High-pressure metal halide lamp

The invention relates to a high-pressure metal halide lamp provided with a discharge vessel with a ceramic wall enclosing a discharge space. The vessel has a cylindrical portion of internal diameter ID which is closed off at either end by end wall portions which form end faces of the discharge space. At least two electrodes are arranged in the discharge vessel, whose respective tips have an interspacing EA such that ##EQU1## The electrodes have lead-throughs which are enclosed in ceramic closing plugs and are connected thereto in a gastight manner by means of ceramic glazing joints. According to the invention, the rated lamp power is at most 100 W, and an electrode tip lies substantially in the adjacent end face. A closing plug is fastened in the relevant end wall portion in a gastight manner at a distance from the end face.

BACKGROUND OF THE INVENTION 
The invention relates to a high-pressure metal halide lamp comprising a 
discharge vessel which encloses a discharge space, which has a ceramic 
wall and a filling which comprises besides Hg, and a rare gas and at least 
one metal halide, and which is formed from a cylindrical portion with an 
internal diameter ID closed off at either end by end wall portions, each 
end wall portion forming an end face of the discharge space while at least 
one end wall portion is provided with an opening in which a ceramic 
closing plug is fastened which narrowly encloses over a length 1 a 
lead-through of a respective electrode provided with an electrode tip and 
is connected thereto in a gastight manner at the side facing away from the 
discharge space by means of a ceramic glazing joint, the discharge vessel 
containing at least two electrodes whose respective tips are situated at a 
mutual interspacing EA such that the following relation is satisfied 
##EQU2## 
A lamp of the kind mentioned in the opening paragraph is known from EP-A-0 
215 524 (PHN 11.485). The term "ceramic wall" is here understood to mean a 
wall of a refractory material such as monocrystalline metal oxide (for 
example, sapphire), polycrystalline metal oxide (for example, 
polycrystalline densely sintered aluminium oxide; yttrium-aluminium 
garnet, or yttrium oxide), and polycrystalline non-oxidic material (for 
example, aluminium nitride). Such materials allow for high wall 
temperatures up to 1500-1600 K and are satisfactorily resistant to 
chemical attacks by halides and by Na. 
The internal diameter is defined in the present description and claims as 
1.12 times the square root of the quotient of the volume of the discharge 
space between the electrode tips and EA. 
The known lamp contains metal halide in excess. The metal halide vapour 
pressure, and thus the partial pressures of the ingredients, are governed 
by the temperature of the free surface of the excess quantity. This 
temperature is called cold spot temperature for short (T.sub.KP). A colour 
temperature T.sub.c in the comparatively low range from approximately 2500 
K to 3500 K can be realised with the known lamp with a high luminous 
efficacy as well as good colour rendering properties. 
A typical characteristic of the lamp of the kind mentioned in the opening 
paragraph is the comparatively great internal diameter ID of the discharge 
vessel in relation to the distance between the electrode tips EA. One of 
the results of this is that the location where T.sub.KP prevails in lamps 
having a prior-art discharge vessel is situated near an end face at the 
discharge vessel wall. 
In the known lamp, the electrodes project over some distance into the 
discharge space, so that there is a considerable tip-to-bottom distance, 
i.e. the distance between the electrode tip and the location of T.sub.KP. 
This is found to result in too low vapour pressures of the halides present 
in lamps of comparatively low power. This manifests itself in a deviating 
colour temperature T.sub.c of the light radiated by the lamp and a 
deviation in colour point in the chromaticity diagram, in particular in 
the form of differences with differing burning positions of the lamp. A 
reduction of the tip-to-bottom distance gives rise to attacks on the 
ceramic discharge vessel wall in many cases, in particular on the ceramic 
closing plug. Fractures also frequently occur in the end wall portion or 
the closing plug, or both. Chemical attacks and fractures form problems in 
the realisation of a lamp with a reliable life expectancy. 
