Continuing education CET Complete course in dispensing Part 6 – Lens selection After last month’s article (November 14, 2008) discussing the basic properties of materials used for the manufacture of spectacle lenses, Andrew Keirl and Richard Payne give an overview of the vast selection of lens types and designs available for the ‘normal’ power range. Module C10361, two general CET points

ith the prevalence of wavelengths. These are the chromatic online information aberrations (6) which were addressed available for consum- in article 5 of this series. The six aberra- ers today, many lens tions are: manufacturers have 1 Spherical aberration -4.00 -2.00 -4.00 -8.00 -6.00 -4.00 W seen the need to enhance their product +8.00 +6.00 +4.00 +4.00 +2.00 +0.00 2 Coma ranges with marketable brands, employ- 3 Oblique astigmatism ing household names and retail market- 4 Curvature of field ing tactics to ensure their lens ranges are 5 Distortion more appealing. With such an extensive +4.00 D lenses -4.00 D lenses 6 Chromatic aberration (TCA) range of options now available, it is A best form spectacle lens is a lens vital that practitioners can see past the Figure 1 designed to minimise the effects of ever increasing lists of brand names and Lens form oblique astigmatism and therefore marketing terminology used to properly comparisons provide the best possible vision in consider the true attributes of any given oblique gaze. Secondary considerations lens design and use the appropriate are distortion and transverse chromatic ‘tool’ for the appropriate ‘job’. Future aberration. articles will discuss lens coatings and In recent years, the analysis of lens ‘complex’ lenses. It is of course impos- form has been revolutionised by the use sible to consider every product available of the personal computer. The possibility Meniscus Meniscus Bi-convex

in an article of this nature. Bi-concave of rapid analysis of lens performance is Plano-convex Plano-concave now a reality. Calculations, which took Spectacle lenses hours to do in the past, can now be done In very simple terms, a spectacle lens is in seconds. This allows us to examine lens an optical device with the purpose of Plus lenses Minus lenses performance by examining the results altering the vergence of pencils of light of exact ray tracing in the oblique gaze transmitted from the lens to the eye. Figure 2 prevent the image formed by a lens positions. Eye care practitioners should Physically, a lens is an optical medium Categories from being a perfect copy of the object. understand these effects and whenever bounded by two polished surfaces, one of lens form Aberrations that afflict spectacle lenses possible should dispense spectacles with of which must be curved. The curva- can be broadly divided into two types, best form theory in mind. There are ture employed may be spherical, cylin- image deforming aberrations and image occasions when the dispenser should drical, toroidal, aspherical or free-form degrading aberrations. In addition, some specify the form required and not leave (surfaces that cannot be defined using aberrations occur with monochromatic it to the manufacturer. There are also a single mathematic equation). The light, the monochromatic aberrations numerous examples of when the practi- usual function of a spectacle lens is the (1-5 in the list below); other aberrations tioner should understand the advantages correction of a refractive error. However, occur with light that contains at least two of one design compared to another, for spectacle lenses are also used to correct example, spherics verses aspherics. In binocular vision anomalies, reducing Series modules summary, lens designers must consider: the amount of light that reaches the eye ● Off-axis performance (oblique (a tinted lens) or to provide mechanical ● Practice development, marketing and astigmatism, distortion and transverse protection (a safety lens). communication chromatic aberration) ● Prescription interpretation ● Appearance Best-form lenses and spectacle ● Frame design ● Spectacle magnification lens design ● Spectacle lenses ● Thickness. What is a best form lens? Put simply, it is ● Tints and coatings The main aberration of interest is ‘a lens designed to minimise the effects of ● Dispensing oblique astigmatism. Put simply, oblique certain stated defects or aberrations in its ● Specialist lenses astigmatism causes a spherical lens to image-forming properties’. Aberrations ● Ordering, checking and collection behave like an astigmatic lens and can can occur with lenses that are free from ● Non-tolerance occur when an eye views through off manufacturing defects and manufac- ● Glossary axis points on a lens. It occurs because of tured using the best materials and can the asymmetry in the refraction of rays

