Discharge lamps

Abstract: A metal halide arc discharge lamp (10) comprising: an arc tube (14) containing an arc generating and sustaining medium and having first and second electrodes (18) sealed at opposite ends thereof; an outer envelope (12) surrounding said arc tube and having first and second terminals for electrical connection thereto; an electrical connector (26) coupling said first electrode to said first terminal; an electrical connector (32) coupling said second electrode to said second terminal; a heat reflecting coating (27) on at least one end of said arc tube; a starting aid (24) operatively associated with said arc tube; and a light transmissive shroud (16) positioned about said arc tube on at least two sides thereof; said arc tube having a given wall thickness of about 0.5 mm and said shroud having a thickness greater than said given thickness, the ratio of said shroud wall thickness to said arc tube wall thickness being about 2. The lamp has an increased CRI over lamps made with thicker-walled arc tubes and the thinner arc tube material allows the use of 1 mm thick shrouds which still provide containment in the event of an arc tube burst.

Description: This invention relates to an arc discharge lamp having an outer envelope containing an arc tube disposed within a light transmissive shroud and more particularly to such lamps having enhanced performance and improved containment in the unlikely event of a capsule burst. Arc discharge lamps, particularly of the metal halide variety, are frequently employed in commercial usage because of their high luminous efficacy and long life. A typical metal halide lamp includes a quartz or fused silica arc tube that is hermetically sealed within a borosilicate glass outer envelope. The arc tube, itself hermetically sealed, has tungsten electrodes sealed into opposite ends and contains an arc generating and sustaining medium. This medium, or fill, includes mercury, metal halide additives which generally include the halides of sodium, cesium and scandium, and a rare gas to facilitate starting. Starting, in the form of auxiliary electrodes, as is well known in the art, or ultraviolet sources, as shown in US-A-4,818,915 and US-A-5,323,091, can also be used. In high wattage lamps the outer envelope is filled with nitrogen or other inert gas at less than atmospheric pressure. In low wattage lamps, such as those operated at about 100 watts, the outer envelope is evacuated and provided with gettering material to maintain the vacuum. These lamps, as they age, develop less lumens, show an increase in voltage and a lowering of their colour rendering index (CRI). Further, it has been found desirable to provide these metal halide arc discharge lamps with a shroud which comprises a generally cylindrical, light-transmissive member, such as quartz, that is able to withstand high operating temperatures. The arc tube and the shroud are coaxially mounted within the lamp envelope with the arc tube located within the shroud. Preferably, the shroud is a tube that is open at both ends. However, in other cases, the shroud is open at one end and has a closed, domed configuration at the other end. Shrouds for metal halide arc discharge lamps are disclosed in US-A-4,499,396; US-A-4,580,989; and US-A-4,281,274, as well as the above-mentioned US-A-5,323,091. The quartz shrouds employed by the prior art lamps are expensive because of their size as characterized by their wall thickness which is 3 mm, this size having been determined to be necessary because of the mass of the arc tubes which are constructed from quartz having a wall thickness of 1 mm. Shrouds of hard glass having a lesser thickness are disclosed in EP-A-0616358. Thus, according to the present invention there is provided an arc discharge lamp having an outer envelope containing an arc tube disposed within a light transmissive shroud, characterised in that the arc tube has a wall thickness of about 0.5 mm, and in that the wall thickness of the shroud is about twice that of the arc tube. Decreasing the thickness of the arc tube reduces the mass of any particles resulting from a burst and thereby allows for a reduction in the thickness of the shroud. Further, lamps constructed with the thin wall arc tube showed increased lumen maintenance and an enhanced colour rendering index (CRI) with no effect upon colour temperature. Preferably, the arc tube is a metal halide arc discharge tube and contains an arc sustaining fill including at least the halides of sodium, cesium and scandium. Both the arc tube and the shroud, in the most preferred embodiment, are made from quartz with the wall thickness of the arc tube being between 0.4 and 0.6 mm, preferably between 0.45 and 0.55 mm, and most preferably equal to 0.5 mm, with the wall of the shroud being substantially twice as thick, ie. between 0.8 and 1.2 mm, preferably between 0.9 and 1.1 mm, and most preferably equal to 1.0 mm. In such a preferred embodiment, this can produce a CRI of greater than 70, and preferably greater than 72 for the majority of the life of the lamp, which may be greater than 4000 hours. Thus, in a preferred embodiment of the present invention there may be provided a metal halide arc discharge lamp comprising: an arc tube containing an arc generating and sustaining medium and having first and second electrodes sealed at opposite ends thereof; an outer envelope surrounding the arc tube and having first and second terminals for electrical connection thereto; an electrical connector coupling the first electrode to the first terminal; an electrical connector coupling the second electrode to the second terminal; a heat reflecting coating on at least one end of the arc tube; a starting aid operatively associated with the arc tube; and a light transmissive shroud positioned about the arc tube on at least two sides thereof; the arc tube having a given wall thickness of about 0.5 mm and the shroud having a thickness greater than the given thickness, the ratio of the shroud wall thickness to the arc tube wall thickness being about 2. Some preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figs. 1-4 are graphs illustrating operating conditions for preferred lamps of the present invention; and Fig. 5 is a side elevation, partly in section, of a preferred lamp in accordance with the present invention. Referring now to the drawings, in Fig. 5 there is shown a metal halide arc discharge lamp 10 having a sealed envelope 12 enclosing a quartz sleeve 14. The sleeve 14 surrounds an arc tube 16 having electrodes 18 located at opposite ends thereof and a fill material capable of generating and sustaining an arc. The fill can comprise mercury, metal halides and argon, as is well known. In a preferred embodiment at least the halides of sodium, cesium and scandium are present. Each electrode is coupled to a molybdenum ribbon 20 which is enclosed in a seal 22,23, preferably a press seal, that hermetically seals the arc tube. Electrical energy is coupled from a lamp base 28 through a lamp stem 30 and leads 32 and 26 to the electrodes 18 of the arc tube. A coating of suitable material, 27, such as zirconium dioxide, may cover the ends of the arc tube to control the cold spot temperature, as is known in the art. A UV enhancer 24 has a sealed envelope 34 that encloses an electrode 25. The electrode 25 is coupled to the lead-in wire 26 and is capacitively coupled to the lead-in wire 32, which may include a conductor that is helically wrapped around the envelope 34. A typical UV enhancer is about 4.0 mm in diameter and 15.0 to 20.0 mm in overall length. Further details of UV enhancers are disclosed in US-A-5,323,091. The shroud 14 shown in Figure 5 has a domed configuration. However, it is to be understood that a shroud comprising a cylinder open at both ends is equally appropriate, such shrouds also being known. The construction of the lamp of the invention is substantially as described above except that the arc tube 16 is constructed of quartz having a wall thickness of 0.5 mm and the shroud is constructed of quartz having a wall thickness of 1.0 mm. This results in a considerable saving of material cost, as quartz is a material sold by weight. Referring to the illustrated graphs, it will be seen that the preferred lamps of the present invention (test A) compare favourably to the standard lamps (Control). Of particular importance, the graph of Fig. 3 shows much improved CRI for the preferred lamps of the present invention as well as comparable lumen loss out to 4000 hours, which is shown in Fig. 4. These improvements are accomplished with the use of less of the expensive materials since the arc tube has a thickness of only 0.5 mm and the shroud a thickness of 1 mm. The reason for the effect of the increased CRI is not known with certainty, but it is believed that it is a result of maintaining a more uniform temperature within the arc tube and, consequently, a lesser amount of heat sinking through the press seal areas owing to the thin quartz in those areas. The less heat sinking, the more heat retention. This heat retention in a region occupied by the condensate generates increased CRI without adversely affecting other operating parameters, for example, such as colour temperature. Further, processing times are reduced when using the thin wall material. Owing to the decreased amount of mass, the material requires less time to heat to a plastic state so as to perform the press sealing of the electrodes. Reduced press sealing time also correlates to a diminished amount of time that the electrode assemblies are exposed to the extreme heat from the sealing fires. This decreased amount of time reduces the potential of electrode oxidation which, in time, will adversely effect lamp performance and life. Containment of the lamps was tested by force failing. Force failing comprises starting a lamp and running to equilibrium and then discharging a 20-30 microfarad capacitor at between 1500-2500 volts connected across the lamp. When lamps having the 0.5 mm arc tube wall and the 1 mm wall thickness shroud where mounted in a standard ED-17 outer jacket were so tested all of the arc tubes failed violently. However, all of the outer jackets remained intact and contained all fragments. Thus, the use of the preferred arc tube and preferred shroud in accordance with this invention increases the CRI of the lamps and provides excellent containment in the event of a burst. Thus, at least in an illustrated embodiment of the present invention, there is provided an arc discharge lamp which obviates the disadvantages of the prior art; has enhanced operation; and has improved containment capabilities.

