Diffraction on the Focal-Conic Texture

For a polycrystalline layer of a cholesteric liquid crystal the expression for the wavelength of the scattering maximum was derived by Fergason [13] under the condition of small optical anisotropy of the material 

With this equation and knowing the angles of incidence 0, and of the reflection ip of monochromatic light, the helical pitch of the cholesteric liquid crystal can be determined (Section 6.1.2). 

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Fig. 4.29 Measurements of the pitch of the helix in a cholesteric, (a) Geometry for mraiochro-matic light diffraction on the focal-conic texture. The pitch is found from the angle 2i between the incident and diffracted beams with wavevectors ko and kr, (b) Spectral measniemtaits of the light transmissicm by a planar cholesteric texture (Jo and 7t are intensities of the incidoit and transmitted beam)...

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