Arc tube for a high pressure discharge lamp

Abstract

 An arc tube (12, 46, 48), provided for a high pressure discharge lamp, includes a discharge chamber (24) containing a fill. Electrical conductors (26, 28) extend into the discharge chamber (24) to form electrodes (32, 34). The electrical conductors (26, 28) are sealed in the arc tube (12, 46, 48) by a single pinch seal portion (30). At the interface (44) of the discharge chamber (24) and the pinch seal portion (30), the discharge chamber has at least in part a non-concave outline. This outline may be rectilinear or convex.

Description

[0001] The present invention relates to an arc tube for a high pressure discharge lamp. In particular, though not exclusively, the present invention relates to a high intensity metal halide discharge lamp.

[0002] Many high intensity discharge lamps, fabricated from fused silica and employing molybdenum foil pinch seals, operate at very high internal pressures, up to pressures of 3.0MPa. Accordingly, one important criterion in determining the quality of the arc tubes used in these lamps is the ultimate bursting strength of the arc tube - the internal pressure to which the arc tube can be subjected before it explodes. Requirements on lamp performance dictate that arc tubes should be fabricated with bursting strength as high as possible consistent with a good geometrical accuracy and freedom from oxidation of metallic components. Both the design of the arc tube and the detailed values of parameters of the seal making process, such as times, pressures, gas flow rates etc, are known to effect the attainable ultimate bursting strength.

[0003] Figures 1(a) and 1(b) are schematic representations of an arc tube for a known single ended low power metal halide lamp; Figure 1(b) is a cross-sectional view of the arc tube of Figure 1(a). The discharge chamber 4 of the arc tube is made as near spherical as possible, although ellipsoidal geometries are also acceptable. A spherical geometry for the arc tube yields a near isothermal temperature profile for the operating lamp. This is a highly desirable feature since it ensures stability of lamp performance, particularly colour and light output, with lamp operating position. Such lamps are classified as 'universal burning' and are suitable for use in display lighting.

[0004] An example of this type of lamp is the 150W Arcstream 3000 lamp which operates at internal pressures of about 2.5MPa. Arc tubes for this type of lamp are fabricated from silica tubing of 11 mm bore with a wall thickness of 1.25 mm. The tubes contain a mixture of mercury together with the iodides and bromides of tin, sodium and thallium. Argon is also present at a cold pressure of 24KPa to act as a starting gas. Each arc tube is pinch sealed into a quartz outer jacket with nitrogen gas in the space between the arc tube and outer jacket. The assembly so produced is mounted onto a bi-pin ceramic base.

[0005] It has been found by experience that arctubes made to this design having an ultimate bursting strength of less than 5.5MPa when tested are more likely to fail violently by explosion in operation. However lamps manufactured according to existing processes produced excessive manufacturing scrap when pneumatically pressure tested at pressures of 5.5MPa. Extensive experimentation to optimise the levels of important parameters of the manufacturing process using the Taguchi method (a partial factorial method of systematic experimentation using orthogonal arrays) failed to increase the ultimate bursting strength (as measured by a hydraulic pressure test) to the required level.

[0006] It is an object of the present invention to at least alleviate the problem outlined hereinbefore.

[0007] According to the present invention there is provided an arc tube for a high pressure discharge lamp, the arc tube comprising a discharge chamber containing a fill and electrical conductors extending into the discharge chamber to form electrodes, the electrical conductors being sealed in the arc tube by a single pinch seal portion wherein, at the interface of the discharge chamber and the pinch seal portion, the discharge chamber has at least in part a non-concave outline.

[0008] The inventors have surprisingly found that arc tubes provided in accordance with the present invention have a significantly higher bursting strength than prior art lamps, as shown in Figure 1, in which, at the interface of the discharge chamber and the pinch seal portion, the discharge chamber has a concave outline. The inventors believe this is due to changes made in the complex way in which the electrode structures interrupt the internal geometry of the discharge chamber as compared with prior art arc tubes. Accordingly, in making the invention, the inventors had needed to appreciate that despite the prior art, it might be worthwhile investigating the role of pinch seal geometry on ultimate bursting strength.

[0009] In a preferred embodiment for use in any orientation, the non-concave outline is a rectilinear outline. Modification of the geometry of the discharge chamber to other than a spherical geometry had been expected to modify the lamp performance to an unacceptable extent, in particular, to increase the variation of lamp colour with operating position. The inventors were surprised to find that the performance of lamps provided in accordance with this preferred embodiment was acceptable for a universal burning lamp.

