FR2557339A1 - Display device comprising liquid crystal cells and improved addressing means - Google Patents

Abstract

Cell for a liquid crystal display panel. It comprises a main liquid crystal cell 1, backlit by ultraviolet radiation. A photoresistive cell 4 comprising zinc sulphide is connected in series with the liquid crystal cell between the supply voltage +V and earth and receives all or part of the radiation transmitted by the cell 1, in such a way as to react on the bias voltage of the cell, and thus adjust the grey level of the cell. In this way the cells of the panel will present a uniform grey level. A layer 2 of photoluminescent material transforms the radiation given off by the cell 1 into visible radiation.

Description

The present invention relates to a liquid crystal display device, comprising a two-dimensional matrix of cells associated with addressing circuits of each of the cells to cause the formation on the panel of a desired image.

Liquid crystal display panels are devices commonly used to produce small-sized displays such as a calculator screen or a watch. They can also be used to produce larger displays to display a television image or data on terminal screens.

In this second application, there are problems in obtaining an image which is of good quality and which has its own luminosity. In fact, in the absence of an image displayed, the gray levels are not uniform. In addition, liquid crystal displays do not have their own brightness, which means that they cannot be used in the absence of ambient light and the visibility of the display depends considerably on the angle of observation. . Crosstalk phenomena can also occur, due to the parasitic capacities of the address lines and the liquid crystal cells which cause the image formed on the screen to be altered.

The display device described in the French patent application published under No. 2,507,803 includes means for avoiding this crosstalk. These means comprise auxiliary switching means which receive a compensation signal having phase characteristics with respect to the phase of the control signals of the switching means normally provided in any display device, such that the effects of the stray capacitances are compensated. and reduced crosstalk.

In addition, in the horned devices, the addressing of the cells arranged in a matrix is done using semiconductor switching devices often transistors with field effect, reported at each point of the matrix. This results in a prohibitive cost and manufacturing difficulties due to the co-existence of two different technologies.

We can also refer to the article published in IEEE Spectrum of July 1978 on pages 26 to 32 under the title "Flat Panel Display: a critique" which shows the problems to be solved in order to produce a good quality display.

An object of the present invention is therefore to produce a liquid crystal display panel whose image elements consist of improved cells making it possible to obtain fidelity of the gray level.

Another object of the present invention is to provide a liquid crystal display panel which has its own brightness.

Another object of the present invention is also to provide a display panel made up of improved liquid crystal cells, in which the switching means provided for addressing the cells are produced using the same manufacturing technology as that used for the manufacturing of the cells themselves.

Another object of the present invention is to provide a liquid crystal display panel in which crosstalk phenomena due to stray capacitances are eliminated.

Summary of the invention
The display device comprises a panel of matrix-addressable liquid crystal cells defining the image elements of the panel. The panel is lit from the quarry by a source of ultra-violet radiation. Means for self-adjusting the gray level are associated with each cell. These means comprise a layer of photoconductive material such as zinc sulphide arranged so as to receive all or part of the radiation transmitted by the cell and to retro-act on the operating point of the cell, thus making the transmission coefficients identical for all of them. the cells of the panel. Additional means intended to transform the ultraviolet radiation coming from the cell into visible radiation, comprise a layer of photoluminescent substance chosen as a function of the desired color for the cell.

Addressing means making it possible to apply to selected cells signals carrying the information to be displayed in said cells.

In a preferred embodiment, these addressing means comprise an auxiliary cell associated with each main cell, comprising an all-or-nothing liquid crystal cell, which is illuminated by the same source of ultraviolet radiation as the panel and whose the transparency is modified by one of the addressing signals, and a layer of photoresist material subjected to radiation from the auxiliary liquid crystal cell and transmitting the second addressing signal carrying the information to be displayed to the main cell addressed.

Brief Description of the Figures
FIG. 1 represents a liquid crystal cell with its self-adjusting and addressing means.

Figure 2 shows the equivalent electrical diagram.

FIG. 3 shows the curves representing, the light flux transmitted by the cells as a function of the bias voltage.

FIG. 4 shows the family of curves for different values of the supply voltage + V, characteristic of the photoresist cell associated with the main cell.

Figure 5 shows the combination of the curves in Figures 3 and 4 explaining the self-adjusting effect.

FIGS. 6 and 7 represent an example of arrangement of cells in a three-color picture element and a monochrome picture element.

Figure 8 shows the display panel with its light source.

Figures 9A and 9B are sections along lines AA 'and BB' of a main and auxiliary cell and the associated cell of the trichromatic picture element.

The display device according to the invention consists of a set of liquid crystal cells, arranged in a panel usable as the screen of a terminal. To constitute a monochrome display panel, each image element (pixel) comprises a cell and for a full color display panel, each image element comprises several cells, one for each color. Each picture element can be addressed and controlled by a matrix addressing device, in a conventional manner, each cell being connected to a horizontal addressing line and to a vertical addressing line to control the display of information. desired in the cell.

