Abstract
The invention describes a gas-discharge lamp comprising an inner vessel enclosing a pair of electrodes separated by a gap; and an outer vessel enclosing the inner vessel; and wherein the lamp comprises a lateral stripe arranged on the surface of a vessel such that the lateral stripe lies below a horizontal plane through a longitudinal axis through the center of the lamp, and wherein the lateral stripe extends essentially only over a region corresponding to the gap between the electrodes.
Claims
1. A gas-discharge lamp for horizontal mounting in a vehicle headlamp comprising: an inner vessel enclosing a pair of electrodes separated by a gap; and an outer vessel enclosing the inner vessel; wherein the lamp comprises a lateral stripe for blocking light of the lamp from appearing above a bright/dark cut-off line of a front beam of the vehicle headlamp, with the lateral stripe being arranged on a surface of the inner or outer vessel below a horizontal plane through a longitudinal axis through the center of the lamp, and wherein the lateral stripe extends over a region corresponding to the gap between the electrodes, and by a distance of at most 1.0 mm beyond the gap in the direction of an inner end region of the lamp, and by a distance of at most 2.0 mm beyond the gap in the direction of an outer end region of the lamp.
2. A lamp according to claim 1, wherein the lateral stripe has a length of at most 6.0 mm, preferably at most 5.0 mm.
3. A lamp according to claim 1, wherein the lateral stripe has a height of 2.00.7 mm.
4. A lamp according to claim 1, wherein the lamp comprises two lateral stripes, one on each side of the inner or outer vessel.
5. A lamp according to claim 4, wherein the lateral stripes have essentially identical dimensions.
6. A lamp according to claim 1, wherein the lateral stripe extends by a distance of at most 0.5 mm in the direction of the inner end region of the lamp.
7. A lamp according to claim 1, wherein the lateral stripe extends by a distance of at most 0.5 mm in the direction of the outer end region of the lamp.
8. A lamp according to claim 1, wherein the lateral stripe comprises an opaque material.
9. A lamp according to claim 1, wherein the lamp is free of any coating about the inner end region.
10. A lamp according to claim 1, further comprising a circumferential stripe arranged on a surface of the inner or outer vessel in the inner region of the lamp, and wherein the circumferential stripe is separate from the lateral stripe by a distance of at least 1.5 mm.
11. A lamp according to claim 1, with a nominal power of 35 W, for which lamp the capacity of the inner discharge vessel is greater than or equal to 15 l and less than or equal to 23 l; the inner diameter of the inner discharge vessel comprises at least 2.2 mm and at most 2.8 mm; and the outer diameter of the inner discharge vessel comprises at least 5.2 mm and at most 5.8 mm.
12. A lamp according to claim 1, wherein the lateral stripe is applied to an outer surface of the outer vessel of the lamp.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 shows a high-intensity discharge lamp according to a first embodiment of the invention;
(2) FIG. 2 shows a prior art R-type high-intensity discharge lamp;
(3) FIG. 3A shows a front beam generated by the lamp according to the invention used in a projector headlamp;
(4) FIG. 3B shows a front beam generated by a prior art lamp used in a projector headlamp;
(5) FIG. 4A shows a shoulder region of a beam profile achieved by a prior art lamp of type D4R used in a reflector headlamp;
(6) FIG. 4B shows a shoulder region of a beam profile achieved by a lamp according to the invention used in the same type of reflector headlamp;
(7) FIG. 4C shows a shoulder region of a beam profile achieved by a prior art lamp with a detachable metal frame used in the same type of reflector headlamp;
(8) FIG. 5 shows further views of an embodiment of the lamp according to the invention;
(9) FIG. 6 shows another preferred embodiment of a lamp according to the invention with a circumferential stripe in the inner region of the lamp.
(10) In the drawings, like numbers refer to like objects throughout. Objects in the diagrams are not necessarily drawn to scale.
