Page 8 - Cahier-Matériaux-et-Traitements-GB.indd

Thinness and weight B Weight The weight of a lens comes from the combination of its thickness and the lightness of the material used in its manufacture. More a) Plastic materials: Brand Refractive Abbe number UV precisely, it is the combination of the volume of the lens and the Categories names index (v e / v d ) Density cut-off density of the material which determines its weight. (n e / n d ) The volume of the lens depends on the geometry of its surfaces, Normal index Orma® (Essilor) 1,502 / 1,500 58 / 58 1,32 355 nm its shape and the dimensions of the template of the lens and the Normal index Trivex® (PPG) 1,533 / 1,530 43 / 44 1,11 395 nm thickness necessary to ensure its robustness and make fitting Mid-index Airwear® (Essilor) 1,591 / 1,586 31 / 31 1,20 385 nm possible (minimum thickness at the centre of minus lenses or at Mid-index Ormix®/ the edge of plus lenses). High index Thin & Lite 1,60 1,596 / 1,592 41 / 421,31400 nm & TREATMENTS to another: from 1.1 for the lightest plastic materials to almost b) Glass materials: 1,734 / 1,728 Abbe number 1,36 400 nm The density itself comes from the nature of the material and its (Essilor) chemical composition. It varies considerably from one material Stylis®/ Thin & Lite 1,67 1,665 / 1,660 32 / 32 (Essilor) 4.0 for the heaviest glass materials (see materials table). Generally speaking, the higher the refractive index of a material, Lineis®/ Very 1,47 high index Thin & Lite 1,74 33 / 33 400 nm the higher its density, since the increase in the refractive index (Essilor) is obtained by introducing heavy atoms into the chemical structure of the material. The lightest lenses are therefore obtained through the best combination of reducing the thickness of the lens and the Refractive UV Brand index Categories names cut-off (n e / n d ) MATERIALS lightness of the material, i.e. by the simultaneous optimisation Normal index Fit 40 (Essilor) 1.525 / 1,523 (v e / v d ) Density 330 nm of the thickness (index + aspherisation + surfacing) and the Stigmal 15 59 / 59 2,61 density. (Essilor) Stigmal 16 1,604 / 1,600 2,63 41 / 42 Mid-index 335 nm (Essilor) 1,705 / 1,701 41 / 42 High index 3,21 335 nm Density and specific gravity of a material – (Essilor) index definitions: Very high Stigmal 18 1,807 / 1,802 34 / 35 3,65 330 nm Very high Density is a value which quantifies the mass of a material per index 19 (BBGR) 1,892 / 1,885 30 / 30 3,99 340 nm unit of volume. It is defined as the relationship between a mass and its volume and is usually expressed in grammes per cubic Figure 4 : Table of the principal materials. centimetre. Specific gravity, also called specific mass, is the relationship between the density of a substance and that of another substance chosen as a reference (water in the case of solids and liquids); it is expressed as a dimensionless number. Since the density of water, chosen as the reference substance, is 1 g/cm , 3 its specific gravity has the same value as its density. The density (or specific gravity) gives a precise measurement of the weight of the material but only gives an approximation of the weight of the lens. It cannot be used as the only In summary, these are materials which combine both a high reference when comparing lenses. Only the weight of the edged refractive index, a low density and the ability to take thin lens and the combination of the exact volume and the density surfacing which make it possible to produce the thinnest, of the material, can make an exact and relevant comparison lightest lenses. In this respect, these are high-index plastic possible. materials and, more particularly polycarbonate, which are the most suitable materials available today. 8 Copyright © 2010 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.