Posts Tagged ‘plastic lenses’

On Your Bike with Varifocals and UV Protection

Friday, September 3rd, 2010

With Boris furnishing the capital with bikes and all of us striving to be greener, cycling is on the upsurge as a serious means of transport and an ever growing hobby. The lanes around our office are frequently enlivened by Lycra clad chaps pedalling furiously, apparently for fun!

The National Cyclists Association, the CTC, recently did some survey work amongst their members to look at safety on the roads, and discovered that one in four questioned had never had an eye exam or hadn’t had one in the last two years. Our governing optical body recommends that everyone has an eye test at least every two years, with some people being recalled sooner if eye health issues are a problem. Clearly, it’s as vital for Cyclists to make sure their vision is up to the driving standard as it is for motorists, so this is a worrying statistic.

As well of standards of vision, the survey looked at eye protection. 42% surveyed did not wear any sun protection, another worry as Cyclists are fully exposed to UV light when out and about, and this can affect the eyes on cloudy and sunny days. Specs of any description, tinted or clear protect the eye against UV, which can damage the retina and increases your risk of Cataracts. Specs also shield they eye from flying debris.

For the occasional Cyclist, large specs with plastic lenses will be protection enough, and photochromic lenses that go light and dark with the conditions will be a good general purpose option. You can wear them in bright sunlight or after dark, knowing they are safe in all light levels, and they give UV protection. The more serious Cyclist may wish to invest in a wrap around frame for full physical protection against the elements. You can also get them with specialist lenses that improve contrast and show up uneven road surfaces. Sides are usually contoured to fit the face under helmets, and the streamlined shape may even make you go faster! Brands such as Oakley have dedicated models for different sports, cycling included.

The bike may be the transport of the future, so if you’re taking to two wheels instead of four, invest that saved petrol money in an eye exam and some glasses that will make you safer.

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Wednesday, June 23rd, 2010

Nowadays plastic is our material of choice for lenses, for strength, durability, and of course excellent vision! More than 80% of lenses sold across the US and European market are plastic, as glass is heavier and doesn’t have the impact resistance of plastic. The market for plastic is still growing, and the most popular form used today actually came about by accident during the Second World War.

During the war raw materials became scarce, and research began to focus on alternatives. Polystyrene resins and nylon had already been used since the 1930s, and industry began to explore the possibilities of other forms man-made materials. A company called PPG industries created a new compound in 1940, which they called 39 – the 39th attempt at this particular clear resin! During the War it was used in the Aviation industry for fuel tanks and clear fuel lines, but when peace arrived the company were left with about 38,000 pounds of excess product in liquid resin form. They began to experiment with uses for the material, and various companies took an interest due to its useful properties of high impact resistance and clarity. Lens manufacturers Essilor, Armorlite and Sola started making CR39 plastic lenses, and from the 1960s onwards  popularity has grown.

CR39 plastic is cast in a mould and then cured. The entire range of lens powers can be worked on CR39, and to make it completely user-friendly it’s mostly treated with a scratch resistant coating. Treat your plastic lenses with care and they’ll give you many years of service. Don’t lay them curve side down on any flat surfaces, wash them with warm soapy water before you polish them. Don’t put anything abrasive in the case with them! Next time you put your specs on and they give you crystal clear vision, take a moment to appreciate the history of a remarkable product that makes our lives so much better and safer today.

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A Material World

Friday, December 4th, 2009

When you’re happily driving along or snuggling up to watch The Sound of Music yet again, you’re probably grateful for your glasses and the job they do. Those two lenses that you peer through make all the difference to your world if you’re quite short or long sighted, so lets take a moment to ponder on the fantastic stuff called plastic…

Once upon a time lenses were glass, which did the job but was heavy and potentially hazardous. During World War 2 plastic development and production was stepped up dramatically as more and more raw materials were needed. One plastic, called CR39 was trademarked as it showed remarkable properties – after the 39th attempt! It was exceptionally clear and had excellent thermo-setting properties. This was highly desired as it was scratch resistant and less impact resistant.

Initially it was used for fuel tanks and lines, and as lenses in reflectors and searchlights. After the war the surplus liquid resin needed a use, and gradually lens manufacturers began to take an interest in it. From the 1960s onwards the optical market gradually began to sell plastic lenses, and finally today over 90% of lenses used are plastic, with glass now a rapidly diminishing option.

The perfect lens material must be strong, light, scratch resistant, and able to absorb UV. The lens must also be optically as sharp as possible, with minimal reflections, and of a suitable density and thickness. CR39 has proved to be an excellent material over the years and is easy to cut, drill and tint.

Polycarbonate has been available as an alternative type of plastic since the 1950s, giving extra impact resistance. Initially it was used in visors, and was first used as a spec lens in 1978. Despite the advantage of extra strength, generally this material is not as good optically. Its main use is in sports eyewear, and for children. In the litigious USA the market share is 30%!

Both plastic and glass can be made in denser versions, which cuts the thickness of the finished lenses. The refractive index number denotes the density, in standard CR39 this is 1.5. Plastic is then available in 1.6, 1.67, and 1.74, which are progressively thinner for higher prescriptions.

So spare a thought for the wonders of this material in our techno world, produced in the post wars years and still going strong – and light!

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