[0010] Alternatively, in an embodiment for use in a single orientation, the preferred non-concave outline is a convex outline. The limitation of use of the arc tube to a single orientation enables the lamp engineer to modify the fill of the arc tube to provide an optimum performance for that single orientation.

[0011] Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of a prior art arc tube as described hereinbefore;

Figure 2 shows a discharge lamp containing an arc tube provided in accordance with the present invention;

Figures 3 and 4 are schematic representations of arc tubes provided in accordance with the present invention.

[0012] Figure 2 shows a high pressure discharge lamp 10 provided in accordance with the present invention. An arc tube 12 is located in a quartz outer jacket 14 by a pinch seal 16 (shown in part). The space 18 between the outer jacket 14 and the arc tube 12 is filled with nitrogen gas. The assembly so produced is mounted onto a bi-pin ceramic base 22.

[0013] The arc tube 12 is fabricated from silica tubing having a bore and a wall thickness dependant on the wattage of the lamp to be produced which may be from 20W to 2.5kW. For a 150W lamp, the bore is 11 mm and the wall thickness is 1.25 mm. The arc tube 12 has a discharge chamber 24 in which is present a fill comprising a mixture of 25 mg mercury together with the iodides and bromides of tin, sodium and thallium. Argon is also present at a cold pressure of 24KPa to act as a starting gas. A pair of electrical conductors 26, 28 are sealed into one end of the arc tube 12 at a single pinch seal portion 30 to provide a pair of spaced apart electrodes 32, 34 extending into the discharge chamber 24. The other end of each of the electrical conductors 26, 28 is joined to a strip of molybdenum foil 36, 38 in the pinch seal portion 30. The strips of foil 36, 38 are in turn joined to a pair of electrical leads 40, 42, for connection to the pins of the ceramic base 22 via the pinch seal 16. The value of the dimensions indicated in Figure 2 is a = 12.8 mm, b = 14.0 mm and c = 15.5 mm.

[0014] The shape of the discharge chamber 24 in the arc tube 12 is defined, inter alia, by the shape of the pinch seal portion 30. In the discharge lamp 10, the discharge chamber 24 is shown as having a rectilinear outline 44 at the interface of the discharge chamber 24 and the pinch seal portion 30. The arc tube is accordingly hereinafter termed a 'rectilinear arc tube'. Alternatively, the discharge chamber may have a convex outline at the interface of the discharge chamber and the pinch seal portion 30 - hereinafter termed a 'convex arc tube'. The radius of curvature of the convex outline can be as low as 7.5 mm (for a 150W lamp) to infinity (ie 'rectilinear'). In convex 'arc tubes' used in the experiments described hereinafter, the radius of curvature of the convex outline was 22.0 mm.

[0015] For ease of comparison, Figures 3 and 4 are schematic representations, similar to Figure 1, of lamps provided in accordance with the present invention. Figure 3a shows a rectilinear arc tube 46 corresponding to the arc tube of Figure 2. Figure 4a shows a convex arc tube 48. Figures 3b and 4b show cross-sections of the arc tubes of Figures 3a and 4a along respectively the lines III - III and IV-IV. The prior art arc tube of Figure 1 will hereinafter be termed a 'concave arc tube'.

[0016] The inventors have experimentally tested the arc tubes hereinbefore described for bursting strength and lamp performance. Sixty arc tubes of each type - concave, rectilinear and convex, were constructed by a process using optimized parameters as determined by the Taguchi analysis. The arc tubes were pressurized hydraulically to failure and the bursting pressures noted. The results of this experiment are shown in Table 1 below.

Table 1

		Convex	Rectilinear	Concave
Bursting pressure MPa	Mean	7.4	7.17	5.45
	Standard deviation	0.98	1.17	1.17
	Maximum	11.2	10.5	8.45
	Minimum	5.5	5.0	2.07

[0017] As can be seen, there is a considerable difference in bursting pressure and hence ultimate bursting strength between arc tubes of the traditional concave type and either the rectilinear or convex types. Statistical analysis of these results show this difference to be highly significant, i.e. it is extremely unlikely that such a result could be obtained by chance. The difference between rectilinear and convex arc tubes was found not to be statistically significant, though it is the inventors' opinion that the results indicate that the convex arc tubes tested were slightly stronger than the rectilinear arc tubes tested. Thus, when ultimate bursting strength only is taken into consideration, the preferred type of arc tube is one of the rectilinear or convex type, not the concave type.

