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Material changes--plastic lenses of the last decade.

Dispensing optician, Phil Gilbert delivers a structured overview on the changes to the plastic lens market in recent times, giving an account of the relative merits of new products in an evolving market.

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With a rapidly changing lens market it can be difficult to keep on top of the vast range of products available. This article gives a concise overview of the latest lens materials in common use, identifies those that are falling out of favour, and looks at what the future may hold.

Introduction

Advances in lens materials in the last half of the 20th century have allowed eyecare professionals to offer their patients higher performing lenses than ever before. The introduction of plastic lenses by Univis in the 1960s began the revolution by presenting a lighter, thinner option than glass lenses. Polycarbonate, introduced in the 1970s, provided an impact-resistant option for patients, although lacked the optical performance of CR39 or glass. Within the past decade, dozens of plastic lens materials in higher refractive indices have been developed, enabling thinner and lighter lenses than ever before, though they have failed to rival the impact resistance of polycarbonate.

Changing times

Past and present copies of the ABDO publication Ophthalmic Lenses Availability (OLA) from 2005 to 2013, demonstrate the incremental changes to the refractive index of plastic lens materials over time. This has led to a completely different menu being offered by major lens casters and optical laboratories over the past 10 years. Referencing the 2005, 2009, 2012 and 2013 copies of the OLA allows us to understand the products offered in 2014.

The most notable change over the last 10 years is how the mid-index market has been affected. Increased availability from 2005 to 2009 was followed by depletion in more recent years, particularly with regard to clear stock and prescription lenses (see Table 1, page 47).

[FIGURE 1 OMITTED]

There were two main reasons for the changes to the mid-index materials market: the introduction of Trivex[TM] lenses and the reduction of production costs of 1.6 index material. Previously, 1.6 index lenses were expensive to produce and represented a considerable increase in retail cost compared with CR39. The subsequent price reduction allowed major manufacturers to migrate from the production of other mid-index materials. More recently, the introduction of Tribrid[TM] 1.6 index material in 2013 has reduced the demand for lower mid-index clear lenses even further.

Types of plastic lenses

Plastic or organic spectacle lenses can be divided into two distinct categories termed thermosetting and thermoplastic, each with their own characteristics and different manufacturing processes. Thermosetting materials--for example, hard resin--are cast in moulds as a liquid monomer and then cured or polymerised. This is performed using either a photo or chemical catalyst that turns the liquid into a solid. Cross-linked molecules form the lens, resulting in a lattice structure, which has good scratch-resistance at the expense of impact-resistance.

Thermoplastic lenses, such as polycarbonate on the other hand are injection-moulded as a hot liquid, which is then cooled; this material forms long chain molecules that can slide past each other. In contrast to thermosetting material, it is often less scratch-resistant but more impact-resistant, though it can be melted down and remoulded.

The arrival of Trivex[TM], a hybrid design, creates a third material category and captures the best properties of thermosetting and thermoplastic. The polymer chains in Trivex[TM] material are lightly cross-linked with strong polar interactions between the polymer chains to allow energy to be absorbed when pressure is applied to the material. The properties of Trivex[TM] allows for drilling, making it a suitable choice for rimless glazing.

Spectacle lens properties

The main material properties of plastic spectacle lenses fall into several categories, including:

* Refractive index

* Specific gravity or density

* Reflectance

* Abbe number or V-value

* UV absorption

* Impact resistance

* Durability.

The main aim of the spectacle lens producer is to manufacture lenses that satisfy all of these criteria to the highest possible standard within the confines of the material being used and the available production techniques. The resultant product must then be usable and perform well, not only across a broad range of prescriptions, but also accommodate the glazing requirements for modern spectacle frames.

Refractive index

Refractive index is the relationship between the speed of light in a vacuum and the speed of light in the lens. At present, in the UK and the USA, refractive index is measured on the helium d-line (wavelength 587.56nm) whereas in Continental Europe it is measured on the mercury e-line (wavelength 546.07nm). Note that the value for lenses followed by ne is a little greater than for nd, so that when the value of ne is given, the material appears to have a slightly higher refractive index.

A principle demand from the consumer is for optimal cosmetic appearance and the thinner the lens, the more acceptable it is likely to be. However, as the manufacturers race to produce thinner lenses, with higher refractive indices, the materials become softer, reflect more light, have lower V-values and incur more dispersion.

Specific gravity or density

Specific gravity refers to the particular weight of a material. Generally, higher refractive index materials have a greater specific gravity, as the material is denser. Ideally, a lens should be produced using material with the lowest possible specific gravity. The density shows the specific weight of the lens material in g/[cm.sup.3]. The weight of a spectacle lens depends also on the refractive index.

