FIELD OF THE INVENTION 
     The invention relates to a discharge lamp. More specifically, the invention relates to a high intensity discharge lamp with a discharge vessel and an outer bulb arranged around the discharge vessel. 
     BACKGROUND OF THE INVENTION 
     Discharge lamps, specifically HID (high-intensity discharge) lamps are used for a large area of applications where energy efficiency and high light intensity are required. Especially in the automotive field, HID lamps are used as vehicle headlamps. 
     A discharge lamp comprises two electrodes arranged at a distance within a discharge vessel. An arc discharge is generated between the electrodes. Different types of fillings within the discharge vessel are known, distinguishing mercury vapor, metal halide and other types of lamps. 
     Commercially available lamps for use in a vehicle headlight have an outer bulb which is arranged around the discharge vessel at a distance therefrom. A known type of such a lamp is designed for a nominal power of 35 W and achieves a high efficacy of 80-90 lm/W. After starting such a lamp, a run-up current of, for example, 2.7-3.2 A is necessary, and a run-up power of 75-80 W is used. Thus, the complete HID system comprising lamp, ballast and igniter must be able to operate as these values. 
     Especially for the automotive field, it would be desirable to have a discharge lamp with lower nominal power, e.g. in the range of 20-30 W, and correspondingly lower demands on the complete HID system. If, however, known lamp designs are simply used at lower power, the lamp efficacy will be dramatically reduced. 
     US-A-2005/0248278 shows an example of an automotive head lighting discharge lamp with a power of 30 W. The lamp has a ceramic discharge vessel comprising the electrodes, which is surrounded by an outer bulb. The distance between the electrode tips is 5 mm. The discharge vessel has cylindrical shape with an internal diameter of 1.2 mm. The wall thickness of the discharge vessel is 0.4 mm. The discharge vessel comprises a filling which is free from mercury and comprises NaPrI and ZnI 2  as well as Xe with a filling pressure of 16 bar. The outer bulb is made of quartz glass and is arranged at a distance of 0.5 mm to the discharge vessel. The outer bulb is filled with N 2  with a filling pressure of 1.5 bar at room temperature. 
     It is an object of the invention to provide a relatively low power HID lamp with high lamp efficacy. 
     This object is achieved by a high intensity discharge lamp according to claim  1 . Dependent claims refer to preferred embodiments of the invention. 
     SUMMARY OF THE INVENTION 
     The inventors have recognized that in order to maintain high efficacy thermal design of the lamp needs to be adapted to the lower power. The “coldest spot”-temperature needs to be maintained at a high level to achieve good lamp efficacy. However, thermal load on a “hot spot” needs to be constrained in order to achieve good durability. This has led the inventors to propose a lamp with a relatively small discharge vessel, leading to reduced heat radiation, while still maintaining a sufficiently thick wall of the discharge vessel to not only withstand high internal pressure, but specifically to allow heat conduction from the hot upper side (“hot spot”) to the colder lower side. 
     According to the invention, a specific geometry is provided in view of the thermal design of the lamp. The discharge vessel is maintained with a substantial wall thickness of 1.4-2 mm, and preferably also a relatively small inner diameter from 2-2.7 mm. 
     An outer bulb is arranged around the discharge vessel. The outer bulb is sealed and has a gas filling with a thermal conductivity λ. The thermal conductivity λ of the outer bulb filling is taken at 800° C. 
     The geometry of the outer bulb (here specifically: the distance d 2  between the discharge vessel and the outer bulb) and the gas filling are chosen to achieve a certain, limited heat flow from the discharge vessel to the outside. The thermal conductivity λ of the gas filling and the distance d 2  are chosen to obtain a desired heat transition coefficient λ/d 2  calculated as the thermal conductivity λ divided by the distance d 2 . According to the invention, this coefficient is below 150 W/(m 2 K). For the purposes of measurement, here, the distance d 2  is measured in cross-section of the lamp taken at a central position between the electrodes. 
