Low pressure inert gas discharge device

A lamp primarily containing neon gas is supplied with alternating electrical power at a frequency of not less than 5 kHz. The discharge current is determined on the basis of the gas pressure such that no striations occur. If necessary, getter means including a metal element belonging to the second, third, fourth or fifth periodic group are provided near each electrode, oriented so as not to interfere with any electron emissions from the lamp electrodes.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates in general to a low pressure inert gas discharge 
device and to a method of operating same, and more particularly to one in 
which the luminescence of neon is utilized. 
2. Description of the Prior Art 
A low pressure inert gas discharge lamp utilizing the luminescence of a 
positive column has numerous advantages such as less deterioration, longer 
life, less temperature dependence, and less flux variation after startup, 
in comparison with a fluorescent lamp. 
As neon emits red light, it is suitable as a light source in a facsimile 
machine or in an optical character reader where a red light source is 
utilized. 
It is well known that there is flickering, commonly termed moving 
striations, in the positive column of a low pressure inert gas. Such 
striations depend upon the value of the discharge current; there are upper 
and lower limits for the discharge current which cause the striations to 
occur. Consequently, it is required that the value of the discharge 
current be below the lower limit or above the upper one in order to obtain 
a stabilized discharge with no striations. 
Usually a discharge current whose value is below the lower limit will not 
produce sufficient light output because of its small value and is thus of 
no practical use, whereby it is required that the value of the discharge 
current be above the upper limit. 
This upper limit is established by the following formula, called Pupp's 
critical current: 
EQU Ic=c/p, 
wherein: 
Ic=critical current, 
c=constant value peculiar to a given inert gas, and 
p=gas pressure (Torr). 
The above formula has been further developed by Rutscher and Wojaczek, as 
follows: 
EQU Ic=c/p.gamma., 
wherein: 
.gamma.=an additional constant value peculiar to a given inert gas. 
For neon, c=7 and .gamma.=1. 
These formulae have been derived from direct current discharge, and are 
therefore not applicable to alternating current discharge because the 
current value so determined may be above the upper limit at a certain 
moment and less than such limit at another moment. 
It is thus difficult to determine the upper and lower limits for critical 
currents in an alternating current discharge mode. With respect to a high 
frequency discharge, however, as the alternating speed of the electrical 
polarity of a discharge current is higher than the speed of ambipolar 
diffusion, the ion density does not vary in accordance with the 
alternation of the polarity of the discharge current; in other words, the 
ion density is almost constant. Therefore, critical lower and upper 
current limits can be established. 
The value of the critical current depends upon the gas pressure, which is 
determined in consideration of luminous efficiency and life, while it is 
required that the value of the discharge current be more than that of the 
upper critical current limit. 
The design of a lamp, a lighting apparatus, or a range where a lamp is 
applicable is limited by the critical current. It is thus desirable to 
reduce the value of the critical current in order to minimize this 
limitation. 
Among low pressure gas discharge lamps where the luminescence of an inert 
gas is utilized, gaseous impurities which have an undesirable effect on 
emitting light, starting, and lighting are minimized using getters. The 
impure gas contained in such a lamp would cause the lamp to start with 
difficulty. If the impure gas contains an atom or a molecule whose 
excitation potential is lower than that of that of the inert gas, the 
energy supplied to the lamp is first consumed by such an atom or molecule. 
Light which is unnecessary or undesirable is then emitted, and 
subsequently the lamp becomes poor in both its colorimetric purity and its 
efficiency. For example, an energy of about 19 (ev) is needed for a low 
pressure neon discharge lamp to emit red light at a wave length of 640 
(nm). If a molecule of nitrogen (the resonance excitation potential for N2 
is 1.6 (ev) and that for N is 10.2 (ev)), of oxygen (the resonance 
excitation potential for 0 is 9.1 (ev)) or of hydrogen (a resonance 
excitation for H is 12.2 (ev)) is contained in the lamp as an impure gas, 
an energy of about 13 (ev) is sufficient for such an impure gas to emit 
light. Consequently, the light emitted from such an impure gas and that 
emitted from the neon gas mix with each other. Under these circumstances, 
a red light emitting neon lamp which has both excellent colorimetric 
purity and a high efficiency cannot be obtained. Additionally, an impure 
gas which is produced in correspondence to the consumption of the cathode 
material causes the discharge to be unstable and reduces the life of the 
lamp. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a low pressure inert gas 
discharge device having a discharge lamp containing neon as its major gas, 
which can be steadily lighted, and a method of operating such a device. 
