Ultraviolet discharge lamp

An ultraviolet lamp provides radiation in the 240 to 313 millimicron range for polymerizing photosensitive inks. Accurate quantities of metals and halides are provided by introducing stable gaseous organic compounds of the halogens and reacting the organic compounds with the desired metals to produce their halides.

BACKGROUND OF THE INVENTION 
The present invention relates to discharge lamps with an internal ionizable 
atmosphere containing mercury vapor, the vapor of at least one metal 
selected from group III A of the periodic table of elements, and at least 
one rare gas. It is characterized by the fact that this atmosphere also 
contains a halogenated organic compound, which combines with the selected 
metal when the tube is fully operating. 
The invention relates more particularly to a lamp operating at medium 
pressure and rich in ultraviolet radiation. This lamp is used as a source 
of ultraviolet radiation for producing photo-chemical reactions, such as 
in particular the polymerization of polymerizable inks and varnishes used 
by printers or for coating the surfaces of objects of various materials 
such as wood, tissue, plastic or metallic materials, etc. 
Among the various known advantages resulting from the processing of 
polymerizable inks and varnishes by ultraviolet radiation, mention may be 
made of the savings in power, the high processing speed and the reduction 
in pollution (noise and odors). 
In general, earlier ultraviolet lamps were designed to meet the 
requirements of photographic or photosensitive papers. Compared to tests 
made with variable-spectrum lamps, it was found preferable for the 
polymerization of polymerizable inks and varnishes to provide a lamp whose 
radiated power in the 240 to 313 millimicrons ultraviolet is high with 
respect to that in the 334 to 408 millimicron ultraviolet range. The 
development of a lamp meeting these requirements constitutes a first 
objective of the invention. 
In earlier techniques for printing drawings or copies, photographic or 
photosensitive paper was exposed by means of mercury-vapor lamps, whose 
maximum radiated energy was in the 365 to 407 millimicron range. In order 
to increase the intensity of the ultraviolet radiation, the spectral width 
had to be enlarged. This was achieved by adding metallic elements and a 
pure halogen, or again by means of metallic halides. A known method was to 
use, for example, metallic gallium and gaseous iodine or chlorine, or 
again a gallium halide in the form of gallium tri-iodide. A known 
disadvantage of these substances is the difficulty in adjusting the very 
small quantities required and of introducing them into the lamp. This is 
particularly true when chemically unstable compounds such as gallium 
tri-iodide are employed. Industrially, there is a danger of excessive 
statistical dispersion of the composition about the ideal or required 
values initially specified. 
In another former type of UV lamp, the radiated spectrum was widened by 
associating metallic gallium and mercury iodide with the mercury. When 
this lamp was started, it was found that the mercury iodide decomposed 
into mercury and iodine, and that the iodine recombined with the gallium 
to produce gallium iodide. This method avoided the handling and measuring 
of gallium iodide or iodine. 
One of the purposes of the invention is to overcome the above sources of 
error, by the introduction of a halide in the form of a stable compound 
easily measured. 
Another purpose is to reduce losses in the lamp by reducing visible and 
infra-red radiation as far as possible, as well as far ultraviolet 
radiation which produces ozone. 
SUMMARY OF THE INVENTION 
The present invention reveals that in the presence of mercury vapor and at 
least one of the following: one halogenated organic compound in the 
discharge atmosphere, one pure metal selected from one of the following 
three in group III A of the periodic table (gallium, aluminum and indium), 
and one rare gas, an electrical discharge lamp could be produced, 
particularly suitable for solving the problem of polymerizing 
polymerizable inks and varnishes. 
One characteristic of the present invention resides in the fact that the 
iodine is provided in the form of a halogenated organic compound which can 
be introduced into the lamp in gaseous form at ambient temperature, and in 
the fact that its volume and pressure may be easily controlled and 
measured in an accurate manner. Another characteristic of the invention is 
due to the fact that the metal selected may be introduced in relatively 
inaccurate quantities into the lamp, the only condition being that the 
amount exceeds a predetermined minimum amount, the upper limit being at 
least double without notably influencing the operation or behavior of the 
lamp. 
For suitable quantities of substances introduced in the lamp, the latter 
produces uniform radiation over the whole length of arc between the 
electrodes, even when its length is increased up to 2 meters. This is an 
advantage over certain former lamps, whose tubes were filled with a free 
halogen, such as iodine, and a metallic element, such as gallium. In these 
lamps, the spectral distribution and ultraviolet radiation level in the 
region close to the two electrodes and the central region of the tube were 
not uniform. 
Another characteristic of this lamp is due to the fact that the halogenated 
organic compound introduced into the tube prevents the formation of opaque 
deposits in the cooler parts of the lamp when the latter is operating 
normally. Consequently, this compound helps to preserve the initial 
uniform radiation. It appears probable that this contribution is due at 
least partially to the role played by the organic radical of this 
compound. 
Other characteristics of the invention will emerge from the detailed 
description below. It should be understood that the description and 
drawing are given as examples only, and in no way limit the scope of the 
invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred practical application of the invention can be seen in FIG. 1 
where the lamp consists of a cylindrical tube 10 made of very pure silica 
and very transparent to ultraviolet radiation. Its diameter is preferably 
18 to 22 mm. It may have a length of up to 2 meters for powerful lamps of 
the order of 25 kilowatts. The tungsten electrodes 11 and 11' are sealed 
in the narrowed ends of the tube, their spirals 12 and 12' in the tube 
space being coated with thorium. The other ends of these electrodes 11 and 
11' are connected to the external end pieces 13 and 13' of nickel or 
nickel-plated steel by means of the molybdenum foil conductors 14 and 14'. 
