Non-uniform resistance cathode beam mode fluorescent lamp

The lamp shown herein is a beam mode fluorescent lamp for general lighting applications. The lamp comprises a light transmitting envelope, having a phosphor coating on its inner surface, enclosing a thermionic cathode for emitting electrons and an anode for accelerating the electrons and forming an electron beam, and a fill material, such as mercury, which emits ultraviolet radiation upon excitation. The cathode configuration provides for the elimination of "hot spots" due to ion bombardment at the low potential end of the cathode and for higher overall cathode emission of electrons. Various methods are employed to accomplish these ends, such as: segmenting the cathode, pitch variation of the cathode winding; ion probes and a non-uniform primary coil wound around a larger mandrel wire.

CROSS REFERENCE TO RELATED APPLICATIONS 
The present invention is an improvement to copending U.S. patent 
application Ser. No. 219,564, filed on Dec. 23, 1980, now abandoned for a 
"Beam Mode Fluorescent Lamp", assigned to the same assignee. 
BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention pertains to beam mode discharge fluorescent lamps and 
more particularly to an arrangement for configuring the cathode within a 
beam mode discharge fluorescent lamp. 
(2) Description of the Prior Art 
U.S. patent application Ser. No. 219,564, filed on Dec. 23, 1980, now 
abandoned for a "Beam Mode Fluorescent Lamp", and assigned to the same 
assignee as the present invention, discloses a particular embodiment of a 
fluorescent lamp suitable for replacing the conventional incandescent 
bulb. Although incandescent lamps are inexpensive and convenient to use, 
they are considerably less efficient than fluorescent lamps. 
In the above mentioned patent application, a single anode and cathode 
configuration is shown. A discharge is formed between the electrodes and 
electrons are emitted. Ions in the cathode potential drop region are 
accelerated by the field and bombard the cathode. This ion bombardment is 
not uniform and concentrates at the most negative end of the cathode. This 
leads to severe localized heating of the cathode with an elevated primary 
electron emission. The localized heating produces evaporation of the 
cathode coating with a resultant shortening of cathode life and darkening 
of the phosphor coating and increased chance of discharge "runaway" and 
cathode "burn-out". 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a beam 
mode fluorescent lamp in which the ion emission along the length of the 
cathode is uniform. 
It is another object of the present invention to provide a non-uniform 
resistance cathode beam mode fluorescent lamp with a longer cathode life. 
It is yet another object of the present invention to provide a non-uniform 
resistance cathode beam mode fluorescent lamp, which substantially 
eliminates darkening of the phosphor coating of the envelope. 
The subject beam mode fluorescent lamp includes a light transmitting 
envelope enclosing a fill material, which emits ultraviolet radiation upon 
excitation. A phosphor coating on an inner surface of the envelope emits 
visible light upon absorption of ultraviolet radiation. 
A thermionic cathode arrangement for emitting electrons is located within 
the envelope. The cathode arrangement is connected to a DC power source by 
two conductors, one conductor connected to each end of the cathode. These 
same conductors also serve to support the cathode at a stationary location 
within the envelope. 
An anode is connected to the positive end of the DC power source. The anode 
extends over and parallel to the cathode. This anode accelerates electrons 
emitted by the cathode to form an electron beam. The anode is constructed 
of a simple round wire segment. The anode is spaced apart from the cathode 
by a distance which is less than the electron range in the fill material. 
The structure of the anode permits acceleration of the corresponding 
electron beam with minimum collection of primary electrons due to the 
anode. 
The fluorescent lamp includes a corresponding drift region within the 
envelope through which the electron beam drifts after passing through the 
anode. Electrons in the electron beam collide with atoms of the fill 
material in a drift region, thereby causing excitation of a portion of the 
film material atoms and emission of ultraviolet radiation and causing 
ionization of another portion of the fill material atoms and thereby 
producing secondary electrons. These secondary electrons cause further 
emissions of ultraviolet radiation. The fill material typically includes 
mercury and a noble gas, such as neon. 
