Electrooptical switching device
This invention relates to matrix-control display devices which make it possible to represent a figure by breaking it down into a set of segments or intersections which vary in appearance under the control of electric signals applied to these latter by means of two sets of crossed connections. These signals serve to define addresses corresponding to said connections. 
     In one known design, a matrix access display device having addressing means which are entirely electric contains two orthogonal arrays of wire electrodes. An electrode array of this type consists of a set of elements designated as lines or columns according to the array considered. Each intersection between a line and a column defines a zone of local excitation of a display element. Electrical excitation of conductor lines and columns involves the use of electric control circuits which comprise a large number of passive and active components in order to deliver the control voltages during a suitable period of time. In fact, when carrying out sequential scanning, the control voltages must be applied during a sufficient period of time to produce a switching operation whereas the addressing instructions are received in the form of signals of short duration. This gives a clear idea of the complexity of all the switching elements, connections and different means for addressing these elements. The bulk of the control equipment placed at the periphery of a display beam becomes rapidly prohibitive in the case of devices of the high-definition type which are being extensively developed at the present time. 
     For similar reasons, conventional switching elements are ill-suited to small-area screens which are particularly advantageous on account of the low switching power required for their control. Display panels of small size can be adapted to large-screen projection by making use of an auxiliary light source whose radiation is optically modulated by changes of state of the panel elements. 
     In order to solve the problem of control of a display panel, consideration has already been given to the possibility of interposing a photosensitive layer between the display elements and one of the electrode arrays whilst addressing is performed by optical means. In the case of projection, however, a device of this type is attended by a disadvantage in that it gives rise to difficulties in regard to optical decoupling between the addressing control and the radiation which is employed in order to form an enlarged image of the display panel. In fact, the photosensitive layer must not react to the reading radiation. If it is sensitive to this radiation, provision must accordingly be made for an opaque protective layer in order to ensure that the image to be projected is not disturbed by any accidental control operation caused by the light which serves to project the image. 
     The present invention is intended to overcome these difficulties and will therefore be particularly applicable to high-definition display systems or to small-area systems. In fact, the invention makes it possible to separate the switching elements proper from the means for carrying out addressing operations and to achieve a considerable reduction in the equipment to be provided around the screen by reason of the fact that, in our invention, the switching elements and the leads which connect these latter to the voltage sources represent the greater part of the equipment whereas all the control circuits react to the incident signals in order to address the switching elements. The zone in which the image to be projected is formed does not contain any photosensitive control elements. It should be added that either a reflection or transmission mode of display can be adopted with equal ease, thus making it possible in particular to carry out projection of an image on a screen by means of an optical system. It is thus possible to design display elements having much smaller dimensions than was the case in the prior art, with all the advantages which arise from this reduction in size. 
     The invention consequently relates to a visual display device whereby a representation formed in a visual display zone by means of data introduced in the form of incident electric signals can be broken down into visual display elements. Said display elements are adapted to cooperate with electrodes connected to voltage sources by means of switching means located outside said zone. Said switching means essentially comprise optically-controlled switching elements and optical addressing means for controlling said switching elements. Said incident signals are intended to control the state of said visual display elements as a function of the light intensity delivered to said switching elements by said optical addressing means. 
    
    
     Other features of the invention will be more apparent to those skilled in the art upon consideration of the following description and accompanying drawings, wherein : 
     FIG. 1 is a general schematic diagram of a device according to the invention; 
     FIG. 2 is a view in isometric perspective of the device of FIG. 1; 
     FIG. 3 is a schematic circuit diagram of the array of electrodes for addressing the lines of the image; 
     FIG. 4 is a schematic circuit diagram of the array of electrodes for addressing the columns of the image; 
     FIG. 5 shows an alternative embodiment of the schematic circuit diagram of FIG. 3; 
     FIG. 6 illustrates an optical addressing means according to the invention; 
     FIG. 7 is an explanatory diagram; 
     FIG. 8 illustrates the electro-thermal mode of control of a liquid crystal in smectic phase. 
