US3390297A - Shield for hollow cathode lamps - Google Patents

Description

June 25, 1968 J. w. VOLLMER SHIELD FOR HOLLOW CATHODE LAMPS 2 a A a 6 y W n/ e 0 7 6 6 9 0 1 z 1 L 2 6 1 M 6 u J 00 1 d "W 6 m z I A 7 1 g 9 x I lil 4 1 1 //1 I I 1 4 1 I11 1 I 1 I! 4 l 1 1 I 1 1 M. I /1 1 1, t

INVENTOR.

John W. Vollmer HTTORNE'Y.

United States Patent 3,390,297 SHlELD FOR HOLLOW CATHODE LAMPS John W. Vollmer, Nor-walk, Conn., assignor to The Yerkin-Elmer Qorporation, Norwalk, Coma, a corporation of New York Filed July 1, 1966, Ser. No. 562,145 Claims. (Cl. 313-209) ABSTRACT OF THE DISCLOSURE Hollow cathode lamps of the type used for example as spectral sources in atomic absorption spectroscopy have generally cup-shaped cathodes, including the glowing or sputtering desired material at least on their interior surfaces. The invention utilizes a generally cylindrical shell of insulating material partly surrounding the cathode so as to both restrict the volume of glowing vapor (thereby increasing its brightness) and blocking the sputtered material from coating other parts of the lamp (which coating tends to cause short circuits within the lamp). In an alternative form the shell partially restricts the open end of the cathode so as to form an effective aperture smaller than the interior of the cathode.

This invention relates to hollow cathode lamps of the type used, for example, as atomic absorption radiation sources. More particularly this invention concerns improving such lamps by shielding the hollow cathode in such a manner as to increase the useful life and the practical available energy therefrom.

In atomic absorption spectroscopy, a source of radiation, having high intensity in the spectral lines of the metal or metals being tested, is required. One preferable source of such radiation comprises a discharge lamp in which the cathode is in the form of a hollow cylinder open at one end. At least the interior surfaces of this hollow cathode includes the one or more metals for which the emission spectrum is desired. When an electrical current is passed from an anode to this cathode (through a low pressure inert gas), the metal or metals will sputter from the cathode surface so as to form a cloud of substantially atomic particles. In the relatively high temperature, electrical field, these particles emit a substantial amount of radiation at the characteristic frequencies of the metals involved so as to form relatively intense radiation at these characteristics spectral lines. It has recently been recognized that restriction of the glow to the interior of these hollow cathode tubes will enhance their useful intensity relative to the amount of electrical energy required. The invention not only provides such shielding of the exterior surfaces of the cathode so as to increase the practical efficiency of the lamp, but also restricts the path of the sputtered metal so as to achieve further advantages.

In conventional hollow cathode lamps a substantial part of the sputtered metal eventually reaches the inner surface of the glass envelope which serves to enclose the inert gas and to form one of the main supporting structures of the lamp. This coating of the sputtered metal will of course occur more rapidly at higher operating currents, but even at moderate currents the coating occurs at a relatively rapid rate. This sputtered metal coating will gradually cover all of the interior parts of the tube in the vicinity of the hollow cathode. Since the material being coated is one or more metals, the electrical insulation between the anode and the cathode (and more particularly between the leads thereto) eventually is greatly reduced or even completely shorted by this coating material. In fact it has now been found that this shorting is often the limiting factor in the hollow cathode lamp life. Hollow cathode 33%,297 Patented June 25, 1968 shields according to the invention greatly reduce this coating action so as to eliminate such shorting as a major contributor to lamp failure. Shields according to the invention also lessen the rate of loss of'the emitting metal or metals from the interior of the hollow cathode, thereby improving another factor in the lamp life. Finally, shields according to the invention actually increase the intensity of the useful emission for a given electrical current density (this effect being quite marked with the form of the invention in which the open end of the hollow cathode is partly restricted by the shield, as will be seen hereinafter).

