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118 Ophthalmic Lenses

The prismatic effect due to segment is:
P=cf
Or, P = 0.6 x 2
Or, P = 1.2Δ BD

So, the total prismatic effect would be 2.8" Base Up, the horizontal prismatic
effect being ignored. Therefore, TCA at this point would be, if the material
is CR with Abbe number 58:
39
TCA = P/V
= 2.8 / 58
= 0.05Δ

This value at the centre of the near visual zone is unlikely to cause any
complaints.

In case of fused bifocal in which the near portion combines a crown
glass element with a flint glass element of lower abbe value, the chromatism
is the sum of three components – distance element, chromatism because of
depression curve and chromatism due to segment element. The distance
element of crown glass represents the power of the distance portion and is
denoted by F, the centered depression curve, also a part of crown glass, is
denoted by – Ak, where A is the reading addition and k is the blank ratio
(n – 1) / (n – n). The power of the segment element is given by A (k + 1).
s
At a point N, in the near portion, three components contribute to the
prismatic effect and hence to the chromatism at that point. Two components
are made from crown glass and the third is the flint glass element of the
near portion. If the distance from the distance optical centre to the point N
is denoted by Y and the distance from N to segment optical centre to N by
Ys which must be given a minus sign when Os lies below N. Then
chromatism at point N is given by:


TCA = y (F/V) – (ys)(AK/V)      (ys)   A[K 1]     
  Vs  
Where V = Abbe value of crown glass
Vs = Abbe value of flint segment
k = (n – 1) / (n – n)
s
So, in our above example:
Y = 0.8 Ys = 0.6
F = 5.00 A = 2.0
V = 58 Vs = 36 (say).
K = ( n -1 ) / ( n – n ) = 1.523 – 1 / 1.654 - 1.523 = 4.
s