Patent Publication Number: US-7901110-B2

Title: System and method for forced cooling of lamp

Description:
BACKGROUND 
     The present technique relates generally to a system and method for cooling a lamp and, more specifically, to a cooling technique for a short arc gas discharge lamp. 
     Gas discharge lamps are used in modern lighting technology including fluorescent lighting, liquid crystal displays, indicator lamps, germicidal lamps, neon signs, photographic electronic flashes, video projectors, or the like. Typically, gas discharge lamps comprise a gas filled inside a glass, quartz, or translucent ceramic arc tube. These lamps also include a pair of electrodes, which are energized to create a discharge within the arc tube to ionize the gas. The ionized gas, in turn, generates visible and/or ultraviolet light. 
     The performance of gas discharge lamps for video projectors depends at least partially on a relatively small arc gap (e.g., on the order of 1 mm) formed between the pair of electrodes located inside the arc tube and, also, a relatively high pressure (e.g., on the order of 100 to 400 atmospheres) of the gas filled inside the arc tube. The use of a ceramic tube rather than a quartz tube enables the gas discharge lamp to operate at higher operating temperatures within the lamp tube. In turn, the ceramic tube enables the gas discharge lamp to operate at a relatively higher vapor pressure with a commensurate reduction in the arc gap between the pair of electrodes. These advantages also lead to improvements in the spectral output of the gas discharge lamp. 
     In operation, these gas discharge lamps generally have temperature differentials, which can lead to stresses that reduce the lifespan of the lamp. For example, tensile stresses are predominant in the ceramic arc tube due to a large coefficient of thermal expansion in combination with a large temperature difference between a top and bottom side of the arc tube. Unfortunately, passive convective cooling of the arc tube is insufficient to reduce the tensile stresses to an acceptable level. 
     Therefore, there is a need for a system and method for reducing temperature differentials in the walls of a ceramic arc tube to reduce potential stresses. 
     BRIEF DESCRIPTION 
     In accordance with one embodiment of the present technique, a gas discharge lamp is disclosed. The gas discharge lamp includes an arc envelope and a cooling mechanism including a cooling passage reorientable towards a top side of the arc envelope in a plurality of different positions of the arc envelope. 
     In accordance with another embodiment of the present technique, a method of operating a lamp is disclosed. The method includes reducing a temperature differential between a top and a bottom side of an arc envelope by channeling airflow towards the top side of the arc envelope. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will be better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a partial front perspective view of an exemplary lamp having cooling mechanisms in accordance with certain aspects of the present technique; 
         FIG. 2  is a cross-sectional view of the lamp of  FIG. 1  illustrating the flow of cooling air through the lamp in accordance with certain aspects of the present technique; 
         FIG. 3  is a partial rear perspective view of the lamp of  FIG. 1  in accordance with certain aspects of the present technique; 
         FIG. 4  is a diagrammatical representation of an electromagnetic/electromechanical mechanism configured to move a circular ring to a desired orientation to control airflow through the lamp of  FIGS. 1-4  in accordance with certain aspects of the present technique; 
         FIGS. 5 and 6  are flow charts illustrating various methods of operation of a lamp having cooling mechanisms in accordance with certain aspects of the present technique; 
         FIGS. 7 and 8  are flow charts illustrating various methods of manufacturing a lamp having cooling mechanisms in accordance with certain aspects of the present technique; 
         FIG. 9  is diagrammatical representation of a system incorporating a lamp having cooling mechanisms in accordance with certain aspects of the present technique; 
         FIG. 10  is a perspective view illustrating a nozzle provided to supply cooling air to a top side of the arc envelope in accordance with certain aspects of the present technique; 
         FIG. 11  is a table illustrating measured temperature data of the arc envelope in accordance with the aspects illustrated in accordance with  FIG. 10 , and 
         FIG. 12  is a graph illustrating distribution of temperature along the surface of the arc envelope in accordance with certain aspects of the present technique. 