An object of the invention is the provision of a shield for the hollow cathode in a lamp which not only restricts the glow to the inside of the cathode, but substantially reduces the amount of material sputtered to the various other lamp parts, and increases the emission efiiciency of the lamp (even in comparison with hollow cathode lamps having other types of shielding of the hollow cathode).

Other objects, features and advantages of the invention will be obvious to one skilled in the art upon reading the following detailed specification in conjunction with the accompanying drawings, in which:

FIG. 1 is a part elevation, part vertical section through one embodiment of the invention; and

FIG. 2 is a similar view of a second embodiment of the invention.

A relatively simple exemplary embodiment of the inventive hollow cathode shield is shown in FIG. 1, as incorporated in a more or less otherwise conventional hollow cathode lamp. The conventional parts of this lamp will first be described. Such a lamp comprises a closed envelope 10, formed by a main cylindrical (typically glass) tube 12, which has been closed at each end by, respectively, a stem assembly 14 and a transmitting window 16.

The stem assembly 14 supports a plurality of pins or rods, assumed (for exemplary purposes) to comprise four pins symmetrically arranged about the central longitudinal axis of the lamp (the pins therefore being in a square or diamond-shaped arrangement). One of these pins, 15, is bent so as to form a portion 17 extending radially to the longitudinal central axis of tube 12, at which point it bends again so as to have a portion at 18 coincident with this axis. Portion 18 supports the hollow cathode assembly 20. Such cathode assembly comprises the hollow cathode cup proper 22, which includes material comprising the element or elements having the spectral lines desired. This material may, for example, be coated (as at 24) on the interior walls of the hollow cathode cup 22. Alternatively it may comprise a cup-shaped insert which is press fit or otherwise secured within the cathode 22; or the hollow cathode 22 itself may be composed of the desired material. The hollow cathode may be supported by an integral narrower portion 2% being crimped or press fit onto center pin portion 18.

Before describing the shield of the invention, which surrounds the hollow cathode, the rest of the more or less conventional components of the lamp will first be described. An insulating base 30 is attached as by cementing to the stem assembly 14 end of the envelope 19. A pair of short pins (of which only the one at 32 is seen in FIG. 1) are at the two vertices of the square or diamond adjacent to that of pin 15 (so that these short pins are diametrically opposite each other in the horizontal central plane of FIG. 1). A horizontal, radially extending bar 34 is attached (as by welding) adjacent the right-hand ends of these short pins (only end 33 of pin 32 being visible in FIG. 1). The middle of the horizontal bar 34 is attached to the central bend 35 (between portions 17 and 18) of pin 15. The two short pins and hori- Zontal bar 34 thereby form a relatively rigid tripod ar- 3 rangement with bent pin 15. A lower conducting pin 36 (diametrically opposite bent pin in the central vertical plane) supports a slightly bent, rod-like anode 33. A different number and arrangement of pins can of course be used. The electrically connecting pins (in this case 15 and 36) will be connected to electrical leads 42 and 46, respectively, as at 48 and 4-4, respectively. Leads 42 and 46 may emerge from base as strands of a cable 4-8, terminating in an electrical plug connector 4-9. Leads 42 and 46 are preferably embedded in slack condition in an 'nsulating tiller 5% within an outer hard casing 52 forming the base A strain release 54 may be provided at emerging point of the cable 3-8 from the casing 52.

A hollow cathode lamp of this generally conventional type (assumed to be without the shield of the invention, indicated at 63) operates as follows. The opposite sides of an electrical power supply are connected (by means of plug 49, cable 48, leads 42. and 46, pins 15 and 36) to the hollow cathode 22 and the anode 38. Typical operating currents are in the range from 10 to milliarnpercs, and typical operating voltages are between 150 v. to 300 v. A cloud of small excited particles (at least some of which are atoms of the desired element) is thereby formed from a material sputtered from the hollow cathode at its elevated temperature within the electrical field. Relatively high intensity emission at the spectral lines characteristic of the desired metallic element is therefore generated (along with other radiation), so as to pass through end window 16 to be used in the, say, atomic absorption spectrometer.

