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 Wseen 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.