Claims: An arc discharge lamp (10) having an outer envelope (12) containing an arc tube (16) disposed within a light transmissive shroud (14), characterised in that the arc tube (16) has a wall thickness of about 0.5 mm, and in that the wall thickness of the shroud (14) is about twice that of the arc tube. A lamp as claimed in claim 1, characterised in that the arc tube (16) is made from quartz. A lamp as claimed in claim 1 or 2, characterised in that the shroud (14) is made from quartz. A lamp as claimed in claim 1, 2 or 3, characterised in that the arc tube (16) is a metal halide arc discharge tube. A lamp as claimed in claim 4, characterised in that the arc tube (16) contains a fill including at least the halides of sodium, cesium and scandium. A lamp as claimed in any preceding claim, characterised in that a heat reflecting coating (27) is provided on at least one end of the arc tube (16). A lamp as claimed in any preceding claim, characterised in that it has a colour rendering index (CRI) of greater than 70. A lamp as claimed in claim 7, characterised in that it has a CRI of greater than 72 for the majority of the life of the lamp. A lamp as claimed in claim 7, characterised in that it has a CRI of between 70 and 75 throughout the life of the lamp. A lamp as claimed in any preceding claim, characterised in that the lamp has a life of greater than 4000 hours.

Inventors: MORIN SANDRA M, SCHOLZ JOHN A, MORIN, SANDRA M., SCHOLZ, JOHN A.
Applicants: FLOWIL INT LIGHTING, FLOWIL INTERNATIONAL LIGHTING (HOLDING) B.V.
Application year: 1995
Main classification: H01J61
IPC codes: H01J61