In order to maintain a relatively high intensity of this esired radiation, it is necessary that the metallic elements incorporated, at 24 or in the hollow cathode 22, are caused to sputter rather violently under operating conditions. For this reason a substantial quantity of the sputtered material leaves the vicinity of the cathode and condenses on the cooler parts of the tube it). Although a major portion of this condensation occurs on those parts of the inner urface of the main cylindrical tube 12 which are near the cathode (e.g., in the vicinity of the area indicated at 56 in PEG. 1), which does little harm, other parts within the interior of the lamp are also coated to some extent.

Since the material condensing is metallic, even a relatively thin coating thereof is a relatively good electrical conductor. Therefore even a very thin coating on the interior surface 58 of the stem assembly 14 can cause a substantial short (directly or indirectly) between the electrically negative pin 35 and the anode pin 35. Use of insulation is only partially effective. Insulation in the form of sleeves or the like become coated with condensed metal vapor so that electrical paths are established along their entire surface from one end to the other. Only if the enclosing insulation is substantially vapor-tight for its entire length (including at its two ends) will the insulation be effective. In general it is extremely difiicult to maintain such vapor-tight connections between dissimilar materials. Further, since the cathode and anode themselves must be free of. insulation at least in those areas in which they are operating, the condensed metallic vapor can eventually establish a conductive path from bare cathode to bare anode regardless of intermediate insulation. Insulating caps, such as shown at 51 and 53 (on the base or" bent cathode pin 15 and short pin 32, respectively) at least inhibit the formation of an electrically conducting path between the (coated) stern assembly and these pins during use. Specifically, these caps create an undercut or re-entrant gap, as at 55 and 57 respectively, between themselves and the adjacent surface of the stem assembly (i.e., thickened mounds 59); this gap is relatively inaccessible to the metallic vapor and therefore tends to maintain a break in the continuity of any coating. Unfortunately, even such a is only partially successful in avoiding coating therein, since the metallic vapor dit es therein is all e. ctions. 0-.erally, the shorting effect of built-up condensed metallic vapor is .S the most common single cause of tube failure upon prolonged use.

One form of a cathode shield according to the invention, which greatly reduces the rate of escape of the vapor and therefore the rate at which a coating of condensed vapor is formed, is shown generally at till. This shield is cupsbaped and generally conforms to the exterior of the cathode 22. However, the shield 69 has cylindrical walls which extend substantially beyond the open end of the hollow cathode to form a hood portion, as may be seen at 62. Additionally this open end part of the shield has thinner walls than the remaining part so as to leave a recess or gap (:4, between the parts of the cathode 22 near its opening and the adjacent portions of the shield. Shield 69 may be mounted by the provision of an aperture in closed end 6-5, which surrounds the outer cylindrical surface of narrower portion 26 of the cathode. A press fit retaining washer 63 may be used to assure adequate support of the shield. Shield 66 may be integrally molded piece of pressed alumina or other suitable refractory electrically insulating material. T.e hood or wall extensions 62 will intercept a substantial portion of the material forming the interior of the cathode which would otherwise sputter away from the cathode vic :ity and eventually condense on the interior parts of the tube.

The above results achieved with the invention not only improved the lamp life, but allow the lamp to be run at higher operating currents (and therefore greater brightness) than is practical with similar lamps not having the inventive shield. In particular essentially the same lamp with the shield may be run at 20 ma. so as to be about five times as bright as the corresponding conventional lamp, which has a practical operating current of 10 ma. Thus one may obtain either three times increase in the lamp life or an even greater factor increase in brightness than this, or a combination of somewhat greater brightness and somewhat greater life by using the shield of the invention.

