Continuing education CET Complete course in dispensing Part 5 – Ophthalmic lens materials

In the latest in our complete course in dispensing, Andrew Keirl and Richard Payne look at lens material properties and how they may be best selected for the final appliance. Module C10130, two general CET points, suitable for optometrists and dispensing opticians

n recent years, the optical profession Table 1 has seen a revolution in spectacle lens Lens materials offered by Hoya technology and in the vast variety of 3 lenses that are commercially avail- Product Refractive index (ne) V-value (Ve) Density (g/cm ) able. Some of us will remember Plastics materials Iwhen there was no need to question the CR39 1.50 58 1.32 V-value of an ophthalmic lens material since we really had little or no choice in PNX (Trivex) 1.53 43 1.11 what we were dispensing. Today, the Eyas 1.60 41 1.32 practitioner has a wide selection of lens designs and lens materials to choose from Eynoa 1.67 31 1.37 and the choice of lens material is often Eyry 1.70 36 1.41 one of the first factors to be considered Glass materials when dispensing a new prescription. To do this effectively the practitioner must 1.5 1.52 56 2.54 have a clear understanding of the proper- LHI-2 1.6 1.6 41 2.63 ties of ophthalmic lens materials and be LHI 1.7 1.7 40 2.99 able to communicate these facts to the client. THI-2 1.8 1.81 33 3.47 THI-1 1.9 1.89 31 3.99 The ‘ideal’ spectacle lens In order to provide our clients with the optimum correction for their refractive error, spectacle lenses should be: using heat. Thermoplastic materials such usually provided by a manufacturer or ● Optically and mechanically stable as polycarbonate are injection moulded supplier. These are refractive index, V- ● Free from aberrations as a hot liquid and then cooled to form value (constringence or Abbe number) ● Free from reflections long chain molecules, resulting in materi- and density. These properties enable us ● As thin as possible als that offer increased impact resistance. to achieve or at least control some of the ● As light as possible Trivex is a urethane based pre-polymer objectives listed above. ● Abrasion resistant that does not fall into either the thermo- ● Lens thickness is controlled by the ● Impact resistant setting or thermoplastics categories. refractive index of the material ● Easily tinted So what do the numbers in Table 1 ● Off-axis vision is affected by the V- ● Available with a range of surface actually mean? When choosing a lens value of the material processes with the ability to retain hard material for a given prescription, the ● The weight of the lens is affected by and reflection-free coatings without practitioner must consider three well- the density of the material. complex chemistry known physical properties that are ● Easily manufactured at a reasonable Refractive index cost. Series modules Refractive index is the term given to Most of the major lens manufactur- the ratio of the velocity of light of a ers and suppliers offer a wide (and ● Practice development, marketing and given frequency in air to the velocity similar) range of lens materials with communication of the same frequency in the refracting various surface processes and options. It ● Prescription interpretation medium. It is important to note that is of course impossible to mention every ● Frame design the refractive index of a material varies product on the market, so as an example, ● Spectacle lenses with wavelength. Two wavelengths are Table 1 gives an overview of the materi- ● Tints and coatings commonly used to measure refractive als offered by Hoya. ● Dispensing index. The helium d-line is used in the Plastics materials can be classified as ● Specialist lenses UK and the US. This has a wavelength thermosetting or thermoplastics. CR39 is ● Ordering, checking and collection of 587.56nm and gives a refractive an example of a thermosetting material. ● Non-tolerance index (nd) of 1.523 for crown glass. In Such materials are cast in moulds as a ● Glossary continental Europe however, refractive liquid monomer and then polymerised index is measured on the mercury e-line,

