Motor vehicle lighting/indicating apparatus using luminescent discharge

A lighting and/or indicating apparatus for a motor vehicle, of the type comprising an external light transmitting element and a base arranged behind the latter, typically consists of a lighting and indicating cluster. The light transmitting element comprises an outer panel, the base comprises an inner wall, and the two walls together constitute a thin gas-tight envelope and define at least one closed cavity. This cavity, or each cavity, contains two electrodes and a suitable gas under low pressure, the apparatus having means for applying voltage for starting and maintaining a luminescent gas discharge in the cavity, so that the envelope itself constitutes at least one light of the gas discharge type. The appearance and light output are improved, and the light beams emitted are highly homogeneous.

FIELD OF THE INVENTION 
The present invention relates, in general terms, to lighting and/or 
indicating (or signalling) apparatus for motor vehicles. More 
particularly, it is directed to a new concept for such an apparatus in the 
form of a multi-function lighting/indicating cluster, also referred to 
herein as a display unit. 
BACKGROUND OF THE INVENTION 
A lighting apparatus or indicating apparatus, for example apparatus for 
interior lighting, traditionally includes an opaque base plate which is 
provided with devices (lamp holders) at appropriate positions, for the 
fitting and electrical connection of one of more filament lamps. Such an 
apparatus also includes a light transmitting element in the form of a 
cover glass or globe which is fitted in front of the base plate 
(considered with respect to the direction in which light is emitted). The 
base plate and the cover glass may, conventionally, include 
light-controlling means for giving the beam or beams produced by the 
apparatus the required photometry. Such light-controlling means may for 
example be in the form of reflective elements on the base plate, or 
lenses, prisms or ribs on the cover glass. The apparatus may be arranged 
to emit one or more beams, and in particular signalling or indicating 
beams. 
However, such a conventional arrangement has a certain number of drawbacks, 
in particular where the apparatus consists of, or includes, indicating (or 
signalling) lights. This is true both from the optical point of view and 
from the aesthetic point of view. First of all, in the optical context, it 
is quite difficult, especially for indicating functions where the 
illuminated zone of the apparatus has a large area, to obtain a 
homogeneous light intensity over the whole of this area. In particular, 
those regions of this illuminating zone which are furthest away from the 
filament will in general receive a quantity of light per unit of surface 
area which is smaller than that received by the region which lies 
immediately in front of the lamp; the light intensity in the illuminated 
zone decreases from its centre towards its edges. 
In addition, from the aesthetic point of view, the light transmitting 
element (i.e. the cover glass) will be made in one or more colours (e.g. 
red, amber and so on), the colour being determined by the colour which the 
emitted beam is required to have. In addition, due to the fact that each 
indicating function may make it necessary to provide particular 
light-controlling means on the cover glass in order to give the emitted 
beam its required shape, the cover glass, when seen as a whole, usually 
looks extremely unhomogeneous when all the various lighting and/or 
indicating functions of the apparatus are extinguished. 
Again, conventional lighting or indicating apparatuses generally give only 
a rather mediocre light output. Besides all this, filament lamps give rise 
to a substantial amount of heat, which in turn leads to design problems, 
especially in respect of the choice and dimensioning of the materials of 
components which are close to these lamps. 
Finally, conventional lighting or indicating apparatus is relatively bulky, 
due especially to the fact that it is necessary to leave around the lamp a 
free space of significant size, mainly because of the heat which is 
produced as mentioned above. 
It is also known to use a discharge lamp in a motor vehicle headlamp. 
Discharge lamps are well known for their high light output. However, since 
such a lamp emits very intense radiation from a highly localised region, 
it is not itself suitable for overcoming the problems mentioned above. 
DISCUSSION OF THE INVENTION 
An object of the present invention is to overcome the disadvantages 
discussed above. 
According to the invention, lighting or indicating apparatus for a motor 
vehicle, of the type comprising a translucent element and a base disposed 
behind the translucent element, is characterised in that it includes: 
-- a thin gas-tight envelope which comprises an outer wall constituting the 
said translucent element, and an inner wall constituting the said base, 
the said walls together defining at least one closed cavity; 
-- a gas contained at low pressure in the said cavity; 
-- at least two electrodes located in the cavity; and 
-- means for applying between the electrodes a voltage for starting and 
maintaining a luminescent gas discharge in the cavity. 
Thus, in accordance with the present invention, the envelope of the 
apparatus, that is to say the equivalent of the combination of base plate 
and cover glass in the apparatus of the prior art, constitutes of itself a 
luminescent discharge cavity.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Referring to the drawings, these show an indicator display unit in which 
each indicator comprises essentially at least one envelope, defining 
within it a closed cavity. In the cavity at least two electrodes are 
disposed, these being connected to an appropriate high voltage source, and 
the envelope contains a gas or a mixture of gases under low pressure. The 
envelope preferably also constitutes the support for the indicator or 
group of indicators. 
