Method of producing a gas discharge light source

A method for producing a gas discharge light source for emitting an electromagnetic radiation comprising a gas discharge tube filled with at least one discharge gas material and having a means for generating a gas discharge includes the following method steps: igniting the gas discharge tube filled with the discharge gas material, performing a forming operation in which the electrical operating power of the gas discharge light source is at least as high as the desired continuous operation power, the gas discharge tube is held at a temperature which is at least as high as the subsequent operating temperature, the intensity of the emitted electromagnetic radiation in the region of the desired wavelengths is monitored and the partial pressure of the vaporized discharge gas material is varied until the intensity of the selected electromagnetic radiation has reached a maximum value, whereupon a reservoir with excess discharge gas material is separated from the gas discharge tube in such a manner that the gas discharge tube under operating conditions does not contain any condensed discharge gas material. A gas discharge tube having a discharge material forming a discharge gas under operating conditions is distinguished in that the discharge gas material is incorporated in the inner wall of the gas discharge tube and there is no longer any condensed discharge gas material in the gas discharge tube.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method of producing a gas discharge light source 
for emitting an electromagnetic radiation comprising a gas discharge tube 
which is filled with at least one discharge gas material, and a means for 
generating a gas discharge. 
2. Description of the Prior Art 
Such a method is known for example from U.S. Pat. No. 2,755,159. To ensure 
the suitable operating voltage of the gas discharge light source 
irrespective of the amount of discharge gas material (mercury) introduced 
and of variations of the dimensions, which both considerably influence the 
operating voltage, an excess of mercury is introduced and the excess then 
expelled again. A voltage source is connected to the gas discharge light 
source and the filled gas discharge tube ignited. The voltage present at 
the electrodes of the gas discharge tube is monitored, the voltage rising 
gradually until the entire mercury has evaporated and then starting to 
drop again when vapour from the overheated main section of the gas 
discharge tube reaches a colder side arm and condenses there. As soon as 
the voltage has dropped to a desired value the voltage source is switched 
off and the side arm serving as reservoir for excess discharge material is 
melted off. The steps described are intended to achieve that independently 
of said variations the operating voltage is fixed to 1 to 2 volts. 
In U.S. Pat. No. 3,309,565 a fluorescence lamp is proposed in which in 
operation a temperature stabilization of a supply of condensed mercury 
takes place (a socalled "cold spot"). For this purpose, with the aid of a 
Peltier element the cold spot is cooled and the vapour pressure of the 
discharge material (mercury) held at a value which permits an optimum 
light emission of the gas discharge light source in operation. 
In U.S. Pat. No. 4,431,947 a regulated light source is described which in 
operation makes available a light flux of constant intensity at a desired 
wavelength by employing a closed-loop control system which holds the 
intensity of a lamp generating light at a predetermined value. Such a 
light source of constant intensity is used for special purposes, for 
example in instruments operating with nuclear magnetic resonance, such as 
a magnetic resonance gyroscope. To achieve the desired constancy of the 
light at the desired wavelength, another wavelength is monitored and a 
control signal is derived therefrom which finally controls the temperature 
of a "cold end" of the light source. 
In DE-PS 2,212,536 a method is proposed for producing a fluorescent lamp 
with which the aim is to ensure that the lamp bulb remains uninfluenced 
from impurities liberated on activation of the lamp cathodes. For this 
purpose the cathodes are activated outside the chamber in which the lamp 
bulb is filled, the impurities are removed and end caps with the cathodes 
are then sealed into the ends of the lamp bulb. 
DE-OS 336,088 describes a production method for gas discharge lamps in 
which for removing impurities from the bulb interior the lamp is ignited 
and/or heated, a continuous stream of an inert ignitable gas being 
conducted through the bulb during the discharge and/or the heating. As 
flushing gas for example argon or another noble gas may be used. 
In the prior art there has been no lack of further attempts to improve 
substantial parameters of such gas discharge light sources, for example 
the intensity of the emitted electromagnetic radiation, the stability and 
the like. 
Thus, U.S. Pat. No. 4,157,485 has already proposed a low-pressure mercury 
vapour lamp in which the aim is to operate the mercury vapour lamp with 
maximum efficiency. For this purpose, it is considered necessary to keep 
the optimum mercury partial pressure of 6.times.10.sup.-3 Torr, 
corresponding to a temperature of the mercury of about 40.degree. C., as 
constant as possible. For if the lamp temperature rises above the ambient 
temperature at which the mercury in the discharge would have a temperature 
of about 40.degree. C. the efficiency drops and consequently the lamp 
power fluctuates with fluctuating ambient temperature. If the electrical 
power supplied to the lamp is increased the temperature rises and this in 
turn leads to a reduction of the conversion efficiency, i.e. the 
conversion of the electrical power supplied into electromagnetic radiation 
power. To ensure that the mercury partial pressure is stable over a wide 
temperature range at about the value 6.times.10.sup.-3, the use of an 
amalgam of bismuth, indium and mercury having a predetermined composition 
is proposed. This is said to be advantageous compared with already known 
amalgam compositions because in the known amalgam compositions the mercury 
proportion in the amalgam drops in operation because part of the mercury 
is bonded, in particular by absorption in a fluorescence layer. 
