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The pitch of a cholesteric liquid crystal (how fast its director rotates through
a full circle) determines how it interacts with different wavelengths (that is, colors)
of light. Being able to control and vary the pitch thus allows one to control and vary
the cholesteric's (CLC's) optical properties, including reflection, polariztion,
and color travel [these could have links within the page].
Mastery over the pitch forms the basis of Chelix's broadband reflective films.
Reflection
The strongest interactions between a CLC and light occur with colors whose wavelengths
are about the same size as the pitch of the CLC. Usually this only happens for one very
narrow range (or "band") of wavelengths, since most CLCs have a single, fixed pitch.
Light in the narrow band of strongly interacting wavelengths is reflected when it strikes
the surface of the CLC, and all the other wavengths are transmitted (pass through) the
material.
 Cholesteric film with narrowband reflection
Chelix, however, has developed a proprietary technology to extend the reflection band to
cover multiple colors and create a broadband reflection, one in which a wide, continuous band of
colors is reflected. Chelix does this by creating CLC materials with a pitch that continuously
varies, instead of the typical fixed pitch. The figure below demonstrates this effect,
which is used to create broadband reflective Solar Control Films.
 Broadband film with full spectrum reflection
Polarization
Another optical property unique to cholesterics is that they
circularly polarize the light they reflect.
While this property is not visible to the naked eye, it is easily revealed by the use of a
polarizer: when viewed through an appropriate polarizer, the reflected light vanishes.
This property can be exploited in making special inks and pigments [link this to products] for
anti-counterfeiting markings on documents or banknotes.
Color Travel/Color Flop
Finally, cholesterics also exhibit the unusual optical property of "color travel," also called
"color flop." The angle that you view a CLC from changes the color that you see. As your viewing angle
increases (viewing from directly above a surface is considered to be at a 0° angle), the color you
perceive shifts to shorter and
shorter wavelengths: red turns into green and green turns into blue, for example.
Transitions when colors either seem to disappear completely or emerge from
nothing are particularly dramatic. Again, as the viewing angle increases, one can see a transition from
blue to ultraviolet (perceived as a disappearance of blue color) or a transition from infrared to red
(the appearance of red color where before there was none). These color effects are can be used to great effect in
Pigment Technologies for paints and coatings for visual and
security applications.
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