This object is achieved by a device comprising a lamp having a bulb in 
which inert gas mostly composed of neon is sealed at a pressure ranging 
from 1.5 to 15 Torr, an electrode structure contained in the said bulb, 
and means for supplying said lamp with electrical power at a frequency of 
not less than 5 kHz, wherein the peak value Iop (A) of the electrical 
current and the pressure P (Torr) of the sealed insert gas satisfy the 
following formulae: 
EQU when 1.5.ltoreq.P.ltoreq.8, Iop.gtoreq.7/P.sup.1.1, 
and 
EQU when 8&lt;P.ltoreq.15, Iop.gtoreq.69/P.sup.2.2 
Another object of this invention is to provide such a lamp which can start 
lighting at a low starting voltage with a high reliability, which can emit 
light with an excellent colorimetric purity, and which has a long life. 
This object is achieved by providing getter means for each electrode having 
a metal component chosen from the group consisting of metal belonging to 
the second, third, fourth or fifth periodic groups with a getter function, 
except at the portion of each electrode where an electron emitting 
substance is attached.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Various embodiments of this invention are described below, referring to the 
drawings and based on the results of experiments by the applicants. First, 
with respect to the equipment used in the experiments, some brief 
descriptions will be given. 
The lamps used contained filament coil electrodes sealed at both end 
portions, neon gas at a pressure ranging from 1.5 to 15 Torr, and 
comprised glass tubes which were 26 mm in diameter and 436 mm in length. A 
high frequency electrical power supply was utilized in order to drive the 
lamps. A current limiting element having an appropriate impedance was 
inserted between the power supply and each lamp, namely a leakage type of 
output transformer. 
In order to determine the critical current, waveforms of emitted light for 
various values of discharge current were detected by a photodiode, and the 
current value at which uniform and stable light was emitted throughout the 
positive column was recorded. 
Since the lamp, the high frequency power supply and the leakage transformer 
which were used in the experiments were conventional, a detailed 
disclosure thereof will not be given. 
The results of the experiments shown in FIG. 1 are concerned with the 
relation between the critical current and the pressure of the sealed gas. 
In FIG. 1 the abscissa shows the pressure, and the ordinate shows the 
critical current on a logarithmic scale. The small circles designate 
experimental values which the bent solid line follows. The peak value of 
the current corresponds to the critical value in FIG. 1. The dotted line 
corresponds to the equation Ic=7/p (Ic=critical current, p=pressure) as 
established by Rutscher and Wojaczek for a direct current discharge in 
neon. 
The solid line showing the relation between the critical current and the 
pressure is approximately described as follows: 
Ic=7/p.sup.1.1 wherein the pressure of the sealed gas is below 8 Torr, and 
Ic=69/p.sup.2.2 wherein the pressure of the sealed gas is above 8 Torr. 
FIG. 1 thus shows that the dotted line corresponding to a direct current 
discharge and the solid line corresponding to a high frequency current 
discharge are close to each other at low pressures, while the difference 
between these two lines becomes larger as the pressure increases. The 
reason for this is not clear, but it might be because the differences 
between a high frequency discharge and a direct current discharge have 
some effect not accounted for in the equation by Rutscher and Wojaczek, 
which is based on experiments where the gas pressure was relatively low. 
Applicants also have researched the case where the starting voltage for the 
lamp discharge is reduced owing to the Penning effect. It is well known 
that the Penning effect can be found in neon which includes traces of 
argon, krypton or xenon. As the critical current for argon, krypton or 
xenon is different from that for neon, the value of the critical current 
for neon mixed with such a gas is also different from that for a pure neon 
gas. More of such gas contained in the neon causes the value of the 
critical current to be larger, and the Penning effect is most notable when 
the neon gas contains such other gas in a range of 0.1 to 1 percent by 
volume. A mixture ratio of at most one percent of neon with argon, krypton 
of xenon is thus sufficient for the Penning effect. In this regard, 
applicants have studied lamps whose mechanical structures were the same as 
those described above, which contained 99% neon gas as a major gas and one 
of argon, krypton or xenon at 1% as a residual minor gas at a total or 
combined pressure ranging from 1.5 to 15 Torr. 