     
    
    
     DETAILED DESCRIPTION 
     As discussed in detail below, embodiments of the present technique provide cooling mechanisms configured to focus a cooling airflow on a lamp module (e.g., a ceramic arc tube) to achieve a desired temperature distribution, thereby improving the life span and performance of the lamp module. For example, hot spot temperature of the lamp module may be reduced by approximately 200 degrees. Specifically, techniques are disclosed for focusing a cooling airflow on a top portion of the lamp module where heat is relatively greater, thereby reducing thermal stresses associated with a temperature differential between top and bottom sides of the lamp module. Also, techniques are disclosed for maintaining the focused airflow in the desired region of the lamp module despite the orientation of the lamp module. In other words, if the lamp module is rotated or flipped over, then embodiments of the cooling mechanisms reorient the cooling airflow to maintain the focus of the cooling airflow on the top portion of the lamp module. Various embodiments of these techniques are discussed in further detail below with reference to  FIGS. 1-12 . 
       FIG. 1  illustrates an enclosed lamp assembly  10  including a short arc lamp module  11 . The lamp assembly  10  may be used in video projectors, fiber optic illuminators, televisions, lighting headsets of surgeons, endoscopic applications, outdoor lighting or stage lighting, automotive headlamps, marine lighting, public transportation lighting (e.g., buses, trains, airplanes, boats, etc.), and other suitable applications. The lamp module  11  includes a hermetically sealed arc envelope  12 . The arc envelope  12  may be formed from a variety of materials such as transparent ceramics and other materials, such as yttrium-aluminum-garnet, ytterbium-aluminum-garnet, microgram polycrystalline alumina, alumina or single crystal sapphire, yttria, spinel, ytterbia, polycrystalline alumina, quartz, or the like. The arc envelope  12  may be a hollow cylinder, a hollow elliptical shape, a hollow sphere, a bulb shape, a rectangular shaped tube, or another suitable hollow light-transmissive body. 
     The illustrated lamp module  11  includes two electrodes  14  provided inside the arc envelope  12 . The electrodes  14  may be formed from tungsten, molybdenum, or any other suitable materials. The lamp module  11  also includes lead wires  16  and  18  extending outside the arc envelope  12  and coupled to the two electrodes  14 , which terminate inside a cavity  13  within the arc envelope  12 . The cavity  13  of the arc envelope  12  is typically filled with a noble gas, such as helium, neon, argon, krypton, xenon, or the like, and dosed with mercury. The cavity is also typically dosed with a halogen like bromine or iodine or chlorine. In addition, the cavity  13  may be dosed or filled with other materials, typically metal halides such as thallium, indium, sodium iodide, or the like. The pressure of gas filled inside the arc envelope  12  is typically above 1 atmosphere during non-operating condition of the lamp module  11 . In certain embodiments, the pressure of gas filled inside the arc envelope  12  may be in the range of approximately 100 to 400 atmospheres during operation of the lamp module  11 . The electrodes  14  are mounted lengthwise along the arc envelope  12 , thereby providing relatively precise control of an arc gap  15  between the tips of the electrodes  14  within the cavity  13 . For example, a small arc gap  15  on the order of approximately 1 mm may be formed between the arc electrodes  14 . This precise control of the arc gap  15  improves the performance of the lamp module  11 . In the illustrated embodiment, the electrode tips are oriented along a centerline  20  of the arc envelope  12 . However, alternative embodiments of the lamp module  11  have the electrodes  14  positioned at an offset from the centerline  20 , such that the arc is substantially centered within the arc envelope  12 . In other embodiments, alternative electrodes  14  may be angled outwardly from the centerline  20 , such that the arc is substantially centered within the arc envelope  12 . 
     The enclosed lamp assembly  10  also includes a reflector  22  disposed about a portion of the lamp module  11 , such that the light generated by the lamp module  11  is focused in a generally outward direction from the lamp assembly  10 . In the illustrated embodiment, the reflector  22  has a parabolic shape. However, other shapes and configurations of the reflector  22  may be employed for a particular application. The illustrated lamp assembly  10  also includes a generally transparent or translucent cover glass  24 , such as a glass or plastic cover, coupled to an outer portion of the reflector  22  opposite from the lamp module  11 . In certain embodiments, the cover glass  24  may be at least partially colored, doped, or filtered, e.g., to remove red, blue, green, ultraviolet, infrared, or combinations thereof. Accordingly, the reflector  22  and the transparent or translucent cover glass  24  cooperatively enclose and protect the lamp module  11 , focus the light output in a desired direction from the lamp module  11 , and color the light output from the lamp module  11  if desired for a particular application. 