A further improvement of the deposition mercury vapour lamp described in 
U.S. Pat. No. 4,157,485 has been proposed in EP-A1-0 157 440. In 
particular, with a higher discharge current and with a lamp tube of small 
diameter the stabilization value of the mercury vapour pressure with the 
amalgam proposed in the American patent specification is too low to obtain 
an optimum efficiency and maximum light output power. For at temperatures 
in the range of 100.degree.-120.degree. C. in the region of the amalgam 
the mercury pressure drops beneath 6.times.10.sup.-3 Torr. For this 
reason, EP-A1-0 157 440 proposes an alloy which forms an amalgam and 
consists of bismuth, lead and silver. This is intended to give an optimum 
light output power at a temperature of 70.degree.-150.degree. C. in the 
region of the amalgam. The alloy of bismuth, lead and silver is provided 
for this purpose at a socalled "cold spot" of the mercury vapour lamp at 
which a condensation takes place. 
In U.S. Pat. No. 3,878,421 a high-power lamp for ultraviolet radiation is 
proposed which has a special electrode arrangement. The lamp tube here has 
a relatively large diameter and is provided with constricted end portions. 
Cold points, that is cold spots, form there in order to achieve partial 
condensation of the mercury vapour and thus prevent an excess vapour 
pressure of the mercury occurring which would reduce the lamp efficiency. 
The length of the end portions is greater or equal to twice the diameter 
of the end portions. 
U.S. Pat. No. 4,005,332 describes a mercury fluorescence lamp which is 
operated with direct current. Whereas with alternating current operation 
an optimum efficiency is obtained at a mercury partial pressure of 
7.times.10.sup.-3 Torr corresponding to a temperature of 40.degree. C. at 
the "cold spot" of the mercury, the exact values depending among other 
things on the lamp tube radius, the optimum temperature is obviously 
different in direct current operation. This optimum temperature depends on 
the poling in direct current operation and for a cold spot at the anode 
end of the gas discharge tube lies beneath 40.degree. C., the optimum 
temperature decreasing with increasing lamp current, whereas with an 
arrangement of the cold spot at the cathode end the optimum temperature is 
greater than 40.degree. C. and rises with increasing temperature. For this 
reason it is proposed in U.S. Pat. No. 4,006,332 to arrange the cold spot 
in a water bath, the temperature of which can be varied. In operation in 
this case enough mercury to ensure that a certain proportion thereof is 
always liquid is always present in the fluorescent lamp. 
In U.S. Pat. No. 4,349,765 a mercury vapour lamp is proposed for 
ultraviolet radiation of high intensity in which mercury and inert gas are 
provided and the discharge takes place at a pressure of 5.times.10.sup.-3 
to 5.times.10.sup.-1 Torr mercury and 5.times.10.sup.-2 to 10 Torr inert 
gas. The gas discharge tube consists of a quartz glass which is so doped 
that the lines of 185 nm and 194 nm of the emitted radiation are absorbed 
whereas the line of 254 nm passes through practically unattenuated. To set 
the optimum vapour pressure of the mercury a cold spot is again provided, 
i.e. a point at which the mercury is present in condensed, i.e. liquid 
form, and the temperature of the cold spot can be adjusted between 
48.degree. and 65.degree. C. With this vapour pressure adjustment an 
intensity adjustment can be achieved. 
An improvement of this ultraviolet gas discharge lamp is proposed in U.S. 
Pat. No. 4,835,442. To avoid fluctuations of the essential parameters of 
the gas discharge lamp a reservoir is proposed which serves as cold spot 
and the temperature of which can be adjusted. The reservoir is 
temperature-isolated with respect to the surroundings to eliminate said 
fluctuations. 
An ultraviolet gas discharge lamp similar to that described in U.S. Pat. 