A different embodiment of the invention is shown in FIG. 2. Since many of the conventional parts of the second embodiment may be the same as those of the already described one, such similar parts are indicated by use of primed numbers corresponding to the unprimed numbers of FIG. 1. Thus envelope 10, tube 12, stem assembly 14', window 16', the hollow cathode 2t) and its component parts 22'26', and insulating base 39' may all be substantially identical to the corresponding parts of FIG. 1. Hollow cathode 20' may be supported on a central pin portion 18' extending along the longitudinal center line of the tube 12'. This portion 18' may be formed by two right angle bends (forming intermediate radially extending pin portion 17') of a pin 15', in a manner similar to pin 15 and portions 17 and 18 in FIG. 1, wherein the view of this corresponding pin (15) is at right angles to the view of pin 15' (and portions 17' and 18') in FIG. 2. The pin (above the plane of the paper and therefore not shown) which is diametrically opposed to pin 15' in the horizontal plane of FIG. 2 may have a radially extending portion (similar to 17 or 17') at 34 which is joined to the most central part of portion 17 so as to form therewith a bipod support of central portion 18'. Alternatively both pin 15 and its unseen paired pin may be short (like pin 32 in FIG. 1) and support at their ends a transverse bar (like 34 in FIG. 1) which in turn rigidly supports at its center a longitudinally extending central pin acting as the cathode support pin 13. Obviously, other arrangements (including the use of more or even less than a total of four pins) may be used to support the cathode. The other pair of diametrically opposed pins (i.e., the upper and lower pins 134, 136 in FIG. 2) are also somewhat different from any pair of pins in the first (FIG. 1) embodiment, and therefore are referenced with dift'c" numerals. in the PM}. 2 embodiment either (or both) of these pins (134, E36) may serve as the anode of the lamp,

and both pins perform the additional function of assisting in supporting the different form of the shield of FIG. 2.

The shield 160 of FIG. 2 comprises a main cylindrical body portion 161, which has a partly closed end portion or hood 162 (at the right in FIG. 2) which has a central aperture at 164. The end or hood portion 162 is recessed at its periphery so as to form an annular shoulder 166. A hold-down plate 170 has a relatively large central aperture fitting this shoulder 166, and also has smaller upper and lower apertures so as to receive the ends 133, 135 of pins 134 and 136, respectively. Retaining washers 1'72 and .176 snugly engage ends 133 and 135 of the pins so as to secure the hold-down plate 170 against movement to the right. Abutment of the interior surface 163 of the radially extending hood portion 162 against the edge of the open end portion 23' of the hollow cathode provides the opposite direction longitudinal stabilization of the position of the shield. The relatively snug fit of the shoulder portion 166 of the shield within the large central aperture of hold-down plate 170 and of the connections between the ends 133, 135 of the pins and this hold-down plate provide relatively firm radial support of the righthand end of shield 160. Additional support of the shield is provided by a large circular disc 180, which has large central aperture so as to snugly engage the shield as indicated at 134. Disc 180 also has smaller apertures (as at 182, 186) so as to allow relatively snug passage of upper and lower pins 132 and 136 (additional similar apertures being provided if additional pins are present). The peiipheral edge 188 of disc 180 is preferably of substantially the same diameter and circular configuration as the interior surface 156 of tube 12, thereby providing additional mechanical support for not only the disc 180 but also pins 134, 136 and shield 160.

As is true of the FIG. 1 embodiment, the FIG. 2 shield 160 is preferably made of a nonconductive material (which for example may also be molded alumina). Holddown plate 170 may be the same or another nonconductive material (and in fact may be integrally formed with shield 160 if desired). On the other hand, in some cases it may be desirable for not only the ends (133, 135) of one or both of the pins 132, 136 to act as the anode, but rather to use electrically conductive material for holddown plate 176 so as to make this plate part of the positive electrode. An annular gap, such as at 192, is preferably maintained between the exterior of the cathode and the interior wall of the shield 16!) to allow for thermal expansion of the cathode during use. The corner edge 165 of end portion 162 of the shield acts as a baffle, so that the outer or right-hand surface 167 of the hood portion .162 is not fully coated with metal vapor even after substantial use of the lamp. The potential electrical path to the plate 170 (and ultimately to the anode 133 and/ or 135) is therefore not established, so that electrical isolation of the cathode and anode is maintained.

