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1.Thinness 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 were aspherised, lenses were surfaced as thin as possible and Thinness and weight Actually, to produce lenses that are both aesthetic due to their reduced thickness and comfortable because they are light in weight, heavy glass materials were replaced by extremely light plastic ones. 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 Thinness The reduced thickness of a lens results from a combination of three a 1.6 index material, allows, for an identical thickness at the centre, factors: the refractive index of the material, the aspherisation of For a lens with a power of -6,00D and a diameter of 65 mm, using the surfaces and working to minimum thickness when surfacing. a reduction of the thickness at the edge by 1.5 mm compared to the same lens produced in 1.5 index material (7.5 mm as against 1. Effect of the material’s refractive index 9.0). The aspherisation produces an additional reduction of 0.4 mm and makes the lens slightly flatter. Thin surfacing then enables This is the main factor behind the reduction in thickness of the an additional gain of 0.8 mm (1.2 mm as against 2.0). In total the lens. For a given power, the higher the refractive index, the thinner reduction in thickness is 2.7 mm (6.3 mm as against 9.0), i.e. 30%. the lens. More precisely, the higher the index, the greater the capacity of the material to deflect light rays, the flatter the & TREATMENTS lens. curvatures required on the front and rear faces of the lens to produce a given optical power and, as a result, the thinner the 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 1.76 (in plastic) and 1.9 (in glass) for the latest materials (see table of materials). 2) Effect of the aspherisation 2. Effect of the aspherisation of the surfaces The aspherisation of surfaces is an indirect factor in reducing thickness: it enables the production of flatter and, as a result, thinner lenses. More precisely, aspherisation 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 centre of the lens can then 3) Effect of the surfacing be slightly reduced by bringing the rear surface closer; in addition, the overall flattening of the lens contributes to the impression of thinness. For minus lenses, naturally flat, the effect © Essilor International of aspherisation on the thickness is less but nonetheless significant. This “optical” aspherisation must not be confused with “geometrical” aspherisation, 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 aspherisation (2) and more to do with geometry than optics. 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.