DISPLAY DEVICE

Abstract: The device comprises a two-dimensional display panel (20) composed of interchangeable modular units (24). Each modular unit is composed of display elements which are nematic or smectic LCD cells. Half-tones and gray-scale tones may be produced by either changing the contrast of the LCD elements or in subdividing each display element into several smaller cells (28, 30, and 32) which are independently switchable to light or dark conditions to provide gray-scale effects in each element. The device can be used to display either fixed or changing images.

Description: Display device This invention relates to a novel display device, namely to such a device which is run in a controlled- automatic mode in order to display different or changing images containing any optical information whatsoever, e.g. portraits, objects, scenes, series of alphanumerical signs, ets., the display being short or long lasting. The display device is based upon visibilizationof signs on the principale of mosaic or dot matrix systems. The display device to be defined and protected should be, before all, a large area advertisement panel v displaying changing information. However, it should also comprise panels capable of the transmission and supplying of any other information as it will be described later. Device for the display and the visibilizationof images and other information are already known. For example. U.S. patent no. 3,273.140 discloses a display panel provided with a great plurality of lamps being supplied with power according to a program so that an image is formed by the combination of light and dark lamps. A similar device is known from U.S. patent no. 2,239,522 wherein coloured lamps are used, and from U.S. patent no. 3,210,757. Another kind of display panels has been pro¬ posed in U.S. patent no. 3,270,447. According to that patent, the display panel is subdivided in a plurality of small cells containing each a reflector which can be set, by electromagnets having multiple windings, on different depths' within its cell. U.S. patent no. 3,486,253 discloses means for the displaying of living images whereby display elements are displaced behind a screen. Furthermore, U.S. patent no. 3,482,344 describes a display panel wherein flat display means are povotable by 180 . When an image is to be formed and to be cancelled, the whole screen must be transported from one station to another where the dis¬ play means are pivoted by fluid means. All these known devices first suffer from the common severe drawback their need for electric energy is tremendously high. Furthermore, they are very expensive in construction. Display panels with electric lamps have the additional disadvantage that lamps must always be replaced since they have a limited lifetime, the replacement being technically difficult. The lighted areas do never cover the entire surface area of the screen since lamps have a circular section and the panel elements a square one, and the light density is furthermore not the same on the circular lamp section. Other known display devices are too expensive in function or too complicated. The display processes and devices of U.S. patents 4,186,394 and 4,328,492 (to the applicant) resolve a number of the cited probelms and have substantial advantages but are still complex in construction. The primary object of the invention is to pro¬ vide a novel display panel which will be able to func¬ tion in an automatic mode and capable of rendering visible different information such as images of persons, scenes, signs, advertisements and so on. A further object of this invention is to provide a display system being well visible at normal daylight as well as in artificially illuminated areas like halls, airport lounges etc., capable of displaying information, e.g. on sport fields, without difficulties. The display panel should furthermore offer the possibility of re¬ producing the displayed images with half-tones. Still a further object of this invention is to render visible the display in the form of a mosaic or a dot matrix pattern by the device to be developed whereby, however, the different points or dots must no be distiguishable by the naked eye but the image should give the impression of a continuous, dot-free one with half-tones. The device to be provided should be arranged, according to another object of this invention, in such a way that it works in a reliable and simple manner and is of uncomplex contruction, and that it renders possible a rapid change of the displayed image. Furthermore, the device should be constructible in a most economic and automatic way. These objects of the invention are fulfilled by a display device as being defined in independent claim 1. Preferred embodiments of the device are the subject of the dependent claims. The invention is based upon the use of LCD (liquid crystal display) systems also called twist cells, which are known since some years and which have acquired in the last time a high degree of reliability and lifetime properties being about 50,000 hours. A condensed review has been published in ELO 1980, fasc. 6, under the title "Drehzellen" , p. 61-64. Nowadays, cell sizes as to the display area of several square centimeters are possible. The service temperature may be in the range of from -15 to +65 C, sufficient in practice. The power consumption goes 2 against 1 ,uW/cm corresponding to a total power of only 0.18 W for an enormous display panel of 6 x 3 m front area; however, a multiple of this value is to be added for running the control- circuitry. Furthermore, the response of the twist cells has been improved by the technical progress and allow shift frequencies between the light and dark states of about 5 to 8 cycles per second. This technique -will certainly permit soon the display of continuously moving pictures for which a picture changing frequency of at least 10 Hz (cycles per second) is necessary, like movies and television. Furthermore, colour device can already be O H imagined . Compared with conventional electro-optical display devices, the present invention has the advantage of an incomparably higher service life and a lower power consumption by several magnitudes. Compared with electromechanical devices, the invention has the advantage of a simpler and more compact construction, a faster image shift, and a better optical resolution. Compared with all known display devices of the kind con¬ sidered here, the invention presents a much simpler and less expensive construction and half-tone formation. The display panel itself, the main component of the device, is composed -of single modules of practical size, e.g. 25 x 35 or 30 x 50 cm (about 10 x 14 or 12 x 20 inch) , which are mounted with mutual contact in a supporting frame structure, forming together the display area. On the rear side, they are electrically inter-connected. They can easily be re¬ placed individually if necessary. Each module is com¬ posed of a plurality of display elements. In order to display halftones (grey values) , each display element may comprise several unit cells, normally three, each of them being either transparent (light) or opaque (dark) According to still another idea of this inven¬ tion, the grey values may also be obtained in another, often simpler way : Normal LCD cells contain liquid crystals in the nematic or the s ectic state, and there exists an opto- electric characteristic curve of the LCD display. The cell contrast is substantial zero below and up to a certain threshold voltage (about IV) and rises at higher voltages up to its maximum.value at the saturation voltage (about 3 V) ; large regions of the curve are non-linear. It has now been found that a desired half¬ tone contrast may be obtained in a very simple manner by the drive control of the cell when applying the control voltage in the conventional phase shift mode, but not using the normal, fixed phase angle of 180 but with a variable phase angle comprised bet- we ' en 0 and 180 , dependent upon the desired grey value. As a result of this method, the liquid crystals are firstly not completely aligned in twisted'position, only partially, so that the contrast does not reach its maximum value, and furthermore, the relation bet¬ ween phase angle and twist angle is, in contrary to the relation control voltage : twist angle, substantially completely linear. The phase angle may be varied by any technique known per se, digital techniques being preferred, it will be sufficient to provide for a limited number of fixed angles, about 10 at most, in the range of from 0 to 180 in order to obtain the necessary grey value variations. Furthermore, digital data storage and supply is much easier than an analoguous one. The LCD cells of each module are preferably controlled and driven, in a manner known per se, by multiplexing methods. The display device of this invention has the further advantage of being relatively lightweight and OMPΪ « o to have only a small construction thickness. These properties permit its use in interiour rooms like airport and railway station lounges, sport halls, gymnasiums, supermarkets etc. Due to the rigid, solid construction free from movable parts and to the large service temperature range of the used LCD cells, the panel is suited for outdoor use; the display area should of course be protected, like the known panels, against direct sunlight in order not to unduly compromise the image contrast. The invention shall now be explained in detail by the aid of examples of execution, with reference to the drawing wherein - FIG. 1 schematically represents, in a per- spective view, the optical principle or large scale display panels; • - FIG. 2 schematically shows, in a perspective view, the modular construction of a display panel; - FIG. 3 in perspective view the principal construction of a module; - FIG. 4 a rear perspective view of the dis¬ play panel according to FIG. 2 and 3; - FIG. 5 a front view of the above left corner region of a module, and - FIG. 6 schematically the forming of half-tones, -g j n_r; The display system of the invention provides a continuous,substantially dot-free image or pattern. The theoretical optical basis therefor is illustrated in FIG. 1. It is well known that the human eye cannot distinguish two