7.9 Materials selection
What properties does a spectacle wearer require from the lenses in the spectacles? Well, first and foremost, the lens must transmit light and not distort the image. Secondly, as spectacles need to be worn for prolonged periods, ideally the lens should be of low weight. The material must be stable over a range of temperatures – the geometry of the lens must not change with variations in temperature. Finally, the material should not become scratched with everyday use.
There are five common choices of materials for spectacle lenses. These are: glass; so-called 'high refractive index glass'; the polymer polymethylmethacrylate (abbreviated to PMMA); CR39 (a thermosetting polymer); and polycarbonate. Both PMMA and CR39 are available with a 'high' refractive index. 'High' simply means that the polymer or glass has a refractive index higher than normal glass. The typical properties of these five materials are listed in Table 2.
|Material||Refractive Index||Dispersive power||Density/ kg m−3||Toughness/ MN m−3/2||Vickers hardness/HV||Abrasion resistance|
|Glass||1.52||1.72 × 10−2||2530||0.8||490||High|
|High index glass||1.60–1.80||~3 × 10−2||3900||0.5||590||High|
|CR39||1.50||1.72 × 10−2||1320||0.75||40||Medium|
|High index polymer||1.54–1.68||~3.1 × 10−2||1350||~0.3||~100||Medium|
The high refractive index materials may produce lighter lenses as they can produce much thinner lenses for the same bending of light. However, as with most material properties there are several trade-offs. Glass provides better scratch resistance than the polymers; however, the lens produced has additional weight. And although glass can be made with a higher refractive index than the polymers, its toughness decreases as its refractive index increases, so making the final lens more brittle.
PMMA and CR39 are generally cast by pouring them into a mould of polished glass plates separated by rubber. PMMA lenses can also be produced by injection moulding, which is faster, but has the drawback that a lower molecular mass PMMA must be used to allow the polymer to flow into the mould and this reduces the final toughness of the lens. However, it is a cheap process and high production rates are achieved.
Glass lenses are also formed in metal moulds which are approximately the correct curvature and dimensions for the lenses. Both plastic and glass lenses are finished using grinding and polishing machines. These dramatically improve the precision and reproducibility of the finished lens, and hence improve the subsequent optical performance of spectacles.