Why do chiral nematic reflective displays tend to be used for static images?
In nematic arrangements the crystals are randomly positioned with all their long axes pointing in essentially the same direction, given by a vector known as the director. In smectic arrangements the crystals are further constrained to parallel planes; in smectic A the director is perpendicular to these planes, while in smectic C the director is at some other angle to the planes.
Chiral nematic reflective displays, also known as cholesteric liquid crystals, are typically utilized for static images because they are relatively slow in their response time.1
These displays function by spontaneously ordering molecules into helical structures that reflect light of a specific wavelength, which corresponds to the pitch of the helices. While these devices offer high resolution and acceptable contrast, the process of switching the orientation of these helices using an electric fieldāto transition between a coloured reflective state and a scattering or black stateāis not fast enough to support the rapid image changes required for dynamic content like video.1