EP0014156B1 - Electroluminescent chromatic transducer with additive synthesis - Google Patents

Abstract

1. Apparatus for generating images in at least two colors comprising an image reproduction screen, means for directing an energizing electronic beam over the entire surface of said screen, the intensity of which beam is dependent on the amplitude of a brightness control signal, and means for imparting a scanning movement to the said energizing beam so that its point of impact on the surface of the screen moves according to two dimensions, the said image generating apparatus being characterized in that it further comprises : a) a superimposition of at least two layers (23-25) each one produced in a luminescent material radiating light of a predetermined wavelength, different from the wavelength of the light radiated by the other layers, the luminescent materials used, radiating light under the action of an electronic bombardment and adapting to an amplification of the light intensity thus radiated by superposition of an electroluminescent effect, b) an appropriate control device, associated to each luminescent layer to generate therein a pulsating electric field, the frequency and amplitude of which are dependent on the amplitude of a corresponding color signal so as to ensure the restitution of the different chromatic components, and c) generators (31, 32) supplying the said signals controlling the color and brightness.

Description

The invention relates to a light current transducer device of a special image tube comprising an electroluminescent assembly associated with a cathode scan allowing the television reproduction of color images.

Since the start of color television, all receivers have been equipped with a picture tube made up of three electron guns (one per color), which each excite an element of the screen where phosphors emitting light are deposited. green, red or blue light. Before the impact on the phosphors, the electron beams pass through a system of grids responsible for focusing the electrons, in order to obtain the excitation of the desired light point. This trichromatic image tube with shadow mask is known as a shadow mask tube (the description and the operating principle are abundantly developed in the technical literature).

Another two-color picture tube, known by the technical name of "penetron", was designed around 1934 and improved by numerous patents (US-A-2,446,248, US-A-2,445,710, US-A-2,704,783, US-A- 2,730,653, US-A-2,590,018). The operating principle is different: two superimposed cathodoluminescent thin films present their maximum emission at distinct levels of the energy of the incident electrons. When the luminescent materials of the two layers are suitably chosen, the color of the light emitted can be varied by varying the acceleration voltage of the incident electrons. The low energy electrons excite the luminescence of the first layer, but cannot penetrate the second. We only observe the luminescence of the first layer. If the energy is increased (increase in the high acceleration voltage), it is possible to excite both the luminescence of the first layer and that of the second layer, and with possible predominance of the luminance of the second layer.

The tube with "shadow mask" requires, for industrial production, significant investments, expensive specific tools, and the cost price of a tube represents 50% of the manufacturing price of a color TV receiver.

• 1/ la convergence dynamique et la pureté des couleurs
• 2/ les aberrations de l'image, que l'on corrige par des circuits complexes, ainsi que:
• 3/ la sensibilité au champ magnétique terrestre, laquelle impose la présence de blindages et de circuits de démagnétisation.

In addition, the reconstruction of a colored image using this image tube presents several difficulties, namely:

• 1 / dynamic convergence and purity of colors
• 2 / the aberrations of the image, which are corrected by complex circuits, as well as:
• 3 / sensitivity to the earth's magnetic field, which requires the presence of shields and demagnetization circuits.

In addition, the image tube and its supply circuits are large consumers of energy, the electrons are intercepted by the color selection electrode (shadow mask).

The latest technological improvements (autoconvergence, precision In Line, PIL) aim to reduce on the one hand the pointed side of the settings, and on the other hand to improve by elaborate grids the mixture of the beams just at the level of the screen. These systems only slightly increase the resolution and stability of the images.

• 1/ réflexion diffuse de l'écran négligeable, donc possibilité d'obtenir une image visible même en présence de lumière extérieure,
• 2/ résolution optimale, car l'absence de diffusion dans les films minces réduit le diamètre du point lumineux au diamètre du faisceau d'électrons incident,
• 3/ géométrie favorable à la diffusion de la chaleur, facilité d'évacuation par le substrat qui forme radiateur.

The "penetron" has the advantages of thin cathodoluminescent films. These films obtained by thermal evaporation or spraying by vaporization on a support, instead of sedimentation of powder for the mask tube, have various advantages.

• 1 / negligible diffuse reflection of the screen, therefore possibility of obtaining a visible image even in the presence of outside light,
• 2 / optimal resolution, because the absence of diffusion in thin films reduces the diameter of the light point to the diameter of the incident electron beam,
• 3 / geometry favorable to the diffusion of heat, ease of evacuation by the substrate which forms a radiator.