SUMMARY OF THE INVENTION 
The invention has for its object to provide a measure for counteracting the 
problems described. According to the invention, a lamp of the kind 
mentioned in the opening paragraph is for this purpose characterized in 
that the lamp has a rated power of at most 100 W, in that at least one 
electrode tip is situated substantially in the adjacent end face, and in 
that the relevant ceramic closing plug is fastened in the end wall portion 
in a gastight manner at a distance from the end face. 
It is found to be possible with the measure according to the invention to 
realise a lamp with an increased T.sub.KP. Since the ceramic closing plug 
does not extend up to the end face but is situated at a distance 
therefrom, problems involving chemical attacks and fractures are found to 
be solved. It is an advantage of the invention, accordingly, that a lamp 
with a very small tip-to-bottom distance can be realised. In an 
advantageous embodiment, the closing plug is fastened in the end wall 
portion in a gastight manner at a distance of 1 mm from the end face. The 
gastight fastening between the end wall portion and the closing plug is 
preferably realised by means of a sintered joint. This type of joint is in 
fact as resistant to high temperatures and attacks as are the ceramic wall 
portions themselves. 
To obtain a substantially straight arc in every burning position, the lamp 
according to the invention preferably complies with the relation 
##EQU3## 
The colour temperature of the light radiated by the lamp will then be 
substantially the same in all burning positions. 
Suitable metals for forming the metal halide in the discharge vessel are 
Na, Tl, Sc, Y, and the lanthanides. 
A further improvement of the lamp according to the invention can be 
realised in that the filling of the discharge vessel also comprises Mg in 
the form of a halide. This favourably affects the maintenance of a good 
luminous efficacy during lamp life. 
The filling of the discharge vessel comprises besides Hg and a rare gas one 
or several halides, usually iodides. A suitable rare gas is, for example, 
Ar which has an ignition-promoting effect. 
A high-pressure metal halide lamp with a ceramic discharge vessel is known 
per se from EP-A-0 011 993 with a narrowed portion at either end, where 
electrode tips lie substantially in one plane with the narrowed portion 
near the relevant electrode. The lamp, which has a power of at least 100 W 
and more, for example 150 W and 250 W, has a considerable interspacing 
between the electrode tips (2 cm), and as a necessary consequence a 
comparatively small diameter. This renders the lamp unsuitable for 
realising a colour temperature in the region between approximately 2500 K 
and 3500 K with at the same time a comparatively high luminous flux and a 
good colour rendering.

FIG. 1 shows a high-pressure metal halide lamp provided with a discharge 
vessel 3 with a ceramic wall which encloses a discharge space 11 and with 
a filling which comprises besides Hg and a rare gas at least one metal 
halide. The discharge vessel is enclosed in an outer envelope 1 which is 
provided with a lamp cap 2 at one end. The discharge vessel is provided 
with internal electrodes 4, 5 between which a discharge extends in the 
operational state of the lamp. Electrode 4 is connected to a first 
electrical contact forming part of the lamp cap 2 via a current conductor 
8. Electrode 5 is connected to a second electrical contact forming part of 
the lamp cap 2 via a current conductor 9. The discharge vessel, shown in 
more detail in FIG. 2 (not true to scale), has a ceramic wall and is 
formed from a cylindrical portion with an internal diameter ID closed off 
at either end by end wall portions 32a, 32b, each end wall portion 32a, 
32b forming an end face 33a, 33b of the discharge space. The end wall 
portions each have an opening in which a ceramic closing plug 34, 35 is 
fastened in the end wall portion 32a, 32b in a gastight manner by means of 
a sintered joint S. The ceramic closing plugs 34, 35 each narrowly enclose 
over a length 1 a lead-through 40, 41, 41a, 50, 51, 51a of an associated 
electrode 4, 5 provided with a tip 4b, 5b. The lead-through is connected 
to the closing plug 34, 35 in a gastight manner by means of a ceramic 
glazing joint 10 at its side facing away from the discharge space. The 
electrode tips 4b, 5b are situated at a mutual distance EA. The 
lead-throughs each comprise a halide-resistant portion 41, 51 made of, for 
example, Mo, enclosed by an Mo coil 41a, 51a, and a portion 40, 50 which 
is fastened to an associated closing plug 34, 35 in a gastight manner by 
means of the ceramic glazing joint 10. Each Mo coil 41a, 51a extends up to 
the relevant lead-through portion 40, 50. The ceramic glazing joint 
extends over some distance, for example approximately 1 mm, over the Mo 
coil 41a, 51. The portions 40, 50 are made of a metal which has a 
coefficient of expansion which harmonizes very well with that of the 
closing plugs. For example, Nb is a very suitable material. The portions 
40, 50 are connected to the current conductors 8, 9 in a manner not shown 
in detail. The lead-through construction described renders it possible to 
operate the lamp in any burning position as desired. 