18 | Optician | 12.12.08 opticianonline.net Continuing education CET

Table 1 astigmatism. Oblique astigmatism is an off-axis aberration caused by asymmet- Lens 1 rical refraction at a spherical surface. BVP = +5.00DS, F2 = -1.00 D, t = 6.53mm Sphero-cylindrical and toric lenses have Ocular rotation(°) Oblique rower (D) Distortion (%) TCA (∆) axial astigmatism. While oblique astig- 0 +5.00 DS 0.00 0.00 matism is a nuisance, axial astigmatism is often induced on purpose, eg in the 5 +5.02DS/-0.02DC 0.16 0.02 correction of astigmatic ametropia. 10 +5.10DS/-0.08DC 0.65 0.05 15 +5.22DS/-0.19DC 1.50 0.07 Lens form To describe the overall form of a lens 20 +5.40DS/-0.35DC 2.78 0.10 we can divide them into several groups 25 +5.63DS/-0.57DC 4.57 0.13 as shown in Figure 2. Flat form lenses though rarely used today consist of 30 +5.94DS/-0.86DC 7.02 0.17 equi-convex, equi-concave, bi-convex, b-concave, plano-convex and plano- Lens 2 concave forms. Lenses where one surface BVP = +5.00DS, F = -3.00 D, t = 6.65mm is spherical and the second surface 2 plano-cylindrical can also be term ‘flat’. Ocular rotation(°) Oblique power (D) Distortion (%) TCA (∆) Curved-form or meniscus lenses differ 0 +5.00DS 0.00 0.00 from flat form lenses in that one surface 5 +5.01DS/-0.01DC 0.14 0.02 is convex and the other concave. Figures 1 and 2 illustrate several lens form 10 +5.04DS/-0.04DC 0.56 0.05 comparisons. 15 +5.08DS/-0.10DC 1.28 0.08 Astigmatic lenses are usually manufac- ture in curved or meniscus form to 20 +5.14DS/-0.18DC 2.36 0.10 provide improved oblique performance. 25 +5.21DS/-0.27DC 3.83 0.13 These lenses are referred to as toric lenses 30 +5.29DS/-0.39DC 5.80 0.17 which consist of one spherical surface and one toroidal surface.

Lens 3 Base curves BVP = +5.00DS, F2 = -6.00 D, t = 7.00mm Eye care practitioners and lens manufac- Ocular rotation(°) Oblique power (D) Distortion (%) TCA (∆) turers use the term ‘base curve’ in different ways. The eye care practitioner is usually 0 +5.00DS 0.00 0.00 interested in the finished lens as opposed 5 +4.99DS/0.00DC 2.99 0.03 to the semi-finished lens. With regard to finished spherical lenses, the base curve is 10 +4.97DS/0.00DC 1.85 0.05 the lower of the two surface powers. As 15 +4.94DS/-0.01DC 1.02 0.08 far as finished toric lenses are concerned, 20 +4.89DS/-0.01DC 1.85 0.11 the base curve is the lowest numerical power found on the toroidal surface. 25 +4.82DS/-0.01DC 2.99 0.14 In the mass production of stock lenses 30 +4.72DS/0.00DC 4.47 0.17 and semi-finished cast moulded bifocal and multifocal lenses it has become commonplace for manufacturers to in two mutually perpendicular planes give the same back vertex power (+5.00 categorise lenses by their base curve. called the tangential and sagittal planes. DS) when measured using a focimeter! They also tend to restrict the number of The visual effect of oblique astigmatism It is important to note that all the lenses base curves available to certain power can be better appreciated by considering in the above example employ spherical ranges that provide ‘best form’ perform- the off-axis performance of the follow- surfaces. ance across as wider range of powers as ing three +5.00 DS lenses. Lens 1 is made In summary, lenses with steeper possible. For the purposes of manufac- in a flat form using a back surface power (spherical) curves: ture, the base curve of a lens is defined of -1.00 D. Lens 3 is made in a much ● Provide good off-axis vision as the spherical surface of a semi-finished steeper form using a -6.00 D back surface ● Control distortion uncut lens which, in most cases, is the power. The ocular rotation is in relation ● Are more bulbous front surface of the lens. So eye care to the principal axis of the lens. ● Are thicker practitioners and manufacturers define Table 1 shows that: ●Produce more spectacle the base curve in different ways. This ● The best off-axis performance (an magnification. difference is important when we are faced oblique power closest to +5.00 DS) is Lenses with flatter (spherical) curves: with suspected lens form intolerance. It is obtained with the lens with the steeper ● Produce poor off-axis vision always wise to contact the manufacturer form ● Produce more distortion to clarify exactly what their definition of ● Steeper forms give better control of ● Are less bulbous base curve is, before placing an order for distortion ● Are thinner a specific base curve. ● The lens form makes little difference ● Produce less spectacle magnification. to the amount of TCA produced. Sphero-cylindrical and toric lenses Aspheric lenses Remember that all three lenses will have properties that remind us of oblique So far we have seen that the lens form