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Table 2 Curvature or Curve Variation Factor. tion unless you know what the client is Classification of refractive Indices The relative curvature of a material is already wearing. If thin lens theory is given by (1.523 - 1)/(nmat - 1) where nmat assumed to be sufficiently accurate then Normal index 1.48 but < 1.54 is the refractive index of an alternative the following procedure can be used to Mid index 1.54 but < 1.64 (usually a higher) refractive index, and estimate the refractive index of a lens. High index 1.64 but < 1.74 1.523 is 4the refractive index of crown The procedure requires the use of a lens glass. However, if a higher refractive measure and a focimeter. Very high index 1.74 and above index material is being compared with ● Record the surface powers F1 and F2 CR39 then (1.498-1)/(nmat-1) should be of the lens using a lens measure used. Relative curvature is used to indicate ● Calculate the power of the lens as Table 3 the degree of flattening achieved by the determined using the lens measure, FL, Illustration of curvature, sag and edge thickness use of a higher refractive index material. using FL = F1 + F2 reduction for a -12.00 D plano-concave lens As it is also a reasonably accurate indica- ● Record the power of the lens using a made using materials of normal, high and tor of relative lens thickness; it can be focimeter, FF very high refractive indices. r2 is the radius of used to predict the reduction in edge ● Calculate the relative curvature using curvature of the back (concave) surface, s is the 2 thickness obtained. Table 4 gives typical RC = FL/FF sag of the back (concave) surface and te is the ● edge thickness. The centre thickness of the lens relative curvature values for materials When the relative curvature is known is 1.0mm compared to spectacle crown glass of the refractive index of the lens can be refractive index n = 1.523. estimated. For example, if RC = 0.75, Refractive r2 s2 te From Table 4, a lens material with the refractive index of the lens must be index a refractive index in the region of 1.7 in the region of 1.7. Alternatively, the 1.523 43.58 mm 7.88 mm 8.88 mm will have a relative curvature of 0.75. RC expression given above can be used 1.701 58.42 mm 5.62 mm 6.62 mm This means that a lens made using this to calculate the refractive index of the material will require only 75 per cent of lens. 1.890 74.17 mm 4.34 mm 5.34 mm the curvature compared to the same lens This method is accurate enough to made using crown glass. In other words, distinguish between materials of refrac- the higher refractive index material will tive indices of 1.5, 1.6, 1.7, 1.8 and 1.9 wavelength 546.07 nm. This wavelength result in a lens that is 25 per cent flatter if the lens measure is used carefully and gives a refractive index (ne) of 1.525 for than the crown glass equivalent lens. accurately. The lens measure must be crown glass. This can be misleading, as This should be evident if the values for held perpendicular to lens surface and the material appears to have a higher r2 given in Table 4 are now compared. if the lens under test is astigmatic, paral- refractive index. Relative curvature can also be used lel to a principal meridian. It must be With the wide range of materials to predict the dioptric appearance of a stressed that this method is an estimation available today, it is important to use a lens made using a material of a higher of refractive index and has a predisposi- consistent system when describing a lens refractive index if the power of the lens tion to inaccuracies! material as ‘mid’ or ‘high’ index, as vague in dioptres is multiplied by the relative generalisations can be ambiguous. Table curvature of the material to be used. Chromatic aberration 2 is taken from the British Standards Using a -10.00D lens as an example, the High refractive index materials are, of publication, Specification for Complete finished lens would ‘look like’ a -7.50D course, used with the aim of improving Spectacles (BS 7394-2 1994). lens (0.75 x -10.00) if it were made from the cosmetic appeal of spectacle lenses, Generally speaking, higher refrac- a material with a refractive index in the particularly in the correction of myopia tive index materials are used to produce region of 1.7. If the same lens were and in general terms, the higher the thinner lenses. This is particularly manufactured using a material with a refractive index the thinner the finished common with myopic prescriptions. A refractive index of 1.8 its appearance lens. However, there is a disadvantage reduction in thickness is achieved by would be similar to a -6.50D lens and that can occur when higher refractive the fact that a lens made using a higher so on. index materials are used. refractive index material requires flatter The third practical use of relative curva- The term chromatic aberration or curves to produce the stated power