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Plastic and glass materials B Glass materials For several centuries, from the origins of optics to the middle of the 20th century, glass was the only material used for lenses for increase in the density of the material which cancelled out the ophthalmic optics. In just a few decades they were eliminated expected weight saving from the reduced thickness of the lens. and replaced by plastics. Once again, the increase in the index was accompanied by an As a result, a glass lens, whatever its index, remains at least twice as heavy as a plastic one. As for thickness, the new very-high- The glass lens is a solid and amorphous material (i.e. of a non- index plastic materials enable the manufacture of lenses whose periodic structure); it is hard and breakable at room temperature reduced thickness rivals that of traditional high-index glass and takes on a viscous state at high temperature. It is obtained lenses (n = 1.7). On the other hand, for high levels of correction, by the fusion at approximately 1500°C of a mixture of oxides very-high-index glass (n = 1.8 or n = 1.9) undeniably retains a & TREATMENTS potassium, lead, barium, titanium, lanthanum, etc. The glass lens such as those of silicon (the main oxide used, as it makes up thickness advantage compared with plastic lenses. approximately 65% of the material), calcium, sodium, does not have a regular chemical structure and, as a result, has no clear melting point at which it suddenly changes from a solid to a liquid state. In addition, with a rise in temperature, glass becomes soft and changes gradually from a solid into a liquid via a state known as "vitreous”, characterised by the absence of crystals. This exclusive special property enables it to be worked when hot and thus molded. Two properties make it interesting MATERIALS for ophthalmic optics: it transmits visible light and its surface can be polished to make it transparent and non-diffusing. 1. Standard glass materials Glass with an index of 1.5 is the traditional material, formerly used in ophthalmic optics. It is made up of 60-70% silicon oxide and the remainder of various components such as oxides of calcium, sodium or boron. Glass with an index of 1.6 is the standard glass material: its higher index is obtained by the addition to the mixture of a significant proportion of titanium oxide. It is usual to separate glass into two categories depending on its chemical composition: - “Sodiocalcic” materials containing significant proportions of sodium and calcium: these are the traditional materials used in optics. Their refractive index is a little higher (ne = 1.525 / nd = 1.523) and their chromatic dispersion low (Abbe number in the region of 60). - “Borosilicate” materials with a high boron content: these are the materials used in the manufacture of photochromics and mid-index glass lenses (ne = 1.604 / nd = 1.600) 2. High-index glass materials Glass specialists have always sought to increase the refractive index of materials, in order to reduce the thickness of lenses and to maintain chromatism at a low level. To do this, metal and rare earth atoms (lead, titanium, lanthanum, etc.) are introduced into the material’s composition. So it was that in around 1975 titanium lenses with an index of 1.7 and an Abbe number of 41 appeared, then around 1990, lanthanum lenses with an index of 1.8 and an Abbe number of 34 and finally, around 1995, niobium lenses with an index of 1.9 and an Abbe number of 30. These materials enabled the production of thinner and thinner lenses but without a significant reduction in their weight. 14 Copyright © 2010 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.