No. 4,349,765 is also disclosed in GB-PS 2,009,493. In CH-PS 570,040 a 
mercury vapour lamp is described as ultraviolet radiation source in which 
the gas discharge tube comprises a pressure equalizing space which 
connects the cathode space to the anode space. The sum of the volumes of 
the cathode space, the anode space and the pressure equalizing space is to 
be greater here than the volume of the discharge space. By a control 
member the mercury partial pressure is regulated so that the radiation 
yield in the region of 254 nm is at least 80% of the yield maximum. In 
this mercury vapour lamp as well a socalled cold spot is provided at which 
a drop of condensed, i.e. liquid, mercury is disposed. 
Thus, in accordance with the prior art described above the desired 
improvement of the properties of gas discharge light sources is achieved 
by specific steps which fix the operation of said gas discharge light 
sources, i.e. the actual operating conditions. 
SUMMARY OF THE INVENTION 
The present invention therefore has as its object to increase still further 
the power of gas discharge light sources, that is to increase the light 
intensity output thereof. 
The knowledge on which the invention is based resides in not changing the 
actual operating conditions but optimizing the operating conditions then 
subsequently occurring in operation by certain method steps in the 
production of a gas discharge light source, i.e. before the actual 
operation. 
The present invention therefore proposes in a method for producing a gas 
discharge light source for emitting an electromagnetic radiation 
comprising a gas discharge tube which is filled with at least one 
discharge gas material, and a means for generating a gas discharge, the 
improvement comprising the method steps of 
a) igniting the gas discharge tube filled with the discharge gas material, 
b) performing a forming operation in which 
b.1) the electrical operating power of the gas discharge light source is at 
least as high as the desired continuous operation power, 
b.2) the gas discharge tube is held at a temperature which is at least as 
high as the subsequent operating temperature, 
b.3) the intensity of the emitted electromagnetic radiation in the region 
of the desired wavelengths is monitored and 
b.4) the partial pressure of the vaporized discharge gas material is varied 
until the intensity of the selected electromagnetic radiation has reached 
a maximum value, 
c) whereupon a reservoir with excess discharge gas material is separated 
from the gas discharge tube in such a manner that the gas discharge tube 
under operating conditions does not contain any condensed discharge gas 
material. 
A further solution according to the invention is achieved by a gas 
discharge tube comprising a discharge gas material which under operating 
conditions forms a discharge gas, the discharge gas material being 
incorporated in the inner wall of the gas discharge tube and no condensed 
discharge gas material then being present in the gas discharge tube. 
Thus, in a surprising departure from the prior art a gas discharge tube is 
produced which does not have a cold spot. 
Due to the complicated conditions obtaining in a gas discharge light source 
in operation and depending on the temperature of the discharge tube, the 
conditions in the ignited discharge gas plasma, but also on the gas flow 
processes in the gas discharge tube, the effective mechanism which results 
in the advantageous properties of a gas discharge light source made 
according to the method of the invention has still not been fully 
explained. It would however appear that the ratio of electron energy and 
electron density in the plasma to the partial pressure of the discharge 
gas always preferably represents certain ranges of the spectrum of the 
discharge gas. 
It is assumed that by the method according to the invention some of the 
discharge gas is deposited in the walls of the gas discharge tube in such 
a manner that later under operating conditions an optimum discharge gas 
concentration arises. The electromagnetic radiation power in desired 
spectral ranges of gas discharge light sources produced by the method 
according to the invention is several times greater than the power which 
can be achieved with the prior art, typically at least 300%. 
Advantageous further developments of the invention are set forth in the 
subsidiary claims. 
Advantageously, as discharge gas mercury is used and may be present as pure 
mercury or amalgam. In addition, the gas discharge tube may be filled with 
an inert gas or a mixture of inert gases, in particular to achieve easier 
igniting and a more stable plasma. 
In an advantageous further development of the method according to the 
invention prior to the firing of the gas discharge tube filled with the 
discharge gas a heating of the gas discharge tube is effected. The period 
for the heating is advantageously about 2 hours, and at a temperature of 
about 250.degree. C. 
To avoid discharge gas depositing in the forming operation at a point which 
in subsequent operation of the gas discharge tube is in connection with 
the latter, during the forming operation the portion of the reservoir 
leading to the gas discharge tube is advantageously held at a temperature 
which is equal to or greater than the wall temperature of the gas 
discharge tube. 
To set the optimum partial pressure of the discharge gas in the forming 
operation the temperature of the reservoir is preferably varied. 
The temperature of the reservoir here is preferably reduced so that the 
partial pressure of the discharge gas diminishes. 
In a particularly advantageous further development of the method according 
to the invention in the forming operation the electrical operating power 
of the gas discharge tube is about 25% to about 50% higher than the 
desired continuous operation power.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The gas discharge tube 10 illustrated in FIG. 1 is known per se from the 
prior art. A tubular quartz tube 12 is fused gas-tight at its ends, 
current leads 14 and 18 for thermoemissive electrodes 16 and 20 
respectively being fused in. A pump tube 24 is in communication with the 
discharge space 22 of the quartz tube 12. Said pump tube can be 
specifically heated, this being done via the flame 28 of a burner 26. 