    
    
     Depending on the type of application desired, a number of different physical phenomena can be contemplated such as, for example, certain optical effects produced by liquid crystals in either smectic, nematic or cholesteric phase. These forms of construction with liquid crystals will be described by way of example but it will be understood that any light emitter is suitable on condition that the sets of leads and electrodes are compatible in order to produce a light emission. 
     FIG. 1 shows the general arrangement or schematic circuit diagram of a visual display device according to the invention. The data S to be displayed are fed to the input of a control circuit 1. The data are signals representing characteristic elements of the information contained in an image to be projected. The circuit 1 serves to control the electric generator 2 and/or the optical addressing means 3 and 4 as a function of the data received. All the combinations of these two control operations are possible according to the nature of the data and the operating mode adopted. 
     By way of example and without any limitation being implied, the incident electric signals can appear in the form of a composite signal of the television signal type; the optical addressing means 3 and 4 operate in synchronism with the synchronization signals contained in said composite signal; the light beam 14 emitted by the optical addressing means is simply subjected in this instance to a recurrent deviation for successively scanning a line of photodetectors 50 1  . . . 50 M . The light beam 13 sweeps a column 80 1  . . . 80 N . The video component of the composite signal S is delivered by the circuit 1 to the control input of the electric generator 2. The electric generator 2 supplies the optical control switching elements 50 1  . . . 50 M  and 80 1  . . . 80 N , the elements 50 . . . 80 supply line electrodes such as those designated by the references 90 1  . . . 90 N , and column electrodes such as those designated by the references 70 1  . . . 70 M . The visual display elements 2 11  . . . 2 MN  are placed at the intersection of the lines 90 and of the columns 70 in order to form a matrix of MN elements addressable by M+N electric connecting leads. 
     The shape and character of the elements are very variable and essentially dependent on the image to be displayed. The elements can consist of any system in which a changing optical characteristic makes it possible to construct an image. Mention can be made of plasma cells, electroluminescent cells. Depending on requirements, control of the cells can be carried out either by current or by voltage. Voltage control makes it possible to produce an electric induction field. Current control can serve to produce transient heating. The lines and columns of electrodes are connected to voltage sources by means of switching elements along lines 80 1  . . . 80 N  and the columns 50 1  . . . 50 M  which are controlled optically by the addressing means 3 and 4. The positioning, geometrical dimensions and physical characteristics of these elements must be selected according to the type of application. 
     In accordance with the invention, optical addressing is directed to switching elements arranged at the periphery of the zone of formation of the image to be displayed. It should be mentioned in addition that these switching elements are much less complex than those which have to be employed in order to perform an electric control operation in accordance with known techniques. The switching elements 50 1  . . . 50 M  and 80 1  . . . 80 N  receive the voltages generated by the generator 2 and deliver these latter to the different line electrodes 90 1  . . . 90 N  or column electrodes 70 1  . . . 70 N . These voltages assume values corresponding to the optical addressing positions of the light beams 13 and 14 emerging from the optical addressing means 3 and 4. The connecting leads which are necessary for the constructional assembly of a device of this type are therefore limited in accordance with the invention to the sum of the number of electrode lines and columns M+N, which therefore leaves the possibility of designing high-definition assemblies with small dimensions. 
     FIG. 2 is a view in isometric perspective of the device of FIG. 1 in which the same elements are designated by the same references. The visual display assembly 50 comprises two supports 10 and 11 consisting of transparent glass plates, for example, if the device is employed in the transmission mode of operation. These supports 10 and 11 enclose a liquid crystal layer 12 which can be written under the action of an electric field. An array of line electrodes 90 1  . . . 90 N  and the array of column electrodes 70 1  . . . 70 M  are placed between each support 10 and 11 and the liquid crystal layer 12. Said lines and columns are obtained by the usual methods of metal deposition. At least one of these sets of electrodes is permeable to the projection radiation. A visual display element such as one element 2 11  . . . 2 MN  is accordingly constituted by that portion of said liquid crystal layer 12 which is placed at the intersection of the line j and the column i. Depending on the electric field which exists between the electrodes j and i, the visual display element will have an appearance of either diffusion or of transparency and the complete assembly of visual display elements such as those designated by the references 2 11  . . . 2 MN  will describe the overall display pattern. The electrode lines 90 1  . . . 90 N  are each connected at one end to the generator and at the other end to switching elements such as the elements 80 1  . . . 80 N . The other end of each switching element 80 j  . . . shares a common ground connection M with the generator 2. 
