Additive colour properties and colour gamut of cholesteric liquid crystals

Abstract

LIQUID crystals are organic compounds in a state of matter intermediate between the isotropic liquid and crystaline solid. They are fluid but at the same time they exhibit molecular order. Depending on the ordering one distinguishes the smectic, the nematic and the cholesteric liquid crystals¹ (CLC). In the latter, the rod-like molecules are arranged in layers with their long axes parallel to each other. In each successive layer the direction of the long axis is rotated by an angle of 10–20 arc min; the molecules then form a helical structure. The spacing between layers differing by an angle of 360° is called the pitch, p. Due to the periodicity of molecular orientation, reflections from all layers separated by p/2 interfere constructively², if the reflected wavelengths equal the product of the pitch and the refractive indices seen by the wave. This condition is met for a band of wavelength that is relatively narrow and steep and appears as a highly saturated colour^2,3. The width of the band equals the product of the pitch and the difference of the two refractive indices of birefringence. Colours produced by such constructive interference, sometimes described as irridescent colours, have also been observed in certain beetles, birds and butterflies⁴. We report here results of experiments showing that colours of superimposed CLC coatings add like coloured lights and produce a colour gamut greater than that obtained with inks, dyes and pigments.