Abstract:
A high intensity discharge lamp, the lamp including a light emitting vessel having a wall made of ceramic material that defines an inner space with a first end portion having a respective first opening formed therein and a second end portion having a respective second opening formed therein, two discharge electrodes, with a first electrode extending therethrough the first opening of the first end portion of the vessel and a second electrode extending therethrough the second opening of the second end portion of the vessel, together forming a gap between ends of the discharge electrodes positioned within the vessel, wherein the light emitting vessel defines an inner space characterized by an inner diameter ranging from and including 1 millimeters to 3 millimeters and an inner length between and including 5 millimeters to 10 millimeters, wherein the wall of the vessel has a thickness ranging between and including 0.3 millimeters to 0.8 millimeters, wherein each tip of the electrodes within the vessel have a shank diameter ranging between and including 0.2 millimeters to 0.55 millimeters, and wherein the gap between the ends of the electrodes positioned within the vessel is smaller than 4 millimeters.

Description:
FIELD OF INVENTION  
       [0001]     This invention relates generally to the field of lighting systems and, more specifically, to high-intensity discharge lamps.  
       BACKGROUND OF THE INVENTION  
       [0002]     Within the automotive industry, High Intensity Discharge (HID) lamps are beginning to replace conventional incandescent halogen lights as lights for headlamps. In an HID lamp, light is generated by means of an electric discharge that takes place between two metal electrodes enclosed within a quartz envelope sealed at both ends. The main advantages of HID lamps are high lumen output, better efficacy and longer life. The HID headlamps available currently are Quartz Metal Halide lamps that are also used for general lighting.  
         [0003]     The discharge medium in Quartz Metal Halide lamps consist of a mixture of xenon, mercury, sodium iodide (NaI) and/or scandium iodide (ScI 3 ), wherein the surrounding envelope, or arc-tube, is made of quartz with tungsten electrodes protruding within the envelope. In operation, the lamp size is kept small enough for optical coupling purposes. Further, the lamps are required to meet the automotive industry standard of starting fast by delivering at least eighty percent of their steady state lumens no later than four seconds from the point at which they are turned on. The small lamp size and fast start requirements result in higher wall thermal loading, which in turn poses some limits on the quartz envelope material, and significant thermal stresses in the arc-tube, especially near the electrode roots. These limitations result in shortening the lamp life and also decreasing reliability of the lamp.  
         [0004]     Because of improved reliability and performance, quartz in HID lamps is being replaced with ceramic material, such as polycrystalline alumina (PCA) and yttrium aluminum garnet (YAG). Ceramic arc-tubes can withstand higher temperatures and the cold spot temperature in ceramic lamps can be driven to a high enough value to evaporate the metal halide dose and produce enough vapor pressure for both the light emitting elements and the buffer gas. However, changing to ceramic material requires a change in the design of HID lamps to best optimize the thermal and structural integrity of the lamps.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0005]     This invention is directed towards a high intensity discharge lamp that provides for a sufficiently large cold spot temperature while at the same time sufficiently small hot spot temperature and also an electrode tip temperature high enough to provide electron emission and low stress within the lamp. Towards this end, a lamp comprising a light emitting vessel having a wall made of ceramic material that defines an inner space with a first end portion having a respective first opening formed therein and a second end portion having a respective second opening formed therein is disclosed. Two discharge electrodes, with a first electrode extending therethrough the first opening of the first end portion of the vessel and a second electrode extending therethrough the second opening of the second end portion of the vessel, together forming a gap between ends of the discharge electrodes positioned within the vessel is also disclosed.  
         [0006]     The light emitting vessel defines an inner space characterized by an inner diameter ranging from and including 1 millimeters to 3 millimeters and an inner length between and including 5 millimeters to 10 millimeters. The wall of the vessel has a thickness ranging between and including 0.3 millimeters to 0.8 millimeters. Each tip of the electrodes within the vessel has a shank diameter ranging between and including 0.2 millimeters to 0.55 millimeters. The gap between the ends of the electrodes positioned within the vessel is smaller than 4 millimeters.  
