March 24, 1970
`
`RICHTER’
`J.
`HIGH mrmnsmy ARC LAMP
`
`'
`
`3.502329
`
`Filed. July 14, 1967
`
`5']
`
`INVENTOR.
`
`JOHN F. RICHTER
`
`BY
`
`/,%.,+/zé9,%
`
`ATTORNEY
`
`ASML 1126
`
`.ASh&L1126
`
`

`
`United States Patent Otfice
`
`3,502,929
`Patented Mar. 24, 1970
`
` 1
`
`2
`
`3,502,929
`HIGH INTENSITY ARC LAMP
`John F. Richter, San Francisco, Calif., asslgnor to Varian
`Associates, Palo Alto, Calif., a corporation of Cali-
`fornia
`
`Filed July 14, 1967, Ser. No. 655,717
`Int. Cl. H01j 5/16, 61/40
`U.S. Cl. 313-111
`
`
`
`17 Claims
`
`ABSTRACT OF THE DISCLOSURE
`
`A novel structure is described for a gas discharge
`lamp adapted for operation in the short are mode and
`capable of producing increased light flux density as well
`as increased total light at a given light flux density. The
`structure includes a compact ceramic envelope to pro-
`vide high power handling capability in terms of current
`and voltage and to enable containment of ionizable gases
`at high pressures. Improvements in efliciency through in-
`corporation of a light reflector within the envelope and
`the use of sapphire and other special window materials,
`are described.
`
`
`
`BACKGROUND OF THE INVENTION
`
`This invention relates to a gaseous discharge device
`and particularly to a novel high intensity short are
`lamp structure.
`In optical projection systems involving the generation
`and precisely controlled radiation of long pulses of non-
`coherent light, such as in spectroscopy, microscopy, and
`solar simulation,
`in addition to the more conventional
`projection systems,
`there is a need for a light source
`capable of producing the highest possible light
`flux
`density, that is, the greatest total amount of light from
`the least possible volume. The ideal would be a point
`source of light with unlimited light output.
`Of the electrical devices for the generation of non-
`coherent
`light in pulses of substantial
`length, gas dis-
`charge devices offer
`the possibility of generating the
`greatest total quantity of light from the least possible
`volume (i.e.,
`light flux density). The light flux density
`which can be produced by incandescent or luminescent
`devices is limited by the amount of power that can be
`concentrated in the solid materials which serve as the
`light emitters before a change of state occurs in such
`material,
`‘whereas in a gas discharge device no such
`change of state can occur in the light emitting medium
`regardless of the concentration of power.
`The amount of power which can be concentrated in
`a gas discharge may be maximized by decreasing the
`spacing between the electrodes of the device and in-
`creasing the pressure of the gaseous medium, the voltage
`at which the discharge operates, and the current carried
`by the arc. It has been found that for any given volt-
`age and current the greatest
`light flux density will be
`obtained when the electrode spacing and gas pressure
`are adjusted to produce an arc discharge which is rough-
`ly spherical (that is, the length of the arc is approximate-
`ly equal to its transverse dimensions). In this mode of
`operation the electrode spacing is less than two centi-
`meters and usually less than one centimeter. Arc dis-
`charge devices designed to operate in this mode are
`called “short arc” devices to distinguish them from other
`forms of arc discharge such as “medium arc” and “long
`arc” devices which may produce larger total quantities
`of light but at much lower light flux density.
`According to the teaching of the prior art short are
`lamps consist of a vacuum-tight, transparent bulb located
`adjacent a reflector. The reflector is often fixed to an
`
`least partially sur-
`outer housing or casing which at
`rounds the bulb, and which has a window through which
`light from the arc passes. The bulb envelope is usually
`made of amorphous fused silica in a tubular shape.