• 1/ commutation difficile des différentes hautes tensions pour obtenir les niveaux distincts de couleur,
• 2/ diffusion d'une couche dans l'autre en cours de fonctionnement et tout se passe comme s'ilny avait qu'une seule couche,
• 3/ accumulation de charges électriques à la surface de séparation des deux couches, il se produit des émissions secondaires indésirables, altérant les couleurs émises.

Despite these advantages, the penetrant presents major technical difficulties, namely:

• 1 / difficult switching of the different high voltages to obtain distinct levels of color,
• 2 / diffusion from one layer to the other during operation and everything happens as if there was only one layer,
• 3 / accumulation of electrical charges on the separation surface of the two layers, undesirable secondary emissions occur, altering the colors emitted.

The present technological innovation is to overcome the difficulties of tubes using color selection masks and the technical disadvantages of "penetration tubes", while retaining the advantages of thin films for maximum resolution.

Thus, the proposed innovation makes it possible to use a single-channel tube, identical electronically to that of a tube used for reception of television in. black and white. Only the electroluminescent front panel is different.

• 1/ une facilité de fabrication,
• 2/ une fiabilité incontestable,
• 3/ une géométrie de l'image parfaite,
• 4/ une résolution optimale,
• 5/ une tension d'accélération et des éléments de déviation plus faciles à construire.

The technology of the single-channel tube is known, and the advantages obtained are:

• 1 / ease of manufacture,
• 2 / undeniable reliability,
• 3 / a perfect image geometry,
• 4 / an optimal resolution,
• 5 / an acceleration voltage and deflection elements easier to build.

The invention, as defined in the claims, makes it possible to improve the resolution and the stability of the dots reproduced, thus avoiding the confusion of the signs B and 8, C and 0, D and 0, S and 8 etc ... in the reproduction of texts in telematics by giving a perfect restitution, and a marked improvement in the reproduction of the image is felt.

To this end, for the production of a color image generator, provision is made in a single-channel cathode-ray tube, a screen constituted by superposed electroluminescent capacitors.

Under the influence of the bombardment of the electron beam, subject to the intensity of the luminance signal, the electroluminescent assembly, having at least two layers, emits lights of different wavelengths.

The metal fittings of each electroluminescent capacitor exit outside the tube and allow the metering of the light intensities by means of an amplification controlled by the signals supplied by the decoding of the chrominance signal.

The color selection and mixing are done point by point on the front of the tube. To do this, a specific physical property of certain cathodoluminescent substances, namely "image amplification" by the action of a high electric field, is used.

The light spot of a point weakly irradiated by an electron beam is amplified when a variable electric field is applied in a capacitor containing phosphors.

The brightness and the mixing of the colors are obtained on the one hand by the cathode current for the luminance, and on the other hand by the electroluminescent transducers for the chrominances.

• - sur les particularités physiologiques de l'oeil,
• - sur les lois de la colorimétrie,
• -sur les propriétés de la cathodolumine- scence, associés à celles de l'électroluminescence en "couches minces".

The principle of the single-channel image tube with chromatic electroluminescent transducers, in accordance with the invention is based on the use of several physical phenomena:

• - on the physiological particularities of the eye,
• - on the laws of colorimetry,
• -on the properties of cathodoluminescence, associated with those of electroluminescence in "thin layers".

Briefly, we recall that since the work of Newton in 1669, we know how to decompose white light into three fundamental colors and we use for reproduction the recomposition of the three superimposed images to obtain all visible hues (color photography, multi-color litographies). On the difference, in television we have so far not found a valid solution allowing to transmit simultaneously all the details of a complete scene. All the points of the image are therefore explored and transmitted one after the other, according to a well-determined law, the analysis consisting in moving the light spot which travels the screen in the same way as the gaze of a reader on a page of a book, starting with the first line of the page, from left to right. The details are transmitted one after the other, exploring point by point, line after line, frame after frame. The details to be transmitted are converted into an electrical signal which varies over time. The different elements that make up the image each have a certain luminance and their own color. These luminances vary from one element to another and produce signals at variable level.

• -la position dans le plan, de chacun des points de l'image explorée,
• - les variations instantanées de luminosité de ces points en fonction du temps.

The function of a television receiver is to transform electrical energy into light energy and to reproduce exactly at reception:

• -the position in the plane, of each point of the explored image,
• - instantaneous variations in the brightness of these points as a function of time.

