Page 7 - Cahier-Materials-and-Treatments-USA.indd

Thickness 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 center of a lens with a power of +4.00D and a diameter of fitting to be used: 65 mm obtained using a material with a refractive index of 1.6 is - for a circular fitting a minimum edge thickness of 0.8 mm is 0.6 mm; the additional gain provided by aspheric design is recommended for the bevelling of the lens; 0.2 mm and is accompanied by a net flattening of the lens; finally a gain of 0.5 mm is provided by thin surfacing. In total the - for a “Nylor” type mounting, the thickness required at the reduction in thickness is 1.3 mm (4.1 mm compared to 5.4) or edge for the grooving of the lens is a minimum of 1.6 mm for a close to 25%. 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 2.2 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, minimized thickness, related exactly to the shape of the lens and its centering; this 1) Effect of the refractive index technique is particularly effective in reducing the thickness of plus lenses. 2) Effect of the aspheric design © 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 aspheric design (2) combination of several factors: the choice of a high-index material makes it possible to gain several millimeters, the use of and thickness of the surfacing (3) for a lens with a power of +4.00D. aspheric design gives an extra reduction of several tenths of a millimeter 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 aspheric surfaces, the reduction in thickness would be even more significant: In addition, the choice of frame and the precalibration of the with an index of 1.74, it would be, compared to an index of 1.5, lenses is added to the previous effects and provides a further 3.8 mm (5.2 as against 9.0) for the -6.00D lens and 2.7 mm (2.7 saving of the order of a millimeter. Thus the combined skills of as against 5.4) for the +4.00D lens, i.e. a reduction of nearly 50%. the manufacturer and the optician make it possible to offer In addition, a judicious choice of frame and precalibration of the wearers the thinnest and therefore most aesthetic edged lenses. 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.