US2297940A - Electric discharge lamp - Google Patents

Description

Oct. 6, 1942.

A. CLAUDE 2,297,940

ELECTRIC DISCHARGE LAMP Filed June 4, 1940 Inventor And v Claude,

y M v His Attorney.

Patented Oct. 6, 1942 S PATENT- OFFICE 2,297,940 ELECTRIC DISCHARGE LAMP Andre Claude,

Anonyme pour Paris, France, assignor to Socie'tc les Applications dc lElectricite et des Gaz ltares-Etablissements Claude-Paz et Silva, Paris, France Application June 4, 1940, Serial No. 338,767 In France June 15, 1939 10 Claims.

The present invention relates to gaseous electric discharge lamps, and more particularly to such lamps having a luminescent material preferably deposited as a thin layer on the inside of the lamp envelope or tube.

It has already been proposed to make' use of rare gases, to the exclusion of mercury vapor, for the gaseous atmosphere of fluorescent tubes. But, amongthe rare gases which are of low electroluminescence, that is to say such that the fluorescent tubes which-contain them supply a luminous emission essentially characteristic of the particular emission of the luminescent material that is employed, argon has not received any practical application up to the present time.

" However, I have found that very remarkable results, much better than those obtained with xenon and krypton, can be obtained by making use of practically pure argon as the filling gas and. by utilizing therewith various means, the

combination of which is the subject matter of the present invention. I

The luminous efliciency of tubes of the type comprising my invention is considerably increased owing, on the one hand, to improvements which have been made in the manufacture of luminescent material the most important of which consists in reducing the percentage of alkaline metals therein, and, on the other hand, to the filling of the tube with a low pressure of argon. However, the use of these low pressures in the case of argon ordinarily results in an increase 'of the absorption of; this gas in the tubes and, consequently, a reduction of the life thereof. to such a degree that they -are not suitable for commercial purposes. .This contrasts with krypton and xenon tubes wherein, at the same pressures, this drawback does not occur andthere is no difliculty in obtaining, with the trodes, tubes having an extremely long life.

The problem of the life of the tubes has been solved by us by taking advantage of a combination'of the following conditions:

(a) The argon pressureswhich, from the point of view of the luminous efficiency must be very low, can however be made sufliciently high to permit an admissible life of the tube; suitable pressures are from 1 millimeter to 3 millimeters and preferably about 2' millimeters;

(b) The use of low current-intensities, lower for a given diameter than the lowest normally utilized in the usual luminescent tubes, such for instance as neon tubes, argon-mercury tubes, or

time ensure a satisfactory luminous efliclency. These current intensities are from 5 miliamperes to 30 milliamperes and preferably about 15 milliamperes. Of course, with such intensities, and in order to obtain a .sufiicient brilliancy, the diameters of the tubes that are used should be those of the usual tubes, generally lower than 25 millimeters, and preferably of the order of 10 millimeters, the diameters of this order of magnitude giving, as a rule, the maximum ofbrilliancy.

(c) Finally, among the various types of known electrodes which can be used in connection with electric discharge tubes, the electrodes which act both as anode and cathode, covered on the parts thereof which are liable to be disintegrated during operation, especially on their edges, with a dielectric coating, and further provided with a deposit which produces a low cathodic voltage drop, for instance a deposit of a metal such as barium or potassium, or an oxide, such as barium oxide, are particularly useful for obtaining commercial lives of several thousands of hours.

The variations of the luminous efficiency as a function of the current intensity is all the more important as the filling'pressure is lower. It follows that even when, fora given current, the. luminous efficiency drops when the pressure is reduced, it is possible, by increasing the current intensity, to bring this luminous efficiency well above that obtained under the same conditions but with a higher pressure.

usual elecrare gas-mercury fluorescent tubes, while permitting the obtaining of a suitable life at the same In all cases it must be borne in mind that an increase of the luminous efficiency resulting from a drop of pressure or from a current increase, or from both of these variations simultaneously, does not always justify (far from it) the loss in life of the tube at the cost of which itphas been obtained.

