EP0519835A1 - Flat matrix screen with method of control and application to image display - Google Patents

Abstract

The invention relates to a flat matrix screen whose two faces include respectively an array of N row electrodes 3 and an array of K column electrodes 4. Between these two faces an electron source 10 provides sufficient electrons to illuminate the selected image points, a confinement electrode 13 as well as a blocking electrode 12 provide for respectively the confinement of the electrons on the column electrodes and the control of the electron charge conveyed from the column electrodes to the selected row electrode, so as to illuminate the image points located at the intersection of the said selected row 3 and of the columns 4. Application to all fields requiring the on-screen visualising of stationary or moving images. <IMAGE>

Description

The present invention relates to a flat matrix screen allowing in particular the display of high definition images. It finds many applications in the fields requiring a visualization on screen of fixed or animated images, in colors or black and white.

Today there are several types of flat screens, in particular cathodoluminescent screens using the guidance of electrons emitted by a known electron source. Such a device is described in the French patent application published under the number FR-A-2 647 580.

In Figure 1, there is shown schematically a section of the screen according to the prior art, previously cited. The device comprises two substrates 1 and 2, at least one of which is transparent, and on which two arrays of electrodes are deposited.

A first network of N electrode-lines 3 is deposited on a first substrate 1, called substrate-line. The line electrodes 3 are largely covered with a layer 5 of a cathodoluminescent material. The ends of the electrodes 3 are not covered with said cathodoluminescent layer 5, thus allowing a contact area, not shown in the figure, and thanks to which electrical contact is ensured with control means external to the device and not shown in the figure.

A second network of K column electrodes 4 is deposited on a second substrate 2, called column substrate. The column electrodes 4 are largely covered with an electrically insulating layer 6 consisting, for example, of SiO₂ or Si₃N₄. The ends of the column electrodes 4 are not covered with said insulating layer, thus allowing a contact area 8 thanks to which electrical contact is ensured with the control means mentioned above.

The array of electrode-lines 3 and the array of electrode-columns 4 are crossed, their points of intersection, the number of N x K, defining the elementary points of the image, also called image points or pixels.

Throughout the rest of the description, we will consider, by way of example, a screen made up of four row electrodes and four column electrodes, thus allowing a screen of 4 × 4 = 16 image points.

The column substrate 2 and the line substrate 1 are assembled by a sealed sealing bead 9 so as to form a vacuum enclosure. This enclosure comprises, in the vicinity of the sealing bead 9, an electron source 10 of known type, for example, a source consisting of microfilaments or of a single filament also supplied externally.

The screen is addressed sequentially line by line.

The non-selected row electrodes 3 are brought to a potential of value identical to that of the potential Vs of the electron source, while the selected row electrode 3a is brought to a potential Vl greater than the potential Vc of the column electrodes 4 (Vc being greater than or equal to the potential Vs of the electron source 10). Furthermore, in order to adjust the brightness of each pixel, each column electrode receives a determined potential Vc making it possible to attract more or less electrons from the source 10. In other words, depending on the desired brightness on the pixels of a same row selected, the columns associated with these pixels may have different potentials Vc but always greater than or equal to Vs. Thus, from the electron source 10 to the column electrodes 4, the electrons, represented by points, are accelerated in the direction of the arrow 11a. Blocked by the insulating layer 6, the electrons accumulate in its vicinity, forming in the vacuum of the enclosure charge zones 7 (a charge zone 7 is associated with each column electrode) making it possible to conduct the electrons towards the selected line.

The electrons accumulated in the vicinity of the insulator 6 then strike the cathodoluminescent layer 5 of the selected line electrode 3a in a direction as indicated by the arrows 11b and 11c.

