Discharge lamp and lighting system having a discharge lamp

The invention relates to a discharge lamp whose discharge vessel (1) is pided with a light-transmitting, electrically conductive layer (4) in order to improve the electromagnetic compatibility of the lamp when it is operated from an electronic operating unit. The light-transmitting, electrically conductive layer (4) is advantageously connected to the circuitry-internal ground potential of the operating unit.

The invention relates to a discharge lamp in accordance with the preamble 
of patent claim 1, and also to a lighting system having a discharge lamp. 
BACKGROUND ART 
A discharge lamp of this type is disclosed for example in the U.S. Pat. No. 
5,420,481. This patent specification describes a discharge lamp which has 
outer electrodes fitted on its discharge vessel, said outer electrodes 
being designed as transparent ITO layers. 
The European Patent Specification EP 0 334 208 describes a discharge lamp 
which is arranged in a reflector and whose discharge vessel is surrounded 
by a cylindrical vitreous heat accumulation tube. The heat accumulation 
tube is provided with an ITO layer in order to reduce the color 
temperature of the lamp by approximately 600 kelvin. 
The abovementioned discharge lamps have the disadvantage that their 
operation from an electronic operating unit, which usually feeds the lamp 
with a medium-frequency supply voltage in the range of from approximately 
20 kHz to 100 kHz, can interfere with the reception of radio sets. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a discharge lamp which avoids the 
disadvantages of the prior art. This object is achieved according to the 
invention by means of the defining features of patent claim 1. 
Particularly advantageous embodiments of the invention are described in 
the subclaims. 
The discharge lamp according to the invention has at least one 
light-transmitting lamp vessel enclosing the discharge space of the 
discharge lamp, a luminous means and electrical terminals for supplying it 
with voltage. According to the invention, the at least one lamp vessel has 
a light-transmitting electrically conductive layer enclosing at least the 
discharge space of the lamp. The discharge space is in this case 
understood to mean only that part of the interior space of the at least 
one lamp vessel which is effective for the gas discharge in the lamp. 
Therefore, the coating according to the invention extends at least over 
those parts of the at least one lamp vessel which enclose the discharge 
plasma. As a result of the at least one lamp vessel being coated according 
to the invention, the medium-frequency electromagnetic radiation emitted 
by the discharge plasma enclosed in the lamp vessel is attenuated by more 
than 50 decibels in the case where the discharge lamp is operated from a 
medium-frequency AC voltage. Interference with the radio reception does 
not take place, therefore, even when the discharge lamp according to the 
invention is operated from an electronic operating unit in proximity to 
the antenna of a radio receiver. 
For reasons of production engineering, the light-transmitting, electrically 
conductive layer is advantageously applied on the outer surface of the at 
least one lamp vessel. In order to ensure satisfactory electromagnetic 
compatibility of the discharge lamp according to the invention, the 
surface resistivity of the light-transmitting, electrically conductive 
layer is advantageously less than 100 ohms per square. 
The surface resistivity of an electrically conductive layer is usually 
measured with the aid of two extensive electrodes which are applied on the 
layer to be measured such that they are arranged opposite one another. The 
distance between the two measuring electrodes is identical to the width of 
the measuring electrodes, with the result that a square patch of the layer 
to be measured is arranged between the two measuring electrodes. A current 
of predetermined current intensity is impressed on the layer via the 
measuring electrodes and the voltage drop across the measuring electrodes 
is determined by means of a galvanometer. The quotient of the measured 
voltage drop and the current intensity of the impressed current yields the 
surface resistivity of the layer to be measured. The surface resistivity 
of the layer is independent of the size of the square areal patch of the 
layer. It depends only on the quotient of the electrical resistivity of 
the layer material and the layer thickness. The unit of surface 
resistivity is usually denoted by ohms per square. 
The light-transmitting, electrically conductive layer is advantageously 
designed as an ITO layer, that is to say as an indium tin oxide layer. The 
particularly preferred exemplary embodiment of the invention concerns a 
discharge lamp which emits predominantly yellow, orange or red light. 
