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Bifocal Lenses 123

Fig. 10.22: Cutting of solid bifocal disk to obtain one semi-finished segment blank

Truly this is very interesting lens which provides a comfortable binocular
vision to many who otherwise would only achieve monocular vision or
revert to two pairs of single vision lenses.

INVISIBLE BIFOCALS
Invisible bifocals are one-piece bifocal lenses having round segment, in
which the line of demarcation between the distance portion of the lens and
the bifocal segment has been obliterated by a polishing process. The result
is an invisible bifocal segment. Optically the lens performs the same as a
standard round shaped bifocal, as only two actual zones of focus exist –
one for distance and one for near. The obliteration process produces
distortion around the bifocal segment which varies in width from less than
3 mm to just under 5 mm, depending on the power of the near addition
and the base curve. This area serves as the boundary of the segment and
permits the lens to be fitted according to the same provisions that affect an

Fig. 10.23: Invisible bifocal-round shape