• la différence de potentiel quasi nulle existant entre les zones de charge 7 est généralement une cause importante de fuites d'électrons d'une électrode-colonne 4 vers sa voisine ne permettant pas d'appliquer des luminosités différentes sur les pixels d'une même ligne ;
• les fuites d'électrons, courantes dans de tels écrans, peuvent également provenir de l'action d'un champ magnétique externe à l'écran tel que, par exemple, le champ terrestre ; et
• lorsque l'électrode-ligne sélectionnée 3a est portée au potentiel Vl, qui est supérieur au potentiel Vc des électrodes-colonnes 4 et donc supérieur au potentiel Vs de la source d'électrons 10, les électrons émis par la source 10 se dirigent directement vers l'électrode-ligne sélectionnée 3a par l'intermédiaire des zones de charge 7, qui se comportent ici comme des conducteurs électriques. De ce fait, la quantité d'énergie frappant la couche 5 cathodoluminescente aux points image adressés dépend du débit de la source 10 et n'est pas précisément contrôlable. L'image affichée risque donc d'être très inhomogène.

This type of known screen has the following drawbacks:

• the almost zero potential difference between the charge zones 7 is generally a major cause of electron leakage from a column electrode 4 towards its neighbor not allowing different luminosities to be applied to the pixels of the same line;
• electron leakage, common in such screens, can also come from the action of a magnetic field external to the screen such as, for example, the earth's field; and
• when the selected row electrode 3a is brought to the potential Vl, which is greater than the potential Vc of the column electrodes 4 and therefore greater than the potential Vs of the electron source 10, the electrons emitted by the source 10 go directly to the selected electrode-line 3a via the charge zones 7, which here behave like electrical conductors. Therefore, the amount of energy striking the cathodoluminescent layer 5 at the addressed image points depends on the flow rate of the source 10 and is not precisely controllable. The displayed image may therefore be very inhomogeneous.

The object of the present invention is precisely a flat matrix screen which makes it possible, in a simple manner, to precisely control the charge of electrons supplied respectively to the image points addressed, as well as to avoid the leakage of electrons from a column electrode 4 towards its neighbors.

• un premier et un second substrats, assemblés par un cordon de scellement étanche, constituant une enceinte sous vide ;
• un premier réseau de N électrodes-lignes construit sur le premier substrat et recouvert d'au moins un matériau cathodoluminescent ;
• un second réseau de K électrodes-colonnes construit sur le second substrat et recouvert d'une couche électriquement isolante, ce second réseau étant croisé avec le premier réseau de façon à définir, par leurs intersections, N x K points image ; et
• au moins une source d'électrons pour injecter des électrons à l'intérieur de l'enceinte sous vide,

caractérisé en ce qu'il comporte des moyens pour confiner les électrons injectés sur l'électrode-colonne de chaque point image adressé, ainsi que des moyens pour contrôler la charge d'électrons fournie à chaque point image adressé.More specifically, the invention relates to a matrix flat screen used, in particular, for the display of high definition images comprising:

• first and second substrates, assembled by a tight sealing bead, constituting a vacuum enclosure;
• a first network of N electrode-lines constructed on the first substrate and covered with at least one cathodoluminescent material;
• a second network of K column electrodes constructed on the second substrate and covered with an electrically insulating layer, this second network being crossed with the first network so as to define, by their intersections, N x K image points; and
• at least one electron source for injecting electrons inside the vacuum enclosure,

characterized in that it comprises means for confining the electrons injected on the column electrode of each addressed image point, as well as means for controlling the charge of electrons supplied to each addressed image point.

Advantageously, the means for controlling the electron charge comprise at least one blocking electrode located between the electron source and the line electrodes, said blocking electrode being able to be brought to a negative potential with respect to the potential of the source, to prevent electrons from the electron source from reaching the electrode arrays.

According to the invention, the means for confining the electrons comprise a confinement electrode having a comb shape, each tooth of which is inserted between two column electrodes in order to prevent, when said electrode is brought to a negative potential with respect to the potential. from the source, any coupling between two neighboring column electrodes.

In addition, each line electrode of the screen is controlled by a line control circuit, this circuit comprising a single transistor including a first electrode is brought to a constant voltage Vl, a second electrode of which is connected to said line electrode and a third electrode of which is connected to means for controlling the conduction state of the transistor.

According to a preferred embodiment of the invention, the screen comprises two electron sources with which two blocking electrodes are associated, each source / blocking electrode assembly being arranged on either side of said screen.

