High intensity discharge lamp with intermediate pressure xenon fill gas

Abstract

 A high intensity arc discharge lamp comprises an outer light transmissive envelope having a light transmissive arc discharge tube positioned therewithin. The arc discharge tube has electrodes at opposite ends thereof and means are provided for electrical connection to the electrodes. An arc generating and sustaining medium is provided within the arc tube, the medium comprising a substantial amount of mercury, at least the iodides of sodium, scandium and lithium, and xenon at a fill pressure at room temperature of between about 130 millibar to about 260 millibar.

Description

TECHNICAL FIELD

[0001] This invention relates to high intensity discharge lamps and more particularly to metal halide lamps having improved lumen maintenance and good glow-to-arc characteristics together with higher efficiencies.

BACKGROUND ART

[0002] Metal halide lamps generally include an amount of mercury and additionally need some gas within the arc chamber in order to initiate the discharge. Typically, argon at pressures from 40 millibars to 130 millibars is the gas and pressure range of choice. Argon is a noble gas and does not react with the other chemicals or the lamp envelope. It is the most abundant of the noble gases and is the least expensive. It is known that lamps filled to the lower pressures ignite easier. Conversely, lamps filled to the higher pressures are more difficult to ignite. Further, it has been observed that the electrodes in lamps having the lower pressures sputter more, causing the lamp envelope to darken and shortening the lamp life. Electrodes in lamps having higher pressure, although being harder to start, sputter less so the lamps experience better lumen maintenance. Thus, argon pressure must be optimized for given applications.

[0003] Other noble gases, particularly xenon, have been utilized in metal halide arc discharge lamps. However, it has not been known to use xenon as a low pressure starting gas. Because of its heavy atomic weight of 131, xenon at pressures greater than one torr has been used as a substitute for all or part of the mercury, see, for example, U.S. Patent No. 4,757,236. Such lamps can reach brilliance quickly since there is less or no mercury to vaporize during warm-up. Starting of these lamps, however, can be difficult, requiring high voltage, low impedance igniters to initiate the discharge

DISCLOSURE OF INVENTION

[0004] It is, therefore, an object of the invention to obviate the disadvantages of the prior art.

[0005] It is another object of the invention to enhance the operation of metal halide discharge lamps.

[0006] Yet another object of the invention is the provision of a metal halide discharge lamp that is easy to start and has excellent lumen maintenance due to reduced electrode sputtering causing darkening of the envelope.

[0007] These objects are accomplished, in one aspect of the invention, by the provision of a high intensity arc discharge lamp which comprises an outer light transmissive envelope having a light transmissive arc discharge tube positioned therewithin. The arc discharge tube has electrodes at opposite ends thereof and means are provided for electrical connection to the electrodes. An arc generating and sustaining medium is provided within the arc tube, the medium comprising a substantial amount of mercury, at least the iodides of sodium, scandium and lithium, and xenon at a fill pressure at room temperature of between about 130 millibar to about 260 millibar.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] 

Fig. 1 is a perspective view of lamp which can utilize the invention; and

Fig. 2 is a graph of Lumen Output versus Time of lamps of the invention relative to prior art controls.

BEST MODE FOR CARRYING OUT THE INVENTION

[0009] For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.

[0010] Referring now to the drawings with greater particularity, there is shown in Fig. 1 a metal halide arc discharge lamp 10 including a lamp envelope 12 and an arc discharge tube 14 mounted within the envelope by mounting frame 16. The arc tube 14, which preferably is fused silica or polycrystalline alumina, may be positioned within a shroud 20 which can also be supported by the mounting frame 16. Electrical energy is coupled to the electrodes (not shown, but which are preferably 2% thoriated tungsten rods having a diameter of 0.43 mm and provided with short coiled coil overwinds, as is known) of arc tube 14 through a base 22, a lamp stem 24 and electrical leads 26 and 28. The arc tube 14 contains an arc generating and sustaining medium characterized generally as a chemical fill or dose of materials to provide light when an arc is initiated therein, as will be explained hereinafter. The shroud 20 comprises a cylindrical tube of light transmissive, heat resistant material such as quartz.

[0011] As noted, in this particular instance, the mounting frame 16 supports both the arc tube and the shroud within the lamp envelope 12. The mounting frame 16 includes a metal support rod 30 attached to lamp stem 24 by a strap 31. The support rod engages an inward projection 32 in the upper end of the lamp envelope 12. The support rod 30 in its central portion is parallel to a central axis of the arc tube 14 and shroud 20. The mounting means 16 further includes an upper clip 40 and a lower clip 42 which secure both arc tube 14 and shroud 20 to support rod 30. The clips 40 and 42 are attached to the support rod 30, preferably by welding.

[0012] Referring now to Fig. 2, the graph depicted shows the results of tests comparing xenon and argon containing fills. The test comprised four groups each containing four 100 watt metal halide lamps. As noted above, the arc tubes were fused silica and the thoriated tungsten electrodes were provided with molybdenum electrical feedthroughs, as is known. The arc length was 14 mm. The arc tubes had a volume of 1.4 cc and the lamps were dosed with 12 mg of mercury, 0.13 mg of pure scandium metal and 10 mg of the iodides of sodium, scandium and lithium in a molar ratio of 24:1:2.5. The lamp of plot A contained xenon at a fill pressure of 260 millibar; the lamp of plot B contained xenon at a fill pressure of 130 millibar; the lamps of plots C and D represented a control. The lamp of plot C contained argon at a fill pressure of 260 millibar and the lamp of plot D contained argon at a fill pressure of 130 millibar.

[0013] As is readily apparent, the xenon filled lamps exhibited superior lumens as compared to the argon filled lamps. At 9000 hours the 260 millibar group (plot A) was still averaging 6400 lumens whereas both argon groups were less than 3500 lumens.

[0014] The lamps were operated with standard ANSI M90 ballasts using standard 100 watt ignitors.

[0015] After the lamps had aged for fifteen minutes, measurements were taken of the glow-to-arc transition times for both electrodes, the peak-to-peak voltage just after the transition to arc, the voltage at peak current (also just after the transition to arc), and the re-ignition spike, a high voltage pulse occurring at current zero crossing during the first minute or so of warm-up. No differences were found in glow-to-arc transition times between gases or fill pressures. However, it was found that both xenon and the higher fill pressures reduced peak-to-peak voltage and the voltage at peak current by as much as 45 volts. Also, both xenon and the higher pressures were observed to reduce re-ignition voltage by 49 and 59 volts respectively. It is believed that these lower voltages reduce the electrode sputtering and thereby contribute to the better maintenance of the high pressure xenon group.

[0016] While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.

Claims

1. A high intensity arc discharge lamp comprising: an outer light transmissive envelope; a light transmissive arc discharge tube positioned within said envelope and having electrodes at opposite ends thereof; means to provide electrical connection to said electrodes; and an arc generating and sustaining medium within said arc tube, said medium comprising a substantial amount of mercury, at least a halide of sodium, and xenon at a fill pressure at room temperature of at least about 130 millibar.

2. The lamp of claim 1 wherein said medium includes an amount of scandium metal.

3. The lamp of Claim 1 wherein said arc discharge tube comprises fused silica.

4. The lamp of Claim 2 wherein said arc discharge tube has a volume of 1.4 cc, said substantial amount of mercury is about 12 mg, said amount of scandium metal is about 0.13 mg, and said halides of sodium and scandium are present in an amount of about 10 mg and in the molar ratio of 24:1.

5. The lamp of Claim 4 wherein said halides are iodides.

6. The lamp of Claim 1 wherein said arc discharge tube is polycrystallin alumina.

Drawing