Page 156 - The Indian Optician Digital Edition Jan-Feb 2020

FIGURE 8 - ISO-CYLINDER AND MEAN POWER PLOTS FOR PROGRESSIVE LENSES FROM US PATENT 6 685 316 (2004)

power on a free form progressive PROGRESSIVE LENSES WITH to Rodenstock which discusses
surface. There may also be a need PRIORITISED ZONES prioritised zones also discloses
to alter the axis direction of the near a cosmetic improvement of the
vision cylinder some eyes exhibiting Several modern progressive appearance of the lens by employing
a measurable cyclorotation upon designs are offered in different a toroidal surface with the steeper
depression and convergence of the forms which offer wider fields of curve along the longest meridian of
visual axes in near vision. view in particular zones of the lens. the lens shape. For example, most
For example, progressive lenses modern spectacle frames have a
Some major lens manufacturers whose progressive surface is “hard” greater A-dimension (horizontal box
now offer to provide spectacle lenses in design, generally with shorter dimension) than the B-dimension of
in which the cylinder power and/or corridor lengths, will offer a wider the shape (vertical box dimension) as
the axis direction in the near portion clear distance zone with less Minkwitz depicted in figure 7(a).
is different from that in the distance astigmatism than “soft” designs, which
portion. Although this specialised generally possess longer corridor Figure 7(b) illustrates a progressive
form of correction has been possible lengths and which aim to increase lens whose nominal DP curves are
for many years with bifocal lenses, the intermediate and/or near zones shown where the shapes of the curves
free form production now makes it of vision. Examples of the first are in the upper portion of the lens are
possible to provide different cylinder marketed under trade descriptions essentially spherical. The power of
powers or axis directions for distance such as “Driver”, “Outdoor”, “Road”, the distance portion is -2.00 D and
or near in progressive lens forms. “Clear” “Freeway” etc. the front curve in the DP is +4.00 D.
A patent for the determination of In Figure 7(c) the shape of the front
an ophthalmic lens for a wearer, Typical differences in the surface in the upper portion is toroidal
for whom near vision and far vision distribution of Minkwitz astigmatism with curves +4.00 DC x 180/+6.00 DC x
prescriptions have been made out, for a progressive design suitable for 90, the steeper +6.00 D curve running
the near vision astigmatism being driving (figure 6(a)) compared to a in the horizontal meridian of the lens.
different from the distance vision design for more general use (figure This means that it will reduce the
astigmatism was assigned to Essilor 6(b)) are illustrated in figure 6. overhang of the front surface from the
as long ago as 2007 . The US 66853167 patent, assigned front rim of the frame when compared
6
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