• des électrodes-lignes portées à un potentiel Vl ;
• des électrodes-colonnes portées à un potentiel Vc fonction de l'image à afficher et croisées avec les électrodes-lignes de façon à définir, par leurs intersections, des points images ; et
• au moins une source d'électrons portée à un potentiel Vs,

caractérisé en ce qu'il consiste, d'une part, à utiliser une électrode de blocage et une électrode de confinement, cette dernière étant portée à un potentiel Vp constant négatif par rapport au potentiel Vs et, d'autre part, à effectuer pour chaque électrode-ligne sélectionnée et pour l'électrode de blocage successivement les commandes suivantes :

• porter les électrodes-lignes et l'électrode de blocage au potentiel Vs de la source, le potentiel Vs étant inférieur au potentiel Vc des électrodes-colonnes ;
• porter l'électrode de blocage au potentiel Vb inférieur au potentiel Vs de la source ;
• porter l'électrode-ligne sélectionnée à son potentiel Vl supérieur au potentiel Vs de la source.

The screen according to the invention implements a control method in which:

• line electrodes brought to a potential V1;
• column electrodes brought to a potential Vc as a function of the image to be displayed and crossed with the line electrodes so as to define, by their intersections, image points; and
• at least one electron source brought to a potential Vs,

characterized in that it consists, on the one hand, in using a blocking electrode and a confinement electrode, the latter being brought to a negative constant potential Vp with respect to the potential Vs and, on the other hand, for each line electrode selected and for the blocking electrode successively the following commands:

• bringing the line electrodes and the blocking electrode to the potential Vs of the source, the potential Vs being less than the potential Vc of the column electrodes;
• bringing the blocking electrode to the potential Vb lower than the potential Vs of the source;
• bring the selected electrode-line to its potential Vl greater than the potential Vs of the source.

This screen can be used for displaying color images. Each image point is then divided into at least three image sub-points each covered with a different cathodoluminescent material.

• la figure 1, déjà décrite, représente schématiquement une section de l'écran selon l'art antérieur ;
• la figure 2 représente schématiquement une section de l'écran plat matriciel selon la présente invention ;
• la figure 3 représente le chronogramme des tensions et luminances rencontrées lors du fonctionnement de l'écran selon l'invention ;
• la figure 4 représente l'écran selon l'invention suivant une vue de dessus partiellement coupée ;
• la figure 5 représente de façon très schématique une section d'un écran plat matriciel comprenant deux sources d'électrons ; et
• la figure 6 représente le circuit de commande ligne tel que mis en oeuvre dans l'écran selon l'invention.

Other characteristics and advantages will emerge more clearly from the description which follows, given by way of illustration and not limitation, with reference to the figures in which:

• Figure 1, already described, schematically shows a section of the screen according to the prior art;
• FIG. 2 schematically represents a section of the flat matrix screen according to the present invention;
• FIG. 3 represents the timing diagram of the voltages and luminances encountered during the operation of the screen according to the invention;
• FIG. 4 represents the screen according to the invention according to a partially cut top view;
• FIG. 5 very schematically represents a section of a flat matrix screen comprising two sources of electrons; and
• FIG. 6 represents the line control circuit as implemented in the screen according to the invention.

FIG. 2 represents a screen in the same section as that of FIG. 1, a screen to which the means for controlling the electron charge according to the invention have been added. The references which appear in FIG. 1 will be kept during the description of said FIG. 2 and of the following figures.

The following description is given for the particular case of a screen allowing the display of high definition images. The screen according to the invention can however allow many other functions.

Thus, in the case where one seeks to display high definition images, it is absolutely necessary to avoid the risks of obtaining an inhomogeneous image; precise control of the charge of electrons striking the cathodoluminescent layer of each pixel is therefore necessary.

These means for controlling the charge of electrons consist for example of a blocking electrode 12. This blocking electrode 12 is placed parallel to the line electrodes, between the electron source 10 and the array of line electrodes 3.

