Page 138 - July-August 2020

Deviation and prism magnification for 4Δ prisms made in flat and
6.00 D base curve forms in CR 39
Ocular Flat Form +6.00 Base Deviation Meniscus
Rotation Deviation (Δ) P mag (%) (Δ) P mag (%)

+40° 5.65 +6.77 4.99 +3.30
+30° 4.78 +3.93 4.70 +2.21
+20° 4.28 +2.12 4.43 +1.24
+10° 4.03 +0.78 4.19 +0.38
0° 4.00 -0.40 4.00 -0.40
-10° 4.18 -1.65 3.84 -1.11
-20° 4.61 -3.25 3.68 -1.77
-30° 5.41 -5.64 3.51 -2.34
-40° 6.83 -9.77 3.27 -2.78

being tilted, the back surface of being tilted to provide a pantoscopic
the prism was normal to the eye’s angle. If it is assumed to be in plan
primary visual axis. It is seen that view, then a face form angle is being
when the prism is in a position of applied to base out prism in front of
minimum deviation, when i 1 = i´ 2 the left eye
(and, therefore, i´ 1 = i 2), the prism Note that the effect of tilting the
magnification is unity.
prism is the same as if the prism had
The following table has been remained stationary and the eye
compiled from this relationship for rotated up or down through the same
a 4Δ prism made in CR 39 material angle as the tilt.
and it is seen that the prism Although this article has restricted
magnification, (which is expressed as the discussion to plano-prisms,
a percentage increase or decrease in remembering that a spectacle lens
size) is quite small for small angles of can be considered to comprise a
tilt, especially for the curved forms, series of plano-prisms placed base-
which are assumed to be mounted to-base, in the case of a plus lens and
27mm in front of the eye’s centre of apex-to-apex in the case of a minus
rotation and to have a base curve of lens, knowledge of these effects is
+6.00 D and centre thickness of 3mm.
essential for the dispensing optician
These results have been found by so that the cause of unhappiness in
accurate trigonometric ray tracing spectacle wearers who are sensitive
methods for the curved forms and the to these changes is understood.
reader interested in the calculation REFERENCES
routines are referred to a text book
on ophthalmic lens theory . 1. Ogle K N (1951) Distortion of the
2
image by prisms, Journal of
Figure 6 shows the difference the Optical Society of America
between a positive tilt and a negative 41:1023-1028
tilt of the prisms. If the figure is taken 2. Jalie M (2016) Principles of
to be a side elevation of a prism Ophthalmic Lenses. 5 Edition, ABDO,
th
before the eye, base down prism is Godmersham.

| JULY-AUG 2020 | 134 LENS TALK