         [0007]     In another exemplary embodiment a high intensity discharge lamp providing for a sufficiently large cold spot temperature while at the same time sufficiently small hot spot temperature and also an electrode tip temperature high enough to provide electron emission and low stress within the lamp is disclosed. The lamp includes a light emitting vessel having a wall made of ceramic material that defines an inner space with a first end portion having a respective first opening formed therein and a second end portion having a respective second opening formed therein. Two discharge electrodes, with a first electrode extending therethrough the first opening of the first end portion of the vessel and a second electrode extending therethrough the second opening of the second end portion of the vessel, together forming a gap between ends of the discharge electrodes positioned within the vessel is also disclosed.  
         [0008]     Further, a reflective coating proximate an outer surface of the vessel near the end portions of the vessel is provided. The light emitting vessel defines an inner space characterized by an inner diameter ranging from and including 1.5 millimeters to 2.1 millimeters and an inner length between and including 6 millimeters to 10 millimeters. The wall of the vessel has a thickness ranging between and including 0.4 millimeters to 0.65 millimeters. Each tip of the electrodes within the vessel has a shank diameter ranging between and including 0.3 millimeters to 0.5 millimeters. The gap between the ends of the electrodes positioned within the vessel ranging between and including 4 millimeters to 5 millimeters.  
         [0009]     In another exemplary embodiment a high intensity discharge lamp comprises a light emitting vessel having a wall made of ceramic material that defines an inner space with a first end portion having a respective first opening formed therein and a second end portion having a respective second opening formed therein. It further comprises two discharge electrodes, with a first electrode extending therethrough the first opening of the first end portion of the vessel and a second electrode extending therethrough the second opening of the second end portion of the vessel, together forming a gap between ends of the discharge electrodes positioned within the vessel.  
         [0010]     The light emitting vessel defines an inner space characterized by an inner diameter ranging from and including 1 millimeters to 1.7 millimeters and an inner length between and including 5 millimeters to 8 millimeters. The wall of the vessel has a thickness ranging between and including 0.3 millimeters to 0.6 millimeters. Each tip of the electrodes within the vessel has a shank diameter ranging between and including 0.25 millimeters to 0.5 millimeters. Furthermore, the gap between the ends of the electrodes positioned within the vessel is smaller than 3 millimeters. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     A more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:  
         [0012]      FIG. 1  is an exemplary embodiment of a schematic of a HID lamp of the present invention without a coating; and  
         [0013]      FIG. 2  is an exemplary embodiment of a schematic of a HID lamp of present invention with a coating;  
         [0014]      FIG. 3  is an exemplary embodiment of a schematic arc-tube heating partition between the arc discharge and the conduction through the electrodes;  
         [0015]      FIG. 4  is an exemplary representation of relative effects of arc tube wall thickness and its diameter on maximal steady state axial stresses generated in the arc tube; and  
         [0016]      FIG. 5  is an exemplary representation of relative effects of arc tube wall thickness and its diameter on maximal steady state hoop stresses generated in the arc tube. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     With reference to the figures, exemplary embodiments of the invention will now be described. As presented below, dimensional ranges are provided for different aspects of the present invention. Though not explicitly stated, the ranges include the values defining the ranges. Thus, a particular dimension may possess the actual range limits discussed below. Additionally, these range limits are approximations only. Towards this end, since the limits are provided with two significant figures, a value outside of these limits that may round up to the next significant two-digit figure should also be considered included within the range limits provided. Also presented herein are actual computation data. Though computational data is presented herein, it should by no means be considered limiting as to the scope of the present invention. Those skilled in the art will readily recognize that depending on experimental conditions that may not be exactly identical case-to-case, the results provided may not always repeat exactly.  
         [0018]     Additionally, though dimensions disclosed herein are presented as though the dimensions appear uniform for a particular element, the dimensions in an element may vary depending on location. For example, an arc-tube, including the arc-tube legs and arc-tube body may have has a uniform wall thickness in one exemplary embodiment. Whereas in another exemplary embodiment, the arc-tube body may have a different wall thickness than the arc-tube legs.  
         [0019]     Furthermore, though ceramic HID automotive lamps are discussed throughout, this invention is applicable to other ceramic HID lamps as well. Thus, the present invention is applicable to other ceramic HID lamps used with transportation vehicles, such as in airplane landing gear, as well as generally used ceramic HID lamps. Additionally, since a ceramic envelope material is used instead of quartz, the HID lamps disclosed herein operate at higher temperature than quartz lamps. This in turn can provide for a more efficient mercury-free lamp.  