     The outer bulb therefore plays an important part in the thermal design of the lamp. While on one hand thermal radiation is limited by the limited size of the discharge vessel, heat conduction in radial direction of the lamp is further limited by the geometry and filling of the outer bulb. As will be explained in relation to the preferred embodiment, the amount of heat transported per time unit between the discharge vessel and the outer bulb, both at their constant operating temperature, is roughly proportional to the defined heat transition coefficient. Thus, by choosing the heat transition coefficient to be below 150 W/(m 2 K), cooling is limited, such that sufficient high coldest spot temperatures, and thus high efficacy are maintained. To achieve a desired, high enough coldest spot temperature the heat transition coefficient is preferably equal to or less than 130 W/(m 2 K), most preferably even lower &lt;100 W/(m 2 K). It is further preferred for the heat transition coefficient to be at least 10 W/(m 2 K), further preferred at least 15 W/(m 2 K). 
     A lamp according to the invention is especially suited for a nominal power of 20-30 W. The filling of the discharge vessel is preferably free of mercury and may comprise one or more metal halides and a rare gas. Preferably, the filling of the discharge vessel comprises one or more of the following: NaI, ScI 3 , ZnI 2 . 
     Preferred embodiments of the invention relate to the outer bulb. The outer bulb is preferably made out of quartz glass and may be of any geometry, e.g. cylindrical, generally elliptical or other. It is preferred for the outer bulb to have an outer diameter of at most 10 mm. The outer bulb is sealed and has a gas filling at a pressure of 10 mbar to 1 bar, preferably below 1 bar, most preferably 50 mbar to 300 mbar. The gas filling may essentially consist (i.e. comprise more than 50%, preferably more than 90%) of one or more of the following: Xe, Ar, N 2 , O 2 . The distance d 2  between the outer bulb and the discharge vessel is preferably 0.1-1.4 mm, most preferably 0.3-0.8 mm. As will be appreciated by the skilled person, the filling gas, pressure and distance d 2  may only be chosen dependent on one another to achieve the desired heat transition coefficient. 
     Other preferred embodiments of the invention relate to the discharge vessel. Preferably, the discharge vessel is made from quartz glass. The distance between the electrodes is preferably 2.5-5.5 mm. Most preferably, the optical distance (i.e. the distance as viewed from the outside, taking into account magnification of the discharge vessel wall acting as a lens) is 4.2±0.6 mm. The discharge vessel has a shape such that in a cross-section taken at the central position between the electrodes the wall of the discharge vessel is at least substantially circular. 
     In a preferred embodiment, the discharge vessel, when viewed in longitudinal section, has at least substantially elliptical outer shape and may have either elliptical or cylindrical inner shape. In this case, it is preferred for the wall thickness w 1  to be in the range from 1.55-1.85 mm. 
     According to an alternative embodiment, the discharge vessel, when viewed in longitudinal section, has elliptical or cylindrical inner shape and concave outer shape, i.e. starting from the central position between the electrodes the outer diameter of the discharge vessel increases towards both sides. In this case, it is preferred for the wall thickness w 1  to be in the range from 1.4-2 mm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments, in which: 
         FIG. 1  shows a side view of a lamp according to a first embodiment of the invention; 
         FIG. 2  shows an enlarged view of the central portion of the lamp shown in  FIG. 1 ; 
         FIG. 2   a  shows a cross-sectional view along the line A in  FIG. 2 ; 
         FIG. 3  shows a side view of a lamp according to a second embodiment of the invention; 
         FIG. 4  shows a side view of a lamp according to a third embodiment of the invention; 
         FIG. 5  shows an enlarged view of the central portion of the lamp shown in  FIG. 4 ; 
         FIG. 5   a  shows a cross-sectional view along the line A in  FIG. 5 , 
         FIG. 6  shows a side view of a lamp according to a fourth embodiment of the invention, 
         FIG. 7  shows a graph representing a heat transition coefficient λ/d 2  for different fillings and distances d 2 , and 
         FIG. 8  shows a graph representing measured values of lumen output over time (run-up) for a lamp according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     All embodiments shown are intended to be used as automotive lamps for vehicle head lights, conforming to ECE R99 and ECE R98. This, specifically, is not intended to exclude lamps for non-automotive use, or lamps according to other regulations. Since such automotive HID lamps are known per se, the following description of the preferred embodiments will primarily focus on the special features of the invention. 