The additive synthesis carried out by the organ of sight, allows by the mixture of the three fundamental colors to reconstitute the white light. This white light can be effected with light sources radiating color directly.

• 1/ par des photons basse énergie, lumière visible ou ultra-violette (photoluminescence),
• 2/ par des photons de haute énergie, rayons X (roentgénoluminescence),
• 3/ par des rayons cathodiques (cathodo- luminescence),
• 4/ par un champ électrique (électroluminescence),
• 5/ par des particules alpha (ionolumin- escence),
• 6/ par des particules ou des radiations nucléaires (radioluminescence).

In addition, it is recalled that luminescence is the sustained emission of light, from special substances, when they are irradiated:

• 1 / by low energy photons, visible or ultra-violet light (photoluminescence),
• 2 / by high energy photons, X-rays (roentgenoluminescence),
• 3 / by cathode rays (cathodoluminescence),
• 4 / by an electric field (electroluminescence),
• 5 / by alpha particles (ionolumin- escence),
• 6 / by particles or nuclear radiation (radioluminescence).

The phenomenon essentially resides in the direct excitation of the luminescent centers by accelerated electrons, in a high electric field.

Electroluminescence is an unfamiliar phenomenon. In 1936 Professor Georges Destriau, of the University of Paris, discovered that zinc sulfide could emit light under the direct action of an alternating electric field. This new phenomenon was of great importance since, for the first time, a solid body directly transformed electrical energy into light energy.

• 1/ un produit de base sous forme cristalline, comme le sulfure de zinc, l'oxyde de zinc, le sulfure de strontium, le tungstate de calcium, le silicate de zinc, le fluorure de calcium, le germanate de magnésium, le sulfure de cadmium, le fluorure de cadmium,
• 2/ des traces d'un élément étranger, dit "activateur", en proportion très faible (10-² à 10-⁴ en poids) tels que le cuivre, l'or, l'argent, le manganèse, le plomb, le chrome, le bismuth et différentes terres rares.

A solid electroluminescent compound comprises:

• 1 / a basic product in crystalline form, such as zinc sulfide, zinc oxide, strontium sulfide, calcium tungstate, zinc silicate, calcium fluoride, magnesium germanate, sulfide cadmium, cadmium fluoride,
• 2 / traces of a foreign element, said "activator", in very small proportion (10- ² to 10- ⁴ by weight) such as copper, gold, silver, manganese, lead, chromium, bismuth and different rare earths.

These elements constitute the "luminogenic" centers of the electroluminescent compound.

A thin film of the above-mentioned luminescent compound coated in an insulator is placed between two plates which are both transparent and conductive, generally two glass plates on which a thin layer of antimony chloride or oxide has been sprayed. tin.

The electroluminescent cell thus produced constitutes a capacitor. If we connect the two conductive strips of this capacitor to a periodic voltage generator we see, through the glass slides, shine the luminescent compound. This brightness increases both with the value of the applied voltage and with its frequency. The light emitted is blue, green, yellow or red, depending on the nature of the basic product and the activator. Luminescent cells comprising such materials arranged in layers from 20 to 50 μm thick, called "powder-light emitting capacitors", have been the subject of numerous experimental tests.

For more than a decade, the average diameter of crystalline products has been less than a micron. The manufacture of thin films (structure "thin layer") is achievable by a process falling within the radio frequency sputtering technique in a vacuum chamber (pressure of the residual gases of less than 10- ⁵ torr) in the presence of an inert gas (argon).

The whole of a cell is in the form of a thin layer having a thickness of 0.4 to 3, u. To increase the luminous efficiency, annealing at 500 ° for 20 to 30 minutes under an argon atmosphere is recommended.

These thin layers have high chemical reliability, stable and homogeneous characteristics over all their surfaces and thicknesses. They operate with relatively low excitation voltages and their minimum lifespan is 10 ⁴ hours.

A new effect, "the shine boosting effect", was discovered in 1953 by Destriau. Certain materials which are luminescent under the action of an external excitation have an increased sensitivity, that is to say become much brighter when they are subjected simultaneously to the action of external excitation and to that of the applied alternating electric field. between the capacitor plates. The enhancement of the brightness depends on the luminogenic products used, and especially on the voltage and the frequency of the electric field.

