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Progressive Addition Lenses 157

spherical surface of curvature required for DP curve. Then the spherical
curve is subsequently made aspheric in accordance with the design
philosophy for the lens. Before that the spherical convex surface must be
smoothed and polished accurately so that its topography has precisely the
known desired characteristics.
CNC diamond tool is used to create the aspheric surface as explained
earlier. If the work piece is glass blank, it must be smoothed and polished,
using floating pad system. It is essential to ensure that the pads do not
remove any more glass material than intended; otherwise an accurate
surface geometry will not be maintained.
Ceramic slumping moulds produced by CNC cutters can be used to
produce progressive power surface. The glass blanks are placed on ceramic
moulds as shown in the Figure 11. 37. The entire assembly is then heated to
the high temperature at which the glass starts to flow.
The glass slumps and the geometry of the convex surface of the mould
is transferred to convex surface of the blank. The entire process has to be
done under highly sophisticated temperature control to ensure that the
glass flows correctly. Slumping can be assisted by vacuum forming also,
whereby the glass is heated to just beyond its softening point, whereupon,
a vacuum is applied to the interface between the forming block and the
concave surface of the blank – which pulls the surface into shape.
It is the dream of the lens designer to create a progressive addition lens
with no areas of distortions at the either side of the lens, which unfortunately
is just not possible. The distortion on the either side is the result of induced
cylinder, which is due to the presence of unwanted surface astigmatism
and is evident during the lateral gaze within the progressive zone. All that
the designer can do is to reduce distortions to the minimum. Progressive
designs vary from one to another only by how these “no go” areas of the
lens are distributed across its surface.

Fig. 11.37: Slumping a glass blank to a required geometry