Therefore, the layer thickness of the light-transmitting, electrically 
conductive layer is advantageously chosen such that the coated lamp vessel 
has the highest possible transparency, that is to say a transmission 
coefficient of greater than 0.8, in the wavelength range of from 550 nm to 
700 nm. This is because the thickness of the light-transmitting, 
electrically conductive layer must be large enough to ensure a sufficient 
electrical conductivity, on the one hand, but also be small enough that it 
still exhibits sufficient light transmission, on the other hand. In 
accordance with the particularly preferred exemplary embodiment of the 
invention, the discharge lamp is designed as a neon gas discharge lamp. 
This lamp produces predominantly orange or red light. Therefore, it may 
advantageously be used as part of a lighting system in a motor vehicle, 
for the purpose of producing the flashing light or the rear and/or brake 
light. 
The lighting system according to the invention has a discharge lamp and an 
operating unit for the discharge lamp, the discharge lamp having at least 
one lamp vessel which encloses the discharge space of the lamp and is 
provided with a light-transmitting, electrically conductive layer, this 
layer extending at least across the discharge space and being connected to 
a predetermined electrical reference-ground potential, which is 
advantageously the circuitry-internal ground potential of the operating 
unit 20 or the ground potential. The abovementioned features of the 
lighting system according to the invention ensure its satisfactory 
electromagnetic compatibility since the medium-frequency electromagnetic 
radiation emitted by the discharge plasma of the discharge lamp is 
attenuated by more than 50 decibels. It is advantageous for the lighting 
system according to the invention additionally to have a reflector. In 
order to obtain a high degree of light reflection, the reflector of the 
lighting system according to the invention advantageously has a metallic 
or metallized reflecting surface. Therefore, the reflector likewise has a 
shielding effect on the electromagnetic radiation generated by the 
discharge plasma in the discharge lamp. It has proved to be particularly 
advantageous to likewise connect the reflector and possibly metallized 
parts of the luminaire housing to the predetermined electrical 
reference-ground potential in order to improve the shielding. As a result, 
the layer thickness of the light-transmitting, electrically conductive 
layer on the wall regions of the at least one lamp vessel which face the 
reflector or the inner space of the luminaire may advantageously be made 
smaller than that on the wall regions of the at least one lamp vessel 
which are remote from the reflector, and, in this way, the light 
transmission of the wall regions facing the reflector can be increased and 
the efficiency of the lighting system according to the invention improved. 
The at least one lamp vessel advantageously has a cylindrical vessel part 
and two ends angled away in the direction of the reflector. This ensures 
that the dark ends of the discharge lamp which are equipped with the 
electrical terminals of the lamp are not visible. As an alternative, the 
dark ends of the discharge lamp may also be installed in shaded regions of 
the lighting apparatus. 
In the particularly preferred exemplary embodiment of the lighting system 
according to the invention, the layer thickness of the light-transmitting, 
electrically conductive layer on the wall regions of the at least one lamp 
vessel which are remote from the reflector is 300 nm. As a result, these 
wall regions have particularly high transparency to light with a 
wavelength of 600 nm. Therefore, this lighting system is advantageously 
suitable for use in a motor vehicle for the purpose of producing the rear 
light and/or the brake light.

The preferred exemplary embodiment of the invention represented in FIG. 1 
concerns a neon gas discharge lamp. This lamp has a tubular, vitreous 
discharge vessel 1 having two ends la which are angled away at right 
angles in the same direction. An electrode system 2 of the neon gas 
discharge lamp is sealed into each of the ends 1a in a gastight manner. 
The power supply leads 2a projecting from the sealing-in region 1aa form 
the electrical terminals of the lamp. The discharge vessel 1 has a 
circular-cylindrical configuration between its angled-away ends 1a. The 
external diameter of the discharge vessel 1 is approximately 5 mm. The 
distance between the power supply leads 2a corresponds approximately to 
the length of the circular-cylindrical discharge vessel part 1b and is 308 
mm. The angled-away ends 1a have a length of 36.2 mm. 