dots as such which have a distance from each other less than about 0.1 mm at the normal visual range, i.e. at about 25 to 30 cm. In FIG. 1, the reader 10 holds a newspaper 12 at the visual distance of 30 cm. She looks at a picture being about 40 mm in height. The newspaper picture is composed, in a conventional manner, of dark and light dots having a distance, in the picture plane, of less than about 0.1 mm so that the picture appears as a continuous one. This picture remains continuous, if it is displayed on a TV screen 14, about 40 cm high, * when a visual distance thereto of about 3 m is selected, although the dark and light dots on the screen are greater. On the TV screen, 625 lines are displayed to that the distance between two adjacent lines is 40 : 625 = 0.064 cm or 0.64 mm. Since the visual distance is about 3 m, the TV picture- gives the same continuous visual impression as a newspaper having a height of 40 mm at a distance of 30 cm; the apparent distance bet- ween two adjacent vertical dots (corresponding to the TV picture lines) will be 0.64 : 10 = 0.64 mm, a value inferior to the resolution limit of 0.1 mm (at the normal visual range) . Now, if a display panel is seen at a distance of about 30 , a dot distance of ■ 0.1 mm in the normal visual range (30 cm) will correspond to a dot distance and, at the same time, to a dot dia¬ meter of 0.1 x 300 = 30 mm. Therefore, if the display on panel 16 should be composed of dark and light dots or elements having a linear dimension in the panel plane of not more than 10 mm, the thus formed image will appear continuous to the viewer at a visual distance of 10 m or more. The display panel of the invention takes ad¬ vantage of this fact in having its surface divided into elements which may have a height and width of only about 5 to 10 mm. Images on such a panel appear continuous at a visual distance of 10 m or more. At shorter distances, the image appears slightly dotted but still perfectly recognizable. The general arrangement of the display panel is shown in FIG. 2. The shape of the panel may be as desired. Examples are rectangular, trapezoidal, squared, triangular, oval and circular panels.' The panel may be fixed to posts.22 but may also be sus¬ pended from above, inclined or curved, may fill in the space above doors, etc. This is the modular sub- division of the panel which offers these substantially unlimited possibilities. The large display panel 20, being typically about 2 to 15 m high and 4 to 30 m broad, is composed of a plurality of identical modules 24. For example, each module has the dimensions 28.5 x 31.7 cm but may also be square shaped, 40 x 40 cm. All modules 24 are of identical construction which will now be described. Each module 24 is composed of a plurality of, for example, rectangular display elements (abbreviated D-elements) 26 (FIG. 5) , and each D-element comprises at least one cell. FIG. 5 and 6 show a preferred embodiment wherein each D-element 26 comprises three cells 28, 30 and 32, which have the same width but whose height is different from each other. The total composition of these three cells forming a D-element 26 has the same width and hight, namely about 6.35 mm. Such a D-element 26 is shown in frontal view in FIG. 5. All D-elements 26 are rectangularly or square shaped. They form a honey-_-comb-like network within the module 24. Each one of cells 28, 30 and 32. is a LCD display (twist cell) . The width of all cells is the same, namely 1/1 in arbitrary units. The hight of the cells is however different from each other. For example, cell 28 has 2/9 (two ninth) of the total height of the element 26, cell 30 has 3/9 (three ninth) , and cell 32 4/9 (four ninth) . The reason for this particular division of element 26 in three differently sized cells is the following : Images of deep black dots on white background appear hard and crude and do not make a three-demen- sional impression. However, when half-tones are intro¬ duced, the image becomes soft and appears plastic and three-dimensional. The invention permits the creation of such a plastic effect by the introduction of half-tones. The described system comprises eight gradations between black and white. As it becomes clear from FIG. 6, a plainwhite colour of the dot being represented by the D-element 26, is obtained by shifting all three cells 28, 30, 32 to white. This corresponds to degree 8 on the grey scale. If cell 28 is switched to black, 7/9 of the total surface area of the element appear white, and 2/9 appear black. This corresponds to degree 7 on the grey scale. In following Table 1, the gradations of the grey scale depend upon the switching state of cells 28, 30 and 32. In this table, the word "off" says that the corresponding cell does not receive a voltage, whereas "on" indicates that the cell received control voltage and thus appears black. ABLE 1 Cell switching state proportion of white degree of (FIG. 6) front' face grey scale 28 30 32 off off off 9/9 8 (white) on off off 7/9 7 off on • off 6/9 6 off off on 5/9 5 on on off 4/9 4 on off on 3/9 3 off on on 2/9 2 on on on 0/9 1 (black) This table shows that all gradations of