       FIG. 1  shows a side view of a first embodiment  10  of a discharge lamp. The lamp comprises a socket  12  with two electrical contacts  14  which are internally connected to a burner  16 . 
     The burner  16  is comprised of an outer bulb  18  of quartz glass surrounding a discharge vessel  20 . The discharge vessel  20  is also made of quartz glass and defines an inner discharge space  22  with projecting electrodes  24 . The glass material from the discharge vessel further extends in longitudinal direction of the lamp  10  to seal the electrical connections to the electrodes  24  which comprise a flat molybdenum foil  26 . 
     The outer bulb  18  is arranged around the discharge vessel  20  at a distance, thus defining an outer bulb space  28 . The outer bulb space  28  is sealed. 
     As shown in greater detail in  FIG. 2 , the discharge vessel  20  has an outer wall  30  arranged around the discharge space  22 . The discharge space  22  is of ellipsoid shape. Also, the outer shape of the wall  30  is ellipsoid. 
     The discharge vessel  20  is characterized by the electrode distance d, the inner diameter d 1  of the discharge vessel  20 , the wall thickness w 1  of the discharge vessel, the distance d 2  between the discharge vessel  20  and the outer bulb  18  and the wall thickness w 2  of the outer bulb  18 . Here, the values d 1 , w 1 , d 2 , w 2  are measured in a central perpendicular plane of the discharge vessel  20 , as shown in  FIG. 2   a.    
     The lamp  10  is operated, as conventional for a discharge lamp, by igniting an arc discharge between the electrodes  24 . Light generation is influenced by the filling comprised within the discharge space  22 , which is free of mercury and includes metal halides as well as a rare gas. 
     In the following examples, the filling of the discharge space  22  comprises about 17 bar cold xenon pressure and as metal halides 36 wt % NaI, 24 wt % ScI 3  and 40 wt % ZnI 2 . 
     In the following, different embodiments of a lamp will be discussed, which are each intended to be used at different (steady-state) levels of operating power. The operating power of the embodiments is within the interval of 25-30 W. For each embodiment, a specific design is chosen with regard to thermal characteristics of the lamp in order to achieve high lamp efficacy. 
     Regarding the thermal behavior of a discharge lamp  10  as shown, it should be kept in mind that automotive lamps are intended to be operated horizontally. The arc discharge between the electrode  24  will then lead to a hot spot at the wall  30  of the discharge vessel  20  above the arc. Likewise, opposed portions of the wall  30  surrounding the discharge space  22  will remain at comparatively low temperatures (coldest spot). 
     In order to achieve good efficacy and, as will become apparent later, also achieve favorable run-up behavior, the geometric design of the lamp  10  is chosen according to thermal considerations. The “coldest spot” temperature should be kept high to achieve high efficacy. The thickness of the wall  30  should be small enough to allow a quick run-up with limited run-up current, but should not be too small in order to still achieve good heat conduction from the “hot spot” in order to reduce thermal load. The inner diameter d 1  should not be too small in order to reduce excessive thermal load at the “hot spot”. 
     In order to reduce heat transport from the discharge vessel  20  to the outside, and to maintain high temperatures necessary for good efficacy, it is thus preferable to use the outer bulb  18  instead of a significant reduction of the thickness w 1  of the wall  30 . In contrast to a simple downscaling of the discharge vessel  20  (reduced inner diameter, reduced wall thickness, reduced outer diameter), this has proven to also serve to maintain a good lamp lifetime. 
     In order to limit cooling from the outside, the outer bulb  18  is sealed and filled with a filling gas of reduced heat conductivity. Especially Argon and Xenon are preferred here, but O 2  or N 2  could be used as well. The outer bulb filling is provided at reduced pressure (measured in the cold state of the lamp at 20° C.). As will be further explained below, the choice of a suitable filling gas has to be made in connection with the geometric arrangement in order to achieve the desired heat conduction from discharge vessel  20  to outer bulb  18  via a suitable heat transition coefficient λ/d 2 . 
     In the following table, measurement results of lamp efficacy are shown for a lamp as shown in  FIG. 1-2   a  with an inner diameter d 1 =2.2 mm, a wall thickness w 1  of 1.65 mm (thus an outer diameter of the discharge vessel of 5.5 mm) and a steady-state operating power of 25 W for different outer bulb fillings: 
     