To facilitate the description of the operation of the screen of the invention, we consider Vs = 0 Volt in the following description.

The electrode 12 and the line electrodes 3 are advantageously made using the same conductive material. If the image does not have to be observable through the substrate 1, a reflective metal will preferably be used.

From a more functional point of view, the blocking electrode 12 makes it possible, when brought to a negative potential Vb, to block the passage of the electrons emitted by the electron source 10 towards the column electrodes.

More precisely, during the charging of the charge zones 7 with the electrons emitted by the electron source 10, the blocking electrode 12 is brought to a potential Vb identical to that of said electron source. The electrons then circulate normally from the electron source 10 to the column electrodes 4, as described previously in the description of the prior art.

When all the charge zones 7 have reached a charge Q = CVc, where C is the column capacity fixed by construction of the column electrodes (Q being specific to each column electrode), the blocking electrode 12 is brought to a potential Vb sufficiently negative, for example -100 Volts. Under the effect of such a potential Vb, the electrons, emitted by the electron source 10, are repelled when they arrive at the blocking electrode 12, then returning to said source 10, as shown on Figure 2 by the dotted arrow 11d. Thus, when the selected line electrode 3a is brought to its potential Vl, only the electrons in charge zones 7 strike the layer 5 of the selected line electrode 3a, the electrons coming from the source 10 being returned to said source 10 thanks to the blocking electrode 12.

FIG. 3 shows the timing diagram of the voltages and luminances encountered when the screen is in operation, that is to say that it describes the control method implemented in the screen. This process has already been described through the descriptions in FIGS. 1 and 2, but the sequence of commands appears more clearly in the timing diagram of FIG. 3.

• du temps t0 jusqu'à t1, les électrodes-lignes (d'indice li) et l'électrode-colonne (d'indice b) sont portées au potentiel Vs de la source. On a alors Vli = Vb = Vs. Sur la figure 3, on a représenté Vs comme nulle. Pendant ce temps, l'électrode-colonne (indicée cj) est portée à un potentiel Vcj (de valeur comprise entre 0 et 10 Volts environ), fonction de l'image à afficher.
• De t1 à t2, l'électrode de blocage est portée à son potentiel Vb, inférieur au potentiel de la source Vs. Ce potentiel Vb a pour valeur, dans l'exemple, -100 Volts.
• De t2 à t3, l'électrode-ligne li est portée à son potentiel Vl supérieur au potentiel Vs, Vl étant de 100 Volts dans l'exemple. L'énergie emmagasinée par l'électrode-colonne (cj) aux temps t0 et t2 est alors déchargée. La luminance Lpij du point image (pij) intersection des lignes li et colonne cj, est maximale (Lmax) à t2, diminue progressivement jusqu'à atteindre 0 au temps t3. La tension Vcj de l'électrode-colonne, de valeur Vmax entre t0 et t3 (soit 10 Volts, dans l'exemple) diminue pendant la durée du temps-ligne suivant atteignant Vmax/2 dans l'exemple. A cette tension Vmax/2, la luminance est plus faible et vaut Lmax/2.

The addressing of the screen is done line by line, with a time-line time Tl comprising the following three phases:

• from time t0 to t1, the line electrodes (of index li) and the column electrode (of index b) are brought to the potential Vs of the source. We then have Vli = Vb = Vs. In Figure 3, we represented Vs as zero. During this time, the column electrode (indexed cj) is brought to a potential Vcj (of value between approximately 0 and 10 Volts), depending on the image to be displayed.
• From t1 to t2, the blocking electrode is brought to its potential Vb, lower than the potential of the source Vs. This potential Vb has the value, in the example, -100 volts.
• From t2 to t3, the line electrode li is brought to its potential Vl greater than the potential Vs, Vl being 100 Volts in the example. The energy stored by the column electrode (cj) at times t0 and t2 is then discharged. The luminance Lpij of the image point (pij) intersection of the lines li and column cj, is maximum (Lmax) at t2, gradually decreases until reaching 0 at time t3. The voltage Vcj of the column electrode, of value Vmax between t0 and t3 (ie 10 Volts, in the example) decreases during the duration of the next line time reaching Vmax / 2 in the example. At this voltage Vmax / 2, the luminance is lower and is equal to Lmax / 2.