In FIG. 2 the quartz tube 12 is introduced into a heatable container 30. 
The pump tube 24 is surrounded by a heating winding 32 which is supplied 
with current via connection leads 34, 36. Furthermore, in FIG. 2 the leads 
for the current supplies 14, 18 are shown for the application of an 
alternating current voltage thereto. 
FIG. 3 shows the gas discharge light source with the quartz tube 12 and a 
means for temperature adjustment of the pump tube 24 as well as a 
measuring means for determining and monitoring the electromagnetic 
radiation emitted by the gas discharge light source in the particular 
spectral range of interest. 
The electromagnetic radiation emerging from a certain region of the 
discharge space 22 passes via a mirror 44 and a diaphragm 46 to a 
schematically illustrated spectral photometer 48, the output signal of 
which, i.e. the electromagnetic intensity in the particular spectral range 
set, is passed to a recorder 50 which records the intensity as a function 
of time. 
The pump tube 24 provided with the heating means 32 already described is 
melted off, i.e. sealed, and at the lower end of the fused pump tube 24 
there is a drop 42 of condensed mercury. This is however dependent on the 
temperature of this portion of the pump tube 24, and for adjusting the 
temperature the pump tube 24 is immersed into a liquid 40 with which a 
temperature-adjustable container 38 is filled. 
FIG. 4 shows a further embodiment of a gas discharge light source in which, 
in contrast to the embodiment described in FIGS. 1 to 3, the electrical 
power is introduced into a gas discharge tube 52 not by means of 
electrodes but (electrodeless) via microwave radiation. 
For this purpose the gas discharge tube 52 according to FIG. 4 is 
constructed as double-wall hollow quartz tube having concentric walls. The 
discharge space 54 is formed by the annular space between the outer and 
the inner tubes. The concentric tubes are fused together in gas-tight 
manner at their ends. 
The supply of electrical energy is by the microwave radiation which is 
generated by a magnetron tube 58 and is emitted by a radially irradiating 
waveguide slot antenna 56. 
The method according to the invention can be used in gas discharge tubes 
provided with electrodes and in electrodeless gas discharge tubes. 
After the usual steps, such as cleaning pumping, evacuation, possibly 
activation of the electrodes 16, 20, the gas discharge tube 12 illustrated 
in FIGS. 1 to 3 is filled in known manner with an inert ignition gas, for 
example argon, krypton, xenon, neon or a mixture of such gases, at a 
pressure of 50-500 Pascal, preferably 150 Pascal. A small amount of about 
15-20 mg pure elementary mercury is then introduced. The pump tube 24 is 
then melted off and sealed so that the remaining length is about 10 cm. 
The already functionable gas discharge light source, which has however not 
yet been optimized, is then introduced into the container 30 constructed 
as heat cabinet and preferably preheated there for about 2 hours at a 
temperature of 250.degree. C. 
The gas discharge in the gas discharge tube 12 is then ignited and operated 
with an electrical power about 25% to about 50% higher than the subsequent 
desired operating power. The sealed pump tube 24 is brought here by the 
separate heating means 32 to at least the wall temperature of the gas 
discharge tube 12. 
Apparently, due to the comparatively high mercury vapour pressure in the 
system mercury atoms and molecules diffuse into the upper molecule layers 
of the inner wall of the hot gas discharge tube 12. After completion of 
this burning-in diffusion operation, which under the conditions given 
above lasts about 30 minutes, the ignition voltage is switched off and the 
tube cools slowly in the heat container 30. A further diffusion of mercury 
into the inner wall of the gas discharge tube 12 apparently takes place 
when this is done. 
As illustrated in FIG. 3, the measuring means 44-50 is then attached to 
record the desired electromagnetic radiation emitted in operation of the 
gas discharge tube 12, i.e. in the present case the 254 nm line of 
mercury. 
It must be ensured that the gas discharge tube 12 is disposed in 
surroundings having a temperature corresponding approximately to the 
ambient temperature of the later intended use. The expediently downwardly 
directed pump tube 24 is heated with the heating means 32 over a distance 
of about 30-40 mm adjoining the gas discharge tube 12. It this region, 
that is the subsequent melting-off and fusing point, condensed mercury is 
able to form, since it could later evaporate back again into the gas 
discharge tube 12. For this purpose, this portion of the pump tube 24 is 
heated to a temperature which is higher than the temperature of the wall 
of the gas discharge tube 12 under operating conditions. 