       
         
               
               
               
             
           
               
                   
               
               
                 Outer bulb filling 
                 Efficacy S-type 
                 Coldest spot temperature (outside) 
               
               
                   
               
             
             
               
                 Air (1 bar) 
                 67 lm/W 
                 810° C. 
               
               
                 Ar (100 mbar) 
                 79 lm/W 
                 840° C. 
               
               
                 Xe (100 mbar) 
                 86 lm/W 
                 900° C. 
               
               
                   
               
             
          
         
       
     
     It is thus clearly visible how the reduced heat conduction to the outside leads to a higher coldest spot temperature, and to a higher lamp efficacy. 
     The heat conduction to the outside may be roughly characterized by a heat transition coefficient λ/d 2 , which is calculated as the thermal conductivity λ of the outer bulb filling divided by the distance d 2  between the discharge vessel  20  and the outer bulb  18 . 
     Due to the relatively small distance between the discharge vessel  20  and outer bulb  18 , heat conduction between the two is essentially diffusive and will therefore be calculated as {dot over (q)}=−λ grad            where {dot over (q)} is the heat flux density, i.e. the amount of heat transported per time between discharge vessel and outer bulb. λ is the thermal conductivity and grad          is the temperature gradient, which here may roughly be calculated as the temperature difference between discharge vessel and outer bulb, divided by the distance:
               grad   ⁢           ⁢   ϑ     =           T   dischargeVessel     -     T   outerBulb         d   2       .           
Thus, cooling is proportional to
 
     
       
         
           
             
               λ 
               
                 d 
                 2 
               
             
             . 
           
         
       
     
       FIG. 7  shows the dependence of the heat transition coefficient λ/d 2  on the distance d 2  for different outer bulb fillings. It is clearly visible how Argon, and especially Xenon (provided here at a reduced pressure of 200 mbar) have significantly lower heat conductivity than air, and that the heat transition coefficient λ/d 2  is further reduced with increasing distance d 2 . The heat transition coefficient was found to differ more strongly with the gas composition, and less with the pressure, if it is in the range from about 10 mbar to about 1 bar. 
     The following examples of lamps with a rated power of 25-30 W are proposed: 
     Example 1 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 25 W lamp 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 discharge vessel: 
                 ellipsoid inner and outer shape 
               
               
                   
                 electrode distance d = 
                 4.2 mm optical 
               
               
                   
                 inner diameter d 1  = 
                 2.2 mm 
               
               
                   
                 wall thickness w 1  = 
                 1.65 mm  
               
               
                   
                 outer diameter = 
                 5.5 mm 
               
               
                   
                 outer bulb distance d 2  = 
                 0.6 mm 
               
               
                   
                 outer bulb filling = 
                 Xe 
               
               
                   
                   
                 100 mbar (λ = 0.014 
               
               
                   
                   
                 W/(m*K) at 800° C.) 
               
               
                   
                 heat transition coefficient λ/d 2  = 
                 23.3 W/(m 2 K) at 800° C. 
               
               
                   
                 outer bulb wall thickness w 2  = 
                     1 mm 
               
               
                   
                   
               
             
          
         
       
     
     Example 2 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 30 W lamp 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 discharge vessel: 
                 ellipsoid inner and outer shape 
               
               
                   
                 electrode distance d = 
                 4.2 mm optical 
               
               
                   
                 inner diameter d 1  = 
                 2.3 mm 
               
               
                   
                 wall thickness w 1  = 
                 1.75 mm  
               
               
                   
                 outer diameter = 
                 5.8 mm 
               
               
                   
                 outer bulb distance d 2  = 
                 0.45 mm  
               
               
                   
                 outer bulb filling = 
                 Xe 
               
               
                   
                   
                 100 mbar (λ = 0.014 
               
               
                   
                   
                 W/(m*K) at 800° C.) 
               
               
                   
                 heat transition coefficient λ/d 2  = 
                 31.1 W/(m 2 K) at 800° C. 
               