The luminescent discharge which is caused to occur in the cavity when a 
voltage is applied across the electrodes is distributed in the cavity in 
an extremely homogeneous manner. It will be understood that it is 
therefore possible to make the indicators so that the illuminated area 
which they present is itself homogeneous. Besides this, the display unit 
now to be described offers numerous other advantages, for example (and in 
particular) an improved light output, the fact that the whole apparatus 
has a very small thickness, and the fact that it generates very little 
heat. 
Preferably, and as will be seen more clearly further on in this 
description, there are different cavities corresponding to different 
indicators, with each cavity being defined by common panel portions of the 
envelope, all of which are made integrally with each other. 
The indicator display unit shown in the drawings comprises a plurality of 
illuminated zones Z1 to Z6, the illuminating functions of which are for 
example as follows: 
zone Z1: amber direction indicator 
zone Z2: red stop light 
zone Z3: red position light (i.e. side light, parking light or rear light) 
zone Z4: red fog light 
zones Z5 and Z6: white reversing light. 
The envelope 100 of the display unit is defined essentially by an outer 
panel 110 and an inner panel (or base panel) 120 which extends parallel to 
the outer panel 100, from which it is spaced over the whole extent of the 
illuminated zones by an amount which is for example (and preferably) of 
the order of a few millimetres. 
The outer panel 110 has an inwardly directed flange, indicated at 111 in 
FIG. 2 and extending over the whole periphery of the panel. The inner 
panel 120 is secured to this flange, to which it is also sealed. 
In addition, the outer panel 110 has a set of inwardly directed ribs 110a 
to 110d (see FIG. 3), against which the inner panel 120 is also secured in 
a sealed manner. The rib 110a has for example a sinuous form, and defines 
the boundary between the flashing direction indicator light and the stop 
light. The rib 110b is essentially straight, and divides the stop light Z2 
from the position light Z3 and also from the zone Z5 of the reversing 
light. The rib 110c, together with part of the rib 110b, defines the 
illuminated zone Z5 itself. This latter is of generally quadrilateral 
shape, and occupies a height which is smaller than the total height of the 
display unit as a whole at this point. Similarly, the ribs 110d and 110e 
are joined together, so as firstly to define the illuminated zone Z6 of 
the white reversing light, and secondly to separate the position light Z3 
from the fog light Z4. 
Thus the panel 110, with its peripheral flange and its ribs, together with 
the base panel 120, define a plurality of closed spaces or cavities E1 to 
E6, which correspond to the illuminated zones Z1 to Z6 respectively. Each 
of these cavities contains at least two electrodes, which may be of very 
diverse shapes and dimensions, as will be seen later on in this 
description. These electrodes are indicated by the reference numerals 201 
to 206 for the cavities E1 to E6 respectively. The electrodes are 
connected in the usual way to ballasts 300, which are arranged to cause 
illumination to take place and to maintain illumination of the various 
indicating functions. 
The ballasts may be disposed in any appropriate location. In a first 
embodiment, they can be mounted behind the display unit within the 
bodywork of the vehicle. In another embodiment, the inner panel 120 may be 
so designed as to define at least partially a specific cavity which 
contains the ballasts. In this latter case, the display unit is in the 
form of a monobloc unit which includes the ballasts. 
As has been indicated, each individual indicator includes, by way of light 
source, a luminescent discharge which is produced throughout the whole of 
its cavity E. It follows that this gives rise to a homogeneous 
illumination of the relevant zone Z. 
The gas or mixture of gases under low pressure contained in each cavity 
E1-E6 of the display unit is so selected that the luminescent discharge 
which is obtained operates directly or indirectly within the range of 
spectral wavelengths of colour required by the appropriate regulations for 
the particular lighting or indicating function concerned (these being 
typically red, amber or white). In order to obtain the colours red and 
amber, the gas may be so chosen that the luminescence gives the required 
colour directly. 
However, it is difficult to obtain a white luminescent discharge. In this 
case, it is possible to provide in the cavity concerned (which in this 
example is the cavity E6 for the reversing light corresponding to the 
illuminated zone Z6) a fluorescent alkaline earth compound which reacts 
with the light discharge that occurs with a gas such as argon, so as to 
emit white radiation in the way which is well known for fluorescent tubes. 
Another way to obtain the colour white consists (as is illustrated for the 
zone Z5) in using three superimposed cavities, each containing a 
respective gas; these gases are so chosen that they emit light of the 
wavelengths of the three primary additive colours red, green and blue. The 
illuminated zone Z5 thereby, by additive synthesis, emits white light. 