`Housed within the fused silica envelope along its tubu-
`lar axis are a pair of electrodes. A small space between
`these electrodes defines an arc gap. A spherical aneurism
`in the envelope surrounds the arc gap to reduce the
`intensity of
`the heat
`to which the fused silica will
`be subjected during lamp operations. The tubular shape
`of the envelope is also based upon heat considerations
`for it provides two ends relatively remote from the
`arc. The electrodes are sealed through the envelope
`at these ends and are thus located at the coolest part
`of the enevelope. Prior to envelope sealing the bulb is
`filled with an inert gas or metal vapor under some 2
`to 15 atmospheres of pressure at room temperature.
`At operating temperatures the gas pressure can be ex-
`pected to increase to some 10 to 50 atmospheres.
`The short are lamps of the prior art, such as that
`just described, are limited both in the .light flux density
`which they are capable of producing and in total amount
`of light
`that
`they can produce at a given light flux
`density. Applicant’s structure disclosed and claimed here-
`in makes possible the production of both greater light
`flux density and a greater total amount of light at a
`given light flux density,
`than short are devices of the
`prior art.
`As explained above, reflectors in the prior art have
`been spacially separated from the bulb. This increases
`the overall size and weight of the lamp. It also creates
`a maintenance of alignment problem under shock and
`vibrant environmental
`conditions. Furthermore,
`light
`passing through the bulb to, and in some designs from,
`the reflector is diffused and refracted thereby lowering
`the collector efliciency of the device. Applicant’s struc-
`ture described and claimed herein largely eliminates these
`disadvantages.
`The electrodes or their wiring, within the bulbs of
`the prior art lamps have had to possess significant length
`in traversing the tubulation between the arc gap and the
`metal-to-fused silica seals located at the cooler ends of
`the tubular bulbs. Inductance of this length of wiring
`impeded the arc current whenever the lamp was operated
`in a pulse mode. Applicant’s structure described and
`claimed herein, enables a reduction in the length of
`this internal wiring and a corresponding reduction in in-
`ductance.
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`25
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`30
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`40
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`45
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`Finally, at least a portion of the bulb envelopes in the
`prior art has served as a bulb window. In some configura-
`tions a second window has been provided in the outer
`housing. Though the housing window could be open air,
`the bulb window was made of the same fused silica as that
`of
`the envelope. This material,
`though transparent
`to
`light in the near infrared, visible and ultraviolet regions
`of the electromagnetic spectrum, is opaque to that of the
`far infrared and ultraviolet regions. Where utility in these
`latter regions has been specified, short are lamps of the
`prior art cannot be used. Applicant’s structure disclosed
`and claimed herein permits use of special window ma-
`terials transparent in all of these regions, such as sap-
`phire, lithium fluoride and magnesium fluoride.
`Accordingly, it is an o-bject of the present invention to
`provide an improved high intensity are lamp.
`It is also an object of the invention to provide a short
`arc high intensity lamp having a composite, integral bulb
`and reflector.
`
`Another object of the present invention is to provide a
`rugged, high intensity are lamp having no glass-to-metal
`seals, and having an envelope composed of materials
`which will withstand severe shock, vibration, heat and
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`65
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`

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`3,502,929
`
`3
`pressure environ-mental conditions, and which will not
`react with the gaseous atmosphere of the lamp.
`Yet another object of the invention includes production
`of a compact high intensity are lamp having low induct-
`ance electrodes and electrode terminals, high collector
`and energy conversion efficiency, and windows pervious
`to the passage of electromagnetic energy in the region
`bounded by the far-ultraviolet and far-infrared.
`SUMMARY OF THE INVENTION
`
`the present invention is a high in-
`Briefly described,
`tensity, short are lamp comprising a sealed envelope, a
`portion of which is ceramic. The envelope houses a
`cathode an an anode which are spaced apart a distance
`less than two centimeters to define a short are gap there-
`between. The envelope also houses an ionizable gas under
`at least two standard atmospheres of pressure. A reflector
`and a sapphire window may also form a portion of the
`envelope.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGURE 1 is an elevational view partly in cross-section
`of a typical high intensity short are lamp structure of the
`prior art.
`FIGURE 2 is a cros.s-sectional view of one embodiment
`of high intensity short are lamp structure of the present
`invention. FIGURE 2:: is a frontal View of the lamp in
`FIGURE 2.
