Patent GB-1401322-A (GB)
Title: Display devices
IPC: C09K19/02
Filed: 1972-07-31
Abstract: 1401322 Modulating light MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 31 July 1972 [29 July 1971] 35742/72 Heading H4F A display cell comprising liquid crystal material between a pair of electrodes, and which undergoes a change in optical transparency when a potential difference greater than a threshold level V th  is applied, is operated by applying to one electrode a gated A.C. voltage of frequency f and to the other electrode an A.C. voltage of frequency f and of a phase # which may be varied relative to that of the gated voltage, whereby the resultant potential difference may be selected to be above or below V th . Although the method may be applied to independent ones of an array of cells, e.g. a 7-segment panel, it is described in detail with respect to a liquid crystal panel having an X-Y crossed electrode array, Figs. 1 and 3 (not shown). The gated voltage of amplitude V x  and phase # 1  is applied to a selected X electrode, e.g. X 4 , for a period T 1 , Fig. 5 (a), and a continuous A.C. voltage V y  is applied to all the Y electrodes, where 1/3 V th  # V y  # V th . Preferably V x  = 2V y , but it may be greater, up to 3 V y , so long as V x  < V th  Initially, Fig. 5 (b) V y  has phase # 1 , but in response to an image signal at a selected Y electrode, e.g. Y 5 , it is switchedto a phase # 2  for a period T 2  < T 1  commencing with T 1 . The potential differences at the addressed cell C 45 , and the other cells in that X-row and Y-column are shown in Figs. 5(c), (d) and (e). The period T 2  may be constant for a black and white display. Alternatively, and if T 1  is less than the response time of the liquid crystal material, T 2  may be varied to control the number of abovethreshold pulses for a grey-scale display, Fig. 6 (not shown). As well as the rectangular A.C. waveforms shown, and for which (# 1 -# 2 ) may take any value other than zero (shown as #/2), sinusoidal, triangular or other forms may be used, but (# 1 -# 2 ) and their amplitudes are then interdependent. The A.C. voltages may be superimposed on D.C. levels, which should be substantially identical for V x  and V y , but may differ by up to 20% of the amplitude of V y . The liquid crystal material used may be (a) nematic with negative dielectric anisotropy; (b) cholesteric mixtures; (c) a mixture of nematic material with positive dielectric anisotropy and cholesteric material; (d) a mixture of (a) and (c). Type (a) exhibits dynamic scattering with a D.C. or low frequency A.C. field (below a critical frequency f c ) and above a low threshold level, so that in this case f < f c  and V th  is low. At f > f c , light scattering occurs at above a relatively high threshold due to dielectric deformation, rather than ion flow, and this phenomenon may also be used. Type (b) undergoes a cholesteric to nematic phase transition with A.C. or D.C. fields above threshold, the contrast being improved by viewing between crossed polarizers. The lifetime of the induced nematic phase is extended due to the continuous voltage V y , even though V y  < V th , reducing flicker. The use of types (c) and (d) in the subject of Specification 1,400,495 which specifically deals with the extension of the life of the optically induced transparent state by application of an A.C. voltage below threshold (V y ) subsequent to the application of an exciting voltage. Both electrodes of the display device may be transparent, or one may be reflective.