Page 113 - PowerPoint 演示文稿

Bifocal Lenses 103

This approach is still in use for prescriptions that cannot be
manufactured using mass production technique, an example being where
a large amount of prism is required at near but not at distance. Following
Franklin’s invention, the bifocal lens evolved through a number of stages
before the development of modern fused and one-piece bifocal lenses early
in this century. Modern version is cosmetically and mechanically improved
to suit the mass production also.

TYPES OF BIFOCAL LENSES
Bifocal lenses can be obtained in four basic constructional types:
Fused bifocals, Solid bifocal, Cemented bifocal and Split bifocal.

FUSED BIFOCAL
The first fused bifocal was the fused kryptok invented by Borsch in the
year 1908. In countries, where glass lens still represent a sizeable section of
the market, the fused bifocal remains the most common form of bifocal
design. The round segment is still in production for some markets but no
doubt, the most commonly used segments are D-shaped flat top or the
C-shaped curved top as shown in Figure 10.2.

Fig. 10.2: Fused bifocal design (A) Round segment (B) D segment flat top
(C) C segment curved top

The segment is permanently bonded onto the convex surface of the
lens by heat fusion process and the required addition depends upon:
1. The refractive indices of the two glass materials.
2. The depression curve.
3. The curve worked on the segment side of the lens.

To fuse a segmented bifocal, a countersink cavity is produced on the
front surface or the convex side of the crown lens having a refractive index
of 1.523. A flint glass of high refractive index usually of 1.654 is taken. The
front surface of the flint button is simply left and a predetermined contact
surface is worked on the rear surface of the button to match the curve on
the crown blank.