GB1335834A - Display devices using liquid crystals - Google Patents

Abstract

1335834 Modulating light NIPPON ELECTRIC CO Ltd 17 Nov 1970 [29 Nov 1969] 54672/70 Heading H4F [Also in Division C4] It is stated that in known devices wherein an electric field is applied across a cholesteric liquid crystal layer to change its colour, the optic axis of the crystal and the field direction are in substantial parallel alignment. When using a cholesteric crystal having a parallel component of electric susceptibility (i.e. in the plane of the molecules and molecular layers) greater than the transverse component (normal to the plane of the molecules and layers), if a D.C. field greater than a predetermined value is initially applied across the crystal (for a time depending on the actual field strength), the optic axis becomes aligned substantially perpendicularly to the field direction. The layer may remain in this alignment for a considerable period after the field ceases, and shows a reduction in response time, e.g. from 10 milliseconds to 1 millisecond, on application of a variable electric field thereacross. The crystal selectively transmits light of a wavelength determined by Bragg reflection from the helically arranged molecular layers, application of the variable electric field varying the pitch of the helix. When the optic axis and field direction are mutually orthogonal, the light may be directed from behind an observer into the crystal and Bragg reflected back to the observer, e.g. after transmission through two transparent electrodes, or by transmission through one electrode and a further reflection at a second electrode to traverse the cholesteric layer twice. As particularly described a 6 Á layer consisting of 42% cholesteryl oleyl carbonate, 30% cholesteryl chloride and 28% cholesteryl nonanoate is disposed between a transparent tin oxide electrode supported on a glass plate, and a coloured aluminium oxide film on an aluminium plate electrode for reflection. 18 volts D.C. is applied across the layer for 3 minutes and subsequently a variable or pulsed signal is applied. The colour of the layer changes continuously with signal amplitude being blue at OV, red at 260 V and giving selective reflection in the infra-red at higher amplitudes. The electric field may be obtained by applying mechanical stress to a piezoelectric element connected to the electrodes.