This principle is described under the name "electro-cathodoluminescence" by DA Cusano (Cathodo, Photo and DC Electroluminescence in Zinc Sulfide Layers "-Luminescence of organic and inorganic Materials-Ed. By H. Kallmann, John Wiley, London (1962) ) Pages 501 et seq. The activator used is manganese (Mn).

In addition, French Patent No. 1,115,162, titled "Cathode Ray Screen and Luminescence Enhancement Process" from Westinghouse Electric Corporation and U.S. Patent A-2,992,349 to DA Cusano, titled "Field Enhanced Luminescence System ", use this physical phenomenon in monochrome screens. However, the reinforcing effect is greater in "thin-film" electroluminescent capacitors and has a high efficiency when the phosphor is zinc sulfide (ZnS) and the activator consists of molecules of one or more oxysulfides, in particular, trivalent fluorides of the lanthanide family (called "rare earths"). The effects of these activators, (like terbium, europium, thulium, praseodymium, neodymium, samarium, gadolinium, holmium, erbium, dysprosium) were called in 1968: "lumocen effects" (Luminescence from Molécular Centers). The phosphors doped with rare earth fluorides emit very bright green, red and blue lights.

• 1/ une couche électroluminescente d'une épaisseur de 6000 A, composée de sulfure de zinc associé à un oxysulfure.
• 2/ deux couches tampons d'oxyde d'yttrium (Y₂0₃) de 2000 A d'épaisseur entre lesquelles est intercalée ladite couche électroluminescente, ces deux couches étant sensiblement transparentes et leurs faces extérieures étant rendues conductrices par un dépôt d'oxyde d'étain (S_n0₂) ou d'oxyde d'indium (I_"O₃).

A chromatic electroluminescent transducer, according to an embodiment of the invention is in the form of an electroluminescent cell with a thickness of approximately one micron. He understands:

• 1 / an electroluminescent layer with a thickness of 6000 A, composed of zinc sulphide associated with an oxysulphide.
• 2 / two buffer layers of yttrium oxide (Y ₂ 0 ₃ ) of 2000 A thickness between which said electroluminescent layer is interposed, these two layers being substantially transparent and their outer faces being made conductive by an oxide deposit tin (S _n 0 ₂ ) or indium oxide (I _" O ₃ ).

If we direct an electron beam with a post-acceleration voltage of 5KV on this transducer, a fine spot of light appears, at the point of impact of the beam on the luminescent layer. This light emission is locally amplified when a voltage pulse is applied across the transducer.

• -la figure 1, est une vue schématique en coupe transversale d'un transducteur électroluminescent,
• - la figure 2, est une vue schématique en coupe d'un tube image utilisant le principe de l'invention,
• -la figure 3, est une coupe suivant AA de la dalle avant du tube image de la figure 2,
• -la figure 4, est le schéma synoptique d'un récepteur de télévision utilisant le tube à transducteurs.

The invention is explained in more detail using drawings representing only one embodiment.

• FIG. 1 is a schematic view in cross section of an electroluminescent transducer,
• FIG. 2 is a schematic sectional view of an image tube using the principle of the invention,
• FIG. 3 is a section along AA of the front panel of the image tube of FIG. 2,
• FIG. 4 is the block diagram of a television receiver using the tube with transducers.

The electroluminescent transducer (Figure 1) comprises a luminescent layer (1) made of a material both cathodoluminescent and electroluminescent, on either side of which are arranged two conductive layers (2 and 3) each connected to a respective terminal a generator (4).

Means not shown, maintain a fairly high vacuum in the space (5) located on one side of the stack of layers (1, 2 and 3). A threadlike electron beam (6) scans the surface (7) on the empty side. These electrons (6) have sufficient energy to penetrate the luminescent layer (1), but the power of the beam is sufficiently low so that in the absence of periodic voltage between the layers (2 and 3) the luminescent material (1 ) emits at the point of impact (8) of the electron beam (6) only a low intensity spot.

If a periodic voltage with an amplitude of 100 to 140 volts is applied between the layers (2 and 3) during a short period of time, for example 10- ⁴ seconds, a flash of light, represented schematically by the arrow ( 10) is emitted at the point of impact (8).

The wavelength of this light depends on the nature of the impurity atoms constituting the luminogenic centers of the material (1).

It is advantageous to interpose between the luminescent layer (1) and each of the conductive layers (2 and 3), a so-called buffer layer (1 and 2) made of a material which is good insulator of electricity and has a high dielectric constant. . These layers (11 and 12) make it possible to apply high voltages between the two conductive layers (2 and 3), in order to obtain a large electric field in the material (1).

