Page 115 - The Indian Optician Digital Edition July-August 2021
P. 115
of the upper of the two rays shown passing
through the combination, differs from that of
the lower ray by the depth of groove, d. The
optical path length is given by the actual path
length multiplied by the refractive index of the
material, so the optical path difference, OPD, is
given by
n d - n d
1
2
Figure 5 showed that in order for the light
from each zone of the full-period, phase zone
plate to arrive at P in phase, each path length
must differ from its neighbour by one wave
FIGURE 9 GROOVE DEPTH OF THE KINOFORM
length, so
OPD = n d - n d = λ.
2
1
touch of the conductive material on the side Hence, the groove depth, d, is given by
switched the current on, and a second touch
switched it off again. If the conductive material d = λ / (n - n )
2
1
on the side was swiped forward it would For the emPower segment the groove
TM
activate a microgyroscope which switched off depth is
the current when the head was upright and
switched it on automatically, to activate the 550 / (1.67 - 1.53) = 3.93μm.
segment when the head inclined to the When the emPower adaptive spectacles
TM
reading position. The wearer could select either were originally launched, it was envisaged that
mode at will.
the electro-active segments would be available
The power of the segment lens in the in the three additions, shown below.
original design was +0.75 with the rest of the Full near Adds
near addition being made up in the form of a
progressive surface on the back of the lens. The 1.25 to 2.00 - segment add +0.50 D
segment was oval in shape with diameter 2.25 to 2.75 - segment add +0.75 D
20 x 12 mm and assuming λ = 550nm the 3.00 to 3.50 - segment add +1.00 D
number of zones required was
The remainder of the addition is provided by
2
m = y / 2 f ' λ = 10 / 2x1333.3x550x 10-6 = 68.2. the progression worked on the back surface of
2
m
the lens.
The diameter of the central zone, 2y , is found
0
from REFERENCES
y = √(2 f ' λ) = √(2x1333x550x 10-6 ) = 1.21mm. 1. US Patent 6491391 (2000), Blum R.D. et
0
al. System, apparatus and method for
so 2y = 2.42mm. reducing birefringence. See also US Patent
0
The depths of the concentric grooves of the 7731358 (2010), Blum R.D. et al. System,
transparent diffractive element are carefully apparatus and method for correcting vision
controlled to provide the correct optical power. using an electro-active lens.
Figure 9 shows how the groove depth of the
diffractive rings can be found. The kinoform, 2. US Patent 7728949 (2010), Clarke R. et
al. Electro-Active Lens. Assigned to Pixel
made from a material of refractive index, n , is in Optics, Roanoke, VA.
2
contact with the nematic liquid crystal, whose
refractive index is n , and the optical path length 3. Both manufactured by Merck & Co. Inc. USA.
1
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