[0018] As already outlined hereinbefore, lamp performance also needs to be considered when determining arc tube geometry. Relevant parameters were measured for a large number of arc tubes of each type. The results are shown in Tables 2 to 4; Table 2 for (prior art) concave arc tubes; Table 3 for rectilinear arc tubes; Table 4 for convex arc tubes.

TABLE 2

CONCAVE ARC TUBES
	x	y	CCT	Lm/W	Vt	W
Cap Down (CD)	.4361	.4078	3042	86.1	97.2	150.6
Horizontal (H)	.4483	.4174	2922	85.6	95.4	150.6
Shift CD to H	.0122	.0096	-120
Cap up (CU)	.4459	.4148	2938	90.2	99.7	153.9
Shift CD to CU	.0098	.0070	-104

TABLE 3

RECTILINEAR ARC TUBES
	x	y	CCT	Lm/W	Vt	W
Cap Down (CD)	.4433	.4067	2916	82.2	94.6	149.3
Horizontal (H)	.4531	.4162	2840	81.8	93.0	148.9
Shift CD to H	.0098	.0095	-76
Cap Up (CU)	.4563	.4153	2785	85.1	95.3	150.2
Shift CD to CU	.0130	.0086	-131

TABLE 4

CONVEX ARC TUBES
	x	y	CCT	Lm/W	Vt	W
Cap Down (CD)	.4334	.4086	3097	81.0	91.4	145.8
Horizontal (H)	.4551	.4180	2825	82.2	91.5	147.1
Shift CD to H	.0217	.0094	-272
Cap Up (CU)	.4572	.4162	2781	85.0	93.3	149.3
Shift CD to CU	.0238	.0076	-316

[0019] Explanation of Terms used in Tables:
x, y colour co-ordinates - a measure of the colour of the light emitted by the arc tube.
CCT correlated colour temperature - a measure of the colour of the light emitted by the arc tube.
Lm/W efficacy of lamp in lumens per Watt.
V_t voltage drop across arc tube in volts.
W Lamp Power in Watts.
Cap down Results for operating orientation in which base of lamp is vertically below arc tube.
Horizontal Results for operating orientation in which base of lamp is on same level as arc tube.
Cap Up Results for operating orientation in which base of lamp is vertically above arc tube.

[0020] The results surprisingly show that colour stability with operating orientation for rectilinear arc tubes is very similar to that of concave arc tubes. As already outlined, it had expected that concave arc tubes would have the best colour stability with operating orientation. Accordingly for lamps to be used for universal burning, the preferred arc tube is the rectilinear type.

[0021] As expected, lamps with convex arc tubes show double the shift in X colour co-ordinates and colour temperature as compared with lamps with concave or rectilinear arc tubes. However, convex arc tubes can be used in lamps intended for use in a single orientation.

[0022] Modifications to the embodiments described hereinbefore will be apparent to those skilled in the art. In particular, the internal pressure within the arc tube is primarily controlled by the amount of mercury dosed into the arc tube and so the gains in ultimate bursting strength illustrated by the specific embodiments described can be expected for any metal halide dose. Similar gains are also expected for lamps of ratings other than 150W. The present invention is expected to be equally applicable to lamps utilising bare arc tubes and to arc tubes sealed into glass or quartz envelopes with or without a gas fill between the arc tube and outer jacket.

Claims

1. An arc tube for a high pressure discharge lamp, the arc tube comprising a discharge chamber containing a fill and electrical conductors extending into the discharge chamber to form electrodes, the electrical conductors being sealed in the arc tube by a single pinch seal portion wherein, at the interface of the discharge chamber and the pinch seal portion, the discharge chamber has at least in part a non-concave outline.

2. An arc tube according to Claim 1 wherein, at the interface of the discharge chamber and the pinch seal portion, the discharge chamber has a non-concave outline.

3. An arc tube according to Claims 1 or 2 for use in any orientation wherein said non-concave outline is a rectilinear outline.

4. An arc tube according to Claims 1 or 2 for use in a single orientation wherein said non-concave outline is a convex outline.

5. An arc tube according to any one of the preceding Claims wherein the fill comprises mercury, an inert gas and at least one metal halide.

6. A high pressure discharge lamp comprising an arc tube according to any one of the preceding claims.

Drawing