The large disc 130 (which may for example be mica), not only provides mechanical support to the various elements, but also inhibits the formation of any condensation of sputtered vapor in the volume enclosed by stem assembly 14', the left-hand interior surfaces of tube 12 and the disc 180 itself. Disc 1S0 therefore acts as an auxiliary shield from sputtered material for the parts within this volume. Substantially identical upper and lower sleeves 204 and 206 may be provided about pins 134 and 136 respectively, which sleeves may provide additional mechanical support for the pins and disc 180 as well as electrical insulation for the pins. Preferably the lefthand ends of these sleeves meet the adjacent surfaces of the supporting mounds 59 of the stem assembly so as to leave undercut or substantially re-entrant gaps 203, 205 (similar to gaps 55, 57 of FIG. 1). Thus, in FIG. 2, the pins 134, 136 (at least one of which acts as the anode) are protected from becoming electrically connected to the stem assembly 14- by the coating action of the vapor; in the FIG. 1 form, the electrically negative (cathode) pin 15 (and the connected short pin 32 and the other, not shown, short pin) were insulated by a similar gap from being electrically connected to the coating on the stem assembly. Obviously the gap accomplishes the same general result either way, namely, insulating the cathode and the electrically negative pins on the one hand from the anode and the electrically positive pins on the other hand (by avoiding a conductive path from one across the stem assembly to the other).

The shield of FIG. 2 thus inhibits the glow which otherwise occurs about the exterior surfaces of the cathode 2t); and the hood portion 162 greatly reduces the amount of sputtered material (from the interior leaving the open front end of the cathode) which otherwise would condense upon the various other interior parts of the lamp. In addition to these functions (which are also performed by the shield 60 of FIG. 1 in a somewhat different manner), the shield of FIG. 2 restricts the size of the opening of the interior of the hollow cathode 22 in that aperture 164 is substantially smaller than the cathode interior diameter. This restriction improves the lamp performance in a number of ways.

First the brightness (ie., the radiant flux density per unit cross-sectional area) is higher than with the same hollow cathode without such a restrictive shield. The smaller cross-sectional of aperture 164 also improves (both theoretically and practically) the efficiency of collection and beam formation of this energy. On the other hand, since the volume bounded by the interior of the hollow cathode and the inner surfaces of the restricting flanges or hood 162 of the shield is relatively large, the total intensity of the glow remains quite large. In other words, the effect of the shield with the restricted opening is to increase the effective concentration of the radiation without diminishing its total intensity, thereby giving a brighter source. In fact, by lowering the rate at which the sputtered material leaves the glowing area interior of the cathode, even the total intensity itself is somewhat increased. Additionally the restriction so reduces the sputtering losses that a given lamp with the shield may be run at substantially higher currents than the same lamp without the shield, thus further increasing the total intensity (and therefore the brightness) of the shielded lamp. Another advantageous effect of the restricted opening at 164 is to cause the active material of the hollow cathode lamp (ie., the interior layer 24') to assume during use a rounded shape approximating that of a moderately eccentric ellipsoid (i.e., departing only moderately from a spherical surface). Since a spherical surface is the ideal one for generating the greatest concentration of sputtered material (and therefore the brightest glow), the restricted opening of the shield 1160 actually improves the inherent efiiciency of the hollow cathode itself, because of this shaping tendency.

It has also been found in practice that a hollow cathode lamp using a restricted opening shield substantially conforming to that of FIGQZ yields a further unexpected advantage when used as an atomic absorption spectroscopic source. The intensity of the atom line radiation (i.e., radiation corresponding to the wavelength emitted and absorbed by the metal in its atomic state) is proportionately greater than the ion line radiation (at the wavelength emitted or absorbed by the metal in its ionized state) of an unshielded, otherwise similar lamp run under the same conditions. The reason for this enhancement of the ratio of the desired (atomic line) radiation relative to the undesired (ion line) radiation is not fully understood. Nevertheless, such restricted opening shielded cathode lamps do outperform conventional lamps in typical atomic absorption spectrometers, where the measurement is made at the .atom line wavelength and the ion line radiation is nonabsorbing interfering signal.