in ture is in the estimation of the refrac- dispersion refers to the inability of differ- comparison to a material with a lower tive index of a lens. This is obviously an ent wavelengths of light within a pencil refractive index, for example, crown important consideration if a client visits of rays to focus at the same point and arises glass or CR 39. The flatter curves result your practice for the first time and you due to the fact that the refractive index in a decrease in the sag of the surface and suspect that the client is wearing a lens of a material varies with the wavelength therefore a reduction in lens thickness. made from a higher refractive index of light under consideration. This is This can be illustrated by considering a material. It is impossible to give the client because light of different wavelength plano-concave lens of power -12.00D, the correct advice as to material selec- travels at different velocities through a diameter 50 mm and centre thickness 1.0 mm. The lens is to be made in three Table 4 refractive indices, 1.523, 1.701 and Relative curvature values. Four higher refractive indices are compared to 1.890. Table 3 has the results. Inspection crown glass of Table 3 shows that as the refractive index increases, the concave surface of Refractive index Relative curvature % Thickness reduction the lens becomes flatter. This flattening 1.6 0.87 13% results in a reduction in the sag of the 1.7 0.75 25% curve with a consequent reduction in edge thickness. 1.8 0.65 35% A very useful ‘spin-off’ from refrac- 1.9 0.58 42% tive index is a value known as Relative opticianonline.net 14.11.08 | Optician | 25 Dispersion produced by a thin plano-prism Continuing education CET Dispersion produced by a thin plano-prism

White light Three wavelengths

C (red) Incident white light Dispersed by the prism into its component colours d (yellow)

F (blue) Illustration of colour fringing as a result of A ray of light is incident at the surface transverse chromatic aberration. of a thin prism. The light is dispersed by the The angle between the ‘red and blue’ end of the spectrum is prism into its monochromatic constituents known as the angular dispersion or chromatic aberration

Figure 1 Dispersion produced by a thin plano-prism Figure 2 Dispersion produced by a thin plano-prism particular medium. As a result, red light is refracted less and blue light is refracted more than yellow light. Consequently, the shortest wavelengths of the visible The object is a dark bar against a bright background spectrum are deviated to a greater extent Appearance when than the longer wavelengths. Figure 1 viewed through a shows the refraction by a prism for three base-up prism wavelengths, while Figure 2 attempts to illustrate the refraction (dispersion) of all wavelengths within the visible Appearance when spectrum. Chromatic aberration can viewed through a be either axial or transverse. However, base-down prism the transverse variety is of most interest when dispensing spectacle lenses. Figure 3 Illustration of colour fringing as a result of transverse chromatic aberration. The object is Transverse chromatic aberration a dark bar against a bright background Transverse chromatic aberration (TCA) is an aberration which creates multi- blur at its ends. This effect will not inter- where nd is the refractive index for a ple images of objects. These images are fere with resolution. However, when the wavelength of 587.56 nm (yellow light), perceived by the spectacle wearer as base-apex line is perpendicular to the nF is the refractive index for a wavelength coloured fringes around the outline of bars, the image formed will suffer from of 486.13 nm (blue light) and nC is the an object and can be observed when a dispersion across the whole of the image, refractive index for a wavelength of subject views through off-axis points causing maximum image degradation. 