A method to reduce color temperature variation in metal halide arc tubes

Abstract: Disclosure is given for a method of forming and treating an arc tube (14), preferably for a metal halide lamp, such that end pockets (10) or other internal tube irregularities are eliminated. In one preferred embodiment the unwanted pockets are eliminated during the manufacturing process of the arc tubes. Therefor a press seal machine with 4 presses (2 pairs) was designed to provide pressing capability in two mutually perpendicular planes. With this machine a first press is made in the conventional manner with two opposing press feet (20, 22) being driven into the hot quartz of the arc tube body. While these feet are still in contact with the hot quartz, a second pair of press feet (24, 26) enter from the sides to flatten the sides of the press, and to dimple in the quartz with the press at arc chamber junction area (16) where end pockets are normally formed.

Description: The invention relates to an arc tube for a high pressure metal vapour electric discharge lamp. In particular this invention relates to arc tubes for lamps having a metallic halide fill. High pressure metal vapour arc discharge lamps generally comprise an elongated arc tube made of quartz or fused silica, the ends of which are sealed either by blow moulding or by having pinched or pressed seals. The arc tube contains a quantity of mercury along with an inert starting gas such as argon and is provided with electrodes at opposite ends extending through the blow mouldings or supported by the pinch or press seals. Metallic halide lamps contain, in addition to the mercury and starting gas, one or more metal halides such as sodium, thallium, and indium iodides, or sodium and scandium iodides. In commercial metal halide lamps, the arc tube is generally enclosed within a vitreous outer envelope or jacket provided with a screw base at one end. Arc tubes have been made utilizing a so-called full press seal, wherein the entire end segment of a piece of quartz or fused silica tubing is collapsed and sealed off. This is done by pinching the ends of the quartz tube while in a heat-softened condition between a pair of opposed jaws to press the quartz about a foliated inlead supporting an electrode on its inner end. The jaws contact and compress only the end portions of the quartz tube, thereby forming the press or pinch seal. The immediately adjacent quartz which is viscous at the instant of pinching assumes a generally rounded shape in the transition zone between the cylindrical main body of the arc tube and the press seal which may be referred to as the end chamber. Another method which is widely used in the lighting industry for shaping the end wells in quartz arc tubes, is commonly known as "blow moulding". In the blow moulding process, a hemispherically shaped cavity is formed by the press feet when the seal is made and a slight excess pressure of inert gas is applied to the inside of the arc chamber. This gas pressure forces the plastic quartz to expand into the mould thus shaping the end of the arc tube. The shape of the end chambers, that is of the space around and behind the electrodes, will vary with the type of quartz, the wall thickness, the heat concentration and the nitrogen pressure build-up during pressing. That the inside surface of the arc tube should be smooth and free of angular crevices, corners, or pockets has previously been reported, see for example, US-A-2965698. However, the realization of this need has not yet resulted in any one method suitable for totally achieving the stated requirement, see, for instance, US-A-3939538 which recites advantages of the blow moulding method over the press seal method. While the blow moulding method does provide some degree of control over the shape of the end well, it does not eliminate or prevent the formation of end pockets where the seal meets the arc tube. Further, it is known from US-A-4396857 to heat the area around the seal and then to squeeze the material between two cup-shaped halves of a mould to produce a cylindrical neck portion around the electrode leads, thereby avoiding the formation of corners in the arc tube. In any metal halide lamp the colour temperature is controlled by the coldest spot temperature in the arc tube, which in turn determines the vapour pressures of the radiating species. To have a lamp with reduced colour temperature variations, what is really needed is an arc tube geometry in which the cold spot temperature does not vary, or at least is less sensitive to changes in the lamp's operating or burning position. Modifications of arc tube inner geometry are thus constantly being explored, due in part, to the difficulty of control experienced during press sealing and blow moulding. Viewed from one aspect, the present invention provides a fused silica