the grey scale and thus all desired half-tones can be re¬ produced and displayed by a corresponding combination of the switched state of said three cells which have corresponding sizes. Now, FIG. 3 schematically shows a module 24 in perspective view. The module 24 is composed, in a sandwich-like manner, of three structural elements or construction units: On the front side, there is first the display unit 34 which comprises, as it has already been described, a plurality of LCD elements, each of which being in turn subdivised into cells. Typically, the commercially available LCD twist cells have connexions available at the rear side. Therefore, it is advantageous that the flat control unit 36 is placed behindthe also flat display unit 34. Control unit 36 contains the necessary multiple control devices, typically as integrated circuits, for each of the cells of the display elements. As a third "layer" behind control unit 36, there is a flat power unit 38 containing the electric power supply of the module as well as microprocessors and memories for treating and storing of the data for the images and signs to be displayed (as far as it will not be preferred to incorporate at least part of such microprocessors and memories in the control unit 36) . The today's techniques allow to limit the depth of the total compoundeddevice to a depth T of not more than about 7.5 cm. The display panel may also be constructed in another manner, not represented here but easily to be understood by the man skilled in the art. Namely, the basic panel 20-may be built up by rigidly incorporated control units 36 (FIG. 2 and 3) and power units 38 whereas each display unit 34 is removably fixed onto the corresponding control unit 36. The mode of placing all units 34 on units 36, assuring simultaneous electric contacts, comprised providing rearwardly directed .contact pins, like such used in integrated circuits, along the four edges of each display unit, in the " edge region of, say 2 to 6 mm (even until 1 cm) wide. These pins are then connected to the columns and rows of the LCD cells 28,30,32 (FIG. 5) in order to energize them in a multi¬ plex manner. This technique, and also the connecting of the cells to the pins, is known per se. The display units 34 may thus be fixed on the panel board 20 in the manner of wall tiles. If should also be noted that the display device may easily be combined with means for producing sound effects which are not represented in the drawing. However, the man skilled in the art will realize that loudspeakers may be incorporated into the display of FIG. 2, for example into the peripheric border zones or behind, above, below or at the sides of the display panel 20. Such sound effects may be human voice, musiC j noises etc. Data or signals corresponding to such sound effects may be stored together with the video signals, e.g. on magnetic tapes, which will be explained later. FIG. 4 shows a rear view of a portion of the display panel. The basic structure is formed by a T profile frame 40, the leg of the T's being directed to the front. The modules 24 (FIG. 3) are inserted from the front into the frame 40 which represents an array of squares. If the power unit is made smaller at its periphery by the thickness of the leg of the T profile, see FIG. 4, the modules 24 enter laterally into contact with their display and control units 34, Γ. OMPI 36 so that the particular modules can no longer be distinguished one from another when viewed from the front. The modules 24 are fixed to the frame 40 by means not represented but known per se, and they can be changed in case of breakdown within a few seconds. Each module 24 has at the rear side of its power unit 38 plug-in connection means 42, 44 which can be connected with information-carrying coaxial cables, current cables, etc. Furthermore, the modules 24 are interconnected by data cables 46 since each module must produce a partial image, all partial images contributing to the formation of the correct overall image on the panel. In order to display an image, it suffices in principle to decompose the original to be displayed into a dot matrix, the number and distribution of the dots corresponding to that of the display elements 26, and to coordinate a digital seven degree grey value to each dot. Such image decomposition or resolution pro¬ cesses have been known since some time, especially in television, and may be realized electronically by microprocessor and memory circuits. The control units 36 of the modules 24 of the display panel 16 are pre- programmed for the display. Now, when applying the collected digitalized information of an image to be displayed, the control units of all cells 26 of the module are energized in concordance with the location and the grey value of each image dot. The depicted methods and steps can be realized with the knowledge of the one skilled in electronics, in a manner known per se, and need not be described in detail here, the more as there is more than one way for the practical realization. Finally, it should be noted that the LCD normally function in the nematic range. The relatively limited angle of