656.27 nm (red light). The term (nd – 1) away from the optical centre of the is referred to as the refractivity of the lens. For example, if a myopic subject V-value and chromatic aberration material and the term (nF – nC) is known observed a dark window bar against The visual performance provided by a as the mean dispersion. The dispersion a bright background through a point spectacle lens material is related to the or spread of light will be greater when above the optical centre of the lenses V-value or constringence of the material a low V-value material is used compared (effectively a base up prism), the subject used. The V-value gives the practitioner to the dispersion produced by a high V- would perceive a blue line above the information regarding the amount of value material. The spread will always be bar and a red/yellow line below. These angular dispersion produced by the along the base-apex line of the prismatic effects would be reversed for a hyper- lens material and is used to calculate effect. Relating this to a spectacle lens, a metrope corrected with positive lenses the amount of TCA or angular disper- greater amount of dispersion causes the as the subject is now effectively looking sion produced when the client wears a image formed to be ‘less sharp’ than it through a base down prism (Figure 3). particular spectacle lens. The V-value is would be with a lower amount of disper- The effect of TCA is to ‘spread out’ the reciprocal of the dispersive power of sion. Vision may therefore be better the image formed of an object. Consider a material and along with the prismatic with high V-value materials. The visual an object in the form of a line emitting effect at a given point on a lens, gives acuity obtained by a client can therefore white light. When a prism with its base- an indication of the visual consequence be influenced to a certain extent by the apex line perpendicular to the line object of TCA to the client. Dispersion is the lens material chosen. The classification is placed before an eye, the retinal image splitting of white light into its compo- of V-value based on dispersion is given formed will comprise the component nent colours and V-value is a function below (BS 7394-2 1994). wavelengths of the spectrum, therefore of refractive index, which of course is a ‘spreading out’ the image over an area of function of the wavelength of the light ● Low dispersion V = 45 and above the retina. This dispersive effect always used to measure it. ● Medium dispersion V > 39 but < 45 occurs along the base-apex line of the ● High dispersion V < 39. prism. If the base-apex line of a prism V-value is given by is placed parallel to the lines of a target Table 1 provides V-values for Hoya’s glass in the form of a bar grating, the disper- nd-1 and plastics ophthalmic lens materials. sive effect of the prism will fractionally Vd = nf -nc When considering glass materials lengthen the image of the bars due to we can state that as the refractive index

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increases the V-value reduces as this is Table 6 always the case when considering glass A comparison of weights for four glass materials. The lens compared is a materials. However, it is not so easy to round 50mm diameter lens make this simple statement for modern plastics materials. Referring to Table 1, Lens Power 1.5 Glass 1.7 Glass 1.8 Glass 1.9 Glass Weight (g) Weight (g) Weight (g) Weight (g) the material with the lowest refractive index and highest V-value is, as expected, -4.00 11.11 12.0 12.8 13.6 CR39. PNX (Trivex) and Eyas have -6.00 13.3 13.6 14.3 15.6 different refractive indices but similar V-values whereas Eynoa and Eyry have -8.00 16.6 16.6 17.2 18.5 similar refractive indices but different V- -10.00 20.1 19.6 20.2 21.4 values. The visual performance of Eyry -12.00 23.5 22.5 23.2 24.4 is therefore theoretically better than that of Eynoa. It is interesting to note that the -14.00 26.4 24.8 25.5 27.4 V-Values of plastics materials are gener- -16.00 30.6 28.1 28.2 30.6 ally lower than that of glass equivalents. Transverse chromatic aberration For example, the V-value of a 1.7 glass -18.00 35.0 31.6 32.3 33.7 material is around 40 whereas the V- -20.00 42.0 35.4 36.0 37.1 value of a 1.7 plastics material can be in the low 30s. Also, the V-value of Eynoa (n = 1.67) has the same V-value (V = 31) as 1.9 glass! Pilkington used to produce a glass material known as Slimline 50. It had a Dispersed by the lens into refractive index of 1.7 with a very respect- Incident white light its component colours able V-value of 50.8. It was taken out c of production due to poor commercial Optical support. It is also interesting to note that centre because of its superior impact resistance, cF TCA = polycarbonate has become the default V material for paediatric dispensing in the US. The major drawback of polycar- F = power of spectacle lens (D) bonate is its low V-value of 30. However, c = distance from optical centre to point of incidence (cm) since the arrival on the market of Trivex V = V-value of the lens material a plastics material manufactured by PPG and used by Hoya as PNX and Younger TCA = transverse chromatic