or quartz arc tube for use in a high pressure metal vapour electric discharge lamp, comprising a sealed arc chamber defined by a main body portion, a seal at one end of the main body portion, and an electrode sealed into the seal and projecting into the arc chamber, characterised in that on either side of the seal, adjacent the junctions between the seal and the arc chamber, there are provided indentations in the wall of the main body portion which project into the arc chamber whereby end pockets in the arc chamber are substantially absent in the regions of said junctions. In a preferred embodiment of the present invention, the unwanted end pockets are eliminated during the manufacturing process of the arc tubes. Thus, viewed from another aspect, the present invention provides a method of manufacturing a fused silica or quartz arc tube for use in a high pressure metal vapour electric discharge lamp, said tube comprising a sealed arc chamber defined by a main body portion, a seal at one end of the main body portion, and an electrode sealed into the seal and projecting into the arc chamber, wherein the method comprises the steps of forming the seal by a pressing operation whilst the silica or quartz is heated, characterised in that subsequent to the pressing operation to form the seal there is carried out a second pressing operation in a perpendicular direction to provide indentations in the wall of the main body portion which project into the arc chamber on either side of the seal, adjacent the junctions between the seal and the arc chamber, whereby end pockets in the arc chamber are substantially absent in the regions of said junctions. Viewed from a yet another aspect, the present invention provides apparatus for manufacturing a fused silica or quartz arc tube for use in a high pressure metal vapour electric discharge lamp, said tube comprising a sealed arc chamber defined by a main body portion, a seal at one end of the main body portion, and an electrode sealed into the seal and projecting into the arc chamber, wherein the apparatus comprises means for heating the silica or quartz, a pair of press feet and means for urging the press feet together in a first direction so as to form the seal by a first pressing operation whilst the silica or quartz is heated, characterised in that a second pair of press feet are provided adjacent the junction between the seal and the arc chamber, and means are provided for urging the feet of the second pair together in a direction perpendicular to the first direction in a second pressing operation subsequent to the first pressing operation, whilst the silica or quartz is still heated and whilst the feet of the first pair are still in contact with the silica or quartz, the feet of the second pair being configured to provide indentations in the wall of the main body portion which project into the arc chamber on either side of the seal in the regions of said junctions, whereby end pockets in the arc chamber are substantially absent in said regions. In yet another preferred embodiment of the present invention, commercially available (or otherwise preformed) arc tubes may be improved by a preferred method of the present invention. Thus, viewed from a further aspect this invention provides a method of modifying a fused silica or quartz arc tube for use in a high pressure metal vapour electric discharge lamp, said tube comprising a sealed arc chamber defined by a main body portion, a seal at an end of the main body portion, and an electrode sealed into the seal and projecting into the arc chamber, there being one or more pockets in the arc chamber in the regions of the junctions between the seal and the arc chamber, characterised in that the arc tube is heated and pressure is exerted on the wall of the main body portion on either side of the seal in the regions of the junctions between the seal and the arc chamber so as to provide indentations in the wall of the main body portion which project into the arc chamber on either side of the seal so as to substantially eliminate the pockets in the regions of said junctions. Embodiments of the present invention allows modification of the internal geometry of arc tubes, for the purpose of reducing colour temperature variations in metal halide arc tubes. A GTE Sylvania M100 arc tube, modified in accordance with the present teachings, may operate with equal efficiency in both the vertical and horizontal positions. Unmodified M100 arc tubes can suffer from both reduced lumen output and increased colour temperature when changed from the normal vertical operating position to the horizontal. Preferred methods of the present invention can remove unwanted end pockets in many lamps, for example GTE M100 lamps, without adversely affecting the output of the lamp and improving the quality of the light output. The