20 | Optician | 12.12.08 opticianonline.net CET Continuing education

Table 2 Hyperbola Ocular rotation(°) Oblique power (D) Distortion (%) TCA (∆) Parabola 0 +5.00DS 0.00 0.00 5 +5.00DS 0.13 0.02

Oblate Circle Prolate 10 +4.98.DS/-0.01DC 0.51 0.05 ellipse ellipse 15 +4.95DS/-0.01DC 1.17 0.07 20 +4.91DS/-0.02DC 2.16 0.10 25 +4.85DS/-0.02DC 3.52 0.13 30 +4.76DS/-0.02DC 5.35 0.16

Figure 3 The conicoids introduces surface astigmatism in order aspheric lenses for the normal power to neutralise the aberrational astigma- range are: (and the lens material) can influence the tism associated with flat form lenses. ● Good off-axis performance quality of vision provided by a spectacle Aspherical surfaces are derived from ● Some control of distortion lens. The visual performance of a lens a family of curves called conicoids, as ● Less spectacle magnification has to be considered when the patient shown in Figure 3. ● Reduced sensitivity to fitting distance looks through points on the lens close An aspherical surface is actually astig- ● Thinner and lighter lens to the principal axis and also when the matic, the negative surface astigmatism ● Noticeably flatter lens form provides patient views through off-axis points on increasing progressively as we move improved cosmesis. the lens. An ideal best-form lens is one away from the vertex of the aspherical In recent years computer aided whose off-axis performance is essentially surface. This negative astigmatism is design has allowed lens manufacturers the same as its back vertex power. The used to counteract the positive oblique to integrate aspheric surface technology aberration oblique astigmatism causes astigmatism arising from off-axis gaze. into a growing number of more complex a spherical lens to behave like an astig- This is how aspheric lenses work! An forms. Elliptico-toroidal or atoroidal matic lens when a patient looks away aspherical surface can have its aspheric- surfaces combine different oblate ellip- from the principal axis. Oblique astig- ity (p-value) chosen to counteract the tical cross sections along each principle matism is a consequence of the form oblique astigmatism for any chosen meridian providing a different degree of the lens. Generally speaking, when form. If a flatter form than the traditional of asphericity for each meridian with restricted to spherical surfaces, flatter spherical form is chosen, the sag of the much improved oblique performance lens forms produce higher levels of necessary asphere will be less than that in higher cylindrical corrections. unwanted oblique astigmatism. Oblique of a spherical surface of the same radius. The integration of aspherical surfaces astigmatism may be controlled if the lens Hence, a thinner, lighter, less bulbous into lenticular lens forms has greatly is made in a more curved form. lens is produced with less magnification improved their optical performance and However, curved lenses tend to be of the eye and surround. appearance. More recently, the introduc- thicker and more bulbous than a lens of The reduction in the thickness and tion higher base curve lens forms for the same power but made using flatter weight of an aspheric lens is the result use in wraparound-style sunglass frames curves. One of the features of current of a two-stage process. Firstly the lens has only been possible with the use of single vision spectacle lenses is the large is flattened (which makes it thinner) specially designed aspherical surfaces. number of designs made in aspheric and then the off-axis performance of form. Originally, aspheric lenses were the flatter form is restored to that of a Individual free-form surfaces only used for the high powered positive steeper spherical design by aspherising The latest innovation and possibly range. However, it was Jalie who pointed the surface, as a result of which the final ultimate breakthrough for lens designers out the cosmetic advantages of using lens form is thinner still. has been the development of free-form lenses with an aspherical surface for low If lens 1 in the previous examples surfacing technology. Free-form surface powers so that nearly flat forms could be (Table 1) is made in aspheric form with creation involves the use of precision used which at the same time give accept- a convex hyperbolic surface (p-value point by point (CNC) grinding and able off-axis performance. Jalie suggested of -2.00) the off-axis performance is as polishing processes to create a complex, that the front surface of positive lenses and shown in Table 2. irregular, asymmetric aspherical surface. the rear surface of negative lenses should It will be noted that the optical Wavefront computer modelling systems be in aspherical form, the other surface performance of this aspheric lens is are used to design each lens surface, being spherical or toroidal depending on very similar to the steep lens with a taking optics-influencing variables and the prescription. Unlike Jalie’s patent, -6.00D back surface. However, it has the wearer’s physiological viewing minus aspheric lenses tend to be made the cosmetic appearance of the fat lens. habits way beyond the usual elements with a front aspherical surface, presum- Hyperbolic surfaces are commonly of just the optical prescription. This ably because the ophthalmic industry is employed in aspheric lenses for the process can eliminate even the small- equipped mostly for the manufacture normal power range. est aberrations and because there are no of concave toroidal surfaces only. As Today we encounter aspherical constraints with free-form surfacing, previously mentioned, any flattening of surfaces in many of the lenses we lens designers have the freedom to create the lens form can introduce unwanted dispense and although there are numer- totally unique lens surfaces and integrate aberrational astigmatism and also distor- ous minus-powered aspherical lenses the a wider range of individual parameters tion. This can be largely overcome by real impact has been on plus-powered into the lens design that are unique to the use of an aspherical surface, which lenses. To summarise, the advantages of the final wearer. opticianonline.net 12.12.08 | Optician | 21 Continuing education CET