By way of example, considering three identicalv tubes filled with argon at the respective pressures of 3 2 mm., and -1 mm. of mercury,

it is found that for the tube at the pressure of 3 mm. the luminous efliciency, expressed in arbitrary units, remains substantially equal to 4.30 when the current is varied from 10 to 25 milliamperes for instance. For the tube filled at the pressure of 2mm. of mercury, it is found that, if the luminous efficiency varies for instance from 6.2 to 7.4 when the current varies from 10 to 15 milliamperes, this efliciency reaches only 1.8 at 25 milliamperes; and as the life of the tube is longer at 15 milliamperes than at 25 milliamperes, it is therefore preferred, according to the present invention, to operate the tubes with an intensity averaging 15 milliamperes. In the case of the tube filled with argon at the pressure of 1 mm. of mercury, it is found that the luminous efllciency is 6 for a current of 10 milliamperes,

7.2 for a current of milliamperes and 9 for a current of .milliamperes. It follows that, up

. to 15 milliamperes a better efficiency is obtained flux (which then varies from 7.4 to 9) averages 15%, this is offset by the fact that the life of the tube is reduced by more than three quarters of its value.

For the luminescent materials, I may make use of tungstates, such as calcium tungstate, and silicates, such as zinc silicate, used at the present time in neon fluorescent tubes, or rare gas and mercury vapor tubes, the materials being obtained from substances which are extremely pure, particularly regarding the percentage of iron and alkaline metals therein. The importance of a low percentage of iron (of the order of 00 0) However, I have has already been pointed out. found that the percentage ofalkaline metals is no less important for the obtainment of a good luminous efliciency and the maintaining of this efllciency during the life of th tube, especially when, as in the present invention, the luminescent materials are then placed in electric discharge. tubes, since experience hastaught that, for instance with a zinc silicate, the luminous efliciency is particularly highwhen the percent- .age of alkaline metals is lower than ,4 and pared with the emission of the luminescent substance, the joints can be made by merely utili'zing a tube element which does not contain lumi- In this way I avoid welds onluminescent portions which generally alter the luminescence.

preferably of the order of Amoco. Also, it is advisable in the case of 'zinc silicate activated by a phosphorogemsuch as manganese, to utilize a substance in which the excess of silica is not toohigh. Previously, silicates were commonly made in which the percentage of silica was for instance equivalent to a proportion of 80 parts of silica for 20 parts of zinc oxide. It is preferable for good luminous efllciency to utilize proportions which are. closer to the theoretical composition of zinc orthosilicate, such for instance as 50 parts of silica for 50 parts of zinc oxide. I may also utilize zinc. beryllium mixed silicates Of course, the tubes according to the present invention have all the quahties of the rare gas fluorescent tubes without mercury, to wit: proper operation at very low temperatures, maintenance of the luminous efficiency during the life of the tube, facility'of forming luminous designs. On the other hand, the linear voltage drop is substantially the same in the case of argon tubes as in the case of the usual rare gas and mercury vapor tubes; Consequently, a similar transformer permits of feeding the same length of the fluorescent argon tube 'as of the usual rare gas and mercury tubes, and the reduction of the current intensity to a low value in argon tubes permits the obtaining of a considerable economy in the cost of the transformer and the consumption of current.

For a further understanding of my invention, I

reference may be made to the accompanying drawing which is a somewhat diagrammatic representation, in section, of one end of a species of fluorescent tube comprising my invention.