               
                   
                 outer bulb wall thickness w 2  = 
                     1 mm 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 3  shows a second embodiment of the invention. A lamp  110  according to the second embodiment comprises a discharge vessel  120  of different internal shape. The remaining parts of the lamp correspond to the lamp  10  according to the first embodiment. Like elements will be designated by like reference numerals, and will not be further described in detail. 
     The discharge vessel  120  of the lamp  110  has external ellipsoid shape, identical to the discharge vessel  20  according to the first embodiment. However, the internal discharge space  22  is cylindrical. Both the length and diameter of the inner discharge space  22  however are as in the above first embodiment. It should be noted that the term “cylindrical” used here refers to the central, largest part of the discharge space  22  and does not exclude—as shown—conical end portions. 
     The wall  130  surrounding the discharge space  22  is consequently of varying thickness, with the thickness being greatest at a position corresponding to the center between the electrodes  24 , and decreasing towards both sides. 
     In the following, a third embodiment of the invention will be described with reference to  FIGS. 3-4   a . A lamp  110  according to the second embodiment again in large parts corresponds to the lamp  10  according to the above first and second embodiments. Like elements will be designated by like reference numerals and will not be further described in detail. 
     The lamp  210  differs from the lamp  10  by the concave outer shape of the discharge vessel  120 . The inner discharge space  22  remains roughly ellipsoidal as in the first embodiment. However, the wall  230  surrounding the discharge space  22  has a varying wall thickness such that its outer shape is concave. 
     Again, geometrical parameters d 1 , w 1 , d 2 , w 2  are measured in a central plane of the discharge vessel  220 . 
       FIG. 6  shows a fourth embodiment of the invention, which in large parts corresponds to the third embodiment according to  FIG. 4-5   a . Again, like elements are designated by like reference numerals and will not be further described in detail. 
     According to the fourth embodiment of the invention, a lamp  310  has a discharge vessel  320  with a concave outer shape, but an inner discharge space  22  of cylindrical shape. 
     Both in the third and forth embodiment, the thickness of the wall  230 ,  330  surrounding the discharge space  22  varies such that it is minimal in a position corresponding to the center between the electrodes  24  and increases towards both sides. This leads to a lens effect, such that the electrode distance d will appear to the outside smaller than it actually is. Thus, to achieve the desired optical electrode distance d of 4.2 mm, the real electrode distance may be, e.g. 4.8 mm in the third and in the forth embodiment. The possibility to thus increase the real electrode distance d but maintain the optical distance gives to the lamp designer a further degree of freedom. Since the operating voltage increases with the electrode distance, it is possible to obtain a higher voltage. 
     This may be used to provide a lamp which is compatible with ECE R99 geometrically (optical distance 4.2 mm), but—as a mercury-free-lamp—fulfills the electric requirements of a D2 lamp (voltage more than 68 V). 
     On the other hand, for the first and second embodiment (elliptical outer shape), it is also possible to provide a larger electrode distance to obtain a lamp, which is not according to ECE R99, but may be operated with higher voltage. 
     The following examples of lamps according to the third embodiment in a range of 25-30 W are proposed: 
     Example 3 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 25 W lamp 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 discharge vessel: 
                 concave outer shape, 
               
               
                   
                   
                 elliptical inner shape 
               
               
                   
                 electrode distance d = 
                 4.2 mm optical 
               
               
                   
                 inner diameter d 1  = 
                 2.2 mm 
               
               
                   
                 wall thickness w 1  = 
                 1.5 mm 
               
               
                   
                 outer diameter = 
                 5.2 mm 
               
               
                   
                 outer bulb distance d 2  = 
                 0.75 mm  
               
               
                   
                 outer bulb filling = 
                 Ar 
               
               
                   
                   
                 100 mbar (λ = 0.045 
               
               
                   
                   
                 W/(m*K) at 800° C.) 
               
               
                   
                 heat transition coefficient λ/d 2  = 
                 60 W/(m 2 K) at 800° C. 
               
               
                   
                 outer bulb wall thickness w 2  = 
                     1 mm 
               
               
                   
                   
               
             
          
         
       
     
     Example 4 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 28 W lamp 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 discharge vessel: 
                 concave outer shape, 
               
               
                   
                   
                 elliptical inner shape 
               
               
                   
                 electrode distance d = 
                 4.2 mm optical 
               
               
                   
                 inner diameter d 1  = 
                 2.2 mm 
               
               
                   
                 wall thickness w 1  = 
                 1.7 mm 
               
               
                   
                 outer diameter = 
                 5.6 mm 
               
               
                   
                 outer bulb distance d 2  = 
                 0.55 mm  
               
               
                   
                 outer bulb filling = 
                 50% Ar/50% Xe 
               
               
                   
                   
                 100 mbar (λ = 0.025 
               
               
                   
                   
                 W/(m*K) at 800° C.) 
               