As eye care professionals it is vital that we keep up to date with this emerging technology and more importantly, become familiar with the additional information required by many lens manufacturers when ordering a free- form surfaced, individual lens. So far we have considered the basic concepts of the best-form lens and the recent innovations that have brought the optical profession closer to this goal. We must also consider the vast range of different lens types available, their uses and common features. Before deciding on the optimum lens for a given prescription the following points should be considered: ● Lens form ● Material Figure 4 The Hoyalog system ● Field of view ● Centre thickness and design options to provide the patient Because of the orientation of the princi- ● Weight with the best cosmetic appearance pal meridians, an ideal frame will be one ● Reflections without compromising visual perform- that has its shortest dimension along ●Availability of surface processes. ance. To illustrate the above options, we the vertical meridian. This of course is will use plastics lens materials from the the case with most modern designs. If Single-vision lenses Hoya catalogue, as listed in Table 3. the principal powers were reversed, a Single-vision lenses can be broadly Inspection of the above prescription little more thought would need to be divided into the following groups: will reveal that the principal meridians given to the selection of an appropriate ● Meniscus. A spherical lens with two of the lens are roughly vertical and frame. In order to equalise the nasal and spherical surfaces, one convex and one horizontal. The principal powers are temporal edge thicknesses the frame concave -3.00D along 170 and -4.25 along 80 should have a box centre distance that ● Toric. A meniscus lens incorporating in the right eye and -3.50D along 10 is close to 64mm (BCD = DCD). The a toroidal surface for the correction of and -4.50D along 100 in the left eye. frame chosen for this example had a astigmatism, the other surface being BCD of 65mm resulting in only 0.5mm spherical in form. The majority of toric of inwards decentration in both eyes. lenses incorporate a concave toroidal Table 3 Plastics lens materials offered by Hoya Excessive inward decentration will surface. Bi-toric lens are occasional cause an increase in the temporal edge Product Refractive V-value Density encountered in practice 3 substance. Practitioners play a very ● Aspheric. One aspherical surface index (ne) (Ve) (g/cm ) important role in helping clients select combined with either a spherical or Plastics materials frames that are cosmetically pleasing toroidal surface CR39 1.50 58 1.32 and appropriate for a client’s facial ● Double aspheric. A lens that employs features. However, practitioners must two aspherical surfaces PNX (Trivex) 1.53 43 1.11 also help clients to select frames that are ● Atoric. A lens that employs an Eyas 1.60 41 1.32 suitable for the given prescription. It is atoroidal surface for the correction of Eynoa 1.67 31 1.37 therefore important that optometrists astigmatism and dispensing opticians can visual- ● Free-form. A free-form lens is a lens Eyry 1.70 36 1.41 ise the mechanical appearance of the that is manufactured without a prede- finished lens. Table 4 gives the edge and termined surface. A free-form surface is a surface that cannot be defined with a Table 4 Edge and centre thicknesses for the lenses used in example 1 single mathematical equation. Single-vision lens availability extends Product Centre thickness Max edge thickness Min edge thickness right across the range of powers and Hilux 1.5 (spherical) R 2mm L 2mm R 4.2mm L 4.5mm R 2.7mm L 2.7mm materials with plastics lenses being avail- Nulux 1.5 (aspherical R 1.8mm L 1.8mm R 3.8mm L 4.0mm R 2.5mm L 2.5mm able in refractive indices from 1.498 to 1.74 and glass lenses from 1.523 to 1.90. Hilux Eyas 1.6 R 1.4mm L 1.4mm R 3.2mm L 3.5mm R 2.0mm L 2.0mm (spherical) Single-vision dispensing example 1 Nulux Eyas 1.6 R 1.1mm L 1.1mm R 2.8mm L 3.0mm R 1.7mm L 1.7mm Client 1 is a male in his early 20s and has (aspherical) been wearing spectacles since his early Hilux Eynoa 1.67 R 1.0mm L 1.0mm R 2.6mm L 2.8mm R 1.5mm L 1.5mm teens. His prescription details are: (spherical) Right -3.00/-1.25 x 170 6/5 Left -3.50/-1.00 x 10 6/5 Nulux Eynoa 1.67 R 1.0mm L 1.0mm R 2.5mm L 2.7mm R 1.5mm L 1.5mm Monocular distance centration (aspherical) distances were 32mm in both eyes. Nulux Eyry 1.70 R 1.0mm L 1.0mm R 2.4mm L 2.6mm R 1.5mm L 1.5mm This example gives the practitioner the (aspherical) opportunity to explore various material