     By way of non-limitative example, the switching element 80 j  is a photoconductive cell and this latter receives a radiation which is capable of reducing its resistance and therefore connecting the electrode 90 j  to ground, said electrode being supplied as a current source. 
     The column electrodes 70 1  . . . 70 M  are each connected at one end, on the one hand to switching elements 50 1  . . . 50 M  and on the other hand to memory capacitors 60 1  . . . 60 M . The other end of each memory capacitor 60 1  . . . 60 M  is connected to a ground which is common to the generator 2. The other end of each switching element 50 1  . . . 50 M  is connected to the electric generator 2 via the leads 30 1 . The design function of the memory capacitors is to apply elementary voltages to the column electrodes 70 1  . . . 70 M  during a predetermined time interval, said elementary voltages being contained in the composite signal carried by the leads 30 1 . The light beams 13 and 14 illuminate respectively the photodetectors 80 1  . . . 80 N  and the photodetectors 50 1  . . . 50 M  in synchronism with the composite signal 15. 
     The beam 13 illuminates the photodetector 80 j  and therefore produces a substantial resistance drop in this latter. The line 90 j  is consequently connected to the ground M which is common with the generator 2. Since the other photodetectors 80 are not illuminated, they have a high resistance and the corresponding lines are at the voltage transmitted by the connecting lead 30 2 . 
     Before the beam 13 illuminates the line photodetector 80 j , the beam 14 must successively illuminate all the photodetectors 50 1  . . . 50 M  in compliance with the synchronization of the composite signal in order to transmit the information to said photodetectors. The elementary signals are delivered to each column in correspondence with the illumination of their photodetector. In fact, when the photodetector 50 i  is illuminated by the light beam 14, the memory capacitor 60 i  is charged to the value of the voltage delivered at that instant via the connecting leads 30 1  ; the electrode 70 i  is therefore at this voltage. 
     Each of the other photodetectors 50 has an appreciable resistance by reason of the fact that it is not illuminated and its column electrode 70 is then at the voltage delivered by its memory capacitor 60. This delivered voltage is the elementary voltage of the signal which the memory capacitor has stored at the time of illumination of the corresponding photodetector by the light beam 14. 
     The light beam 13 subsequently controls the element 80 j  in such a manner as to ensure that the potential difference which is present between the line j and the column i makes it either possible or not possible, according to its value, to produce the diffusing state of the element 2 ji . 
     If Z designates the total addressing time of the image to be displayed, each line is connected sequentially to ground during the time interval Z/N and brought to the voltage delivered by the connecting lead 30 2  during the time interval Z (1-1/N). The columns are addressed by the beam 14 at the frequency M.N/Z. At the level of a column, the information must be stored at the level of the capacitive element 60 i  during at least the time taken to scan one line, that is, Z/N. Furthermore, in order to charge the information rapidly at the level of said capacitive element, the time constant which characterizes the charge when the photoconductor is illuminated must be of low value compared with the time-duration of said illumination. The capacitors permit line-by-line access of the screen. 
     FIG. 3 is a schematic circuit diagram of the array of electrodes for addressing the lines of the image. The elements 90 1&#39;   . . . 90 N&#39;  are resistive elements which produce a voltage drop when the electrodes 90 are connected to ground or in other words when their photodetector is subjected to illumination by the beam 14. It is an advantage to place these resistors between the electric generator 2 and the electrodes 90 in order to avoid overcrowding of areas reserved for the photodetectors. 
     FIG. 4 is a schematic circuit diagram of the array of electrodes employed for addressing the columns of the image. The references are identical with those of the preceding figures. 
     An alternative embodiment of a device according to the invention can be provided by making use of liquid crystal cells in smectic phase. 
     FIG. 8 illustrates the mode of electro-thermal control of a liquid crystal in smectic phase. 
     The curves C 1  and C 2  are time-dependent representations of the temperature of a smectic liquid crystal. The reference θ s  is the threshold temperature above which the liquid crystal is in a non-diffusing isomorphous phase. 
