Page 151 - The Indian Optician Digital Edition Jan-Feb 2020

Figure 1 illustrates cross-sectional
views through the horizontal meridian
of a prism controlled progressive lens
which incorporates base in prism for
the right eye. For simplicity, the cross-
sections through the lens shown on
the right-hand side assume that the
front and back surfaces in the distance
FIGURE 2 - PROGRESSIVE ADDITION OF portion are plane and viewed in plane
HORIZONTAL PRISM IN THE INTERMEDIATE
AND NEAR ZONES OF A PPL (ESSILOR) elevation when the gradual increase
in thickness is seen to occur on the
nasal side. It can be seen that the eye
would encounter a gradual increase
in thickness as it rotates downwards
through the intermediate portion until
it reaches the near zone of the lens.

Figure 2 shows how the prism
increases from the distance reference
point, D, to the near reference point,
N, as the eye moves down the lens.
The prism power profile indicates
that there is no prismatic effect in the
FIGURE 3 - PROGRESSIVE LENS WITH INTERMEDIATE ZONE BENEATH NEAR
PORTION (ESSILOR) distance portion, the prism power
increasing to 3.5Δ in the near portion.
PRISM CONTROLLED The power profile down the lens, not
PROGRESSIVE LENSES shown, would of course, depend upon
out prism would be helpful to relieve
A patent for a prism controlled symptoms. The patent suggests that the near addition.
progressive lens in which horizontal the invention might also be useful PROGRESSIVE LENSES
2
prism of ever increasing strength, for myopia control in young people. DESIGNED TO REDUCE THE
either base in or base out, can be A result of studies undertaken by RISK OF FALLS
obtained between the distance and the Applicants on slowing down
near portions. The object of the patent progress in myopia has shown Several papers have appeared
is to provide horizontal prismatic that prescribing only an addition in recent years which discuss the
effect of at least 2Δ in strength, the of power in the correcting lens is increased risk of falling by those who
prism being defined as the difference not effective for all subjects with wear bifocal and progressive power
3
between the horizontal prismatic myopia. They point out that in near spectacles . In the main, the authors
effect, if any, at the distance reference vision both accommodation and the point to the lack of clear vision in the
point, D, and the horizontal prismatic convergence may be overworked. lower portion of the lens resulting in
effect at the near reference point, N. One aim of the invention is to subjects stumbling on stairs or other
The patent mentions that the prism provide a mechanism whereby both obstacles at floor level. Indeed, in the
power should lie within the range 2Δ accommodation and convergence reference quoted, it was concluded
to 5Δ and preferably be of the order can be balanced simultaneously in that less risk of falls would be secured
of 2.5Δ to 3.5Δ. This amount of prism, order to prevent or, at least slow if subjects were prescribed multifocals
the patent suggests, if ordered base down the onset of myopia. The with just an intermediate addition.
in would be helpful to subjects who patent also suggests that in cases of This would provide the ability to see
exhibit symptoms of exophoria at presbyopia, the addition of base in both the ground and occasional,
near, whereas, in the case of subjects prism to the near portion may offer if short-term, near vision. It will
who exhibit esophoria at near, base relief from convergence insufficiency. be recalled that, in the past, one

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