Reflectance

A disadvantage of higher refractive index material is the proportional increase in surface reflectance. Whereas the light reflected per surface on a 1.5 index is just over 4% per surface, it reaches a very high 9% per surface on a 1.9 index glass lens. This denotes the need for anti-reflection coatings on all lenses with higher indices. Light is reflected from the front and the back surfaces and generally most plastic lenses have transmission levels of between 85-92%. The formula for calculating reflectance is shown below:

p = [(n - 1).sup.2]/[(n + 1).sup.2] x 100%

An example of the reflectance per surface of a 1.67 index lens would be:

P = (1.67-1 / 1.67 + 1)2 x 100 = 6.3%

Abbe number or V-value

A further disadvantage of higher indices is the lowering of the Abbe number or V-value. This indicates the amount of dispersion that a lens produces. Lenses with a low refractive index have high V-values but an increase in refractive index lowers the V-value, creating greater dispersion and results in noticeable chromatic aberration towards the lens periphery.

UV absorption

The issue of ultraviolet (UV) protection is becoming increasingly important due to public awareness of the thinning of the ozone layer and the correlation between UV exposure and its association with changes to the eye such as cataract and pterygia. UVA and UVB rays are considered to be the most harmful and lens casters are now adding UV inhibitors into their higher index lenses. Inhibiting a plastic lens up to 380nm can usually be done without affecting the clarity of the lens; however, up to 400nm it usually creates a yellowing of the lens, which can be masked by over tinting with brown. The UV absorption of lens materials differs between casters, depending upon the UV inhibitor that is added to the monomer. The various properties of current plastic lenses are detailed in Table 2, page 46.

[FIGURE 2 OMITTED]

Material review

Today, a number of materials are available on the market.

1.5 CR39

At present, CR39 is still the material of choice for standard plastic lenses. Every lens supplier offers products made of CR39 as this material has a good optical quality, with a high V-value and is easy to surface, edge, drill and coat. The main drawbacks of the material are its low refractive index and low tensile strength. CR39 still has the highest market share of materials in Europe because the prices are attractive and the properties are adequate for most users. But the 'run for highest index' is very important as it enables the lens manufacturers to demonstrate their capability.

In the last decade the main focus for manufacturers has been to increase the refractive index of lens materials to satisfy the demand for thinner lenses, but at the same time taking care to ensure that this is not to the detriment of other lens properties.

1.53 Trivex[TM]

Trivex[TM] has a high impact resistance, and due to the crosslink design has a much better chemical and stress resistance than polycarbonate, coupled with an acceptable V-value of 43. The basic Trivex[TM] monomer price is high, approaching the level of 1.6 index materials. The monomer can be difficult to cast and did lead to a few early processing problems with regard to surfacing, coating, tinting and edging, although these difficulties have now largely been overcome. In comparison to polycarbonate the chemical resistance is very good and there is no cracking due to stress, essential for rimless glazing. Overlaying a polycarbonate lens with a polarising filter demonstrates the high degree of inherent internal stress within this type of lens (see Figure 1). In contrast, a similar test with a Trivex[TM] lens shows minimal internal stress (see Figure 2).

1.54 Index

Rodenstock now exclusively offer lenses produced in 1.54 index for their photochromic ColorMatic IQ. It is mass tinted and activates from 8%-85% absorption. It lightens to 57% after two minutes and to 45% after four minutes. It is available in single vision, bifocal and progressive formats.

1.56 Index

This material is manufactured by Corning and distributed as Sunsensors photochromic lenses. It is available in single vision, aspheric, bifocal and progressive lenses and a number of suppliers offer this material. The photochromic molecules are within the lens body as opposed to a surface coating with an 86% faded indoor light transmission factor.

1.59 Polycarbonate

Polycarbonate is renowned for its impact resistance. It was developed in 1957 by General Electric (Lexan) and used in the 1960s for helmet shields. The first spectacle lens was made in 1978 by Gentex USA. Polycarbonate lenses are manufactured using an injection moulding process with high volumes produced in a short time. However, they have a lower optical quality compared with other lenses and suffer with higher levels of internal stress. Due to its high impact resistance the market share of polycarbonate in the USA is now over 30%. In Asia and the majority of Europe the market penetrance of this material is subdued by its low V-value, poor chemical resistance, and susceptibility to cracking, particularly on contact with metal. The material requires a unique dry edging process in the glazing lab and applying a robust, tintable, hardcoat is difficult to achieve.

1.6 Index

1.6 Index material is rapidly becoming the entry-level index for modern practitioners. Recommended for rimless and in-line supra glazing, the benefits of increased resistance to breakage, combined with a thickness reduction of 17% compared with CR39, this index is gaining market share. To produce lenses with refractive index of 1.6, there are different materials in use. Notable materials are MR6 and MR8 from Mitsui Chemical in Japan. It is important to remember that even if different casters use the same materials, the final lens can differ in optical design and optical quality. The lenses are produced in a casting process as liquid monomer where different reactive components are mixed and a defined temperature controls the polymerisation process. MR6 was the most popular material for 1.6, prior to the development of the next generation, MR8, with a higher V-value. These materials have a very high tensile strength, good optical properties and can be tinted using a defined process.

1.6 Index Tribrid[TM]

Newly launched in 2013, the Tribrid[TM] lens material merges elements of Trivex technology with traditional high index processes to deliver a lens with excellent clarity, reduced thickness and strength, while also being lightweight. The arrival of this material allows the eye-care professional to make lens recommendation with limited compromise.

