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Bifocal Lenses 117

The prismatic effect due to the distance lens would be:
P = 0.5 × 3.00
= 1.5Δ BU
The total prismatic effect would be 2.5Δ BU.
In case of minus power, the distance portion will exert base down prism.
Consequently the prismatic effect will change depending upon the type of
the segment shape. E-style bifocal will exert least prismatic effect in minus
power. In case of straight top D shape bifocal which has an optical centre
coinciding with the NVP, the vertical prismatic effect at reading level is
due to the distance power only. Therefore, it does not change when the
wearer switches from single vision to bifocal lens. The total vertical prismatic
effect at the reading level is of practical importance as it determines the
amount of transverse chromatic aberration the wearer will be subjected to.

CHROMATIC ABERRATION

Transverse Chromatic Aberration (TCA) arising out of the oblique rays is
the potential problem for the wearer of the single vision lenses as well as
bifocal lenses. The effect of TCA is the colour fringes to be seen surrounding
the image of the high contrast targets, while looking through a point in the
lens away from its optical centre. As a result of the fact that bifocal segment
is viewed through the edge of the distance lens, problems can arise due to
TCA arising from the distance vision prismatic effect. When the prismatic
effect at a point is known, the TCA at that point can be found by dividing
the prismatic effect by the Abbe Number for the material which is denoted
by V, i.e.
TCA = P/V
(Where, P stands for the prismatic effect).
This paraxial relationship expresses the chromatism in prism dioptres
and the average threshold value is in order of 0.1Δ. Chromatism less than
this value are unlikely to give rise to complaints. In case of solid bifocals
including both glass and plastic, the chromatism can be found simply by
dividing the total prismatic effect at the point in question by the Abbe
Number for the material. Thus, in case of solid bifocal of + 5.00D with
reading addition + 2.00D, made with a 24mm diameter segment and a
segment drop of 2mm, the vertical prismatic effect at a point 6 mm below
the segment top is given by:
P=cf
Or, P = 0.8 × 5
Or, P = 4.00Δ BU