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Thinness and weight Under the same conditions, the reduction in the thickness at the In addition, the thickness of the lens also varies with the type of centre of a lens with a power of +4.00D and a diameter of 65 fitting to be used: mm obtained using a material with a refractive index of 1.6 is 0.6 - for a circular fitting a minimum edge thickness of 0.8 mm mm; the additional gain provided by aspherisation is 0.2 mm and is recommended for the bevelling of the lens; is accompanied by a net flattening of the lens; finally a gain of 0.5 mm is provided by thin surfacing. In total the reduction in - for a “Nylor” type mounting, the thickness required at the thickness is 1.3 mm (4.1 mm compared to 5.4) or close to 25%. edge for the grooving of the lens is a minimum of 1.6 mm for a nylon wire fitting and 2.2 mm for metal wire; - for a drilled fitting, the minimum thickness required at the drilling point is 1.5 mm for a polycarbonate lens, 1.8 mm for a high index and 2.3 mm for traditional CR39. Note that these are minimum values that have to be observed and that it is generally advisable to add 0.2 to 0.3 mm. Finally, since the thickness which matters is that of the edged & TREATMENTS lenses, the choice of frame by the optician and the optimisation of the thickness of the lenses play important roles. In order to obtain the thinnest lenses, the frame must be chosen with a view to minimising the diameter of the lens necessary for centering, i.e. it must be small, symmetrical and of a size close to the wearer’s pupillary distance. Also, the lenses must be MATERIALS “pre-calibrated”, i.e. have a calculated, minimised thickness, related exactly to the shape of the lens and its centring; this 1) Effect of the refractive index technique is particularly effective in reducing the thickness of plus lenses. 2) Effect of the aspherisation © Essilor International 3) Effect of the surfacing © Essilor International Figure 3: Effect of pre-calibration on lenses. In summary, the reduced thickness of a lens is the result of the Figure 2b: Effects of the refractive index (1), of aspherisation (2) and combination of several factors: the choice of a high-index material makes it possible to gain several millimetres, the use of thickness of the surfacing (3) for a lens with a power of +4.00D. aspherisation gives an extra reduction of several tenths of a millimetre and a minimum thickness produced by surfacing can still save several tenths. In total, comparing a spherical lens with an index of 1.5 to an aspherical lens with an index of 1.74, the thickness is on average reduced by almost 50%. It is self-evident that by using a higher refractive index and aspherised surfaces, the reduction in thickness would be even more In addition, the choice of frame and the precalibration of the significant: with an index of 1.74, it would be, compared to an index lenses is added to the previous effects and provides a further of 1.5, 3.8 mm (5.2 as against 9.0) for the -6.00D lens and 2.7 mm saving of the order of a millimetre. Thus the combined skills of (2.7 as against 5.4) for the +4.00D lens, i.e. a reduction of nearly the manufacturer and the optician make it possible to offer 50%. In addition, a judicious choice of frame and precalibration of wearers the thinnest and therefore most aesthetic edged lenses. the lenses enables the thickness to be reduced still further. 7 Copyright © 2010 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.