Faculté des sciences

Improved red color with cholesteric liquid crystals in Bragg reflection mode

Kipfer, Peter ; Klappert, Rolf ; Herzig, Hans-Peter ; Grupp, Joachim ; Dändliker, René

In: Optical Engineering, 2002, vol. 41, no. 3, p. 638-646

The helical pitch of the cholesteric liquid crystal (CLC) can be adjusted to reflect the colors red, green, and blue. Additive mixing of these colors in displays results in multicolor images and it is easy to use pure primary blue and green colors, but the red color is in general very unsaturated. We show by simulations that this poor red color performance is due to reflection sidebands on the... More

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    Summary
    The helical pitch of the cholesteric liquid crystal (CLC) can be adjusted to reflect the colors red, green, and blue. Additive mixing of these colors in displays results in multicolor images and it is easy to use pure primary blue and green colors, but the red color is in general very unsaturated. We show by simulations that this poor red color performance is due to reflection sidebands on the smaller wavelength side of the normal red Bragg reflection band. We discuss five approaches to improve the red color performance, namely, two types of spectral filtering (dyes or filters), a very low birefringence CLC, a gradient in the birefringence of the CLC, and the use of a bluish reflector. The two methods of spectral filtering are also experimentally tested.