Glass lenses –lenses made from natural mineral glass, according to their professional classification – used to be the norm. They still have their place in optometry today thanks to their exceptional scratch resistance. Consumers will also like the fact that they are less expensive than comparable plastics. In cases of severe ametropia, they can also provide the correction needed with relatively thin lenses - an aesthetic aspect that is not to be underestimated.
Natural glass is also recommended for bifocal or trifocal lenses because various materials can be melted together without forming a noticeable cutting edge. In principle, the increased thickness of the material makes it optically purer; the glasses appear cleaner and are free from disruptive color fringes (so-called dispersion).
When light strikes an eyeglass lens, it is broken down into its component parts and dispersed. This creates a disruptive visible color spectrum, similar to a prism. The intensity of this effect, known as dispersion, depends on the condition of the material used: high-quality material means minimal dispersion. The color fringe effect is measured based on what is known as the Abbe number: The higher the Abbe number for an eyeglass lens material, the lower the dispersion. The advantage of natural glass: It produces considerably weaker color fringes even when the refractive index is identical to that of plastic lenses.
The greater the refractive index range (also called the refraction index) of the eyeglass lens material, the thinner the finished glass. For high dioptric values, it is therefore advisable to use a lens material with a high refractive index, as this will reduce the thickness of the lenses and thus the weight of the glasses. For example: A lens with a refraction index of 1.6 is always thinner than one with a refraction index of 1.5 for an identical dioptric value.
Natural glass has a clear advantage here: Its refractive index range extends from 1.5 to 1.9, while the refractive index range of organic glass (= plastic) is only 1.5 to 1.74. Natural glass also has a greater density than plastic. The result: Even when the refraction index is the same, eyeglass lenses made of glass are always thinner than those made of plastic – but they are also substantially heavier.
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