Each electrode 4, 5 comprises an electrode rod 4a, 5a which is provided 
with a winding 4c, 5c near the tip 4b, 5b. The electrode tips lie 
substantially in the planes defined by the end faces 33a, 33b of the end 
wall portions. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The closing plugs do not extend up to the end faces but are fastened in the 
end wall portions in a gastight manner by means of a sintered joint S at a 
distance a from the end faces. 
In a practical embodiment of a lamp according to the invention as depicted 
in the drawing, the rated lamp power is 70 W. The filling of the discharge 
vessel is 4.4 mg Hg and 8 mg NaJ, TlJ, and (Dy+Ho+Tm)I.sub.3 in a mass 
ratio of 65:10:25. The lamp also contains Ar as an ignition gas. The lamp 
was designed to supply a colour temperature of 3000 K with colour point 
coordinates (x,y) (437,404) and a general colour rendering index Ra above 
80. 
The discharge vessel is made of polycrystalline aluminium oxide, has an 
internal diameter ID of 6.85 mm and an interspacing between the electrode 
tips EA of 7 mm. The closing plugs were sintered in the end wall portions 
at a distance a of 1 mm from the end faces formed by the end wall 
portions. The end wall portions have a height of 3 mm so that the sintered 
joint with the closing plugs extends over a length of 2 mm. Such a length 
of the sintered joint was found to be sufficient in practice for realising 
a sufficiently strong and gastight fastening between the end wall portion 
and the closing plug also in the case of large-scale mass production. The 
electrode tips lie in the end face planes. The electrodes are made from a 
W rod which is provided with a W winding at the tip. 
The lamp was subjected to a life test. The colour temperature of the light 
radiated by the lamp is 3150 K after one hour of operation, 3144 K after 
100 hours, and 3096 K after 1000 hours. The luminous efficacy after 100 
hours of operation is 88 lm/W, falling to 75 lm/W after 1000 hours of 
operation. The following colour point coordinates were measured for the 
light radiated by the lamp (x,y): (430,407); (431,410); (433,408). 
A comparison between vertical and horizontal burning positions was made for 
a similar lamp after 100 hours of operation. The luminous efficacy in 
horizontal position was 85 lm/W and in vertical burning position 88 lm/W. 
The accompanying T.sub.c, values and the coordinates of the colour point 
were 3013 K and 3096 K, and (437,405) and (431,410). The general colour 
rendering index R.sub.a was 82 in both cases. 
For comparison, the data measured for a prior-art lamp after 100 hours of 
operation: a luminous efficacy in horizontal position of 84 lm/W and in 
vertical position 88 lm/W. The accompanying T.sub.c values are 3033 K and 
3240 K, and the colour point coordinates are (431,396) and (423,404). A 
value of 82 was measured for the colour rendering index R.sub.a. 
It is apparent from these data that the differences in colour temperature 
T.sub.c and in the colour point coordinates resulting from differences in 
burning position of the lamp are much smaller in the lamp according to the 
invention than in the known lamp. Since the lamp is designed for use as an 
interior lighting lamp (for example, shop window lighting) this is a major 
importance.