These are sealed in the axial holes of the silica tube 10. The molybdenum 
conductors 15 and 15' connect the molybdenum foils 14 and 14' to the 
copper braids 16 and 16'. Each of these braids 16 and 16' passes through 
an axial hole in the corresponding end-piece 13 or 13', and are braised at 
their exits on these end-pieces. These end-pieces 13 and 13' are 
preferably of the type described and claimed in the French patent filed by 
the present applicants on Apr. 11, 1974 under the French Pat. No. 74 12 
746. 
It is obvious that the distance D between the electrodes is a function of 
the required lamp power. If a lamp having a total power of 7.5 kilowatts 
is required, this inter-electrode distance is preferably 970 mm. 
The tube is filled as follows. Predetermined quantities of mercury and of a 
metallic element selected from one of the elements: gallium, indium or 
aluminum are introduced by a neck 17, and the tube is then pumped out in 
the conventional mannner. A certain volume of a halogenated organic 
compound in the gaseous state at ambient temperature is added by a tube 
connected to the neck. This compound may be an iodized organic compound 
such as methyl iodide (CH.sub.3 I) or ethyl iodide (C.sub.2 H.sub.5 I). 
They are fed into the enclosure at a suitable pressured adjusted between 
0.1 and 0.8 torr. The enclosure is then filled by at least one of the two 
rare gases, argon or neon, and the neck 17 is then sealed. 
In the previous example of a lamp designed to have a power of 7.5 
kilowatts, the metallic element selected and introduced in the tube is 
preferably gallium, its quantity being proportional to that of the mercury 
and the selected halogenated organic compound. The typical composition in 
the enclosure of a 7.5 kilowatt tube consists of at least 0.15 mg of 
gallium, 430 mg of mercury, a methyl iodide filling at a pressure of 0.15 
torr and afterwards of argon under a pressure of 20 torr. 
For a tube having an internal diameter of 20 mm, these quantities represent 
approximately 6 .times. 10.sup.-.sup.4 mg of gallium and 1.80 mg of 
mercury per cm3 of the tube. This quantity of gallium per cm3 is the 
minimum necessary value for correct operation of the tube, but this 
quantity may be greater, even double for example, without affecting the 
operation or behavior of the lamp. 
At ambient temperature, the gallium and mercury are condensed, while the 
methyl or ethyl iodide and the rare gas are gaseous. The same results may 
be obtained when the quantity of gallium is replaced by a corresponding 
quantity of aluminum or indium, i.e. by 2.4 .times. 10.sup.-.sup.4 to 4.8 
.times. 10.sup.-.sup.4 mg of aluminum or 1 .times. 10.sup.-.sup.3 to 2 
.times. 10.sup.-.sup.3 mg of indium per cm3. 
The lamp possessing the means described above may be operated by applying a 
supply voltage of 1225 V across the two electrodes, the load current being 
6.75 A. An arc is first established in the rare gas between the two 
electrodes. The energy dissipated then heats the tube, evaporating the 
mercury and gallium. After operating for approximately one minute, the 
lamp behaves as a lamp with mercury vapor only. After two minutes, the 
gallium reacts with the halogenated organic compound, and its ultraviolet 
spectrum is superimposed on that of the mercury. The lamp reaches its 
normal operating condition after three minutes when the total internal 
pressure is optimum, i.e. between 1 and 2 atmospheres. The cold points of 
the silica envelope of the lamp then reach a temperature of 600.degree. C. 
to 750.degree. C. 
The radiated power in watts per inch (1 inch = 25.4 mm) at the main wave 
lengths by two examples of lamps fed with a power of 200 watts per inch 
and 300 watts per inch respectively are given in the following table. 
______________________________________ 
Radiated power in watts per inch at the main wave-lengths. 
______________________________________ 
Power supplied 
Wave-length in millimicrons 
200 W/inch lamp 
300 W/inch Lamp 
______________________________________ 
1357/1393 0.9 1.8 
1189/1213 
IR 1119/1129 2.4 5.0 
1014 8.3 15.0 
691 0.3 0.5 
577/9 14.0 25.0 
546 15.4 29.0 
Visible 492 0.2 0.5 
436 12.2 24.0 
417 8.0 15.0 
408 1.0 2.0 
405 6.1 12.0 
UV C 403 4.0 8.0 
391 0.2 0.5 
366 12.1 24.0 
334 1.4 3.0 
313 5.5 10.0 
302 3.2 6.5 
297 2.0 4.0 
294 4.0 7.0 
UV B 292 0.4 0.7 
289 0.7 1.2 
287 4.0 7.0 
280 1.4 2.5 
275 0.4 0.6 
270 0.7 1.2 
265 1.5 2.5 
257 2.1 4.0 
UVA 254 1.0 2.0 
248 1.3 2.5 
240 1.2 2.0 
______________________________________ 
The table shows that the sums of the ultraviolet radiated powers A and B 
reach high values in watts per inch, compared with the ultraviolet 
radiated powers C. This lamp is therefore suitable for solving the problem 
of polymerizing polymerizable inks and varnishes. 
Compared with a lamp containing only mercury vapor, the improvement in the 
efficiency of ultraviolet radiation is more than 40% in the useful region 
between 240 and 408 millimicrons. At the same time, radiation is decreased 
in the regions of visible light and far ultraviolet beyond 240 
millimicrons, thereby decreasing the production of ozone. 
The improved efficiency of the lamp in watts radiated with respect to watts 
supplied also decreases the losses on the silica envelope, dropping its 
temperature by 50.degree. C. to 100.degree. C, thereby increasing the life 
of the lamp. 
Although the principles of the present invention are described above in 
relation with specific practical examples, it should be clearly understood 
that the said description is given as an example only and does not limit 
the scope of the invention.