Page 158 - The Indian Optician Digital Edition Jan-Feb 2020

with the horizontal spherical curve requirement that the upper portion The uncut diameter shown in
which is only +4.00 D. Naturally, the of the lens provides a wide area for Figure 9 is 60mm and a spectacle lens
astigmatism introduced by the convex distance vision and the intermediate with a typical shape and size has been
toroidal surface has to be neutralised zone provides a wide zone of clear superimposed upon the uncut. It can
by a concave toroidal surface worked vision down to about one metre from be seen that the astigmatism within
on the back whose nominal curves the subject. There is no near portion the cut lens shape is very small only
are seen in figure 7(c) to be -6.00 DC x on the lens. reaching 0.75 DC in the lower nasal
180/ -8.00 DC x 90. The DP power of Consider the prescription +0.50 side of the lens, which is unlikely ever
the lens remains -2.00 DS. Needless Add +1.50 for near which is to to be used by the wearer.
to say, if the progressive surface is to be dispensed using lenses of this
be combined with the toroidal surface invention. The power of the upper Whether any of the designs
on the back of the lens, the concave distance portion of the lens is described above will ever be brought
surface would become a free form +0.50 D and the power of the lower into production will depend upon the
atoroidal, progressive surface. Figure intermediate portion is +1.25 D, it manufacturer’s perceived demand
8 taken from the patent compares the being assumed that an intermediate from the marketplace. It is certain that
iso-cylinder and mean power plots for addition of 50% of the full near the development of new progressive
the two designs, one with a nominally addition has been supplied. Figure lens designs will continue, with the
spherical front surface and one with 9(a) shows an iso-cylinder plot for best features which emerge with each
a nominally toroidal front surface and a progressive lens made to the new generation being combined into
it is seen that there is no discernible prescription +0.50 Add +0.75 given the latest state-of-the-art designs.
difference in the optical performance in the patent with the astigmatism REFERENCES
of each design. exhibited by the lens plotted in 0.25 D 1. Mori T, et al, (2013) US Patent 8550622
PROGRESSIVE DESIGNS intervals. The iso-cylinder plots are B2 Progressive power lens and
progressive power lens design method.
FOR DISTANCE AND given for the through power, eye
INTERMEDIATE USE plus lens, rather than just the surface 2. Poulain I. et al, (2007) US Patent
astigmatism. Figure 9(b) shows an 7216977 Ophthalmic lens with
A further patent assigned to iso-cylinder plot for a progressive lens progressive addition of power
and prism.
Rodenstock discloses a progressive made to the same specification +0.50
8
lens designed for distance and Add +0.75 according to the patent. 3. Elliott DB. et al, Intermediate addition
multifocals provide safe stair
intermediate vision but without an It can be seen that the new design ambulation with adequate short term
area for a full near vision addition has a wider distance field where the reading, Ophthalmic Physiol. Opt. 2016
being provided. In principle it’s Minkwitz astigmatism is less than Jan; 36(1) 60-8, Epub 2015 Aug. 25.
modus operandi is similar to that of 0.25 D and narrows down to a funnel 4. Giraudet G. et al, (2011), US Patent
a degressive power lens described shape in the intermediate zone, rather 8061838 Progressive lens for
ophthalmic spectacles having an
in Part Six of this series except that than the hourglass shape of a typical additional zone for intermediate
particular attention is paid to the progressive lens. vision.
5. Jalie M. (2016) Principles of
Ophthalmic lenses, 5th Edition, ABDO,
Godmersham.

6. Donetti B. et al, (2007),US Patent
7249850, Method for determination
of an ophthalmic lens using an
astigmatism prescription for far sight
and for near sight.

7. Baumbach P. et al, (2004), US Patent
6685316 Method of Manufacturing
Progressive Ophthalmic Lenses.
8. Dorsch R. et al, (2006), US Patent
7033022 B2, Progressive spectacle
FIGURE 9 - COMPARISON OF ISO-CYLINDER PLOTS FOR PROGRESSIVE LENSES lens for seeing objects at a large or
FROM US PATENT 7033022 B2 (2006) average distance.

| JAN-FEB 2020 | 154 LENS TALK

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