EP0383672B1 - Flat light source - Google Patents

Description

The present invention relates to a planar light source and relates more generally to the production of wide planar sources of small thickness such as they are used for the rear lighting of display devices (liquid crystal screen), the rear lighting of photographic films, etc.

To date achieve plane light sources of a certain extent, the skilled person usually uses two different techniques.

The first technique consists in using fluorescent sources and in particular such sources in the form of tubes which are juxtaposed one beside the other in more or less number. In practical terms, fluorescent tubes of the discharge tube type are used which are juxtaposed side by side, which leads to illuminating surfaces whose uniformity of lighting is imperfect and whose thickness is at a minimum. of the order of 1 cm, taking into account the minimum dimensions of fluorescent tubes existing in commerce.

The second technique consists in using sources of the electroluminescent type. Unlike fluorescent sources, there are electroluminescent sources consisting of plates but these devices have a very poor efficiency and the heat they give off to obtain a certain intensity of lighting is quite significant. Furthermore, the life of such devices is limited. The two previous drawbacks have considerably limited the use of electroluminescent sources outside very specific applications such as night use.

The subject of the present invention is a flat light source, which is easy to produce using means that are simple to implement and which leads to a device of small thickness (approximately 2 mm) with high gloss (several thousand candela per square meter). with very good uniformity of lighting and a very long service life.

• une enceinte sous vide limitée par une paroi latérale et deux parois planes parallèles et isolantes,
• sur chaque paroi plane précédente, et à l'intérieur de l'enceinte, une électrode conductrice recouverte d'une couche d'isolant, l'un au moins de ces deux ensembles paroi-électrode-couche d'isolant étant transparent,
• sur l'une des couches isolantes, une couche d'un matériau cathodoluminescent,
• au voisinage de la paroi latérale et extérieurement aux deux électrodes conductrices, une source d'électrons, et
• une source de tension permettant d'appliquer alternativement aux deux électrodes conductrices deux potentiels distincts tels que les électrons émis par ladite source d'électrons soient alternativement collectés par lesdites électrodes.

The planar light source, object of the invention, is characterized in that it comprises:

• a vacuum enclosure limited by a side wall and two parallel and insulating flat walls,
• on each preceding flat wall, and inside the enclosure, a conductive electrode covered with an insulating layer, at least one of these two wall-electrode-insulating layer assemblies being transparent,
• on one of the insulating layers, a layer of cathodoluminescent material,
• in the vicinity of the side wall and externally to the two conductive electrodes, a source of electrons, and
• a voltage source making it possible to apply alternately to the two conductive electrodes two distinct potentials such that the electrons emitted by said electron source are alternately collected by said electrodes.

As can be seen, the planar light source which is the subject of the invention uses the cathodoluminescence effect already employed for example in the cathode ray tubes of our television sets. A material is called cathodoluminescent when under the effect of a bombardment by electrons of a certain kinetic energy, it emits radiation luminous. Such known cathodoluminescent bodies are often wrongly called "phosphors".

According to the invention, a conventional cathodoluminescent material covers the internal face of one of the armatures of a planar capacitor, the corresponding electrode consisting of a conductive material covered with a layer of electrical insulator, such as the electrode of the opposite armature of the planar capacitor, moreover.

When the source object of the invention is made to illuminate from only one of its planar walls, at least the corresponding wall as well as the electrode and the insulating material placed on this wall must be transparent, in other words capable of allowing passage the light emitted by cathodoluminescence. When this source is made to light from its two plane walls, these as well as the electrodes and the corresponding insulating materials must be transparent.

Inside the vacuum enclosure containing the reinforcements of the condenser is placed a source of electrons of a type known per se (heated filament, points, etc.) which allows, after high tensioning of the armature chosen as anode, to charge the previous plane capacitor by depositing electrons which will localize in the form of a cloud of negative electricity in the vicinity of this anode, the insulating material deposited on the electrode preventing these same negative electric charges from flow through the anode. When the charge of the capacitor is thus carried out, if the electrons are made to oscillate by a voltage source applying alternately to the two conducting electrodes two distinct potentials such that the electrons are alternately collected by these electrodes, the electrons then oscillate at the frequency of the signal applied between the reinforcements in the zone which separates them thus causing an excitation of the cathodoluminescent material which they come to strike at each period by causing an emission of light. In stable operating conditions, the electron source located in the vacuum enclosure practically no longer delivers current, except to compensate at all times for the leakage of electrons by electrical faults in the insulators, keeping them at a constant number.

The usable electron source can be either a hot source (heated filament) or a cold source (photoemission, field effect).