The gas discharge is now ignited and maintained with the desired operating 
power. After some time, temperature equalization with the surroundings 
establishes itself. 
The lower portion of the pump tube 24 is now dipped into the vessel 38 with 
the liquid 40. In the present case the liquid is cooling liquid, that is a 
salt solution at about -3.degree. C. Thereupon, in the cooled portion of 
the pump tube 24 a condensate 42 forms, that is a mercury droplet, and the 
mercury content in the plasma in the gas discharge space 22 is depleted. 
Now, if the mercury partial pressure in the plasma becomes smaller than the 
diffusion pressure of the mercury incorporated in the inner wall of the 
gas discharge tube 12, the diffusion of the mercury back into the 
discharged space 22 begins. In this connection, in particular the wall 
temperature of the gas discharge tube 12 and the amount of diffused-in 
mercury are of significance. Due to further depletion of the plasma 
mercury content by continuous condensation of the mercury in the cooled 
part of the pump tube 24 and the likewise progressive depletion of the 
diffused-in content of mercury in the walls of the gas discharge tube 12 
and the resulting fusion back, an equilibrium state of the mercury partial 
pressure in the plasma and the diffusion pressure from the inner wall of 
the gas discharge tube at continuously decreasing values occurs. 
The intensity of the 254 nm line of mercury recorded by the recorder 50 is 
very low in the initial phase and in spite of the comparatively high 
operating power of the tube reaches only a fraction of the values which 
would be reached with a standard low-pressure lamp of the same dimensions. 
With progressive reduction of the mercury concentration in the plasma the 
electromagnetic radiation intensity rises and finally reaches a peak value 
which in comparison with a standard low-pressure lamp is approximately 
proportional to the increase of the electrical operating power in the 
present case. 
As soon as the resulting intensity maximum value is reached, which is 
recorded by the photometer 48 and the recorder 50 connected thereto, the 
pump tube 24 with the condensed mercury contained therein is melted as 
quickly as possible off the gas discharge tube to fix the optimum state 
reached. The melting takes place in the heated zone of the pump tube 24 in 
which it is certain that no condensed mercury has deposited, as closely as 
possible to the gas discharge tube. 
When the gas discharge tube 12 cools after switching off the discharge the 
mercury atoms or molecules are obviously again bound by the wall of the 
gas discharge tube 12. This is clearly apparent on restarting the (cold) 
gas discharge tube 12. After the ignition initially only a pure inert gas 
discharge is obtained. The emission of the desired mercury radiation at 
254 nm does not occur intensively again until the wall of the gas 
discharge tube 12 has again approximately reached its desired operating 
temperature which also obtained during the forming operation at the 
instant at which the intensity maximum of the emitted radiation was 
reached, and the wall liberates the bonded mercury atoms exactly in the 
amount which results in optimum conditions in the plasma. This is 
confirmed in that after switching on again the occurrence of the 
electromagnetic radiation with a wavelength of 254 nm is initially 
detectable at the points of the wall of the gas discharge tube 12 which 
are located in the immediate vicinity of the glowing electrodes 16, 20 
because these points of the wall of the gas discharge tube 12 reach an 
adequately high temperature very rapidly. 
It is thus seen that by the forming operation according to the invention in 
the production of a gas discharge light source optimum conditions are 
created which result in predetermined operating values which in subsequent 
practical operation of the gas discharge light source must again be 
substantially observed. It is however possible to carry out an adaptation 
to appreciably changed ambient temperatures of the gas discharge light 
source; this can for instance be achieved by suitable increasing or 
reduction of the electrical operating power. 
The method according to the invention is not restricted to mercury or 
mercury amalgam but can be used with numerous discharge gases. In every 
case, by the forming operation according to the invention a concentration 
of the discharge material obviously arises in practice which in operation 
of the gas discharge light source assumes a value optimum for the desired 
wavelengths, apparently by diffusion back from the inner walls of the gas 
discharge tube. 
The method according to the invention does not depend on the type of gas 
discharge tube used. In particular, it is possible to use both gas 
discharge tubes provided with electrodes as well as socalled 
"electrodeless" gas discharge tubes, an example of which is illustrated in 
FIG. 4. 
The method according to the present invention will be explained in further 
detail with the aid of the following tests I to III. 
