive lens availability in high plus powers CONTINUING CET EDUCATION & TRAINING Sponsored by: This issue CET: Pay as you learn  To gain more standard CET points for this year’s PAYL series, enter online at: www.otcet.co.uk or ¥ 0207 878 2412

cylinders, the finished lens can become more difficult to visualise and it is vital to match and compare Summary the principal powers of the lens with the frame dimensions. If the Complex dispensing presents us with additional challenges to those in simple dispensing. These include: prescription • Care when selecting an appropriate frame in order to avoid decentration. • The prudent consideration of lens material and form. RT +0.50/+5.50 x 45 LT +0.50/+5.50 x 135 The above points are vital if we are to achieve a visually and cosmetically acceptable solution for our patients. It is the practitioner’s professional duty to advise the patient 38 was matched with a frame that had having considered all the physical, mechanical and optical problems presented with a a quadra or contour shape, the nasal particular prescription. In the last twenty years or so, the optical profession has seen a transformation in spectacle lens dispensing and in the variety of lenses that are CET edges of the lens would be very thick commercially available. Today, the practitioner has a wide selection of lens designs long the 135 meridian of the right lens and materials to choose from. It is therefore vital that the practitioner is clear about and the 45 meridian of the left lens. the optical and mechanical properties of products. As professionals we have a duty The thin edge would of course be at to advise our patients of the advantages and disadvantages of products available. It 22/02/08 ninety degrees to the thick edge. If is therefore important that practitioners keep up to date in terms of their product the principal powers were reversed knowledge regarding lens design and availability. Question the patient carefully, listen to the responses and make a considered decision. i.e. axis 135 in the right eye and axis 45 in the left eye, the differences between the thick and thin edges p-value of a symmetrical About the authors would be less extreme. When dealing hyperboloidal surface would only be Optometrist Andrew Keirl and with prescriptions of this nature it is correct for one principal meridian of Dispensing Optician Richard Payne necessary to employ minimum the lens. In the other meridian, the work in private practice in Cornwall. substance surfacing techniques asphericity of the surface must be resulting in an elliptical shaped increased so that it is appropriate for References uncut lens. It is also helpful to talk to this meridian of the lens. We have 1. Tunnacliffe A H (1995) Essentials your surfacing laboratory for advice now described an aspherical surface of Dispensing. ABDO, London, 29-30 on what may and may not work! that has two p-values at right angles 2. Tunnacliffe A H (1995) Essentials to each other. Carl Zeiss employed a of Dispensing. ABDO, London, 28 Bitoric lenses surface of this type for their original 3. Jalie M (2003) Ophthalmic Lenses & For very high cylindrical powers we Hypal design (1986). This more Dispensing 2nd Edition. Butterworth- occasionally encounter a complicated aspherical surface is not Heinemann, Oxford 39 manufacturing problem when it a rotationally symmetrical surface of 4. Jalie M (2003) Ophthalmic Lenses & becomes physically impossible to revolution, but like a toroidal surface Dispensing 2nd Edition. Butterworth- work the full cylindrical correction has a different shape along its two Heinemann, Oxford, 102-103 onto a single surface. To overcome principal meridians. The geometry 5. Keirl A (2007) The properties of this, a prescription laboratory may evolves from a minimum p-value ophthalmic lens materials. Optometry consider using a bitoric lens. In this along one meridian to a maximum p- in Practice 8, 127-138 form a toroidal surface is worked onto value along the other. The surface 6. Jalie M (2003) Ophthalmic Lenses & both sides of the lens, effectively employed on the original Zeiss Hypal Dispensing 2nd Edition. Butterworth- splitting the prescribed cylinder lens was not strictly “atoroidal” as Heinemann, Oxford, 111-112 across both surfaces. the surface did not incorporate the 7. Jalie M (2003) Ophthalmic Lenses & cylindrical correction which was Dispensing 2nd Edition. Butterworth- Atoroidal Surfaces worked, as usual, on the concave Heinemann, Oxford, 107-108 Conicoidal and polynomial surfaces surface of the lens. When the 8. Tunnacliffe A H (1995) Essentials are rotationally symmetrical surfaces toroidal surface itself is aspherised, it of Dispensing. ABDO, London, 14-15 of revolution (they have the same will have both different powers and 9. Jalie M (2003) Ophthalmic Lenses & degree of asphericity along all different asphericity along each Dispensing 2nd Edition. Butterworth- meridians) and can be used principal meridian. This type of Heinemann, Oxford, 174-175 successfully for spherical surface is particularly useful when 10. Jalie M (2003) Ophthalmic Lenses prescriptions. For example a +2.00 D dispensing astigmatic lenses with a & Dispensing 2nd Edition. lens made with a +5.00 D front curve high cylindrical power11. Butterworth-Heinemann, Oxford, would be point-focal in form if the 176-178 convex surface is a hyperboloid with Acknowledgements 11. Jalie M (2003) Ophthalmic Lenses a p-value of -0.1. The p-value of the The authors would like to thank & Dispensing 2nd Edition. aspherical surface would be the same Duncan Counter for his helpful Butterworth-Heinemann, Oxford, (-0.1) along all meridians. When the comments on an earlier version of 214-215 prescription contains a cylinder the this article. CONTINUING CET EDUCATION & Sponsored by: TRAINING This issue CET: Pay as you learn  To gain more standard CET points for this year’s PAYL series, enter online at: www.otcet.co.uk or ¥ 0207 878 2412

