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122 Ophthalmic Lenses

Figs 10.21A to E: Stages in the production of an E-style bi-prism bifocal
from the semi-finished lens
(a) Semi-finished lens
(b) Front view
(c) Base up prism is applied across whole of the rear surface
(d) Lens with base up prism
(e) Base down prism is applied to front surface, distance only

PRISM CONTROLLED SOLID BIFOCAL

Prism controlled solid bifocal lenses, surely are the most versatile prism
segment lens of all, allows to achieve separate control of prismatic elements
in both the distance and near portions. Unlike lenses for the control of
anisometropia whose correction is only for the vertical prismatic element,
a prism controlled solid bifocal lens can correct vertically, horizontally and
any oblique angle.
The segment surface of this bifocal saucer blank shows the same axis of
revolution as the DP surface, the centration of the reading portion is
dependent upon the segment diameter and the prescription of the lens.
The segment does not incorporate a separate prismatic effect and is
constructed by depressing the reading portion surface below the level of
the DP surface and dropping a prism into the depression. The apical angle
of this prism is the same angle as that which the blank must rotate through
to obtain the prescribed prism in the segment.