The results of the experiments where the above lamps were used show that 
the value of the critical current in the lamps containing such minor 
amounts of argon, krypton or xenon is smaller than in lamps containing 
neon only. 
In conclusion, it has been made clear that, in lamps where the Penning 
effect is utilized, lighting the lamp at a current whose value is not less 
than that of the critical current for a lamp containing neon only enables 
a stabilized discharge having no striations. 
Generally speaking, a fluctuation in the electron density may occur at a 
low lighting frequency whose lower limit has not yet been clarified. 
The value of the critical current is constant when the lighting frequency 
is not less than 5 kHz. 
The reasons why the value of the critical current is expressed as a peak 
value is that in experiments where sinusoidal high frequency electric 
power was applied to the lamp, current distortion sometimes occurred 
because of electrode damage, for example. The peak values of the critical 
current were always constant, however. 
Directing attention to the constancy in the peak value, applicants 
conducted experiments where the shape of the high frequency electric power 
was a square wave. It was found that the value of the critical current was 
almost the same as the peak value of the critical current for sinusoidal 
high frequency electrical power signals. The reason for this fact may be 
that the electron density is affected by the peak value of the current 
rather than by the root-mean-square value of the current. 
The pressure of the gas contained in the lamp is determined based on the 
following reasoning. A pressure which is below 1.5 Torr requires too large 
a critical current, which reduces the life of the lamp. A pressure which 
is above 15 Torr is also not suitable because the luminescence efficiency 
becomes lower as the pressure becomes higher. 
Another embodiment of this invention is described using FIG. 2, which 
relates to a discharge lamp where neon gas is mixed with argon, krypton or 
xenon. Before a detailed description of this embodiment, a general 
description of a lamp which contains two mixed inert gases will be given. 
In general, the critical current for striations in gas depends upon the 
kind of gas, and it is supposed that a mixture of two inert gases has a 
critical current whose value is between those of the two individual gases. 
Argon, krypton, and xenon have critical current values which are smaller 
than that of neon. These inert gases have ionization potentials which are 
lower than that of neon, and consequently when one of them is mixed with 
neon it emits light before the neon. Thus, the amount of argon, krypton or 
neon which may be added to a lamp containing neon is extremely limited. 
With respect to a low pressure inert gas discharge lamp containing a 
mixture of neon and argon, the condition where the neon emits most of the 
light is described in Japanese patent application No. 56-167502 in 
relation to the pressure of the sealed gas and the ratio of the mixture, 
as below: 
EQU A.ltoreq.5P.sup.-2, 
where: 
P=the pressure of the sealed gas (Torr), and 
A=the mixture ratio for argon (%). 
The mechanical structure of the lamp in this embodiment is the same as that 
in the first embodiment. The lamp in this embodiment contains neon-argon 
mixed gas, in a pressure range of 1.5 to 8 Torr, and the relation between 
the pressure and the mixture ratio is given by the above formula. 
The relation between the critical current and the pressure of the sealed 
gas based upon the results of experiments is shown in FIG. 2, where the 
shadowed portion indicates the region in which the values of the critical 
current lie. 
The upper straight line I in FIG. 2 shows the relation when the lamp 
contains only neon, and it corresponds to the left portion of the solid 
line in FIG. 1. 
The vertical difference L between lines I and II indicates the amount of 
reduction in the value of the critical current, which is given by the 
following formula: 
EQU 5.3 p.ltoreq.Ic.ltoreq.7/P.sup.1.1 
at the region 1.5.ltoreq.P.ltoreq.8, 
wherein: 
P=the pressure of the sealed gas (Torr), and 
Ic=the value of the critical current (A). 
Similar to the first embodiment, the lower limit of the lighting frequency 
where the value of the critical current varies is not certain, but a 
frequency which is not less than 5 kHz does not induce any variations in 
the value of the critical current. In FIG. 2 the value of the critical 
current indicates the peak value of the current, similar to that in the 
first embodiment. 