According to the invention, the number of electrons in oscillation in the light source corresponds to the capacity of the planar capacitor thus produced and is therefore entirely determined by the dimensions of the capacitor, the thickness of the insulators and the voltage applied to the armatures. In particular, it does not depend on the emission characteristics of the auxiliary electron source used. In other words, during permanent operation, the light sensation felt by an observer of the source therefore depends only on the frequency of oscillation because the quantity of light emitted in each period is constant. This guarantees the uniformity of the illumination produced by cathodoluminescence.

One of the advantages of the planar light source which is the subject of the invention lies in the fact that its structure is perfectly compatible with the manufacture of planar sources of small thickness (up to 2 mm) and of very large surface (several square decimetres per example without difficulty).

Like the electrons that oscillate between the two armatures of the source are used a large number of times, the energy expended to produce them using the electron source can be made very low.

The plane light source which is the subject of the invention can emit with a very high brightness which is adjustable both by the voltage imposed on the armatures and the frequency of the source, two parameters which influence this same brightness approximately linearly.

Finally, a non-negligible advantage of this light source lies in its very long lifespan, which is practically that of the cathodoluminescent material placed in the best operating conditions (potential difference of the order of 1 to a few kilovolts and good electrical insulation ).

• la figure 1 représente le schéma de principe général d'une source lumineuse plane conforme à l'invention,
• les figures 2 illustrent la phase de chargement du condensateur plan de la source à l'aide de la source d'électrons auxiliaire ; - la figure 2a montre la répartition des charges selon que l'on choisit comme anode l'électrode supérieure (partie gauche) ou l'électrode inférieure (partie droite) ; - la figure 2b montre la répartition de densité des électrons sur l'anode constituée par l'électrode supérieure et, - la figure 2c montre la répartition de densité des électrons lorsque l'on choisit comme anode l'électrode inférieure,
• la figure 3 illustre le principe de l'émission de lumière lors de l'inversion des potentiels entre les deux électrodes conductrices de la source.

In any case, the invention will be better understood by referring to the following description of an example of implementation of the plane light source which is the subject of the invention, a description which will be given by way of explanation and without limitation, with reference to Figures 1 to 3, in which:

• FIG. 1 represents the general block diagram of a plane light source according to the invention,
• FIGS. 2 illustrate the charging phase of the planar capacitor of the source using the auxiliary electron source; - Figure 2a shows the distribution of charges depending on whether the upper electrode (left part) or the lower electrode (right part) is chosen as the anode; - Figure 2b shows the density distribution of electrons on the anode formed by the upper electrode and, - Figure 2c shows the density distribution of electrons when choosing as anode the lower electrode,
• FIG. 3 illustrates the principle of the emission of light during the inversion of the potentials between the two conductive electrodes of the source.

In Figure 1, there is shown in a vacuum enclosure 1 limited by a side wall 2 and two flat, parallel and transparent walls of glass for example, respectively upper 3 and lower 4, the elements of a flat light source conforming to the invention and which comprises: a transparent conductive electrode 5 situated inside the enclosure 1 on the wall 3; a conductive electrode 6 inside the enclosure 1 on the wall 4; two layers of insulating material 7 and 8 respectively covering the conductive electrodes 5 and 6 and on one of the armatures, here the lower armature, a layer of cathodoluminescent material 9. A voltage generator 10 makes it possible to control the potential of the electrodes 5 and 6.

The device is completed by the electron source 11, for example of the heating filament type at the terminals of which the voltages V _1s and V _2s are applied.

In the example of FIG. 1, the flat walls 3 and 4 are produced by glass plates sealed in a sealed manner on the side wall 2.

The upper glass substrate 3 is covered with the transparent conductor 5, consisting of indium oxide doped with tin, with a thickness of approximately 10⁻⁵ cm (1000 Angstroms) and the insulating layer 7 which covers the conductor 5 previous is a layer of silica of about 5 micrometers.

The lower glass substrate 4 is covered with a metallic conductor 6. When, as this is the most general case, this conductor 6 does not need to be transparent, it can be produced by an aluminum deposit with a thickness of around 10⁻⁵ cm (1000 Angstroms). On the conductor 6 is a thin insulating layer 8, produced, like the homologous layer 7, by a deposit of silica about 5 micrometers thick. On the insulating layer 8 is a layer 9 of cathodoluminescent material, produced either by screen printing from a powder, or directly by deposition in a thin layer, with a thickness of the order of 1 micrometer. A person skilled in the art is perfectly aware of cathodoluminescent bodies which can be used in the context of the invention and he may for example have recourse to oxysulfide Y₂O₂S doped with europium to obtain a light emission in red, or to zinc sulfide ZnS doped with copper and aluminum for a light emission in green, and zinc sulfide ZnS doped with silver for a light emission in blue.

