Note: Descriptions are shown in the official language in which they were submitted.
CA 02280556 1999-08-20
Attorney Docket No.: 98P5548
High-pressure discharge lamp and associated illuminating system
Technical Field
The invention proceeds from a high-pressure discharge lamp in accordance with
the preamble of claim 1. It relates in particular in this case to a metal
halide lamp
with a two-end pinch, above all of high power.
Prior Art
The printed publication DE-A 44 43 354 has already disclosed a high-pressure
discharge lamp which has a radially asymmetric coating of the bulb. An
aperture
lamp is referred to in this case. This means that an increased emission is
achieved
from a noncovered narrow bulb region by providing the remaining bulb surface
with a reflecting coating. The coating in this case covers a substantial part
of the
bulb surface.
Axially asymmetric coatings of the bulb have been described in various ways.
For
example, DE-U 94 O1 436 has disclosed a metal halide lamp which is pinched at
two ends and is installed axially in a reflector in order to achieve a high
luminous
flux and a high uniformity in the emission. In addition to the known heat-
2 5 concentration spherical caps at the ends, this lamp also uses as radially
symmetric
layer a completely circumferential annulus in the middle of the bulbous
discharge
vessel.
Summary of the invention
It is the object of the present invention to provide a high-pressure discharge
lamp
in accordance with the preamble of claim 1 in which the vi~all loading of the
discharge vessel, and thus the temperature of the cold spot, is increased.
This object is achieved by means of the characterizing features of claim 1.
Particularly advantageous refinements are to be found in the dependent claims.
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A high wall loading increases the proportion of the filling substances in the
discharge arc and is therefore desired, because thereby the electrical and
lighting
data of the lamp are substantially improved.
An improvement in these data has so far been attempted by changing the
composition of the filling substance and reducing the volume of the discharge
vessel. To date, this has meant a high outlay on development in conjunction
with
an uncertain result. However, it has not as yet been possible thereby to set
the light
distribution curve of the lamp.
According to the invention, a radially asymmetric reflecting coating is
applied to
an essentially radially symmetric discharge vessel (which is, therefore,
essentially
circularly cylindrical in the radial direction). This coating is applied in a
locally
limited region which includes the cold spot, and has a limited extent whose
size is
preferably between 5 and 40% of the surface of the discharge vessel, in order
to
keep the shading as low as possible.
This reflecting coating can be produced by means of a physical and/or chemical
treatment of the surface of the discharge vessel. For example, a coating known
per
2 0 se with metallic (aluminum) or nonmetallic materials (in particular
zirconium
oxide), or a layer produced by means of sandblasting or etching is suitable.
By
selecting the layer geometry and the coating method, the lamp-specific data of
in
particular the color rendition, color temperature and light distribution curve
as well
as the color locus can be varied and set exactly as desired.
This improvement in the electrical and lighting data is achieved by means of a
specific asymmetric coating of the discharge vessel in the region of the cold
spot.
Particularly large improvements are achieved in the case of high-power (at
least
400 W to power far in excess of 1000 W) metal halide lamps without outer bulb,
3 0 since here the heat loss is particularly critical because of the lack of
the outer bulb.
The cold spot in these lamps is normally to be found behind the electrodes.
For this
reason, the ends of the discharge vessel are frequently fitted with heat-
concentration spherical caps. The effect of this is that the cold spot (which
then,
3 5 however, is at a higher temperature than when a spherical cap is dispensed
with) is
to be found at the deepest point of the discharge vessel, something which is
conditioned by the effect of gravity. The point is that the discharge arc is
deflected
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upward by the lift. A discharge vessel which is as isothermal as possible is
desired.
The dimension of the cold region about the cold spot (and consequently the
temperature gradient) in this case determines the extent of the asymmetric
coating.
This can either be a limited spot or axially elongated, for example like a
strip. The
spot is round or oval or elliptical.
In some circumstances, the geometrical arrangement of the asymmetric coating
is
determined by an associated illuminating system (for example a luminaire)
since
this system can influence the position and the dimension of the cold region
about
the cold spot. The constituent of an illuminating system which is essential
for this
purpose is mostly a reflector which, for example, is constructed as a trough
reflector or ellipsoidal reflector.
The position of the asymmetric coating can advantageously be defined relative
to
the exhaust tip of the discharge vessel, since this is best positioned upward
during
operation of the lamp as a potential heat sink. Consequently, the asymmetric
coating is fitted exactly opposite the exhaust tip. For practical reasons, as
well, no
coating is desired in the region of the exhaust tip, in order to avoid
distortions and
adhesive problems of the coating at the exhaust tip.
The lamp is preferably operated in a horizontal mounting position (referred to
the
longitudinal axis) and is distinguished by a coating which extends in the
axial
direction at least over 30% of the length of the discharge volume and which
extends in the radial direction over at least a center angle of 30°
with the lowest
2 5 point of the discharge vessel as the middle of the center angle. The
maximum
length of the coating in the axial direction is limited by the overall length
of the
discharge vessel, and is limited in the radial direction by a center angle of
at most
180°, since otherwise the effect of the shading would be too great. In
this case, a
circular shape or elliptical shape which is close to the circular shape is
assumed in
3 0 the radial direction for the cross section of the discharge vessel. In the
axial
direction, the discharge vessel is elongated and preferably has the shape
either of a
body of a barrel, a cylinder or an ellipse.