     As appreciated by those skilled in the art, an arc is generated between the electrodes  14  by applying voltage across the electrodes  14 , causing ionization of the gas filled in the arc envelope  12 . The ionized gas, in turn, generates light. In the illustrated embodiment, the arc envelope  12  is formed from ceramic instead of quartz. In comparison, the maximum allowable gas pressure in a quartz arc envelope may be in the range of 150 to 200 atmospheres, whereas the maximum allowable gas pressure in a ceramic arc envelope may be in the range of 200 to 400 atmospheres. The performance of the lamp module  11  is dependent on the arc gap  15  and the gas pressure. The higher temperature capability of a ceramic arc envelope enables higher operating pressures. As known to those skilled in the art, tensile stresses are predominant in the ceramic arc tube due to a large coefficient of thermal expansion in combination with a large temperature difference between a top and bottom side of the arc tube. During operation of the lamp module  11 , gas currents are generated inside the arc envelope  12  by convection causing temperatures at a top side  26  of the envelope  12  to be higher than a bottom side  28 . In certain other embodiments, the top side  26  may be referred to as “bottom side” and the bottom side  28  may be referred to as “top side” depending on the orientation of the lamp module  11 . In other words, heat rises within the cavity  13  due to gas circulation, thereby creating a significant temperature differential between the top side  26  and the bottom side  28 . The life of a quartz envelope is typically limited by devitrification of quartz, which is driven by a hot spot temperature, but not by temperature differentials between top and bottoms sides of the quartz envelope. In contrast, the life limitation of a ceramic envelope may be driven by high circumferential tensile stresses generated on an outer side of the envelope  12 . The circumferential tensile stresses are generated due to a large coefficient of thermal expansion in combination with the temperature differentials between hot and cold spots in the envelope  12 . Additionally compressive stresses are generated inside the envelope  12 . Accordingly, in certain embodiments discussed below, forced cooling that selectively cools the top side  26  to a greater extent relative to the bottom side  28  of the envelope  12  reduces these temperature differentials, thereby reducing thermal stresses in the ceramic envelope  12  to desirable levels. 
     In the illustrated embodiment, the arc envelope  12  is mounted to a neck portion  30  of the reflector  22 . The lamp assembly  10  also includes a cooling mechanism  31  to cool the arc envelope  12  in a focused manner as discussed in further detail below. The cooling mechanism  31  includes cooling passages  32  and  34  defined between the arc envelope  12  and the reflector  22 . Although two passages  32  and  34  are illustrated, any number of passages may be provided in other embodiments. One cooling passage is above the other cooling passage depending on the orientation of the lamp module  11 . In the illustrated embodiment, if the lamp module  11  is mounted in an upright position, the passage  32  is referred as an upper passage and the passage  34  is referred as the bottom passage. If the lamp module  11  is mounted in an inverted position, the passage  32  is referred as the bottom passage and the passage  34  is referred as the top passage. The cooling mechanism  31  also includes an airflow blocking structure  36  rotatably attached to the arc envelope  12 . An open portion of airflow blocking structure  36  is above the closed portion of the airflow blocking structure for various orientations of the lamp module  11 . Together, the open portion of the airflow blocking structure  36  and the upper one of the passages  32  or  34  define a passage that allows airflow to blow through the lamp module  11  and onto a top side of the lamp module  11 . The airflow blocking structure  36  includes a ferrule  38  attached to the arc envelope  12  and located proximate to the cooling passages  32  and  34 . The airflow blocking structure  36  further includes a circular ring  40  disposed concentrically about the ferrule  38  with a suitable clearance, such that the circular ring  40  is rotatable about the ferrule  38 . The illustrated circular ring  40  has a generally tubular or cylindrically shaped structure  41  and a partial disk-shaped or semi-circular structure  42  protruding outwardly from the tubular structure  41 . An open portion of the semi-circular structure  42  and the unblocked passage or unblocked portion of the passage may be referred to as the upper passage. The semi-circular structure  42  is positioned at a bottom side of the lamp module  11 , such that airflow cannot pass through the cooling passage  34 . Due to the effect of gravity and also due to the clearance formed between the ring  40  and the ferrule  38 , the semi-circular structure  42  is always located at a bottom side of the ferrule  38  despite the orientation of the lamp assembly  10 . For example, the illustrated lamp assembly  10  could be rotated 360 degrees about the axis  20  and the semi-circular structure  42  would reposition itself downward toward the bottom portion of the passages  32  and  34 . The illustrated airflow blocking structure  36  also includes a protrusion  44  formed in the ferrule  38  in a position that restricts axial movement of the circular ring  40 . Specifically, the protrusion  44  prevents the ring  40  from sliding outwards along the ferrule  38 . 