Several electroluminescent transducers identical to that of FIG. 1 can be used to obtain colored images from video-color signals.

For this purpose, as shown in FIG. 2, a television picture tube with a single electron gun is used, and 3 chromatic electroluminescent transducers.

We can see on the schematic section in Figure 2, the filament (13) of the electron gun, the cathode (14) on which the luminance signal Y is applied, the wenehlt (15) for adjusting the brightness of the image, the accelerating grid (16) and the concentration grid (17). Electromagnetic deflection coils, not shown, provide a suitable deflection of the electron beam (18), coming from the electron gun (13 to 17), so that this beam (18) scans in a known manner the surface of the "slab" (19) .

This slab (19) is formed by the outer wall of the image tube (20) located opposite the barrel (13 to 17).

The result would be equivalent, if instead of an electromagnetic deflection tube an electrostatic deflection tube was used, the bulb would then have plates X ,, X ₂ and Y _{1 '} Y ₂ to ensure the deflection of the electron beam along two dimensions.

As shown in Figure 3, this glass slab (21) is provided on its inner face with a filtering layer (gray filter) (22), and a stack of thin layers comprising three superimposed light-emitting transducers. These three transducers consist of three luminescent layers (23, 24 and 25) and four conductive layers (26, 27, 28 and 29), the luminescent layers (23, 24 and 25) are made of a material that is both electroluminescent and cathodoluminescent, each of them containing a doper of different nature, allowing respectively the emission of green, red and blue light. For this purpose, each light-emitting layer (23, 24 and 25) is located between two conductive layers 26-27, 27-28 and 28-29 respectively. As in the case of FIG. 1, buffer layers (30) are interposed between the luminescent layers (23 to 25) and the conductive layers (26 to 29).

The conductive layer (29) located on the side of the interior of the tube (20) may be a thin layer of aluminum or copper, brought to a high positive potential. The purpose of this thin layer is to increase the conductivity of the screen and to act as a mirror for the light rays emitted by the luminescent layers, the light rays all being reflected forward.

Layers 23, 24 and 25 are formed from zinc sulfide (ZnS), associated with dopant molecules and containing oxysulfides to limit the interactions of said molecules and zinc sulfur.

• - pour 23 du terbium (Tb), donnant une lumière de longueur d'onde 545 nm (nano- mètre) lumière verte,
• -pour 24 de l'europium (Eu), donnant une lumière de longueur d'onde 620 nm (rouge),
• - pour 25 du Thullium (Tm), donnant une lumière de longueur d'onde 450 nm (bleu).

The activators are:

• - for 23 terbium (Tb), giving a light of wavelength 545 nm (nanometer) green light,
• -for 24 of europium (Eu), giving a light of wavelength 620 nm (red),
• - for 25 Thullium (Tm), giving a light of wavelength 450 nm (blue).

The layers 26, 27 and 28 are layers of tin oxide Sn0 ₂ or indium oxide In ₂ 0 ₃ .

The layers 30 are layers of yttrium oxide Y ₂ 0 _{3 '}

The conductive layer 29 is a 1000 A aluminum layer.

As shown in FIG. 4, the tube (20) reproduces the color images of a television program in the following manner: a reception and demodulation block (31) picks up the HF waves on its terminal (31 A) and delivers the video signal of luminance Y on the terminal 31 B connected to the cathode (14) of tube (20). The video signal from chrominance delivered on the terminal (31 C) is applied to the input (32 A) of the chrominance decoding block (32). This decoding assembly processes the green, red and blue chrominance signals respectively on the output terminals (32 B, 32 C and 32 D). These voltages delivered in 32 B, 32 C and 32 D are proportional to the green, red and blue chromatic components of the emission and are applied respectively between layers 26 and 27, 27 and 28, 28 and 29.

For compatibility in all standards, in black and white, the chrominance signal decoder 32 does not deliver any signal at the output of the green, red blue amplifiers at 32 B, 32 C, 32 D. The television works like a black television and plain white. At the point of impact of the cathodic bombardment, the kinetic energy of the electrons on the electroluminescent parts of the three transducers causes a white point of light to appear, with a brightness proportional to the voltage of the video-luminance signals applied to the tube cathode.

