Neon discharge lamp

A lighting unit includes a discharge vessel having a wall provided with a first luminescent layer, a gas fill consisting essentially of neon at a pressure less than 30 mbar, and a means for providing a constant DC current for exciting the fill in the discharge vessel to emit UV light for exciting the luminescent layer. DC operation offers substantially higher luminous flux over AC operation. The lighting unit may further include a housing having a reflective surface, a second luminescent layer, and a filter incorporated within the discharge vessel.

BACKGROUND OF THE INVENTION 
The invention relates to a lighting unit comprising 
a discharge lamp provided with a gas filling comprising mainly neon, 
a housing having a reflecting surface, and 
means for positioning the discharge lamp in the housing. 
The invention also relates to a discharge lamp suitable for use in such a 
lighting unit. 
A discharge lamp provided with a gas filling comprising mainly neon is here 
understood to be a discharge lamp having a gas filling composition 
comprising neon such that red light is generated in the plasma during 
stationary lamp operation whose colour point in the C.I.E. chromaticity 
diagram lies within the region bounded by the lines y=0.300, y=0.350, 
y=-x+1, and y=-x+0.99. 
A lighting unit as indicated above is known from European Patent EP 
0562679. The lighting unit described in this European Patent is very 
suitable for acting as a stop light on or in a vehicle. The gas filling of 
the known discharge lamp comprises exclusively neon, and the life of such 
a lighting unit is long compared with traditional stop lights in which 
incandescent lamps are used. It is also possible to give the lighting unit 
a comparatively flat shape through a suitable choice of the dimensions of 
the discharge lamp. This flat shape increases the application 
possibilities of the lighting unit considerably because such a shape can 
be used in combination with, for example, a very wide variety of shapes of 
the portion of a vehicle on or in which the lighting unit is accommodated. 
Further advantages of the known lighting unit over traditional stoplights 
are the comparatively high luminous efficacy (1 m/W) and the fact that the 
discharge lamp emits light comparatively soon after the brake pedal has 
been operated, even at comparatively low temperatures. A disadvantage of 
the known lighting unit is the fact that the discharge lamp generates red 
light, which renders the lighting unit less suitable or unsuitable for a 
large number of applications. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a lighting unit which has the 
advantageous properties described above but whose application 
possibilities are considerably increased. 
According to the invention, a lighting unit as mentioned in the opening 
paragraph is for this purpose characterized in that a wall of the 
discharge lamp is provided with a luminescent screen comprising a first 
luminescent layer. 
Operation of the discharge lamp generates besides the visible red light 
emitted by the lamp also short-wave ultraviolet light. This short-wave 
ultraviolet light is converted into visible light by the first luminescent 
layer. The total quantity of visible light generated by the discharge lamp 
is now composed of the visible light generated in the lamp plasma and the 
visible light generated from the short-wave ultraviolet radiation by the 
first luminescent layer. The colour of the total quantity of visible light 
generated by the lamp may be adjusted through a suitable choice of the 
composition of the first luminescent layer. It is possible to generate 
light of widely differing colours with a comparatively high luminous 
efficacy with the use of a comparatively simple first luminescent layer 
which generates no or only a small quantity of red light. The red light 
generated by the discharge lamp in this case is exclusively or 
substantially exclusively that originating from the plasma. Since part of 
the short-wave ultraviolet light that is generated in the plasma is 
converted into visible light, the luminous efficacy of the lighting unit 
according to the invention is considerably higher than that of the known 
lighting unit. 
If the lighting unit is designed, for example, for use as a signal light 
source, such as a direction indicator on a vehicle, for example a 
motorcar, it is desirable for the lighting unit to radiate amber light 
when energized. Such a lighting unit is also very suitable for serving, 
for example, as a traffic light, as a backlight of an LCD screen, or for 
use in reprographic applications and image scanners. This amber light can 
be realised in that the first luminescent layer comprises a 
green-luminescing substance. Zinc silicate activated by bivalent 
manganese, yttrium-aluminium garnet activated by trivalent cerium, and 
yttrium silicate activated by trivalent terbium were found to be suitable 
for this application. More in particular, it was found to be possible with 
the use of yttrium-aluminium garnet activated by trivalent cerium to 
realise a discharge lamp with a comparatively high luminous efficacy whose 
colour point complies with the E.C.E. requirement for direction indicator 
lights to be used in/on a motorcar without the use of a filter. According 
to this requirement, the colour point of the radiated light must lie 
within a region in the I.E.C chromaticity diagram bounded by the lines 
y=0.429, y=0.398, y=-x+1, and y=-x+0.993. 