A potential drop exists between the anode and all points along the cathode. 
The cathode arrangement is divided into three segments. Two cathode 
segments are connected in parallel at the low potential end of the 
discharge space and a first end of this parallel connection is connected 
to the negative conductor. A third cathode segment is connected between 
another conductor, which is connected to ground, and the second end of the 
parallel connection of the first and second cathode segments. 
This arrangement allows the third cathode segment to assume a higher 
temperature due to ohmic heating than the first and second cathode 
segments. The area where ion bombardment takes place is expanded. As a 
result, a relatively uniform temperature distribution and ele-tron 
emission is achieved along the length of the cathodes. In addition, 
cathode life is prolonged and darkening of the coating phosphor is 
inhibited. This arrangement applies equally well to two terminal or single 
electrode beam mode fluorescent lamps described in its cross referenced 
patent applications. 
An AC version of the present invention is provided by arranging a cathode 
segment between to parallel cathode network segments. Two cathode segments 
are connected in parallel and to the first AC conductor. These two cathode 
segments are further series connected to a single cathode segment. The 
single cathode segment is connected to two other parallel cathode 
segments, which are further connected to a second AC conductor. On 
alternate half cycles of the AC as the low potential of the cathode 
alternates, ion discharge bombardment is regulated by the appropriate 
parallel connection of cathode segments. 
Another alternative for uniform ion discharge is a non-uniformly wound 
cathode. The winding density is greatest at the high potential (negative) 
end of the cathode and decreases uniformly to the low potential end of the 
cathode. This cathode is then immersed in a highly emissive coating and 
binder. 
Another alternative for uniform cathode heating is the use of one or more 
ion collecting probes electrically connected at the low potential end of 
the cathode. These probes are L-shaped and extend parallel to the length 
of the cathode, although not to the full extent of the cathode. The length 
of the probes may be adjusted to control the ratio of ion collection 
between the probe and cathode. 
Another alternative for uniform ion discharge is the use of a uniformly 
wound mandrel wire and a non-uniformly wound primary coil around the 
mandrel wire. This primary coil has a high coil density at the high 
potential end of the cathode with a progressively lower coil density with 
distance from this end. A non-uniform resistance cathode is formed with 
the two coils electrically in parallel.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a segmented cathode non-uniform cathode beam mode 
fluorescent lamp for DC use is shown. A vacuum type lamp envelope 31 made 
of a light emitting substance, such as glass, encloses a discharge volume. 
The discharge volume contains a fill material which emits ultraviolet 
radiation upon excitation. A typical fill material includes mercury and a 
noble gas or mixtures of noble gases. A suitable noble gas is neon. The 
inner surface of the lamp envelope 31 has a phosphor coating 37 which 
emits visible light upon absorption of ultraviolet radiation. Also 
enclosed within the discharge volume by the lamp envelope 31 is an anode 
7, conductors 35 and 36 supporting cathode segments 4, 5 and 6. 
In general, the function of the cathode segments 4, 5 and 6 is to emit 
electrons, while the function of the anode 7 is to accelerate the 
electrons emitted by these cathode segments, while collecting only a 
minimal amount of primary electrons. Anode 7 is L-shaped and extends 
upwardly and parallel to cathode segments 4, 5 and 6. 
Supporting conductors 35 and 36 provide for electrical connection of the 
external DC power supply 40 through the envelope 31 in a vacuum tight 
seal, as well as providing support for the structure of cathode segments 
4, 5 and 6. Conductor 35 connects the negative output and conductor 36 the 
ground output of power supply 40 to the cathode segments 4, 5 and 6. Anode 
7 is connected to the positive output of power supply 40. Alternatively, 
ground and the positive output may be common in which case only two 
conductors are necessary. Cathodes segments 4, 5 and 6 are of a thermonic 
type Cathodes segments 4 and 5 are connected in parallel and have one end 
32 of their parallel connection connected to conductor 35. The other end 
of their parallel connection is connected in series with cathode 6. 
