LOW-PRESSURE DISCHARGE LAMP, PARTICULARLY COMPACT FLUORESCENT LAMP, ESPECIALLY FOR OUTDOOR AND INDOOR USE

LAMPE A DECHARGE A BASSE PRESSION, EN PARTICULIER LAMPE FLUORESCENTE COMPACTE, NOTAMMENT POUR USAGE A L'EXTERIEUR ET A L'INTERIEUR

English Abstract

To permit operation of a compact fluorescent lamp in an
outdoor environment, for example at an annual average temperature
of 10°C, a U-shaped discharge vessel has two straight legs of
essentially circular cross section, connected by a cross-
connecting portion (10) forming a 180° bend, which, to permit
such outdoor use, has its inner diameter constricted from a
transition with the longitudinal legs to an apex point of the
bend. Preferably, the constriction results in an essentially
elliptical cross section, in which the relationship of the minor
axis (d) to the major axis (D) is
<IMG>
the minor axis (d) extending in the direction of the legs, and
the major axis (D) having a dimension corresponding about to the
inner diameter of the legs. Preferably, the elliptical shape is
obtained by thickening the outermost wall (10a) of the cross-
connecting portion (10). The arrangement provides for insulation
of the region where the cold spot forms so that, even under lower
ambient temperatures, a suitable cold-spot temperature, which
controls vapor pressure and vapor density in the lamp in
operation, is obtained.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Low-pressure discharge lamp, especially compact
fluorescent lamp, having
a discharge vessel comprising at least one U-bent glass
tube defining two tube legs of essentially circular cross section,
and a cross-connecting portion bent essentially 180°;
electrodes located at terminal ends of the tube legs of
the discharge vessel;
a fill gas including mercury and at least one noble gas
within the discharge vessel;
a luminescent coating at the interior of the discharge
vessel; and
wherein, in accordance with the invention,
the inner diameter of the essentially 180° cross-
connecting portion decreases from a transition with the legs to an
apex (I) of said cross-connecting portion to have, in cross
section at said apex, essentially elliptical shape, in which the
relationship of the minor axis (d) of the ellipse to the major
axis (D) of the ellipse is defined by
<IMG>
the minor axis (d) is positioned in a plane essentially parallel
to the direction of said legs; and
wherein the major axis (D) has a dimension which
corresponds essentially to the dimension of the diameter of said
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legs.
2. The lamp of claim 1, wherein said relationship of the
minor axis (d) to the major axis (D) is:
<IMG>
3. The lamp of claim 1, wherein the wall of the 180° cross-
connecting portion, at the region remote from a base of the lamp
has a wall thickness s which is related to the wall thickness w of
the legs of the discharge vessel by the relationship
<IMG>
4. The lamp of claim 3, wherein said relationship is:
<IMG>
5. The lamp of claim 1, wherein said discharge vessel
comprises two U-bent glass tubes, each one of said U-bent glass
tubes having one electrode in one of the respective legs, only;
and
a through-interconnection connecting the legs of the two
U-bent tubes which do not carry electrodes, to form a continuous
discharge path through both U-bent glass tubes.
6. Low-pressure discharge lamp, especially compact
fluorescent lamp, having
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a discharge vessel comprising two essentially parallel
tube legs of essentially circular cross section, and a cross-
connecting portion interconnecting said tube legs;
electrodes located at the terminal end of the tube legs
of the discharge vessel;
a fill gas including mercury and at least one noble gas;
a fluorescent coating at the interior of the discharge
vessel,
said electrodes, when energized, establishing a
continuous discharge within the vessel, which discharge extends
through said essentially parallel legs and, in and about 180°
bend, through the cross-connecting portion,
said legs and said cross-connecting portion being formed
to establish, in the presence of the discharge, cold spots in
regions remote from the legs where the electrodes are located,
and wherein at least one of: the tube legs and the
cross-connecting portion are dimensioned and shaped to result in
cold spots, in operation of the lamp, when the lamp is subjected
to an average ambient surrounding temperature in the order of
about 10°C.
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Description

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FIELD OF THE INVENTION
The present invention relates to compact fluorescent
lamps which, technologically, are low-pressure discharge lamps in
which the discharge excites a fluorescent coating, and more
particularly to such lamps which are especially adapted for
outdoor use, although they have universal application for outdoor
and indoor use.
BACKGROUND
United States Patent No. 4,481,442, (Albrecht et al..)
and United States Patent No. 4,853,591, Klein et al., describe
compact fluorescent lamps. They are small in size and, up to now,
have been used mostly for indoor illumination. The lamps are
designed to have maximum light output at a temperature of about
25°C, which is a frequent indoor temperature. This temperature of
about 25°C is frequently an average temperature at which these
lamps are used. The maximum light output is determined by the
density and the vapor pressure of mercury within the lamp at the
coldest point, or cold spot within the discharge vessel. The
discharge vessel, in order to provide the requisite density, or
the requisite vapor pressure, respectively, are so constructed
that corners of 180° bends, interconnecting parallel legs of a
generally U-shaped discharge vessel are not within the path of the
discharge arc, so that mercury can condense at the corners. Other
constructions than unitary U-shaped discharge vessels are known.
