With an increase in the prevalence of contact lens wear in the UK and worldwide, more contact lens designs are being produced.
Combined with the developments in lens design and manufacturing processes, ever more complex products are coming to market. It is, therefore, important to have a good understanding of the key details of design, manufacture and the subsequent verifications of contact lenses. A sound knowledge of these aspects will enable practitioners to understand where issues of discomfort and variations in clinical performance may arise and how to try to mitigate these occurring through contact lens specification.
Contact lens manufacture
The purpose of this article is to review contact lens verification techniques; however, to have an understanding of the purpose and methods of inspection, the design and manufacture of contact lenses should also be understood. Since there are many comprehensive and existing resources detailing contact lens design and manufacture,3-5 only a brief overview will be given here.
There are two principal methods used in contact lens manufacture: cast moulding and lathe cutting. For cast moulded manufacture two moulds are created, one each for the front and back surfaces of the lens. Contact lens monomer is injected and spun into the cavity between the two moulds to form the contact lens shape. Typically the monomer is then polymerised in situ before removal from the mould and subsequent hydration and packaging takes place.3
For lathe production, lenses are manufactured from lens blanks where a rod or button of pre- polymerised material is machined in the pre-hydrated form into the desired lens shape. The process typically involves two cutting stages: one for the front surface and one for the back. Polishing is usually conducted after cutting the lens surfaces, although this may not be needed with advanced lathe tooling.6 The final machined lens is then hydrated (for a soft lens) or finished with any additional polishing (for a rigid lens) before packaging.3
Cast moulded manufacture is used for high volume lens manufacture, for example, daily disposable lenses, although the number of parameters available is often restricted due to a different mould being required for each lens- parameter design combination. However, lathe manufacturing is still employed for rigid gas permeable and lower volume custom soft lens designs, which are now typically produced using computer- controlled lathes.
With both manufacturing processes, parameter and quality verification typically occurs at two stages: pre- and post-hydration. Pre-hydration checks are usually simpler and are primarily for the purpose of rejecting defective lenses. Where the two methods differ is in the final lens inspection, with moulded manufacture typically relying on batch inspection post packaging and sterilisation and lathe manufacturing usually involving 100% inspection of all lenses before packaging and sterilisation.
Contact lens design
There are several different contact lens types, including hybrids,1,3-5 but the predominant types are rigid, soft and scleral contact lenses (Figure 1: Illustration of the predominant types of contact lenses, running from top to bottom: scleral, spherical soft, prism ballast soft and rigid. (Right) Key contact lens parameters as detailed in ISO 18369-1:2009, ISO 18369-3:2006 and ISO 18369-3:2006). Within each category there are further sub-designs, such as spherical, aspherical, toric or multifocal,7 and also more complex designs, such as reverse geometry lenses, available.
All lens designs have a number of key parameters in common, with additional parameters required for the more complex designs. Figure 1 shows the key parameters of lens designs as specified within the ISO standards (ISO 18369- 1:2009,7 ISO 18369-2:2012,8 and ISO 18369-3:2006,9) for contact lens manufacture, as detailed in Table 1 (Table 1: Summary of lens design parameters with abbreviations, symbols and tolerances from ISO 18369-1:2009, ISO 18369-2:2012 and ISO 18369-3:2006). However, only a small subset of the ISO specified lens design parameters are typically utilised by practitioners to specify a contact lens: back optic zone radius (BOZR), usually referred to as base curve on lens packaging, total diameter (TD), and back vertex power (BVP). For lathe manufactured soft and rigid lenses, practitioners need to decide on which contact lens material is most suitable for any given patient with regard to oxygen transmissibility and water content.
Before detailing how the design parameters are inspected, it is important to understand how variations in the key parameters will affect the contact lens itself and its potential fit and performance. Typically, the BOZR is reduced (creating a steeper curve) to create a tighter fit to eye as a result of an increase in lens sagittal depth (sag), and increased to loosen the fit due to a decrease in lens sag. However, the BOZR may not be an accurate predication of fit; despite nominally identical BOZR values, a variation in back surface geometry between manufacturers will cause different lens sag values to be present.10,11 The TD may also be altered to attempt to obtain a better fit, but careful attention must be paid to its combination with the BOZR, as variation of diameter with constant BOZR will alter lens sag.3 In addition, the package diameter may not represent the actual diameter on the eye with many materials showing significant changes of temperature, which can also apply to BOZR.12-14
The final parameter that can be specified is the BVP. The choice of BVP affects on the lens design through the lens shape particularly for the extreme ends of power. Often the front optic zone diameter and BOZR are adjusted to reduce or increase the centre thickness (CT) to ensure suitable lens handling, optical performance and oxygen transmission. For example, high minus power lenses typically are thin and, therefore, will have a thicker periphery and smaller optical zone to counteract the reduced CT. Higher plus power lenses will typically have a higher CT, therefore, optic zone diameters may be decreased to minimise the CT (see Figure 2: Illustration of the changes in thickness that can occur with varying the BVP with a constant BOZR and TD). As a result lens designs will have varying thickness changes across the lens and issues of lens handling and oxygen transmissibility must be considered,15 in addition to differing fitting characteristics.10 Manufacturers typically specify a CT for a -3.00D lens, however, there is little to inform on how the CT may vary for other power values.
