Electric discharge lamp with reduced internal photoelectron production

Abstract: A high pressure discharge lamp having a metallic frame (30,31) for supporting a discharge vessel (25) within an outer envelope (22), has a frame portion (30) along the discharge vessel (25). To suppress photo-emission of electrons said frame portion is coated with a granular oxide (38). In an outwardly bulged outer envelope (22) the coated frame portion (30,38) may be bent outwardly so as to extend adjacent the outer envelope (22).

Description: The invention relates to a high pressure electric discharge lamp comprising a discharge vessel having a filling comprising inert gas, mercury and metalhalide, an outer envelope surrounding said discharge vessel, a metallic frame within said outer envelope, keeping said discharge vessel positioned within said outer envelope, a portion of which frame extending along said discharge vessel and having an envelope to shield said portion of the metallic frame from ultraviolet radiation. Such a lamp is known from US Patent 3,484,637. The cause of photoelectron production in electric lamps, and the consequent problems are well documented. See, for example, Waymouth, Electric Discharge Lamps (MIT Press, 1971), Section 10.5. As Waymouth describes in detail, some electric lamps, particularly electric discharge lamps containing ionized mercury, emit a strong flux of ultraviolet radiation. These lamps typically are comprised of a discharge vessel in which an arc discharge occurs, mounted within an outer envelope by metal support structure. The ultraviolet radiation from the discharge vessel strikes the metal support causing the emission of photoelectrons. Photoelectron emission can be very detrimental to certain electric lamps. In metal halide discharge lamps the discharge vessel is generally made of quartz glass and contains during lamp operation an ionized plasma of mercury, a small metal atom such as sodium, a halogen such as iodine, and other metals such as scandium and thallium and various compounds of these elements. Small ions such as sodium ions have a high rate of diffusion through heated quarz glass. Photoelectrons which collect on the outer surface of the discharge vessel create a negative potential that attracts the positive sodium ions and accelerates their diffusion through the wall of the discharge vessel. The production of photoelectrons substantially accelerates the depletion of sodium within the discharge vessel and thus shortens the useful life of the lamp. Different measures have been taken in order to diminish the effect of the photoelectrons. U.S. Patent3,484,637 discloses a mercury vapor discharge lamp in which a portion of the discharge tube support frame is enveloped by a tube comprised of alumina or silica. The tube shields the metal rod from ultraviolet radiation and reduces the production of photoelectrons. A similar approach is disclosed in U.S. Patent 3,780,331 which discloses a discharge lamp in which a ceramic or fused quartz tube covers a support conductor which supports the lamp discharge tube. This patent also teaches the addition of a photoelectron collector and the use of a stainless steel support conductor with a chrome oxide surface, in substitution for the nickel plated iron support conductor usually used. U.S. Patent 4,171,498 (Fromm et al) likewise teaches the use of a quartz glass tube surrounding the support conductor for reducing photoelectron emission. All of the lamps disclosed in the above-mentioned references include a straight frame portion. The frame portion is straight because the ceramic or glass tube covering the frame portion is straight. It would be impracticable to fabricate curved ceramic or glass tubes so as to allow the use of frames having a curved portion. An altogether different approach to reducing photoelectron emission is to eliminate the frame portion extending along side the discharge tube. U.S. Patent 3,424,935 discloses a metal halide lamp having metallic structures for supporting respective ends of the discharge tube at the opposite ends of the lamp outer envelope. No metal support rod extends along the length of the discharge tube for providing mechanical support, but a fine tungsten wire provides a conductive path between the lamp base and the far end of the discharge tube. The elimination of the metal support rod eliminates the source of a substantial portion of the photoelectrons produced by the ultraviolet radiation emitted from the discharge tube. The mounting of the discharge vessel within the outer envelope is however more complicated and less rigid. Another measure for reducing photoelectron production, applicable to both lamp types just mentioned, is the introduction of a gas, such as nitrogen, into the outer envelope. The nitrogen reduces the number of photoelectrons that reach the discharge tube and thus collect on it and impart a negative potential to the tube outer wall. However, a gas filling in the outer envelope is not desirable in all lamps. Still another technique is to construct the metal frame so that its elements are as far from the discharge tube as is possible. This appears to be of limited effectiveness, however, without other measures being taken . Waymouth reports that the photoelectric current in a lamp having an evacuated outer envelope and a metallic frame three inches away from the discharge tube is greater than in a lamp having a nitrogen atmosphere in the outer envelope and the metallic frame running along the discharge vessel only one-half inch away from the discharge