GB2097181A - Cathodoluminescent lamps - Google Patents

Abstract

In a cathodoluminescent lamp of the form described in Application No. 8102935, which includes (Fig. 1) a conductive anode coating 5 on the interior of the bulb wall 1, a phosphor coating 6 overlying the anode coating, a dome shaped grid 8, preferably of metal wire mesh supported on a hollow metal cylinder 9 disposed coaxially within the envelope neck, and a cathode 10 located within the grid/cylinder assembly, the cathode 10 consists (Fig. 2) of a directly heated body of metal mesh or perforated metal foil, located coaxially within the grid- supporting cylinder, supported on wires constituting the cathode leads, and having a coating 11 of electron emissive material on a surface 12 of the mesh or foil facing towards the grid. Preferably the mesh or foil is in the form of a U- shaped strip, mounted with the base of the U, which carries the emissive coating, spaced one or two millimeters from the base of the grid. The phosphor coating 6 may be covered with a reflecting layer 7 of metal or white particulate material. <IMAGE>

Description

SPECIFICATION Cathodoluminescent light sources and electric lighting arrangements including such sources This invention relates to cathodoluminescent light sources in the form of electric lamps which are suitable for general lighting purposes, and to electric lighting arrangements including such lamps.

In the specification of our co-pending Patent Application No. 8102935 there is described a cathodoluminescent lamp which includes an anode constituted by an electrically conductive coating on the interior surface of the bulb wall, at least part of which wall is light transmissive, a phosphor on the whole of the bulb wall interior, a dome-shaped grid located within the bulb and supported on a hollow metal cylinder disposed within the envelope neck, and an electron emissive cathode mounted within the assembly of the grid and its supporting cylinder.The assembly including the grid and cylinder, the cathode, and conducting leads to the grid and cathode, constitutes an electron gun from which, in operation, electrons are emitted to bombard the phosphor coating on the bulb wall, resulting in the emission of light from the bulb wall, or from the light-transmissive part thereof.

In one form of lamp described in the abovementioned specification, the cathode is of effectively circular form, consisting either of a directly heated coil filament of "wreath" form incorporating electron emissive material, or of an indirectly heated hollow metal cylinder capped with a metal disc coated externally with electron emissive material, in either case the electron emissive part of the cathode being located in the region of the junction between the grid and its supporting cylinder.

In another form of lamp described in the aforesaid specification, the cathode consists of a directly heated linear coil filament located within the grid supporting cylinder in a position relatively remote from the grid.

It is an object of the present invention to provide a cathodoluminescent lamp of similar construction to the lamps described in Application No. 8102935, but incorporating an improved form of directly heated cathode.

According to the invention, in a cathodoluminescent lamp which includes an evacuated bulbous glass envelope with an integral glass neck terminated by a cap, an anode constiuted by an electrically conductive coating on at least part of the interior surface of the bulb wall, such that at least part of the bulb wall is light-transmissive, a layer of phosphor on the whole of the interior of the bulb wall and overlying the said anode coating, which phosphor is excitable to luminescence by electron bombardment, a dome-shaped metal grid lo cated within the bulb and supported by a hollow metal cylinder disposed coaxially within the envelope neck, an electron emissive cathode mounted witin the assembly of the grid and its supporting cylinder, and electrically conducting leads connecting said anode, grid and cathode to respective contacts carried by the said cap, which contacts are adapted to be connected to a circuit arrangement for operation of the lamp from a source of electric current supply, the said cathode consists of a directly heated body formed of metal mesh or perforated metal foil, located coaxially within the grid-supporting cylinder, supported on wires constituting the cathode leads, and having a coating of electron emisive material on a surface of the mesh or foil facing towards the grid.

The cathode is preferably in the form of a strip of metal wire mesh or perforated foil bent into a U shape, and mounted with the base of the U, which carries the electron emissive coating, facing towards the grid, the lateral arms of the U extending in the direction away from the grid and being attached to the lead wires: preferably the electron emissive coating is applied only on the base of the U, but in some cases a small amount of electron emissive material may be present on the arms of the U.