According to the invention, the electron emitting source 11 can be produced from any known means, such as for example the heated filaments emitting by thermoelectric effect, the conductive microdots emitting by field effect and the thin layers emitting by photoemissive effect .

• 1) - de porter le conducteur électrique transparent 5 situé sur la paroi supérieure 3 de la source à un potentiel que l'on appellera V_sup ;
• 2) - de porter le conducteur métallique 6 déposé sur la paroi inférieure 4 à un potentiel électrique que l'on appellera V_inf ;
• 3) - la source d'électrons 11 peut être connectée à un ou plusieurs potentiels, qui doivent être inférieurs à V_sup ou V_inf. Dans le cas où la source est constituée d'un filament chauffé, deux connexions (c'est le cas représenté sur la figure 1) la relient à l'extérieur et sont soumises respectivement aux potentiels V_1s et V_2s. Dans le cas où la source est constituée de micropointes, deux connexions sont encore nécessaires, mais l'une est utilisée pour la cathode portant les micropointes, et l'autre pour la grille de commande d'extraction des électrons.

The assembly thus represented in FIG. 1 is provided with electrical connections with the outside which allow:

• 1) - to bring the transparent electrical conductor 5 located on the upper wall 3 of the source to a potential which will be called V _sup ;
• 2) - to bring the metallic conductor 6 deposited on the lower wall 4 to an electrical potential which will be called V _inf ;
• 3) - the electron source 11 can be connected to a or more potentials, which must be less than V _sup or V _inf . In the case where the source consists of a heated filament, two connections (this is the case shown in FIG. 1) connect it to the outside and are subjected respectively to the potentials V _1s and V _2s . In the case where the source consists of microtips, two connections are still necessary, but one is used for the cathode carrying the microtips, and the other for the electron extraction control grid.

In the case where the electron source 11 consists of a photoemissive layer, only one connection with the outside is necessary.

In all cases, the person skilled in the art will in any case be able to use the electron source 11 to obtain, after having chosen one of the two conductors 5 or 6 as the anode, the charge of the planar capacitor formed by these two same conductors 5 and 6.

In the remainder of the text, the explanations will be provided, limiting themselves to the most frequent case, which is that where the electron source 11 consists of a heated filament, the two ends of which are brought to the respective potentials V _1s and V _2s. .

In the example of FIG. 1, one of the wall assemblies 3 - conductive electrode 5 - insulating layer 7 alone is transparent and the source emits only on one side. It would also be possible, without departing from the scope of the invention, to produce a planar source emitting on the two faces by manufacturing the two walls 3 and 4, the two electrodes 5 and 6 and the two insulating layers 7 and 8 in transparent materials.

• d'abord une étape que l'on qualifiera de régime statique et durant laquelle la source de tension 10 porte les électrodes 5 et 6 à des potentiels constants et la source d'électrons 11 est utilisée pour charger le condensateur formé par les deux électrodes conductrices 5 et 6 précédentes. Durant ce chargement en régime statique, la source n'émet aucun rayonnement lumineux. Ce régime statique sera décrit en se référant aux figures 2 (2a, 2b, 2c).
• ensuite, un deuxième régime, qualifié de dynamique, qui correspond en fait aux périodes de fonctionnement de la source d'émission de lumière et sera décrit ensuite en se référant à la figure 3.

We will now describe the operation of the planar light source as just described with reference to FIG. 1 and keeping in mind that this operation comprises two stages, namely:

• first a step which will be called a static regime and during which the voltage source 10 brings the electrodes 5 and 6 to constant potentials and the electron source 11 is used to charge the capacitor formed by the two conductive electrodes 5 and 6 above. During this static loading, the source emits no light radiation. This static regime will be described with reference to Figures 2 (2a, 2b, 2c).
• then, a second regime, termed dynamic, which in fact corresponds to the periods of operation of the light emission source and will be described next with reference to FIG. 3.

In the operation in static mode, during which the capacitor formed by the reinforcements 5 and 6 is charged, the voltage source 10 delivers constant potentials V _sup and V _inf . In FIG. 2a, the two possibilities available to the user are shown, namely: On the left half, the use of the upper electrode as anode, the latter being brought to a potential V _sup of l 'order of 1 kilovolt (V _sup = V _anode ) and the lower electrode 6 being brought to a rest voltage V _inf very little different from 0 (V _inf = V _rest ); on the right part of the same figure 2a, the opposite option, in which the lower electrode is used as the anode electrode (V _inf = V _anode ) and the upper electrode is put at a resting voltage ( V _sup = V _rest ). These two ways of practicing are roughly equivalent except that it is generally preferable to choose the option on the left side of FIG. 2a corresponding to the accumulation of electronic charges on that of the armatures of the capacitor which does not comprise cathodoluminescent material.