aberration as Trilogy, practitioners now have an alternative to polycarbonate as Trivex Figure 4 Transverse chromatic aberration has the following properties: ● It passes FDA testing at 1mm centre ● It requires no special edging equip- Prentice’s rule, the prismatic effect can be thickness ment required expressed as the product of the decentra- ● It meets BS EN 166-F and ANSI ● It can be manufactured with a centre tion in cm and the power of the lens in Z87.1 standards for eye protection thickness of 1.3mm for minus lenses. dioptres. It is therefore more useful to ● It withstands a pulling force of 80kg express TCA in the form of expression ● It withstands pressure of 10kg The calculation of TCA in spectacle 1 below: ● It has a higher V-value (43) when lenses compared to polycarbonate (30) In the case of a spectacle lens, angular cF (1) ● It displays a good resistance to dispersion is referred to as transverse TCA = scratching chromatic aberration (TCA) in the UK or V ● It absorbs UV to 395nm lateral colour in the US, to distinguish it ● It displays resistance to high from axial chromatic aberration, which This is illustrated in Figure 4. temperatures is not considered in spectacle lens design. The amount of TCA produced by a ● It displays resistance against solvents TCA is measured in prism dioptres (∆) spectacle lens therefore depends on three or in minutes of arc and is given by the factors: Table 5 prismatic effect at the point of incidence ● The power of the lens (F in dioptres) Density values for five glass lens upon the lens divided by the V-value of ● The distance from the optical centre to types the lens material. the oblique visual point (c in cm) 3 TCA in spectacle lenses (when ● The V-value of the material used. Lens type Density (g/cm ) measured in prism dioptres) is given TCA can manifest itself in two ways. 1.5 Glass 2.55 by: The increased dispersion with low V- 1.6 Glass 2.67 P value materials causes a spread of light TCA= within the image formed. This spread is 1.7 Glass 3.19 V along the base apex line of the prismatic 1.8 Glass 3.62 effect at the oblique visual point. This is 1.9 Glass 4.02 where P is the prismatic effect at the the tangential meridian and results in off- oblique visual point on the lens (Δ) and V axis blur known as tangential blur. High is the V-value of the material used. From contrast objects are seen with coloured opticianonline.net 14.11.08 | Optician | 27 Continuing education CET

Table 7 Table 8 Surface reflectance values for Reflectance and transmittance of various optical materials various indices Material Reflectance % Transmission % n P % Crown 1.523 4.5 91.6 1.523 4.3 Mid index 1.6 4.8 89.6 1.600 5.3 High index 1.701 5.3 86.9 1.700 6.7 High index 1.74 5.2 85.9 1.800 8.2 Chromatism produced by a -5.00D lens made Very high index 6.2 84.2 1.900 9.4 in three plastics materials Very high index 6.7 82.1 fringes when a client looks through a point on the lens away from the optical centre. Clients have been known to 9 V = 58 complain about this effect but it is often 8 V = 41 treated as novelty rather than a nuisance. V = 36

c) 7 Under conditions of low contrast, the ar 6 effect of TCA has more serious conse- of quences as it causes a reduction in visual 5 performance. This complaint often 4 presents in the form, ‘these lenses are fine when I view through the centres 3 CA (minutes (minutes CA but are blurred when I view through the T 2 edge.’ Any complaint is usually directed 1 towards distance vision as opposed to 0 near vision although the visual effects 0 5 10 15 20 25 30 of TCA can result in non-tolerance to Ocular rotation (degrees) progressive power lenses. It is generally accepted that the average threshold value Figure 5 Chromatism produced by a -5.00D lens made in three plastics materials. Data provided for TCA is 0.1∆. by Hoya Lens (UK) It is useful to be able to estimate the point of a spectacle lens where a client will notice the TCA threshold. This can easily be calculated using expression 1. the optical centres of high-powered, low It should also remembered that myopes It is generally agreed that if prescrip- V-value lenses at the zero pupil position do not need to use as much ocular rotation tions of 5.00D and above are dispensed with a zero pantoscopic tilt will reduce as emmetropes or hypermetropes to using materials with a V-value less than the effects of TCA and hence avoid observe at oblique angles in the object 58, then the client should be warned reduced visual acuity in primary gaze space because of the