Module questions Course code: c-8180 Please note, there is only one correct answer. Enter online or by form provided

An answer return form is included in this issue. It should be completed and returned to CET initiatives (c-8180) OT, Ten Alps plc, 9 Savoy Street, London WC2E 7HR by March 21 2008.

1. A prescription reads +8.00/+2.00 x 90 at 14mm. The final lens is to be 7. When dispensing for high myopia, which of the following products is likely to dispensed at a vertex distance of 10mm. The most appropriate final lens to produce a lens with the thinnest edge substance? order is: a. Essilor Lineis a. +8.25/+2.00 x 90 b. Seiko SPG b. +8.25/+2.25 x 90 c. Zeiss Lantal c. +8.00/+2.25 x 90 d. Zeiss Tital 39 d. +8.00/+2.00 x 90 8. Which of the following statements is most correct? 2. When dispensing high plus powers which of the following is not a reason for a. Ring diplopia is a common complaint from high myopic patients.

keeping the vertex distance as small as possible? b. A 44 year old medium to high myope might enjoy more comfortable near CET a. To maximise the field of view vision in spherical as opposed to aspheric lenses. b. To reduce convergence demand c. The Rodenstock Lentilux lens is manufactured using a resin material. c. To reduce the retinal image size d. When dispensing medium to high plus lens, the uncut size used is of no d. To reduce the centre thickness of the lens consequence. 22/02/08 3.Which of the following is not a reason for correctly centring a spectacle lens? 9. Which of the following statements is most correct? a. To position the lens where the paraxial prescription is most effective. a. If a high powered plus lens with a spherical back vertex power is b. To reduce unwanted differential prismatic effects. manufactured with a convex prolate ellipsoidal surface, aberrational oblique c. To reducing the possibility of the formation of ghost images. astigmatism will be reduced. d. To control spectacle magnification. b. If a high powered plus lens with a spherical back vertex power is manufactured with a convex prolate ellipsoidal surface, both aberrational 4. A pair of plus, distance aspheric lenses is to be dispensed to a subject with an oblique astigmatism and distortion will be reduced. inter-pupillary distance of 62mm. The frame chosen has a 48mm x 42mm oval c. Ring scotoma and the Jack-in-the-box effect are not eliminated with shape. When the head is in the primary poison the subject’s pupil centres lie polynomial aspheric designs. 5mm above the horizontal centre line. The frame has a boxed centre distance of d. The lens of choice for a high powered plus lens of spherical back vertex 66mm and the standard optical centre position coincides with the boxed centre. power is one that incorporates an atoroidal surface. The pantoscopic angle of the frame is 10º. With regard to lens centration, which of the following would be ordered? 10. Which of the following statements regarding near vision effectivity error a. Decentre 2mm in. (NVEE) is most correct? b. Decentre 2mm in and 5mm up. a. Lenses of the same power but manufactured in different forms are c. Optical centres to coincide with the pupil centres. interchangeable for near vision. d. Optical centres to coincide with the boxed centres. b. NVEE is of significance with medium/strong plus spectacle lenses and is more significant when an aspheric lens is used. 5. Which of the following will not assist in keeping the effects of TCA to a c. NVEE is of significance with high powered minus lenses. minimum? d. NVEE is a dispensing and not an optometric problem. a. Use materials with a high V-value. b. Apply correct horizontal and vertical centration and pantoscopic tilt. 11. An atoric lens would be the lens of choice for which of the following c. Fitting the spectacle frame with as large a vertex distance as possible. prescriptions: d. Use best form designs. a. +2.00DS b. +6.00/+3.00 x 180 6. Which of the following statements is most correct? c. -4.00DS a. For all mineral and resin spectacle lens materials, as the refractive index d. +8.00 DS increases, so does the density. b. The term density simply relates to the weight of a finished spectacle lens. 12. Which of the following lens shapes is most suitable for the prescription c. The density of a material is given by mass/volume. 5.00/-1.50 x 180? d. A lens manufactured using a low refractive index material will always weigh a. Round less than the same lens manufactured using a very high refractive index b. Oval with the long axis horizontal material. c. Oval with the long axis vertical d. Aviator

Please complete on-line by midnight on July 10 2008 - You will be unable to submit exams after this date – answers to the module will be published in our July 11 issue