Cathodoluminescent light sources and electric lighting arrangements including such sources

A cathodoluminescent lamp for use for general lighting service includes an anode constituted by an electrically conducting coating, which may be partly internally reflective, on the interior surface of the bulb wall, a phosphor coating over the whole of the bulb wall interior, a dome-shaped metal mesh grid located near the junction of the bulb with the envelope neck and supported on a hollow metal cylinder, and an electron emissive cathode mounted within the grid/cylinder assembly. The cathode may be a "wreath" filament or indirectly heated disc located near the grid, or a linear filament located near the open end of the cylinder remote from the grid. In the latter case a metal disc, connected to the negative lead to the cathode, is located near the open end of the cylinder, to repel electrons emerging therefrom. A circuit, which may be incorporated in the lampholder, converts the supply to unidirectional operating voltages applied to the anode, grid and cathode.

This invention relates to cathodoluminescent light sources and to electric 
lighting arrangements including such sources. 
It is an object of the invention to provide a cathodoluminescent light 
source in the form of a lamp which is suitable for use for general 
lighting purposes. 
According to the invention a cathodoluminescent light source consists of an 
electric 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 grid located within 
the bulb adjacent to the junction between the bulb and neck of the 
envelope, 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, which cathode is so 
shaped, and/or so located within the said grid assembly, that the 
electrons emitted from the cathode in operation of the lamp are 
substantially uniformly distributed over the whole of the surface of the 
grid on which they impinge, and electrically conducting leads connecting 
said anode, grid and cathode to respective contacts, the grid and cathode 
contacts being constituted by or carried by the said cap and the anode 
contact being located on the exterior of the envelope neck, which contacts 
are adapted to be connected to a circuit arrangement for operation of the 
lamp from a source of electric current supply. 
The anode preferably covers the whole of the interior surface of the bulb 
wall and may consist of a known type of light transmissive electrically 
conducting coating formed, for example, of tin oxide and/or indium oxide. 
Alternatively, part of the bulb may be provided with an internally 
reflective metal coating which also serves as at least part of the anode, 
the remainder of the bulb either being free from any conductive coating 
or, preferably, having a light transmissive conducting coating to provide 
continuity of the anode over the whole of the bulb wall. 
The presence of the dome-shaped grid, together with the provision, within 
the grid assembly, of an electron emissive cathode so shaped and/or 
located that the elctrons emitted from the cathode in operation of the 
lamp are substantially uniformly distributed over the surface of the grid 
on which they impinge, promotes substantially uniform distribution of the 
bombarding electrons over the whole of the phosphor-coated bulb surface, 
thus ensuring substantially uniform light output from the whole of the 
light transmissive surface area of the bulb. The grid is preferably formed 
of metal mesh, for example of nickel or stainless steel. The cylindrical 
grid support member may be formed of any gas-free metal conventionally 
used for a similar purpose in, for example, thermionic valves or cathode 
ray tubes, such as a nickel-iron alloy; however, the cylinder is 
advantageously formed of a metal having gettering properties, such as 
titanium, tantalum, or zirconium. If desired, a second metal cylinder may 
be mounted concentrically around the grid-supporting cylinder, and this 
second cylinder may also support a second grid, for example in the form of 
a second metal mesh dome, or a plate with a single aperture, such plate 
being located within the mesh grid. Such a double cylinder or double grid 
arrangement may be required, in some cases, for modifying the electron 
distribution and the electrical characteristics of the lamp. 
In a first form of lamp in accordance with the invention, the cathode is of 
effectively circular form and is located coaxially within the grid 
assembly so that the electron-emissive part thereof lies in the region of 
the junction between the grid and its supporting cylinder. The term 
"effectively circular" is to be understood to mean that the cathode is so 
shaped and arranged that, in plan view as observed from the center of the 
bulb interior, it has the appearance of a circle or a nearly closed 
circle. The cathode may be of the directly heated type, preferably of 
filamentary form, consisting of a single coil or coiled coil, of 
refractory metal wire, incorporating electron emissive material; the 
filament is of "wreath" form, that is to say the coil is effectively 
circular, as defined above, forming a nearly closed circle: this effect 
can be achieved by a circular or zig-zag form of mounting, the zig-zag 
arrangement enabling a greater length of coil to be used. Alternatively, 
the cathode may be of the indirectly heated type, suitably consisting of a 
hollow metal cylinder capped with a metal disc which is coated externally 
with electron emissive material, with a heating coil located within and 
insulated from the cylinder. 
