US 3657588 A
The lamp envelope is formed of a metallic reflector having at each of its ends a cylindrical ceramic insulator and, closing the open ends of the ceramic insulators, metallic cathode and anode support assemblies, the anode support assembly including an optical window sealed across an aperture. Cathode and anode electrodes extend into the envelope from the cathode and anode support assemblies concentrically with the reflector to form an arc gap at the focal point of the reflector. A third electrode connected to the reflector extends into the region of the arc gap for starting purposes. A cylindrical metallic heat shield extends from the cathode end of the reflector into the cylindrical ceramic insulator at that end in order to shield the joint between the cylindrical insulator and the reflector from thermal radiation and convection from the adjacent arc gap.
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Description (OCR text may contain errors)
United States Patent McRae [l5] 3,657,588 [451 Apr. 18,1972
 ENVELOPE STRUCTURE FOR HIGH INTENSITY THREE ELECTRODE ARC LAMPS INCORPORATING HEAT SHIELDING MEANS 3,502,929 3/1970 Richter ..3l3/220X Primary Examiner- Roy Lake Assistant Examiner-Palmer C. Demeo Attorney-Stanley Z. Cole and Gerald L. Moore [3:] inventor: :ussell MjRae, ulpe;tlino,CCz1l1f. ABSTRACT t l Sslgnee ssoc a a o to, a l The lamp envelope 15 formed of a metallic reflector having at  Filed: Jan. 19, 1970 each of its ends a cylindrical ceramic insulator and, closing the open ends of the ceramic insulators, metallic cathode and [2!] App! 3655 anode support assemblies, the anode support assembly including an optical window sealed across an aperture. Cathode and  U.S. C1 ..313/113, 313/197, 313/219, anode electrodes x en into he nvelope from the cathode 313/220 and anode support assemblies concentrically with the reflec- 511 1m. 01. ..H01j6l/30,H01j 61/36 tor to form an are gap at the focal point of the refleeter- A 1531 w m, 313/1 10, 1 13, 7 193, 5 third electrode connected to the reflector extends into the re- 3 13/206 207 219 220 gion of the arc gap for starting purposes. A cylindrical metallic heat shield extends from the cathode end of the reflector into the cylindrical ceramic insulator at that end in order to shield  References Cited the joint between the cylindrical insulator and the reflector UNITED STATES P E S from thermal radiation and convection from the adjacent arc a 3,450,924 6/1969 Knochel et a] ..313/220 g p 3,450,928 6/1969 Cobine ..3 13/220 X 4 Claims, 2 Drawing Figures t l 48 52 r 54 l 24 50 I 38%; 41)
BACKGROUND OF THE INVENTION This invention relates generally to high intensity short-arc lamps such as those disclosed in U.S. Pat. No. 3,502,929, issued Mar. 24, 1970, and assigned to the same assignee as the present invention. Sealed beam high intensity short-arc lamps typically contain two axially aligned electrodes spaced apart a short distance to form an arc gap located in a gas-tight envelope. The are gap may measure, for example, 0.1 to millimeters in length. The envelope, which confines an ionizable gas under pressure, usually comprises a metallic arcuate reflector incorporating the cathode electrode, a cylindrical ceramic insulator joined to the open end of the reflector, and an anode support assembly joined to the open end of the ceramic insulator and supporting the anode in close proximity to the cathode at the approximate focus of the reflector. A flat optical window is sealed within a concentric aperture in the anode support assembly.
Initiation of the arc discharge is caused by momentary application of a high voltage pulse between the cathode and anode while subsequent continuous operation is achieved by a low voltage high current supply connected between anode and cathode. For instance the starting pulse might be 50 kv while the power supply for continuous running might provide-l2 volts at 12 amperes. For some applications such as pulse or amplitude modulated light sources, it is desirable to supply a third electrode extending into the region of the arc gap for starting purposes. In this way, the high voltage starting pulses can be isolated from the low voltage steady state power supply circuit.
In providing the lamp with such a third electrode, it has been found desirable to connect the third electrode mechanically and electrically to the reflector while electrically isolating the cathode and its support assembly from the reflector by an additional ceramic insulator. However, this construction usually has resulted in locating the brazed ceramic-to-metal joint between the reflector and the additional ceramic insulator in close proximity to the arc gap. Strong thermal convection currents'from the are often have heated the brazed joint to such a degree that cracking and failure of the joint have occurred. The present invention is directed to the solution of this problem.
SUMMARY OF THE INVENTION According to the present invention, excessive heating of the metal-to-ceramic joint between the reflector and insulator can be avoided by providing the reflector with a cylindrical metallic sleeve extending from the cathode end of the reflector into the cylindrical insulator on that end. By dimensioning the sleeve smaller than the cylindrical insulator an annular space is provided between the sleeve and the insulator. Convection currents of heat proceeding outwardly from the arc region are thus forced to travel a considerably longer path around the end of the sleeve to reach the ceramic-to-metal joint; moreover, the joint is shielded from radiated heat from the arc region.
Thus the principal object of the present invention is to provide an improved three electrode arc lamp in which the reflector is electrically connected to the third electrode while the joint between the reflector and the cathode insulator is effectively shielded against heat from the arc region. This and other objects, features and advantages of the present invention will become more apparent upon reading the following detailed description and examining the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of a high intensity short-arc lamp according to the present invention.
FIG. 2 is a view of the lamp of FIG. 1 taken along lines 2-2.
