REFRACTIVE INDEX OF LENSES
The higher the refractive index, the thinner, lighter and more attractive the lens at the same dioptre value. This not only increases comfort, but also makes them look good. The advantage of thin lenses is that you are relatively free in your choice of frame. The lenses are also more comfortable to wear due to the reduced weight. Spectacle wearers with high values should therefore always opt for a higher index. Click on “spectacle advice” to find out more about the various characteristics of the indices.
CAN BE USED FOR LENSES?
Although we usually refer to the spectacles we wear as “glasses”, we don’t realise that most lenses are now made of plastic. In principle, opticians have the option of choosing between mineral glass or plastic. These two materials have different properties that play an important role in the purchase decision.
Mineral glass is the traditional material for spectacle lenses. Like conventional glass, it consists of a special mixture of quartz, soda and lime. It breaks more easily and is heavier than plastic. The advantages: it is robust and scratches less easily. Protection against scratches is now achieved by special coatings, even on plastic lenses. That’s why about 95% of all spectacle wearers choose plastic lenses. (Source: Kuratorium Gutes Sehen)
Glass (silicate) is the classic material for spectacle lenses. Glass lenses are approx. 40% heavier than plastic lenses, very fragile but naturally scratch-resistant and acid-proof. Especially if the lens wearer comes into contact with a lot of vapours, acids and dust, lenses made of glass can still be a sensible choice. For the average wearer of glasses, however, the disadvantages far outweigh the benefits. Glass is completely unsuitable for children’s glasses or sports glasses, as the risk of them breaking is too great.
Spectacle lenses made of plastic are on average about 40% lighter than lenses made of mineral glass and are unbreakable. In contrast to mineral lenses, plastic lenses do not have any natural scratch resistance. However, this is a problem that can be solved – even car headlights are now mostly made of plastic and when was the last time you saw a scratched headlight? The solution is hard lacquers in which plastic lenses are soaked several times until they have almost the same scratch-resistant properties as lenses made of mineral glass.
Polycarbonate is also a type of plastic and the strongest material. Polycarbonate lenses can withstand even very heavy impacts and offer optimum protection in sports and industrial safety. So the question is: why is polycarbonate not used for all lenses? The answer is simple: because there is also a downside. The optical performance of the material is significantly inferior to that of other lens materials and the lenses are quite thick due to the relatively low refractive index (which determines the lens thickness).
There are an enormous number of names for this category of lens in the optical retail trade. These range from extra-thin, through high-index, to thin-ground. The fact is that, with these lenses, the higher quality material is all that matters. The material has a higher refractive index and it is therefore possible to produce thinner and lighter lenses with the same effect. The best known so-called high refractive indices are 1.60, 1.67 and 1.74.
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