         [0020]     In designing a ceramic HID lamp, consideration should be given to circumferential and axial tensile stresses that may develop on the outside part of an arc-tube during operation of the lamp. These stresses may result from significant temperature gradients within the arc-tube that result from heat flux from the discharge to the walls. In view of this issue, one design goal described herein is to have a lamp with decreased temperature gradients within the arc-tube as well as along the arc-tube length. Another design goal is to limit stresses and temperature increases on the inside of the arc-tube. Limitation of the stresses and temperature will reduce a possibility of creep deformation within the arc-tube. Towards this end, since the temperature of an HID lamp is controlled at least in large part by the arc-tube and electrode dimensions, the dimensions of these elements can be concurrently optimized relative to one another.  
         [0021]      FIG. 1  is an exemplary embodiment of a schematic of a HID lamp of the present invention without a coating. As illustrated, the ceramic HID lamp  5  has a straight cylindrical arc-tube body  10 , also referred to as an envelope or vessel. The central part of the arc tube is preferentially cylindrical geometry but may also be elliptical, spherical, or intermediate shapes. Co-sintered cylindrical ceramic legs  12  are located at opposite ends of the arc-tube body  10 . In another exemplary embodiment a single piece ceramic arc-tube may be used wherein the legs  12  are part of this single piece ceramic arc-tube. Within the HID lamp  5 , a metal electrode  20 , typically made from tungsten, is inserted and sealed inside each leg  12  and extends into the arc-tube body  10 .  
         [0022]     The input power for HID automotive lamps is generally between 20 W and 50 W, preferably between 25 W and 45 W, and most preferably 35 W. In one embodiment, the input power for HID automotive lamps incorporating teachings of the present invention is about 35 W. However, the input power can be varied depending upon the desired lamp life and light output. For example, by reducing the input power, the lamp life can be extended albeit with a decrease in light output. Conversely, by increasing the input power, the light output can be increased albeit with a decrease in lamp life.  
         [0023]     The arc-tube body  10  has an inner diameter  15  less than or equal to 2.0 mm, preferably less than 1.7 mm, and a wall thickness  18  between 0.3 mm and 0.6 mm. The reduction of inner diameter  15  is beneficial for the reduction of both axial and hoop stresses developed in the lamp. This benefit is evident from the table below, Table 1, and further illustrated in  FIGS. 4 &amp; 5 , which illustrate exemplary computational fluid dynamic and structural analysis results for axial stress and hoop stress when the present invention is utilized.  
                                                                                           TABLE 1                           Effect of Arc-Tube Thermal Gradients on       Maximal Steady State Stresses (in Mpa)                                            Axial           T 34     T 32     T 33′     S 34     S 32     S 33′     Sp   Stress                        id = 2.4 mm   6.7   139.7   194.5   10   21   29   15   75       id = 1.6 mm   7.3   36.6   208.1   11   5   30   10   56       Unit   K   K   K   Mpa   Mpa   Mpa   Mpa   Mpa                  
 
 From detailed computational fluid dynamic (CFD) and structural analyses, it is found that, both hoop and axial stresses in the arc-tube at the location of the maximum tensile stresses at the outside top center of the arc-tube (at location T 4  illustrated in  FIG. 2  where T stands for Temperature) are related to the key temperature differences and pressure by the following relations:
 
σ 4   =S   34   +S   32   +S   33′   +S   p 
 
 where S 34 , S 32 , S 33 ′ and Sp are monotonic functions of T 34 , T 32 , T 33′  and pressure respectively. The exact functional forms for both hoop and axial stresses are obtained from the results of CFD and structural analyses. In these expressions, T 34 =T 3 −T 4 , T 32 =T 3 −T 2 , T 33′ =T 3 −Ttop-corner (Ttp). Approximate locations of T 1 , T 2 , T 3 , T 4 , Ttop_corner and Tbottom_corner on the arc-tube body  10  are illustrated in  FIG. 2 . For example, for an id of 2.4 mm and 1.6 mm the corresponding values of S 34 , S 32 , S 33′  and Sp contributing to the axial stress are given in Table 1. It is seen from Table 1, that the reduction of the inner diameter id  15  down to 1.6 mm reduces substantially T 32  and therefore the axial stresses; and in general the reductions of the T 32  and T 33′  are helpful for stress reduction. Through analysis, and as illustrated in  FIGS. 4 and 5 , it has also been shown that mostly the inner diameter and much less the bulb wall thickness affects both the axial and the hoop stresses in the lamp. In particular, the reduction of the inner diameter from 2 mm down to 1.4 mm helps decrease the maximal axial stress by more than 10% and the maximal hoop stress by about 30%. 