Referring to the drawing, the tube comprises an elongated tubular envelope l0 having sealed into the end thereof a sheet metal electrode ll, of iron or nickel for example, provided with a coating which produces a low cathodic voltage drop, such as a, coating of barium or potassium or barium oxide. The forward edge of the said electrode is protected from disintegration by suitable means, such as a dielectric coating 12 of a suitable glaze. The opposite end of the .tube may be provided with a similar electrode. The tube contains a filling of argon at a pressure of from 1 to 3 mm. and a coating l3 of luminescent material as described above.

with these fluorescent substances to obtain very beautiful golden yellow luminescent emission. V

Experience also teaches that, contrary to what takes place with fluorescent argon-mercury tubes, it is of advantage to make use of the luminescent material in the form of a film as thin as possible,

A remarkable characteristic of th present invention is the considerable increase in the luminous efliciencies that are obtained. The argon tube with a pressure of 2 mm. gives, when it is not fluorescent, a light flux equal to 0.53 arbitrary unit, and to 7 in the contrary case. This result gives rise to a practical application of considerable interest. As a matter of fact, when such fluorescent tubes serve to form luminous advertisements or ornamentations, for instance, it happens often that, for reasons of facility of utilization,'a single tube is employed for making two distinct tubes. Under these conditions,.a portion of the tube is to be concealed, either by hiding it behind the background of the motive or by covering it with'an opaque coat. With the tube comprising the present invention, as the luminous emission of argon is very low as comelectrodes having a coating thereon producing a low cathode drop and being provided with means for protecting the portions thereof which are subject to disintegration, and means for supplying current to the tube at an intensity between 5 to 30 milliamperes.

2. A. discharge tube as set forth in claim 1 wherein the luminescent material comprises zinc silicate in which the proportions of the components correspond approximately to the formula of zinc orthosilicate.

4 3'. A discharge tube as set forth in' claim '1 wherein the luminescent material comprises a tungstate. l

4. An electric discharge tube comprising an envelope containing a gaseous atmosphere consisting of argon at low pressure and a layer on the inner surface of said envelope of luminescent material containing less than .001 per cent alkaline meta-ls.

5. An electric discharge tube comprising an elongated tubular envelope containing widely.

spaced electrodes and a gaseous atmosphere consisting of argon at a mercury, a layer on the inner surface of said envelope of luminescent material containing less than .001 per cent alkaline metals, said envelope having a diameter less than 25 mm., and means for supplying current to said tube at an intensity between 5 to 30 milliamperes.

6. An electric discharge tube comprising an elongated tubular envelope containing widely spaced electrodes and a gaseousatmosphere consisting of argon at a pressure of 1 to 3 mm. of

mercury. a layer on the inner surface of said envelope of luminescent silicate containing less than .001 per cent alkaline metals, said envelope having a diameter less than 25 mm., and means for supplying current to said tube at'an intensity between 5 to 30 milliamperes.

'7. An electric discharge tube comprising an elongated tubular envelope containing widely spaced electrodes and a gaseous atmosphere consisting of argon at a pressure of '1 to 3 mm. of mercury, a layer on the inner surface of said envelope of luminescent tungstate containing less than .001 percent alkaline metals, said envelope having a diameter less than 25 mm., and

means for supplying current to said tube at an intensity between 5 to 30 milliamperes.

8. An electric discharge tube comprising an elongated tubular envelope containing widely spaced electrodes and a gaseous atmosphere consisting of argon at a pressure of 1 to 3 mm. of

pressure of 1 to 3 mm. of-

mercury, a layer on the inner surface of said envelope of luminescent material containing less than .001 per cent alkaline metals, said envelope having a diameter of the order of 10 mm., and means for supplying current to said tube at an intensity between 5 to 30 milliamperes.

9. An electric discharge tube comprising an elongated tubular envelope containing widely spaced electrodes and a gaseous atmosphere consisting of argon at a pressure of 1 to 3 mm. of mercury, a layer on the inner surface of said envelope of luminescent material containing less than .001 per cent alkaline metals, said envelope having a diameter of the order'of 10 mm., and means for supplying current to said tube at an intensity of -the order of 15 milliamperes.

10. An electric discharge tube comprising a tubular envelope containing a gaseous atmosphere consisting of argon at a pressure of 1 to 3 mm. of mercury, a layer on the inner surface of said envelope of luminescent material containing less than .001 per cent alkaline metals, a pair of electrodes adjacent the ends of said envelope serving alternately as anode and cathode, said electrodes having a coating thereon producing a ANDRE: CLAUDE.

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