US2703374A - Stroboscopic light source - Google Patents

Description

March 1, 1955 F. FRUENGEL Filed Jan. 15, 1951 STROBOSCOPIC LIGHT SOURCE 2 Sheets-Sheet 1 INVENTOR. FRANK FRUENGEL March 1, 1955 F. FRUENGEL STROBOSCOPIC LIGHT SOURCE 2 Sheets-Sheet 2 Filed Jan. 15 1951 INVENTOR.

FRANK FRUENGEL AGENT United States Patent STROBOSCOPIC LIGHT SOURCE Frank Fruengel, Hamburg-Rissen, Germany Application January 15, 1951, Serial No. 206,049

Claims. (Cl. 313-149) The present invention relates to electrical spark discharge devices and more particularly to gaseous discharge devices in which light flashes of extreme brightness are eflfected by sparkovers between electrodes immersed in suitable gases of suitable pressure.

A general aim ofthis invention is to provide an improved electrical spark discharge circuit and a device therefore having a discharge vessel of demountable construction which can be manufactured economically and with which spark discharges of highest intrinsic brilliance, extreme brightness, and extreme short duration can be eifected at controllable frequencies in the range of from single flashes up to 1000 discharges per second; thus, making the device, besides other usefulness, especially suitable for the emission of light flashes in ultrahighspeed photography and for stroboscopic illumination under normal ambient light condition.-

It is well known in the art that sparks of high brightness can be eifected by discharging a condenser by spark-over across a spark gap and that the brightness of such sparks can be enhanced by having them occur within a gaseous atmosphere. The basic circuit of spark discharge devices, permitting controlled spark-overs, includes a condenser, permanently connected to a suitable high-voltage direct-current source through a resistance or impedance limiting the charging rate to a reasonable value; and a discharge path for the condenser through a grid-controlled thyratron and a spark gap of suitable construction. In such arrangements, the condenser is continually charged from the direct-current source through a current-limiting resistance or impedance; and discharge surges withresulting breakdown sparks are tripped as desirable by firing the thyratron tube by means of voltage impulses to its grid. .The instantaneous brilliance or brightness ofthe resulting sparks depends thereby not only on the charge and capacity of the condenser, but largely on the constants of the discharge circuit or path. In a discharge arrangement of the kind referred to, it is, however, difiicult tochange the constants of the circuit in such a manner that an optimum of light emission, i. e., highest instantaneous brightness of the spark, is achieved and the duration of the flash reduced to an order of about onemicrosecond. Such instantaneous discharge and thereby conversion of the total energy stored in the condenser into a brilliant spark can only be realized, when making the spark resistance substantially equal to the aperiodic limit resistance of the discharge path which includes the impedance of the condenser and of the connecting lines inclusive of their inductance. Theoretical calculations have revealed, that to approach an optimum of spark brightness instantaneously, it is necessary to reduce the inductance of the discharge path to the order of 0.03 microhenry. Such small inductances, however, are not realizable when compelled to connect voluminous units, such as thyratrons and spark gaps within gaseous sealed envelopes, in the circuit; because connecting such units requires leads of considerable length having an inductance of at least 0.5 microhenry. With conventional equipment it is practically impossible to reduce this value considerably and to obtain higher spark brightness than the order of to 10 candles per square centimeter or stilbs.

To eifect the spark discharge within a suitable gaseous atmosphere of increased pressure or Within high-pressure mercury vapor and thereby enhance the brightness of the spark as already mentioned, sealed gaseous discharge tubes are generally applied; Such seale'd't'ubes have, howvolume 7. Cover plate ever, several drawbacks and a limited life, especially, if for emission of very bright flashes large currents, amounting up to 10,000 amperes, have to be discharged; because such large current pulses are a heavy burden on the lead-in connections sealed in the walls of the glass or quartz envelope of the tube on account of the fact that the connections are considerably heated by such currents resulting in stresses at the seals and possible cracks and destruction of the tube. Besides, such lead-in connections for large currents through glass envelopes are voluminous, hard to produce, and therefore expensive. The life of such tubes is also decreased by particles thrown off from the electrodes, which deposit on the inner walls of the glass envelopeand cause noticeable diminution of light penetration already after 10 to hours of operation. As sealed discharge tubes, designed for the emission of high-intensity flashes as required in ultra-high-speed photography and for stroboscopic observation in normal ambient light, have, for reason mentioned above, relatively short and uncertain life, it is necessary for the user to stock spare tubes. Furthermore, access to the electrodes for exchange or replacement of the same, or for adjustment of the spark gap in order to alter the intensity of the spark according to requirements, i. e., less light if it suffices, and to thereby prolong the life of tube and electrodes, is impossible in all sealed spark discharge devices.