present invention can thus be considered as being a method directed towards the substantial elimination of end pockets in metal halide arc tube envelopes (or other irregularities) in order to improve the colour stability thereof. Preferred embodiments of the present invention will now be discussed by way of example only, and with reference to the accompanying drawings, in which: Figure 1 shows a side elevation of a standard M100 watt arc tube. Figure 2 shows a sectional view of the standard M100 watt arc tube of Figure 1 revealing the contours of the inner chamber. Figure 3 shows a longitudinal view of a M100 watt arc tube modified by crimping the ends thereof with a carbon rod. Figure 4 illustrates the lamp of Figure 3, sectioned to reveal the improved contours of the inner chamber. Figures 5a-5c illustrate a preferred pressing sequence of the present invention. The first frame (step a) represents the press feet approaching the quartz blank. The second frame (step b) shows the main press feet in contact with quartz as the seal is formed. The third frame (step c) shows the side press feet completing the press. Figures 6, 6a and 6b illustrate the side press foot operation to remove end pockets. Figure 6 - The press seal has been formed by the main press feet (not shown) and the side press feet are approaching the hot quartz. Figure 6a - Side press feet contact the quartz and crimp (dimple) the quartz at the press - arc tube joint region. Figure 6b - Press feet withdrawn. Figure 7 shows a side elevation of a conventional Sylvania 1000 watt arc tube with a blow moulded end. Figure 8 illustrates a sectional view of a Sylvania 1000 watt arc tube with a blow moulded end, revealing the contours of the inner chamber, and showing the presence of end pockets. Figure 9 illustrates a side elevation of a conventional General Electric 400 watt arc tube with a blow moulded end. Figure 10 illustrates a sectional view of a General Electric 400 watt arc tube with a blow moulded end revealing the contours of the inner chamber, and showing the presence of end pockets. One aspect of the present invention is directed to a method of forming or treating an arc tube, preferably for a metal halide lamp, such that end pockets or other internal tube irregularities, e.g., angular corners on the interior of the arc tube, are substantially eliminated. Since the cold spot temperature is generally located in the end wells of the lamp, attention was directed to the shape of these end wells to see if there was any simple way to improve their geometry to achieve the desired results. The M100 watt arc tube, recently developed by GTE Sylvania, was chosen as the experimental vehicle. When these lamps were examined in neatsfoot oil, it was discovered, with much surprise, that there were present in every lamp, deep end pockets 10 in the area where the press seal 12 meets the body 14 of the arc tube (herein referred to as the arc chamber - seal junction 16). (See Figures 1 and 2). Thus, while the external portions of the lamps described in the figures may be in the "prior art," the discovery of unwanted end pockets and/or corners on the inner surface of said lamps is not. These pockets were not intentionally designed into the arc tube, and are, in essence, an artifact of the seal manufacturing process. This closure geometry is what is generally obtained in the lighting industry when press seals are made, and the only type of crevice that most engineers worry about, or even look for, are crevices directly adjacent to the electrode. In most preferred embodiments, the method of analyzing the arc tubes should be a non-destructive method, although destructive methods, such as breaking the tube may also be employed if desired. Prior to our invention, the arc tube ends were press or pinch sealed using conventional techniques such as that described in US-A-2965698, and commercial production was generally conducted by a quartz lamp sealing machine such as that disclosed in US-A-2857712. In such processing, a quartz tube is supported in a head of a pinch sealing machine and has its exhaust tubing accommodated in a gas supply port providing nitrogen to prevent oxidation of the inleads during the sealing operation. The sealing or pinching is performed at a station where oxy-hydrogen burners heat the lower end of the quartz tube to a plastic state. At the proper moment, the burners are withdrawn and a pair of pinching jaws are rapidly moved firstly up into alignment with the lower end of the quartz tube and then in horizontally against the sides of the tube. If this is the first end of the quartz tube to be sealed, the other end is stoppered. At the moment of pinching, the end chamber is formed by the back pressure of nitrogen which forces the soft quartz outward. The shape of the end chamber is determined by the viscosity of