24 | Optician | 12.12.08 opticianonline.net CET Continuing education

centre thicknesses for appropriate Hoya Table 5 Lens details: Stock Eynoa Hilux products for this prescription. The lens of choice, both mechani- Centre thickness Max edge Min edge cally and optically, is the Nulux Eyry. thickness thickness The Nulux Eynoa produces almost the Right 5.3mm 4.6mm 3.0mm same edge thickness but this material Left 4.7mm 4.1mm 2.7mm has a lower V-value (31 compared to 36) which may result in reduced off- axis performance. When comparing the spherical and aspherical options, the use material which has a refractive index of specified with an intermediate addition of an aspherical surface results in a reduc- 1.67. Table 5 has the details. or a trifocal lens can be specified. Trifocal tion in lens substance in all cases. The lenses (Figure 5) contain an intermedi- aspherical surface will also produce an Option 2: Minimum edge thickness ate portion usually located between the improvement in off-axis performance. surfaced, spherical surfaces distance and near portions. Examples of Again the material used was Eynoa bifocal and trifocal availability are given Single-vision dispensing example 2 with a refractive index of 1.67. The in Tables 8 and 9. Client 2 is female and in her mid 30s, who use of minimum edge thickness surfac- Due to a decline in demand, bifocals is generally dissatisfied with her last pair ing techniques produces an appreciable and especially trifocals are not available of distance spectacles. A stable refraction improvement in weight, thickness and in as wider range of lens types and materi- of right +5.50 DS and left +4.75 DS was cosmesis. Table 6 has the details. als as was once the case. Lenses are avail- found. Visual acuities were 6/6 in both able in spherical, toroidal, aspheric and eyes. Discussion with the client revealed Option 3: Minimum edge thickness reduced-aperture lens forms.Although that she finds her current lenses to be far surfaced, aspheric lens the range of available indices is gener- thicker and heavier than a previous pair The refractive index of the chosen lens ally smaller, 1.74 spherical and atoroi- despite having paid an additional fee to material was 1.70 (Eyry). The use of an dal curve-top bifocal lenses have been ‘upgrade’ them. Poor vision in oblique aspherical surface (along with a slightly recently introduced. gaze was also an issue, with her feeling higher refractive index) has produced a Most bifocal and trifocal plastics unsteady when using stairs. Inspection of further reduction in lens substance and lenses are cast moulded as front-surface the lenses suggested that a higher refrac- will ensure good off-axis performance. segment lenses. Glass bifocal lenses are tive index lens had been dispensed with Table 7 has the details. either solid or fused. Examples of current a distance centration distance of 62mm The lens of choice is therefore option bifocal and trifocal lenses are given in and a frame box centre distance of 74mm 3. A further option would be to use a Tables 8 and 9. resulting in 6mm of inward lens decen- bi-aspheric (Eyry Nulux EP). This lens, Specialist bifocal and trifocal lenses tration. Points for consideration are: using free-form technology, offers a wide will be covered later in this series. ● Control of the lens substance by visual field with clear and stable vision Although bifocal lenses are not exten- considering frame size and decentration in oblique gaze and reduced spectacle sively used, particularly for first-time ● Lens material, form and surfacing. magnification. Hoya’s Hoyalog system presbyopic clients, they still provide a In this case, the client had previously (Figure 4) was used to determine lens simple and sometimes more appropriate worn spherical CR39 lenses in a frame thickness in the above examples. form of correction for presbyopes. As with a box centre distance that was much a general rule, if the distance prescrip- closer to the required centration distance Bifocal and trifocal lenses tion is minus, flat-top bifocal segments resulting in less decentration. The newer, Along with separate single-vision correc- provide better control of centration for larger frame and subsequent 6mm tions, bifocal lenses provide the presbyope near as the base-up prism produced by decentration had resulted in a greater with the traditional solution of combin- the segment will neutralise some of the lens substance despite the use of a higher ing distance and near vision corrections base-down prism generated by the main refractive index material. in a single lens with the reading addition lens. However, if the distance prescription On this occasion a new, smaller frame incorporated into a specific section of the is positive, round down-curve segments with swept out lugs was chosen to lens. For vocational purposes the distance provide better control of centration at avoid decentration and reduce the lens or near portions of the bifocal can be near. substance. The frame BCD was 64mm. To illustrate the above options, we will Table 6 Lens details: Surfaced Eynoa Hilux again use Hoya lenses. If we continue to D/S725 +728 use a high refractive index material, possi- Centre Max edge Min edge ble lens options available are: thickness thickness thickness 1 Stock lenses, spherical surfaces Right 3.3mm 2.6mm 1.0mm (maximum refractive index 1.67) Left 3.0mm 2.4mm 1.0mm 2 Minimum edge thickness surfaced, D spherical surfaces (maximum refractive index 1.67) I 7 3 Minimum edge thickness surfaced, Table 7 Lens details: Surfaced Eyry Nulux N aspheric lens (1.70 refractive index). Centre Max edge Min edge thickness thickness thickness Option 1: Stock lenses, spherical surfaces Right 3.0mm 2.4mm 0.9mm 25 or 28 From a cosmetic, mechanical and optical Left 2.8mm 2.3mm 1.0mm point of view, this is not a good choice. The material used was Hoya’s Eynoa Figure 5 Schematic of a trifocal lens opticianonline.net 12.12.08 | Optician | 25 Continuing education CET