2 DESCRIPTION OF A PREFERRED EMBODIMENT Referring now in more detail to the drawing, FIG. 1 illustrates a sealed beam, high intensity short-arc lamp in which the electrodes are oriented axially. The lamp envelope includes a cylindrical ceramic insulator section 10 which may be made of polycrystalline alumina for example. One end of cylinder 10 is brazed to a ductile metallic ring 12 which in turn is brazed to a metallic reflector 44 which may be spherical, ellipsoidal, parabolic, or any other suitable shape. The ductile metallic ring serves as a stress relieving portion of the envelope.
The ceramic member 10 is brazed at the other end to a ductile metallic ring 18 which in turn is brazed to a rigid metallic anode ring 20. Ring 20 then is brazed to another ductile metallic ring 22 which in turn is brazed to a flange of a tubular rigid metallic window support 24. As in the case of ring 12, the ductile metallic rings 18 and 22 serve to relieve stresses generated by the high temperatures encountered in operation. A disc shaped planar window 26, which may be sapphire or other suitable material, is received within and brazed to window support 24. A cylindrical metallic anode electrode 28 which for example may be tungsten, is supported along the axis of cylindrical ceramic insulator l0 and window 26 by three metallic struts 30 which may be made of molybdenum for example. The struts 30 are trapezoidal rather than rectangular in shape in order that the distance between the struts and the reflector 44 may be increased to avoid possible are breakdowns. Each strut has a notch into which anode ring 20 is brazed. Struts 30 and anode ring 20 together comprise an anode support assembly which provides an electrically conductive path between anode electrode 28 and anode ring 20.
A cylindrical metallic cathode 32 which may be made of thoriated tungsten, for example is supported adjacent anode electrode 28 by a metallic cathode support plate 34. Cathode electrode 32 may be supported in plate 34 by being brazed within a central aperture in plate 34. A hollow threaded stud 36 is brazed to one surface of plate 34 concentrically therewith. Stud 36 serves as a mounting means for the arc lamp. A tubulation 38 is brazed within an aperture in stud 36. During manufacture tubulation 38 is used to fill the interior of the lamp envelope with an ionizable gas, for example, xenon. After this step, tubulation 38 is pinched off" to form a coldwelded seal. Apertures 40 in plate 34 provide passageways for the filling of gas.
A cylindrical ceramic insulator 42 joins plate 34 and a spacer ring 46. Ductile metallic rings 48 are provided at each of the ceramic-to-metal brazed joints for stress relief. Spacer ring 46 is joined to reflector 44 by brazing for example. A third or starter electrode 50 is joined to reflector 44 by means of a tab 52 by brazing for example.
In accordance with the present invention, the ceramic-tometal joint between cylindrical ceramic insulator 42 and spacer ring 46 is protected from the intense heat generated in the are between cathode 32 and anode 28 electrodes by a cylindrical metallic sleeve 54 joined to spacer ring 46 by brazing for example. In operation, the electrodes of the lamp are energized with suitable potentials connected to the metallic external parts of the envelope by means of a suitably designed socket (not shown). Inone example, a 12 volt, l2 ampere dc power supply is connected between anode ring 20 and cathode support plate 34 to provide operating potential, while a 50 kv pulse source connected to reflector 44 is sufficient to energize third electrode 50 for starting the arc.
Since the temperature of the arc is approximately 5,000 K, strong convection currents are generated within the envelope. However, because of the presence of sleeve 54 these convection currents and thermal radiation from the are are prevented from impinging directly upon the ceramic-to-metal braze joint between elements 42 and 46. The path traveled by convection currents from the arc to the above-mentioned joint is considerably lengthened by the baflling effect provided by sleeve 54 such that the path length for heat travel between the arc and the joint may be doubled for example. In this way, intense heating which formerly caused premature failure of the brazed joint is avoided.
It should be understood that the above described embodiments are merely illustrative of the application of the principals of this invention. Obviously, many modifications may be made without departing from the spirit and scope of the invention as setforth in the following claims.
What is claimed is:
1. An arc lamp having a sealed envelope filled with an ionizable gas, said envelope comprising three conductive members separated by and joined gas tightly to first and second insulative members, one of said members having an optical window gas tightly sealed within an aperture therein, each of said conductive members having a conductive electrode connected thereto and extending within said envelope to form an arc gap between a pair of said electrodes, and thermal shielding means mounted within said envelope and positioned to lengthen the spatial path between said arc gap and the joint between one of said conductive members and one of said insulative members.
2. An arc lamp in accordance with claim 1 wherein one of said conductive members comprises a reflector member interposed between the others of said conductive members and separated therefrom by said insulative members, and wherein said thermal shielding means comprises a conductive sleeve 5 joined to said reflector member and extending over the joint between said reflector member and an adjacent insulator member.
3. An arc lamp in accordance with claim 2 wherein said other conductive members comprise a cathode support and an anode support, said anode support having said optical window mounted therein.
4. An arc lamp according to claim 3 wherein said insulative member separating said reflector and cathode support is a cylinder and wherein said cathode support includes a metal plate closing one end of said cylinder, and wherein said reflector is joined to the other end of said cylinder concentrically therewith, said conductive sleeve projects from the end of said reflector into said cylinder concentrically therewith, said sleeve having an external diameter smaller than the internal diameter of said cylinder, whereby a concentric gap is formed between said sleeve and cylinder.