     In the illustrated embodiment, a cooling device  46 , such as an axial fan or a centrifugal fan, is located at a rear side of the enclosed lamp assembly  10 . In the illustrated embodiment, the cooling device  46  forces air toward the reflector  22 , the airflow blocking structure  36 , and the arc envelope  12 . In operation, the airflow blocking structure  36  functions to substantially reduce or block airflow through the cooling passage  34 , while allowing the airflow to pass through the passage  32 . In this manner, the airflow blocking structure  36  focuses the airflow on the top side  26  of the arc envelope  12 , thereby reducing hot spots and temperature differentials between the top and bottom sides  26  and  28  of the arc envelope  12 . 
     Referring to  FIG. 2 , an embodiment of the lamp assembly  10  is illustrated inside an enclosure  47  of an electronic device  49 , such as a video projector, a television, a fiber optic illuminator, a lighting head set of surgeon, a outdoor lighting or stage lighting, a automotive headlamp, a marine lighting, a public transportation lighting, or another suitable application. The semi-circular structure  42  of the circular ring  40  is located at the bottom side of the ferrule  38  due to the effect of gravity irrespective of the position of the electronic device  49 . In another embodiment, the rotation of the ring  40  may be controlled by an electromagnetic/electromechanical mechanism to a desired orientation described in greater detail below. The cooling device  46  forces air in a direction toward the reflector  22 , the airflow blocking structure  36 , and the arc envelope  12 . A portion of the forced air enters a reflector cavity  48  through the cooling passage  32 , while the semi-circular structure  42  blocks the airflow through the cooling passage  34 . Moreover, the airflow forces the ring  40  to close the cooling passage  34 , thereby providing better blockage of the airflow at the cooling passage  34 . The air selectively cools the top side  26  of the arc envelope  12  while minimal collateral cooling of the bottom side  28  of the arc envelope  12  occurs. The air inside the reflector cavity  48  is allowed to exit through plurality of openings  50  formed in the reflector  22 . In certain other embodiments, cooling fan  46  may be provided outside the reflector  22  and adjacent to the top opening  53  of the reflector  22  to draw air outside through the top opening  53  to selectively cool the top side of the arc envelope  12  while minimal collateral cooling of the bottom side  28  of the arc envelope  12  occurs. 
     The focused cooling on the top side  26  of the arc envelope  12  reduces the temperature difference between the top side  26  and the bottom side  28  of the arc envelope  12 . This arrangement reduces the hot spot temperature of the arc envelope  12 . This, ultimately, reduces circumferential thermal stresses generated in the arc envelope  12  irrespective of whether the electronic device  49  and the internal lamp assembly  10  is mounted in a normal position or an upside down position. As a result of this reduced temperature differential, the cooling device  46 , the airflow blocking structure  36 , and the cooling passages  32  and  34  reduces the likelihood for cracks and increase the life of the lamp module  11 . The reduced temperature differential also enables the lamp module  11  to operate at much higher temperatures and operating pressures, thereby improving the performance of the lamp. Thereby fracture of the arc envelope  12  is prevented at higher temperature and operating pressure of the arc envelope  12 . 
     Referring to  FIG. 3 , this figure illustrates a rear perspective view of the lamp assembly  10  in accordance with certain embodiments of the present technique. As illustrated, the plurality of openings  50 ,  53  are formed in the reflector  22  to allow the cooling airflow, after heat is convectively transferred from the lamp module  11  to the cooling airflow, to flow outwardly from the reflector cavity  48  (shown in  FIG. 2 ). The arc envelope  12  is fixed to the neck portion of the reflector  22  using cement  45 . Also illustrated are the cooling passages  32  and  34  formed between the arc envelope  12  and the reflector  22 . Two rods  51 , such as plastic rods, may be used to form the cooling passages  32  and  34  respectively. The technique used for forming the cooling passages is illustrated in greater detail below. Again, the airflow blocking structure  36  is provided for allowing airflow through the cooling passage  32 , while blocking airflow through the cooling passage  34 . 