view of about 45 is disadvantageous. Therefore, it is also possible to have a display in the smectic range where an angle of view of more than 160 can be obtained. However, this service range is limited to a relatively small temperature range, and special liquid crystals must normally be selected for smectic applications. These facts which may sometimes bring about drawbacks will certainly be changed by the progress of the LCD art. The man skilled in the art will understand that the methods described or suggested here to transform an original image into a displayed one, can also be carried out by other means than the disclosed one without influencing the invention since the steps appropriate or necessary for the data production and treatment belong to the normal knowledge of the well trained electronic engineer. or expert. The new device for the visibilization mosaic¬ like images of this invention presents many advantages, as already pointed out. The displayed image is well visible in daylight and also in. artificially lighted spaces and need not specially be illuminated. It can rapidly be changed and renewed since the display is sub¬ divided into modules. This advantage allows thedisplay of rapidly changing information; for example, sport results may be displayed followed by advertisement which,in turn, may rapidly be replaced by other sport results. Furthermore, the information for the image composition can easily be stored electrically or magnetically so that an image can be displayed repeatedly without need for anew scanning the original. Ar- tificial images may also be produced and displayed in a very easy manner, and there are sound indications to show soon LCD movies on the panel of the invention without need for projection means.

Claims: CLAI S 1. - Display device, especially large area display panel, for the controlled and automatic display of optical information of the dot matrix kind, comprising a two-dimensional display panel subdivided into a plurality of display modules which, in turn, are com¬ posed of a plurality of display elements, characterized by the fact that said display elements (26) are liquid crystal twist cells capable of being switched between a "light" position giving a white optical impression, and a "dark" position giving an approximated black optical impression, and that the display elements provide for the production of half tones between light and dark. 2. - The display device of claim 1, characte¬ rized by the fact that each display module (24) is composed, in a sandwich-like manner, of three bi- dimensional units disposed one behindanother, namely (a) a display unit (34) comprising said display elements, (b) a control unit (36) comprising those parts which control the display elements, and (c) a power unit (38) providing for current supply and data transfer. 3. - The display device of claim 1 or 2, characterized by the fact that the display elements (26) are rectangular or square shaped and provide for a honeycomb-like structure of the display module (24) , and that each one of said display elements (26) is composed of three twist cells (28, 30, 32) having a rectangular front surface, said three surfaces having an area ratio of 2 : 3 : 4. 4. - The display device of claim 2 or 3, characterized by the fact that the modules of the display panel are in touch with each other on their periphery and are inserted from the front into a sup- porting frame (40) , the power unit (38) of the device having a smaller size than the remainder of the module in order to make possible said insertion into the supporting frame (40) . 5. - The display device according to any one of the preceding claims, characterized by the fact that the modules (24) are provided on their rear faces ' with interconnecting means (46) and with means (42, 44) for connexion with auxiliary devices. 6. - The display of claim 1, characterized by the fact that a continuous or digital light/dark control means for the production of half-tones is provided for each display element (26) . 7. - The display device of claim 6, characterized by the fact that said control means comprises a phase shift angle control between the field electrodes of the LCD, the phase shift of the electrode alternating voltage being adjustable between 0 and 180 . 8. - The display device of claim 1, characteri¬ zed by the fact that the modules (24) contain micro- processors capable of being programmable and able to store and to deliver light/dark data and half-tone data in the form of a grey value for each display element and for each image to be displayed. 9. - The display device according to any one s of the preceding claims, characterized by the fact that said twist cells are provided with nematic phase liquid crystals or to work with such crystals. 10. - The display device according to any one of claims 1 to 7, characterized by the fact that said twist cells are provided with smectic phase liquid crystals or to work with such crystals.

Inventors: BOBAK TADEUSZ, BOBAK, TADEUSZ
Applicants: BOBAK TADEUSZ, BOBAK, TADEUSZ
Application year: 1984
Main classification: G09G3
IPC codes: G02F1, G09F9, G09G3, H04N3