Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J29/00—Details of cathode-ray tubes or of electron-beam tubes of the types covered by group H01J31/00
  • H01J29/02—Electrodes; Screens; Mounting, supporting, spacing or insulating thereof
  • H01J29/10—Screens on or from which an image or pattern is formed, picked up, converted or stored
  • H01J29/18—Luminescent screens
  • H01J29/185—Luminescent screens measures against halo-phenomena
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J29/00—Details of cathode-ray tubes or of electron-beam tubes of the types covered by group H01J31/00
  • H01J29/02—Electrodes; Screens; Mounting, supporting, spacing or insulating thereof
  • H01J29/10—Screens on or from which an image or pattern is formed, picked up, converted or stored
  • H01J29/18—Luminescent screens
  • H01J29/28—Luminescent screens with protective, conductive or reflective layers

Definitions

• the invention relates to a cathodoluminescent screen of the thin film type, for cathode ray tube, and it particularly relates to means for improving the light output of the cathodoluminescent thin film.
• a cathode ray tube includes a vacuum enclosure in which a source of electrons produces a beam. The electron beam is accelerated and focused before bombarding a luminescent screen; a bright image can be formed on the screen surface by deflecting the beam.
• One of the manufacturers' constant concerns is to give the image the best definition or resolution possible, as well as to improve the light output.
• the resolution depends in particular on the focusing of the beam as well as on the characteristics of the luminescent screen, this luminescent screen also having effects on the light output and the luminance in general.
• the luminescent screen comprises at least one luminescent layer generally formed of phosphor grains; this layer is formed on a transparent face of a substrate which most often is a glass slab.
• the phosphors used in cathode ray tubes are materials that emit visible light when they are bombarded with electrons (cathodoluminescence effect).
• Several types of luminophores can be used in cathode ray tubes, for example: luminophores using crystals of a single luminescent material, material chosen according to the wavelength to be emitted; there are also multicomponent phosphors using a mixture of crystals of several luminescent materials; or phosphors for cascading screens; or even phosphors that can produce color variations.
• cathodoluminescent layer Taking for example the case of a single cathodoluminescent layer, the latter can be produced in particular, either in the form of a thin film, or in the form of a plurality of phosphor grains which are deposited on the glass substrate. , one beside the other so as to form a layer the thickness of which comprises several superimposed phosphor grains; this last type of cathodoluminescent layer is called phosphor layer in the following
• the cathodoluminescent layer is formed of a film called a luminescent thin film.
• lacquerEe average particle size is generally of the order of 2 to 8 micrometers, depending on the applications.
• a relatively thin metallic layer for example of aluminum, which has the function of both applying
• Such a phosphor layer makes it possible to obtain a
• cathodoluminescent screen also has another drawback which resides in the fact that there is a
• the luminescent thin film it is known to produce it in the form of a film in a thin layer, deposited for example on a glass slab by a conventional method of depositing a film in a thin layer, for example by evaporation, or by a gas phase chemical decomposition (CVD) method.
• CVD gas phase chemical decomposition
• the lumincescent thin film can be of the YAG (yttrium-aluminum-renat) type, produced by a liquid phase epitaxy method on a YAG single crystal; in this case, the epitaxial thin tick can be doped with terbium Tb for example to obtain a green luminescence.
• YAG yttrium-aluminum-renat
• the luminescent thin film obtained by the thin film deposition method or by the epitaxial method has the advantage of having a high resolution, and also of having good thermal contact with the substrate, but it has the drawback of '' have a very low light output.
• FIG. 1 shows a conventional cathodoluminescent screen of the luminescent thin film type.
• the cathodoluminescent screen 1 comprises a substrate 2, a glass slab for example, an inner face 3 of which carries a thin luminescent film 4 which forms a continuous and homogeneous layer with a thickness E of the order of about one micrometer.
• the thin film 4 is of the type formed on the glass slab 2 by a conventional method of depositing a film in a thin layer, for example by evaporation.
• the thin film 4 is made of a conventional luminescent or luminophore material such as for example a compound formed of zinc combined with sulfur and with silver impurities ZnS: Ag. Above the thin film 4 is deposited, in a conventional manner, a layer 5 of aluminum.