A cell with addressing means is shown in Figure 1, and the equivalent electrical diagram is shown in Figure 2.

A main cell 1 with conventional type liquid crystal is intended for display and an auxiliary cell 2 with conventional type liquid crystal is intended for addressing.

Cell 1 has one of its plates (9) connected to a supply potential which is the ground in the embodiment described and its other plate (8) connected to a terminal with a capacity C. The other terminal capacitance C being connected to ground.

This cell is illuminated by the quarry by a source of continuous and uniform ultraviolet radiation in order to make the display panel bright.

The ultraviolet radiation passing through the liquid crystal cell will excite a photoluminescent substance 3 chosen as a function of the color which it is desired to obtain at the point considered.

To vary the quantity of light emitted by the photoluminescent substance, it is necessary to vary the voltage U at the terminals of the cells.

Because of the dispersion of the transmission characteristics of.

different cells which will constitute the panel, each liquid crystal cell 1 has its own curve representing the quantity of light passing through it, proportional to the transmission coefficient of the cell, as a function of the bias voltage U which is applied to it. As can be seen in FIG. 3, on which three curves corresponding to three cells are represented, for a voltage Uo for example, three distinct transmission coefficients are obtained.

Between cell 1 and the photoluminescent substance 3, a cell 4 is placed, made up of a thin layer of photo-resistant material which will act as a detector and feedback element to standardize the gray levels of the cells of the panel.

This material whose electrical resistance Z (see the equivalent electrical diagram in Figure 2) is a function of the light passing through it is chosen to have maximum efficiency over the wavelength emitted by the chosen UV radiation source and very low in visible light so that the incident light has no disturbing effect.

In a preferred embodiment, the photoresist material chosen is zinc sulfide, the photoresist effect of which has a maximum on the 0.35 p line emitted mainly by the conventional W source.

The cell 1 and the photoresist cell 4 are arranged in series between the mass and a supply potential + V.

FIG. 4 represents the family of lighting / voltage curves of the photoresist cell for different value of the supply potential + V. They show that the voltage left by cell 4 at the terminals of cell 1 decreases when the illumination increases.

Figure S is a combination of the curves in Figure 3 and a curve in Figure 4 for a given value + V.

It shows that the point of polarization U of each of the liquid crystal cells of the panel is self-adjusting to give an almost uniform illumination.

For a first cell there is obtained a point of polarization a, characterized by a voltage U1, for a second cell a point of polarization b, characterized by a voltage U2 and for a third cell, a point of polarization c characterized by a voltage U3.

Cell 4 therefore has a feedback effect which means that when the illumination increases, the voltage across the terminals of cell 1 increases, which causes the illumination to decrease until equilibrium is reached.

In the preferred embodiment of the invention, special addressing means are used which include an auxiliary cell 2, associated with each cell 1. It is understood that any other conventional addressing means, for example with a switching element semiconductor can be used.

Auxiliary cell 2 is lit from behind by the same radiation
UV than the main cell. It constitutes with another photoresist cell 5 an opto-electric switch which works in all or nothing.

Cell 2 has one of its plates connected to ground and its other plate connected to an address line 6, for example the horizontal address line.

A control voltage applied to the address line to select the associated main cell makes cell 2 transparent so that cell 5 becomes conductive (ON state).

The cell 5 is connected on the one hand to the node X and on the other hand to a second address line 7 for example, vertical line.

The voltage to cause the desired display at the selected point is applied to line 7 and forces via cell 5 a bias voltage across the main cell to obtain the desired display level in the cell main.

An additional capacitance C is connected to the terminals of the main liquid rate cryocell to ensure that the display voltage is stored for a sufficient duration between two successive addresses.

An optical mask is intended to shield the photoresist material from the cell from the action of ambient light. As in all matrix addressing, a single cell is selected, this cell being subjected to the control voltages on the vertical and horizontal lines to which it is connected.

Indeed, in the absence of an addressing signal on line 6, the liquid crystal cell 2 is not transparent and cell 5 has an infinite resistance (OFF state), which means that no display can be performed in the associated main cell.

By these opto-electric addressing means, the crosstalk is eliminated because the switching is done by all or nothing and that the circuit RC constitutes by the capacitance C and the resistance R of the cell 5 has a low impedance at the time of switching, as opposed to the impedance of field effect transistors used in the prior art.

By way of example, FIGS. 6 and 7 show the arrangement of the three main and auxiliary cells to constitute a three-color picture element and of a main cell associated with an auxiliary cell to constitute a picture element. monochrome.

In this example, the three-color dot has a height varying from 360 to 600 microns and the height of the cell varies from 120 to 200 microns. The height of the monochrome point is 125 microns.

