Page 116 - PowerPoint 演示文稿

106 Ophthalmic Lenses

Fig. 10.5: Stages to manufacture round segment bifocal

The quantity (F – F ) is the function of the refractive indices of the
3
1
respective materials and hence can be shown to be equal to F (n – n) /
1
f
(n – 1). Similarly, F con can be shown to be equal to – Fc ( n – n ) / ( n – 1).
f
Substituting the values:
 n  n  n  n
A= F  1  n  f 1    F c   n  f 1  
Or, A = F (K) – F (K) n  1 
1
c

Or, A = K (F – F )   n   n 
1
c
Or, A/K = F – F c f
1
Or, F c =F – A/K
1
∴ F c =F – A
1
Example:
(1) Consider a lens with F of + 6.00D and refractive index of 1.523 and
1
1.654 for n and n respectively. If required addition is + 2.00, then
f
F = 6 – (0.523/0.131) 2
c
= – 1.98D
SOLID BIFOCAL
Solid bifocal can be considered as one piece bifocal, made from single piece
of material. They can be made by molding or casting one surface and
working the other (usually plastics), by molding or casting both the surfaces
(almost always plastics) or by working both the surfaces (almost always
glass). Most segment shapes and sizes, from E style to round are available
as solid plastic bifocals. Blended bifocals are solid, usually round segment
bifocals in which the segment edge blends into the distance portion.