The sagittal depth of the lens is emerging as a more suitable parameter for controlling the fit due to being a better comparator between different lens designs, rather than BOZR and TD. In addition, sagittal depth has more relevance to clinical eye measurements with calls for it to be included together with details on BOZR values.11
Contact lens inspection
Lens manufacturers will typically gear production quantities to meet the projected requirements of the marketplace. Regardless of the numbers produced, there will be variance in processes used, making the inspection and verification aspect of manufacture essential in ensuring that the final lens matches the desired specification.
A key aspect of manufacturing is tolerances, which are used to ensure that a product is delivered to a sufficient degree of accuracy for its intended purpose. To achieve 100% accuracy is extremely difficult and often practically impossible, so a tolerance specifies the acceptable amount that a dimension can vary above or below its nominal value and which does not impact on the resulting product being fit for purpose. For contact lens manufacture, ISO 18369-2:2012 provides the recommended tolerances, although often manufacturers will specify their own tighter tolerances. As an example, a soft contact lens of 14mm diameter has to be manufactured to within a tolerance of ±0.2mm following ISO 18369-2:2012, as this gives an achievable reproducibility in manufacture in addition to giving suitable performance on the eye. Table 1 summarises the tolerances for key contact lens parameters, although there are additional tolerances for other dimensional features of scleral and toric lenses not included for brevity.
ISO 18369-3:2006 defines a range of methods that can be used for inspection and verification for each contact lens parameter, as shown in Table 2 (Table 2: Summary of methods of contact lens dimensional inspection with reference to ISO 18369-3:2006 where appropriate. An example instrument is also included for reference). Due to the effects of temperature and state of hydration on contact lens materials, particularly soft materials,12,14,16; the ISO standards specify that as a minimum all inspection should be conducted at 20°C ± 5°C and, where possible, for soft lenses to be measured in a hydrated state with saline manufactured to specifications from ISO 18369- 3:2006. These precautions are particularly important for soft contact lenses due to being manufactured pre-hydration. The desired, hydrated, lens parameters are used to back calculate the pre-hydration manufacturing parameters based on material specific hydration expansion factors. The environment of manufacture and inspection must be carefully controlled to ensure that the final hydrated lens is within tolerance.
For each dimensional parameter, a brief summary of the key measurement methods will be given, highlighting potential issues with the methodologies, before a final summary of where variations in these parameters can occur.
Ophthalmometers (also known as keratometers) can be used for the measurement of rigid lenses, and soft lenses with some adaptation,3 however they are rarely used in practice. Optical microspherometers (also known as radiuscopes) use optical principals to measure the BOZR and are relatively simple to use with rigid lenses measured in air. However, soft lenses present difficulty due to the flexibility and dehydration that can occur when measuring in air,16 as they require the surface water to be removed before measurement.3 As a result, alternative method of BOZR measurement are more typically used, which rely on calculation of BOZR from the measurement of the chord sagittal height (S) using the following equation:
r2=(r-S)2+0.25y2 (equation 1)
Where y is the length of a chord on the lens surface, r is the radius of curvature and S is the chord sagittal height (Figure 3: Illustration of the chord sagittal depth measurement for calculation of BOZR. r=radius of curvature of lens. s=sagittal depth, y=Outside (chord) diameter of lens support, see Equation 1). By using a chord of a known diameter, the equation can be rearranged to calculate the BOZR from S, which is commonly measured using a mechanical probe but ultrasound techniques can also be used. Careful positioning is required to ensure the lens is central. Although commonly used, the sagittal height method can be inaccurate for more complex lens back surface designs as it assumes a spherical back surface.3
For measuring the TD, both a V-groove gauge and projection- based methods are suitable for rigid lenses and will utilise a scale that can be read manually. Projection- based methods typically provide a magnified (17x) image of a lens, with markings either on the projection screen or etched onto the surface the lens rests on. For soft lenses, projection based methods are more appropriate as these are simple to utilise in a wet cell and provide an accurate method of inspection.17 Although automated measurements employing video imaging techniques are possible, they are not within the current standards.
The CT can be measured using a contact method (dial and force gauges) where a low force mechanical probe will touch a contact lens placed onto a ball anvil, with the gauge giving the offset from the anvil as the CT. For soft lenses, CT measurement using these methods requires the lens to be dabbed dry and placed on the anvil before measurement, risking dehydration altering the measurement and potential adhesion of the lens to the anvil. In addition, correction factors are required due to the compressions that take place at the probe and lens interface.18
Although not within ISO standards to date, a range of alternative methods are available based on optical measurement methods. Ptychography involves a number of sequential diffraction patterns being produced using a laser aimed through a sample, which can then be used to calculate the power and thickness of lenses.19 Confocal measurement involves using a laser, typically of 1,310nm wavelength, to pass through the centre of the lens, recording the distance between peaks of the returned signal to provide a non-contact measurement of thickness.20 Two similar methods also use a single laser source: low coherence interferometry,21 and beam profile reflectometry.22 Although promising more precise measurements in addition to compatibility with a wet cell, temperature control should be utilised to try and control the sample adequately for measurement. Additional lens data in the form of the refractive index is also required to obtain accurate measurements.