increased field of about the effects of off-axis blur and ● Use best form designs view with high minus lenses. Hence, colour fringing in oblique gaze. When ● Be sensible with frame selection less ocular rotation is needed which may considering the off-axis performance of – consider shape and size carefully mitigate the effects of TCA. a spectacle lens, the maximum ocular ● Fit the spectacle frame with as small a rotation is usually taken to be 30∆. At vertex distance as possible. The larger the Density and weight a fitting distance of 27mm, this angle vertex distance, the greater the distance The weight of a spectacle lens depends equated to a distance of only 15mm from the optical centre to the visual point on its shape and size, volume and the from the optical centre of a spectacle for a particular ocular rotation. So fitting density of the lens material. Density lens. The TCA produced by a -5.00D the lenses as close as possible to the eyes is given by mass/volume (g/cm3) and lens is illustrated in Figure 5. keeps the visual points as close to the gives an indication as to the weight of The major ophthalmic lens manufac- optical centres as possible and minimises the finished spectacle lens. In general turers are now producing higher the effects of TCA. terms, and for glass materials in particu- refractive index plastics materials Other advantages are to be gained by lar, the density of a material increases with reasonably respectable V-values. keeping the vertex distance as small as with refractive index. This does not However, there is a trend among possible. The field of view is larger, with mean, however, that the finished lens manufacturers to supply newer lens a smaller vertex distance. Both TCA and will be heavier. Naturally, we want designs only in plastics materials, which distortion increase with the distance of spectacle lenses to be as light as possi- have a refractive index of 1.6 or greater. the chief ray from the optical axis. So, ble. This means that the density of the These materials invariable have V-values with a smaller vertex distance, both material must be as low as possible. The in the region of 32. distortion and TCA are marginally less density of plastics materials are about To keep effects of TCA to a minimum, because the chief ray meets the lens half that of glass so that plastics lenses the following points should be consid- closer to the optical axis for a given eye are about half the weight of glass lenses. ered by practitioners: rotation. These effects are small but not However, in reality we cannot just ● Use materials with a high V-value insignificant and can be demonstrated by compare the densities of the materials ● Apply correct horizontal and vertical the application of accurate trigonometri- since higher refractive index materials centration and pantoscopic tilt. Placing cal ray tracing. will have less volume, owing to the

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fact that the same curve will produce a Reflectance chemist, fantastic advances in ophthal- higher surface power on a higher refrac- A further consequence of increasing mic lens technology have been made in tive index material. Unfortunately, the refractive index of a material is an the last 10 years. Lens materials chosen some low-density materials have very increase in surface reflectance. Surface are generally a compromise between low V-values. This means that a poten- reflectance (P) is calculated using the vision and thickness or weight. It is also tial reduction in off-axis vision must expression apparent that mid-index materials will be considered when dispensing low- soon become the ‘standard’ material. It density materials with low V-values. (n-1)2 is, however, important to ensure that the Polycarbonate is a good example here P = x 100% material best suited to the client’s require- because it has a low-density (1.2) and (n+1)2 ments is always dispensed. a low V-value (30). An example of a In some cases, this material may low-density material with a much more For a lens made from a material of be glass. While the use of glass as an respectable V-value is PNX which has a refractive index 1.5 the surface reflect- ophthalmic lens material is in decline, density of 1.1 and a V-value of 43. ance would be 4.00 per cent at the first there are some areas where glass can The weight of a spectacle lens can be surface and 3.84 per cent at the second outperform plastics. The demise of predicted by considering the density of surface. The total light transmitted by the glass