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104 Ophthalmic Lenses

Fig. 10.3: Construction of fused bifocal: (A) Components prior to fusing,
(B) Rough fused blank (C) Semi-finished fused blank

The curve on the contact surface of the button is deliberately made some
0.25D to 0.50D steeper than the depression curve on the corresponding
crown blanks so that initially, contact is only made at the centre of the
curves. The button is placed on the depressed crown portion and the
assembled components are then placed on a carborundom block, the surface
of which has been molded to match the curve on the back of the crown
glass major and the blocks are then kept into the furnace. At around 640ºC
the button softens and sags under its own weight, gradually expelling air
towards the wire feeler. The steeper curve on the button aids this procedure
by tending to unroll the depression curve towards the feeler. Eventually,
the whole surface of the button is in contact with the depression curve
except for two very small areas around the supporting wires. The resultant
fused blanks are then put into rigorous inspection. If, by chance, some dust
has remained between the surfaces, a bubble is formed, which may result
in rejection of the blank. Since, the front curve is continuous over the distance
and near portion; the dividing line of the fused bifocal segment cannot be
felt.
Shaped bifocals like D segments, B segments, C segments and curve
top bifocals can be produced by means of composite buttons, made up of
two or three different glasses. In each of these cases the composite button
consists of a crown component and a flint component. The crown portion
must have exact refractive index as the main crown glasses. In case of shaped
bifocal, the shape of the segment must be checked and if necessary it must
be corrected so that it has the required segment diameter and segment
depth. This can be achieved by grinding a small amount of vertical prism
relative to the segment side surface.