Cathode segment 6 is connected at its other end 33 to conductor 36. The 
ohmic resistance of cathodes segments 4, 5 and 6 is such that their total 
equals the single cathode which they replace with the resistance of 
cathodes segments 4 and 5 being approximately equal. 
When the electrons have passed anode 7, they enter into a drift region 30 
which extends from the anode to the bounds of the enclosing envelope 31. 
The lamp further includes a base 38 which externally is of a conventional 
type suitable for inserting into an incandescent lamp socket. 
When a DC voltage is applied by power supply 40, a potential difference 
exists between anode 7 and all points along cathodes 4, 5 and 6. A 
potential drop also exists between ends 32 and 33 of the cathode 
structure. Since cathodes 4 and 5 are connected in parallel at the lower 
potential end 32 of the discharge, cathode 6 will be at a higher 
temperature due to ohmic heating than cathodes 4 and 5. This heating 
difference results in a relatively uniform temperature distribution and 
uniform electron emission distribution along the length of the cathodes 
between points 32 and 33 as shown by FIG. 3B. FIG. 3A shows the 
non-uniform electron emission distribution expected from a uniformly warm 
cathode. 
Referring to FIG. 1A, an AC arrangement of the present invention is shown. 
Cathodes 11 and 12 are connected in parallel with one end connected to the 
first conductor at point 32. The other end of cathodes 11 and 12 is 
connected in series connected to cathode 13. Cathode 13 is series 
connected to the parallel connection of cathodes 14 and 15. Cathodes 14 
and 15 are connected to the second conductor at point 33. On one-half 
cycle of the AC voltage, point 32 will be negative and cathodes 11 and 12 
will operate to increase the temperature and electron emission of cathode 
13 as similar to the DC operation indicated above. On the alternate half 
cycle of the AC voltage, point 33 will be negative and cathodes 14 and 15 
will operate to raise the temperature and electron emission of cathode 13. 
Thereby during both half cycles, the electron emission is made relatively 
uniform as shown in FIG. 3B. 
FIG. 2A shows another embodiment of the present invention. A non-uniformly 
wound cathode is connected between points 32 and 33. The coil density and 
electrical resistance is greatest at the high potential end 32 of the 
cathode. The winding pitch is then varied while the coil is wound so that 
the winding density and electrical resistance is relatively less at the 
low potential end 33 of the cathode. 
FIG. 2B shows another embodiment of the present invention employing two ion 
collecting probes 24 and 25 connected to the low potential end 32 of the 
cathode. The probes 24 and 25 extend parallel to the cathode and opposite 
to each other. One or more ion probes serve to collect some of the ions in 
order to more uniformly heat the filament, producing more uniform electron 
emission along the cathode from end 32 to end 33. 
FIG. 2C depicts another alternative embodiment of the present invention. 
Primary coil wire 28 is non-uniformly wound around mandrel wire 25. Then 
mandrel wire 25 is uniformly wound. Primary coil wire has a high winding 
density at the high potential end 32 and a relative lower density at low 
end 33. Wires 28 and 25 are electrically in shunt. As a result, a 
non-uniform resistance cathode is formed. 
FIG. 3A depicts the electron emission along the length of a single segment 
cathode, of uniform resistance, such as those mentioned in the prior art. 
FIG. 3B depicts the electron emission along the length of the cathode 
between the same points with the cathode arrangements of FIGS. 1 and 2. 
Although a preferred embodiment of the invention has been illustrated, and 
that form described in detail, it will be readily apparent to those 
skilled in the art that various modifications may be made therein, without 
departing from the spirit of the invention or from the scope of the 
appended claims. For example, the dual cathode beam mode fluorescent lamp 
which is the subject of cross referenced patent application Ser. No. 
337,046, filed Jan. 4, 1982 may have dual non-uniform resistance cathodes.