Shields generally conforming to each of the embodiments of FIG. 1 and FIG. 2 have been successfully constructed and tested. A shield conforming generally to that shown at 68- in FIG. 1 had an internal diameter of its main body portion just larger than the external diameter of the hollow cathode in which it was to be used (namely, .460 and .440 inch, respectively). The center walls defining the open end or hood portion 62 extended about of an inch beyond the open end of the cathode itself. The recess at 64 provided an additional radial clearance of about .01 to .02 of an inch, and was formed by continuing the thin wall portion 62 for a total length of about of an inch (so that recess 64 had a longitudinal length of about A; of an inch before being opposite the open end of the cathode).

A magnesium (at 24) lamp incorporating such a shield was compared with an unshielded, otherwise substantially identical magnesium lamp. After the same, moderate amount of test use, the unshielded magnesium lamp contained a coating on the interior wall of tube 12 from the stem 14 to within about an inch of window 16 (a total longitudinal distance of about 5 inches of the sixinch length of tube 12.). After similar use, the shielded lamp of the invention had only a quite light ring of magnesium coating on the glass tube in the area generally indicated at 56, the ring being only approximately one and a quarter inch wide (being spaced therefore about one and one-half inches from the stem and about 3% inches from the window end of tube 12.

After running the shielded lamp for the additional substantial period of 500 hours (at ma. 150 volts, DC), the sputtered coating on the glass increased to less than a two-inch wide band and was somewhat thicker. The interior portion of the shield (i.e., the inside surface 63 of the hood or end walls 62) was relatively heavily coated with magnesium, but there was neither magnesium coating nor any frame glow on any part of the outside of the shield. The radially narrow and relatively deep (in the longitudinal direction) recess 64 was successful in creating a gap in the coating on the interior of the shield and the cathode itself, since no tendency to short the shield to the cathode occurred. Such electrical connection of the shield and cathode would be readily recognizable since it would cause in effect the shield to become an extension of the cathode, so that the relatively heavy coating on the interior of end wall portion 62 would then become active. Such. undesired effect did not occur. Therefore the shield continued to act solely as a shield even after its extensive use, so as to continue to minimize the amount of magnesium being deposited on other parts of the lamp.

Two versions of the second embodiment (of FIG. 2) were made and tested. Each had an effective overall longitudinal length of about 95 of an inch, with wall thicknesses of about .05 inch. The version most closely conforming to that illustrated included an extension of the hood or end wall 162 of about .03 inch to form the shoulder 165, which received a very thin (about .01 inch) hold-down plate 170.

The other version of this type of shield was generally similar, except that instead of a recessed shoulder portion and separate hold-down plate, it had integrally formed at the right-hand end a relatively extensive flangelike portion, conforming both in general dimensions and function with the hold-down plate 170. The exterior diameter of both of these forms of the shield 160 was .585 inch, so that the internal diameter was .485 inch. The restricting aperture 164 had a diameter of .187 7 inch, the unrestricted internal diameter of the hollow cathode being about twice this of an inch). The mica disc 18-0 was approximately 1 /2 inches from the stem assembly base 14, and surrounded the shield 160 at approximately its longitudinal center. The right-hand, apertured end 1162. of the shield was therefore somewhat less than one-half inch on the other side of the disc 180.

Thus shields of both the FIG. 1 and the FIG. 2 types achieve the objective of increasing the useful life of the lamp and/or increasing the usable intensity (and brightness) of the lamp. Although shields of either type may be used in various hollow cathode lamps, such shields are especially useful for hollow cathode lamps in which the sputtered active -metals include the more violently sputtering and lower boiling point metals (such as, for example, magnesium, cadmium, selenium, and the like).