 
         [0024]     If the ceramic legs  12  are cosintered, their insertion length into the arc-tube body is between 0.5 mm and 3 mm. The gap  22  between the electrode tips is smaller than 5 mm, such as between 2.8 mm and 3 mm. With respect to automobiles, the current electrode gap is standardized at 4 mm to 4.5 mm. However, it has been advantageously recognized that reducing the electrode tip gap  22  in association with the other lamp and electrode dimensions disclosed herein provides for an improved HID automotive lamp  5 .  
         [0025]      FIG. 3  is an exemplary embodiment of a schematic arc-tube heating partition between the arc discharge and the conduction through the electrodes. As illustrated, as a ceramic bulb envelope is heated both directly from the arc discharge and by the heat conducted through the electrode, the electrode dimensions depend on the arc tube dimensions. The arrows  21  in the legs  12  further illustrate that heat is conducted from a location of the electrode within the leg  12  to the arc-tube  5 . For instance, a larger electrode shank diameter  24  is used in the lamps with larger inner diameter and it is preferably less than 0.5 mm but larger than 0.2 mm.  
         [0026]     Moreover, vehicle forward lighting applications demand brighter lamps with high luminance in order to make their optical system as small as possible, and thereby reduce the system cost and enhance its overall performance (luminance). The luminance is defined as the ratio of the amount of lamp lumens to the “etendue”, E (optical extent) of the application (L=lumens/E). It is known that the etendue is proportional to the product of the arc gap and arc diameter. For this reason, typically the shorter the arc gap (arc length), the higher the lamp luminance. Similarly, for the wall stabilized arcs, the smaller the bulb inner diameter, the larger is the lamp luminance.  
         [0027]     Exemplary design rules have been developed. These rules are established to provide for a HID lamp to have a sufficiently large cold spot temperature that is equivalent to having high vapor pressure of the metal halide gases. These design rules and provide for sufficiently small hot spot temperature, and large enough electrode tip temperature. Thus these designs rules allow for electron thermoionic emission. In particular, it has been determined that the arc-tube body  10  wall thickness  18  depends on the inner diameter  15 . Accordingly, the wall thickness  18  should be increased if the inner diameter  15  is decreased. For example, in accordance with one embodiment of the invention, a wall thickness larger than 0.3 mm and smaller than 0.45 mm is suitable for an arc-tube  10  having an inner diameter of 1.6 mm. However, if the arc-tube  10  has an inner diameter of 1.1 mm, the wall thickness should be smaller than 0.6 mm, such as 0.48 mm. Similarly, the minimal electrode shank diameter  24  should be increased if the inner diameter  15  is increased. Thus, the most preferable design space is an inner diameter  15  between 1.1 mm and 1.7 mm, a wall thickness  18  between 0.3 mm and 0.6 mm, a shank diameter  24  between 0.28 mm and 0.52 mm, and an arc-tube inner bulb length (ibl)  26  between 6 mm and 10 mm. All dimensional measurement ranges are inclusive and are intended to be satisfied at the same time in order to provide an efficient HID lamp  5 .  
         [0028]      FIG. 2  is an exemplary embodiment of a schematic of the HID lamp of the present invention with a coating. The coating  30  has several functions. First, by reducing the amount of thermal radiation coming from the arc-tube, it controls the thermals of the legs where the metal halide dose typically resides, thus helping vaporize more light-emitting dose. Second, the coating reduces the axial arc tube temperature gradients. This benefit is further illustrated in Table 2 in view of the difference T 3 −Ttop_corner.  