The provision of an effectively circular cathode, located adjacent to the 
grid, as described above, ensures that the electrons emitted from the 
cathode in operation of the lamp are substantially uniformly distributed 
over the surface of the grid. However, in some cases it is desirable to 
employ a filamentary cathode in the form of a linear coil of wire, in 
particular because linear coil filaments are more readily mass produced by 
automatic means than "wreath" filaments. 
In a second form of lamp in accordance with the invention, therefore, the 
cathode consists of a linear wire coil filament incorporating electron 
emissive material, the coil being disposed orthogonally to the 
longitudinal axis of the assembly of the grid and its supporting cylinder, 
and the cathode is located within the grid supporting cylinder in a 
position nearer to the open end of the cylinder remote from the grid than 
to the end thereof on which the grid is supported. The cathode may be a 
single coil or coiled coil of refractory metal wire, and is supported on 
two or more wires so that it lies straight. 
As a result of the positioning of the linear cathode within the grid 
support cylinder, a large proportion of the electrons emitted by the 
filament in operation of the lamp are initially attracted by the cylinder, 
which is maintained at a positive potential. The electrons are thus 
diffused in all directions around the filament and therefore, on impinging 
on the grid, are distributed substantially uniformly over the interior 
surface thereof. 
Since the linear cathode is located relatively near to the open end of the 
grid supporting cylinder remote from the grid, there is a tendency for 
some of the emitted electrons to flow out from the said open end of the 
cylinder and to be attracted to the glass neck of the envelope, which is 
positively charged in operation, thus heating the neck and possibly 
causing it to crack. In order to prevent the occurrence of this effect, it 
is preferred, in an additional feature of the said second form of the lamp 
of the invention, to provide a metal disc of substantially the same 
diameter as the cylinder, located close to, and either inside or outside, 
but out of contact with, the said open end of the cylinder and disposed 
coaxially with the cylinder, and connected to that lead to the cathode to 
which a negative potential will be applied in operation, the disc having a 
central aperture permitting the passage of the cathode leads therethrough. 
With this arrangement, a negative potential will be applied to the disc in 
operation, which potential will be of sufficient magnitude to repel 
electrons and thus prevent them from flowing out from the open end of the 
cylinder. 
In either of the above-described forms of lamp, the wires supporting a 
filamentary cathode may be sealed into or through a glass bead in 
conventional manner. The cathode assembly so formed, or a cylindrical 
indirectly heated cathode, and the grid cylinder or cylinders may be 
supported on wires sealed into the closure of the neck of the lamp 
envelope, which closure is suitably a pinched foot tube of the form 
conventionally used in the manufacture of incandescent lamps, and current 
conducting leads from contacts on the lamp cap to the cathode and grid may 
be constituted by, or may incorporate, such support wires. A metal disc 
incorporated in the aforesaid second form of lamp, for repelling electrons 
emerging from the open end of the grid supporting cylinder, may also be 
supported by a wire sealed into the neck closure. The lead to the anode is 
located along the exterior of the envelope neck, being connected to the 
anode by an end of the lead wire sealed through the envelope wall, and 
said lead extending to a contact provided on the exterior of the neck. 
We have found that glasses of the type generally used for incandescent lamp 
envelopes, such as conventional soda-lime glasses, may be unsuitable for 
use for the envelope of a lamp in accordance with the invention, in 
particular because such glasses permit the transmission of X-rays 
generated by the electron bombardment of the envelope. The envelope is 
therefore preferably formed of an X-ray inhibiting glass, for example a 
lead glass. 
The invention further provides an electric lighting arrangement consisting 
of a lamp of either of the forms 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 unidirectional operating potentials of required magnitudes for 
application respectively to the anode, cathode and grid of the lamp. 
For operation of the lamp, a high potential, suitably in the range of 5 to 
15 kilovolts, is required to be applied to the anode: the circuit 
arrangement, for operation from a standard mains supply, therefore 
includes step-up means, suitably comprising a high frequency oscillator, a 
transformer, and a voltage multiplying system. The circuit will also 
include current limiting means for applying a low voltage, usually between 
6 and 40 volts, which is negative with respect to the grid potential, 
across the cathode, and usually also means for reducing the supply voltage 
for the application of a suitable potential to the grid. The grid 
potential may be from 40 to 250 volts, the magnitude of the grid potential 
required depending upon the configuration of the cathode-grid assembly: 
thus the greater the spacing apart of the cathode and grid, the greater is 
the grid potential required. If, as is usual, the lamp is to be operated 
from an alternating current supply, rectification will of course be 
required, and where an oscillator is employed as part of the step-up 
arrangement for providing the anode potential, further rectifying means 
will be required to be inserted between the transformer and the connection 
to the anode. 
