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Bifocal Lenses 109

Fig. 10.8: Round segment bifocal

This gives rise to “jump” at the edge of the segment when the line of gaze
passes from distance to near.
The kryptok bifocal is one of the most popular round bifocals. The design
as developed by John L Borsch Jr, in 1908 made use of distance lens of
crown and a segment lens of flint glass. The process involved fusing of
flint glass into a countersink curve made on the convex side of the crown
lens under a high temperature. If the heat is not carefully controlled during
the fusing operation, the countersink curve will warp and will produce
unwanted optical aberration.
The round Barium (Nokrome) bifocal differs from the kryptok bifocal
in the use of high index barium glass for the segment instead of flint glass
and in the close matching of the countersink curve and the rear surface of
the button. Close matching of curves cuts down on the heat required for
fusing process. This in turn results in more accurate curves and better lenses.
Round segment bifocal, especially those with low power having small
segments are among the least conspicuous of conventional bifocals. Only
the cosmetic or blended bifocals and the progressive addition lenses are
less conspicuous multifocals.

D-SHAPED BIFOCAL
D segment bifocals are available in solid plastics and fused glass forms.
They are a nice compromise lens, giving a relatively wide reading area at
the point of visual entry into the segment, with less of a thickness problem
than E-style. Cosmetically, the straight edge of D segment is little more
noticeable and therefore cosmetically less attractive to many patients, than
edge of round segment. There is still some jump at the edge of most D