     Referring to  FIG. 4 , an electromagnetic/electromechanical mechanism  52  is illustrated for use with the lamp assembly  10  and electronic device  49  of  FIGS. 1-3 . In the illustrated embodiment, the ring  40  is moved by the mechanism  52  to a desired orientation. The movement of the ring  40  is based on a signal transmitted from an electronic image control unit (not shown). The signal from the electronic image control unit is transmitted based on a flipped position of the electronic device  49 . 
     A motor  54  is mounted to a lamp fixture  56 . An electromagnetic signal from the image control unit is transmitted to the motor  54 . If the electronic device  49  and/or lamp assembly  10  is mounted in a normal position, the motor  54  is not rotated. If the electronic device  49  and/or lamp assembly  10  is mounted upside down, the motor  54  is rotated by 180 degrees. The motor  54  rotates a pinion  58  and the ring  40 . The ring  40  may have a semi-annular groove. The rotation of the ring  40  through 180 degrees results in opening of passage  34  and blockage of passage  32 . The mechanism  52  allows airflow through the passage leading to a hot spot region of the arc envelope  12  irrespective of the position of the projector. 
     Referring to  FIG. 5 , a method of operation of the lamp assembly  10  is illustrated. Electric power is supplied to the lamp assembly  10  to generate an electric arc as represented by step  60 . Voltage is applied between the electrodes  14  so that gas in the arc envelope  12  is ionized. The ionized gas generates light. The cooling passage is moved by gravitational or magnetic force relative to the arc envelope  12  depending on the position of the lamp  10  as represented by step  62 . For example, the air blocking structure  36  rotatably mounted to the arc envelope  12  is moved by gravitational force or magnetic force. The open/unblocked cooling passage is oriented toward a top side of the arc envelope  12  as represented by step  64 . The cooling device configured to supply the cooling medium is provided to the rear side of the lamp assembly  10 . The cooling medium is forced through the cooling passage to the top side of the arc envelope  12  as represented by step  66 . Cooling medium flows parallely along the top side of the arc envelope  12 , thereby forcing convective heat transfer away from the top side of the arc envelope  12  and into the passing cooling medium. The passing cooling medium, having collected heat from the top side of the arc envelope  12 , is then allowed to exit through a plurality of openings  50 ,  53  in the reflector  22 . While the cooling medium is allowed to flow along the top side of the arc envelope  12 , the cooling medium flow relative to the bottom side of the arc envelope  12  is blocked by the airflow blocking structure  36  as represented by step  68 . This results in selective cooling of the top side relative to the bottom side of the arc envelope  12 . The temperature differential between the top side and bottom side of the arc envelope  12  is reduced as represented by step  70 . 
     Referring to  FIG. 6 , one embodiment of the method of operation of the lamp assembly  10  is illustrated. The air blocking structure  36  rotatably mounted to the arc envelope  12  is moved by gravitational or magnetic force depending on the position of the lamp assembly  10  as represented by step  72 . In one example, the semi-circular structure  42  of the circular ring  40  is located to the bottom side relative to the ferrule  38  due to the effect of gravity. In the present embodiment, two cooling passages  32  and  34  are provided between the arc envelope  12  and the airflow blocking structure  36 . The cooling passage  32  is maintained in an orientation toward the top side of the arc envelope  12  as represented by step  74 . The cooling fan  46  is provided to the rear side of the arc envelope  12 . The cooling air is forced through the cooling passage  32  to the top side of the arc envelope  12  as represented by step  76 . The cooling airflow relative to the bottom side of the arc envelope  12  is blocked as represented by step  78 . In the illustrated example, the semi-circular structure  42  of the circular ring  40  is located to the bottom side relative to the ferrule  38 . The cooling passage  34  is blocked by the semi-circular structure  42 . The cooling air forced through the cooling passage  32  is allowed to exit through plurality of openings  50 ,  53  formed in the reflector  22  as represented by step  80 . This results in selective cooling of the top side relative to the bottom side of the arc envelope  12 . The temperature differential between the top side and bottom side of the arc envelope  12  is reduced as represented by step  82 . 