More in particular satisfactory results were obtained in case the 
green-luminescing substance yttrium-aluminium garnet activated by 
trivalent cerium is of general formula Y.sub.3-x Al.sub.5 O.sub.12 
:xCe.sup.3+, wherein 0.01.ltoreq.x.ltoreq.0.20 preferably 
0.02.ltoreq.x.ltoreq.0.10. 
It is noted that part of the aluminium in the yttrium-aluminium garnet 
activated by trivalent cerium may be replaced by gallium and/or scandium 
as described in European Patent EP 124175. If for instance half of the 
aluminium is replaced by gallium a luminescing substance of general 
formula Y.sub.3-x Al.sub.2.5 Ga.sub.2.5 O.sub.12 :xCe.sup.3+ is obtained. 
The colour point of this luminescing substance has a lower x-value than 
the colour point of Y.sub.3-x Al.sub.5 O.sub.12 :xCe.sup.3+. It was found 
that in case the first luminescent layer of the discharge lamp consists 
substantially of Y.sub.3-x Al.sub.2.5 Ga.sub.2.5 O.sub.12 :xCe.sup.3+ the 
visible light radiated by the discharge lamp was almost white. 
In case yttrium silicate activated by trivalent terbium was used in the 
first luminescent layer it was necessary to incorporate a filter in order 
to meet the E.C.E. requirements for indicator lights mentioned hereabove. 
The filter is used to filter blue light radiated by the first luminescent 
layer. Satisfactory results were obtained using short wavelength blocking 
filters having 50% transmission at a wavelength within the range 450-550 
nm. 
Instead of using a filter in a lamp containing yttrium silicate activated 
by trivalent terbium in the first luminescent layer, it is also possible 
to meet the E.C.E requirements for indicator lights in case the 
luminescent screen comprises a second luminescent layer present between 
the first luminescent layer and the wall of the discharge vessel, said 
second luminescent layer comprising the green-luminescing substance 
yttrium-aluminium garnet activated by trivalent cerium. In this 
luminescent screen the green-luminescing substance yttrium-aluminium 
garnet activated by trivalent cerium in the second luminescent layer 
absorbs blue radiation generated by the yttrium silicate activated by 
trivalent terbium in the first luminescent layer. An important advantage 
of this composition of the luminescent screen is that the colour point of 
the light radiated by the discharge lamp can be adjusted over a relatively 
wide range within the area indicated hereabove within which the colour 
point meets the E.C.E requirements for indicator lights. Another important 
advantage of this composition of the luminescent screen is that it is 
possible to increase the amplitude of the lamp current and thereby the 
light output of the discharge lamp to a relatively high value while the 
colour point of the light radiated by the discharge lamp still meets the 
E.C.E requirements for indicator lights. 
Of the yttrium-aluminium garnet activated by trivalent cerium present in 
this second luminescent layer part of the aluminium can be replaced by 
gallium and/or scandium. 
In case the first luminescent layer comprises a blue-luminescing substance 
but no other luminescing substances the visible light radiated by the 
discharge lamp is a mix of the red light generated in the plasma and the 
blue light generated by the first luminescent layer. By a proper choice of 
the blue-luminescing substance it is possible to adjust the colour of this 
light over a wide range to suit a range of applications, while the 
luminescent layer as well as the gas filling of the discharge lamp are of 
a very simple composition. 
In many applications it is desirable that the colour of the light radiated 
by the discharge lamp is white or nearly white. It was found to be 
possible to cause the colour of the light radiated by the discharge lamp 
to be white or nearly white in case the first luminescent layer comprises 
a green-luminescing substance and a blue-luminescing substance. Lighting 
units with a comparatively high luminous efficacy can be realised also in 
this case, the luminescent layer generating no or only a very small 
quantity of red light. Since the luminescent layer need not comprise a 
substance which luminesces in red, this layer may be of a comparatively 
simple composition. Very satisfactory results have been obtained in case 
the blue-luminescing substance comprises one or more of the luminescent 
materials belonging to the group formed by MgWO.sub.4, Y.sub.2-x O.sub.2 
S:xTb.sup.3+ and Y.sub.2-x SiO.sub.5 :xCe.sup.3+. 
It has been found that, in case the pressure of the gas filling of the 
discharge lamp at ambient temperature is less than 30 mbar and the inner 
diameter of the lamp vessel is smaller than 5 mm, DC-operation of the 
discharge lamp wherein the lamp current is a DC-current of substantially 
constant amplitude surprisingly produces enough short-wave ultraviolet 
light to excite the luminescent substances in the luminescent screen. 
