High-pressure sodium discharge lamp
BACKGROUND OF THE INVENTION 
     The invention relates to a high-pressure sodium discharge lamp comprising a discharge vessel enclosed with intervening space by an outer bulb and having a ceramic wall, in which two electrodes are present with respective tips spaced apart by a distance D and in which at least over the distance D the discharge vessel has a substantially circular cross-section with an internal diameter d i , the lamp radiating light with a colour temperature T c  of at least 2400K under nominal operating conditions. 
     A lamp of the type described in the opening paragraph is known from GB-A-2,083,281. The known lamp radiates white light with a good colour rendering expressed as the colour rendering index R a  with a value of more than 80. Generally, the region in the colour triangle bounded by straight lines through points having coordinates (x; y); (0.400; 0.430), (0.510; 0.430), (0.485; 0.390) and (0.400); 0.360) can be regarded as representing &#34;white&#34; light in the case of light radiated by high-pressure sodium lamps. The colour temperature T c  lies between approximately 2300K and 4000K in this case. 
     The known lamp can be used to replace an incandescent lamp, for example in accent lighting applications. The colour temperature T c  of the known lamp, however, is relatively low for this in comparison with the light radiated by incandescent lamps. A colour rendering index R a  above 80 is necessary for incandescent lamp replacement. The maximum achievable colour rendering index value for practical high-pressure sodium lamps is between 80 and approximately 85. 
     SUMMARY OF THE INVENTION 
     The invention has for its purpose inter alia to provide a lamp with which light can be radiated having a colour temperature T c  considerably higher than 2400K, the colour rendering index R a  being &gt;80. 
     According to the invention, this object is achieved in that the lamp of the type described in the opening paragraph in that the ceramic wall of the discharge vessel has a wall load of at least 60 W/cm 2  under nominal operating conditions, the space between the outer bulb and the discharge vessel contains a gas filling, and the ratio D/d i  is &gt;6. 
     With the lamp according to the invention it is possible to generate light with a considerably higher colour temperature than 2400K, while a colour rendering index value R a  of above 80 is retained. It is found that the luminous efficacy is thereby at least maintained with respect to the cited prior art lamp having a lower color temperature. The following can be remarked in this connection. 
     A high-pressure sodium discharge lamp radiates light with a spectrum which is characterized by an absorption band near 589 nm, with spectral flanks having maxima at a mutual distance Δλ on either side. The mutual distance Δλ is between approximately 40 and approximately 55 nm in the case of a colour rendering index R a  above 80 of the radiated light. It is known that a further widening of the absorption band, so a further increase of the mutual distance Δλ, is capable of increasing the colour temperature T c  of the radiated light further to above 2500K. This, however, is to the detriment of the colour rendering and the luminous efficacy. In addition, widening of the absorption band while the interior diameter of the discharge vessel remains the same implies a rise of the sodium pressure in the discharge vessel. A rise in the sodium pressure is unfavourable for lamp life because it is especially the sodium pressure which influences the speed of the various corrosion processes in and of the discharge vessel. 
     It should be noted that the term &#34;wall load&#34; in the present description and accompanying claims is defined as the ratio of the nominal lamp power in W to the interior surface area of the discharge vessel wall over the distance D. 
     In the lamp according to the invention, the nominal lamp voltage corresponds substantially to the lamp voltage of a known lamp of corresponding nominal power. This is particularly favourable for use of the lamp according to the invention in an existing installation. An increase of the wall load through reduction of the distance D leads to a reduction of the lamp voltage. A reduction of the internal diameter d i , on the other hand, leads to an increase in lamp voltage. 
     A ceramic wall in the present description and accompanying claims is understood to mean a wall made of crystalline metal oxide or crystalline metal nitride which is highly resistant to the attack by sodium at high temperatures, such as, for example, monocrystalline sapphire polycrystalline gas-tight sintered Al 2  O 3 , or polycrystalline gas-tight sintered AlN. The known wall materials are capable of withstanding temperatures up to approximately 1400K during long periods at the sodium pressure prevalent in the lamp. At substantially higher temperatures, the prevalent sodium pressure leads to a considerable degree of corrosion of the ceramic wall. The use of a gas filling in the space between the discharge vessel and the outer bulb achieves an increased heat transport, so that the temperature of the discharge vessel wall remains within acceptable limits, also in the case of higher wall loads. Suitable gases are, for example, rare gases and nitrogen, since these gases are inert to a high degree under the prevailing circumstances. The gas filling may be composed of a single gas, but a mixture of gases is also possible. Where safety is of exceptional importance, the filling pressure of the gas filling is so chosen that the pressure of the gas filling is approximately 1 at under nominal operational conditions of the lamp. 
