Patent GB-1451076-A (GB)
Title: Operational method for displaying images with liquid crystal electro optical devices and apparatus therefor
IPC: G09F9/30
Filed: 1973-09-18
Abstract: 1451076 Controlling light G BARZILAI C M OTTAVI P MALTESE and P REALI 18 Sept 1973 [22 Sept 1972] 43835/73 Heading G2F [Also in Division G5] A layer of homotropically aligned nematic liquid crystal material having a negative dielectric anisotropy is arranged between polarizers so that no light is transmitted, and between electrodes on either side of the layer is applied a pulsed electrical potential difference of sufficiently great amplitude and short duration such that the arrangement transmits light which appears white to an observer in the absence of any coloured filters, while at the same time avoiding dielectric breakdown of the layer. As explained in the Specification, when a single short high amplitude pulse is applied across such a layer the white light transmittance of the arrangement follows a curve such as that I-VII shown in Fig. 2 for a pulse of duration #, and the corresponding molecular alignments are believed to be those indicated in the lower part of the Fig. For any duration # the pulse amplitude may be adjusted to a corresponding threshold value A such that the point III is just reached at the end of the pulse, the intensity subsequently falling monotonically. For values less than A the peak intensity of the transmitted light is reduced, and for values greater than A the intensity slows the first peak III and possibly further subsidiary peaks during application of the pulse. As claimed the invention is limited to the application of pulses of a duration not exceeding 1.3 times a value # and an amplitude not exceeding 1.3 times the threshold value related to #. When the pulses are applied repetitively, the duration and amplitude required to reach any point on the curve I-VII may both be reduced by a factor of up to approximately 2, depending on the repetition frequency. As can be seen in Fig. 2, the total time T for which the arrangement transmits light is much greater than #, i.e. a temporary memory effect, enabling the arrangement to be formed, e.g. as an X-Y coincidence addressed matrix array. The time (T-#) for the response to return to zero is a function mainly of the thickness of the liquid crystal layer but also of the pulse repetition frequency. Preferably in a matrix display, the response immediately prior to a pulse should have fallen to a value not exceeding one half of the peak response for the pulses then in use, so that the thickness of the layer, normally less than 5Á, is basically determined by the matrix size and frame scanning speed (pulse repetition frequency). Given the layer thickness the threshold pulse amplitude is then determined by the pulse duration, nominally equal to the line period. In a matrix arrangement, the presence of cross-talk voltage also tends to reduce the pulse amplitude and duration for a desired light intensity and it is desirable for example to add to each column and the selected line a waveform selected so that the average voltage on each column is zero. Alternatively waveforms are added to those on the columns which are not correlated with the line voltages so that the RMS voltage on every column over a frame is constant, and the line voltages are correspondingly reduced. The arrangement will generally appear dark if the amplitude of the repeated pulses is reduced by more than 25% below the threshold value. In an arrangement intended for colour display, for example colour television, each column conductor in a substrate 11 is divided into three lines 15, 16,17, Fig. 4A (not shown), and the other surface of the substrate is shaped and carries sets of three primary colour filters 12, 13, 14 such that the coloured beams in each set are separately controlled and fall on a common area of a diffuser 22. In a modification, Fig. 4B (not shown), the filters lie on a planar surface and integration of the colour is done by the eye. The liquid crystal cell may be transmissive or reflective. In the latter case the polarizers are disposed on the same side of the cell and may be separate devices or the same device. The polarizers may be linear, elliptical or circular. The liquid crystal cell may include further layers between the electrodes which have an optical,  chemical or electrical function, for example a dielectric or electrically non-linear layer. The homotropic alignment may be normal or inclined to the surfaces of the liquid crystal layer and as described is produced by a leather or polyamide surfactant on a chemically cleaned surface. By rubbing such a surface with a very fine abrasive, the alignment can be inclined, and/or the deformation of the alignment in the presence of an electric field occurs in a predetermined direction.