Page 113 - The Indian Optician Digital Edition March-April 2021

Now consider how the thickness of the
Alvarez lens changes when, starting from the
afocal position of the elements, the components
are slid horizontally in relation to one another
(Figure 3).
It can be seen that along the vertical
meridian of the combination the thickness does
Figure 1. Coomponents of the Alvaarezz lens not change, the Alvarez lens remains afocal
A
in the vertical meridian. Along the horizontal
meridian, however, as the separation increases
from the zero position shown in Figure 3(b), the
thickness of the combination starts to increase.
It will be realised that this movement produces
a plano-cylinder. The Alvarez lens now behaves
like a plano-cylinder whose cylindrical effect
increases as the separation of the components
is increased along the horizontal meridian. It will
be seen that the axis lies at 45°.
F Figure 2. Alvarez lens arrangedd to form plus and minus lenses
n
s
a
It follows that if the two components are
separated in both the vertical and horizontal
lens would still be afocal if its two components
meridians the resulting Alvarez lens will possess
were placed together, with their plane surfaces
both spherical and cylindrical power, i.e., it has
in contact. Once again, the absence of material
become a variable power sphero-cylindrical lens.
from one component is precisely compensated
As stated above, the axis of the cylinder will lie
for by the presence of material from the second.
along the 45° meridian of the elements shown in
Now consider the cross-sectional shapes Figures 2 and 3.
of the resulting element when the two The contact surface between the two
components are separated by sliding the elements does not need to be a plane surface
two components along their plane surfaces as can be seen in Figure 4 which shows a
as shown in Figures 2(b) and 2(c) where the cross-sectional view of a curved form of Alvarez
components have been separated by sliding lens. However, in designing a curved interface,
the second component in relation to the first, sufficient space must be allowed between the
the plane surfaces remaining in contact. It
can be seen that an optical element is formed
which has either two convex surfaces (Figure
2(b)) where it is seen that the thickness of this
element varies in every meridian in exactly
the same way as the thickness of a biconvex
spherical lens. The further the movement of one
component in relation to the other, the greater
the increase in the power of the lens.

In Figure 2(c), the two components of the
Alvarez lens have been slid in the opposite
direction, when the resulting optical element is
seen to possess two concave surfaces forming
a minus lens. Once again, the further the
movement of one component in relation to
the other, the greater the increase in the
p
g
e
n
Figure 3. Alvarez lens arranged to form a plano--cylinder
r
power of the lens.
| MAR-APR 2021 | 109 LENSES

Mar-Apr 2021 SK.indd 57 26-04-2021 13:31