     Referring to  FIG. 7 , a method of manufacturing a lamp is illustrated in accordance with certain embodiments of the present technique. The arc envelope  12  is located inside the reflector  22  as represented by step  84 . The arc envelope  12  is coupled to the neck region of the reflector  22 . At least one cooling passage is provided in the lamp assembly  10  in a reorientable configuration, which is maintained toward the top side of the arc envelope  12  as represented by step  86 . An airflow blocking structure  36  is rotatably mounted to the arc envelope  12  in such a way that the airflow blocking structure  36  is located opposite the cooling passage as represented by step  88 . A device adapted to supply cooling medium through the cooling passage is provided at the rear side of the arc envelope  12  as represented by step  90 . 
     Referring to  FIG. 8 , one embodiment of the method of manufacturing the lamp assembly  10  is illustrated. The arc envelope  12  is located in the reflector  22  as represented by step  92 . In the illustrated example, the arc envelope  12  is an arc tube. A fixing material such as cement is filled between the reflector  22  and the arc envelope  12  as represented by step  94 . The fixing material is provided to secure the arc envelope  12  to the reflector  22 . In the illustrated embodiment, two rods  51  (shown in  FIG. 3 ) are inserted between the reflector  22  and the arc tube  12  to form cooling passages  32  and  34  as represented by step  96 . For example, plastic rods may be used to form the cooling passages. In certain other embodiments, any number of other devices as known by those skilled in the art may be used to form the cooling passages. The fixing material is dried as represented by step  98 . The plastic rods are removed after drying of the fixing material as represented by step  100 . The cooling passages  32  and  34  are formed between the arc tube  12  and the reflector  22  in the vacancies of the plastic rods as represented by step  102 . The airflow blocking structure  36  is rotatably mounted to the arc tube  12  to block the cooling passage  34  as represented by step  104 . The cooling fan  46  is provided to the rear side of the arc envelope  12 . The cooling fan  46  is adapted to supply cooling air through the cooling passage  32  to the top side of the arc tube  12  as represented by step  106 . 
     Referring to  FIG. 9 , a system  108  including the lamp assembly  10  is illustrated in accordance with embodiments of the present technique. The system  108  may include a video projector, a television, a fiber optic illuminator, a headset of a surgeon, an endoscopic application, or the like. As discussed above, the lamp assembly  10  includes the arc envelope  12  provided inside the reflector  22 . Two electrodes  14  are provided inside the arc envelope  12 . The rotatable structure  36  is attached to the arc envelope  12  to provide selective cooling of top side relative to bottom side of the arc envelope  12 . The cooling fan  46  is provided to force cooling air through the cooling passage to the top side of the arc envelope  12 . 
     Referring to  FIG. 10 , the arc envelope  12  is illustrated in accordance with embodiments of the present technique. In the illustrated embodiment, a nozzle  110  is provided to inject cooling air to the top side  26  of the arc envelope  12  operated without the reflector. The nozzle  110  is located focusing at the hot spot of the arc envelope  12 . As the cooling air blows onto the top side  26 , the temperature of the top side  26  of the arc envelope  12  is substantially reduced without significantly cooling the bottom side  28  of the arc envelope  12 . In other words, the cooling air reduces temperature differential between the top side  26  and the bottom side  28  of the arc envelope. If the arc envelope is rotated or repositioned (e.g., rotated about axis  111 ), then the nozzle  110  maintains its topside position (or reorients itself to a topside position) relative to the arc envelope  12 . The temperature data of the top and bottom sides  26  and  28  of the arc envelope  12  may be measured using a thermocouple or an imaging pyrometer. 