The reason why the pressure of the sealed gas is selected in a range of 1.5 
to 8 Torr is that the lower the pressure, the larger the value of the 
critical current. Consequently, lower pressures reduce the life of the 
lamp. 
When the pressure is above 8 Torr, the critical current becomes close to 
that in a lamp containing only neon, and its value is quite small. 
Consequently, in this case it is unnecessary to reduce the value of the 
critical current. 
Another embodiment of this invention is described below, which relates to a 
lamp containing neon as a major component and krypton as a minor one. 
With such a low pressure inert gas discharge lamp, the condition where the 
neon emits most of the light is described in Japanese patent application 
No. 56-167503 in relation to the pressure of the sealed gas and the 
mixture ratio, as below: 
EQU A.ltoreq.8P.sup.-2 
wherein: 
P=the pressure of the sealed gas (Torr), and 
A=the mixture ratio for krypton (%). 
The lamps in this embodiment contain neon-krypton mixed gas at a pressure 
range of 1.5 to 8 Torr, in which the relation between the pressure and the 
mixture ratio is given by the above formula. 
The relation between the critical current and the pressure of the sealed 
gas based on the results of experiments is shown in FIG. 3, where the 
shadowed portion indicates the region in which the values of the critical 
current lie. 
FIG. 3 shows that the reduction in the value of the critical current is 
given by the following formula: 
EQU 4.5/P.sup.0.9 .ltoreq.Ic.ltoreq.7/P.sup.1.1 when 1.5.ltoreq.P.ltoreq.8, 
wherein: 
P=the pressure of the sealed gas (Torr), and 
Ic=the value of the critical current (A). 
Similar to the first and second embodiments, the lower limit of the 
lighting frequency where the value of the critical current varies is not 
certain, but a frequency which is not less than 5 kHz does not induce any 
variations in the value of the critical current. In FIG. 3 the value of 
the critical current indicates the peak value of the current, similar to 
the first and second embodiments. 
The reason why the pressure of the sealed gas is selected in a range of 1.5 
to 8 Torr is similar to that of the FIG. 2 embodiment. 
The following embodiments relate to the structure of the discharge lamp in 
general, and more particularly to the arrangement of getters which avoid 
the luminescence of gaseous impurities and undesirable effects on the 
starting or life of the lamp. As shown in FIG. 4 an inert discharge lamp 1 
comprises an elongate glass bulb 2 having no coatings on its inner 
surface, and a stem 3 which is tightly bonded at the end of the bulb. Two 
electrode supports 4 whose ends mount a preheating electrode 5 are 
attached to the stem 3. One of the electrode supports also mounts a getter 
holder 6 to which a metal getter structure 7 is secured containing one or 
more getters belonging to the second, third, fourth or fifth group near 
the preheating electrode 5. 
Where a the flash getter such as barium (Ba) or magnesium (Mg) is used, it 
is desirable that the getter emission surface should face in a direction 
opposite to the electrode 5 in order to prevent the getter emissions or 
sputterings from having an undesirable effect on the electrode. The lamp 1 
is equipped with a similar getter structure and preheating electrode at 
its other end. 
The electrode supports 4 pass through the stem 3 and connect electrically 
to pins 9 of a lamp base 8. In manufacturing such a lamp containing a 
getter, where a non-vaporizable metal or an alloy belonging to the second, 
third, fourth, or fifth group such as thorium (Th), titanium (Ti), 
zirconium (Zr), or tantalum (Ta) is used, it is important and desirable to 
heat the lamp sufficiently to exhaust the unwanted gas by fully activating 
the getter material. 
Where a flash getter is used, it is desirable to heat the getter emitting 
structure 7, for example by high frequency induction heating to flash the 
barium metal which is a major component of the getter. The getter material 
is thereby sputter coated onto the device over a region which covers an 
inner wall of the end portion of the glass bulb 2 and the edge of the stem 
3, as indicated by reference numeral 10 in FIG. 4. 
In a lamp equipped with plural preheating electrodes, a sufficient effect 
cannot be obtained by adsorbing an impure gas contained in the lamp by 
means of one getter structure located near the electrode. As the 
preheating electrodes gradually consume themselves they emit or evolve 
impure gases, which if close to the electrode will reduce or hinder its 
capability for emitting electrons. This shortens the life of the lamp and 
impedes the switchover from a glow discharge to an arc discharge on 
startup. 
