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!