`FIGURE 3 is a cross-sectional view of another embodi-
`ment of a high intensity short are lamp structure of the
`present invention. FIGURE 3a is a frontal View of the
`lamp in FIGURE 3.
`DESCRIPTION OF PRIOR ART
`
`Referring now in detail to the drawing, there is illus-
`trated in FIGURE 1
`the structure of a high intensity
`short are lamp of the prior art comprising a sealed, fused
`silica, tubular envelope 101 having an elliptical aneurism
`102 surrounding an arc gap between two coaxially aligned
`and axially spaced electrodes 103 and 104. Two metallic
`cups 105 may be fixed to the ends of the tubular envelope
`and to the adjacent electrodes to protect the seals between
`the electrodes 103 and 104 and to provide electrical ter-
`minals. A reflector, not shown,
`is typically located near
`the bulb.
`Short arc discharge structures of the prior art, such
`as described above, are limited both in the light flux
`density at which they can operate and in the total amount
`of light they are capable of producing at a given light
`flux density. As pointed out above, the amount of power
`which can be concentrated in a gas discharge is an in-
`verse function of the spacing between the electrodes and a
`direct function of the pressure of the gaseous medium,
`the voltage at which the discharge operates and the cur-
`rent carried by the are. It has been found that the total
`amount of light which a structure of the prior art is cap-
`able of producing at a given light flux density is limited
`by the amount of current which can be passed through
`the device due to the mismatch in thermal coeflicient of
`expansion between the electrodes and the envelope. Ex-
`pensive and structurally complex graded seals have been
`used in order to reduce such mismatch but such seals are
`
`limited in the amount of temperature which they can
`withstand. Thus,
`if the current flow through electrodes
`of given size is increased the electrodes tend to be heated
`to a higher temperature resulting in destruction of the
`seal. If the size of the electrodes is increased to accom-
`modate greater current flow,
`the mismatch in thermal
`coeflicient of expansion is accentuated and the ultimate
`temperature which the seals can withstand is reduced.
`The above structural
`limitations also limit
`the light
`flux density which can -be produced. However, limitations
`imposed by the tensile strength of the envelope material
`on the amount of gas pressure which can be used adds a
`further limitation with respect to light flux density. If the
`Size Of the device is reduced in order to enable the use of
`
`4
`higher unit pressures without exceeding the total tensile
`stress which the envelope material can withstand, then the
`electrodes must be made correspondingly smaller, limiting
`the electrical current which can be passed through the de-
`vice for the reasons previously discussed. In addition,
`making a smaller device tends to bring the envelope ma-
`terial closer to the arc discharge resulting in limitations
`imposed by the increased heating of the envelope by the
`arc.
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`50
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`70
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`Other disadvantages of the prior art structure including
`the electrical inductance introduced by attempts to elon-
`gate the electrodes in order to remove the seals from the
`high temperature of the arc, the tendency of the envelope
`material to devitrify and the optical inefliciency of the
`envelope material have been mentioned above. In addi-
`tion,
`the mechanical weakness of the structure of the
`prior art should be noted. Not only is the envelope ma-
`terial structurally weak but the elo-ngated form of the
`structure imposes practical limitations both on the ulti-
`mate size and power handling capability thereof and on
`the applications in which the structure may -be used.