Important advantages of such a DC-operation are that it can be realized 
with relatively simple and inexpensive means and causes no or only a 
relatively small amount of electromagnetic interference.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows the C.I.E. chromaticity diagram. The x coordinate of the 
colour point is plotted on the horizontal axis, the y coordinate of the 
colour point on the vertical axis. The reference word "neon" indicates the 
colour point (0.666, 0.332) of the red light radiated by a DC operated 
discharge lamp whose gas filling consists of neon. The colour point 
(0.440, 0.543) of the green light radiated by the luminescent material 
yttrium-aluminium garnet activated by trivalent cerium is indicated with 
YAG:Ce, this luminescent material being excited by ultraviolet radiation. 
The colour point (0.235, 0.705) of the green light radiated by the 
luminescent material zinc silicate activated by bivalent manganese is 
indicated with "willemite", this luminescent material being excited by 
ultraviolet radiation. Similarly YST indicates the colour point (0.341, 
0.586) of the light radiated by yttrium silicate activated by trivalent 
terbium. YAGAG:Ce indicates the colour point (0.328, 0.563) of the light 
radiated by yttrium-aluminium-gallium garnet activated by trivalent 
cerium. The molar quantity of alumminium is approximately equal to the 
amount of gallium. MgWO.sub.4 indicates the colour point (0.222, 0.309) of 
the light radiated by MgWO.sub.4 and YSC indicates the colour point 
(0.180, 0.210) of the light radiated by yttrium silicate activated by 
trivalent cerium. 
Colour points of the light radiated by discharge lamps provided with a gas 
filling that substantially consists of neon and comprising a luminescent 
layer containing one or more luminescent materials are indicated by 
reference numerals 1-8. These colour points will be further referred to as 
a colour point of a discharge lamp. The filling pressure of the neon was 
15 mbar and the discharge lamps were operated with a DC current of 5 mA. 
The inner diameter of the discharge lamps was 2.5 mm and the length of the 
discharge vessel was 40 cm. The colour point (0.593, 0.396) of a discharge 
lamp provided with a neon gas filling and a luminescent layer comprising 
zinc silicate activated by bivalent manganese is indicated with 1. The 
colour point of such a discharge lamp is determined by the red light 
directly generated in the plasma of the discharge lamp and the green light 
obtained by means of the luminescent material, and lies on a straight line 
interconnecting the colour points "neon" and "willemite". The exact 
position of the colour point 1 on this line follows from the ratio in 
which green light and red light are mixed by the discharge lamp. This 
ratio is influenced, for example, by the filling pressure of the neon gas 
present in the discharge lamp and the current flowing through the 
discharge lamp during lamp operation. The colour point 1 corresponds to 
amber light so that a lighting unit containing such a discharge lamp is 
very suitable, for example, for use as a direction indicator on a vehicle 
such as a motorcar. In a similar manner, it is possible to manufacture 
discharge lamps with a gas filling consisting of neon whose colour points 
lie on a straight line between the colour points "neon" and YAG:Ce through 
the use of the luminescent material yttrium-aluminium garnet activated by 
trivalent cerium in the luminescent layer. Colour point 2 (0.590, 0.400) 
is the colour point of such a discharge lamp. Since colour point 2 has a 
somewhat higher y-value than colour point 1 it is easier to meet the 
E.C.E. requirements for direction indicator lights, when use is made of 
yttrium-aluminium garnet activated by trivalent cerium than when use is 
made of zinc silicate activated by bivalent manganese. Similarly colour 
points 3 (0.525, 0.438), 4 (0.497, 0.443), 7 (0.503, 0.328) and 8 (0.533, 
0.301) are the colour points of discharge lamps comprising a luminescent 
layer that contains the luminescent material that lies on the line through 
the colour point "neon" and the colour point of the discharge lamp. Colour 
point 5 is the colour point of a discharge lamp with a luminescent layer 
comprising a mixture of yttrium-aluminium-gallium garnet activated by 
trivalent cerium and MgWO.sub.4. In the garnet the molar quantity of 
aluminium is approximately equal to the molar quantity of gallium. The 
resulting colour point of the discharge lamp (x=0.512, y=0.412) is near 
white. Colour point 6 is the colour point of a discharge lamp with a 
luminescent layer comprising a mixture of yttrium silicate activated by 
trivalent terbium and MgWO.sub.4. Also in this case the resulting colour 
point of the discharge lamp (x=0.525, y=0.393) is near white. 