spectacle lenses over recent years the material used. Manufacturers quote lens is therefore 92.16 per cent. The total has left many entrants into the optical the density of a material in g/cm3. It is light transmitted by the lens made from profession with little knowledge or convenient for the practitioner to assume a material of refractive index 1.8 would understanding of the attributes of glass that the density of the material is the be 84.3 per cent. It goes without saying as an ophthalmic lens material. This has same as the weight in grammes as one that lenses made using higher refrac- probably contributed the current lack cubic centimetre of the material. Table tive index materials should be supplied of recommendation for glass lenses. As 5 gives density values for five glass lens with multi-layer, reflection free coatings. with any product, there are advantages types. Indeed, some lenses are only available and disadvantages. Table 5 shows that, as the refractive with reflection free coatings. It is worth index of the lens increases, so does the mentioning at this point the fact that Advantages of glass lenses density. A density of 2.55 means that a surface reflections are increased if a low- ● Refractive indices from 1.5 to 1.9 one centimetre cube of the 1.5 material powered aspheric lens is used. Again, the ● Less constringence than with plastics will weigh 2.55 grammes. A one centi- solution is to use a multi-layer, reflection materials of equivalent refractive index metre cube of the 1.6 material will weigh free coating. The reflectance and trans- ● Resistant to surface abrasion 2.67 grammes and so on. mittance of various optical materials is ●Increased durability It is therefore true to say that high shown in Tables 7 and 8. ● Very high refractive indices means refractive index glass materials are that very high powers can be surfaced. heavier if we compare like with like, Conclusion for example, a one-centimetre cube. This An ideal spectacle lens material should Advantages of plastics lenses does not always translate to the finished have: ● Refractive indices currently from 1.5 spectacle lens. It is therefore incorrect to ● A high refractive index. The higher to 1.74 say that high refractive index glass lenses the index, the thinner the lens ● Lightweight and comfortable to are heavier than normal refractive glass ● A high V-value. Higher V-value wear lenses. materials exhibit less chromatism ● Most products can be tinted Because a high refractive index glass ● A low density. The lower the density ● Ideal for rimless mounts lens is thinner than a normal refractive the lighter the lens ● Resistant to solvents glass lens its volume will be reduced ● Low surface reflectance. Surface ● Stable up to about 100°C – there is less of it! This means that reflectance can be eliminated by apply- ● Ideal for use in protective eyewear. even though the lens is made from a ing a multi-layer reflection-free coating denser material its finished weight may to the lens. Further reading be less than the same lens made using Strategies for obtaining thinner and Cripps A J. Ophthalmic lens materials and a normal refractive glass material. In lighter lenses include the careful selec- properties Dispensing Optics, 2008; 23 5 lower powers however, it is probably tion of frame size and shape to ensure pp 4-10. true to say that high refractive index minimum decentration and the use of Fowler C and Latham Petre K. Spectacle glass lenses will be heavier. This is illus- aspheric lenses. Other factors which Lenses: Theory and Practice. Butterworth trated in Table 6. need to be considered when selecting a Heinemann Oxford UK, 2001. It is interesting to note that the 1.5 and lens material are: Jalie M. Principles of Ophthalmic Lenses 1.9 glass lenses weigh the same when ● Abrasion resistance 4th edition The Association of British the power reaches -16.00D, and the 1.9 ● Impact resistance Dispensing Opticians London UK, 1984. lens is actually lighter for -18.00D and ● Ease of tinting and availability of tints Jalie M. Ophthalmic Lenses & Dispensing 3rd -20.00D. This is due to the reduction in and filters Edition Butterworth Heinemann Oxford UK lens volume brought about by the use of ● Availability of surface processes and pp 34-36, 2008. a higher refractive index material. If the the ability to retain hard and reflec- Keirl AW. The properties of ophthalmic lens weight of the finished lens is the main tion-free coatings without complex materials Optometry in Practice, 2007; 8 4 priority for the