The present invention aims at overcoming some or all of the shortcomings enumerated above and has as its specific objects, first, to provide a demountable gaseous discharge vessel aflording access to its interior for replacement of electrodes and for cleaning of the inner walls of the envelope; and, second, to make possible a discharge path of extremely low inductance resulting in light flashes of optimal brightness efiected by spark-overs between electrodes immersed in a gaseous filling above atmospheric pressure.

Another object of this invention charge device which can handle up to 10,000 amperes and above afiecting its structure.

Another object is to provide improved electrodes for a discharge vessel of the kind above referred to.

A further object of this invention resides in the construction and arrangement of an ignition and control circuit for initiating or tripping spark discharges either by means of an electronic oscillator tuned to a desired frequency, by a mechanical contactor actuated by a timer in connection with a machine under observation, or by a manually operated switch. The ignition circuit according to this invention includes an ignition electrode placed in properrelation to the main electrodes within the gaseous vessel and means for inflicting high-tension impulses to said electrode from a secondary-type induction coil whose primary is instantly energized by action of either one of the above named actuating means.

Other advantages and features of this invention will be more fully explained in the following description and more particularly pointed out in the appended claims. In the accompanying drawing, showing diagrammatically a preferred embodiment of this invention, Fig. 1- is a front elevation of the stroboscopic light source with reflector; Fig. 2 is a side elevation of the device, shown is to provide a dislarge current pulses without detrirnentally as section on the line 2-2 of Fig. 1; Fig. 3 is an en larged sectional side view of the spark Fig. 4 is a diagrammatic view of the main and ignition circuits as provided by this invention; and Fig. 5 is an enlarged sectional detail of a modified electrode adjusting and sealing means. I

Referring to the drawing, the demountable discharge vessel (Fig. 3) comprises generally a hollow cylinder 1 of suitable transparent'material such as glass, Plexiglas, or quartz; a cover plate 3; and a bottom 4. The cover plate 3 is preferably of metal and the bottom plate 4 may be of metal or a suitable insulating material. Suitable sealing gaskets 2 are preferably inserted in both joints between plates and cylinder or other means of sealing, such as plane ground surfaces slightly greased or otherwise prepared, may be applied to create a gas-tight enclosure or vessel for-the introduction of a suitable gas 3 and bottom plate 4 are firmly discharge vessel;

pressed against the respective sealing surfaces at the ends of cylinder 1- by a number of rods 5 threaded at both ends and projecting through suitable holes in top and bottom plates and tightened by nuts 6. Bottom plate 4 may be of considerable larger diameter than the diameter of cylinder 1 and may serve as a mounting base for other parts of the unit as can be clearly seen in Fig. 2. The rods 5, according to this invention, are part of the current-carrying system and therefore of suitable conducting material such as copper, brass or the like. Fastened to the inside of the cover plate 3 is an offset bracket Shaving a threaded opening at the center axis of the cylindrical vessel into which the outside-threaded electrode holder 9 is screwed. Thus, by rotating electrode holder 9 in its bracket, axial adjustment of the former, holding electrode 14, is obtained. In axial alignment with the electrode holder 9,- a conical aperture is provided in cover plate 3 into which a tapered plug 10 is gas-tightly but rotatably fitted from the inside of the cover plate. Engaging means between electrode holder 9 and tapered plug 10, comprising, for example, a projection 11 on plug 10 and a suitable groove in holder 9, are provided so that electrode holder 9 and electrode 14 can be adjusted from the outside of the vessel by rotating plug 10. For manipulation of plug 10, carries the same a knobor finger piece 13 which assists also in retaining the plug in its place. A valve 12 of the regular pneumatic-tire type is provided in plug 10 serving for introducing or replenishing the gas filling within the vessel. It is, however, to be understood that said valve may also suitably be located at any other point in the wall of the vessel. The threaded portions of bracket 8 and electrode holder 9 are preferably silver-plated so they will not be affected by chemical reaction of the gaseous filling within the vessel. The engaging surfaces of the aperture and tapered plug 10 are preferably lapped and lubricated with a temperatureindiiferent lubricant so that a gas-tight scaling is assured. As the pressure of the gas filling within the vessel is maintained above atmospheric pressure it will assist in pressing the sealing surfaces firmly together.