DETAILED DESCRIPTION OF THE EMBODIMENTS
(11) FIG. 1 shows a HID lamp 1 according to an embodiment of the invention. The lamp 1 comprises an outer vessel 10 or envelope 10 enclosing an inner vessel 100 or burner 100. Two electrodes 12, 13 extend into the burner 100 and their tips face each other across a narrow gap G. A return lead 130 provides an electrical connection between the outer electrode 13 and a base 11 or ballast housing 11 of the lamp 1, so that both electrodes 12, 13 can be electrically connected to a ballast (not shown). The geometrical construction details of this lamp 1 correspond essentially to those of a standard D2R lamp. When the lamp 1 is ignited, a discharge arc D is established between the electrode tips. The discharge arc D will have a curved bow shape extending above a longitudinal axis X extending through the center of the lamp 1. The diagram shows a lateral stripe P applied to an outer surface of the envelope 10. The stripe P is arranged such that it does not extend above the level of the longitudinal axis X, i.e. the stripe P lies below a horizontal plane through the center of the lamp 1. The height H.sub.P of the lateral stripe P is in the range 2.0 mm0.7 mm, i.e. at least 1.3 mm and at most 2.7 mm, in keeping with the specification. However, in the lamp 1 according to the invention, the lateral stripe P is wide enough to just cover a region corresponding to the gap G between the electrodes 12, 13, with at most a slight overlap into the regions beyond the electrode tips. On account of this design, the lamp 1 according to the invention can be used in both R-type and S-type headlamps in countries that allow a deviation from the applicable regulations.
(12) For comparison, FIG. 2 shows a prior art D4R R-type lamp 2. However, this known type of lamp 2 makes use of two types of stripe S.sub.H, S.sub.V applied to the outer vessel 10. A horizontal stripe S.sub.H serves to achieve a desired bright/dark cut-off, while a vertical stripe S.sub.V, applied around the outer vessel 10 in the region near to the lamp base 11, serves to reduce glare above the cut-off line. Because of the significant manipulation of the beam that is the result of these stripes S.sub.H, S.sub.V, this type of bulb 2 may only be used in an R-type headlamp. A front beam generated by this lamp 2, when used in a projector headlamp, would be severely deficient.
(13) FIG. 3A shows a front beam, generated by the lamp of FIG. 1 installed in a projector headlamp, and cast onto a screen at 25 m. The screen extends from 60 to +60 along the X-axis, and from 10 to +5 along the Y-axis. FIG. 3B shows a front beam generated by a conventional S-type HID lamp installed in the same type of projector headlamp, also cast onto a screen at 25 m over the same angular range as in FIG. 3A. The diagrams illustrate that there is negligible difference between the two beam profiles. Even though the lamp of FIG. 1 bears lateral stripes, its performance is comparable to the conventional S-type lamp, which does not have any stripes at all. This may be explained by the physical effect of the lateral stripe, which raises the burner temperature. Therefore, even though the lateral stripes effectively block some of the light from the discharge arc, the increase in burner temperature results in a brighter discharge arc. The result is a beam profile that deviates at most only slightly from the beam profile achieved by the standard S-type lamp.
(14) FIGS. 4A-4C show the most critical region in the beam profile of a reflector headlamp (at 25-100 m in front of a vehicle, covering an angular range of 5 to +5 along the X-axis, and 1.2 to +1.2 along the Y-axis) as achieved by a standard D4R lamp used in a reflector headlamp (FIG. 4A); for the lamp according to the invention used in the same type of reflector headlamp (FIG. 4B); and for a HID lamp with an detachable metal frame, also used in the same type of reflector headlamp (FIG. 4C). The region shown in the diagrams corresponds to the shoulder region of the bright/dark cut-off line CO. A relevant measurement point HV is given by the intersection of the horizontal line H and vertical line V that divide the projection into four quadrants. Each diagram shows nine ISO candela lines 4_a, 4_b, . . . , 4_i indicating regions of luminous intensity, with values of 400; 630; 1,000; 1,600; 2500 cd; 4,000; 6,300; 16,000; and 25,000 cd respectively. It is important that the front beam is not corrupted by stray light above the cut-off line, particularly in the shoulder region in the upper left corner of the lower right quadrant. FIG. 4A shows that the conventional D4R lamp satisfies the front beam requirements, with a favorably high level of luminous intensity at a distance of 70-80 m outward from the headlamp, as indicated by the lowest contour line 4_i corresponding to 25,000 cd. Glare levels of 625 cd at the intersection of the