The base of the U, forming the cathode face, may be flat of slightly curved towards the grid, and is preferably located a short distance, for example one or two millimetres, from the plane of the base of the grid. If desired, the back of the electron emissive portion of the cathode (that is to say the surface of this portion opposite to the cathode face) may be coated with insulating material, for example alumina applied as a suspension of alumina powder, to prevent backwards emission of electrons through the mesh or perforated foil and away from the grid. Furthermore, if desired, the sides of the cathode face at right angles to the U-arms may be extended to include portions which are bent so as to form a shallow box with the U-arms, to increase the rigidity of the cathode face.

Preferably a baffle plate, connected to one of the cathode leads, is provided within the grid supporting cylinder, in a position between the cathode and the open end of the cylinder (that is to say the end of the cylinder remote from the grid), to repel any electrons emitted by the cathode which might otherwise emerge from the open end of the cylinder.

The baffle is in the form of a metal disc with a central aperture permitting the passage of the cathode leads therethrough. A portion of the interior surface of the grid cylinder adjacent to the disc is covered with a coating or sleeve of suitable insulating material, to insulate the cylinder from the disc, the disc fitting closely within the insulated cylinder surface.

The envelope neck is suitably closed by means of a pinched glass foot tube of conventional form, incorporating an exhaust tube, and wires constituting supports for, and leads to, the cathode and grid are sealed through the foot tube, also in conventional manner.

The baffle disc, when present, is also supported by a wire sealed into the foot tube. The lead to the anode coating on the bulb wall preferably consists of a wire which lies mainly along or close to the interior surface of the envelope neck, and passes through the exhaust tube to a contact carried by the cap, as described in the specification of our co-pending Patent Application No. 8040346.

The anode coating preferably covers the whole of the interior surface of the bulb wall and may consist of a known type of light transmissive electrically conductive coating formed, for example, of tin oxide and/or indium oxide. Alternatively, part of the bulb wall may be provided with an internal reflective metal coating which also serves as at least part of the anode, the remainder of the bulb wall either being free from any conductive coating or, preferably having a light transmissive conductive coating to provide continuity of the anode over the whole of the bulb wall.

In a lamp in which the anode consists of a light transmissive conductive coating covering the whole of the interior surface of the bulb wall, the phosphor layer overlying the anode coating may be covered with a reflecting coating, which may consist of a film of metal such as aluminium, or any highly reflective white substance which can be produced in the form of a smoke and deposited as very fine particles on the phosphor layer, for example titanium dioxide or magnesium oxide. Such a coating, which does not impede the bombardment of the phosphor layer by electrons in operation of the lamp. reflects any of the light emitted by the phosphor which would other wise be directed into the bulb, thus ensuring that all the light emitted by the phosphor is transmitted through the bulb wall, and hence improving the light output of the lamp.

The invention further provides an electric lighting arrangement consisting of a lamp of the form described above, and a circuit arrangement for operating said lamp from a source of electric current supply, which circuit arrangement includes means for converting the supply voltage to undirectional operating potentials of required magnitudes for application respectively to the anode, cathode and grid of the lamp. Suitable operating potentials are 5 to 1 5 kilovolts applied to the anode, 40 to 250 volts applied to the grid, and a cathode potential lower than that applied to the grid.

The circuit arrangement may be contained within a housing which is detachably mounted on the lamp cap and is insertable into a lampholder, or may be incorporated in a lampholder, the housing or the lampholder being provided with contacts arranged to cooperate with the contacts on the lamp cap.

A specific form of lamp in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which, Figure 1 shows the lamp in part-sectional elevation, including a portion of the bulb, and Figure 2 is an enlarged perspective view of the cathode incorporated in the lamp of Fig.

1.