       FIG. 11  is a table  112  illustrating data representing differences between the temperature of the top side  26  and the bottom side  28  of the arc envelope  12  relative to the airflow rate directed toward the top side  26 . As illustrated, column  114  refers to the airflow rate in units of standard cubic feet per hour (scfh) directed toward the top side  26  of the arc envelope  12 . Columns  122 ,  124 , and  126  refer to the temperatures in degrees Celsius of the sides (between top and bottom sides  26  and  28 ), the bottom side  28 , and the top side  26  of the arc envelope  12 , respectively. Column  128  refers to the temperature gradient in degrees Celsius of the top side  26  relative to the bottom side  28  of the arc envelope  12 . As indicated by rows  130 ,  132 ,  134 ,  136 ,  138 , and  140 , the values in each of the foregoing columns  122  through  128  decrease in response to the increasing airflow rates of 0, 1, 2, 3, 4, and 5 standard cubic feet per hour set forth in column  114 . More importantly, the temperature at the top side  26  decreases more rapidly than the temperature at the bottom side  28 , thereby leading to a gradually smaller temperature gradient as indicated by rows  130  through  140  of column  128 . This decreasing temperature gradient is attributed to the focused airflow toward the top side  26  of the arc envelope  12 . In other words, the airflow carries heat away from the top side  26 , whereas the convective heat transfer at the bottom side  28  is relatively less due to the lack (or relatively lesser amount) of airflow at the bottom side  28 . For example, if the airflow rate is 1 standard cubic feet per hour, then the temperature of the bottom side  28  of the arc envelope  12  is 987.1 degree Celsius and the temperature of the top side  26  of the arc envelope  12  is 1039.2 degree Celsius. The temperature difference between the top and bottom sides  26  and  28  of the arc envelope  12  is 52.1 degree Celsius. If the airflow rate is 5 standard cubic feet per hour, then the temperature of the bottom side  28  of the arc envelope  12  is 906.1 degree Celsius and the temperature of the top side  26  of the arc envelope  12  is 909.1 degree Celsius. The temperature difference between the top and bottom sides  26  and  28  of the arc envelope  12  is 2.9 degree Celsius. The stresses generated in the arc envelope  12  are reduced proportional to the reduction in difference between the top and bottom sides  26  and  28  of the arc envelope  12 . In the illustrated embodiment, the thermal stresses generated in the arc envelope  12  may be substantially reduced by reducing the temperature difference between the top and bottom sides  26  and  28  of the arc envelope  12 . 
     Referring to  FIG. 12 , a graph  142  representing distribution of temperature along the length of the arc envelope  12  provided inside the reflector, is illustrated in accordance with the embodiments of the present technique. As known to those skilled in the art, the temperature distribution along the length of the arc envelope  12  is obtained through numerical simulation technique. Y-axis  144  of the graph  142  represents distribution of temperature in Kelvin and X-axis represents length of the arc envelope  12  in centimeters. A line  147  represents center along the length of the arc envelope  12 . Two portions on either sides of line  147  along the X-axis represents two halves on either side of the center respectively. A curve  148  illustrates distribution of temperature along the top side of the arc envelope  12  for an airflow rate of zero scfh. A curve  150  illustrates distribution of temperature along the bottom side of the arc envelope  12  for an airflow rate of zero scfh. Another curve  152  illustrates distribution of temperature along the top side of the arc envelope  12  for airflow rate of 5 scfh directed to the top side of the arc envelope  12 . Yet another curve  154  illustrates distribution of temperature along the bottom side of the arc envelope  12  for airflow rate of 5 scfh. In the illustrated example, if no cooling air is supplied during the operation of the lamp module, the temperature at the top side of the arc envelope  12  is 1278 Kelvin and the temperature at the bottom side of the arc envelope  12  is 1135 Kelvin. The temperature difference between the top and bottom side of the arc envelope  12  is 143 Kelvin. If cooling air of 5 cubic feet per hour is supplied to the top side of the arc envelope  12 , the temperature at the top side of the arc envelope  12  is 1082 Kelvin and the temperature at the bottom side of the arc envelope  12  is 1002 Kelvin. The temperature difference between the top and bottom side of the arc envelope  12  is 80 Kelvin. Unlike as illustrated in  FIG. 11 , the airflow is not exactly focused at the hot spot of the arc envelope  12  provided inside the reflector. In fact, the airflow diffuses after entering the cooling passage  32  (shown in  FIG. 2 ). As a result, the temperature gradient of the lamp module may be reduced from 10,000 degrees/meter to 6000 degrees/meter. This increases the life of the lamp. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.