Progressive power lenses Disadvantages of PPLs For the modern presbyope and many ● Peripheral surface astigmatism results eye care professionals alike the progres- in a narrower near and intermediate field sive power lens or varifocal has become of view when compared to a bifocal or the lens of choice. Progressive power trifocal lens lenses (or PPLs) are designed to provide ● Adaptation can be unpredictable and clear vision across a range of distances. can take a few days or even weeks In addition, wearers of PPLs experience ● Intolerances are more common place. no image jump between the different zones. PPL markings To assist practitioners in the identifi- The basic progressive lens cation of a particular lens, a universal In very simple terms, a PPL can be marking system is used for all progres- thought of as having two spherical sive lenses (Figure 8). With reference to surfaces, one for distance and one Figure 8, A is the distance power check- Figure 6 for near, connected by an aspherical ing point, B is the near power checking is applied to most PPLs in order to surface. The change in power within point, C is the nasal reference point, D the improve cosmesis, particularly the lens the progression corridor is achieved temporal reference point, E the fitting thickness at the upper edge of the lens. by continuously altering the radii of cross and F the prism checking point. Approximately 0.6Δ base-up prism is curvature between the distance and The nasal and temporal reference points removed for each dioptre of reading near zones. This change in power is are usually two engraved circles 34mm addition, effectively adding equal base brought about by the use of an aspheri- apart. The reading addition is engraved down prism in both lenses equivalent cal surface. The process of changing the under the temporal reference point with to 2/3 of the reading addition. This surface power creates unwanted aberra- the manufacturer’s identifying mark thinning prism should always be taken tional surface astigmatism either side the under the nasal reference point. into account when neutralising a progres- progression corridor, resulting in image A process known as prism thinning sive lens at the prism checking point. blur and restricting the usable width of the intermediate and near portions of the lens to a central ‘corridor’ (Figure 6). Table 8 Bifocal lens types The second consequence of a progres- Segment type Abbreviation Range of sizes Characteristics sive surface is that of skew distortion Flat-top S seg 25mm–45mm Good field of view at near (Figure 7) which occurs because a Reduced image jump change in power is accompanied by a Improved centration at near with minus change in magnification. Skew distor- lenses tion is responsible for the ‘swimming More obvious visible segment design effect’ reported by new wearers of Used in aspheric lens forms PPLs. Early PPL designs like the original Varilux, introduced by Essel in 1959, had Curve–top C seg 25mm-40mm Good field of view at near Reduced image jump a cast-moulded front surface progression Improved centration at near with minus with a fairly hard boundary between the lenses usable and unusable zones. Less obvious segment design Despite early limitations, technologi- Used in aspheric lens forms cal improvements in both lens design and manufacturing techniques have led Round R seg 15mm–45mm Poorer field of view at near to a rapid growth in PPL availability More image jump and performance. Modern progres- Improved centration at near with plus lenses sives have a far wider progression ‘Invisible’ segment corridor, with the boundaries between Executive E style Widest field of view at near the zones on the lens being far softer. No image jump This has allowed a far greater number The most obvious segment design of clients with a much wider variety Very small range of indices available of optical corrections to successfully adapt to PPLs. The advantages and some disadvantages for the presbyopic Table 9 Trifocal lens types wearer over the more traditional forms Segment type Intermediate seg depth Available Intermediate/near ratio of lens are: sizes