The lamp shown in Fig. 1, which is designed for general lighting service comprises an evacuated envelope formed of any suitable glass which will inhibit the transmission of Xrays generated by the electron bombardment of the envelope, and consisting of a bulb 1, suitably of oblate spheroid shape, and an integral neck 2 in which an electron gun assembly is mounted, and which is closed by a pinched glass foot tube 3 and surmounted by a brass cap 4. The whole of the interior surface of the bulb 1 is coated with a transparent film 5 of electrically conducting material such as tin oxide and/or indium oxide, and this film is overlaid by a layer of electronresponsive phosphor 6.The phosphor layer is coated with a reflecting coating 7, suitably a film of alumium or a thin layer of fine particulate titanium dioxide, the latter being deposited from a titanium dioxide smoke produced, in known manner, by hydrolysis of titanium tetrachloride vapour by bubbling the vapour through water.

The electron gun assembly comprises a dome-shaped grid 8 formed of metal wire mesh, supported on a hollow metal cylinder 9, a cathode 10 in the form of a strip of fine metal mesh or perforated foil bent into the form of a U with a flat base and having a coating 11 of electron emissive material on the surface 1 2 (Fig. 2) of the U-base facing the grid, the cathode being positioned within the cylinder 9 a short distance from the grid, and a metal disc 1 3 with a central aperture 14, located within the cylinder 9 between the cathode and the open end of the cylinder 9 and insulated from the cylinder by a sleeve 1 5 of insulating material which is fitted inside the cylinder and within which the disc 1 3 fits closely. The cylinder 9 and disc 1 3 are preferably formed of titanium, but may be of other metal such as stainless steel or nickel. The cathode may be formed of stainless steel, nickel, or tungsten or other refractory metal, and the coating 11 may consist of any known suitable electron emissive material such as, for example, one or more of the oxides of calcium, strontium, and barium. The sleeve 1 5 is suitably composed of lead glass, silica or alumina.

The cathode 10 is mounted on two nickel lead wires 1 6 which pass through the aperture 14 in the disc 1 3 and are sealed through the foot tube 3 and connected to contacts 1 7 carried by the insulating closure of the cap 4.

The cylinder 9 is supported by a nickel wire 1 8 which constitutes the lead to the grid and is sealed through the foot tube and connected at 1 9 to the cap 4 which thus constitutes the grid contact. The disc 1 3 is supported by a nickel wire 20 which is sealed into the foot tube and is connected to one of the cathode leads at 21.

The anode coating 5 is connected to a pin contact 22 carried by the cap insulation, the connection being made by means of a carbon coating 23 applied to the interior surface of the neck-bulb junction region of the envelope so as to cover the edge region of the anode coating, and a 48% nickel-52% iron lead wire 24 which is attached to the carbon coating and lies close to the interior surface of the neck 2 to a point beyond the open end of the grid supporting cylinder 9, then passes through the exhaust tube 25 in the foot tube 3. The anode lead is suitably attached to the carbon coating 23 by a silver contact 26, formed by applying a drop of silver paint over the end of the lead in contact with the carbon, the paint being hardened by heating during subsequent processing of the lamp.The anode lead is partially insulated by a glass sleeve 27, and is sealed into the exhaust tube by heating after evacuation of the lamp.

Barium/aluminium getter rings 28, of known form, are supported within the envelope neck by wires 29 attached relatively respectively to the wire supports for the grid cylinder 9 and the disc 1 3. The cap 4 is provided with locating pins 30, preferably three in number to ensure correct orientation of the lamp in a lampholder or housing for connection of the respective contacts to the operating circuit.

The lampholder, or housing insertable into a lampholder, into which the lamp is inserted for operation, contains a circuit arrangement for operating the lamp from an electric current supply, and carries internal contacts arranged to co-operate with the contacts 17, 1 9 and 22 on the lamp cap 4, for connection of the cathode, grid and anode respectively to the operating circuit. The circuit arrangement employed may be substantially of the form described in the specification of Application No.8102935 with reference to Fig. 4 of the drawings accompanying that specification.