`DESCRIPTION OF PREFERRED
`EMBODIMENTS
`
`There is illustrated in FIGURES 2 and 2a a high in-
`tensity short are lamp having a ceramic cylinder section
`10 and a ceramic ring 12 forming portions of a sealed
`envelope. A transparent window 14 of sapphire, for ex-
`ample, in the form of a disc is peripherally brazed to a
`metallic ring 16 having a flange 18 sandwiched between
`and brazed to one end of ceramic members 10 and 12. A
`spherical metallic back plate 20, having a flanged periph-
`ery 22,
`is brazed to a ductile metallic ring 24 which in
`turn is brazed to the other end of the ceramic cylinder
`section 10. Spherical back plate 20 is preferably made of
`stainless steel or Kovar. The surface of the reflector 26
`is preferably coated with rhodium. The reflector is mount-
`ed on back plate 20 by copper support block 28. A cath-
`ode 30 and an anode 32 radially penetrate opposing aper-
`tures in the sides of the ceramic cylinder section 10 along
`a common axis. Metallic stress relief rings 33 are brazed
`at their inner periphery to these electrodes 30 and 32 in
`vacuum tight relation and are sealed at their outer periph-
`ery to the periphery of the associated aperture in the side
`wall of ceramic cylinder section 10. The electrodes are
`made of a refractory metal—the anode preferably of tung-
`sten and cathode of thoriated tungsten. The ends of these
`electrodes are axially spaced a distance of less than one
`centimeter to provide a short are gap. The lamp is filled
`with an ionizable gas which may be under 25 atmospheres
`of pressure, for example, and sealed at pinch—ofl" 34 in ex-
`haust tubulation 36.
`The ceramic may be either alumina (A1203) or beryl-
`lia (BeO) which are commercially available in various
`degrees of purity. Beryllia has the advantage of ‘being a
`better heat conductor than alumina but it is more expen-
`sive. The ceramic-to-metal seals may be made with melt
`alloy brazes in which copper-silver braze fillers are placed
`between the metallic and ceramic members after the sur-
`face of the ceramics has first been coated with a molyb-
`denum manganese metalizer for example, or the ceramic-
`to-metal seals may be made with active alloy brazes such
`as titanium and nickel, in which case the ceramics need not
`be first metalized. Various other ceramic-to-metal seal
`materials and methods are well known and used extensive-
`ly in the electron tube art. In a specific embodiment of the
`high intensity lamp of FIGURE 2 the inner diameter of
`the ceramic cylinder section 10 is 11/2 inches, the elec-
`trodes are spaced approximately 4 mm. apart and the
`ionized gas is xenon at 25 atmospheres. Such a lamp con-
`sumes approximately 150 watts of power at 20 volts DC;
`drawing approximately 7.5 ampers of current. In opera-
`tion the envelope can withstand an internal gas pressure
`of 30 atmospheres with a safety factor of three.
`FIGURES 3 and 3a illustrate another embodiment of
`
`

`
`3,502,929
`
`5
`the present invention in which the electrodes are oriented
`along an axis parallel to the lamp beam. One end of the
`ceramic cylinder section 40, which preferably is made of
`polycrystalline alumina,
`is brazed to a ductile metallic
`(copper, for example) ring 42 which in turn is brazed to a
`metallic (Kover or stainless steel, for example) member
`44 of the lamp envelope. The metallic member 44 may be
`spherical, ellipsoidal or parabolic. The ductile metallic
`ring serves as a stress relieving portion of the envelope.
`The inner surface of member 44 serves as integral reflector
`46. The other end of ceramic member 40 is brazed to a
`ductile metallic ring 48 which in turn is brazed to one side
`of a rigid metallic terminal ring 50. The terminal -ring is
`then brazed to another ductile metallic ring 52 which in
`turn is brazed to the flange of a tubular rigid metallic
`window support 54. As in the case of ring 42, the ductile
`metallic rings 48 and 52 serve to relieve stresses. The pe-
`riphery of a disc-shaped window 56 which may be sap-
`phire, for example, is slightly recessed within and brazed
`to window support 54..
`A rod-shaped metallic anode 58 (tungsten, for example)
`is supported along the axis of tubular ceramic member 40
`and window 56 by three triangular, metallic supports 60
`which may be of molybdenum, for example. Each support
`has a notch into which terminal ring 50 is brazed. Sup-
`ports 60 provide electrically conductive paths between
`anode 58 and terminal ring 50. Each of the metallic sup-
`ports 60 is bent into the shape of a spiral for stress re-
`lease during high temperature expensive states.
`A rod-shaped cathode 62 (e.g. thoriated tungsten) is
`supported adjacent anode 58 and the axis thereof by a
`metallic cup 64. This cup, which may be Kovar, for ex-
`ample, and which forms a portion of the sealed envelope,
`is brazed about the periphery of an aperture in ellipsoidal
`member 44. A copper exhaust tubulation 66 communicates
`through the cup into the interior region of the envelope.