In a modified construction shown in Fig. 5, plug 10 is omitted and adjustment of electrode 14 attained by applying a metal bellow 35. Said bellow is at its one end gas-tightly secured to cover plate 3 and at its other end to electrode holder 9. A threaded bolt 36 extends from said electrode holder through an opening in the cover plate and is threadingly engaged by finger piece 13 at its outer extremity. Thus, bolt 36 with attached electrode holder 9 and electrode 14 may be axially moved by turning finger piece 13 againstthe action of spring 37. Other modifications in actuating means for adjustment of electrode 14 and other sealing means between cover plate 3 and actuating means will occur to those skilled in the art, but it is to be understood that such modifications fall within the rightful scope of this invention.

The actuating. means for electrode adjustment may be also applied to actuate a wiper 22 serving to remove dust particles, thrown oif by the electrodes, from the inside wall of the glass cylinder without having to demount the vessel. For this purpose, a radially extending sleeve 23 is rigidly fixed to the forward extremity of electrode holder 9 over which another sleeve 24, rigidly fixed to wiper 22, is telescoped. A spring 25, confined Within sleeve 23, urges sleeve 24 outwardly and thereby wiper 22 against the inner surface of cylinder 1. Wiper 22 is preferably faced with a rubber or leather strip 26 to increase wiping action. If now the cylinder needs cleaning from the inside, this can be accomplished by turning finger piece 13, and in connection therewith wiper 22, to and fro for the amount of one revolution. The adjustment of the electrode 14 need not be affected thereby as the original position of the actuating means can be easily reestablished after cleaning.

Electrode 14 is suitably inserted and secured in holder 9, whereas the counter-electrode is carried by holder 16 projecting through bottom plate 4 and being gastightly sealed therein by a suitable sealing bushing 17. An annular electrode 18 is arranged concentrically around and substantially in the radial plane of the tip of electrode 15 leaving an annular gap around the tip amounting in distance to about A to /3 of the spark gap distance between the main electrodes 14 and 15. Electrode 18, being the ignition-electrode, is supported by suitable means such. as stud 19 which projects through bottom plate 4 and is sealed therein by a sealing bushing 20. The annular shape of. the ignition. electrode is. a preferred form and it will be understood that this electrode may assume other suitable shapes such as a bar or needle placed in proper spaced relation to the main electrode, or the main electrode may be given an annular configuration and the ignition electrode arranged at the center axis without materially impairing the results of this invention.

To provide insulation for the lead-in connections of the electrodes, the bottom plate 4 is preferably made of heat-resisting plastics or ceramic material. It is, however, also possible to make the plate 4 of metal and apply insulating bushings around the lead-in connections. In discharge vessels intended for heavy and frequent current discharges, heating of the current-carrying parts may be considerable and it is advisable to provide cooling ribs or fins 21. on the cover plate 3 and on the projections of the lead-in conections to dissipate the heat more readily. Also forced-air ventilation may, in some cases, be applied for cooling purposes to some advantage. The leadin connections, as represented by stud 19 and electrode holder 16, are preferably made of copper or other heatconducting material to carry the heat more readily to the outside of the vessel. Proper care has to be taken in designing the insulating parts, especially the bottom plate 4, giving it ample over-surface distance between current-carrying parts to prevent fiashovers and leakage currents. It is good practice to provide ridges in the surface of said plate to increase the length of the leakage path or, if the plate is of metal, to surround the leadins with bushings in substantially a similar manner as applied in spark plug and other high-voltage construction; in fact, it is possible to use a suitable spark plug as main electrode.

A suitable reflector 27 is attached to the discharge vessel just described, as can best be seen in. Fig. 2. The main spark gap between electrodes 14 and 15 has thereby to be adjusted so that the focal point of the reflector lies substantially in its center. The discharge condenser 28 is arranged directly adjacent to the vessel in order that the discharge path can comply to the requirements of this invention. A cylindrical housing 33, encloses the condenser 28, a secondary-type induction coil 29 and the other elements of the circuits, and carries at its bottom a threaded receptacle or bushing 34 adapted for mounting on a tripod or another suitable stand. The induction coil 29 is of standard construction and need not be described.