Miniaturisation of electronic circuitry makes it possible to provide a 
compact package consisting of the lamp and its operating circuit 
arrangement. Thus the circuit arrangement may be contained within a 
housing which is detachably mounted on the lamp cap and which is 
insertable into a lamp-holder, the circuit-housing assembly incorporating 
contacts arranged to co-operate with the contacts on the lamp cap and/or 
neck, and the housing also carrying external contacts for connecting the 
circuit arrangement to the supply by co-operation with contacts in a 
lampholder. Alternatively the circuit arrangement may be incorporated in 
or associated with a lampholder which includes a socket having contacts 
arranged to co-operate with the contacts on the lamp for connection of the 
circuit thereto. 
A lamp in accordance with the invention has the additional advantage that 
it can be of a robust and relatively simple construction which is not 
critical dimensionally.

Like parts in FIGS. 1, 2 and 3 are indicated by the same reference 
numerals. 
The lamp shown in FIG. 1 comprises an evacuated envelope 1 formed of lead 
glass, consisting of a bulb 2 of substantially paraboloid shape, with a 
slightly curved front face 3, and an integral neck 4 in which an electron 
gun assembly is mounted, and which is closed in conventional manner by a 
pinched glass foot tube 5. The paraboloid rear portion 2 of the bulb wall 
is internally coated with aluminium, 6, and the front face 3 is internally 
coated with a transparent film 7 of conducting material, suitably tin 
oxide and/or indium oxide: these coatings together constitute the anode, 
the aluminium coating also serving as a reflector, and are overlaid by a 
layer of phosphor 8, which may be of any known electron-responsive type. 
The thicknesses of the coatings are exaggerated in the drawing. 
The electron gun assembly includes a cathode and a grid. The cathode is in 
the form of a wreath type filament 9, composed of a single coil of 
tungsten wire activated with one or more of the oxides of barium, 
strontium and calcium, mounted in zig-zag manner on five support wires 
sealed into a glass bead 10, the wires 11 to which the ends of the coil 
are attached being extended through the glass bead and joined to nickel 
wires 12 which are sealed through the foot tube 5 and which constitute 
supports for, and conducting leads to, the cathode. The grid consists of a 
dome-shaped structure 13 of nickel wire mesh, supported on a titanium 
cylinder 14 which is carried by a nickel bracket 15 attached to a nickel 
lead wire 16 sealed through the foot tube. Barium/aluminium getter rings 
17 are supported by wires 18 attached to the lead wires to the cathode and 
grid. 
The closed end of the bulb neck is cemented into a brass cap 19, in known 
manner. The cap carries contacts 20, insulated from the brass, to which 
the cathode leads 12 are connected, and the grid lead 16 is connected to 
the cap itself, which thus constitutes the grid contact. The cap is 
provided with locating pins 21, preferably three in number to ensure 
correct orientation of the lamp in a lampholder for connection of the lamp 
contacts to the operating circuit. 
Connection to the anode 6, 7 is made by means of a wire 22 of metal having 
a suitable thermal expansion match to the envelope glass, which wire is 
sealed through the envelope at the junction of the neck and the aluminised 
portion of the bulb, and a coating of carbon 23 is applied to the interior 
of the neck-bulb junction region, covering the internal end of the wire 22 
and part of the aluminium coating, to ensure reliability of the anode 
connection. The wire 22 extends along the exterior of the neck as shown at 
24, and is covered by a strip coating of carbon 25 which constitutes the 
anode contact. 
The envelope of the general service lamp shown in FIG. 2 consists of an 
evacuated bulb 26 of oblate spheroid shape with an integral neck 27, 
formed of lead glass, the neck being closed by a pinched glass foot tube 5 
and surmounted by a brass cap 19. The whole of the internal surface of the 
bulb has a transparent film coating 28 of conducting material forming the 
anode, covered with a layer of electron-responsive phosphor 29. If 
desired, the phosphor may be coated with an aluminium film, in known 
manner, to enhance the light output of the lamp. The arrangement of 
cathode, grid, and conducting leads and contacts for the anode, cathode 
and grid is similar to that employed in the reflector lamp described above 
with reference to FIG. 1, and getters and locating pins as described with 
reference to FIG. 1 are also provided. 