In FIGS. 2a and 2b respectively, colour points and luminous fluxes of 
discharge lamps provided with a neon gas filling and with a luminescent 
layer containing yttrium silicate activated with trivalent terbium are 
shown. FIG. 2a also shows the region in the I.E.C chromaticity diagram 
bounded by the lines y=0.429, y=0.398, y=-x+1 and y=-x+0.993. The E.C.E. 
requires that the colour point of (discharge) lamps that are used as 
direction indicator lights in or on a motorcar must be within this region. 
The neon filing pressure was 15 mbar, the inner diameter of the discharge 
vessel was 2.5 mm and the length of the discharge vessel was 40 cm. Colour 
point 1" represents the colour point of such a discharge lamp not 
comprising a filter. It can be seen in FIG. 2a that the colour point 1" 
does not meet the E.C.E. requirements. Colour points 1-3 and 1'-3' were 
measured for discharge lamps having the same luminescent layer, but being 
additionally equipped with a short wavelength blocking filter. Colour 
point 1 and 1' were measured for a discharge lamp equipped with a short 
wavelength blocking filter having 50% transmission at 495 nm. Similarly 
colour points 2 and 2' were measured for a discharge lamp equipped with a 
short wavelength blocking filter having 50% transmission at 515 nm. Colour 
points 3 and 3' were measured for a discharge lamp equipped with a short 
wavelength blocking filter having 50% transmission at 530 nm. In case of 
the colour points 1', 2' and 3' the discharge lamps were operated with a 
DC current of approximately 8 mA. In case of colour points 1, 2 and 3 the 
discharge lamps were operated with a DC-current of approximately 10 mA. It 
can be seen that the colour points 1-3 and 1'-3' all meet the E.C.E. 
requirements for direction indicator lights for motorcars. The luminous 
fluxes (.PHI.) of the discharge lamps equipped with a filter are also 
shown in FIG. 2b for both operation with a DC current of 8 mA and 
operation with a DC current of 10 mA. It can be seen that the luminous 
flux of the discharge lamps equipped with a filter are relatively high 
when these discharge lamps are operated with a DC current of 10 mA. 
In FIGS. 3a and 3b respectively, both colour points and luminous fluxes for 
discharge lamps having a neon gas filling and a luminescent layer are 
shown. The neon filling pressure was 15 mbar, the inner diameter of the 
discharge vessel was 2.5 mm and the length of the discharge vessel was 40 
cm. Again the colour point region corresponding to the E.C.E. requirements 
for direction indicator lights is shown. Colour point 1 was measured for a 
discharge lamp having a luminescent layer consisting of yttrium aluminium 
garnet activated with trivalent cerium and operated with a DC current of 8 
mA. Colour point 2 was measured for a discharge lamp with a luminescent 
layer consisting of yttrium silicate activated with trivalent terbium and 
operated with a DC current of 10 mA. Colour points 3 and 4 were measured 
for discharge lamps having a first luminescent layer LU consisting of 
yttrium silicate activated with trivalent terbium and a second luminescent 
layer LU present between the first luminescent layer and the wall of the 
discharge vessel, said second luminescent layer consisting of yttrium 
aluminium garnet activated with trivalent cerium see FIG. 7d. Colour point 
3 was measured when the discharge lamp was operated with a DC current of 
10 mA and colour point 4 was measured when the discharge lamp was operated 
with a DC current of 14 mA. It can be seen that colour point 1 and colour 
point 4 are within the region corresponding to the E.C.E. requirements. 
The luminous fluxes that were measured at the same time as the colour 
points are also shown in FIG. 3b. It can be seen that the discharge lamp 
with two luminescent layers can be operated in such a way that the colour 
point meets the E.C.E. requirements for indicator lights for motorcars 
while at the same time the luminous flux of the discharge lamp is 
relatively high. 
The data shown in FIGS. 4 and 5 were obtained for discharge lamps whose 
discharge vessels had an inner diameter of 2.5 mm and a length of 40 cm. 
Electrodes made from a chromium-nickel-iron alloy were provided at the 
ends of the discharge vessel. The discharge lamps were filled with 25 neon 
and the discharge vessel wall was coated with approximately 2.5 mg 
luminescent material per cm.sup.2 wall surface. The luminescent materials 
used were zinc silicate activated by bivalent manganese (supplier Philips; 
type G210) and yttrium-aluminium garnet activated by trivalent cerium 
(supplier Philips; type U728). The results shown in FIGS. 4 and 5 were 
obtained with a direct current of approximately 10 mA flowing through the 
discharge lamps during stationary lamp operation. 