In a specific example of a lamp of the form described above with reference to the accompanying drawings, the cathode 10 is formed of stainless steel wire mesh having 400 wires, of diameter 0.036 mm, to the linear inch; the mesh strip is 2 mm wide, the activated surface 1 2 being 2 mm square and the lateral arms 31 (Fig. 2) being 4 mm long. The grid 8 is formed of stainless steel wire mesh having 100 to 300 wires to the linear inch, and the grid supporting cylinder 8 is 20 mm in diameter. The surface 1 2 of the cathode is located approximately 2 mm from the plane of the base 32 of the grid. In operation of the lamp, the cathode is operated from an earthed single turn overwind on the transformer included in the operating circuit, and the electrical characteristics of the cathode, for a suitable emission temperature, thus obtained are 0.9 volt at 1.3 amps.

A cathode of the form incorporated in the lamp in accordance with the invention is advantageous in that it is easily assembled with accuracy. Moreover substantially all the electron emission from a cathode of this form is directed forwards, that is to say towards the grid, as in the case of an indirectiy heated cathode of the form referred to in Application No. 8102935, giving substantially uniform, and concentrated, electron distribution over the surface of the lamp bulb and hence substantially uniform distribution from the bulb.

Claims (11)

1. A cathodoluminescent lamp which includes an evacuated bulbous glass envelope with an integral glass neck terminated by a cap, an anode constituted by an electrically conductive coating on at least part of the interior surface of the bulb wall, such that at least part of the bulb wall is light-transmissive, a layer of phosphor on the whole of the interior of the bulb wall and overlying the said anode coating, which phosphor is excitable to luminescence by electron bombardment, a dome-shaped metal grid located within the bulb and supported by a hollow metal cylinder disposed coaxially within the envelope neck, an electron emissive cathode mounted within the assembly of the grid and its supporting cylinder, and electrically conducting leads connecting said anode, grid and cathode to respective contacts carried by the said cap, which contacts are adapted to be connected to a circuit arrangement for operation of the lamp from a source of electric current supply, wherein the said cathode consists of a directly heated body formed of metal mesh or perforated metal foil, located coaxially within the grid-supporting cylinder, supported on wires constituting the cathode leads, and having a coating of electron emissive material on a surface of the mesh or foil facing towards the grid.

2. A lamp according to Claim 1 wherein the cathode is in the form of a strip of said mesh or foil bent into a U shape and mounted with the base of the U, which carries the electron emissive coating, facing towards the grid, and the lateral arms, of the U extending in the direction away from the grid and being attached to the lead wires.

3. A lamp according to Claim 2, wherein the cathode face, formed by the base of the U, is located one to two millimetres from the plane of the base of the grid.

4. A lamp according to Claim 2 or 3, wherein the sides of the U base at right angles to the lateral arms of the U are extended to include portions which are bent so as to form a box with the said arms.

5. A lamp according to Claim 1, 2, 3 or 4, wherein the back surface of the portion of the cathode carrying the electron emissive material is coated with insulating material.

6. A lamp according to any preceding Claim, wherein a baffle connected to one of the cathode leads is located within the gridsupporting cylinder, in a position between the cathode and the open end of the cylinder remote from the grid, said baffle being in the form of a metal disc with a central aperture permitting the passage of the cathode leads therethrough.

7. A lamp according to Claim 6, wherein a portion of the interior surface of the gridsupporting cylinder adjacent to the said baffle is covered with a coating or sleeve of insulating material.

8. A lamp according to any preceding Claim, wherein the anode consists of a light transmissive conducting coating covering the whole of the interior surface of the bulb wall, and the phsophor layer overlying the said coating is covered with a reflecting coating.

9. A cathodoluminescent lamp according to Claim 1, substantially as shown in, and as hereinbefore described with reference to, the accompanying drawings.

10. An electric lighting arrangement consisting of, in combination, a lamp according to any preceding Claim, and a circuit arrangement for operating said lamp from a source of electric current supply, which circuit arrangement includes means for converting the supply voltage to unidirectional operating potentials of required magnitudes for application respectively to the anode, cathode and grid of the lamp.

11. An electric lighting arrangement according to Claim 10, wherein the said circuit arrangement is designed to apply operating potentials of, respectively, 5 to 1 5 kilovolts to the anode. 40 to 250 volts to the grid and, to the cathode, a potential lower than that applied to the grid.