client a plastics material chemistry 123-138. should be considered. If the refractive ● Mechanical stability indices and V-values of two materials ● Ease of processing. ● Optometrist and dispensing optician are similar then it is sensible to opt for The ideal lens material does not Andrew Keirl runs his own independent the lower density material. However, unfortunately exist. Perhaps it never practice in Cornwall. Richard Payne is a one should not forget other factors such will if we keep moving the goal posts. dispensing optician working in private as impact resistance and durability. However, with the aid of the polymer practice in Cornwall opticianonline.net 14.11.08 | Optician | 29 Continuing education CET

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Which of the following statements is incorrect? A -8.00D lens is made using a plastics material with a V-value 1 7of 32. The transverse chromatic aberration produced when an A Lens thickness is affected by the refractive index of an ophthalmic eye looks through a point 10mm below the optical centre of the lens material lens is: B Off-axis vision is affected by the V-value of an ophthalmic lens A 0.25Δ material B 0.20Δ C The weight of the lens is affected by the density of an ophthalmic C 0.15Δ lens material D 0.10Δ D Lenses manufactured using high refractive index materials are always heavier than lenses manufactured using low refractive index A subject looks through a point 14mm below the optical materials 8centre on a lens of power -5.00D. If the transverse chromatic aberration at that point is 0.175∆ the V-value of the lens material When measuring the refractive index of an ophthalmic lens will be: 2material, which wavelength is used in continental Europe? A 58 A 479.99nm B 40 B 486.13nm C 36 C 546.07nm D 30 D 587.56nm Which of the following statements is most correct? Which if the following terms best describes Hoya’s Eynoa 9 3material (ne= 1.67)? A For all mineral and resin spectacle lens materials, as the refractive A Normal refractive index index increases, so does the density B Mid refractive index B The term density simply relates to the weight of a finished spectacle C High refractive index lens D Very high refractive index C The density of a material is given by mass/volume D A lens manufactured using a low refractive index material will The relative curvature for polycarbonate (ne = 1.591) when always weigh less than the same lens manufactured using a very 4compared to a normal index material (ne = 1.502) is: high refractive index material A 1.13 B 1.18 The surface reflectance for a material with a refractive C 0.85 10index of 1.85 is: D 0.88 A 5.7% B 6.8% A plano-concave lens is made using a material of refractive C 8.9% 5index 1.7. A lens measure, placed on the concave surface of the D 9.4% lens gives a reading of -6.00D. The true power of the lens is: A -6.00D With reference to PNX (Trivex), which of the following B -7.00D 11statements is true? C -8.00D A It requires special processing equipment D -9.00D B It does not meet the requirements of BS EN 166-F C Its V-value is lower that that of polycarbonate A thin prism has the following refractive indices: nC = 1.527, D It is neither a thermosetting nor a thermoplastics material 6n d = 1.530 and nF= 1.536. Which of the statements regarding values for the mean dispersion, refractivity, dispersive power and A subject is dispensed with a pair of -5.00D lenses made V-value (Vd) of the prism are correct? 12using a material with a V-value of 50. Through what angle A Mean dispersion = 0.530, refractivity = 0.009, dispersive power = (to the nearest degree) would the subject need to rotate his eyes 58.88, Vd = 0.01698 away from the principal axis of the lens to experience the 0.1Δ B Mean dispersion = 0.530, refractivity = 0.009, dispersive power = threshold for transverse chromatic aberration? The fitting distance 0.01698, Vd = 58.88 is 27 mm and the pantoscopic angle is zero. (Hint: This question C Mean dispersion = 0.009, refractivity = 0.530, dispersive power = requires the use of trigonometry). 58.88, Vd = 0.01698 A 17° D Mean dispersion = 0.009, refractivity = 0.530, dispersive power = B 18° 0.01698, Vd = 58.88 C 19° D 20°

To take part in this module go to opticianonline.net Successful participation in each module of this series counts as two credits towards the GOC CET scheme administered by Vantage and one towards the Association of Optometrists Ireland’s scheme. The deadline for responses is December 11 2008

30 | Optician | 14.11.08 opticianonline.net