In alternative discharge lamps, the lamp vessel wall was coated with 
yttrium-aluminium garnet activated by trivalent cerium (again 2.5 mg per 
cm.sup.2), and the neon filling pressure was 15 mbar. These alternative 
discharge lamps were partly provided with electrodes without emitter 
material and partly with electrodes having emitter material. It was found 
for these alternative discharge lamps that the colour point of the light 
radiated by the discharge lamp at a direct current of approximately 8 mA 
or less complied with the above E.C.E. requirements for direction 
indicator lights for use in/on a motorcar. The colour point remained 
within the required region also when the discharge lamp was aged. 
In FIG. 4, the burning time (t) in hours is plotted on the horizontal axis 
and the luminous flux (.PHI.) in lumens on the vertical axis. The 
discharge lamps had a comparatively high luminous flux which is well 
maintained with an increasing number of burning hours. It is apparent that 
the discharge lamps having a luminescent layer comprising 
yttrium-aluminium garnet activated by trivalent cerium (indicated with 
YAG-Ce in FIGS. 4 and 5) produce a considerably higher luminous flux than 
do the discharge lamps whose luminescent layer comprises zinc silicate 
activated by bivalent manganese (indicated with G210 in FIGS. 4 and 5). In 
addition, the luminous flux of the discharge lamps with YAG-Ce increases 
slightly during the first 250 burning hours, whereas the luminous flux of 
the discharge lamps with G210 decreases during the first 100 burning hours 
and then remains approximately constant. 
In FIG. 5, the burning time (t) in hours is plotted on the horizontal axis 
and the y coordinate of the light radiated by the discharge lamps on the 
vertical axis. It is apparent that the y coordinate of the discharge lamps 
with YAG-Ce rises slightly during the first 250 burning hours, whereas the 
y coordinate of discharge lamps with zinc silicate activated by bivalent 
manganese decreases substantially during approximately the first 100 
burning hours. 
In FIG. 6a colour points are shown that were obtained by operating a 
discharge lamp having a neon gas filling with a pressure of 5 mbar. The 
inner diameter of the lamp vessel was 3.5 mm and the length of the lamp 
vessel was 40 cm. The lamp vessel was equipped with a luminescent layer 
consisting of yttrium-aluminium garnet activated by trivalent cerium. The 
region corresponding to the E.C.E. requirements for indicator lights is 
also shown in FIG. 6a. It can be seen that the colour point is within the 
E.C.E. requirements in case the discharge lamp was operated by means of a 
DC-current with a substantially constant amplitude. The amplitude of the 
DC-current was 10, 15 and 20 mA and the corresponding colour points are 
indicated as DC-10, DC-15 and DC-20 respectively. In case the discharge 
lamp was operated with a AC-current having an rms value of 10, 15 and 20 
mA respectively, the obtained colour points AC-10, AC-15 and AC-20 are 
outside the region corresponding to the E.C.E. requirements. 
In FIG. 6b the luminous flux (.PHI.) of this discharge lamp for both DC- 
and AC-operation is plotted as a function of the rms value of the lamp 
current. It can be seen that the luminous flux obtained by DC-operation is 
substantially higher than the luminous flux obtained by AC-operation. It 
can therefore be concluded that DC-operation offers substantial advantages 
over AC-operation both in terms of luminous flux as well as in terms of 
the position of the colour point. Furthermore DC-operation can be realized 
using relatively simple means and causes no or only a very small amount of 
electromagnetic interference. 
In FIG. 7, FIG. 7a is a front elevation of a lighting unit according to the 
invention. FIG. 7b is a side elevation of the same lighting unit. La is a 
discharge lamp bent in a plane and provided with a gas filling consisting 
of neon. The discharge lamp wall is provided with a luminescent layer LU. 
H is a housing with a rectangular aperture. A mirroring reflector R is 
provided in the housing, forming the reflecting surface in this 
embodiment. The rectangular aperture of the housing is closed off with a 
light-transmitting cover D. Clamps K1-K5 in this embodiment form means for 
positioning the discharge lamp in the housing. FIG. 7c is a cross-section 
of the lighting unit of FIGS. 7a and 7b taken on the cross-section line 
shown in FIGS. 7a and 7b, perpendicular to the plane in which the 
discharge lamp La was bent.