Page 152 - The Indian Optician Digital Edition Jan-Feb 2020

advantage claimed for ribbon bifocal
segments was that they provided
clear distance vision beneath the
segment for ease of negotiating stairs.
A patent for a progressive lens in
which the power increases to provide
the full near addition at a point some
12mm below the fitting cross position,
and then decreases, to provide a
zone beneath the near portion which
allows clear vision at intermediate
distances between one and one- FIGURE 4 - POWER PROFILE FOR PROGRESSIVE LENS WITH INTERMEDIATE ZONE
BENEATH NEAR PORTION (ESSILOR)
and-a-half metres . The purpose of rotates along the meridian line. In the D, the addition at the prism reference
4
the design is claimed in the patent distance zone of the lens the mean point is +0.75 D.
to assist with negotiating stairs and addition power remains constant
to reduce the risks of falls. Figure 3 until about 8mm above the geometric Below the geometric centre of
illustrates the four different zones of centre (the prism reference point) the lens, the addition increases to its
the lens and the power profile for when it starts to increase. At the full value at the near reference point,
the design for a lens of power Plano, position of the fitting cross, marked FC N, some 8mm below the point G.
Add +2.00 D. It is seen that the power in figure 4, the addition has increased The near portion where the addition
increases from the distance portion, to about +0.25 D and at the prism is within 0.25 D of the prescribed
through the first progressive zone reference point, G, to a value equal to addition is thus a narrow strip some
to the near portion which lies some 6mm in depth centred on the point
12mm below the fitting cross. The (A - 0.50) / 2 N. Below N, the addition power
fitting cross itself lies some 4mm where A is the full near addition. decreases by about one-quarter of
above the geometric centre of the For the design illustrated in figure 4 its full magnitude to a point E which
lens. At the near reference point the where the full near addition is +2.00 lies 5mm below N, where the addition
lens provides the full near addition indicated in figure 4 has reduced to
of +2.00 D. On moving further down +1.50 D.
the lens, the power then degresses to
a value of about 50% of the full near Eventually, at a point some 15mm
addition, +1.00 D in this example, below the prism reference point the
to provide an intermediate addition power has reduced to about
zone at the bottom of the 50% of its full value, here to +1.00 D.
lens through which subjects The patent suggests that
should be able to see progressive lenses which have the
obstacles on the ground at feature of an intermediate zone
their feet thus avoiding trips beneath the near portion are only
and falls.
necessary for the addition range
Figure 4 illustrates starting at +2.00 D, eg, Adds +2.00 to
the power profile for the +4.00 D.
lens Plano Add +2.00 and PROGRESSIVE LENSES
indicates how the addition WHICH FULLY CORRECT
power varies as the eye
ASTIGMATISM IN NEAR VISION

It is a well-known fact that except
FIGURE 5 - OCULAR ASTIGMATISM IN DV in cases where the eye cannot
AND NV FOR +5.00/+2.00 X 90 accommodate, eg, aphakia, the

| JAN-FEB 2020 | 148 LENS TALK

January-February 2020 SK.indd 68 02/15/2020 15:45