Page 114 - The Indian Optician Digital Edition July-August 2021

The diameter of the central zone, 2y0, is seen so 2y = 2.11mm.
0
in Figure 5 to be
In order to provide the on-off state of
y = 2 f ' λ. the electro-active segment, the kinoform, is
2
0
sandwiched between a nematic liquid crystal
The focal length of the kinoform can also be such as BL037 whose refractive indices are, n
3
found from the design equation, E
= 1.808, and n = 1.526, to which is added the
O
3
2
f ' = y / 2 m λ chiral doping solution ZLI-4571 to form the
m
cholesteric liquid crystal whose n , (equal
AVG
The details of a kinoform, which has a power to (n + n )/2) is 1.667. This value matches that
O
E
+1.00 D and diameter 10mm are determined as of several available plastics materials (such
follows. The value of λ = 555nm or 555x10 mm as MR-10). The diffractive segment and liquid
-6
has been chosen from the middle of the visible crystal are, themselves, cast just beneath the
spectrum.
convex surface of a semi-finished blank, with
The number of zones required, m, is first electrodes connecting the liquid crystal to the
found from top of the semi-finished blank (Figure 8).
2
m = y / 2 f ' λ = 5 / 2x1000x555x10 = 22.5. The semi-finished blank for the Pixel Optics
2
-6
m
emPower lens is illustrated in Figure 8.
TM
The diameter of the central aperture, 2y , is The thin diffractive element is sandwiched
0
found from between the substrate and the adaptive
y = √(2 f ' λ) = √(2x1000x555x10 ) = 1.05mm. liquid crystal described above, and an invisible
-6
0
electrode which passes up through the lens
to the frame rim, which has further electrodes
connecting each eye of the frame to a small
battery contained in the tip of the right side of
the frame. An important requirement of the
frame design is to ensure that the conductivity
of the frame is not impaired by fluid such as
wearer perspiration or water splashes which,
for example, might occur in a rain shower.
When no current is passed to the nematic
liquid crystal its refractive index, n AVG , is about
1.67, which is the same as that of the material
FIGURE 7 DESIGN EQUATION FOR THE KINOFORM from which the diffractive element and the
surrounding substrate is made. As a result, the
liquid crystal and kinoform, have no optical
effect. In this state the effect of the segment
is cloaked.
When current is passed to the nematic
liquid crystal, its refractive index changes
instantly to 1.53 and the power of the segment
is thus activated. When the current is switched
off again, the refractive index of the nematic
liquid crystal reverts to 1.67, once again
cloaking the segment element.
In the emPower frame design, the current
TM
was switched on and off when the wearer
FIGURE 8 THE EMPOWER SEMI-FINISHED BLANK
TM
tapped the side of the spectacle frame. One

| 110 LENS TALK