FIG. 8 schematically represents a display panel comprising a set of picture elements (for example trichromes) arranged as in FIG. 6.

The cells constituting these elements are arranged between a front panel which is not transparent to ultraviolet 10 and a rear panel 11 which is transparent to ultraviolet. The two panels are separated by spacers 12.

The source of ultraviolet radiation can consist of a panel 13 comprising two glass plates separated by spacers 14, enclosing low pressure mercury vapor, excited by a high frequency electric field. This panel is the same size as the entire display panel to illuminate it evenly. Any other homogeneous source of UV rays can be substituted for it.

Figure 9A shows a section along line AA 'of a cell of the type shown in Figure 6 and Figure 9B shows a section along line BB'.

The front panel 10 comprises a glass substrate 20 on which are placed the photoluminescent substance 3 partially covered with a strip 4 of zinc sulfide to constitute the photodetector 4 of FIG. 1.

The main cell 1 of FIG. 1 is formed above the element 3, and is formed between the two metal plates 8 and 9. The plate 8 is separated from the cell 4 by a dielectric 21.

The auxiliary cell consists of the layer of zinc sulfide 5 coated with a metallization, which constitutes the horizontal addressing line 6.

The capacitor C is formed between two metallizations 22 and 23 separated by a dielectric 24, the metallization 22 being in contact with the plate 8 at point X of FIG. 1, an insulating material 26 separates the picture elements.

The dielectric layer 24 below the cell 5 constitutes the optical mask.

The addressing line 7 consists of a metallization placed next to the zinc sulfide layer 5. The auxiliary liquid crystal cell 2 is formed above the layer 5. The main and auxiliary cells are partially separated by a dielectric 27.

The rear panel consists of a glass substrate 30 covered with a metallic layer constituting the plate 9 and with a dielectric 28.

Liquid crystal material is placed between the two front and back panels.

FIG. 93 shows the section along line BB> on which the spacers 12 of FIG. 8 can be seen, the photoconductive layer constituting cell 3, and the addressing line 6.

All the successive operations necessary for obtaining a display panel which require masking positioning will be carried out on the same substrate 20, the other substrate 30 comprising only a uniform metallization covered with a dielectric layer, the completely transparent to incident UV rays. I1 therefore does not pose alignment problems during the final assembly of the two substrates.

To produce such a display device, only one simple technology is used since it is not necessary to have to install semiconductor elements in the panel.

In summary, the device which is the subject of the invention allows matrix addressing of display cells without using complex electronic components connected to the nodes of intersection of the address lines.

Each cell emits light in a given color. We can act independently and continuously on the brightness of each main cell, so that we can achieve video image display or digital display.

The dispersion of the characteristics of each cell due to manufacturing imperfections is eliminated by the photoresist cell 4.

The overall brightness of the screen can reach the levels required for projection on the big screen.

The resolution can greatly exceed 100 points / mm2 and even reach 1000 points / mm2.

Consequently, the panel of the present invention can be used for displaying a color television image and in all traditional computer display applications up to very high resolution graphic screens.

Claims (6)

1. Display device comprising a crystal cell panel  addressable liquids in a matrix, characterized in that it comprises  means (13) for producing an illumination of the panel by a  ultraviolet radiation,  main liquid crystal cells (1) defining the  panel image elements,  means (4) for self-adjusting the gray level associated with each  cell, comprising a layer of photoresist material arranged  so as to receive all or part of the illumination transmitted by the  associated cell and to react on the operating point of the  cell, thus making the transmission coefficients of  all the cells of the panel,  means intended to transform the ultra-violet radiation coming from  the main cell in visible radiation. 2. Device according to claim 1 wherein the material  photo-resistant is zinc sulfide. 3. Device according to claim 1 or 2 characterized in that the  means intended to transform the ultra-violet radiation coming from the  main cell in visible radiation, include a layer of  photo-luminescent substance chosen according to the desired color  for the 'cell. 4. Device according to claim 1, 2 or 3 comprising means  addressing the different cells characterized in that these means  understand  an opto-electric auxiliary cell associated with each cell  main comprising a liquid crystal cell (2) lit by  the means of lighting the panel, and the transparency of which is  modified by a first addressing signal,  a layer of photo-resistant material (5) subjected to the illumination from  of the auxiliary liquid crystal cell and transmitting a second  addressing signal carrying the information to be displayed in the cell  main address placed at the intersection of the address lines  receiving the first and second addressing signals. 5. Device according to claim 4 characterized in that the means  addressing comprise a storage means (C) connected to the  main cell terminals, intended to store information at  display between two successive addresses of the same cell. 6. Device according to claim 4 or 5 characterized in that a mask  optic is placed behind the layer of photo-resistant material to  subtract said material from the action of ambient light.