This arrangement in which there are three cavities behind the illuminated 
zone Z5 is shown more clearly in FIG. 3b, to which reference is now made, 
and in which the cavities are indicated at E5, E5' and E5" respectively, 
the last mentioned being the one furthest away from the outer panel 110. 
The cavity E5 is defined between the main panels 110 and 120 and the ribs 
110b and 110c. The other two cavities E5' and E5" are defined by auxiliary 
panels 130, 140 and 150 which are carried on the back, or inner side, of 
the envelope 100 defined by the main panels 110 and 120. More precisely, 
one auxiliary panel 130, corresponding in size to the illuminated zone Z5, 
has an inwardly directed peripheral flange 130a, against which the other 
auxiliary panel 140 is fixed. The panel 110 has the same dimensions as the 
panel 130, and defines the middle cavity E5'. The panel 140 again has an 
inwardly directed peripheral flange, 140a, against which the innermost or 
terminal panel 150 is secured so as to define the rearmost cavity E5". The 
electrodes 205', 205" are disposed in these cavities. 
Although FIG. 1 shows two zones Z5 and Z6, by way of example, in the same 
indicator (namely in this case the reversing light), emitting white light 
in two different ways, it will of course be understood that it is 
preferable to use the same technique for producing white light in each 
part of the indicator where the latter has more than one part. 
One advantage of the additive synthesis solution for producing which light, 
as described above, lies in the fact that the light intensities of the 
three luminescent discharges are cumulative, so that the light which is 
emitted is particularly intense, as is indeed called for by the 
regulations. In addition, this design having superimposed cavities can 
also be used when any given lighting or indicating function (for example 
that of a stop light) is required to emit light of high intensity. For 
example, it is possible to superimpose on the cavity E2 which defines the 
red illuminated zone Z2 of the stop light, an additional cavity which is 
arranged again to emit red light, thus doubling the quantity of light 
produced through the zone Z2. 
The outer panel 110 is of course made of a transparent material, or if 
desired a coloured translucent material, so as to allow the light 
generated to pass to the outside. As to the inner panel 120, this can also 
be transparent or translucent, but it may also be given any desired 
colour, and may for example be self-coloured, or coloured by varnishing, 
painting or in any other suitable way. Thus, when a light-emitting 
function is not activated, the low pressure gas being uncoloured, it is 
the colour of the base panel 120 which is visible. This coloration of the 
base panel cannot be seen when the corresponding zone of the display unit 
is emitting light. 
In addition, it is possible to design the internal surface of the base 
panel 120 in such a way that it performs a particular optical function. 
FIG. 3d, to which reference is now made, illustrates an example of this. 
In FIG. 3d, three catadioptric trihedra 121 are formed integrally with the 
base panel 120 in the cavity E2 of the stop light. The display unit is in 
this way given, when extinguished, the catadioptric function which is 
required by the regulations. These elements 121 are preferably provided 
with a reflective coating such as vacuum metallisation, in order to 
produce the required catadioptric retro-reflection. When the outer panel 
110 in line with the elements 121 is uncoloured, this reflective coating 
can also be given the required colour, which is generally red or amber. 
In a modification, suitable catadioptric elements may be provided on the 
inner face of the outer panel 110. 
Optical reflectors may also be provided on the inner face of the base panel 
120. FIG. 3e shows such reflectors at 122. These reflectors 122 are 
adapted to improve the light output for the lighting or indicating 
function concerned, by directing the light emitted by the luminescent 
discharge outwardly through the front panel 110. These reflectors 122 are 
concave as shown, and may also contribute to the reinforcement of the 
corresponding indicating beam along the main emission axis, which is 
generally parallel to the longitudinal axis of the vehicle. 
The panels 110 and 120, and if appropriate the auxiliary panels 130, 140 
and 150, may be made of glass, for example by moulding, pressing or even 
blowing. However, they are preferably made of a thermoplastic material, 
for example by high pressure or low pressure injection, or by extrusion or 
extrusion-blow methods. Other materials that may be used include, in 
particular, methyl polymethacrylate, polycarbonate, polystyrene, or 
mixtures of these plastics materials. Modified or grafted plastics 
materials may also be used, for example those used in connection with food 
storage, these materials being well known for their excellent gas sealing 
properties. 
It is however also possible, of course, to use thermosetting plastics 
materials, in particular for those parts of the envelope which are not 
required to transmit visible light. 
The envelope of the display unit may be made either as one piece, or, as 
shown, in the form of two main components or, in more general terms, two 
half shells which are secured together in a sealed manner. They can be 
secured together by one of the following conventional methods: hot mirror 
welding, vibration or ultrasonic welding, or adhesive bonding. The 
assembly does of course have to be perfectly sealed under vacuum 
conditions. 
The optical elements 121, 122 described above are preferably made 
integrally at the time of forming the base panel 120. 