The economical use of energy by these compact
fluorescent lamps make them suitable also for outdoor
illumination. The average outdoor temperature, however, which, in
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a temperate zone, is at an average of about 10°C, does not permit
maximum light emission from these lamps, since the maximum light
emission capability is not utilized at the lower temperature.
THE INVENTION
It is an object to provide a low-pressure discharge
lamp, and especially a compact fluorescent lamp, which has optimum
light output even at lower ambient temperatures than those for
which the lamps were heretofore designed.
In one aspect, the invention provides low-pressure
discharge lamp, especially compact fluorescent lamp, having a
discharge vessel comprising at least one U-bent glass tube
defining two tube legs of essentially circular cross section, and
a cross-connecting portion bent essentially 180°; electrodes
located at terminal ends of the tube legs of the discharge vessel;
a fill gas including mercury and at least one noble gas within the
discharge vessel; a luminescent coating at the interior of the
discharge vessel; and wherein, in accordance with the
invention, the inner diameter of the essentially 180° cross-
connecting portion decreases from a transition with the legs to an
apex (I) of said cross-connecting portion to have, in cross
section at said apex, essentially elliptical shape, in which the
relationship of the minor axis (d) of the ellipse to the major
axis (D) of the ellipse is defined by
0.5 < d < 0.9;
D
the minor axis (d) is positioned in a plane essentially parallel
to the direction of said legs; and wherein the major axis (D) has
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a dimension which corresponds essentially to the dimension of the
diameter of said legs.
In another aspect, the invention provides low-pressure
discharge lamp, especially compact fluorescent lamp, having a
discharge vessel comprising two essentially parallel tube legs of
essentially circular cross section, and a cross-connecting portion
interconnecting said tube legs; electrodes located at the terminal
end of the tube legs of the discharge vessel; a fill gas including
mercury and at least one noble gas; a fluorescent coating at the
interior of the discharge vessel, said electrodes, when energized,
establishing a continuous discharge within the vessel, which
discharge extends through said essentially parallel legs and, in
and about 180° bend, through the cross-connecting portion, said
legs and said cross-connecting portion being formed to establish,
in the presence of the discharge, cold spots in regions remote
from the legs where the electrodes are located, and wherein at
least one of: the tube legs and the cross-connecting portion are
dimensioned and shaped to result in cold spots, in operation of
the lamp, when the lamp is subjected to an average ambient
surrounding temperature in the order of about 10°C.
Briefly, the lamps have a discharge vessel which is
arranged to provide for cold spots in regions remote from the base
of the lamp. In one embodiment, the lamps have two leg portions
and a cross-connecting portion to form an essentially 180° bent U-
tube. In accordance with a feature of the invention, the inner
diameter of the cross-connecting portion providing for an
essentially 180° turn discharge arc decreases from a transition
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region with the legs by thickening the glass walls in the region
of the cross connection. In unitary lamps with an integral cross
connection, the inner cross section of this cross connection
decreases from the essentially circular cross section of the leg
portions to assume essentially elliptical shape. The smaller axis
d of the ellipse extends in a direction, or is located in a plane,
which is essentially parallel to the direction of the legs. The
larger axis D of the ellipse has a dimension which corresponds
essentially to the diameter of the leg portions. The relationship
of the smaller axis d to the larger axis D is given by the
formula:
0.5 _< _d _< 0.9
D
In a particularly preferred embodiment, the relationship
of the diameters is given by:
0.6 _< _d _< 0.8
D
The wall thickness s of the cross connection at the apex
of the bend is related to the wall thickness w of the legs of the
discharge tube by the relationship:
1.0 x w < s < 3.0 x w
In accordance with preferred feature, the relationship
is:
1.5 x w < s < 2.5 x w
By constricting the inner diameter of the discharge
vessel in the region of the 180° bend to an ellipse, the cold
spots are placed closer to the discharge arc. If this lamp is
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operated indoors, that 1s, with the higher normally present
interior temperature, the temperature would be too high for
maximum light output. Yet, when operated outdoors, with the lower
average outdoor temperature, the appropriate temperature range for
maximum light output is obtained. Since lamps are used at night,
higher daytime temperatures do not matter.
Thickening the discharge vessel in the region of the
180° bend, by thickening the outer glass wall, is particularly
suitable to improve the utility of the compact fluorescent lamps
at low ambient temperatures. Thickening the wall improves the
heat insulation, so that even extremely low external ambient
temperatures do not have a large influence on the light output.