The asymmetric coating preferably stretches to near the ends of the discharge
3 5 volume, where the electrodes are seated. In this case, the known heat-
concentration
spherical cap can be constructed additionally at both ends.
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Suitable above all as illuminating system is a wall luminaire which has an
elongated reflector fitted laterally behind the horizontal lamp. This prevents
the
asymmetric coating from causing an appreciable shading of the radiation
emitted
by the luminaire.
The coating according to the invention renders it possible to raise the
lighting data
of a lamp without large changes to other parameters. It is even possible to
use the
same reflector paste as is already used for the end silvering. Again, there
are no
extra costs in the production, since no additional process step is required.
The extra
consumption of the reflector paste can be neglected.
A particularly valuable and environmentally friendly point of view is that,
because
of the higher operating temperature, it is possible to reduce by approximately
10-
mg the absolute quantity of Hg required to fill a lamp. Apart from this, there
is
15 no need for any change in the composition of the filling for the purpose of
improving lighting data.
In order to ensure the required unambiguous orientation of the lamp in the
correct
operating position, it is possible to fall back on a known orientatable
base/holder
2 0 system such as is described, for example, in US-A 5 731 656. It is
possible thereby
to ensure very easily the installation position which is required to maintain
an
optimum effect and for which the asymmetric coating points downward.
Figures
The invention is to be explained below in more detail with the aid of a
plurality of
exemplary embodiments. In the drawing:
Figure 1 shows a metal halide lamp in a side view,
Figure 2 shows a cross section through the metal halide lamp of Figure 1
Figure 3 shows a further exemplary embodiment of a metal halide lamp in plan
view of a pinch, and
Figure 4 shows a side view, rotated by 90°, of the lamp from
Figure 3.
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Description of the drawings
Figures 1 and 2 represent diagrammatically a 2000 W high-pressure discharge
lamp 1 without an outer bulb with a length of approximately 190 mm, such as is
described in more detail, for example, in US-A 5 142 195. It is intended for
use in
reflectors although being now arranged horizontally and transverse to the
reflector
axis instead of axially.
The discharge vessel 2 made from silica glass defines a longitudinal axis X
and is
designed as a barrel-shaped body whose generatrix is a circular arc. The
discharge
volume is approximately 20 cm3. The bar-shaped tungsten electrodes 6 with a
filament pushed on are aligned axially in pinches 5 at both ends of the
discharge
vessel. The electrodes 6 are fastened in the pinch 5 on foils 8 at which outer
supply
leads 9 start. A ceramic base 10 is fastened with the aid of cement on the end
of the
pinch 5 remote from the discharge. The discharge vessel 2 contains a filling
made
from an inert gas, mercury (180 mg now suffices instead of approximately 200
mg)
as well as metal halides. The ends of the discharge vessel are provided with a
heat-
concentration spherical cap 13 made from zirconium oxide.
2 0 In addition, an asymmetric coating 4 is applied to the discharge vessel 2,
specifically opposite the exhaust tip 3 in the region which is situated lowest
in the
installed state and contains the cold spot T. The asymmetric coating 4
likewise
consists of zirconium oxide and is an elongated, oval spot which has an axial
width
a of approximately 50% of the width of the barrel-shaped body. Its maximum
radial extent covers a center angle of approximately a = 55° (see
Figure 2). Only
the front half of the spot is to be seen in the side view of Figure l, in
which the
exhaust tip 3 is at the top.
A further exemplary embodiment is shown in Figures 3 and 4. This is a 1000 W
3 0 metal halide lamp 19 similar to that described in Figure 1. Figure 3 shows
a plan
view of a pinch 23. The lamp 19 is seen from below in Figure 4. The asymmetric
coating 20 extends over the entire axial length of the volume of the discharge
vessel 21 and is connected at the ends to the heat-concentration spherical
caps 22.
The radial extent corresponds to a center angle of approximately 90°.
The lamp is
3 5 intended for installation in a luminaire whose elliptical reflector 25 is
seated
laterally next to the lamp 19. The main direction of emission forward is
represented by an arrow 26.
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The color locus, the color rendition and the luminous flux, in particular, are
significantly improved by the asymmetric coating by comparison with the prior
art.
Moreover the operating voltage of the lamp is raised on average by 20 V to
approximately 125 V. A comparison between an uncoated and a coated lamp is to
be found in Table 1, where the luminous flux ~ (in klm), the color coordinates
x
and y and the color rendition index Ra are given for both lamps. There is a
substantial improvement in the luminous flux (by 7%) and the color rendition
index (by 15%). The color locus is plainly closer to white (x,y=0.333) or to
the
curve for the Planckian radiator.
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Table 1
Uncoated lamp Coated lamp
~ (klm) 80.7 86.8
x 0.297 0.332
y 0.377 0.368
Ra 75 86
A wipe-resistant reflector paste based on zirconium oxide was used as coating
material. Its layer thickness and homogeneity correspond to the usual value
for
heat-concentration spherical caps. The lamp was coated in a region about the
cold
spot at which filling condensate forms during operation. This region is
situated in a
special case on the side opposite the exhaust tip.