Starting from the block diagram in FIG. 4, it can be seen that by replacing the monochrome picture tube of a black and white television receiver with a picture tube with light-emitting transducers according to the invention and by associating with said receiver a decoder stage of the type previously described, it is possible to produce a color image whose resolution and restitution of tints and shades are optimal. For reproduction in color television, the different shades to be reproduced are, yellow, cyan, purple, green, blue, red, white. The green, red, and blue transducers by their separate operation or simultaneously 2 by 2 determine the colorimetric unit.

The brightness amplification coefficient, an essential characteristic of the invention is from 10 to 15 for each transducer. This allows, for a brightness equivalent to normal color television reception, to decrease the video amplification. It is even conceivable, instead of using an analog amplifier operating at a supply voltage of 150 to 180 volts, to use a digital amplifier operating at 15 to 25 volts.

soit environ 30% de rouge, 60% de vert, 10% de bleu.A suitable order of stacking of the transducers makes it possible to respect the fundamental equation of the luminance signal, containing all the information necessary for the reproduction of the image in "black and white", ensuring the principle of compatibility of the emissions "black and white "and colors as stated by Georges Valensi; to know:

about 30% red, 60% green, 10% blue.

The light emitted by the first transducer, towards the observer is green, and it undergoes no attenuation; the second light emitted by the second transducer is red, it undergoes a natural attenuation of approximately 40% by crossing the first transducer, as for the third transducer, its light emission is blue and it undergoes an approximation attenuation from 60% to 80% .

Another advantage of the invention is that the brightness of the tube with transducers is very important, and by slightly increasing the supply voltages of the tube, projection on a large screen by Schmitt optics is possible.

The use for color television is an important but not limiting application of the invention.

Applications in very diverse branches such as: data visualization, oscillography, radar, telematics, computer science, office automation must bring new industrial outlets.

The adaptability of this technological innovation to the reception of television signals in all world standards, its compatibility with black and white reproduction, its high reliability, the ease of construction it allows and the lowering of the cost price. resulting, should lead to rapid development, especially in the field of color television receivers.

Claims (8)

1. Apparatus for generating images in at least two colors comprising an image reproduction screen, means for directing an energizing electronic beam over the entire surface of said screen, the intensity of which beam is dependent on the amplitude of a brightness control signal, and means for imparting a scanning movement to the said energizing beam so that its point of impact on the surface of the screen moves according to two dimensions, the said image generating apparatus being characterized in that it further comprises:

a) a superimposition of at least two layers (23-25) each one produced in a luminescent material radiating light of a predetermined wavelength, different from the wavelength of the light radiated by the other layers, the luminescent materials used, radiating light under the action of an electronic bombardment and adapting to an amplification of the light intensity thus radiated by superposition of an electroluminescent effect,

b) an appropriate control device, associated to each luminescent layer to generate therein a pulsating electric field, the frequency and amplitude of which are dependent on the amplitude of a corresponding color signal so as to ensure the restitution of the different chromatic components, and

c) generators (31, 32) supplying the said signals controlling the color and brightness.

2. Apparatus according to claim 1, characterized in that each luminescent layer has a thickness between 0.4 and 2µ and preferably between 0.4 and 0.8₁₁.

3. Apparatus according to one of claims 1 and 2, characterized in that the material constituting the luminescent layers is selected from the following materials: zinc sulfide, zinc oxide, strontium sulfide, calcium tungstate, zinc silicate, calcium fluoride, magnesium germanate, cadmium sulfide, cadmium fluoride doped with at least one of the following activators: praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolimium (Gd), terbium (Tb), holmium (Ho), erbium (Er), thulium (Tm) fluorides.

4. Apparatus according to one of claims 1 to 3, characterized in that, to increase the electric field in the electroluminescent layer, one or two buffer layers (30), produced in an insulating material of high electrical constant, are placed on one side or on either side of the electroluminescent layer.

5. Apparatus according to claim 4, characterized in that, each buffer layer has a thickness between 0.2 and 0.8µ and in that the material used is yttrium oxide (Y_z0₃).

6. Apparatus according to one of claims 1 to 5, characterized in that the control device associated to an electroluminescent layer and the buffer layers comprise two semi-transparent conducting layers (26-27, 27-28, 28-29) placed on either side and connected to the generator (32) of the signal controlling the chrominances.

7. Apparatus according to claim 6, characterized in that the conducting layers (26-29) are produced in a material such as tin oxide (Sn0₂) or indium oxide (In0₃).

8. Apparatus according to one of the preceding claims, characterized in that the superimposition comprises three luminescent layers (23, 24, 25), radiating respectively in green, red and blue.

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