Advantages of PPLs Flat–top 7mm 28mm 50% of full reading add ● No visible segment Flat–top 8mm 35mm 50% of full reading add ● Lens appears to be a single vision thus Flat–top 14mm 35mm 60% of full reading add offering improved cosmesis ● No image jump as there is no abrupt Curve–top 8mm 28mm 50% of full reading add change in power Round Concentric 7 & 8mm Various 50% of full reading add ● Provides a good range of clear vision E style 7mm 50% of full reading add at all distances ● More natural use of available Double D Segs top & bottom 28mm 60% or same as full reading add accommodation.

26 | Optician | 12.12.08 opticianonline.net CET Continuing education

Flat-top bifocal Progressive power lens

A

E C F 225 D

B

Figure 7 Skew distortion Figure 8 Universal marking system for varifocals

Different PPL designs from modern design and manufactur- panels by his feet when landing and To compare the often subtle differences ing technologies. Many lens designs finds the increased head movement between various PPL designs we can now feature back surface and bi-surface required tiring by the end of the day. His study isocylinder charts that plot the progressions, anti-fatigue lenses incorpo- current lenses are older in design. Areas points on the progressive surface that rate smaller reading additions beneficial for consideration are: have the same amount of surface aberra- to the early and pre-presbyopic wearer ● Design of PPL tional astigmatism. Available from most requiring accommodative relief, and ● Working distances manufacturers, they provide us with an with individual lens designs, wearers ● Sun protection. illustration of the width and character- can benefit from greater fields of view, Further discussion with the client istics of the progression corridor for any improved visual comfort and quicker reveals that he rarely needs to read at given lens. Generic examples of isocylin- adaptation. 40cm, with his instruments being about der plots are shown in Figure 9. 90cm in front of him and he only needs With so many brand names and market- PPL dispensing example to use the lower quarter panels to view the ing terms in use today, a general appre- The client is a male in his late 40s who ground when it is about a meter away. ciation of the variety of different PPL flies helicopters for a living. With his He is dispensed with a free-form designs can help avoid the risk of intoler- presbyopia increasing he is finding it surfaced individual progressive design ance caused when switching lens brands, more difficult to cope with his current to provide a wide field of view. For especially if neither the practitioner nor tinted PPLs especially when viewing improved safety and sun protection, a the client expected a problem. the wide instrument panels in front of polycarbonate photochromic lens with As with single-vision surfacing, him. He also needs to be able to view a reflection-free coating is specified. progressive lenses have benefited greatly the ground through lower quarter-light The reading addition is also reduced to

Figure 9 Graphical isocylinder plots of the evolution of progressive addition lenses

Figure 10 The differing types of vocational degressive and progressive lenses opticianonline.net 12.12.08 | Optician | 27 Continuing education CET

increase working distances in the near Multiple-choice questions – take part at opticianonline.net and intermediate zones of the lens.