The lamp shown in FIG. 3 is similar to that shown in FIG. 2 in respect of 
its outward form, consisting of an evacuated oblate spheroid lead glass 
bulb 26 and integral neck 27, with a pinched glass foot tube closure 5 and 
a brass cap 19, the whole of the internal surface of the bulb having an 
anode coating consisting of a film 28 of transparent conducting material, 
overlaid by a layer of electron-responsive phosphor 29. The electron gun 
assembly, mounted within the neck 27, includes a dome-shaped grid 13 of 
nickel wire mesh, supported on a titanium cylinder 14, a cathode 30 in the 
form of a straight single coil of tungsten wire incorporating electron 
emissive material, which is located in the lower part of the cylinder 14, 
and a titanium disc 31 which has a central aperture 32 and which is 
located immediately below the open lower end of the cylinder 14. 
The cathode coil 30 is mounted on four support wires 33 sealed into a glass 
bead 34, the wires to which the ends of the coil are attached being 
extended through the aperture 32 in the disc 31 and joined to nickel wires 
35 which are sealed through the foot tube 5 and which constitute supports 
for, and conducting leads to, the cathode, being connected to contacts 20 
carried by and insulated from the lamp cap 19. The disc 31 is supported by 
a nickel bracket 36 and nickel wire 37, the latter also being sealed into 
the foot tube; the wire 37 is connected to the negative lead to the 
cathode, as shown at 38. 
The conducting leads and contacts for the grid and the anode, and the 
getters and locating pins, incorporated in the lamp of FIG. 3, are similar 
to those described above with reference to FIG. 1. 
An electron gun assembly of the form described with reference to FIG. 3 may 
also be incorporated in a reflector lamp which in other respects is of the 
form shown in FIG. 1. 
For operation, a lamp of any of the forms shown in FIGS. 1, 2 and 3 is 
inserted into a lampholder, or a housing insertable into a lampholder, in 
which a circuit arrangement for operating the lamp from an electric 
current supply is mounted, and which includes a socket formed of 
insulating material extending along substantially the whole length of the 
lamp neck, so as to cover the anode lead wire 22 and the carbon strip 25 
on the exterior of the neck. The socket carries internal contacts arranged 
to co-operate with the said carbon strip, the contacts 20 and the cap 19, 
for connection of the anode, cathode and grid respectively to the 
operating circuit. 
FIG. 4 shows, by way of example only, one form of circuit arrangement which 
can be employed for operating a lamp of any of the forms described above 
with reference to FIGS. 1, 2 and 3, from a 240 volts alternating current 
supply. This circuit arrangement is made up of conventional components, 
and includes essentially a full wave rectifier 39, connected across the 
supply terminals 40 and 41, a step-up transformer 42 and voltage 
multiplying arrangement 43 for applying the requisite potential in the 
range of 5 to 15 kilovolts to the lamp anode 44, and subsidiary circuits 
for applying suitable potentials to the lamp cathode and grid. 
A current limiting arrangement is provided for reducing the current from 
the rectifier 39 to the lamp cathode 45, this arrangement including a 
smoothing condenser 46 and a zener diode 47 for maintaining the cathode 
potential constant, and negative with respect to the grid potential, 
suitably at 10 volts. The connection to the lamp grid 48 is in a line in 
parallel with the cathode circuit; the grid potential is reduced, suitably 
to 100 volts in the case of a lamp of either of the forms shown in FIGS. 1 
and 2 or to 150 volts in the case of a lamp of the form shown in FIG. 3, 
by resistors 49 and 50, and is smoothed by condenser 51. 
The arrangement for supplying the anode potential includes, in addition to 
the transformer and the voltage multiplier, an integrated oscillator 
circuit 52, with means including diode 53 and condenser 54 for applying a 
suitable d.c. voltage thereto; the oscillations produced, at a suitable 
frequency of about 10 kilohertz, are transmitted to the transformer 42 via 
the transistor 55. A slow start arrangement, including condenser 56 and 
resistor 57, is associated with the oscillator for preventing the 
application of the high potential to the anode before the cathode has 
attained its operating temperature. Feed-back to the oscillator from the 
voltage multiplier is provided via resistor 58. The use of the oscillator 
in this manner enables a transformer of small size to be employed for 
obtaining the desired step-up ratio. 
This circuit arrangement provides a safety factor in the operation of the 
lamp, in that the cathode is so connected that if the cathode circuit is 
broken there is no current supply to the oscillator, and hence no high 
potential can be applied to the anode.