                                                                                                   TABLE 2                           Temperatures in K for the bulb dimensions: id = 1.4 mm, wall = 0.44 mm, ibl = 6 mm                                Maximum   Maximum   Cold   Bottom   Top                           Seal   Electrode   Spot   Corner   Corner       Description   T1   T2   T3   T4   Temp.   Temp.   Temp.   Temp.   Temp.                    No coating   1391   1402   1446   1431   716   2413   980   1217   1228       With a Coating   1442   1453   1499   1478   719   2426   1065   1308   1318                    
 Reducing the axial arc tube temperature gradients is also beneficial for the thermal stress reduction, further illustrated in Table 1, and therefore longer life of the lamp. Third, an opaque coating covering the ends of the arc tube body results in eliminating the undesirable portion of the light that causes glare in the projected beam, such as when directed at a ground covering such as a paved road. 
 
         [0029]     In one exemplary embodiment, a coating is made of high temperature opaque oxide (e.g. Zirconia or Alumina). In another exemplary embodiment, a thin (e.g., thickness less than 200 micro-meter) reflective coating  30 , such as any high temperature metal with suitable corrosion properties is applied on the outer surface of the arc-tube covering. For example, Platinum (Pt) is applied approximately 0.5 mm on each end of the arc-tube body  10  and approximately 1-3 mm on each leg surface  12 , if legs are provided.  
         [0030]     The design rules of the present invention for when a coating  30  is used include having the inner diameter preferably less than 2.3 mm. The design rules further dictate that the arc-tube wall thickness  18  is a function of the inner diameter  15  and the arc-tube wall thickness  18  should be increased if the inner diameter  15  is decreased. For example, a 0.4 mm wall thickness  18  is suitable for the arc-tube body  10  having an inner diameter  15  of 2.25 mm. However, for an arc-tube body  10  with an inner diameter  15  of 1.6 mm, the wall thickness  18  is larger than 0.69 mm. As another example, for an arc-tube body  10  with an inner diameter  15  of 1.8 mm, the wall thickness  18  is larger than 0.54 mm.  
         [0031]     The design rules further dictate that the electrode  20  shank diameter  24  should be between 0.25 mm and 0.5 mm if the inner diameter 15 of the arc-tube body  10  is in the range of 1.1 mm and 2 mm. Table 3 depicts the combined effect of the electrode shank diameter, the bulb inner diameter and the wall thickness on bulb thermals.  
                                                                                           TABLE 3                           Effect of the id, wall thickness and shank diameter (in mm) on the       bulb key temperatures (in K), ibl = 6 mm, arc gap = 3 mm            id   wall   Shank_d   T1   T2   T3   T4   T bottom     —     corner     T top     —     corner                      1.1   0.3   0.25   1617   1629   1646   1633   1287   1291       2   0.3   0.25   1213   1215   1384   1376   1127   1170       1.1   0.6   0.25   1332   1356   1371   1344   1131   1134       2   0.6   0.25   1058   1061   1176   1158   1003   1032       1.1   0.3   0.5   1640   1652   1669   1655   1313   1315       2   0.3   0.5   1227   1229   1411   1403   1151   1193       1.1   0.6   0.5   1361   1387   1401   1373   1163   1165       2   0.6   0.5   1081   1084   1208   1189   1032   1061                  
 
 As a large portion of heating energy, approximately 23% of the input power reaches the arc tube  10  through the electrodes  20  the smaller the inner diameter  15  of arc-tube body  10  is, the smaller the electrode  20  shank diameter  24  needs to be as well. For example, for an inner diameter  15  of 1.75 mm, the electrode  20  shank diameter  24  is smaller than 0.35 mm. Whereas, for the arc-tube body  10  inner diameter  15  of 1.85 mm, the electrode  20  shank diameter  24  is smaller than 0.45 mm. The preferred design specifications are for the inner diameter  15  to be between 1.5 mm and 2.1 mm, the wall thickness  18  to be between 0.4 mm and 0.65 mm, the shank diameter  24  to be between 0.3 mm and 0.5 mm, and the ibl  26  to be between 6 mm and 10 mm. 
 
         [0032]     While the invention has been described in what is presently considered to be a preferred embodiment, many variations and modifications will become apparent to those skilled in the art. Accordingly, it is intended that the invention not be limited to the specific illustrative embodiment but be interpreted within the full spirit and scope of the appended claims.