Page 112 - The Indian Optician Digital Edition July-August 2021
P. 112
WITH A PROGRESSIVE
POWER LENS, THE EFFECTS
OF MINKWITZ ASTIGMATISM AND
SKEW DISTORTION INCREASE
WITH THE NEAR ADDITION,
THEREBY HINDERING WEARER
ADAPTATION TO PROGRESSIVE
POWER LENSES FIGURE 4 BLAZED FULL-PERIOD PHASE
ZONE PLATE (KINOFORM)
only 50% of the full addition was present for major portion of the lens, it would be necessary
most of the time, the aberrations in the lower to employ a strong positive depression curve on
peripheral portions of the lens were greatly the major portion (Figure 3). In practice, such a
reduced and clear vision of the ground or steps curve could only be manufactured by means of
at the wearer’s feet was not impeded by the solid bifocal machinery.
presence of a full near addition. Although not
currently available, the method of construction For example, the fused bifocal blank ratio,
may return to the optical profession. The k, for a 1.67 index major portion and 1.53 index
principle of the emPower lens design is shown segment glass is
TM
in Figure 2. k = (n-1) / (n - n) = 0.67/-0.14 = -4.786
s
The basic principle of how the extra power For a +5.00 D front curve and a near addition
provided by the segment is obtained is similar of +1.00 D, the power of the depression curve
to that of a fused bifocal. If it were required to would be
make a simple fused bifocal using a segment
glass of lower refractive index than that of the F = F - Ak = 5 + 4.786 = +9.786 D
C
1
and the radius, r = 670 / 9.786 = +68.465mm.
C
FIGURE 3 - PRINCIPLE OF
ELECTRO-ACTIVE SEGMENT Note in Figure 3, how prominent the segment
BASED UPON FUSED
BIFOCAL CONSTRUCTION dividing line has become in this fused bifocal
whose segment glass has a lower refractive
index than the major distance portion. Clearly
such a design is impractical.
However, since it is the near portion that
requires to be switched on and off, and must
have a lower refractive index than the main
lens, since the electro-active action is to cause
a reduction in the refractive index of the
material, the designers of the lens realised that
a refractive segment element would not be
practical and turned their attention towards
a diffractive segment element, which can be
produced in a very thin form.
A diffractive lens is an Imaging Fresnel lens
being a buried low-index segment in the form
| 108 LENS TALK