the quartz which is temperature dependent, the nitrogen back pressure, and the jaw closing speed. During operation of a typical lamp of the prior art, the corners run significantly cooler than the remainder of the body of the arc tube and the colour temperature is adversely affected thereby. By eliminating the angular corners, the arc tube operates closer to isothermal and colour temperature is improved. In at least one preferred embodiment of the present invention, the arc tube differs structurally from the prior art in that there are notches or "dimples" at the juncture of the seals and arc tube body (and the interior is smooth, i.e., without angular corners). As stated above, existing Sylvania M100 arc tubes were utilized as the test vehicles for developing an arc tube with improved colour temperature stability. The standard Sylvania M100 design arc tube was chosen since it was originally designed for high efficiency (90 LPW) and low colour temperature (3000K) when operated in the vertical position only. When operated in the horizontal position it gives typical results summarized in Table 1. By conventional metal halide standards this type of variation in performance (efficacy down 7 LPW, colour temperature up 400K) when going from vertical to horizontal would be considered to be quite good. However, in keeping with the needs of modern society and the quest for the perfect metal halide lamp, this degree of output change is unacceptable, and certainly can be improved upon. Towards this end, the present invention, which to an extent can be described by the following two preferred methods for removing end pockets, were developed. The first method starts with an already pressed arc tube which contains end pockets. Heat a small area around each pocket with a torch and depress the plastic quartz with a carbon rod, thus collapsing and sealing the end pocket. This method gives a rather crude looking arc tube, but the desired result is achieved - the end pockets are gone. An example of a conventional arc tube after being modified by this method is shown in Figures 3 and 4. Figure 3 illustrates the exterior "dimples" 18 formed by the carbon rod in the arc tube body 14. Figure 4 illustrates how the dimples 18 eliminate the angular end pockets 10. As a second, and more elegant, way of removing the end pockets, there was designed a sealing machine with 4 presses (2 pairs) to provide pressing capability in two mutually perpendicular planes. With this machine a first press is made in the conventional manner with two opposing press feet 20 & 22 being driven into the hot quartz of the arc tube body 14. While these feet are still in contact with the hot quartz, a second pair of press feet 24 & 26 enter from the sides to flatten the sides of the press, and to dimple in the quartz at the press - arc chamber junction area 16 where end pockets are normally formed. An end on view of this sequence is shown in Figure 5. A side view of the secondary (side) pressing sequence is shown in Figure 6 to indicate how the dimpling is accomplished. For either way of forming the arc tube, the end result is the same. In each case the colour uniformity of a group of lamps is improved, and the changes in lamp colour temperature when the burning position is changed are reduced. For lamps made by the first method (carbon rod) typical data is shown in Tables 2, 3 and 4. Comparison of this data with that in Table 1 shows that by eliminating the end pockets, a substantial improvement in lamp operating efficiency has been achieved. Some data for lamps made by the second method (4 way press seal) are shown in Table 5. These data show that by eliminating the end pockets these lamps also maintain a low colour temperature when operated in the horizontal position. Based upon this disclosure, it will be apparent that the techniques of the present invention may be extended to other possible sizes (wattages) of metal halide lamps, and that improvements similar to those shown here are would be obtained by removing the end pockets in any other wattage lamp. For example, as illustrated in Figures 7 & 8, a Sylvania 1000 watt arc tube made with blow moulded ends 28 shows the presence of end pockets 10. Similarly, as illustrated in Figures 9 & 10, a General Electric 400 watt arc tube also made with blow moulded ends 28 shows the presence of end pockets 10. These illustrations represent typical blow moulded lamps. Such end pockets can be substantially eliminated by the first method described herein, and similarly, the blow moulding process can be adapted as described in the second method to introduce a dimpling effect. It has also been discovered that the magnitude of this improvement will increase with decreasing arc tube size, since in smaller arc tubes the ends represent a larger fraction of the inner surface of the device.