1.67 Index

The materials that are used for 1.67 index lenses are also made using the thermosetting process. The lenses are produced in a casting process with a liquid monomer similar to the 1.6 materials. Almost all lens manufacturers that sell lenses in Europe use the same material for their 1.67 lenses. The material can be manufactured using monomer MR7 with good tensile strength, or MR10, which is more temperature resistant.

1.74 Index

Formerly the highest commercially available refractive index for some time, a 1.74 material can yield an impressive 40% saving in edge thickness compared to the same prescription in CR39. It has a creditable V-value of 33 but can appear to have a yellowish tinge to the lens, is more brittle when drilled, and has a distinctive, unpleasant odour when being edged.

1.76 Index

Manufactured by the Japanese company Tokai Optical, this material, which has the ingredient Alkylene Sulfide Polymer, is currently the highest available refractive index plastic, and is supplied by several UK laboratories. Tokai was established in 1939 in Aichi, Japan and the company launched their 1.76 material in 2006. Relatively unknown in the UK, they started trading in Europe through Tokai Optecs N.V. in Belgium in 1995. Tokai supply both stock single vision and progressive designs and the material can be tinted in the mass up to 15% light transmission and is up to 47% thinner than standard CR39.

Conclusion

Needless to say the research and development of plastic lenses with even higher refractive indices is taking place as we speak and the race is on to develop the next generation of Super High Index materials. Watch this space.

MORE INFORMATION

References Visit www.optometry.co.uk/clinical, click on the article title and then on 'references' to download.

Exam questions Under the new enhanced CET rules of the GOC, MCQs for this exam appear online at www.optometry.co.uk/cet/exams. Please complete online by midnight on March 14, 2014. You will be unable to submit exams after this date. Answers will be published on www.optometry.co.uk/cet/exam-archive and CET points will be uploaded to the GOC every two weeks. You will then need to log into your CET portfolio by clicking on 'MyGOC' on the GOC website (www.optical.org) to confirm your points.

Reflective learning Having completed this CET exam, consider whether you feel more confident in your clinical skills-- how will you change the way you practice? How will you use this information to improve your work for patient benefit?

Course Code: C-34968 | Deadline: March 14, 2014

Learning objectives

To be able to guide patients towards appropriate lens choices by meeting their expectations (Group 1.2.1)

To be able to interpret and respond to patient records and provide a suitable optical correction (Group 2.2.5)

To be able to dispense a range of lens forms appropriate to the refractive error (Group 4.1.5)

Learning objectives

To be able to guide patients towards appropriate lens choices by meeting their expectations (Group 1.2.1)

To be able to interpret and respond to patient records and provide a suitable optical correction (Group 2.2.5)

To be able to dispense a range of lens forms appropriate to the refractive error (Group 4.1.2)

Phil Gilbert is a qualified dispensing optician with over 40 years' experience. He currently works as an ophthalmic lens consultant for Carl Zeiss Vision UK Ltd. He is a committee member of BSI TC/172 Ophthalmic lenses and the chairman of the Standards Panel of the Federation of Manufacturing Opticians (FODO). He has numerous publications in peer- reviewed journals and is the editor of ABDO's, Ophthalmic Lenses Availability, which lists and describes every spectacle lens available in the UK.
Table 1 Plastic lens availability from 2005 to 2013

Materials
2005   1.54         1.55       1.56              1.7

       AO                      Nikon
       SOLA                    Essilor
                               Signet Armolite
2009   1.54         1.55       1.56              1.7
       AO                      Essilor           Hoya
       SOLA                    Shamir
       Rodenstock              Lentoid
                               Norville
                               Signet Armolite
2012   1.54         1.55       1.56              1.7
       AO           Norville   Lentoid           Norville
       Lentoid                 Norville          Hoya
       Rodenstock              Shamir
                               Signet Armolite
2013   1.54         1.55       1.56              1.7
       Rodenstock              Lentoid           Hoya
                               Shamir
                               Signet Armolite
                               Jai Kudo

Table 2 Properties of plastics lenses

Index   Lens type                  Density   V-value   UV abs

1.50    CR39                       1.32      58.0      355nm
1.53    Trivex                     1.11      43.0      400nm
1.54    Rodenstock ColorMatic IQ   1.21      43.3      400nm
1.56    Corning Sunsensors         1.19      40.0      380nm
1.59    Polycarbonate              1.20      31.0      385nm
1.60    Plastics mid index         1.30      41.0      395nm
1.60    Tribid                     1.23      41.0      400nm
1.67    Plastics high index        1.36      32.0      395nm
1.74    Plastics very high index   1.47      33.0      395nm
1.76    Tokai                      1.49      30.0      400nm
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Title Annotation:CONTINUING EDUCATION & TRAINING: 1 CET POINT
Author:Gilbert, Philip
Publication:Optometry Today
Article Type:Essay
Geographic Code:4EUUK
Date:Feb 14, 2014
Words:2837
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