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1.Thickness and weight For as long as spectacle lenses have existed, manufacturers have continued to try to make them thinner and lighter in response to the demands of wearers. So, refractive indices were increased, lens surfaces with aspheric design were used, lenses were surfaced as thin as Thickness and weight Actually, to produce lenses that are both aesthetic due to their reduced thickness and comfortable because they are light in weight, possible and heavy glass materials were replaced by extremely light plastic materials. numerous parameters have to be combined. Let us examine closely those that reduce the thickness of lenses and, then, those that reduce their weight. A Thickness The reduced thickness of a lens results from a combination of three a 1.6 index material, allows, for an identical thickness at the center, factors: the refractive index of the material, the aspheric design of For a lens with a power of -6,00D and a diameter of 65 mm, using the surfaces and the reduction to minimum thickness when a reduction of the thickness at the edge by 1.5 mm compared to surfacing. the same lens produced in 1.5 index material (7.5 mm as against 9.0). The aspheric design produces an additional reduction of 0.4 mm and makes the lens slightly flatter. Thin surfacing then enables 1. Effect of the material’s refractive index an additional gain of 0.8 mm (1.2 mm as against 2.0). In total the This is the main factor behind the reduction in thickness of the lens. reduction in thickness is 2.7 mm (6.3 mm as against 9.0), i.e. 30%. For a given power, the higher the refractive index, the thinner the lens. More precisely, the higher the index, the greater the capacity & TREATMENTS optical power and, as a result, the thinner the lens. of the material to deflect light rays, the flatter the curvatures required on the front and rear faces of the lens to produce a given Refractive index – definition It characterises the speed of propagation of light through a transparent medium in relation to the speed of light in a vacuum. Thus it measures the capacity of a transparent medium to refract, that is to say deflect light at the surface between two media. It therefore gives an assessment of the capacity of the material to produce an optical effect. MATERIALS The refractive index of a transparent medium is expressed in the 1) Effect of the refractive index relationship n = c / v between the speed of propagation of light in a vacuum (c) and the speed of propagation of light in this medium (v). This index is a number – dimensionless and always greater than 1 – which quantifies the refractive power of the medium: the higher the refractive index, the greater the deflection of a beam of light passing from air into the medium. The refractive indices of the materials used in ophthalmic optics vary from 1.5 for the more traditional materials to 1.74 (in plastic) and 1.9 (in glass) for the latest materials (see table of materials). 2) Effect of the aspheric design 2. Effect of the aspheric design of the surfaces The aspheric design of surfaces is an indirect factor in reducing thickness: it enables the production of flatter and, as a result, thinner lenses. More precisely, aspheric design makes possible the use of flatter bases – or curvatures on the front face – without affecting the optical qualities of the lens. For plus lenses, the sag of the front surface (i.e. its “height”) is therefore less and the thickness at the center of the lens can 3) Effect of the surfacing then be slightly reduced by bringing the rear surface closer; in addition, the overall flattening of the lens contributes to the impression of thickness. For minus lenses, naturally flat, the © Essilor International effect of aspheric design on the thickness is less but nonetheless significant. This “optical” aspheric design must not be confused with “geometrical” aspheric design, a sort of peripheral flattening sometime added to the edge of high power lenses and which has Figure 2a: Effects of the refractive index (1), of aspheric design (2) more to do with geometry than optics. and thickness of the surfacing (3) for a lens with a power of -6.00D. 3. Effect of surfacing on the thickness An important factor in reducing the thickness of a lens is the ability for the manufacturer to surface it as thin as possible. Depending on the mechanical properties of the material – rigidity and solidity – the possibilities vary considerably: thus, the minimum thickness that can be produced at the centre of a minus lens can vary from 1.0 mm to more than 2.0 mm, depending on the material and the power; similarly, the minimum thickness at the edge of a plus lens at its thinnest point, can vary from less than 0.5 mm to more than 1.0 mm. 6 Copyright © 2010 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.