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156 Ophthalmic Lenses

Fig. 11.36: Concept of a progressive lens surface. A section of an oblate ellipsoid
is inserted between two hemispheres of radius of curvature r for the distance
D
portion and r for the near portion
N
discontinuities. Along the meridian line, DD , a cross section through the
1
surface would be circular and the radii of the circles in a plane parallel
with either the distance or near portion circles do indeed decrease from r D
to r continuously.
N
The ability to cut surfaces of such a complex nature was made possible
by the use of computer numerically controlled (CNC) grinding machines.
The single point diamond tool cuts the surface under computer control,
sweeping in an arc over the work piece with the program positioning the
cutter in exactly the right place as the cutting tool traverses the work piece.
The work piece could be a glass blank or a glass mould from which
plastic lenses might eventually be cast, or it could be a ceramic block upon
which glass blanks or finished lens could be slumped with or without
vacuum assistance.
The important stages in the production of progressive power surface
are as under:
1. Surface designing and numeric description.
2. Generating the aspheric curve.
3. Grinding, smoothing and polishing
4. Inspection.
Surface designing and computation of the surface topography is
translated into numeric data in the form of co-ordinates that can be fed
directly into the CNC generators. As much as 50000 numeric data point
may be required for an individual progressive power surface. The basic
reference for the progressive power surface is usually taken to be the