`Once the envelope has been filled with xenon, for example,
`and pressurized, the copper tubulation is pinched olf there-
`by confining the pressurized gas within the sealed envelope.
`In addition to xenon, mentioned above, it is known that
`other inert gases such as argon and krypton may be used
`in short are lamps. Mercury vapor has also been used in
`short are lamps, usually with a metal halide additive such
`as the iodides of thallium, gallium, indium and thorium
`which have excited states of relatively low energy com-
`pared to mercury, thereby increasing the total light out-
`put and the light flux density. In structures according to
`this invention, it is possible to use in addition to the above
`sodium, potassium, lithium, rubidium and cesium in pure
`metal vapor form, or in halide form. These metal vapors
`could not be used according to the teaching of the prior
`art because of their high degree of reactivity with silica.
`The fact that they can be used in structures according to
`this invention enables a further increase in light output and
`light flux density to be obtained in embodiments of this
`invention.
`The shape of the reflector in embodiments of this in-
`vention may be varied to provide the desired beam out-
`put. For example, a spherical or elliptical reflector would
`be used if a converging beam is desired, the elliptical re-
`flector providing the least aberration. A parabolic re-
`flector would -be used if a parallel beam is desired. IRe-
`flectors of other shapes may also be used.
`It will be seen that according to the teaching of this
`invention the electrodes of a short are lamp may be made
`short and massive in order to pass large amounts of elec-
`trical current. The ceramic-to-metal seals used in embodi-
`ments of this invention can withstand much higher tem-
`peraturés and much greater differential in thermal coefli-
`cient of expansion than glass-to-metal seals. Thus,
`the
`seals may be located much closer to the arc in addition to
`passing more electrical current. Large area electrical ter-
`minals and multiple supports for the electrodes may also
`be used to reduce electrical inductance and increase the
`electrical current handling capacity of embodiments of
`
`6
`this invention. Embodiments of this invention are also
`capable of withstanding higher unit and total
`internal
`pressures than devices of the prior art because of their
`improved overall strength. All of these features make
`possible the generation of greater light flux density and
`greater total light at a given light flux density. In addi-
`tion, embodiments of this invention may be more com-
`pact, for the reasons mentioned above, avoiding the prac-
`tical problems associated with the elongated shape of de-
`vices according to the prior art.
`Furthermore, embodiments of this invention may have
`an integral optical reflector and may use an optical win-
`dow of any desired material depending on the application
`for which the embodiment is designed. Finally, the mate-
`rials used according to the teaching of this invention offer
`many structural advantages due to their strength and
`chemical characteristics as pointed out hereinabove.
`It should be understood that the above-described em-
`bodiments are merely illustrative of applications of the
`principals of the invention. -Obviously, many modifica-
`tions may be made in the two illustrated examples with-
`out departing from the spirit and scope of the invention
`as set forth in the following claims.
`.
`What is claimed is:
`1. An arm lamp having a sealed envelope comprising,
`a ceramic cylinder section, hermetically closed at each
`end by envelope members, one of said envelope members
`being an optical window, said envelope housing a cath-
`ode and an anode which are spaced apart a distance less
`than two centimeters to define a short are gap therebe-
`tween, an ionizable gas under at least two standard at-
`mospheres of pressure filling said short are gap, and
`wherein an optical reflector is mounted within said en-
`velope on the opposite side of said are gap from said
`optical window.
`‘2. An arc lamp as claimed in claim 1 wherein said
`reflector is spherical.
`3. An arc lamp having a sealed envelope comprising, a
`ceramic cylinder section, hermetically closed at each end
`by envelope members, one of said envelope members
`being an optical window, said envelope housing a cath-
`ode and an anode which are spaced apart a distance less
`than two centimeters to define a short are gap therebe-
`tween, an ionizable gas under at least two standard at-
`mospheres of pressure filling said short are gap, and
`wherein the other of said envelope members is in optical
`reflector.