Within the ISO standards the principal method of BVP inspection is the focimeter, which enables measurement of sphere, cylinder and prism powers. For rigid lenses, measurement is simple with an adapter to hold the lens, however for soft lenses all surface liquid must be removed before measurement, meaning that there is only a short time available to measure the lens power before dehydration occurs.23
For soft lens measurement,the remaining methods in the ISO standards are more suitable due to allowing for hydrated measurement through a wet cell. Moiré deflectometry uses a collimated light source and gratings to map deflections of light rays while Shack-Hartmann wavefront sensing utilises a laser source to generate beam deflections from a wavefront. A final method not currently within the standards is phase-shifting schlieren,24 using deflections from projected concentric rings to analyse power of lenses. All of these methods require knowledge of the contact lens refractive index (RI), CT and BOZR to obtain in air power readings, in addition to taking into account the influence of the wet cell itself. To ensure accurate measurements, the additional inputs should be measured for each lens rather than rely on package information, requiring additional instrumentation.
Importance of verification
The variations in design that are often requested by practitioners have been discussed, however what is received may not always be what was ordered, despite rigorous verification processes. Various studies have looked at the variation between the intended lens specification and their labelled parameters,10,18 in addition to the consistency of production,17,18,25 highlighting differences that can occur in a single batch of nominally identical lenses, which could have clinical implications.18,26
Furthermore, the choice of material and manufacturing method has also been shown to impact on the clinical performance of contact lenses.27 Finally, the environment in which the lenses are manufactured, inspected and worn can have a significant impact on the dimensions of a lens.13,14,16 All of these factors highlight why contact lens verification is a key process in the manufacture of contact lenses, and should be a major consideration when prescribing lenses to patients.
With the continued developments in contact lens design, in particular complex lenses for treatment of keratoconus and myopia through orthokeratology, the link between practitioner and manufacturer will continue to develop. Verification will become increasingly important as manufacturers continue to improve their production processes, in addition to ensuring that products closely match increasingly complex design specifications.
The article has illustrated the key aspects of contact lens design and manufacture, highlighting areas where variations in contact lens dimensions may occur and what impact this may have. It has also illustrated the methods that are available to verify contact lens dimensions, in addition to where these methods may not be suitable. It is hoped that with this information, practitioners will be more aware of where problems with contact lens fit and comfort may arise and in collaboration with manufacturers be able to mitigate these issues through improved contact lens specification.
About the author
Dr Benjamin Coldrick PhD, is a metrology engineer and researchers currently working at Optimec Ltd on developing novel contact lens inspection instrumentation through a knowledge transfer partnership with Aston University.
- Kerr C, McParland M (2013) The ACLM Contact Lens Year Book 2013
- Euromcontact (2012) A Comparison of European Soft Contact Lens and Lens Care Markets in 2012
- Efron N (2010) Part II: Soft Contact Lenses, in Contact Lens Practice. Butterworth Heinemann
- Efron N (2010) Part III: Rigid Contact Lenses, in Contact Lens Practice. Butterworth Heinemann
- Douthwaite WA (2005) Contact Lens Optics & Lens Design. Butterworth Heinemann
- O'Brien C, Charman WN (2006) Relative performance of soft contact lenses having lathe-cut posterior surfaces with and without additional polishing. Contact Lens & Anterior Eye 29(2): 101-107
- EN ISO18369-1 (2009) Ophthalmic optics - Contact lenses - Part 1: Vocabulary, classification system and recommendations for labelling specifications
- EN ISO18369-2 (2012) Ophthalmic optics - Contact lenses - Part 2: Tolerances
- ENISO18369-3 (2006) Ophthalmic optics - Contact lenses - Part 3: Measurement methods
- Young G, Holden B, Cooke G (1993) Influence of soft contact lens design on clinical performance. Optometry & Vision Science 70(5): 343-345
- Van der Worp E (2014) Decoding soft lens fitting. Contact Lens & Anterior Eye 37(6): 393-393
- Ozkan J, Ehrmann K, Meadows D et al. (2013) Lens parameter changes under in vitro and ex vivo conditions and their effect on the conjunctiva. Contact Lens & Anterior 36(4): 171-175
- Young G (2008) Daily disposable soft lens diameters. Optician 07.03.08
- Young G, Garofalo R, Peters S et al. (2011) The Effect of Temperature on Soft Contact Lens Modulus and Diameter. Eye & Contact Lens 37(6): 337- 341