vessel. Ideally, one would construct a lamp using more than one photoelectron reduction technique in the lamp. Typically, metal halide lamps are made with a nitrogen atmosphere in the lamp outer envelope, and with a tubular envelope, such as quartz glass or alumina, over the support rod of the metallic frame. Because such tubular glass or ceramic covers are only practicable if made straight, their use constrains the metallic frame shape to straight. The only other alternative, then, is to dispense with the support rod altogether and to use structures embodying the concept disclosed in U.S. Patent 3,424,935, mentioned above. It is an object of the invention to provide a lamp of the kind defined in the opening paragraph having simple means to suppress photo-emission of electrons. According to the invention this object is achieved in that the envelope of said portion of the metallic frame is a coating of a granular zirconium oxide layer. Such a layer can easily be applied from a suspension e.g. by dipping. ZrO₂ has a high work function of well above 5.0 eV, i.e. 5.8 eV as compared to e.g. SiO₂ having a work function of 5.0 eV or A1₂O₃ (4.7 eV). In one embodiment the lamp outer envelope has an outwardly bulged bulbous shape. The metallic frame mounting the discharge vessel within the outer envelope comprises a rod extending along the length of the lamp envelope away from the discharge vessel and adjacent the outer envelope to maximize the distance between the metallic frame and the light source. The layer of zirconium oxide may cover substantially the entire rod. An embodiment of the lamp according to the invention is shown in the drawing. Therein is Fig. 1 an elevation of a known discharge lamp having a conventional sleeve enveloping a portion of the metallic frame to reduce photoelectron production; and Fig. 2 is an elevation of a discharge lamp according to the invention. The prior art lamp of Fig. 1 is comprised of a light source 1 housed within an outer envelope 2. The light source 1 has discharge electrodes 3,4 sealed within a quartz discharge vessel 5 and which contains a discharge sustaining filling. The filling comprises a rare gas sodium iodide, mercury and other metal halides such as thallium iodide. Portions of the discharge vessel 5 adjacent the respective electrodes 3,4 are coated with a zirconium oxide layer 6 which suppresses thermal radiation from the coated portions to avoid cooling of the discharge vessel ends. This prevents the discharge vessel from cooling the plasma within it during lamp operation to a lower than optimum temperature. Conductive lead throughs 8 and 9 are connected to respective discharge electrodes 3,4, and extend through the discharge vessel 5 for external connection. The conductive support rods 10, 11, 14 constitute a metallic frame and define a conductive path for applying a voltage to the discharge electrodes, and also provide mechanical support for suspending the discharge vessel 5 within the outer envelope 2. When a voltage is applied to the lamp base 13 that voltage will be applied to the lead through conductors 8, 9 for establishing a potential difference across the discharge electrodes 3,4 and an electrical discharge is developed between the pair of discharge electrodes 3,4. Additionally, a strong flux in the ultraviolet region is emitted from the mercury vapor ionization within the discharge device 1. Ultraviolet photons which strike the metallic frame cause the emission of photoelectrons from the metal. The free photoelectrons can accumulate on the outer surface of the quartz discharge tube 5 and impart a negative charge to it. The negative charge will accelerate the diffusion of small ions, such as sodium ions, through the wall of the discharge tube 5 resulting in the progressive depletion of the sodium concentration within it. This phenomena is referred to as sodium clean-up and is deleterious to lamp quality. As the sodium concentration within the discharge envelope decreases the lamp voltage increases. The major part of the support rod 10 as a portion of the metallic frame is enveloped by a quartz glass sleeve 18. The quartz glass sleeve 18 has a high photoelectric work function. Consequently, it shields a substantial portion of the metal rod 10 and does not contribute to the production of photoelectrons. Thus, there will be fewer photoelectrons available to contribute to sodium cleanup than if the quartz glass sleeve 18 were not present. In order to reduce the production of photoelectrons the conductor rod 30 of the metallic frame 30,31 is coated with a layer 38 of granular zirconium oxide (ZrO₂). The zirconium oxide is applied mixed with an organic binder for adhering the zirconium oxide to the metal support rod 30. The binder may be of the same type as that used for adhering the zirconium oxide layer 26 to the discharge vessel 25. After the coating has dried it is heated to drive off organic material and left behind is the layer of zirconium oxide 38 adherent to the metal support rod 30. To further reduce photoelectron production the metal support rod 30 is nonlinear and bowed or curved to generally follow the contour of the outer envelope 22. This is effective to maximize the distance between the disdischarge device 21 and the metal support rod 30 and thereby minimize the production of photoelectrons. It also imparts mechanical rigidity to the support rod 30 to allow it to support the discharge device 21 without the need of being anchored at its end opposite the lamp stem press. Although Fig. 2 shows the metal support rod 30 to