The electrodes situated in the respective cavities are preferably cold 
cathodes, although hot cathodes can also be suitable, the cathodes being 
adapted as necessary to the requirements of the particular functions they 
are to perform. The cold cathodes are preferably made of copper, iron or 
silver, or of alloys based on these metals. 
Referring once again to the drawings, the electrodes may be given a variety 
of shapes, dimensions and positions. In the case of the cavities E1, E2, 
E3 and E5--E5", the corresponding electrodes 201, 202, 203 and 205--205" 
are in the form of bands, the width of which is substantially equal to the 
thickness of the cavities in question. These bands are carried vertically 
on the two opposed side edges of the cavities, namely on the side faces in 
facing relationship with the corresponding ribs 110a to 110d. In a 
modification, it is of course possible to arrange them horizontally on the 
respective upper and lower edges of the cavities. 
By way of non-limiting example, if the thickness of the cavities is of the 
order of 2 mm, the width of the band-shaped electrodes may be about 2 mm, 
with a thickness in the range 0.2 to 0.4 mm and a length in the range 50 
to 100 mm, this length being chosen in particular as a function of the 
size of the associated cavity and the length of its opposed edges. 
In addition, in the case of a cavity having a sinuous contour (such as the 
cavity E1), one of the electrodes 201 may match the shape of the 
separating rib 110a, extending along the whole of the latter, while the 
opposed electrode occupies a substantial part of the height of the display 
unit along the periphery of the front panel 110. 
In addition, a pair of electrodes may be provided with one of the 
electrodes in the form of a band, or again with one electrode being in the 
form of a band and the other in the form of a point. This arrangement is 
illustrated in FIG. 3b, in which the opposed electrodes are indicated at 
205". In this case, the point effect is beneficial in that it facilitates 
initiation of the gas discharge. 
In another modification which is illustrated in respect to the cavity E5', 
each electrode consists of one or more points 205'. 
The panels with their various electrodes may be made by moulding over the 
metallic electrodes, the latter having been previously placed in position 
and held in the mould. 
In a variant, the electrodes may be applied on the panels after the latter 
have been made, or may again be made by selective deposition of material. 
In this last case, the depositions will be suitably localised by the use 
of masks, and will be carried out by low pressure application of a metal 
or an alloy, typically by vacuum evaporation, sublimation or cathodic 
powder coating. Conventional techniques for reactive evaporation or 
reactive powder deposition may also be used. 
In another arrangement, electrodes are formed on the internal surfaces in 
facing relationship with the panels 110 and 120. More precisely, and 
referring now to FIG. 3d in this connection, the two electrodes 206 which 
are situated in the cavity E6 are formed over the whole area of the panels 
110 and 120 corresponding to the illuminated zone Z6. The electrode on the 
panel 120 (i.e. the base electrode) may be opaque or transparent. By 
contrast, the electrode formed on the outer panel 110 is transparent so as 
to allow the emitted light to pass through that panel. This electrode is 
preferably made by reactive powder deposition of indium and tin with a gas 
containing oxygen, so as to produce a transparent conductive deposit of 
indium oxide and tin oxide (ITO). This method of deposition is well known 
per se. 
In addition, for the cavity or cavities which are to emit white light by 
fluorescence, the fluorescent substances are preferably deposited by the 
use of one of the known techniques for physical deposition in the vapour 
phase, such as those mentioned above. 
The method which is most preferred is to make all of the panels 110, 120 
and (if provided) 130, 140 and 150 of a transparent plastics material, 
with the faces of these panels being essentially smooth, and with the 
electrodes being made of a transparent conductive material (e.g. layers of 
indium oxide and tin oxide as described above). In this way an indicator 
display unit is obtained which leaves the part of the bodywork C (FIG. 2) 
of the vehicle that lies behind the display unit actually visible. The 
rear of the vehicle thus appears in a colour which is essentially 
homogeneous and uniform over its whole width when the lighting and 
indicating functions are not in use. 
The present invention is of course not in any way limited to the embodiment 
described above and shown in the drawings: the person skilled in this 
technical field will be able to conceive of any variant or modification 
within the spirit of the invention. In particular, the various features of 
the present invention enable a display unit in the form of a lighting and 
indicating cluster for the rear of a vehicle to be provided with a side 
position light and/or a side direction indicator repeater light. In an 
industrial vehicle, it is similarly possible to provide a clearance 
indicating light or a beacon. 
Other lighting functions which can be provided in this way include a light 
source for the cabin of the vehicle, a high level stop light, and so on. 
In this latter case, the sealed envelope is preferably made in a shape 
which is adapted for any possible inclination of the rear window of the 
vehicle, so that its light output window extends preferably parallel to 
the plane of the rear window of the vehicle, and close to the latter.