Test I 
With a measuring arrangement according to FIG. 3 for determining the 
intensity of the 254 nm line of mercury the intensity of a gas discharge 
light source made by the method according to the invention and of a 
commercially usual 40 Watt UVC mercury low-pressure lamp of "ozone-free" 
quartz glass as comparison normal were recorded. The gas discharge light 
sources to be compared were shielded by a gap diaphragm arranged 
perpendicularly to the respective longitudinal axis of the particular gas 
discharge light source in such a manner that the entire diameter of the 
gas discharge light source was scanned but only an always constant length 
of only a few mm thereof. In all the comparison measurements the 
arrangement of the diaphragms, the measuring range and the sensitivity of 
the spectral photometer remained the same. 
The commercially available low-pressure tube has the following dimensions: 
total length about 850 mm, distance between the electrodes about 770 mm, 
outer diameter 19 mm and wall thickness 1 mm. 
The comparison gas discharge light source was ignited and after burning for 
15 minutes the radiation intensity of the 254 nm spectral line was 
recorded. The intensity measured value obtained was taken as a base value 
of 100%, the measured value being checked again before each measurement. 
The comparison gas discharge light source was operated with a burning 
voltage of 103 volt and burning current of 0.4 ampere. 
The gas discharge light source made by the method according to the 
invention consisted of exactly the same materials as the comparison gas 
discharge light source and had the same dimensions but with fused-in 
tungsten step electrodes designed for a continuous load of about 3 ampere. 
The gas discharge source made according to the invention will be referred 
to hereinafter briefly as "test tube". 
The test tube was heated in known manner, evacuated and flushed, and the 
electrodes thereof were annealed by ion bombardment and formed. The test 
tube was then filled with pure argon at a pressure of about 200 Pascal. 
The pump tube was located about 10 cm away from the one end of the test 
tube and had an outer diameter of 4 mm and an inner diameter of 2 mm. The 
pump tube was curved downwardly to a U-shaped loop so that on introducing 
20 mg pure mercury under vacuum the resulting small mercury sphere could 
not drop down into the test tube but remained lying in the U-shaped loop 
of the pump tube. After the pump tube had been sealed and cooled, the 
mercury sphere could be rolled along the sealed fused end of the pump 
tube. At the extension of the pump tube leading to the test tube several 
narrow strips of aluminium foil were wound on in such a manner that the 
heating of the test tube also heated an about 30 mm long portion of the 
pump tube. The test tube was then introduced into the measuring apparatus 
and ignited. 
The electrical operating current of the gas discharge was regulated via a 
throttle ballast with the aid of a mains regulating transformer to 3 
ampere. After a burning duration of 10 minutes the temperature of the test 
tube had risen to about 170.degree. C. and remained largely constant. In 
this stage practically only a pure argon discharge was present and the 
measuring means set to the 254 nm line exhibited such low values that no 
clear allocation was possible. 
The pump tube with the mercury droplet disposed at the end thereof was then 
introduced into a water bath with a temperature of 50.degree. C. Although 
at this temperature adequate vapour pressure of the mercury should arise, 
as apparent from the values indicated at the beginning, even after a 
period of 10 minutes no appreciable intensity on the 254 nm line could be 
detected. 
The water temperature was now increased to 90.degree. C. After a few 
minutes it was possible to observe that near the entrance of the pump tube 
into the test tube a luminous zone formed in the plasma but did not appear 
to spread. The radiation intensity recorded by the measuring means was 
about 17% and remained relatively constant. The water bath was then 
removed from the pump tube and shortly afterwards the radiation intensity 
diminished appreciably. Even when a temperature of the water bath of 
50.degree. C. was intermediately employed, the luminous zone in the plasma 
near the pump tube disappeared. The small amounts of mercury rising from 
the pump tube were obviously bonded by the hot walls of the test tube 
consisting of quartz or diffused into said walls and in this manner were 
taken from the plasma. As soon as a temperature of the water bath of 
90.degree. C. was used again, the original phenomena again occurred. 
In the course of the next 50 minutes the radiation intensity on the 254 nm 
line increased slowly but continuously and finally reached 100% relative 
intensity. This continuous intensity rise continued obviously because the 
hot walls of the test tube absorbed progressively less mercury and 
consequently more mercury was available for the plasma, and after about 
120 minutes the radiation intensity had risen to about 325% and remained 
constant for about 10 minutes. 
When the intensity then exhibited a slight drop, the discharge was quickly 
switched off, the heat insulation (aluminium strips) round the beginning 
of the pump tube removed, and the latter then fused off and sealed near 
the joint to the test tube. 
Renewed ignition of the not yet completely cooled test tube and starting 
operation thereof again with 3 ampere current strength soon led to the 
maximum intensity value of 325% which however then clearly diminished. 