During the duration of this next line time, the next line electrode (li + 1) is selected and the method described above is applied again.

Thus, upon reception of an energy of value C x Vc x Vl, there is emission of light at the image points addressed.

The luminance L of each image point depends, not only on the efficiency r of the cathodoluminescent material, on the frame frequency f, namely the frequency of display of the image, on the surface of the image point, on the capacity of column C, potential Vl of the selected row electrode 3a, and also potential Vc of the corresponding column electrode 4. For each pixel, this luminance L is expressed in the form: $$\text{L =}\frac{\text{rf C Vl}}{\text{πS}}\text{Vc}$$

In this equality, only Vc can vary from one pixel to another and therefore allow the brightness to be adjusted.

In Figure 4, the screen of the invention is shown in a top view. In this figure, the substrate-line 1 is partially cut in order to better show the different electrodes located between the two substrates 1 and 2.

The means for confining the electrons appear clearly in this figure under the reference 13. In fact, said means for confining the electrons comprise a confinement electrode 13 making it possible to avoid any crosstalk, that is to say any coupling between an electrode -column 4 and its neighbors.

We will talk indifferently of coupling between column electrodes or of electron leakage from a column electrode to neighboring column electrodes.

The confinement electrode 13 has the shape of a comb, each tooth 13a of which is interposed between two column electrodes. The confinement electrode 13 is conductive and can be transparent or opaque, so that said confinement electrode 13 is, if possible, in harmony with the array of column electrodes 4 (since they can be produced at the same time ). The column substrate 2 and, if possible, the column electrode array 4 are transparent when the row substrate 1 is opaque and / or the row electrode array 3 is also opaque, and vice versa.

This electrode 13 is able to be brought to a negative potential Vp generally varying between -100 and -500 volts, this potential Vp depending on the pitch of the column electrodes 4, the vacuum volume between the two substrates 1 and 2, potentials Vc applied to column electrodes 4 and potential Vs.

Brought to a sufficiently negative potential Vp, each column electrode 4 keeps in its charge area 7, the electrons received from the electron source 10. Under the action of the negative potential Vp, the electrons cannot escape from the load zone 7 in which they are confined. There is therefore no possible electron leakage from a column electrode 4 to a neighboring electrode 4.

The present invention, described for the display of black and white images, also applies to the display of color images. The known method of dividing each pixel into several parts is used for this application. Each pixel part is then covered with a cathodoluminescent material of different nature making it possible to emit in blue, green or red depending on the nature of said material.

The pixels can be divided according to several arrangements. According to a first arrangement, each pixel is divided in the direction of the line electrodes into three independent parts each covered with a different cathodoluminescent material (in other words, each column electrode is divided into three sub-column electrodes). These materials are: ZnCdS: Ag, ZnS: Ca, Ac or ZnS: Ag, Ac, to emit, respectively, in red, green or blue. According to a second arrangement, each pixel is divided in the direction of the column electrodes into three independent parts each covered with a different cathodoluminescent material (in other words, each line electrode is divided into three sub-line electrodes). Finally, according to a third arrangement, each pixel is divided both in the direction of the row electrodes and the column electrodes, each pixel then being made up of four independent parts, two of which are covered with the same cathodoluminescent material, the other two parts being each covered with a different cathodoluminescent material (in other words, each electrode-line is divided into two sub-electrodes-lines and each electrode-column is divided into two sub-electrodes-columns).

In Figure 6, there is shown the line control circuit which comprises a single transistor. The circuit shown is of the "open drain" type. It could also be of the "open collector" type. This circuit includes only one transistor 14 of the MOS type; the first electrode of said transistor is connected to a control logic 15 for the conduction state of transistor 14, its second electrode to a voltage line V1 and its third electrode to the line electrode 3.

Indeed, known line-control circuits generally use two transistors: one to bring the voltage to Vl and another to bring the voltage to 0.