The outer surface of the discharge vessel 1 is provided with a so-called 
ITO layer 4--that is to say an indium tin oxide layer--which extends 
across the entire discharge space 3 of the neon gas discharge lamp, as far 
as the sealing-in regions 1aa of the electrodes 2. The discharge space 3 
is in this case defined by the discharge-side ends of the two electrodes 2 
and the internal diameter of the discharge vessel 1. The ITO layer 4 has a 
surface resistivity of 14 ohms per square, as measured by the method of 
four-point measurement. It comprises 90 percent by weight of indium oxide 
In.sub.2 O.sub.3 and 10 percent by weight of tin oxide SnO.sub.2. The 
transmission curve 1 shows the light transmission of the discharge vessel 
1 with ITO layer 4 as a function of the wavelength, while the transmission 
curve 2 shows the light transmission of the discharge vessel without an 
ITO layer. The layer thickness of the ITO layer is coordinated in such a 
way that the transmission curve 1 has a transmission maximum at a 
wavelength of 600 nm, that is to say for red light, which is predominantly 
emitted by the neon gas discharge lamp. The layer thickness of the ITO 
layer 4 is therefore approximately 300 nm. In the wavelength range of from 
550 nm to 700 nm, the transmission of the coated lamp vessel 1 is more 
than 80% of the light impinging on the inner wall of the discharge vessel 
1, that is to say the transmission coefficient is greater than 0.8 in this 
wavelength range. A transmission coefficient of more than 0.85 is achieved 
at the wavelength of 600 nm. 
The neon gas discharge lamp described above is preferably part of a 
lighting system, in particular of a motor vehicle rear luminaire, and 
serves for producing a rear light and/or or a brake light. In addition to 
the neon gas discharge lamp, this rear luminaire also comprises an 
electronic operating unit 20 for the neon gas discharge lamp and a 
groove-shaped reflector 5 arranged between the angled-away ends 1a of the 
lamp. The circular-cylindrical vessel part 1b of the discharge vessel 1 is 
arranged approximately in the optical axis of the reflector 5. The 
reflecting surface 5a of the reflector 5, which surface faces the lamp, is 
metallic or metallized and connected to the circuitry-internal ground 
potential of the operating unit. The ITO layer 4 of the discharge vessel 1 
is likewise connected to the circuitry-internal ground potential of the 
operating unit. The lighting apparatus also has a housing (not 
represented) whose metallized parts are likewise connected to the 
circuitry-internal ground potential, with the result that star contact is 
made at a common ground point. The ITO layer 4 has a smaller layer 
thickness on the wall regions 10a of the discharge vessel 1 which face the 
reflector 5 than on the wall regions 10b of the discharge vessel 1 which 
are remote from the reflector 5. The layer thickness of the ITO layer 4 
has a value of approximately 300 nm on the wall regions 10b remote from 
the reflector 5, while it measures approximately 100 nm on the wall 
regions 10a facing the reflector 5. 
The invention is not restricted to the exemplary embodiment explained in 
more detail above. By way of example, the ITO layer 4 need not extend 
across the entire discharge vessel 1. It is enough to provide those wall 
regions of the discharge vessel 1 which enclose the space between the 
discharge-side ends of the two electrodes 2 with the ITO layer 4. 
The invention can also be applied to other types of discharge lamps, for 
example to low-pressure discharge lamps or to high-pressure discharge 
lamps and to lighting systems having a high-pressure discharge lamp such 
as, for example, a motor vehicle headlight furnished with a high-pressure 
discharge lamp. What is concerned, in particular, in this case is a 
high-pressure discharge lamp having a base at one end and having a 
discharge vessel enclosed by a vitreous outer bulb, the outer bulb being 
provided with a light-transmitting, electrically conductive 
layer--preferably an ITO layer--extending across the entire discharge 
space of the lamp. The high-pressure discharge lamp is preferably part of 
a motor vehicle headlight and is operated from an electronic operating 
unit. The light-transmitting, electrically conductive layer on the outer 
bulb of the high-pressure discharge lamp is connected to the 
circuitry-internal ground potential of the operating unit. 
Instead of an ITO layer, it is also possible to use light-transmitting, 
electrically conductive layers which are composed of a different material, 
for example of tin oxide SnO.sub.2 or of fluorine- or antimony-doped tin 
oxide SnO.sub.2 :F or SnO.sub.2 :Sb, respectively.