Thereupon the current strength was reduced to about 2.5 ampere and as a 
result the temperature of the test tube decreased to about 145.degree. C., 
the intensity of the radiation on the 254 nm line then stabilizing itself 
at a value of about 300%. This result was reproduceable because a renewed 
increase of the current strength to 3 ampere caused the intensity value to 
rise temporarily rapidly to 325%, which however then diminished again. A 
return to a discharge current of 2.5 ampere stabilized the radiation 
intensity again at a value of 300%. 
The spectrum was then recorded in a range between 280 and 230 nm. The 
result was a single peak at 254 nm with an intensity of 300%. 
24 hours later a control measurement was performed. In it, immediately 
after the ignition of the cold test tube at 2.5 ampere discharge current 
the typical bluish-white glowing of the plasma could be observed only at 
the glowing electrodes. The major cold portion of the test tube exhibited 
only the watery blue glow of the argon discharge. The measuring apparatus 
did not shown any appreciable radiation on the 254 nm line for this 
portion of the test tube. However, it was possible to obtain such an 
intensive radiation in locally limited form immediately on heating the 
cold portion of the test tube at any point with the flame of a gas burner. 
The resulting brightly glowing plasma spot soon disappeared again after 
removing the flame. After expiry of about 7 minutes the typical 
bluish-white glow of the electrodes spread towards the middle of the test 
tube and finally filled the latter uniformly, the intensity measured value 
rising rapidly and then stabilizing at 300% radiation intensity. 
The electrical operating data were 61 volts for the operating voltage of 
the test tube and 2.5 ampere for the operating current. 
Test II 
The test tube had the following dimensions: total length about 850 mm, 
distance between the electrodes about 770 mm, outer diameter 23 mm and 
wall thickness 1 mm. As electrodes, the same tungsten step electrodes for 
3 ampere permanent load were fused in as in test I. The test tube was 
heated out in known manner, evacuated, and after annealing and forming of 
the electrodes filled with pure argon at a pressure of about 250 Pascal; 
20 mg pure mercury was then introduced. The straight pump tube was sealed 
off to leave a length of about 10 cm. The mercury droplet was in the test 
tube and the latter was introduced into a heatable container as 
illustrated for instance in FIG. 2, being exposed therein for 2 hours to a 
temperature of 250.degree. C. 
After recording the "comparison normal" with the aid of the commercially 
available 40 Watt UVC comparison tube, the test tube was introduced into 
the intensity measuring apparatus, ignited and operated with 4 ampere 
discharge current. Once again the pump tube was wrapped with aluminium 
foil close to the body of the test tube as in example I. 
Firstly, the known phenomena of the discharge were observed; at the high 
current intensity of 4 ampere the plasma column glowed bright white, was 
somewhat constricted and exhibited a slight ripple and motion. The 
radiation intensity of the test tube on the 254 nm line was about 30% 
(relatively to the comparison tube). 
After 15 minutes burning duration, occasionally also heating the pump tube 
with a gas flame to avoid initially any condensate formation of the 
mercury, the pump tube was immersed into a water bath at room temperature. 
During the next 25 minutes the radiation intensity on the 254 nm line 
increased only negligibly. The cooling water for the pump tube was then 
replaced by iced water with numerous pieces of ice floating therein. A 
slow but continuous rise of the radiation intensity was now observed and 
after about 80 minutes a maximum value of almost 400% had been reached. 
The plasma column had assumed the typical blue-white colour, burnt 
uniformly without motion and filled the entire diameter of the test tube. 
The ice water was now removed, the aluminium strips of the pump tube near 
the body of the test tube were taken away and the pump tube rapidly sealed 
off at this point. Re-ignition of the test tube resulted in a stable 
radiation power on the 254 nm line of about 375% when the current strength 
had been reduced to 3.6 ampere. 
After 24 hours a control measurement was made which showed the phenomena 
known from test I. After ignition of the test tube a radiation-intensity 
bluish-white plasma zone was observed firstly only in the immediate 
vicinity of the brightly glowing electrodes and with increasing 
temperature of the test tube propagated itself slowly in the forward 
direction, filling the body of the test tube uniformly after about 8 to 10 
minutes. The heating of a still relatively cool portion of the test tube 
with a gas flame generated at the point concerned a radiation-intensive 
brightly glowing plasma zone which after removal of the flame and cooling 
of the respective point diminished again. After about 15 minutes thermal 
equilibrium with the surroundings was reached and a constant radiation 
intensity on the 254 nm line of 370 to 375% for a current strength of 3.6 
ampere was observed. 
When, on renewed ignition of the cold test tube the current strength was 
first set to 4.2 ampere it took only about 6 to 7 minutes until a high 
radiation intensity of about 400% could be measured in the middle of the 
test tube as well, this intensity however soon diminishing again. 