Vocational and degressive lenses Which of the following aberrations is of Which of the following bifocal options will The traditional idea of a progressive 1most concern in spectacle lens design? 8provide the best control of centration for lens incorporating correction for just A Coma near? distance, intermediate and near vision B Oblique astigmatism A Distance prescription -4.00D, round 38mm has been challenged in recent years by a C Distortion downcurve segment new breed of lens that offers practition- D Transverse chromatic aberration B Distance prescription -4.00D, flat-top S28mm ers the opportunity to select, from various segment lens designs, a far more tailored solution Which of the following statements is C Distance prescription +4.00D, flat-top S28mm for specific vocational tasks. The degres- 2most correct? segment sive, or enhanced near vision lens, has at A Spherical lenses with flatter surfaces give D Distance prescription -4.00D, round 28mm a stroke revolutionised the prescribing of better control of oblique astigmatism downcurve segment lenses for intermediate and near correc- B Spherical lenses with flatter surfaces give tions, and provides practitioners with a better control of distortion  Which of the following statements valuable new tool for occupational and C The form of a spectacle lens has a significant 9regarding progressive power lenses is vocational dispensing (Figure 10). effect on transverse chromatic aberration most correct? Through careful investigation at the D The form of a spectacle lens has an A Modern PPLs are usually ‘hard’ in design point of dispensing, often in the form of a insignificant effect on transverse chromatic B All new wearers of PPLs adapt to their lenses visual task analysis, practitioners can now aberration within 10 days select from a wide range of vocational C PPLs display no image jump progressive and degressive lens designs Which of following lenses cannot be D Peripheral surface astigmatism results in a that truly meet the demands of a modern 3described as a flat lens? wider near and intermediate field of view workplace. Examples of occupational A F1 = +4.00D, F2 = -1.00D when compared to a bifocal or trifocal lens and degressive lenses are the Essilor B F1 = +2.00D, F2 = +2.00D Interview, Essilor Computer 2V and 3V, C F1 = +4.00D, F2 = plano Which of the following statements Rodenstock Nexyma, Zeiss Gradal RD D F1 = +4.00D, F2 = +1.00D 10regarding PPLs is most correct? and the Hoya Hoyalux Workstyle. A Skew distortion is a result of surface Which of the following statements is aberrational astigmatism Summary 4correct? B Lens wearers often describe surface This article has only provided a very brief A The base curve of a finished toric lens is aberrational astigmatism as a ‘swimmimg overview of the many lens designs and the lower of the two powers worked on the effect’ materials available. As eye care profes- toroidal surface C Skew distortion is also troublesome in bifocal sionals, a good working knowledge of B The base curve of a finished toric lens is the lenses the available lens types, designs and their higher of the two powers worked on the D Skew distortion occurs because a change characteristics is an essential part of our toroidal surface in power is accompanied by a change in role. Not only must we remain familiar C The base curve of a finished toric lens is magnification with the more traditional lens designs taken to be the curve worked on the spherical and materials available but we must also surface Which of the following statements keep up to date with the ever changing D The base curve of a finished meniscus lens is 11regarding PPLs is most correct? range of new products and technologies the higher of the two surface powers A The distance between the engraved circles on that make our profession so diverse and a PPL is different for every brand so interesting to work in. ● Which of following is not an advantage of B The reading addition is engraved under the 5a low-powered aspheric lens? temporal reference point Further reading A Good off-axis performance C The manufacturer’s identifying mark is Fowler C and Latham Petre K. Spectacle B Reduced surface reflections engraved under the temporal reference point Lenses: Theory and Practice Butterworth C Less spectacle magnification D Prism thinning is always beneficial Heinemann Oxford UK, 2001. D Reduced sensitivity to fitting distance Jalie M. Principles of Ophthalmic Lenses 4th Which of the following lenses is best edition The Association of British Dispensing Which of the following surfaces is 12described as an enhanced reading Opticians London UK, 1984. 6commonly employed in a low-powered lens as opposed to an occupational or Jalie M. Ophthalmic Lenses & Dispensing 3rd aspheric lens? workstation lens? Edition Butterworth Heinemann Oxford UK pp A Parabola A Hoyalux Workstyle 34-36, 2008. B Hyperbola B Zeiss Gradal RD Keirl AW. Occupational Dispensing for the C Oblate ellipse C Rodenstock Nexyma 80 Presbyope, Optician, 2005; 5988 (229) 25-31. D Prolate ellipse D Essilor Interview Tunnacliffe AH. Essentials of Dispensing. ABDO, London, 1995. Which of the following bifocal segments Take part at www.opticianonline.net Norville Optical Prescription Companion. 5th 7will produce most image jump? Successful participation in each module of this series Edition. A A flat-top S28 segment counts as two credits towards the GOC CET scheme B A flat-top S35 segment administered by Vantage and one towards the ● Optometrist and dispensing optician C A round 38mm downcurve segment Association of Optometrists Ireland’s scheme. Andrew Keirl runs his own independent D A round 28mm downcurve segment The deadline for responses is January 8 2008 practice in Cornwall. Richard Payne is a dispensing optician working in private practice in Cornwall

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