horizontal and vertical line were measured for this lamp. Here, the contour lines 4_c-4_g in the range of 1,000-6,300 cd are closely grouped, indicating a favorably abrupt transition between dark (above the cut-off line CO) and bright (below the cut-off line). FIG. 4B shows that the lamp according to the inventioneven though it only has shorter lateral stripes and no circumferential stripestill produces a satisfactory beam profile. In this case, favorably low glare levels of only around 700 cd were measured, as indicated by the closely grouped contour lines 4_c-4_g (similar to FIG. 4A). Here also, the front beam is characterized by a favorably high level of luminous intensity at a distance of 70-80 m outward from the headlamp, as indicated by the lowest contour line 4_i corresponding to 25,000 cd. The diagrams illustrate that the lamp according to the invention can be used in an R-type headlamp since it provides a beam profile similar to a standard D4R lamp, and is characterized by a glare level that is significantly below the glare level achieved by a prior art Xenon HID lamp with detachable metal frame, as illustrated in FIG. 4C, which shows that the alternative solution (with such a metal frame) fails to deliver a satisfactory front beam. The detachable metal frame is too far away from the discharge arc to be able to achieve a sharp cut-off. Here, the contour lines 4_c-4_g are relatively wide apart, indicating an unfavorable transition between dark and bright about the cut-off line CO. This results in a significantly lower illumination close to the cut-off line far in front of the vehicle (at a distance of 40-80 m). Additionally stray light due to the reflections from the metal frame is scattered which increases the light intensity at the measurement point HV to a level of 1500 cd, which is significantly higher compared to what is achieved by the inventive lamp.
(15) FIG. 5 shows further views of the lamp according to the invention, showing a cross-section taken through a vertical plane (upper part of the diagram) and a cross-section through a horizontal plane (lower part of the diagram). The diagrams show the burner 100 enclosed by the outer vessel 10 or envelope 10, and a longitudinal axis X extending through the electrodes 12, 13 and the center of the lamp 1. The diameter of the outer vessel is about 8.7 mm in the case of a 35 W lamp constructed according to the D4R specifications. The position of the lateral stripes P on each side of the lamp 1 is shown relative to a horizontal plane 50 through the center of the lamp 10. In the lower part of the diagram, the electrodes 12, 13 are shown to be separated by a gap G, which has a gap length L.sub.G of about 4.2 mm. The lateral stripes P are shown to be applied in a region corresponding to the gap G, and to extend by small amounts towards an outer region 18 at the front of the lamp 10 (i.e., facing outward from the headlamp arrangement) and towards an inner region 17 towards the base 11 or ballast housing of the lamp 10, so that the lateral stripe P has an overall length L.sub.P. A lateral stripe P extends slightly further towards the outer region 18 of the lamp 10 than towards the inner region 17, so that the lateral stripe P is offset from the gap G. Taking the center of the gap G as a reference point, the lateral stripe P comprises a first portion with length L.sub.1 extending towards and beyond a point corresponding to the cathode tip, and a second portion with length L.sub.2 extending towards and beyond a point corresponding to the anode tip. As explained above, the effect of applying the lateral stripe P in this way is to ensure that the lateral stripe P always extends along the entire length of the discharge arc, in case the arc lengthens due to burnback, and/taking into account positioning tolerances of the electrodes 12, 13.
(16) Another preferred embodiment is achieved by adding a 2-4 mm circumferential stripe P.sub.R in the inner region close to the base of the lamp 1 as shown in FIG. 6. The inner edge 60 of the circumferential stripe P.sub.R corresponds to the position of the inner edge of a circumferential coating on a standard D4R lamp. The inner edge 60 of the circumferential stripe P.sub.R (i.e. the edge closest to the discharge arc) is separated from the corresponding short edge 61 of the lateral strip P by a distance of about 2 mm. In the lamp according to the invention, the circumferential stripe P.sub.R reduces the glare experienced by oncoming drivers approaching at a distance of 20-100 m by a factor of two.
(17) Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.
(18) For the sake of clarity, it is to be understood that the use of a or an throughout this application does not exclude a plurality, and comprising does not exclude other steps or elements. The mention of a unit or a module does not preclude the use of more than one unit or module.
