Page 89 - The Indian Optician Digital Edition January-February 2022

vision in the lower portion of
the surface. The two surfaces
are shown in Figure 6 where
the distance portion surface
is represented by an oblate
ellipsoid with a p-value of +26.0
(Figure 6(a)) which changes
smoothly to an ellipsoid with
a p-value of +12.5 in the lower
portion of the lens (Figure 6(b)).
The combination is depicted in
Figure 6(c).

The vertex radii of the two
ellipsoids are the same so
that there is no change in
power or discontinuity in the FIGURE 5 - OFF-AXIS
centre of the field. The off-axis PERFORMANCE OF ASPHERIC
+4.00 D POINT FOCAL LENS
performance of the lens in the DESIGNED FOR NEAR VISION
distance and near portions is
shown in Figure 7, the distance to differ on moving away from the vertex along the horizontal
and near field diagrams being meridian. The difference in z-heights is shown in Table 2 for 2
shown separately. mm intervals along the 180 meridian. All z-values are shown in
millimetres
Because this new surface
design requires different Note that the figures given in the difference column are very
asphericities for the distance small. Even at 20 mm from the pole of the surface, the difference
and near zones, the z-heights only amounts to 35 micrometres (microns). Such small differences
(sags) of the surfaces will start are easy to blend with free form manufacturing techniques.

TABLE 2 Z-HEIGHTS FOR DP Figures 8 to 10 give a similar analysis for a -4.00 D lens made
AND NP CONCAVE SURFACE FOR in CR 39 material. The base curve for this design is +2.00 D and
+4.00 D ASPHERIC LENS the astigmatism eliminated for distance vision by means of a
concave hyperboloid with a p-value of -1.05 (Figure 8). The field
Vertex radius = 211.3684 mm diagram shown in Figure 8(a) shows that the lens is point focal
p = +26.0 p = +12.5
distance near
FIGURE 6 - OBLATE ELLIPSOIDAL SURFACES WITH THE SAME
x z distance z near difference VERTEX RADII BUT DIFFERENT ECCENTRICITIES
2 0.009 0.009 0.000
4 0.038 0.038 0.000

6 0.086 0.085 0.001
8 0.153 0.152 0.001
10 0.240 0.238 0.002
12 0.348 0.344 0.004
14 0.478 0.470 0.008

16 0.630 0.617 0.013
18 0.806 0.785 0.022
20 1.009 0.974 0.035

85 | LENS TALK