Page 111 - The Indian Optician Digital Edition July-August 2021
P. 111
ordinary value, n . If the electric field is strong
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enough, the directors of the cholesteric liquid
crystalline material will be substantially parallel
to the applied electric field, whereas a wave
travelling in a direction perpendicular to the
layer of material will experience a refractive
index of the ordinary value, n .
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If the cholesteric liquid crystalline material
is in contact with an alignment layer and no
electric field is applied, the director of the
material at the interface between the alignment
FIGURE 1 REFRACTIVE INDEX CHANGE BY PASSING
AN ELECTRIC CURRENT THROUGH THE MATERIAL layer and the cholesteric liquid crystalline
material will lie in the same direction as the
alignment direction. An incoming light wave
which is travelling in a direction perpendicular
to the material will experience a refractive index
of n AVG . If sufficient current is then applied,
the director will rotate through 90°, so that
an incoming wave will experience a refractive
index of n .
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One of the first applications of this
technology in the ophthalmic field was
employed by Dr. Ronald Blum* of Pixel Optics
in the USA under the trade name, emPower .
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With a progressive power lens, the effects of
Minkwitz astigmatism and skew distortion
increase with the near addition, thereby
hindering wearer adaptation to progressive
FIGURE 2 PRINCIPLE OF THE PIXEL OPTICS power lenses. Following research by a team of
EMPOWER LENS optometrists in the USA, it was concluded that,
TM
as a rule, the requirement for near vision was
wavelengths comparable to the twist pitch only some 20% to 30% of the day. For most daily
and travelling in a direction perpendicular to activities, subjects required mainly distance
the liquid crystal’s director, (i.e., polarized in the and intermediate vision. Hence, in practice,
same direction as the director) will experience use of the near portion of multifocal lenses or
an average refractive index, n AVG , equal to (n + progressive lenses was restricted to only some
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n )/2. Because the value of n AVG is substantially 25% of the time that the spectacles were being
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constant with respect to the polarization state worn. For most of the time, only the distance
of the incident waves, the cholesteric liquid and intermediate zones were being used, and
crystalline material is insensitive to the state therefore, it was concluded, why not develop a
of polarization. In the presence of an electric lens where the near portion could be “switched
field applied in a direction perpendicular to the off” when not required?
alignment layer, most of the material’s directors
align with the field, effectively unwinding The final lens design which came to the
the director of the helix. As such, a wave market was a progressive lens whose near
travelling along the director axis of rotation addition was some 50% of the full near addition,
will experience a continuous and polarization- with an electro-active segment near the bottom
insensitive change in the value of the refractive of the lens which could be brought into use,
index between the average value, n AVG , and the simply by tapping the side of the frame. Since
107 | LENS TALK