When a value of 3.6 ampere discharge current was then again set, the test 
tube stabilized at a radiation intensity of 370 to 375% again. This 
intensity was achieved with a burning voltage of 55 volts and a burning 
current of 3.6 ampere. 
Admittedly, the value of the radiation intensity was somewhat lower than 
expected This was however apparently due to the fact that the electrodes 
designed for 3 ampere were loaded beyond their capacity when the test tube 
was operated with a burning current of 3.6 ampere and consequently the 
electrodes started to emit vaporizing and degassing products which 
contaminated the composition of the plasma in the test tube. 
Test III 
The purpose of this comparison test was to determine whether the properties 
explained above could also be observed with an "electrodeless" test tube. 
The aim was not a quantitative determination of the individual parameters 
but only to check the qualitative similarity. 
A double-wall test tube was made consisting of two nested concentrically 
disposed quartz tubes. The outer tube had a diameter of 23 mm. The ends of 
the two tubes were fused together in gas-tight manner; the passage of the 
inner tube was free so that the double-walled tube could be pushed over a 
rod in the manner of a sleeve. To the outer tube, about 10 cm away from 
the end, a pump tube was attached which was connected to an annular cavity 
between the two tubes, thereby enabling the latter to be evacuated. 
This hollow jacket tube was heated out in known manner, evacuated, filled 
with pure argon at a pressure of about 200 Pascal and a mercury amount of 
20 mg introduced. The pump tube was then sealed to leave a length of about 
8 cm. 
The test tube had a total length of 750 mm, an outer diameter of 23 mm, an 
inner diameter of 15 mm and a wall thickness of 1 mm in each case. 
This "electrodeless" test tube was pushed onto an aluminium tube having a 
diameter of 14 mm and a length of 100 cm. This aluminium tube was pushed 
onto a magnetron tube (microwave tube) with a power of 450 watts and 
served as antenna. 
This arrangement comprising the test tube, antenna and magnetron tube was 
introduced into the measuring apparatus for determining the intensity of 
the radiation on the 254 nm line, the magnetron tube cooled with an air 
fan and the current supply for the magnetron tube then switched on. A few 
seconds after switching on the current the test tube started to glow 
spontaneously and very rapidly (within 30 seconds) with the bright light 
of the mercury discharge. Within one minute the spectral photometer 48, 50 
set to the 254 nm line recorded a radiation intensity of about 280% which 
then diminished again almost as rapidly and stabilized at about 20 to 25%. 
Even after a further burning duration of 20 minutes the radiation 
intensity did not increase again. 
The current supply for the magnetron tube was then switched off and the 
pump tube immersed in a vessel with ice water. The tube was again ignited 
and initially the behaviour of the test tube was as in the first test. A 
rapid rise of the radiation intensity on the 254 nm line was followed by 
an almost equally rapid drop to low values. However, after about 10 
minutes the intensity of the radiation on the 254 nm line increased slowly 
and after 40 minutes reached a maximum value of about 290% which remained 
constant for a relatively long period and then again slowly diminished. 
The test was broken off here because the quantitative information was not 
important; to obtain such information the microwave energy coupled into 
the test tube would for instance also have had to be determined. The 
present test provided the desired proof, i.e. that the method according to 
the invention can be used both with test tubes provided with electrodes 
and with "electrodeless" test tubes. 
The method according to the invention is not restricted to mercury as 
discharge gas but can be generally employed, for example also for alkali 
metal vapour gas discharge tubes. 
Although the mechanism on which the method according to the invention is 
based has not yet been completely explained, the essence thereof would 
appear to be that under operating conditions, that is for a certain 
discharge with a certain discharge voltage, a certain discharge current 
and the resulting electron energy and electron density, initially making 
available in the plasma of the gas discharge enough concentration of the 
discharge gas for the latter to be incorporated partially in the hot walls 
of the gas discharge tube, establishing equilibrium with the plasma. The 
partial pressure of the "free" discharge gas not incorporated in the walls 
of the gas discharge tube is then rapidly reduced until no more condensed 
gas discharge material is present in the gas discharge tube, in particular 
no reservoir of condensed liquid gas discharge material and in particular 
no socalled cold spot. A predetermined partial pressure of the remaining 
gaseous discharge material in conjunction with the electron density and 
electron energy obtaining then appears to provide preferred conditions for 
generation of desired wavelengths. 
The above considerations apply of course accordingly to a gas discharge 
tube according to the present invention having a substance which under 
operating conditions forms a discharge gas and in which the substance is 
substantially incorporated in the inner wall of the gas discharge tube. 
The substance may also be incorporated in a fluorescent layer at the inner 
wall of the gas discharge tube.