However, in the absence of voltage applied to the lines, the electrons emitted from the columns tend to drop the potential of said lines. An open drain (or open collector) circuit is therefore used which allows the line to be brought to potential + Vl, the return to 0 taking place gradually when the transistor 14 is not conductive.

Such a circuit saves not only one transistor per image point, but also the circuitry around this transistor. The control-line circuit of the invention ensures a cost much lower than the usual manufacturing cost.

A preferred embodiment of the invention is shown in FIG. 5, in a section identical to that of FIG. 2. This embodiment consists in introducing, into the vacuum zone, a second source of electrons 10a located at the opposite of the first electron source 10, with respect to the array of electrode-lines 3.

Like the electron source 10, the second electron source 10a is placed in the vicinity of the sealing bead 9 and parallel to the line electrodes 3. In addition, a second electrode blocking device 12a is installed between the array of electrode-lines 3 and said second electron source 10a. This source 10a and its blocking electrode 12a play the same role as that established for the source 10 and the blocking electrode 12 in the description of FIG. 2.

This second electron source 10a supplies the same quantity of electrons as the electron source 10, which halves the charging time of the charging zones 7, thus allowing an improvement in the time for displaying an image. on the screen according to the invention.

Claims (7)

Matrix flat screen including: - a first and a second substrate (1, 2), assembled by a tight sealing bead (9), constituting a vacuum enclosure; - a first network of N electrode-lines (3) built on the first substrate and covered with at least one cathodoluminescent material (5); - a second network of K column electrodes (4) built on the second substrate and covered with an electrically insulating layer (6), this second network being crossed with the first network so as to define, by their intersections, N x K image points; and - at least one electron source (10), brought to a potential Vs, for injecting electrons inside the vacuum chamber, characterized in that it comprises means (13) for confining the electrons injected on the column electrode of each addressed image point, as well as means (12) for controlling the charge of electrons supplied to each addressed image point. Flat matrix screen according to claim 1, characterized in that the means for controlling the electron charge comprise at least one blocking electrode (12) situated in parallel between the electron source and the line electrodes, said electrode blocking which can be brought to a potential (Vb) negative with respect to the potential of the source, in order to prevent the electrons coming from the source of electrons from reaching the networks of electrodes. Flat matrix screen according to any one of claims 1 or 2, characterized in that the means for confining the electrons comprise a confinement electrode (13) having a comb shape, each tooth (13a) of which is interposed between two electrodes -columns to prevent, when said electrode is brought to a potential (Vp) negative with respect to the potential of the source, any coupling between two neighboring column electrodes. Flat matrix screen according to any one of Claims 1 to 3, characterized in that it comprises two electron sources (10, 10a) with which two blocking electrodes (12, 12a) are associated, each source / electrode assembly blocking being arranged on either side of said screen. Flat matrix screen according to any one of Claims 1 to 4, characterized in that each line electrode is controlled by a circuit comprising a single transistor, a first electrode of which is brought to a constant voltage (Vl), of which a second electrode is connected to said line electrode and a third electrode of which is connected to means for controlling the conduction state of the transistor. Method for controlling a matrix flat screen according to any one of Claims 1 to 5, in which: - line electrodes brought to a potential V1; - column electrodes brought to a potential Vc as a function of the image to be displayed and crossed with the line electrodes so as to define, by their intersections, image points; and - at least one electron source brought to a potential Vs, characterized in that it consists, on the one hand, in using a blocking electrode and a confinement electrode, the latter being brought to a negative constant potential Vp with respect to the potential Vs and, on the other hand, in performing, for each line electrode selected and for the blocking electrode successively, the following commands: - Bring the line electrodes and the blocking electrode to the potential Vs of the source, the potential Vs being less than the potential Vc of the column electrodes; - bring the blocking electrode to the potential Vb lower than the potential Vs of the source; - bring the selected line electrode to its potential Vl greater than the potential Vs of the source. Application of the flat matrix screen according to any one of claims 1 to 6, to the display of color images by dividing each image point into at least three image sub-points each covered with a different cathodoluminescent material.

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