Claims: A fused silica or quartz arc tube for use in a high pressure metal vapour electric discharge lamp, comprising a sealed arc chamber defined by a main body portion (14), a seal (12) at one end of the main body portion, and an electrode sealed into the seal (12) and projecting into the arc chamber, characterised in that on either side of the seal (12), adjacent the junctions (16) between the seal (12) and the arc chamber, there are provided indentations (18) in the wall of the main body portion (14) which project into the arc chamber whereby end pockets in the arc chamber are substantially absent in the regions of said junctions (16). An arc tube as claimed in claim 1 characterised in that the seal (12) has a generally planar form and each indentation (18) projects into the arc chamber in a direction parallel to the plane of the seal (12). An arc tube as claimed in claim 1 or 2, characterised in that the seal (12) is a press seal. An arc tube as claimed in claim 1 or 2, characterised in that the seal (12) has been formed in a blow moulding process. An arc tube as claimed in any preceding claim characterised in that a seal (12) with associated indentations (18) is provided at each end of the main body portion (14). A method of manufacturing a fused silica or quartz arc tube for use in a high pressure metal vapour electric discharge lamp, said tube comprising a sealed arc chamber defined by a main body portion (14), a seal (12) at one end of the main body portion, and an electrode sealed into the seal (12) and projecting into the arc chamber, wherein the method comprises the steps of forming the seal (12) by a pressing operation whilst the silica or quartz is heated, characterised in that subsequent to the pressing operation to form the seal (12) there is carried out a second pressing operation in a perpendicular direction to provide indentations (18) in the wall of the main body portion (14) which project into the arc chamber on either side of the seal (12), adjacent the junctions (16) between the seal (12) and the arc chamber, whereby end pockets in the arc chamber are substantially absent in the regions of said junctions (16). A method as claimed in claim 6, characterised in that the indentations (18) are formed whilst press feet (20,22) to form the seal (12) are still in contact with the heated silica or quartz. A method as claimed in claim 6 or 7, characterised in that in the second pressing operation the sides of the seal (12)are flattened. Apparatus for manufacturing a fused silica or quartz arc tube for use in a high pressure metal vapour electric discharge lamp, said tube comprising a sealed arc chamber defined by a main body portion (14), a seal (12) at one end of the main body portion, and an electrode sealed into the seal (12) and projecting into the arc chamber, wherein the apparatus comprises means for heating the silica or quartz, a pair of press feet (20, 22) and means for urging the press feet together in a first direction so as to form the seal (12) by a first pressing operation whilst the silica or quartz is heated, characterised in that a second pair of press feet (24,26) are provided adjacent the junction (16) between the seal (12) and the arc chamber, and means are provided for urging the feet (24, 26) of the second pair together in a direction perpendicular to the first direction in a second pressing operation subsequent to the first pressing operation, whilst the silica or quartz is still heated and whilst the feet (20, 22) of the first pair are still in contact with the silica or quartz, the feet (24, 26) of the second pair being configured to provide indentations (18) in the wall of the main body portion (14) which project into the arc chamber on either side of the seal (12) in the regions of said junctions (16), whereby end pockets in the arc chamber are substantially absent in said regions. Apparatus as claimed in claim 9, characterised in that the feet (24, 26) of the second pair are further configured to flatten the sides of the seal (12). A method of modifying a fused silica or quartz arc tube for use in a high pressure metal vapour electric discharge lamp, said tube comprising a sealed arc chamber defined by a main body portion (14), a seal (12) at an end of the main body portion, and an electrode sealed into the seal (12) and projecting into the arc chamber, there being one or more pockets (10) in the arc chamber in the regions of the junctions (16) between the seal (12) and the arc chamber, characterised in that the arc tube is heated and pressure is exerted on the wall of the main body portion (14) on either side of the seal (12) in the regions of the junctions (16) between the seal (12) and the arc chamber so as to provide indentations (18) in the wall of the main body portion (14) which project into the arc chamber on either side of the seal (12) so as to substantially eliminate the pockets (10) in the regions of said junctions (16). A method as claimed in claim I1, characterised in that a heated carbon member is used to heat the wall of the main body portion (14) in the regions of the junctions (16) and to apply the pressure to form the indentations (18). A method as claimed in claim 11 or 12, characterised in that prior to the modifying operations the arc tube is analysed to detect the presence of pockets in the regions of the junctions (16) between the seal (12) and the arc chamber.

Inventors: KEEFFE WILLIAM M, MORRIS JAMES C, WHITE PHILIP J, KEEFFE, WILLIAM M., MORRIS, JAMES C., WHITE, PHILIP J.
Applicants: GTE PROD CORP, GTE PRODUCTS CORPORATION
Application year: 1987
Main classification: H01J9
IPC codes: H01J9, H01J61