Size and volume of a discharge vessel according to this invention, depend to a large degree on the electrical energy stored in the discharge condenser and the amperage of the discharge pulses. The efficiency of visible-light emission is about 20% with additional emission of about 20 to 30% in infra-red and ultra-violet rays, so that about one half of the total energy introduced into the circuit is converted into heat which has to be dissipated and for which ample radiating surface or other cooling means have to be provided. A considerable amount of heat is carried off by radiation and convection through the wall of the transparent cylinder; and it has been found important to give this cylinder a surface commensurate with the incurring losses.

To reduce the consumption of the electrodes 14 and 15 to a minimum and to increase their life by a given length and diameter, this invention provides electrodes moulded or sintered from powdered hard metal, such as, for exam ple, a mixture of silver and tungsten or copper and tungsten.

The stroboscopic light source as above described is preferably used in an electrical spark discharge circuit as shown in Fig. 4, in which the condenser 28 is continually charged from a suitable high-voltage direct-current source through an adjustable impedance or resistance 32, and in which spark discharges of condenser 28 are effected by inflicting voltage impulses on ignition electrode 18 by means of the ignition circuit including secondary-type induction coil, or peaking transformer, 29, condenser 30, and resistor 31.

The function of a discharge device and circuit using the demountable light source of the present invention is explained in a divisional application, Serial No. 275,157, filed March 6, 1952, to which reference may be made for further details.

Practical experience has further shown that fiashovers across the insulating surfaces of the discharge vessel are more likely to occur in a gaseous filling of pure noble gases than if electronegative gases or vapor such as for instance H2O, CO: or content of the vessel.

While I have described what I at present consider the preferred embodiment of my invention, it will be apparent that various changes and modifications may be made without departing from the true spirit and scope of my invention, and it is therefore my aim to cover all such changes and modifications by the appended claims as fully as rightfully possible.

What I claim is:

1. A demountable spark discharge device for a source of short-time light flashes comprising a transparent hollow cylinder of suitable material, cover and bottom plates forming closures for said cylinder and constituting in combination therewith a gas-tight vessel, two discharge electrodes coaxially mounted at the axis of said cylinder within said vessel and facing oppositely a spark gap, leadin connections for said electrodes, mechanical adjusting means projecting through said cover plate for changing the length of said spark gap from the outside of said gastight vessel, and valve means in one of the walls of said vessel for introducing a gaseous filling and for changing the gas pressure within said vessel.

2. A demountable spark discharge device as in claim 1, in which the filling of said gas-tight vessel constitutes a noble gas with admixtures of electrically negative gases or vapors, which filling can be replenished during regular service of the device.

3. A demountable spark-discharge device for a source of short-time light flashes comprising a gas-tight cylindrical vessel having a transparent cylindrical wall and opaque cover and bottom plates, two discharge electrodes mounted coaxially and coincident with the axis of and within said vessel facing oppositely a spark gap, an annular ignition electrode arranged concentrically around and substantially in the radial plane of the spark-emitting face Freon are admixed to the gaseous of said main electrode and in spaced relation thereto, insulated separate lead-in connections for said main electrode and the surrounding ignition electrode, and conducting connecting means-for said other electrode to the cover plate.

4. A demountable spark discharge device for a source of short-time light flashes comprising a gas-tight cylindrical vessel having a transparent cylindrical Wallan insulating bottom plate and a conducting cover plate, two discharge electrodes mounted coaxially and coincident with the axis of and within said vessel facing oppositely a spark gap, a tapered plug rotatably inserted in an aperture at the center of said cover plate, and gas pressure within said vessel assisting in pressing said tapered plug into said aperture to eifect sealing of the joints therebetween.

5. A demountable spark discharge device as claimed in claim 4, having a wiper secured to said tapered plug and resiliently pressed against the inside of said transparent cylindrical wall, and having manipulating means attached to the outer end of said tapered plug adapted to serve simultaneously for actuating said wiper and for adjusting said spark gap.

' References Cited in the file of this patent UNITED STATES PATENTS 1,159,914 Farnsworth Nov. 9, 1915 1,418,022 Reisz May 30, 1922 2,007,939 Braselton July 9, 1935 2,130,077 Dorgelo Sept. 13, 1938 2,169,723 Desorrneaux Aug. 15, 1939 2,351,254 Elmendorf June 13, 1944 2,404,116 Wolowicz et a1 July 16, 1946 2,454,761 Barrow et a1. Nov. 30, 1948 2,516,326 Knowles July 25, 1950 2,567,491 Mitchell Sept. 11, 1951

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