• the glass slab 2 has a refractive index n of the order 1, 45, and the refractive index ni of the luminescent thin film 4 can vary depending on the material of which it is made, but generally this index ni is the order of 2.
• the present invention relates to a cathodoluminescent screen, the luminescent substance of which is formed by a thin film, either of the type formed by a film in a thin layer deposited on the substrate, or of the type formed by an epitaxial layer, a screen whose new arrangement brings significant improvement to light collection efficiency, and therefore the overall luminance of the cathode ray tube.
• a cathodoluminescent screen comprising, a substrate on which a thin luminescent film is formed, a film reflecting the light produced by the luminescent thin film being disposed above the latter, opposite the substrate, characterized in that it comprises a diffusing layer formed of a plurality of grains, this diffusing layer being disposed between the thin luminescent film and the reflective film.
• the diffusing layer is of the monolayer type in order to maintain an excellent resolution.
• monolayer we mean a layer whose thickness includes a single grain, this for the entire surface of the layer (although in practice there may be some exceptions to this rule, without degrading the resolution too much).
• the grains which form this diffusing layer are practically in contact with the face of the thin luminescent film opposite the substrate, so that this diffusing layer tends to constitute a rough face of the thin luminescent film, this roughness being linked to the particle size of the grains. It follows from this arrangement that a large number of light rays which in the prior art would be lost as a result of multiple total reflections, are on the contrary reflected or re-diffused towards the use, that is to say towards the outside of the tube after passing through the luminescent thin film and the substrate. This can also be obtained without significantly losing resolution, as mentioned above, by the fact that the grain diffusing layer is a monolayer, and that this monolayer can be compact, that is to say say with grains located very close to each other, and with relatively fine grains.
• FIG. 2 shows partially and schematically, in a sectional view, a cathodoluminescent screen structure according to the invention
• FIG. 3 shows partially and schematically, a preferred version of the cathodoluminescent screen of the invention.
• FIG. 2 shows a cathodoluminescent screen 10 according to the invention, intended to form the screen of a cathode ray tube; Using the basic structure already described in the figure
• the screen 10 comprises a substrate 12 formed by a glass slab, an inner face 13 of which carries a thin luminescent film 14 of the same type for example (layer of ZnS: Ag) as the thin luminescent film described with reference to Figure 1.
• the cathodoluminescent screen 10 comprises a layer called the diffusing layer 15 preferably constituted by grains L1, L2,. . . Ln) thin forming a monolayer applied to the thin luminescent film 14.
• the diffusing layer 15 is also useful for the diffusing layer 15 to be formed of grains Ll to Ln relatively close to each other or touching each other, so as to form a compact layer, with relatively fine grains whose average diameter dl is for example less than or equal about 1 micrometer; this in order not to compromise the resolution of the whole.
• the diffusing layer 15 is covered with a thin layer of an electrically conductive material such as aluminum for example, so as to constitute in a way in itself conventional a film 16 which is both reflective and conductive making it possible to reflect light towards the diffusing layer 15 and the thin luminescent film 14, and to apply the accelerating voltage.
• an electrically conductive material such as aluminum for example
• the grains L1 to Ln can be made of materials of various natures, it is however recommended of course that they absorb little of the light emitted by the thin luminescent film 14.
• limit angle 0_ formed between a 0 first axis x normal to the plane of the luminescent thin film 14 and a limit axis xl, and any photon emitted in a direction having with the first axis x an angle 0 personallygreater than the limit angle 0 n , undergoes a total reflection. But in the present invention this is only valid on the side of the interior face 13, at the interface between the luminescent film 14 and the glass slab.
• FIG. 2 This is illustrated in FIG. 2 by a photon marked p ⁇ which is emitted towards the diffusing layer 15 by forming an angle 0 "with the x axis normal to the upper face 17, this angle 0" being greater than the limit angle 0 n : it is observed that this photon p leaves the luminescent thin film 14 to penetrate into a grain Ll in which it undergoes several reflections before coming out again along a trajectory substantially parallel to the first axis x.