The foregoing description of the invention includes two somewhat difierent practical embodiments thereof. Obviously various parts and details of either of the specific embodiments may be changed without departing from the invention. In particular it should be obvious that the various corresponding parts of one embodiment may be used in the other. For example, a large disc, analogous to that shown at in FIG. 2, may be used in the FIG. 1 embodiment as well (effecting both an additional supporting and an additional shielding function therein). The exact pin positioning and its electrical lead connections are of course merely exemplary and may be varied as desired. Obviously changes in size (and to a certain extent in shape as well) may be made in the two types of shields so as to adapt them to particular, different hollow cathodes. Therefore the invention is not limited to any of the details of the foregoing description, but rather is defined solely by the appended claims.

I claim:

1. In an enclosed hollow cathode lamp of the type having a generally cup-shaped cathode, the interior surface of which includes a material which sputters to form a radiation-emitting elemental cloud, the improvement comprising:

a hollow shield of refractory, electrically insulating material, generally surrounding and conforming to the general shape of said hollow cathode;

said shield comprising a hood portion, extending beyond and at least partially surrounding the open end of said cup-shaped hollow cathode;

said hood portion being of such shape and in such relationship with said cathode open end as to intercept most of the sputtered material which otherwise would escape from said open end;

whereby coating of the various other internal parts of said lamp by said sputtered material is greatly reduced.

2. An improved hollow cathode lamp according to claim 1, in which:

the part of the hollow cathode defining its open end and the hood portion of said shield closely adjacent thereto are of such size and shape as to cause a continuous gap in any deposited metal coating therebetween,

whereby electrical conductivity between said cathode and at least remote parts of said shield is avoided, even after a substantial coating of said sputtered, electrically conductive material has been coated onto said hood portion by use of the lamp.

3. An improved hollow cathode lamp according to claim 1, in which:

said hood portion of said shield comprises a generally cylindrical portion extending substantially beyond the open end of said cathode,

whereby a major fraction of the sputtered material escaping from said cathode open end is intercepted by and deposited on the inside of said cylindrical hood portion.

4. An improved hollow cathode lamp according to claim 3, in which:

said hood portion of said shield is of such size and shape relative to the adjacent part of said cathode as to be spaced apart therefrom to leave a continuous gap therebetween,

whereby a gap in the electrically conductive coating formed between said cathode and said shield hood portion is maintained, even after substantial deposition of said sputtered, electrically conductive material has occurred on said hood portion during lamp use.

9 5. An improved hollow cathode lamp according to claim 1, in which:

said shield comprises mounting means for directly engaging a part of said hollow cathode remote from said open end, whereby said shield is directly supported by said cathode. 6. An improved hollow cathode lamp according to claim 1, in which:

said hood portion of said shield comprises an end wall portion extending radially inwardly, beyond but near said cathode open end, so as to partially restrict the opening therein, whereby a major fraction of the sputtered material escaping from said cathode open end is intercepted by and deposited on said radially extending end wall portion. 7. An improved hollow cathode lamp according to claim 6, in which:

said end wall portion is symmetrically arranged about said cathode open end to define a substantially circular reduced diameter aperture, whereby the flux density brightness of said hollow cathode is increased. 8. An improved hollow cathode lamp according to claim 1, in which:

said shield is supported by mounting means,

10 said mounting means comprising, at least in part, an

auxiliary shield to inhibit deposition of said sputtered material on various other internal parts of said lamp. 9. An improved hollow cathode lamp according to claim 8, in which:

said auxiliary shield directly contacts and assists in directly supporting said shield. 10. An improved hollow cathode lamp according to claim 8, in which:

said auxiliary shield is in the form of an extensive disc,

which is positioned so as to divide the lamp interior into two substantially separate enclosures.

References Cited UNITED STATES PATENTS 2,177,714 10/1939 Hagen 3l3--239 2,431,226 11/1947 Berkey 313-239 X 2,433,809 12/ 1947 Clapp 313-209 2,592,556 4/1952 Ger-meshausen 313204 X 3,264,511 8/1966 Yamasaki 313-209 JAMES W. LAWRENCE, Primary Examiner.

S. D. SCHLOSSER, Examiner.

R. JUDD, Assistant Examiner.

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