     The curves T 1  and T 2  represent the time-dependent values of voltages applied to a smectic liquid crystal. 
     The curves P 1  and P 2  show the variation of the coefficient of diffusion D of a smectic liquid crystal in accordance with the temperature cycles C 1 , C 2  and the tensions applied during the cycles V 1 , V 2  ; the temperature rise during the time interval AB and the level temperature stage BC bring the liquid crystal to a non-diffusing isomorphous phase. During the cooling time CD, the application of the voltage V 1  to the liquid crystal element endows this latter with a predetermined structure having a diffusion coefficient D 1  which will tend towards a zero value at the time of the following temperature rise. A voltage V 2  which is then applied during the cooling stage C&#39;D&#39; which is different from the voltage V 1  produces a diffusion coefficient D 2  which is also different from the coefficient D 1 . It is thus possible to obtain a continuous sequence of states of diffusion of the liquid crystal layer at the time of cooling and to cancel said states by increasing the temperature to a value above the threshold temperature θ s . 
     FIG. 5 shows an alternative arrangement of the schematic circuit diagram of FIG. 3, this variant being applied to the case of liquid crystal cells in smectic phase. 
     The line electrodes are constituted by resistive elements 90 1&#34;   . . . 90 N&#34;   which serve to heat that portion of the liquid crystal which is located in the proximity of said elements. When the light beam 13 sweeps the conductive element 80 j  (the resistance of said element is negligible compared with the resistance of the element 90 j&#34; ), the voltage of the connecting leads 30 2  is applied to the terminals of the element 90 j&#34; , thus producing a heating action and having the effect of erasing the liquid crystal cells 2 ji . At the time of cooling, the voltages delivered to the column electrodes 70 1  . . . 70 M  in accordance with the composite signal serve to establish variable electric charges and therefore to obtain different states of diffusion which compose the overall display pattern. The rest of the device is identical. 
     FIG. 6 illustrates a device in which provision is made for optical addressing means and is similar to the device 3 or 4 described with reference to the preceding figures. However, any practical arrangement for obtaining a light radiation of sufficiently pointlike character which can be modulated in intensity and can undergo rapid deflections may be employed as optical addressing means in accordance with the invention. 
     By way of example, the device 3 comprises a light radiation source 31 such as a semiconductor laser, a beam-shaping optical system 32, a tunable oscillator 35 which receives a tuning signal from the control circuit 1 and produces action on a transducer 34 which forms part of a deflector 33 of the acoustooptical type, for example, in order to carry out a Bragg-effect deflection of the beam emerging from the system 32. The beam 13 issuing from this device controls the different switching elements 80 1  . . . 80 N  for optical control of the visual display device in accordance with the invention. This mode of operation of the device in accordance with the invention can be a reflection mode in which the viewing light is the ambient light. The advantage of a device of this type lies in the fact that an increase in viewing light intensity does not produce a reduction in contrast of the image to be displayed but results on the contrary in enhanced contrast. The device also operates in the transmission mode and makes it possible to project the entire visual display on a screen for more convenient viewing. By means of this method, display assemblies having very small dimensions can be constructed, this being made possible by the invention after simplifying the necessary control connections. 
     One example of an optical system for carrying out a projection of the type just described is illustrated in FIG. 7. This system comprises a light source 30, a light radiation being caused to pass through a beam-shaping optical system 40 and to illuminate the display assembly. When the elements are liquid crystal cells and assuming that these latter are in the non-diffusion phase, the light rays pass through the charge lens 60, converge on the object-lens 70 and are stopped by the opaque spatial filter 80. A diffusing element such as the element 100 emits light radiations which are not all stopped by the spatial filter 80 and will be projected onto the screen 90 which will therefore perform the function of a display screen. The optical-control switching elements have been described as photoconductors. It would not constitute a departure from the scope of the invention, however, to use phototransistors or any other device in which the effects produced by the action of a suitable light beam on the electrodes would be identical to those described. 
     The invention covers all fields of application of visual display panels and is particularly applicable to high-definition display screens or to small-size devices.