Consequently, it is necessary to remove the impure gas which has evolved as 
quickly as possible. This embodiment resolves not only the problem of 
striations but also the problem of impure gas evolving from the 
electrodes. 
The results of experiments by applicants are shown below. The lamps 
contained neon gas at a pressure of 4 Torr, and were 25 mm in diameter and 
436 mm long. These dimensions are those of an FL 15 type of fluorescent 
lamp. 
Two kinds of lamps were used in the experiments. One was equipped with 
getters near the electrodes, the other had no getters. 
The getter structure 7 in FIG. 4 comprises a barium-aluminum alloy buried 
in a groove on an iron base shaped like a doughnut, is clad with nickel, 
and contains barium at a ratio of 55 percent. The getter structure was 
heated to a temperature of about 1100.degree. C. by high frequency 
induction heating so that the getter flashed and was thereby sputter 
coated over a region excluding the electrode 5. 
Experiments were performed in which the lamps were equipped with various 
amounts of getter material to the same amount of cathode substance. The 
results of the experiments show that a lamp equipped with no getter needs 
a high lighting voltage of 150 (v) and emits light which includes other 
than neon red in its spectrum, which is not desirable in terms of light 
purity. On the other hand, the lamp equipped with a getter functioned at a 
low voltage of 100 (v), which is the usual voltage for a common FL 15 type 
of lamp, and emitted pure red light peculiar to neon. 
In these lamps, the life of the lamp depends upon whether the getters are 
located near either one or both electrodes, and upon the amount of the 
getter, as is clear from Table 1 below. In Table 1, the amount of the 
getter means the ratio of the getter substance to the cathode substance of 
each electrode. 
TABLE 1 
______________________________________ 
Amount of getters 
Location of the 
in each getter 
Life 
Lamp No. 
getter structure 
structure (ratio) 
Hours 
______________________________________ 
1 None 85 
2 at one electrode 
1/25 400 
3 at one electrode 
1/5 700 
4 at both electrodes 
1/25 1,200 
5 at both electrodes 
1/5 not less 
than 2,000 
6 at both electrodes 
1/2 not less 
than 2,000 
______________________________________ 
As is shown by Table 1, the life of a lamp equipped with no getter 
structure or with one getter structure nearly only one electrode is much 
shorter than that of a lamp which is equpped with a getter structure near 
each electrode. 
These experiments confirm that an amount of getter which is not less than 
one twentieth of that of the cathode substance in an electrode is needed 
to ensure a lamp life beyond two thousand hours; otherwise an impure gas 
such as oxygen would gradually evolve in correspondence to the consumption 
of the cathode substance and would saturate the capability of the getter. 
That is, it would reduce the capability of electron emission or establish 
a light spot which would emit electrons on restriking, and consequently a 
direct current component would be produced in the discharge which would 
shorten the life of the lamp. 
A lamp having a getter structure as shown in FIG. 5 is also practicable, 
which is similar to that in FIG. 4 except for the getter structure and the 
sealed gas. In FIG. 5 the getter structure 7 has a getter consisting of a 
zirconium (Zr)--aluminum (Al) alloy attached to an iron plate located near 
the electrode 5 and clad with nickel. The getter holder holds the iron 
plate and is directly supported by the stem 3. The lamp contained argon 
gas at a pressure of 3 Torr. This lamp produced line spectrum with a 
wavelength ranging from 700 to 900 mm, which is near infrared radiation. 
Similar to the embodiment of FIG. 4, such a lamp with no getter has a high 
starting voltage, a short life and is not practical. 
While the lamps with the getter started at a low voltage and lit steadily, 
those equipped with getter amounts not less than one twentieth near both 
electrodes performed a steady discharge for a long time; in other words, 
had a long life. 
It was also confirmed that lamps having getters comprising such components 
as magnesium, titanium, barium, thorium, and vanadium belonging to the 
third, fourth or fifth periodic group had an effect similar to that 
described above. 
The lamps in the previous two embodiments contained neon or argon as an 
inert gas while the lamps containing other gases, for example, helium 
krypton, xenon, or mixed inert gas, which are applicable for specific 
usages, had a similar effect. A lamp containing hot cathode type of 
electrode is also applicable.