Cholesteric liquid crystal mixtures sensitive to different ranges of solar UV irradiation

Three photosensitive cholesteric liquid crystal mixtures have been investigated as UV sensors. Each of them is sensitive to a well definite part of the solar LW radiation spectrum, the UV A or the UV B or the LW C range, with suitable absorption spectra. The photosensitive elements in the mixtures are either photoisomerizable nematic hosts or photoisomerizable optically active dopants. The selective reflection peaks of these cholesteric mixtures are in the visible part of the light spectrum. The LW exposure changes the cholesteric pitch and hence the position of the selective reflection peak. The consequence is that the colour reflected by each mixture varies under influence of the LW and it can be used as an indicator of the UV exposure itself.