• this second photon p undergoes total reflection; as a result, this second photon p, is reflected in the direction of the upper face 17 of the luminescent film 14, and this second photon p, passes through the thin luminescent film 14 to penetrate into a grain L2 where it undergoes several reflections, before coming out for cross the luminescent thin film 14 again along a trajectory substantially parallel to the first axis X, so that it can pass through the glass slab 12 and leave the latter.
• the grains L1 to Ln can consist of materials of various natures, but according to a characteristic of the invention, these grains L1 to Ln are made of a luminescent or luminophoric material, so that these grains L1 to Ln can themselves generate light since they are subjected to the same electron bombardment as the thin luminescent film 14; the grains L1 to Ln are then made of a material chosen to produce the desired light.
• the grains L1 to Ln are at least partially made of the same material as that of which the thin luminescent film 14 is made.
• FIG. 3 shows a preferred version of a screen cathodoluminescent according to the invention.
• the screen 10 comprises, as in the previous example, a substrate 12 on which the thin luminescent film 14 is formed, which thin film 14 in turn carries the diffusing layer 15 formed of grains L1 to Ln; the diffusing layer 15 itself being covered with the reflecting and conducting film 16.
• the cathodoluminescent screen 10 comprises a bonding layer 20 which is both in contact with the upper face 17 of the luminescent film 14 and in contact with the grains L1 to Ln of the diffusing monolayer 15; these grains L1 to Ln being preferably, in this version of the invention, phosphors.
• the bonding layer 20 has an average thickness E3 clearly less than the average diameter dl of grains Ll to Ln, so as to constitute a relatively weak absorbent with respect to electrons passing between two neighboring grains, while partially coating the grains L1 to Ln so as to produce an effective thermal junction between the grains L1 to Ln of phosphors and the thin luminescent film 14.
• the bonding layer 20 makes it possible to improve the collection of light, by preventing by its presence that the light rays do not undergo a total reflection at the level of the upper face 17 of the thin luminescent film 14, for light rays which would be emitted along an axis X4 forming an angle 0 personallygreater than the limit angle 0 classroom, and which in addition would reach the upper face 17 in a point O located between two neighboring grains L1, L2, as illustrated by way of example in FIG. 3 by a photon marked
• the photon p In the absence of the bonding layer 20, the photon p would be reflected as it is represented by the arrow in dotted line marked 30, except of course if the point O is sufficiently close to a luminophore grain, the second grain L2 for example, so that the phenomenon of evanescent wave can manifest itself and allow the photon p to leave the luminescent thin film 14 and to penetrate into the second grain L2.
• the photon p Even if it arrives at the upper face 17 at a point of the latter relatively distant from the first and second grains L1, L2, this photon leaves the thin luminescent film 14, and the bonding layer 20 picks up this photon and channels it towards the second grain L2, for example where it is diffused towards the outside.
• the bonding layer 20 has a refractive index n2 greater than that which would exist in its absence (partial vacuum); the most favorable conditions being that the bonding layer 20 has a refractive index n2 equal to or greater than the refractive index neither of the luminescent thin film 14 (nor of the order of 2), the index ni being itself even greater than the index n of the substrate (n of the order of 1.45).
• the bonding layer 20 is preferably as transparent as possible to the light produced by the luminophore grains L1 to Ln.
• this bonding layer 20 can for example be titanium oxide TiO 2 (whose refractive index n 2 is of the order of 2.35); this bonding layer 20 of titanium oxide being obtained for example by a method of dipping alcoholate from a titanium alcoholate Ti (OC 2 H 5 ) 4 .
• the substrate 12 may be made of a material other than glass.
• the substrate 12 can be a YAG carrying a thin epitaxial layer YAG, doped, which forms the luminescent thin film 14; and on the other hand, the grains L1 to Ln of the diffusing layer 15 can be grains YAG possibly doped to constitute phosphors.

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• Cathode-Ray Tubes And Fluorescent Screens For Display (AREA)

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ID=9381917

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• 1989
  • 1989-05-23 FR FR8906719A patent/FR2647592B1/fr not_active Expired - Lifetime
• 1990
  • 1990-05-18 WO PCT/FR1990/000354 patent/WO1990014680A1/fr not_active Application Discontinuation
  • 1990-05-18 EP EP19900908540 patent/EP0427842A1/de not_active Withdrawn

Non-Patent Citations (1)

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