               
                   
                 heat transition coefficient λ/d 2  = 
                 45.5 W/(m 2 K) at 800° C. 
               
               
                   
                 outer bulb wall thickness w 2  = 
                     1 mm 
               
               
                   
                   
               
             
          
         
       
     
     Example 5 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 30 W lamp 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 discharge vessel: 
                 concave outer shape, 
               
               
                   
                   
                 elliptical inner shape 
               
               
                   
                 electrode distance d = 
                 4.2 mm optical 
               
               
                   
                 inner diameter d 1  = 
                 2.2 mm 
               
               
                   
                 wall thickness w 1  = 
                 1.9 mm 
               
               
                   
                 outer diameter = 
                 6.0 mm 
               
               
                   
                 outer bulb distance d 2  = 
                 0.35 mm  
               
               
                   
                 outer bulb filling = 
                 50% Ar/50% Xe 
               
               
                   
                   
                 100 mbar (λ = 0.025 
               
               
                   
                   
                 W/(m*K) at 800° C.) 
               
               
                   
                 heat transition coefficient λ/d 2  = 
                 71.4 W/(m 2 K)at 800° C. 
               
               
                   
                 outer bulb wall thickness w 2  = 
                     1 mm 
               
               
                   
                   
               
             
          
         
       
     
     In the above examples, only discharge vessels of elliptical inner shape were used. However, the same measurements may be used for cylindrical inner shape. 
       FIG. 8  shows measurement results of run-up tests, where a 25 W lamp according to the above example 1 was compared to a reference lamp (35 W lamp). The lumen output was measured and is shown in  FIG. 8  over the time since ignition of the lamp. As is known for starting the lamps, in a first phase, the current is limited to a maximum value, and in a second phase, the power is controlled. 
     As shown in  FIG. 8 , the reference lamp reaches about 50% of the total lumen output after 4 seconds. But this requires a maximum run-up current of 3.2 A, resp. a maximum power of around 75 W. The 25 W lamp according to example 1 was first driven with a current limitation in the first phase of 1.1 A. Here, the results (less then 30% after 4 seconds) were not satisfactory. However, with a run-up current limitation of 1.5 A (maximum power about 50 W), the lamp according to example 1 shows a quite comparable behavior to the reference, whereas the run-up current is less then half and the maximum run-up power is reduced by about 30%. 
     The remaining examples where found to also show satisfactory behavior with a run-up current significantly lower then necessary for the reference lamp. This is due to the fact that the smaller discharge vessel is heated up quickly by the arc discharge. 
     As lifetime tests have shown, the lifetime performance within the first 1500 hours of operation for lamps according to the above embodiments corresponds to the reference (a 35 W lamp). 
     Thus, it has been shown that the above embodiments provide lamps with good lifetime, good efficacy and good run-up behavior, which all correspond to the reference lamps, but at lower required run-up current and lower steady-state power. 
     The invention has been illustrated and described in detail in the drawings and foregoing description. Such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 
     In the claims, the word “comprising” does not exclude other elements, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.
In order to achieve a discharge lamp suited to operate under reduced nominal power of e.g. 20-30 W, a lamp is proposed with two electrodes ( 24) arranged at a distance in a discharge vessel ( 20, 120) for generating an arc discharge. The discharge vessel ( 20, 120) has a filling with a substantially free of mercury and comprises a metal halide and a rare gas. The lamp ( 10, 110) further comprises an outer bulb ( 18) arranged around the discharge vessel at a distance (d 2). The outer bulb ( 18) is sealed and has a gas filling of a thermal conductivity (l). The inner diameter (d 1) of the discharge vessel is preferably in a range from 2-2.7 mm. The wall thickness (w 1) is in a range from 1.4-2 mm. A heat transition coefficient (l/d 2) is calculated as thermal conductivity (l) at 800deg C. Of the outer bulb filling divided by the distance (d 2). The so-defined heat 10 transition coefficient is below 150 W/(m 2 K).