     A further improvement regarding the control of the maximum wall temperature of the discharge vessel can be achieved through the choice of the wall thickness. An increase in wall thickness leads to an increased heat radiation of the wall and promotes further heat transport from the area between the electrode tips to the relatively cool ends of the discharge vessel. 
     On the other hand, an increase in wall thickness adversely affects the luminous flux. In addition, manufacture becomes more difficult with increasing wall thicknesses owing to the increasing risk of irregular crystal growth and the increasing risk of internal fractures. This is why the wall thickness is preferably chosen to be smaller than 3 mm. 
     A choice in favour of a comparatively great D/d i  ratio leads to a comparatively long discharge vessel. It is known, however, that the maximum wall temperature of the discharge vessel is higher in proportion as the discharge vessel is longer, the wall load remaining the same. For present practice, therefore, it is preferable to choose the D/d i  ratio to be not greater than 10. An additional advantage of the discharge vessel dimensions being restricted in this way is that a desired light distribution can be realized in a simpler and often better way by means of a light-distributing optical system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of a lamp according to the invention will be explained in more detail with reference to a drawing. In the drawing 
     FIG. 1 shows a lamp provided with an outer bulb in side elevation; 
     FIG. 2 shows a lamp in longitudinal section; and 
     FIG. 3 shows another lamp in longitudinal section. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, reference numeral 1 denotes a discharge vessel having a ceramic wall which is enclosed with intervening space 8 by an outer bulb 6. The space 8 contains a gas filling. Inside the discharge vessel 1 there are two pin electrodes 2 and 3 with respective tips having a mutual distance D, the discharge vessel 1 having a substantially circular cross-section between the electrodes 2 and 3. The electrodes 2 and 3 are connected to current conductors 4 and 5, respectively. The outer bulb is provided with a lamp cap 7, to which the current conductors 4, 5 are connected. The discharge vessel, which contains a filling of sodium, mercury, and rare gas, has an internal diameter d i  over the distance D. 
     In FIGS. 2 and 3, corresponding parts have reference numerals which are ten and twenty higher than those in FIG. 1, respectively . The electrodes 12, 13 and 22, 23, respectively, consist of tungsten/rhenium (97/3 weight ratio), while the current conductors 14, 15, 24, 25 consist of Nb. The discharge vessels 11, 21 are sealed off with melting ceramic 18, 28, respectively. 
     Lamps according to the invention were manufactured with discharge vessels having the shape of FIG. 2, the data being listed in the table. Data of a commercially available lamp (no. 3) are included in the table for comparison. This is a lamp of the Philips SDW 50 type. 
     
                       TABLE                                                       
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lamp no.       1         2         3                                      
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D (mm)         13        11        16.6                                   
d.sub.i (mm)   2.1       1.7       3.5                                    
D/d.sub.i      6.2       6.5       4.7                                    
lamp power (W) 55        55        53                                     
wall load (W/cm.sup.2)                                                    
               64        94        29                                     
T.sub.c (K)    2680      2800      2500                                   
R.sub.a        82        82        82                                     
luminous efficacy (lm/W)                                                  
               48        50        47                                     
max. wall temperature (K)                                                 
               1350      1370      1430                                   
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     The discharge vessels were filled with Na/Hg=15/40 (weight ratio) and with Xe having a pressure of 530 mbar at 300K (53 kPa). The lamps no. 1 and 2 had a gas filling consisting of N 2  in the space 8 with a pressure of approximately 1 under nominal operating conditions. The space 8 in the known lamp was evacuated. 
     Under nominal operating conditions, the lamp voltages of the lamps 1, 2 and 3 were 91 V, 93 V, and 90 V, respectively. The difference in lamp voltage of max. 3 V is inside the lamp voltage spread of mass-produced lamps of the Philips SDW 50 type. The discharge vessels had an interior length of 18 mm (lamp 1); 17 mm (lamp 2) and 24 mm (lamp 3). The wall thickness of lamp 1 was 1.4 mm, of lamp 2 1.5 mm. The wall thickness of the known lamp is 0.8 mm.