`
`4. An arc lamp as claimed in claim 3 wherein said
`cathode and said anode are axially spaced along the axis
`of said ceramic cylinder section, and one of said cathode
`and said anode penetrates said reflector in hermetically
`sealed relation thereto.
`5. An arc lamp according to claim 4 wherein said re-
`flector is elliptical and said are gap is located at the near
`focal point of said reflector.
`6. An arc lamp as claimed in claim 4 wherein the
`other one of said cathode and said anode is mounted on
`a metallic ring -by a plurality of metallic supports, said
`metallic ring comprising a portion of said one envelope
`member and providing an electrical terminal in said en-
`velope.
`7. An arc lamp as claimed in claim 3 wherein said
`ceramic is selected from the group consisting of alumina
`and beryllia.
`8. An arc lamp as claimed in claim 3 wherein the
`other of said envelope members is metallic.
`9. An arc lamp as claimed in claim 1 wherein said
`cathode and said anode are disposed perpendicular to the
`axis of said ceramic cylinder section.
`10. An arc lamp as claimed in claim 9 wherein said
`cathode and anode penetrate the side ‘wall of said ce-
`ramic cylinder section and are sealed thereto.
`11. An arc lamp as claimed in claim 3 wherein said
`cathode and said anode are disposed along the axis of
`said ceramic cylinder section.
`
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`3,502,929
`
`7
`12. An arc lamp as claimed in claim 3 wherein said
`one of said envelope members comprises a material per-
`vious to the passage of electromagnetic energy in the
`spectral range bounded by the far-infrared and far—ultra—
`violet.
`13. An arc lamp as claimed in claim 3 wherein said
`one of said envelope members is a sapphire window.
`14. An arc lamp as claimed in claim 3 wherein said
`ionizable gas includes an inert gas selected from the group
`consisting of xenon, argon and krypton.
`15. An arc lamp as claimed in claim 3 wherein said
`ionizable gas includes a metal vapor selected from the
`group consisting of mercury, cesium, rubidium, sodium,
`potassium and lithium, and combinations thereof.
`16. An arc lamp as claimed in claim 3 wherein said
`ionizable gas includes an inert gas selected from the
`group consisting of xenon, argon, and krypton and a
`metal vapor consisting of mercury, cesium,
`rubidium,
`sodium, potassium and lithium,and combination thereof.
`17. An arc lamp having a sealed envelope comprising
`a ceramic cylinder section hermetically closed at each
`end by envelope members, one of said envelope members
`comprising an optical window in the form of a circular
`disk having a metallic flange hermetically sealed to the
`outer periphery thereof, said metallic flange being also
`hermetically sealed to one end of this ceramic cylinder
`section and providing an electrical connection through
`said envelope,
`the other envelope member providing an
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`elliptical optical reflector within said envelope, said en-
`velope housing a cathode and an anode which are axially
`spaced apart a distance less than two centimeters along
`the axis of said ceramic cylinder section to define a short
`are gap therebetween at the near focal point of said el-
`liptical
`reflector, an ionizable gas under at
`least
`two
`standard atmospheres of pressure filling said short are
`gap, one of said cathode and said anode being mounted
`on and electrically connected to said metallic flange and
`the other of said cathode and said anode being mounted
`on and extending through said other envelope member in
`hermetically sealed relation thereto.
`
`References Cited
`
`UNITED STATES PATENTS
`
`2,714,687
`2,945,146
`2,971,110
`3,022,444
`3,054,921
`3,304,457
`
`Isaacs et al. _______ __ 313———214
`8/1955
`7/1960 Meyer ___________ __ 313—113
`2/1961 Schmidt __________ __ 313—221
`2/1962 Fischer ________ __ 313——231 X
`9/1962 Lye _____________ __ 313—112
`2/1967 Mastrup _________ __ 3l3—184
`
`JAMES W. LAWRENCE, Primary Examiner
`
`RAYMOND F. HOSSFELD, Assistant Examiner
`
`U.S. Cl. X.R.
`
`3l3——ll3, 184, 214. 22