be bent as a series of straight segments it could have been made with a smoother bend if desired. It would be impossible to cover the curved metal support rod 30 with a quartz glass sleeve or an alumina sleeve because of the rigidity and brittleness of the quartz or alumina. Quartz or alumina sleeves can only be used to cover straight segments of rod. Consequently, the use of quartz or alumina sleeves places a constraint on the support structure design in that the support structure cannot be made curved or bowed and at the same time be covered by a rigid and brittle sleeve. Another advantage of a granular zirconium oxide is its relative low cost. Tubular quartz glass is more expensive and tubular alumina even more expensive. The adherence of an oxide layer 38 to the support rod 30 is affected by preparing the rod before coating by sandblasting, by the temperature at which the coating is baked, and by the use of multiple coatings. To determine how to obtain the best adherence of the coating, metallic support rods made of nickel plates iron were coated with, and in one case without, the support rods first being sandblasted, with the coatings baked at different temperatures, and with one or two coatings. Table I summarizes data concerning ZrO₂ - coatings. Samples Sandblast First Coating Temp.(°C-1hr.) Second Coating Temp.(°C-1hr.) A No 400 - B Yes 400 - C Yes 400 400 D Yes 500 500 E Yes 600 600 F Yes 625 - It was found that the zirconium oxide coatings of samples B had the best adherence, with samples C also having very good adherence. The zirconium oxide coatings of samples A, which were not on a sandblasted support rod, did not adhere as well as those of samples B or C. Samples D, E, and F had zirconium coatings that did not adhere as well as those of samples B or C and which exhibited a tendency to flake more the higher the temperature at which the coating was baked. In order to evaluate the effectiveness of the invention, lamps were made having the structures shown in Figs. 1 and 2 of the application. The lamp according to the invention had a granular zirconium oxide layer on the nickel plated iron support rod coated as in sample B of Table 1, above. One type of prior art lamp had a quartz glass sleeve over the support rod and another prior art lamp had an alumina sleeve over the support rod. The lamps were otherwise identical 100 watt metal halide high intensity discharge lamps. The discharge vessel fill was 10 milligrams of mercury, 10.5 milligrams of sodium iodide (NaI), 2.0 milligrams of mercury iodide (HgI₂) and 0.5 milligrams of scandium (Sc). The discharge vessel also contained argon at 13.3 kPa, and the lamp outer envelope contained nitrogen at 26.7 kPa. The lamps were operated for 3000 hours and the lamp voltage, change in lamp voltage, efficacy and maintenance were determined. This data is shown in Table II for comparison. Lamp Parameter Layer Composition Lamp Operating Time (hr) 100 1000 2000 3000 Voltage (V) ZrO₂ 91.2 93.2 91.9 87.9 SiO₂ 95.7 99.2 99.8 102.0 Al₂O₃ 95.9 96.8 101.2 101.9 Change in Voltage (V) ZrO₂ - 2.0 0.7 -3.3 SiO₂ - 3.5 4.1 6.3 Al₂O₃ - 0.9 5.3 6.0 Efficacy (lm/W) ZrO₂ 82.5 79.5 71.2 65.3 SiO₂ 91.6 82.6 72.3 60.0 Al₂O₃ 88.9 85.2 76.7 73.0 Maintenance (%) ZrO₂ - 96.4 86.3 79.2 SiO₂ - 90.2 78.9 65.5 Al₂O₃ - 96.2 86.6 82.1 As was expected, in both the lamp having a quartz glass sleeve and the lamp having an alumina sleeve, the lamp voltage increased during the first 3000 hours of operation. On the other hand, in the lamp according to the invention, after an initial voltage rise at 1000 hours, the lamp voltage progressively decreased. This is an unexpected result and is markedly different behavior compared to the prior art lamps. The decrease in lamp voltage with time in the present invention is inconsistent with substantial sodium clean-up occurring due to photoelectron production. Accordingly, there is a strong basis for inferring a substantial diminution in photoelectron production. The change in lamp efficacy, or maintenance, for the different lamps is also shown in Table II. The lamp according to the invention having the zirconium oxide coating demonstrated substantially improved maintenance throughout the test period, as compared to the lamp having a quartz glass sleeve. Surprisingly, the maintenance of the lamp having an alumina sleeve was slightly better than even the present invention. The lamp having a quartz glass sleeve exhibited the drop in efficacy that is characteristic of metal halide lamps.

Claims: A high pressure electric discharge lamp comprising a discharge vessel having a filling comprising inert gas, mercury and metal halide, an outer envelope surrounding said discharge vessel, a metallic frame within said outer envelope, keeping said discharge vessel positioned within said outer envelope, a portion of which frame extending along said discharge vessel and having an envelope to shield said portion of the metallic frame from ultraviolet radiation, characterized in that the envelope of said portion of the frame is a coating of a granular zirconium oxide layer. A high pressure electric discharge lamp as claimed in Claim 1, characterized in that the outer envelope has an outwardly bulged bulbous shape and the portion of the metallic frame extending along the discharge vessel is bent outwardly to as to extend adjacent the outer envelope.

Inventors: RAMAIAH RAGHU, SHUMWAY DANIEL, RAMAIAH, RAGHU, SHUMWAY, DANIEL
Applicants: PHILIPS CORP, NORTH AMERICAN PHILIPS CORPORATION
Application year: 1989
Main classification: H01J61
IPC codes: H01J61