Tnvestigations have shown that optimum results are obtained when
the wall thickness varies between 1-3 times, and particularly
between 1.5 to 2.5 times the wall thickness of the straight leg
portions of a U-bent unitary discharge vessel.
DRAWINGS
Figure 1 is a highly schematic side view of a compact
fluorescent lamp embodying the present invention, and
illustrating, also schematically, the lamp in a base;
Figure 2 is a schematic side view, with part of one lamp wall cut
away and partly in section, of the discharge vessel of the lamp of
Figure 1;
Figure 3 is a cross section through the apex of the
discharge vessel at the section lines I-I' in Figure 2;
Figure 4 is a cross-sectional view through the discharge
vessel using multiple, in the example two interconnected U-tubes;
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and
Figure 5 is a graph relating temperature to light flux
of lamps of the prior art and the present invention.
DETAILED DESCRIPTION
The lamp of Figure 1 is a compact fluorescent lamp
having a power rating of 18 W. The lamp 1 has a discharge vessel
2, the length of which is not to scale and shown by the chain-
dotted cut lines in reduced size. The discharge vessel 2 is
retained in a base 3 of the type 2 G 11. The base 3 has four
connecting pins or connecting lugs 4, 5, 6, 7 which provide
electrical energy to the lamp from a suitable socket which may,
also, contain accessory circuits, ballasts, starters and the like.
The discharge vessel 2 is shown in Figure 2, again in
foreshortened representation. It has a length of 19.6 cm and is
formed of two straight, parallel legs 8, 9, coupled by a part-
circular cross-connecting portion 10, forming a 180° bend. The
free ends of the two leg portions 8, 9 have current supply leads
11, 12 and 13, 14, respectively, pinch-sealed therethrough. The
current supply leads 11, 12 and 13, 14, respectively, are
connected to electrode filaments 15, 16, retained in position by
glass beads 17, 18 within the interior of the discharge vessel. -
The longitudinal legs, in cross section, are essentially circular
and have an inner diameter of 15.5 mm. The wall thickness is
about 1 mm. A fluorescent coating 22 is applied to the interior
of the legs as well as the cross-connecting portion 10. In one of
the two pinch seals 19, 20, as shown in the pinch seal 20, an
exhaust-and-fill tube 21 is melt-sealed which, after filling the
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discharge vessel 2, is tipped off. The discharge vessel retains a
fill of mercury and a noble gas.
In accordance with a feature of the invention, the 180°
cross-connecting bend 10 constricts towards the central region or
the apex of the cross connection, as shown by section line I-I' in
Figure 3. The inner diameter of the bent cross connection has a
transition region to the longitudinal legs 8, 9 and their
essentially circular inner diameter of 15.5 mm. Towards the
center of the cross connection, the transition region constricts
to an elliptical form, having a major axis D of 15.5 mm, that is,
the same diameter as the longitudinal legs, and a minor axis d of
10 mm. The minor axis d extends essentially parallel to the
direction of the longitudinal legs 8,9.
In accordance with a feature of the invention, the wall
of the discharge vessel 10, at the outer surface of the 180°
connecting bend 10 is thickened. At the portion of the ellipse
closest to the base 3, the wall thickness is about 1 mm. At the
remote, or outer side 10a, the wall thickness increases to about 2
mm.
The invention is not limited to a low-pressure discharge
tube having a discharge vessel with only one U-shaped glass tube.
The discharge vessel may be formed of a plurality of U-shaped
glass tubes, as described in detail, for example, in the
referenced United States Patent x,853,591, Klein et al. In that
case, the 180° bend of each one of the U-tubes is formed in
accordance with the present invention, so that the lamp will have
the high light output even at low ambient operating temperatures.
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In a two-tube unit, one unit will have first and second
longitudinal legs 8a, 8b, and the other unit legs 9a, 9b. One of
the legs of each unit has a filament 114, 115 therein. The two
units are interconnected by a cross connection 23, close to the
lower or base end of the units, to form a two-tube interconnected
fluorescent tube lamp 1'.
Figure 5 illustrates the relative light outputs as a
function of the ambient temperature for two different compact
fluorescent lamps having a rated wattage of 24 W, in base-up
operating position.
Curve H illustrates the light output of a conventional
compact fluorescent lamp having a U-shaped discharge vessel with
rectangular corners such as is described, for instance, in United
States 4,481,442, Albrecht et al. Curve A illustrates the light
output of the compact fluorescent lamp of the present invention
having a U-shaped discharge vessel for the use in outdoor
fixtures. The maximum light emission of Curve B is at 25°C to
30°C; in Curve A the maximum light emission is about 15°C lower.
Various changes and modifications may be made within the
scope of the inventive concept.
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Representative Drawing