Page 114 - The Indian Optician Digital Edition May-June 2023

substituting the various values for y, also shown
FIGURE 2 - THICKNESS OF MINUS LENSES
THICKNESS OF SHAPED
SPHERICAL LENSES
It will be appreciated that, so far, it has been
assumed that lenses are round in shape. Each
point at the lens periphery is then equidistant
from the optical centre and the thickness,
therefore, is constant all around the lens
periphery. However, modern lens shapes are
rarely symmetrical. Consider the shape depicted
in Figure 3. The horizontal lens size is 50mm
and the vertical lens size is 40mm. Assume
the power of the lens to be 5.00 DS and that it
is made up in plano concave form, in plastics
material of refractive index, 1.498.
The distances of various points on the lens
periphery from the optical centre are shown
on the diagram together with cross − sectional
views taken through these meridians. It will be
noticed that the optical centre does not lie at
the geometrical centre of the horizontal centre
line but has been shifted nasally by 3mm. If
the thickness of the lens at the optical centre
is 2.0mm, the thickness round the periphery
will vary as shown by the values marked on
the figure. These have been calculated by
110 | THE INDIAN OPTICIAN | MAY-JUNE 2023
in the figure, into the sag equation. The thinnest
point on the edge lies in the vertical meridian
where the edge substance, t , is 3.83mm. The
E
thickest point on the edge has a substance
6.95mm. These points on the lens periphery
represent the closest and farthest points from
the optical centre, respectively.
THICKNESS OF SPHERO − CYLINDRICAL
AND TORIC LENSES
Figure 4(a) represents a positive cylinder with
its axis in the vertical meridian. If the curve at
right angles to the cylinder axis has a radius, r,
then the thickness in the axis meridian can be
found in the same way as shown for spherical
lenses, using the sag relationship: z = r - √(r – y ).
2
2
It should be noted from the diagram that
the thickest edge of the cylinder lies at the
extremities of the axis meridian. If the power of
the cylinder is +5.00 DC x 90, the thickest edge
of the lens lies along the plus axis meridian, i.e.,
along 90. If the cylinder axis lies along 30, the
thickest edges would lie at the extremities of the
30 meridian.
Figure 4(b) represents a negative cylinder
with axis vertical. If the radius of curvature of
the cylindrical surface is known, then the sag
of the surface can be found as before. If the
power of this negative cylinder is 5.00 DC x 90 it
can be seen that the thickest edge of the lens,
FIGURE 3 - VARIATION OF THICKNESS WITH LENS SHAPE
LENS TALK