If we return to the case of the left-hand side of FIG. 2a, the upper conductor 5 plays the role of anode and if we put the electron source 11 into operation it then collects, as symbolically indicated on the drawing, the electrons which are emitted by this source 11, which works at the potentials V _1s = 0 volts and V _2s = 5 volts. Under these conditions, the source 11 is practically at the same potential as the lower electrode 6, and it is the upper electrode 5 which plays the role of anode and collects the cloud of electrons e⁻ emitted by the source 11. The FIG. 2b shows the variation 12 of the density of these same electrons in the vicinity of the upper wall 3. In fact, the electrons thus collected by the upper conductor 5 are not eliminated by the latter since the insulating layer 7 prevents them from s '' flow directly into the condenser circuit. These same electrons therefore accumulate at the interface between the vacuum of the enclosure 1 and the insulating layer 7, until the local potential reaches the same value as the potential of the emissive source. When this condition is fulfilled, the potential in the vicinity of the insulation layer is approximately that which is applied between the emissive electron source 11 and the upper conductor 5 acting as an anode. This potential is therefore, in the example chosen, of the order of 1 kilovolt, which justifies the thicknesses of 5 micrometers chosen for the insulating layers 7 and 8.

Thus, at the end of this charging phase of the source capacitor, the number of electrons collected by the upper anode conductor 5 to the state of equilibrium is proportional both to the difference in potential between the source 11 and the collecting electrode 5 and, conversely, to the thickness of the insulator 7, as is the capacity of the capacitor thus formed.

The right part of FIG. 2a as well as FIG. 2c illustrate the symmetrical choice in which the user would have put the upper electrode 5 at rest and would have chosen to bring the lower electrode 6 to a potential of 1 kilovolt to make it the 'anode electrode. This mode of implementation, which is rigorously symmetrical to the previous one and will be deduced therefrom without difficulty for the skilled person, will not be described any further.

Referring now to FIG. 3, the dynamic regime of the source will be described, that is to say the regime during which, after the previous static charge phase, the potentials of the conducting electrodes 5 and 6 are periodically reversed. to obtain, by impact on the cathodoluminescent layer 9 of negative electrical charges, the light emission effect.

If, from the state of the potentials represented on the left half of FIG. 2a, the potentials respectively applied to the upper and lower conductors 5 and 6 are reversed, we obtain the diagram of FIG. 3 in which the electronic cloud moves towards the lower electrode 6 and strikes the cathodoluminescent layer 9 thus causing the emission of photons h ν to the outside of the source. The electrons which strike the cathodoluminescent material at the time of the inversion of the charge zones cause the emission of light and it is sufficient that the voltage source 10 alternately delivers the potentials V _anode and V _rest at the electrodes 5 and 6 to make the phenomenon periodic and cause the continuous light emission from the source.

If Q / mm² denotes the charge stored per square millimeter on in the vicinity of the collector electrode, by f the frequency of inversion of the potentials due to the voltage source 10 between the upper and lower electrodes, the directed current towards the cathodoluminescent material 9 can be written i = Qf.

In a practical example of implementation of the invention, the thicknesses of insulation 7 and 8 are given a dimension of 5 micrometers, by making them of silica with index = 5; a potential difference of 1 kilovolt between the two conductive electrodes and an alternating frequency of 1 kHz for the voltage source, leads to a charge Q per square millimeter close to 10⁻⁸ Coulomb and to a charge current per square millimeter close to 10 microamps.

A gloss of several thousand candela per square meter is thus obtained, taking into account the usual conversion yields of cathodoluminescent materials commonly used.

Furthermore, the static regime described above as a phase prior to the dynamic regime can be eliminated. The charge Q required for operation is then gradually established during the dynamic regime.

Claims (1)

1. Planar light source, characterized in that it comprises a vacuum enclosure (1) bounded by two parallel, insulating, planar walls (3,4) and a side wall (2),
   on each of the planar walls and within the enclosure (1) is placed a conductive electrode (5,6) covered with an insulating layer (7,8) and at least one of these two wall-electrode-insulating layer assemblies is transparent, on one of the insulating layers (8) is placed a cathodoluminescent material layer (9), in the vicinity of the side wall (2) and externally of the two conductive electrodes (5,6) is provided an electron source (11) and a voltage source (10) is also provided for alternatively applying to the two conductive electrodes (5,6) two different potentials (V_anode, V_rest), so that the electrons emitted by said electron source are alternatively collected by said electrodes.

Images